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THE

MICROSCOPE

AND

ITS REVELATIONS

BY THE LATE

WILLIAM B. CAEPENTEE, C.B., M.D., LL.D, F.B.S,

SEVENTH EDITION

IN WHICH THE FIRST SEVEN CHAPTERS HAVE BEEN ENTIRELY REWRITTEN
AND THE TEXT THROUGHOUT RECONSTRUCTED, ENLARGED, AND REVISED

BY THE REV.

W. H. DALLINGEB, LL.D., F.E.S., &c.

WITH TWENTY-ONE PLATES AND EIGHT HUNDRED WOOD ENGRAVINGS

PHILADELPHIA

P. BLAKISTON, SON, & CO.

1012 WALNUT STREET
1891

42Q\oQ

SCIENCE

QH

205
C3

PEEF A C E

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 conditions on which they can be em-
ployed with the best results. Beyond this it should provide an
outline of the latest and best modes of preparing, examining, and
mounting objects, and glance, with this purpose in view, at what is
easily accessible for the requirements of the amateur in the entire
organic and inorganic kingdoms.

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

But the Microscope in its very highest form has become — so far
at least as objectives of the most perfect construction 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 1 high powers ' is a want very widely felt.

The advances in the mathematical 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
have, 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

vi

PEEFACE

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 understands 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.' No one can work properly with either
if he does not understand the theory of their construction and the
principles upon which to interpret the results of their employment.
If he is familiar with these the employment of any range of magni-
fying power is simply a question of care, experiment, and practice;
the principles applicable to the one are involved in the other. Thus,
for example, a proper understanding of the nature and mode of
optical action of a 'sub-stage condenser ' is as essential for the very
finest results in the use of a 1-inch object-glass as in the use of a
2 mm. with N.A. 110 or (he :>•"> mm. with N.A. 1-G0, 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 Tj^-th inch achromatic objectives, and especially the -^th inch
and Jnth 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, tlie 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
mounting of objects, and a review of the whole Animal, Vegetable,
and Inorganic Kingdoms specially suited for microscopic purposes.

PREF.W E

vii

must be essentially a cyclopaedic work. This was far more possible
to one man when' Dr. Carpenter began his work than it was 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 I have been most generously aided. In
no department, not even that in which for twenty years I have
been specially at work, have I acted without the cordial interest,
suggestion, and enlightenment afforded by kindred or similar workers.
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 familiarised me,
more or less, with every department of Microscopy, and with the
great majority of branches to which it is applied. I have therefore
given a common form, for which I take the sole responsibility,
to the entire treatise. The subject might have been carried 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 subjects 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 be
read by anyone familiar with the use of algebraic symbols and the
practice of the rule of three. They are not in any sense abstruse,
and they are everywhere practical.

In the second chapter, on The Principles and Theory of Vision
with the Compound 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, I 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
words which show his cordial friendliness, he says : 'I find the whole
. . . much more adequate to the purposes of the book than I should

Vlll

PREFACE

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 lie 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, \ must place my friend Mr. E. M. Nklson. 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 tome. He placed^nis know-
ledge, instruments, and experience at my disposal, fully and without
limit or condition ; and his except ional 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 Diatoinacca, to he used at my discretion ; to Dr.
van Heurck I am .dsn under much obligation for his eourtesv in
preparing Plate XL pi this hook, giving some of his photo-micro-
graphic work with the new object glass of 2*5 nun. 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
lamented Dr. H. B. BRADY, K.R.S., I am under obligation for
valuable suggestions regarding the Foramin ifera.

PREFACE

ix

From Dr. Hudson I have received cordial aid in dealing with
his special subject, the Rotifera ; and to Mr. Albert Michael j 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. Langley, of Trinity College,
Cambridge — from both of whom special processes of preparation
for histological work were sent.

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

To Professor W. Hicks, F.R.S., Principal of Firth College,
Sheffield, I am indebted for the revision of special sheets ; so also I
owe acknowledgments to Dr. Henry Clifton Sorby, F.R.S., and to
Dr. Groves, as well as to others, whose suggestions, advice, or con-
firmation of my judgments have been much esteemed ; and prominent
amongst these are Professor Alfred W. Bennett, B.Sc, and Professor
F. Jeffrey Bell, M.A., whose constant advice in their departments
of Biology I have received throughout • while in Micro-geological
subjects 1 have been aided by the suggestions and experience of
Professor J. She arson Hyland, 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 sections,
in fact, have been wholly rewritten and illustrated from new and
original sources : this may be seen in the sections on the History
as well as the Construction and Use of the Microscope and its appli-
ances, as also in those on Diatomaceae, 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 the 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 f for elegant and instructive lelaxaticn and
amusement. For this there can be nothing but commendation, but it is
desirable that even this end should be sought intelligently. The social
influence of the Microscope as an instrument employed for recreation

X

PEE FACE

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 smd 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 ;i practical actempt — 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 : lain.

EXPLANATION OF PLATES

FRONTISPIECE

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 ^-inch objective of "95 N.A. x 3 projection eye-piece ;
but it was illuminated by a cone of small angle, viz. of 0-1 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 minute hairs on the
lining membrane of the extremity of the blow-fly's proboscis. In this picture
the focus has been adjusted for the long central hair. It will be observed that
this hair is very fine and spinous ; it has not the ring socket which is common
to many hairs on insects, but grows from a very delicate membrane, which in
the balsam mount is transparent. This photograph was taken with a Zeiss
apochromatic j of "95 N.A. x 3 projection eye-piece. The illumination 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 a lamb, x 77 diams.. by apochromatic 1-inch
.3 N.A. This preparation was courteously supplied to the present Editor by Dr.
Hill, whose imbedding and staining processes for these tissues it beautifully
illustrates.

Fig. 5. Amphipleura pellucida x 18(50 diams., by apochromatic £ 1*4 N.A.
illuminated by a very oblique pencil in one azimuth along the valve.

Fig. 6. A hair of Polyxenus lagurus, a well-known and excellent test
object for medium powers x 490 diams. by apochromatic ~ "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 1 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

Xll

EXPLANATION OF PLATES

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

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

PLATE I

Fig. 1. The inside of a valve of Pleurosigma angulatmn, showing a
' postage stamp ' fracture, x 1750 diams., with an apochromatic ^ LI N.A. by
Mr. T. P. Smith, and illustrating his view of the nature of the Pleurosigma
valve.

Fig. 2. The outside of a valve of Pleurosigma angulatum, showing a dif-
ferent form of structure, x 1750 diams., with an apochromatic ^ LI 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 £1*4 N.A.

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

Fig. G. A small portion in the centre of an Aulacodiscus Sturtii, x 2000,
by an apochromatic £ LI 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

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

PLATE III

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

PLATE IV

THE METHOD OF USING THE SILVER SIDE REFLECTOR OR PARABOLOID

1 A section of this diatom will be found in the Transactions of the County of Middlesex
Natural History Society for 1889, Plate I. fig; 2.

EXPLANATION OF PLATES

x i i i

PLATE V

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.

PLATE VI

SEXUAL GENERATION OF VOLVOX GLOBATOR. (After Cohnj

Fig. 1. Sphere of Volrox glohator at the epoch of sexual generation : a,
sperm-cell containing cluster of antherozoids ; a', sperm-cell showing side-
view of discoidal cluster of antherozoids : a3, sperm-cell whose cluster has
broken up into its component antherozoids ; a1, sperm-cell partly emptied by
the escape of its antherozoids: b b, flask-shaped germ-cells showing great
increase in size without subdivision ; b-, ft2, 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. Sperni-cell, with its contained cluster of antherozoids, more
enlarged.

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

PLATE VII

OSCILLARIACE.E AND SCYTONEMACEJE

Fig. 1. Lyngbya cestuarii, Lieb. x 1G0.

Fig. 2. S/nriilhia Jennrri, Ktz. x 400.

Fig. 3. Tolypothrix cirrho&a, Garm. x 400.

Fig. 4. Oscilloria insigitis, Thw. x 400.

Fig. 5. O. Frolichu, Ktz. x 400.

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

PLATE VIII

DESMlDIACEiE, R1VULARIACE.E, AND SC\ TOXEM ACEJE

Fig. 1. Zygosperm of Mlerasterias denticulata, Breb. (After Palfs.)

Fig. 2. Cosmarium BrebissopJi, Men. (Af'ier Cooke.)

Fig. 3. Euastrum pectinatum, Breb. (After Ralfs.)

Fig. 4. Zygosperm of Staurastrnm hirsutum, Breb. (Aftar Ralfs.)

Fig. 5. 8. graeUe, Ralfs. (After Cooke.)

Fig. 0. Xanthidium aculeatum, Ehrb. (After RalfsA

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

Fig. 8. R. dura, Ktz. x 400. (After Cooke.)

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

Fig. 10. Sto.uraxtrum hirsutxim, Breb. (After Cooke.)

xiv

EXPLANATION OF PLATES

PLATE IX

DESMIDIACE^E

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

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

Fig. 3. Desmidiwm Swartzii, Ag. (After Cooke.)

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

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

Fig. 6. Spirotcsnia condensates Breb. (After Cooke.)

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

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

PLATE X
PLEUROSIGMA ANGULATUM

This is a direct photo-micrograph, taken by Dr. R. Zeiss, as magnified 1900
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.

PLATE XI

This plate has a twofold purpose. It is designed, first, to justify the
opinions held by Dr. Henry van Heurck upon the structure of the valves of
diatoms, and also to show how the usual microscopical tests present them-
selves when examined with the new objective with N.A. 1-60, lately constructed
b}- the Firm of Zeiss. This objective is believed by Dr. van Heurck to realise
what he considers the highest results of photographic optics, which in his
judgment could only be surpassed by finding a new immersion liquid of still
higher refractive index presenting all the necessary qualities, and which at the
same time would not affect the very delicate flint of which it is necessary to
make the front lens of thisobjective. This mediumhe 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 1 is formed by two membranes or thin plates and
by an intermediate septum. By this he understands a plate pierced with
openings. The superior membrane, often very delicate, may be destroyed
in the treatment by acids in the washings, by rubbing, &c. It is possible also
that it sometimes only exists in a very rudimentary state. The majority of
the students of diatoms agree in believing that these membranes ma}r 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 sepl ma 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 suc-
cessively 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.

1 ' The structure of the Valve of Diatoms ' In Uncords of the Belgian Society, v. siii. 1890.

EXPLANATION OF PLATES

XV

Such, in brief, is the view held by Dr. van Heurck as an interpret ation 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. AmpMpleura jxllucida, Kiitz, 1 and 2, valve resolved into
pearls. Fig. 2 x 2000 diarns. Fig. 1 x 3000 diams. Fig. 3. Valve resolved
in striae at about 2300 diams.

Fig. 4. Ampldpleura Lindheimeri, Gr., x 2500 diams.

Fig. 5. Pleurosigma angulation, 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. 1 -6
cannot be used here.

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

Fig. 9. Van HevrcJna crassinervis, Breb. (Frustulia saxonica, Rabh) x 2000
diams.

All the photo-micrographs (except fig. 7) have been done with the new
inch N.A. 1-60 of MM. Zeiss.

These micro-photographs have been produced by sunlight in a monochro-
matic form, the special compensating eve-piece 12, and the Abbe condenser of
N.A. 16

Covers and slides in flint of 1*72 ; 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 tine corrections of at least one
of the lenses of N.A. 1*6, we are convinced that much of its real perfection
in image-forming is destroyed by uncorrected sub-stage illumination. Upon
the corrections and large aplanatic area presented by the condenser and its
careful and efficient employment depends entirely the nature of the image
presented by the finest objective ever constructed ; and as the perfection of the
objective, with a high amplification and a great aperture, is more nearly
approached, the more dependent are we upon perfect corrections in the con-
denser to bring out the perfect image-forming power of the objective. No
image formed by such an objective as that possessing N.A. 1-60 can be consi-
dered reliable until a condenser corrected for all abberrations like the objective
itself is produced ; and so convinced are we of the possible value of this objec-
tive that we trust its distinguished devisor 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 of 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
arachnoidiscus japonicus. (After R. Beck)

The specimens attached to the surface of a sea-weed 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

COMPLETE LIFE-HISTOKIES OF TWO SAPROPHYTES

(Drawn from nature by Dr. Dallinger)

xvi

EXPLANATION OF PLATES

PLATE XIV

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 liaryoln-
nesis, and proving, as established in detail by the text, that all the steps in
the cvclic changes of these unicellular forms are initiated in the nucleus befce
being participated in by the whole body of the organism. (Drawn from nature
by Dr. Dallinger.)

PLATE XV

EOTIFERiE

Fig. 1. Floscularia campanuhita.

Fig. 2. Stephanoccros Eiehhornii.

Fig. 3. Jlelicerta ringens.

Fig. 4. Pedalion mirum (side view).

Fig. 5. P. mirum (dorsal view, showing muscles).

Fig. 6. Copevs 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 XVI

FORAMINIFERA

Fig. 1. Milwlina seminulivm (a and b, lateral aspects).

Fig. 2. AlveolVna Boscii (a, lateral aspect ; b, longitudinal section).

Fig. 3. Astrorliiza limicola (ay lateral aspect; b, portion of the test more
highly magnified, showing structure).

Fig. i. Haliphysema Tumanorviczii, showing the pseudo-polythalamous
foot.

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

Fig. 6. Haplojyhragmium agglutinans (a, lateral aspect; b, longitudinal

section).

Fig. 7. //. ncmwm (a, superior aspect ; b, peripheral aspect).
Fig. 8. Te.rtularia gramen {a, lateral aspect ; b, oral aspect).
Fig. 9. T. gramcn (peripheral aspect).

Fig. 9«. Paroninu Habclliformis (a, lateral aspect; b, oral aspect).
Fig. 10. J In I mi/I itt spinvlosa.

Fig. 11. Chilostomella oroidea (a and b, lateral aspects; e, specimen
mounted in Canada balsam and seen with transmitted light).

T>LATE XVII

FOEAMINIFERA

Fig. 12. Lageva sulcata.
Fig. 13. L. sulcata.
Fig. 14. L. sulcata.

Fig. 15. L. sulcata (a, lateral aspect ; b, oral aspect).
Kig. 1<>. Kodosaria raplianvs.

Fig. 17. Cristellaria. calcar (a, b, c, lateral aspects).
Fig. 18. Ramulina glolndifera.
Fig. 19. It. globulifera.

Fig. 20. Globigerina bulloides (var. triloba, pelagic specimen).
Fig. 21. G. bulloides (a, b. o, adult typical shell).

EXPLANATION OF PLATE S

xvii

Fig. 22. Rotalia Becearii.

Fig. 23. Polystomella craticulata.

Fig. 24. Ampliistegina Lessonii (at superior lateral aspect'; b, inferior lateral
aspect ; <?, peripheral aspect).

Fig. 25. Nummulitcs laevigata (&, lateral aspect ; a, vertical section).
Fig. 2G. Portion of Orbitoides nvmumlitica.

PLATES XVIII, XIX, XX

ACARINA

All the figures, except fig. 4, Plate XX., 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 XVIII. , and 1 to 3, Plate
XIX., are from ' British Oribatidae,' published by the Ray Society ; fig. 7, Plate

XVIII. , from the • Journal of the Linnean Society ' ; fig. 4, Plate XIX., and fig. 3,
Plate XX., from the 1 Journal of the Royal Microscopical Society ' ; fig. 5, Plate

XIX. , and figs. 1 and 2, Plate XX., from the ' Journal of the Quekett Micro-
scopical Club.' Fig. 4, Plate XX., is drawn after Furstenberg by the Editor.

PLATE XVIII

OEIBATID^E

Fig. 1. Anatomy of Nothrus theleproctiis (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 oesophagus. The ventri-
culus is coloured pink ; part of it and the whole of the caeca are covered with
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). Thechitin
at the side and the fatty tissue and muscles have been removed. Alimentary
canal pink ; caeca of the ventriculus spotted ; preventricular glands brown ;
supercoxal gland white ; its vesicles yellow ; expulsory vesicle, between
supercoxal and ovaries, grey ; ovary and oviducts white shaded with blue and
yellow. The genital and anal plates are open, and the genital suckers pro-
truding. One maxilla, white, is seen between the legs.

Fig. 3. Tegeocramis lotus (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 tracheae, 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 tegeocramis ( x about 25), Vigt.
Central ovary, oviducts with eggs, vagina and ovipositor.

Fig. 5. The same of Damceus geniculatvs ( x about 20). The genital plates
and the muscles and tendons which move them, and the genital suckers, are
shown.

These two figures are reduced from the originals.

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

TYKOGLYPHIDiE

Fig. 7. Hypopial (travelling) nymph of Bhtzoglgphits Robini (ventral
aspect, x 100).

xviii

EXPLANATION OF PLATES

PLATE XIX

OKIBATIDiE

Fig. 1. Leiosoma palmieinetwn (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 dorsol
abdominal skin. The other rows of scales belong to the successive nympha-
skins.

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

CHEYLETIDiE

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

MYOBIIDiE

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

PLATE XX

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

GAMAS1DJE

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

ANALGINiE

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

Errata

Page 375, line 25, for oscillariae read Oscillariacece.
,, 677, „ 7, for Coccodia read < 'occidia.
„ 702, ,, '.i from bottom, for < iphridiu in read Ophrpdinm.
„ 706, „ 3, for Ophnjdu read Ophrydia.
„ 770, „ 14, for single read simple.

PLATE 1.

X270

■1

X2000

THE MICROSCOPE

CHAPTER I

ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

'To be the owner of a well-chosen and admirably equipped micro-
scope, 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
instrument, nor 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 are
needful to enable the owner to use either instrument. In the cas<
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 essential,
the neglect of it being the constant cause of loss of early enthusiasm
and not infrequent total failure.

In the following pages we propose to treat the elementary
principles of the optics of the microscope in a practical manner,
not merely laying down dogmatic statements; but endeavouring to
show the student how to demonstrate and comprehend the applica-
tion of each general principle. But in doing this we are bound to
remember 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 found

2 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

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 formulae.

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 angle with the normal.
Conversely, a ray passing out from a dense medium into a rarer one,
making a certain angle with the normal will, on emergence from the
dense medium, make a greater angle with the normal.

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

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

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

When one of the media is air (accurately a vacuum) the ratio of
these sines is called the absolute refractive index of the medium.
As every known medium is denser than a 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.

Further, the absolute refractive index for any particular substance
will differ according to the colour of the ray of light employed. The
refraction is least for the red, and greatest for the violet. The
difference between these refractive values determines what is called
the dispersive power of the substance.

This will be understood by fig. 1. Let I C, a ray of light travel-
ling in air, meet the surface A B of water at the point C. Through
C draw N at right angles to the surface of the water A B. The
line N W is called the normal to the surface A B. The ray I C will
not continue its path through the water in a straight line to Q ; but,
because water is denser than air, it will be bent to B, that is
towards N;. The whole course of the ray will be I C B, of which
the part I C is called the incident ray, and C B the refracted ray.

1 Vide Chambers's Mathematical Tables.

THE LAW OF SINKS

3

The angle IC makes with the normal NN', viz. IC X, is called the
angle of incidence ; and the angle R C makes with the normal N N,
viz. R C N', is called the angle of ref raction.

Conversely, if a ray R C, travelling in water, meet the surface of
air AB in the point C, it will not continue in a straight line, but
will be bent to the point I farther away from N. Thus, when a
ray passes from a 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 figure were glass instead of
water, the refracted ray R C would be bent still nearer N', and,
conversely, if the ray passed out of glass into air, it would be

AT

r

Fig. 1. — The refraction of light. The law of sines.

more bent away from the normal than if it had passed out of water
into air.

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

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

sine I CN » ,. • t £

— ^ = u. = the retractive index or water.

smeRCN'

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

b 2

4 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

]? T F H

Now, as sine ICN = and sine R C N' = — — , then, by

PC F (J

Snell's law.

As any points may be taken in I C.and It 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

KS "

sine I C N = and sine RON' = J "P , and therefore ^ ® = ix.

kg ec inr

EC

But as KC = EC by construction, we can write K C for E C
K_S

thus : a. K C is cancelled, which leaves =-|t = m.

ttet 1 ed

As /x 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 /x and the angle of incidence
are known, the angle of refraction can be found ; and if [x and the
angle of refraction are known, the angle of incidence can be found.
The unknown quantity can be found either geometrically or by cal-
culation when the other two terms are given.

It will, of course, be understood that, for the same medium in
every case, a red ray would be bent or refracted less than a violet
ray. The value therefore of \x for a red ray will be less than that of
fx' for a violet ray. As a practical illustration : The refractive in-
dex for a red ray in crown glass is 1 *51 24 = /<, and for a violet ray
is 1*5288 = fi', the difference being it' — /t =-01G4.

The refractive index for a red ray in dense Hint glass is 1*7030
= /j, and for a violet ray is L7-r)01 = the difference being it' — ii
= -0-171.

Consequently there will be a greater difference between 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 normal to a plane surface is always the perpendicular to it ;
the normal to a spherical surface is the radius of curvature, The
angle of the incident ray and the angle of the refracted ray are
always measured with the normal, and not with the surface.

Fig. 2, «, 6, shows the normals A, B to both a plane and a
spherical surface, C 1).

In the case of the spherical surface, B is the centre of curvature,E F

PROBLEMS OX REFRACTIVE INDEX

5

is the incident ray in air, FG the refracted ray in crown glass. The
angle A F E is the angle of incidence, B F G the angle of refraction.

Sine A F E divided by sine B F G is equal to the refractive indea
of air into crown glass, or, in other words, the absolute refractive
index of crown glass, /x ; thus in this particular case :

(Problem) I. :

sin A F E _ sin 45° _ _ -707

smlTF G ~ sin 28° ~~ 7±T2

= I1-

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

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

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

find

fx', the refractive

index from glass into air.

(Problem) II. :

'-- 1 - - 1 - - 2

fi 3 3

2

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 fx of crown
glass is 1*5, and the ab-

A

Fig. 2. — The normals to a plane and a curved
surface.

solute refractive index ix

of flint glass is 1*6 ; find the relative refractive index fx" from crown
to flint.

(Problem) III. : n 1*6

M ~ fx ~ 1-5 ~
The relative refractive index fx"' from flint to crown is determined
by (problem) ii. :

1 1

1-06G

= -938

6 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

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

Fig. 8. — The phenomenon of total reflexion. (From the ' Forces of Nature,'

published by Macmillan.)

creased in a greater proportion, and the ray F E will approach the
surface F D.

When F E coincides with F D, G F is said to be incident at the
critical angle of the medium. When this critical angle is reached,
none of the incident light will pass out of the denser medium, but it

PROBLEMS ON REFRACTIVE INDEX

7

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

A simple illustration of this is shown in fig. 3. It represents
n 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 denser

medium is given, the critical angle for that medium can be found,

thus : The absolute refractive index of water is l'33 = /i ; find its

.critical angle 6.

(Problem) IV. : a. a 1 1

v 1 Sin 0 = -= =-75 ;

fl LOO

d = 48i° (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 • > v
the formula

fx sin (p = jx' sin 0',

where /j, is the absolute refractive index of the first medium, <p the
angle of the incident ray in it, // the absolute refractive index of
the second medium, and <p' 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 /u = 1 fx' = -, the absolute refractive indices of both the

media, air and glass respectively, being given, find <p', the angle of
the refracted ray in glass.

(Problem) V. 1 :

t«. , u sin 0 1 x sin 45° 1 x *707

0' = 28° (found by table).

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

(Problem) V. 2 :

a. u' sin <b' 1*5 x sin 28° 1*5 x -741

Sm = " — = = — -= *7Uoo.

H 1 1

<p = 45° (found by table).

Now, suppose the A side of CD (fig. 2) is crown glass, li=] -\
-and the B side of C D is flint glass, /t' = T6. The angle of the
incident ray AF E <j> = 45°, find the angle of the refracted ray <(>' or
BFG.

8 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

Problem V. 3 :

« . , u sin 0
fern (jj ='—

1-5 x sin 45 1-5 x -707 1-0605

p' 1-6
0' = 4H° (found by table).

1-6

1-6

= -663.

As a final instance. Suppose the ray to be travelling in the-
opposite direction, so that G F is the incident ray and B F G, or
<j/ =: 41 be given, the media being the same as in the last case,.
ja' = 1*6 and /x = 1*5, find 0, or the angle of the refracted ray.

(Problem) Y. 4 :
Sin 0 = ^sin f = 1-6 sin 411°

^ fJL 1*5

0 = 45° (found by table).

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

Its geometrical form in per-

1-6 x -663 1-0608

1-5

1-5

to;

spective and in section is shown
in fig. 4.

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

In fig. 5 let A B C repre-
sent a prism of very dense flint
glass whose absolute refractive
indices \j! for red light is 1-7,
and a" for blue light is 1*75.

Let the refracting angle BAG
>°, and let the

Fig. 4. — The geometrical form of the prism.
(Prom the ' Forces of Nature.')

of the prism = 50c
angle of incidence of a ray of
white light D E = 45° = 0 in
air, n = 1 . The dotted lines show
the normals. Then by (problem)
v. 1 for red light we have for
the angle of refraction 0'.

. , ul sin 0 1 sin 45° *707 Ain
«m<l>== / = 1-7 =T-7 =*416;
f =24^° (found by table).

And for blue light :

Sin 0"

fx sin 0 _ 1 sin 45° _ -707 _ .

= = '404 :

p 1-75 1-75

0//=23f> (found by table).

Now, for the red ray draw E F (fig. 5), 24^° to the normal, and let
it meet the other side of the prism AC in F. At F draw another
normal.

On the scale of our diagram it is not possible to draw two lines
E F, one for the red ray and the other for the blue, for they are too>
close together, their angular divergence being only |°. But by

PATH OF LIGHT TIIKOI'OII A PRISM

9

measurement it will be found that E F makes, with tke normal at
F, an angle <// of 25J°, and for the blue ray an angle of 26|°.

It should be remembered, however, that if the refracting angle
of the prism is known, there is no necessity for this measurement,
because it is always the difference between this and the angle of
refraction before determined, thus 50° — 24.1,0 = 25.1,0.

A

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, we can, by (problem) v. 2,
find the angles of refraction for each colour.

If we take red light :

a. a sin <// 1-7 sin 251° 1-7 x -43 ™

JSin 0 = 1— -2- = — — ~ = = •< .32 ;

fx 1 1

<p, the angle of refraction = 47° (found by table).
If we take blue light :

a. ix" sin A" 1-75 sin 26i° 1*75 x 442

Sm 0 = — = — — - = = • n 4 ;

fX 1 1

0, 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 D or sodium lines is the point in the
spectrum often chosen.)

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

_ fi! - fx" . . 1-52395 - 1-51535 _ '0086 =

lJ

-1 1-5179 -1 '5179

10 ELEMENTAEY PKINCIPLES OF MICKOSCOPICAL OPTICS

The values of the same lines for the flint glass are as follows :

D, 1-7174 = p; F, 1-73489 = //; C, 1-71055 = /*".

_ ^ - fx" _ 1-73489 - 1-71055 _ -02434

•0339.

fx - 1 1-7174 - 1 -7174
So the dispersive power of the flint between the lines C and F is
slightly more than twice that of the crown for the same region of
the spectrum. In the above formula the expression p! — jx" is usually

C JUL

written d p • in full it is therefore w =

Having thus traced a ray
•experimentally through a
prism, our next step is to show
that a convex lens is only a
curved form of tioo such prisms
with their bases in contact,
.as is shown in A, fig. 6,
where the curved line shows
the lenticular character and
the shaded elements the two
prisms. A concave lens is in
effect two prisms reversed,
that is, with their apices in
contact, as in B, fig. 6, where,
again, the curved line shows
the form of the lens and the

- 1

A

i

!

. i ■ ■

- _ V J i

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

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

shaded parts its relation to a pair of prisms. The fact that a lens is,
in effect, as such, but an assemblage of 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 let O P be the axis in each case ; then, from what 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 O P. But in the other case, where
the prisms have their apices together, as in fig. 9, acting as a con-
cave lens, the light is refracted away from the axis OP.

Fig. 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, ivhich is due to the
different position of the normals.

In the prisms (figs. 8, 9) the incident surface AB is a plane ;
and as the normals are perpendicular to it, they must be parallel to
one another, whether near the base or near the apex. Thus the
normal at E is parallel to the normal at K ; therefore, 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 radii ;,
parallelism is therefore impossible, and parallel incident rays will not
make equal angles with them, and so the refracted rays will not be
parallel.

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

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

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

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

Now, if we study the four following figures, we shall see
the principal action of lenses on light incident on their surfaces.
Fig. 13 shows that if a radiant is placed at the principal focus of a
converging 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. 14 shows that if a radiant be placed beyond the principal

THE FOCI 01- 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 conjtig<Ue
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, Fig. 12. — Biconcave, plano-concave,

and converging meniscus lenses. and diverging meniscus lenses.

(From the ' Forces of Nature.') (From the ' Forces of Nature.')

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

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

Fig. 13.-

A radiant at the principal focus of a biconvex lens makes the refracted

rays parallel.

Fig. 11.— A radiant placed beyond the principal focus causes rays to converge
beyond the principal focus on 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 9ize\ 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 and 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 equal
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 divergent, and will never
meet in a focus on that side. This is seen in fig. 15 ; but if they are
traced backwards, as in the dotted lines of fig. 15, they will then

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

meet in a point. This is called the virtual 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 virtual.1 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 virtual principal focus of the lens.

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

1 A real image can be received on a screen, but a virtual image cannot.

SPHERICAL ABERRATION

IS

Figs. Hand 18 show the refraction of rays of monochromatic
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 rays
of light fall upon the plane surface, and least where, as in fig. 18,
they fall upon the spherical surface. For spherical aberration!,* the

n

F FF

n2

nz

M'

/

Fig. 17. —Spherical aberration.

distance of the focus for any ray jwssing through a lens from the
principal focus of that lens.

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

FFF

\ R
\ R2

\f til

Fig. IS. — Spherical aberration.

spherical aberration of the rays R1 Rl and that of the rays R2 R2 is
FF' — F F, which is F' F".

Thus F F and F F" in (fig. 17), a/= - | . ^ ; FF and FF"
in (fig. 18) J/= - I . t

(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, AF. 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.

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, when the more curvi d
surface 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 become famili ar
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

f ' V 1 r3 \ f r'j \f

where f = principal focal length ; y = semi-aperture ; jx — refr.
index • and r, —r', radii. . ■

In an equi-convex of crown, where /z = ~t r = — r' ,

3 f

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

2 2

3f=——. y-z . Here parallel rays are incident on the convex

6 J

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

fi = r = oo , — r' = ^ , cf= — . V- ' consequently the sphe-

2 - 2 2 jr.,

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

When —r'=oo, and ju= 1*69, the plano-convex becomes the
form of minimum aberration.

3

In a crossed2 biconvex lens, where — r' = 6 r, and /,i = — ,
1 0 'J/2

3 jf = — — - . M- , the parallel rays being incident on the more
curved surface.

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

1 = 1 21- A

f 7 f ff

In figs. 5, 8, and 9 we see that when the incident ray D E con-
sists of ivliite light, the colours of which it is composed are unequally
refracted ; the two extremes, H (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 Eucy clop ce dia Brit. vol. xvii.

2 A biconvex lens is said to be ' crossed ' when the radii of its surfaces are not
equal to each other.

HOW ACHROMATISM MAY UK OBTAINED

'7

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

The white light, A A", falling on the peripheral portion of the Lena,
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
FF ; whilst the red rays are not brought to a focus until they reach
the point D, crossing the divergent violet rays at E E. The foci of
the intermediate rays of the spectrum (indigo, blue, green, yellow,
and orange) are intermediate between these two extremes. The
distance C D, limiting the violet and the red, is termed the longitu-
dinal chromatic aberration of tin A ns.

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

Fig. 19. — -Chromatic aberration.

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

The line E E joins the points of intersection between the red and
the violet rays which marks the mean 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 uncorrected lenses are stopped out, and
the centre only often used that the chromatic aberration may be
reduced to a minimum.

Manifestly, therefore, the correction or neutralisation of this
chromatic 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
BAC.

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 BAC 503, as in this case, the separa-

c

1 8 ELEMENTARY 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 angle, 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 reversed,
but refraction also
is neutralised, the
emergent ray being
parallel to the in-
cident ray. Therefore
the equal and inverted
system of prisms can
be of no possible use

Fig. 20. — Recomposition of light by prisms. (From ^? ^ *\ P^^lCal opti

the ' Forces of Nature.') cian in the correc-

tion of lenses because

the convergence and divergence of rays are both essential to the
construction of optical instruments. The dispersion, in fact, must
be destroyed without neutralising 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
part of the convergence is maintained while the whole of the
dispersion is destroyed.

The spherical lenses which answer to these prisms are a crown
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 R and V (fig. 5). Let us select
one, green, and represent it by G. Now if G lies midway between
R and V in the prism of 50° of angle, and also between R and V in
the prism of 25° of angle, its dispersion will also be neutralised.
This means that when the dispersion between the three colours in

ACHROMATIC OIMKCTIVKS

one kind of glass is proportionMl 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
It and V in one kind of glass, but in the other it may lie, for
instance, much nearer R, say a third instead of half the distance
of R from V. If now the dispersion of R V be destroyed, G will
be left outstanding. If a different angle of prism be chosen, so that
R and G are neutralised, then V must be left outstanding.

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

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

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

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

The rays radiating from the object 0 in every direction fall upon
the cover-glass CC (yu. = 1 *6). On tracing two definite rays, such
as O A and O B, it will be found that they will be refracted to R
aild 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 N.

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

Similarly, by tracing two of the less divergent rays from 0 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 ravs radiating apparently from tiro dis-

0 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
aplanatic. This word (derived from d = privative, and 7rA.ai/aa>, 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.

Fig. 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
spherical 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 flattened,
while that of the flint has been
deepened, which increases the cor-
rective power of the flint, and thus
destroys the balance of the com-
bination 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 focussed at F. This is what is known as over-correction.

Fig. 24. — Over-corrected system.

COLLAR CORRECTION — FOCI OF LENSES

An oplanatic objective (fig. 22) can be made, into an -"nder-
•corrected objective (tig. 23) by either (1) causing the back tenses of
which it is composed to approach the front lens. This is the device of
Andrew Ross, and is now effected 1 by means of a special ' collar '
arrangement, which, by the action of a screw, approximates or sepa-
rates the suitable lenses. But for this a special device is needed for
•each objective. (2) The result can moreover be secured by causing the
eye-piece to 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, that is, bringing them together, whether in the
objective itself or in the microscope as a whole, by shortening the
^distance between the eye-piece and the objective uuder-corrects the
■objective, that is, gives positive aberration ; while the separation of
lenses over-corrects or gives negative aberration.

In using the collar correction 1 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, AF. 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 radius
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-fourth the thick- Fig. 25. — The radius 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, {x the refractive index of the medium, then

p+p=(^-i) (g+g^
i=(/i-1)(g+i);

P P' F

Also, if A is the distance of a focus from F, the principal focus,

' See Chapter V.

22 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

and B, the distance of its conjugate from F', the other principal
focus on the other side, then

AB=FF;

or, in an equiconvex lens,

A B= F2.

In an equiconvex lens of crown glass /u = L5 if F = radius of
curvature. But in a plano-convex lens of crown glass ^ = 1*5 if
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 determi-
nation of the principal focal length of an equiconvex lens may be made
by subtracting from the result obtained by the foregoing formula?
one quarter of the thickness of the lens.

Example. — Equiconvex lens of crown glass /x = 1 "5, r = thick-
ness = J. By above formula F = J. Subtracting from this one-
quarter of the thickness of the lens we get F = T7^- as the distance
between the focus and the surface of the lens. This is only ^ inch
from the truth. If the lens were a sphere it would be accurate.

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.

Example. — In a hemispherical lens of crown glass /i — 1*5, radius
= 3T, thickness = the principal focus on the convex side will be
one inch from the curved surface and on the plane side § inch from
the plane surface.

Similarly, in an equiconcave lens subtract from the principal
focal length, obtained by the above formula, half the thickness of
the lens. In other words, measure the focal length from the centre
of the lens. The focus is of course virtual.

But a plano-concave lens follows the plano-convex. The principal:
focus on the curved side requires no alteration, but from that on the
plane side two-thirds of the thickness of the lens must be subtracted.
The focus is also virtual.

Example. — Equiconcave of dense flint /x = l"75, radius = —
thickness ^, F by formula = — ^. This is about T^ inch too much,
but by subtracting half the thickness of the lens, or ±, we obtain

— which is only T!L inch too little.

Plano-concave of dense flint //. = 1'75, radius = — J, thickness \,
F by formula = — J. Subtract from this two-thirds of thickness of
Jens. Then F = — \. This is only ^ inch too small.

The principal focus of a combination of two or more lenses, whose
principal foci and distances are known, can be found from the formula
111

- + — = -. by assigning for the value of p the distance of the prin-

If^ 1^ J

cipal focus of the first lens from the second, and so on.

Example. — Parallel rays fall on a 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

THE FOKMATION OF A "KKAL I MAO K '

23

last lens, to which the rays will be brought. It is evident that tin-
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 :

-2 p' 6'

p = .

5

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

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

1. The formation of a real image means the production of a
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 tig. 26,
where A B is an object placed beyond P, the principal focus of the
aplanatic combination. From every point of A B are rays radiating
at every possible angle. Let A F and A H be two such rays

P

Fig. 26. — The formation of a real image.

radiating from the point A. Now if the refraction of these rays be
traced, in the manner already indicated, through the aplanatic com-
bination, it will be found that the rays which before inmiergence
were diverging are by the refraction of the combination on emer-
gence rendered converging. Thus the ray F C meets II V 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 C, there will be an image of A. In the same manner the ravs

24 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

issuing from every point along A B may be traced, and will be found
to have each one its respective conjugate lying on C D, so the con-
jugate of B is at D. Hence it is at once manifest that an inverted
conjugate image of the object A B is formed at C D. Further, it
will be noticed that, although the object is straight, the image of it
is curved towards the lens.

If the object AB 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 AF and AH, 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 joarallel 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 tig. 26 the two rays, A F and A H, are
traced through the lens to determine the point C, but in the lower
part of the figure only the ray B K is traced, and the intersection of
'this ray by the straight line B D passing through the Optical centre
gives the point D.

2. An image is said to be virtual when it cannot be received on
a screen. Fig. 27 shows how a virtual image is formed. The
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 beticeen 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,
the rays emerging from the lens are still divergent even after their
refraction through the lens ; consequently they will never intersect,
and as there is no focal point, there can be no screen image.
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.

1 In the 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.

FORMATION 01- A 'VJKTl'AL IMA(iK'

25

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 raws which
would have diverged from the point C had it been an entity, the
.retinal image therefore will be an image of a non-existent picture
CD.

The method of drawing this is exactly similar to that of the
preceding figure. The rays A F and A H are traced through the
lens, and their prolongation backwards (see the dotted lines in the
figure) gives the point C. Also, as in the preceding figure, any
point of the picture can be found by tracing one ray, such as K ;
then the intersection of its backward 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

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

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
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.

1 Journ. Ii.M.S. vol. iv. ser. ii. p. 348.

26 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

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

system varies with the length of I, 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 t linear amplification ' of a system is, of course, different in
the case of a short-shorted 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 power ' 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

i

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

the course of the rays for a short-sighted eye, and the thin lines for
a long-sighted one, the eye in each case being supposed at the pos-
terior principal focus of the system.

The other generally adopted expression of the power by N = --

may be put on a somewhat more rational basis than is generally
done by defining the 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 value of N has no real signification at all in regard to an observer
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

AMICI'S U.SE or 'IMMERSION' LENSES

2/

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.

Tlie only difference is that in the former case we must take the
centres of the circles of indistinctness instead of the sharp ima-v
points in the latter case. If the conventional length of / = 10 inches
is interpreted in this way (as distance of projection, independently
of distinct vision) the absurdity at least of a real influence of the
accommodation on the power of a microscope is avoided. It becomes
obvious that for long-sighted and for short-sighted eyes the same X
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. 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 modifi-
cation to suit the new condition. This modification seems never to
have been successfully effected by Amici himself ; and his idea
remained unfruitful until it was taken up by Hartnack and Nachet,
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 1 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 reflection is greatly
reduced, and the benefit derivable from the large aperture is pro-
portionally augmented. Again, the 1 immersion system ' allows of a
greater working distance between the objective and the object than
is otherwise attainable with the same extent of aperture ; and this
is a great advantage in manipulation. Further, the observer is
rendered less dependent upon the exactness in the correction for the
thickness of the covering glass, which is needed where objectives of
large aperture are used ' dry ' ; for as the amount of 1 negative
aberration ' is far smaller when the rays which emerge from the
covering glass pass into water than when they pass into air, varia-
tions in its thickness produce a much less disturbing effect. And it

28 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

is found practically that ' immersion ' objectives can be constructed
with magnifying powers sufficiently high, and apertures sufficiently
large, for the majority of the ordinary purposes of scientific investi-
gation, without any necessity for cover-adjustment ; being originally
adapted to give the best results with a covering glass of suitable
thinness, and small departures from this in either direction occasion-
ing 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
1by 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 was to Professor Abbe we are indebted for its practical appli-
cation, through whom it is now known as the homogeneous system.
1 The idea of realising the various advantages of such ' a system by
constructing a certain class of homogeneous objectives had, Professor
Abbe says,1 'for some time presented itself to his mind.' 'The
matter assumed, however, subsequently, a different shape in conse-
quence of a suggestion made by Mr. John Ware Stephenson, . . .
of London, who independently discovered the principle of homoge-
neous immersion.' 2

This method consists of the replacement of water between the
covering glass of the mounted object and the front surface of the
object-glass by a liquid having the same refractive and dispersive
power as crown glass. With such a fluid taking the place of air, it
follows that the correction collar, though still a refinement and aid in
the attainment of the finest critical images, would be a necessity no
more.

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

He had, in fact, as we have hinted, already approached the con-
sideration of the subject from another point of view, believing that
petrographic work— the study of thin sections of mineral substances— r-
could be far more efficiently accomplished by the use of homo-

1 On ' Stephenson's System of Homogeneous Immersion for Microscopic Objec-
tives' (Abbe), Journ. B.M.S. vol. ii. 187<J, p. 257.
a Ibid.'

ABBE'S CONSTRUCTION OF FIRST IIOM.OO FN KOl'S OBJECTIVE 2<>

geneous lenses. But in the new aspect in which fche problem was
presented by Mr. Stephenson it carried with it Dew interest to Abbe,
not only as promising to largely dispense with the ' correction colli r,
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 Abbe's calculations based on Mr. Stephenson's sug-
gestion was the construction by Carl Zeiss of aT\jtb with a N.A.1
of 1*25 of fine quality, and still higher promise, and subsequently
of a ^th and a ^th in. objective of a like character.

It may be well to note that Amici suggested the use of oil
instead of water prior to 1850, and Mr. Wenham again revived
the suggestion in 1870. 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 resolvin g
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
suggested 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.

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 k
Lealand, of London, very soon made a ^-th inch and a -l0th inch
objective on the homogeneous principle, with numerical apertures
respectively of 1*38, and during the year 1885 produced lenses of an
excellence impossible to any previous system of }.t\\ inch, r*jth inch,
and Tfrth inch power, having respectively numerical apertures of
1'50, while 1*52 is the theoretical maximum.

The use of a ' correction collar' is continued in the best English
and German homogeneous object-glasses, especially as aiding in tin1
delicate corrections required to get the exact length-relation between

1 The meaning of this expression will he found on p. 48 hut the whole of Chapter
II. must he carefully read.

2 Monthly Micro. Journ. vol. hi. p. 303.

30 ELEMENTARY PRINCIPLES OE MICROSCOPICAL OPTICS

the object-glass and eye-piece. But this must also be aided in
endeavouring to secure the most perfect ' critical images ' by a body-
tube provided with rack and pinion motion. When the two are
combined, if the object-glass is of perfect construction and of latest
form (apochromatic, q.v.), results never before attainable can be got
with comparative ease. And this, be it observed, does not in the
least compromise our admission of the perfect accuracy of the
theoretical principle of the homogeneous system.

With such evidence of advance in the optical construction of
microscopes, dependent apparently on such accessible conditions, the
question 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 resources. 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 widely
overstep our present methods of optical demonstration, there can be
little reason to question. But 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 difficult 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-
geneous lenses were, in a comparatively short period of time, carried
from a N.A. of 1*25 to 1*50 ; and this carried with it the capacity
theoretically indicated.

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 ;

NEW VITKKOI'S OPTICAL COMPOUNDS

31

one, as we have just demonstrated, arises from the residual spherical
and chromatic aberrations, the other takes origin in the want ofliomo-
yeneity, absolute precision of curve, and perfect centeriny of the systi //;
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 dissipation1 — 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 very
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-
correction for the red, and over-correction for the blue and violet
rays, presenting a want of balance between the chromatic corrections
for the central and marginal zones of the objective. Although
perfect chromatic corrections for the central rays may be effected,
giving images of great beauty, the chromatic over-correction for the
peripheral rays with oblique illumination will show the borders of
the image with distinct chromatic fringes.

To compensate these aberrations in the construction of an object-
glass, what is needed is a vitreous material applicable to optical
purposes possessed of such properties that a relatively smaller re-
fractive index could be united with a higher dispersive power, or a
higher refractive index with a relatively lower dispersive power.
By proper combination of such materials if they be provided with
ordinary crown and 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, 1 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 com-
pounds possessing different relations of refractive and dispersive
power by means of which the secondary spectrum could be re-
moved.

For practical purposes the matter was in abeyance until 1 SSI,
but since that time Dr. Schott and Professor Abbe, with the active

1 Hoffman, A. "W., Bericlit iiber die irissenschnfthchen A^paratc auf dcr Lon-
doner Internationale n AusstcUung im Jahre 1870.

32 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

co-operation of the optical workshops of Zeiss, have undertaken the-
laborious and prolonged investigation into the improvement of
optical glass, to which we have alluded ; the result has been the pro-
duction of £ crown ' and 1 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
different parts of the spectrum 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. On the
other hand, the kinds of glass which can be used for optical purposes
have been so increased in variety that, while the mean index of re-
fraction 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 ac-
companied by a high dispersion in flint glass, but may be retained
in crown glass with a low degree of dispersion.

The j)ractical consequence of this is that both the imperfections
inalienable from an objective constructed of ordinary 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 may be ' dry ; or 'homogeneous'
we have so great a freedom from colour defect as to admit of their
being designated apochromatic lenses (d = privative ; ^pw/xa = colour ;
u7ro= from, away from ; ^pcu/xa = colour).

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

ADVANTAGES OF APOCHROMATIC OBJECT-GLASSES

form, these objectives must apparently command a high price. Jim,
given such an object-glass — which is the production of a thoroughly
competent practical optician — and its advantages, theoretical and
practical, are great.

1. The aperture of the objectire can be utilised to it* full r.rh ,,.t.
In the best of the older object-glasses at least one-tenth of the
available aperture was useless j 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 th<5se objectives it has never been possible to realise the amount 1
of resolving power indicated by theory with a given aperture. But
in a well-constructed apochromatic objective — the secondary spec-
trum being removed, and the spherical aberration being uniformly
corrected for different parts of the spectrum — there is a practically
perfect focal concentration of the rays in the image.

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

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.

2 ' On the Relation of Aperture to Power,' Journ. B.M.S. 1883, p. 803.

D

34 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS

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, and depends on the increase in the
total magnifying power of the microscope obtained by means of the
eye-piece as compared with that given by the objective alone. The
number which denotes how many times an eye-piece increases the
magnifying power of the objective, when used with a given body-
tube, gives the proper measure of the eye-piece magnification, and at
the same time the figures for rational numeration.1

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 J ena, on
this important system, viz. :

Dry ...oi

Water Immersion
Homogeneous Immersion

ApocJtromatic Objectives.

Aperture Focal Length
.0-30
0-30
0-65
0-65

0- 95
v0-95

1- 25
rl-40
11-40

English. Equivalent

24-0 mm.

1 inch.

160 „

2

3 »

120 „

1

8-0 „

! "

3 '»

6-0 „

1

4 »

4-0 „

1

6 »

2-5 „

1

10 "

3-0 „

1

8 »

2-0 „

1

12 "

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

It is of interest to note that Messrs. Powell and Lealand, on
receiving the special glass from Germany proceeded immediately to
the production of a ^-inch objective with compensating eye-piece
on a formula devised by Mr. T. Powell, which were supposed to
be apochromatic. The workmanship was of the high class for which,
in the manufacture of lenses, that firm have become distinguished ;
and this objective has, together with those subsequently produced,
brought out admirably the quality of the work; for we now know
that the perfect apochromatic objective requires fluorite lenses in
its combinations to obtain the needful corrections. But without
the use of these this firm came so near the earlier apochromatic
objectives of Zeiss, for visual purposes, that it was not easy to discover
their deficiency. This was due entirely to perfect workmanship.
The same firm have since produced a remarkable lens on the same
system, having a 1ST. A. of 1*50, with a power of y^th of an inch.
Object-glasses are also now made by other makers, English, Euro-

1 ' On Improvements of the Microscope with the aid of new kinds of optical glass'
(Abbe), Journ. B.M.8. 1887, p. 25 ct seq.

♦

FORMULAE JJELATJXd TO HKAL I .MACKS 35

pean, and American, known as 'apochromatic'; but we doubt, in
the majority of cases, if the apochromatism is attained, for it has
only recently been made known that, in addition to the special glass
used in their construction, the Abbe apochromatic systems had in-
serted also the jluorite lenses referred to above, which materially
affects the result. It is remarkable, however, how near some makers
have brought their results to those of Abbe without the advantages
of the fluorite lens. With this material employed in their con-
struction Messrs. Powell and Lealand are making beautiful object-
glasses? We have recently used a new ^-in. made by them, to which
we have seen no successful rival.

d 2

36

VISION WITH THE COMPOUND MICROSCOPE

CHAPTER II

THE PBINCIPLES AND TEEOBY OF VISION WITE TEE
COMPOUND MICBOSCOPE

We are now prepared to enter upon the application of the optical
principles which have been explained and illustrated in the foregoing
pages to the 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
observer proceed directly from the object itself, after having been
subjected only to a change in their course, as we have shown by
fig. 26, which fully explains the action of the simple lens ; whilst in
the compound microscope an enlarged 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
best combined with another lens interposed between itself and the
object-glass, the two together forming what is termed an eye-piece.
The compound microscope must be the subject of careful and de-
tailed consideration ; but it must be remembered that the shorter
the focus of the simple magnifying lens, the smaller must be the
diameter of the sphere of which it forms part ; and, unless its
aperture be proportionately 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 everyone
who studies the history of microscopic inquiry. An important im-
provement on the single lens was introduced by Dr. Wollaston, who
devised the doublet, still known by his name, which consists of two
plano-convex lenses, whose focal lengths are in the proportion of one
to three or nearly so, having their convex sides directed towards

PRINCIPLES AND THEORY OK MICROSCOPIC VISION

the eye, and the lens of shortest focal length nearest the object. I u
Dr. Wollaston's original combination no perforated diaphragm (or
'stop') was interposed, and the distance between the lenses was left
to be determined by experiment in each case. A great improvement
was subsequently made, however, by the introduction of a 'stop'
between the lenses, and by the division of the power of the smaller
lens between two (especially when a very short focus is required), so
as to form a trijilet, 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 achromatisation of its object-glasses.

Another form of simple magnifier, possessing certain advantages
over the ordinary double-convex lens, is that commonly known by
the name of the ' Coddington ' lens.2 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, jo
that they are but little subject to aberration. The idea was further
■improved upon by Sir D. Brewster, who pointed out that the same
end would be much better answered by taking a sphere of glass, and
grinding a deep groove in its equatorial part, which should be then
filled with opaque matter, so as to limit the central aperture. Such
a lens 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, or even of a
doublet. This form is chiefly useful, therefore, as a hand-magnifier,
in which neither high power nor perfect definition is required, its
peculiar qualities rendering it superior to an ordinary lens for the
class of objects for which a hand-magnifier of medium power is
required. Many of the magnifiers sold as 'Coddington' lenses,
however, 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 ' Coddington.' The ' Stanhope '
lens somewhat resembles the preceding in appearance, but differs
from it essentially in properties. It is nothing more than a double-
convex lens, having two surfaces of unequal curvatures, separated
from each other by a considerable thickness of glass, the distance
of the two surfaces from each other being so adjusted that when the
most convex is turned towards the eye minute objects placed on the

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

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

38

VISION WITH THE COMPOUND MICEOSCOPE

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 1 eels ' in paste or vinegar &c. A modified
form of the ' Stanhope ' 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, etc., 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
Heichart'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 required, and when the use of even the lower powers is
continued for any length of time, great advantage is derived from
the employment of achromatic combinations, now made 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. It has
two lenses for the objective, and a concave eye lens.
It is illustrated in fig. 29.

To remedy the inconvenience of the lens being too
close to the object in all but low powers, Charles
Chevalier, in his 'Manuel du Micrographe ' (1839),
Briicke lens, proposed to place above a doublet a concave achro-
matic lens, the distance of which could be varied at
pleasure. The effect of this combination is to increase the magnifying
power and lengthen the focus. Thus arranged, this instrument will
be the most powerful of all simple microscopes, and the space
available for scalpels, needles, &c. will be much greater than

COMPOUND MICKOSCOI'K

39

Avith a doublet alone. The further the concave lens is removed from
the latter, the greater will be the amplification.1

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 amplification
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-hulder, with from
3 to 8 cm. between the object and objective.

Compound microscope. — The compound microscope, in its most
simple form, consists of only two lenses, the object-glass and the
rye-glass. 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 a greater distance on the other side ; whilst the
latter receives the rays which are diverging from this image, as if
they proceeded from an object actually occupying its position and
enlarged to its dimensions, and brings these to the eye, so altering
their course as to make that image appear far larger to the eye, pre-
cisely as in the case of the simple microscope. It is obvious that,
in the use of the very same lenses, a considerable variety of magnify-
ing power may be obtained by merely altering their position in regard
to each other and to the object. For if the eye-glass be carried farther
from the object-glass, whilst the object is approximated nearer to the
latter, the image will be formed at a greater distance from it, and its
dimensions will consequently be augmented ; whilst, on the other hand,
if the eye-glass be brought nearer to the object-glass, and the object
removed farther from it, the distance of the image will be a much
smaller multiple of the distance of the object, and its dimensions
proportionately diminished. We shall hereafter see that this mode of
varying the magnifying power of compound microscopes may be
turned to good account in more than one mode ; but there are limits
to the use which can be advantageously made of it. The ampli-
fication may also be varied by altering the magnifying power of
the eye-glass ; but here, too, there are relative limits to the in-
crease ; since defects of the object-glass which are not perceptible
when its image is but moderately enlarged are brought into injurious
prominence when the imperfect image is amplified to a much greater
extent. Hence, in proportion as the object-glass approaches more
nearly a perfect combination does it admit of higher eye-pieces. In
practice, and with the old achromatic system, it is generally found

1 Robin, C, Traite du Microscope ct cles Injections, 2nd ed. 8vo, pp. 33, 84.
Paris, 1887.

4Q

VISION WITH THE COMPOUND MICEOSCOPE

much better to vary the power by employing object-glasses of dif-
ferent foci, an object-glass of long focus forming an image which
is not at many times the distance of the object from the other side
of the lens, and which, therefore, is not of many times its dimension ;
whilst an object-glass of short focus requires that the object should
be so nearly approximated to it that the distance of the image is a
much higher multiple of the object, and its dimensions are propor-
tionably larger. In whatever mode increased amplification may be
obtained two things must always result from the change : the pro-
portion of the surface of the object of which an image can be formed
must be diminished, and the quantity of light spread over that
image must be proportionally lessened. But, as we have stated, this
is dependent on the aperture and total magnifying power, and not
upon (amongst other things) the focal length of the eye-piece.

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 Y.) 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. Three rays are drawn through
the eye-piece, which gives a magnified virtual image of the real
image from the objective, in order to suggest that with the eye-piece
there is a commencement, as it were, de novo, 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 to
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-glass and the field-glass being together termed
the eye-piece, or ocular. Yarious forms of this eye-piece have been
proposed by different opticians, and one or another will be preferred

IF

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

compound microscope.

42

VISION WITH THE COMPOUND MICKOSCOPE

according to the purpose for which it may be 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 Huyghenian, 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
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 mm., a 6 mm., and a 4 mm., all dry apochromatics by
Zeiss, and especially with a ^th by Powell and Lealand. It is, how-
ever, a matter of moment and interest to note that with good objec-
tives of the ordinary achromatic construction of large 1ST. A. the com-
pensating 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 ' is a con-
venient and useful eye-piece. For viewing large flat objects, such as
transverse sections of wood or of echinus- spines, tinder low magni-
fying powers, the eye-piece known as Kellner's may be employed,
but there is little advantage to be gained. This construction will
be fully described in Chapter V. A flat, well-illuminated field of as
much as fourteen inches in diameter may thus be obtained with very
little loss of light ; 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 unpleasantly 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 solid eye-piece made on the principle of the ' Stanhope ' lens
is sometimes used in place of the ordinary Huyghenian, when high
magnifying power is required for testing the performance of objec-
tives. The lower surface, which has the lesser convexity, serves as
a i field-glass ' ; whilst the image formed by this is magnified by the
highly convex upper surface to which the eye is applied, the advan-
tage supposed to be derived from this construction lying in the
abolition of the plane surfaces of the two lenses of the ordinary eye-

THK FORMATION ()!■' MICROSCOPIC IMAGES

43

piece. 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 — was formerly used for the purpose of micro-
metry, 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, however, may be so readily attained with the Huyghenian
eye-piece that no essential advantage is gained by the use of that
of Ramsden, the field of which is distinct only in its centre.

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 years this entire subject
has undergone a rigorous and exhaustive reinvestigation by one
of the most competent and masterly mathematical and practical
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.

One of the foremost errors relates to the mode in which micro-
scopic 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 com-
plex 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 micro-
scope was determined by the geometrically traceable relations of the
refracted rays of light.

With the telescope what is known as its ' aperture ' is simply
estimated by the diameter of the object-glass. But a close exami-
nation in theory and practice of the conditions of vision with micro-
scopic objectives shows that such an estimate of aperture is wholly
wrong in principle. The front lens of a ^y-in. objective may be no
more than the ^th 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.

44 VISION WITH THE COMPOUND MICKOSCOPE

receives. The apparent problem of practical optics is to be able,
by means of lenses, to gather up and bring to a focus as many of the
unadmitted rays as possible. The general manner in which lenses
act in doing this we have endeavoured in an elementary manner to
show.

Soon after achromatic object-glasses were first made, Dr. Goring
found that the markings on special objects — such as the scales of
the wings of insects — could be seen by some object-glasses, while
with others, although the magnifying power was equal, it was im-
possible to discern them. In every case the greater ' angle ' was
shown to possess the greater ' resolving ' or delineating power ; and
this led to the important conclusion that power of 4 resolution ' in a
lens was dependent upon 'angular aperture.5

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 objec-
tive 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
maximum aperture of any kind of objective, which could not, it
was held, be exceeded, but only equalled, by 180° in water or oil ;
in other words, that a radiant pencil has exactly the same value,
when the angles are equal, no matter what the refractive index of
the medium through which the pencil might be passing.

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

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

A solution of the difficulty was (as we have indicated above)
discovered by Professor Abbe ; and it is to Mr. Frank Crisp's

II()W 'APERTURE' IS OliTAINKI)

45

lucid exposition of Abbe's elaborate monographs that the Kngli.di
student is immensely indebted.1

The definition of 1 aperture' in its legitimate sense of 'opening'
is shown by Abbe to be obtained when we compare the diameter <>i
the pencil emergent from the objective with the focal length of that
objective.

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

Taking in the first case a single-lens microscope, the number of
rays admitted within one meridional plane of the lens evidently in-
creases as the diameter of the lens (all other circumstances remaining
the same), for in the microscope we have at the back of the lens the
same circumstances as are in front in the case of the telescope. Tlx;
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 ad-
mitted ?

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 smaller portion of the object. Thus, if the focal
lengths of two lenses are as 2 : 1, and the first amplifies N diameters,
the second will amplify 2N with the same distance of the image, so*
that the rays which are collected to a given field of 1 mm. diameter

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

and of ——mm. in the second. Inasmuch as the 'opening' of the

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 <>/»')tiitg and the focal
length, in order to define the same thing as is denoted in the telescope
by the absolute opening

1 'On the Estimation of Aperture in the Microscope' (Abbe), Journ. B.M.S.
ser. ii. vol. i. 388 ; ' Notes on Aperture, Microscopical Vision, and the Value of wide-
angled Immersion Objectives,' ibid. 303 ; ' The Aperture of Microscope Objectives,'
English Mechanic

46

VISION WITH THE COMPOUND MICKOSCOPE

Consider now the compound objective — the most important case
in the microscope. What is the opening of this composite system 1
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-
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 on focal
length also remains the same as in the case of the single lens. An
objective with a focal length equal to half that of another admits,
with the same linear opening, twice as many rays as the latter,
because the amplification of the image at one and the same distance
is doubled, and the same number of rays consequently are admitted
by the higher power from a field of half the diameter. And this
will hold good whether the medium around the object is the same
in the case of both objectives or different ; for an immersion system
and a dry system always give the same amplification when the focal
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 is in inverse proportion to the focal
length — that is, the objective which has the wider pencil relatively
to its focal length has the larger aperture.

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

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

So 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 96° 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.

COMPARISON OF OBJECTIVES OF THE SAME POWEB 47

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
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 |-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
180° oil-angle. The inner dotted
circles in the two latter cases are
of the same size as that corre-
sponding to the 180° air-angle.

A dry objective of the full

Numerical Aperture
1*52

= 180° oil-augle.

maximum

air-angle

of 180° is

Numerical Aperture
1-33

= 180° water-angle.

an

0

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
immersion lens of even only 100
(in glass) requires and utilises a
larger diameter, i.e. it is able
to transmit more rays from the
object to the image than any
dry objective is capable of trans-
mitting. Whenever the angle of
an immersion lens exceeds twice
the critical angle for the immer-
sion-fluid, i.e. 96° for water or
82° for oil, its aperture is in ex-
cess of that of a dry objective of
180°.

Having settled the principle,
it was still necessary, however,
to find a proper notation for com-
paring apertures. The astrono-
mer can compare the apertures

of his various telescopes by simply expressing them in inches ; but
this is obviously not available to the microscopist, who has to deal with
the ratio of two varying quantities.

Professor Abbe here again conferred a boon upon mieroscopists 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

Numerical Aperture
1-00

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

Numerical Aperture
•75

= 97° air-angle.

Numerical Aperture
•50

= 60° air-angle.

Fig. 31. — Relative diameters of the (util-
ised) back lenses of various dry and
immersion objectives of the same
power (+) from an air- angle of 60° to
an oil-angle of 180°,

48

VISION WITH THE COMPOUND MICEOSCOPE

from the back of the objective, so that the ratio of the seini-diameter
of the emergent pencil to the focal length of the objective could be
expressed by the sine of half the angle of aperture (u) 1 multiplied
by the refractive index of the medium (n) in front of the objective,
or n sin u (n being 1-0 for air, 1-33 for water, and 1-5 for oil or
balsam).

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

Let O and O* (fig. 32) be the conjugate aplanatic foci of a wide-
angled system; u, TJ 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 '

N.A. of objective^ sin <^> = 1-5 x •573 = ,86

N.A. of condenser =w* sin w

sin «=1"5 x,573 = ,86 = K.A. of objective..

Angular aperture of
objective = 35° +
35° = 70° in glass,,
which is equiva-
lent to the angular
aperture of the
condenser = G0° +
60° s= 120° in air.

.■■ = l-0x-86 = '86.

ix' sin <//=l-0 x -8G = -8G = N.A. of condenser.

Fig Al —Identity of n sin u (German math, form) with n sin 0 (English). Also N.A. and

angular 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 w*, U* the angles of the same rays on their
emergence ; then we shall have always

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

sin U* sin u*

or, — — — — = — : = const. = c ;

sin U sin u

that is, the sines of the angles of the conjugate rays on Loth sides
of an aplanalic system always yield one and the same quotient c,
whatever rays may be considered, so long as the same system and
the same foci are in question.

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

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

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

tains in our Universities, and is thoroughly understood amongst University men.
But to those unaccustomed to mathematical formula? 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 u ' with ' /j. sin <f>.'

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

In the figui'e the upper hemispherical lens represents the /Vow £ 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 1*5, 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 AB 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) — >

jx sin <j) = yu' sin <p' ;

• ~i u sin * 1'5 'x "573 ,oa .

sin d> = - j- — = 86 ;

r It! 10
</>' = G0° (from Table I.)
Therefore the ray on emerging from the under surface of the cover-glass will make
an angle of 603 from the normal.

The dotted lines show the path of the ray where the German symbols arc used,
n sin a = n* sin u* ;

* ii sin u 1"5 x '573 ,Qr

sin it = = = 8b.

11* 10

u* = 60° (from Table I.)

Numerical 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.

I

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-angie, 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°, and 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 be preferred to a dry objective.

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

2. If the objectives whose apertures are compared work in the
same medium, and admit angles of, say, 60°, 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 tivice 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 = 1, so that n sin u
= 1 also, whilst with the immersion objective n is greater than 1 (say
1*5 for oil), and the angle w may therefore be much less than in the
case of the dry objective, and yet the value of the expression n sin u
(i.e. the aperture) may be greater than 1 -0.

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

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

Difference of aperture involves a different quantity of light ad-
mitted to the objective provided all other circumstances are equal.
Hence 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-
tration of numerical aperture, and is thus of great service in its
exposition. It is manifest that aperture cannot be based on quantity

UNEQUAL INTENSITY OF EMITTED LIGHT

51

of light alone — more light can always be .obtained hi the Image by
throwing more upon the object but no increase; in the amount of i 1 1 u
mination can make a dry lens equal in performance? as regards aperl ur<
to a wide -angled immersion len s.

The popular notion of a pencil of light maybe; illustrated by
fig. 33, which assumes that there is equal intensity e>f emission in
-fill 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 quantity of light contained in any given
pencils may be compared by simply comparing the contents of the
solid cones.

When, however, aperture is considered, and the values of // sin u
are worked out for particular cases, they are seen te> 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 aelmits.

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

In the last century ( 10) Bouguet 1 and Lambert 2 e;stablished
the important fact that
with any surface of uni-
form radiation the inten-
sity of the emitted rays is
not the same in all direc-
tions. The power of emis-
sion and the intensity of
the rays (i.e. the quantity
.of light emanating from
a given surface - element
within a cone of given
narrow angle) varies in
the proportion of the co- Fm M _The intengitv ^ emitted rayfl is uofc ^
sine of the angle of obliejui- same in all directions,

ty under which the ray is

emitted; in other words, in the proportion of the cosiif of the angle'
of deflection from the perpendicular to the luminous surface under

1 Traite d'Optique sur la Gradation dc la Lumurc, 17<i<).

2 Photomciria, 1700.

52

VISION WITH THE COMPOUND MICEOSCOPE

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, but by fig. 34, the density of the rays decreasing continuously
from the vertical to the horizontal.

m Owing^ 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 w', 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 no 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 u)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, a (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 cone of the same angle u , as a similar point of a (the slight differ-
ence of the 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 whole 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'

RADIATION OF LI 0 I IT IN 1)1 F F FRFNT MKIHA

53

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

This, as is well known, is not the case, the projection of the
sphere showing equal brightness. The quantity of light, 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 w when the
obliquity w 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-
jangle 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 4 numerical ' aperture)
give 1*0 for the former and 1*17 for the
latter, which is therefore represented to
have a larger aperture than a dry ob-
jective of the greatest possible angle.

In this case also considerable per-
plexity has arisen. It has been assumed
that the total amount of light emitted
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 10 (which represents
180° in air) is consequently supposed to be clearly erroneous and
misleading. Here the whole difficulty lies in the absolutely false
-assumption that there is identity of radiation in different media.

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

54 VISION WITH THE COMPOUND MICROSCOPE

In 1864 R. Clausius established, by distinguished research, the propo-
sition 1 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 : n2 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
refractive indices of the media, viz. as 1*0, 1*77, and 2*25.

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

It thus follows that the estimation of the quantity of light is
found to be in complete accordance with the expression of aperture?

We are now 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

Fig. 37.

lines in fig. 38 will be totally reflected by the cover-glass, and only
those within a smaller angle of 82° will pass out. Although these -
are expanded into 180° on emerging into air, of which the objective
takes up 170°, yet this 170° contains, it is supposed, less light than
the 1 70° in fig. 37, as it has been 'diluted ' by the refraction.

1 ' Ueber die Concentration von Wiirme- und Lichtstrahlen &c.' Fogg. Anna leu
(l. Physik, cxxi. 1804.

'' Fig. A 2 •jive- ;i good, practical illustration of the relative illuminating power of
objectives of varying apertures, and at the same time affords a simple, explanation
of the reason why ( u sin tif is a measure of this illuminating power. Let the circles
A and B represent the hacks of two objectives of the same power but of different
apertures ; then the radii C D and E F will represent the angle n sin u for [x sin
in fig. Al (p. 4n; note). Now because the areas of circles are to one another in the

KADJATiOX IN A I K AND I5ALSA.M

55

A dry objective was therefore supposed to be placed at ;i disad-
vantage when, used upon balsam-mounted objects, its aperture being
supposed to be ' cut down' by the balsam, and the advantage of tlx;
immersion objective was considered to rest on the fact that it
restored, in the case of the balsam mounted object, the same condi-
tions as subsisted in the case of the dry-mounted object, allowing as
large (but no 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 radiatioii
in air and balsam. If there were in fact any such identity, the

Fig. 88.

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

When, however, the increase of radiation with the increase in
the refractive index of the medium is recognised, the mistake of the
preceding view is appreciated. The 170° in air of fig. 37 is not
equal to, but much less than, the 170° in 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 u (or [a. sin <p), the area of the circle A will be to the area of the circle B as the
square of the radius of A is to the square of the radius of B, or as (n sin u)- is to
(ti' sin u')'.

B

Fig. A 2.— The hacks of two ohjectives of the same power but different apertures.

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

56

VISION WITH THE COMPOUND MICROSCOPE

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

The immersion objective, therefore, which is able to receive the
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 5 is relative opening. However
denned, 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, «, and that another solid cone of rays, B, cannot be trans-
mitted through the same opening, a, but requires a wider one, ft,
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 ([3 — a) of the
wider opening, /3 conveys rays to the image which are certainly not
conveyed by the smaller opening a. From the radiant only can this
surplus come, and the pencil B which requires the additional opening
must embrace more rays, even if it should not be of greater angle.

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

RADIATION IN AIR AND I5ALSAM

57

air almost to the entire hemisphere, and it then utilises a definite
opening double its focal length. JJut 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 N2 ,
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 ichen 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 tin1 radiating
light when the medium is changed j 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 cone of rays of nearly
82° within the slide, and the
slide has air above in the first
case and oil in the second, it is obvious that the same ray which is
incident on the object at nearly 41° will always emerge in air at an
angle of nearly 90° (a'), and in oil at nearly 41° (a"), so that the
same rays which in air are expanded over the whole 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 diffraction is considered, then
Fraunhofer's law shows that the same diffracted beams which in air

1 Juurn. B.3T.S. ser. ii. vol. i. p. 303.

Fig. 80. — Comparative compression of
.light rays in two different media.

58 VISION WITH THE COMPOUND MICEOSCOPE

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

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

Fig. 40. — Diffracted beams in air.

us l ioe, ,«••-.;• <

; ,. /

Fig. 41. — Diffracted beams in oil.

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

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

Fig. 42.

the pencil radiating from an object in air, and capable of being
taken up by that objective. This pencil, on its emergence from the
back lens of the combination, will present a diameter somewhat less
than twice the focal length of the objective presented in fig. 43.

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

Fig. 43.

Fig. 44.

denote equal apertures, and equal angles in different media denote
different apertures.

That ' immersion ' objectives may have greater apertures than
the maximum attainable by a dry objective is capable of equally
simple 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

DIFFRACTION RAYs A RE LMAGE-FORMING

59

OBJECT IN AIR
SLIDE.

FRONT LENS

IMMERSION FL UID
| COVER CLASS

Fig. 45. — Diagram illustrating difference of emer^in-
pencil without and with balsam.

haveac//-y object-glass of nearly 180° angular apertuiv. This is readily
seen by fig. 45. By the arrangement presented in the figure the co\ er
glass is practically the
first surface of the ob- ^
jective, for the front
lens, the immersion
fluid, and the cover-
p-lass are all homo-
geneoiis, 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 larger than it was when the
angle of the pencil was 180° in air, and is represented
by the outer circle in fig. 46. In both these cases
the power is identical ; it follows, therefore, that the
greater diameter of the emergent pencil from the
back of the combination denotes the greater aperture
of the immersion objective over that of the dry one,
although it possessed an angle of 180°. From this escape is impos-
sible, and it is for this reason that opticians make the back lenses of
their immersion object-glasses larger than those of dry ones of the
same power.

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

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

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

Fig. 4(5.

Fig.

on

r. — Back of lens
removing eye-
piece when A. pcllu-
dda has been resol-
ved, showing spot of
bright light and faint
bluish spot Opposite;

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
im age - form ing.

The refractive indices of (cedar) oil, water, and air are respec-
tively 1"52, 1-33, and 1*0. 'Angular aperture' claimed that the
angles of the admitted pencils to lenses of these three constructions
expressed equal 1 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
facts and devoid of the wedding garment of deduction, is a triumph
which will make the name of Abbe long and gratefully remem-
bered.

The principle upon which increase of numerical aperture 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 twenty-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, proves that it is hopeless to attempt to attach
any value to angle as angle.

About twenty 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 structurr. 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 was 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 the * angle ' of light depended the delineating power.
It was taken as a self-evident 'proposition that the formation of the
image in the microscope took place in every particular according to
the same dioptric laws by which images are formed in the telescope,
and it was tacitly taken for granted that every function of the

' AXdLK ' AS SI.'CII OF No VAU'K

6l

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 vasl
majority of objects, and especially those with which the highest
qualities of an objective are called into operation, the production of
the microscopic image is wholly and absolutely dependent, oof upon
the obliquity of the rays to the object, as it had been so long and so
stoutly maintained, but upon their obliquity to the axis of the micro-
scope.

Such coarse objects as require only a few degrees of aperture
to disclose them are dependent on ' shadow effects ' j 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 inclining 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
inclination of the object to the axis of the instrument will enable the
objective of 80° to do the work easily done by one of 90°. This
may be tested by anyone 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 hindoi 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 formation
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 sui generis. There is,
and can be, no comparison between microscopic and macroscopic
vision. . The images of minute objects are not delineated microscopi-
cally by means of the ordinary laws of refraction • they are not
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 be 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
be held in a pencil of diverging light, those undulations which pass

DIFFRACTION PHENOMENA

63

directly through the aperture interfere with those passing «*l>li< jut*Iy
at tlie edge of the disc and produce, at certain distances, ;i 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 tmdulatory
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 weaves of sound, an acoustic shadow is
only produced if the obstacle be many times greater than the length
of the sound-waves. If the obstacle is reduced, the waves 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
w^ave-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 striae 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 twenty 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, relieved
the editor in his consciousness of great responsibility by saying that
lie distinctly approves of the 1 lively interest and care which (the
present editor) has bestowed on the exposition of his (Dr. Abbe's)
views,' and that he feels 'the greatest satisfaction in seeing (his)
views represented ... so extensively and intensively.'

But beyond this, an original 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. Abbe.

In the expositions of Dr. Abbe's views on the diffraction theory

64 VISION WITH THE COMPOUND MICEOSCOPE

of microscopic vision given up to this time, it has been usual to
state that he held and taught that the microscopic image consists of
two superimposed images, 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 whose elements lie so close together
as to occasion diffraction phenomena can alone be formed, because
these could not be geometrically imaged. So that in case of an
object with lines closer than the -g-gVo 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
was considered a 2^ositive 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
"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
long as the objects are depicted by means of transmitted or reflected
light, 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 inch. 1

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

RECENT MODIFICATIONS i)F AiiiiE's VIEWS

$5

'My present views may be thus expressed : With coarse objects
the diffracted (bent off) rays belonging to an incident ray or pencil
are all confined within a very narrow angular space, around that
incident ray, and do not appear separated from this except with
objectives of very long focus. The whole of such a narrow diffraction
pencil is consequently always admitted to the objective together with
the direct (incident) beam, whatever may be the direction of inci-
dence, axial or oblique. According to the proposition of p. 72 (])
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 completely,
independently of the limiting action of the lens opening, and hence
the corresponding parts of the object (outlines lire.) 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.
this part of the total diffraction pencil will be subject to a more or
less incomplete admission to the objective, and the corresponding
image will therefore show the characteristic traces of the diffraction
image, that is to say, change of aspect with different 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 principle, 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 our purpose is a plate
of glass ruled with fine parallel lines. If the dame of a candle be so
placed that its image may be seen through the centre of the plate, this

1 Letter from Dr. Abbe.

2 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
woidd 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

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 be produced by dust scattered
over a glass plate and by other objects whose structure contains very
minute particles, the light suffering a characteristic change in pass-
ing through such objects, that change
consisting in the breaking up of a
parallel beam of light into a group of
rays diverging with wide angle, and
forming a regular series of maxima and
minima of intensity of light due to difference of phase of vibration.

In the same way in the microscope the diffraction pencil origin-
ating 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 fig. 49 will be observed at the back of the

objective on removing the eye-piece and
looking down the tube of the microscope.
The central circle is an image of the dia-
phragm opening produced by the direct,
so-called non-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 images
agree in size and form, but in white light
the diffracted images are radially drawn
out with the outer edges red and the
inner blue (the reverse of the ordinary
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 by the
fact that the rays collected at the back of the objective, depicting
there the direct and spectral images of the source of light, reach in
their further course the plane which is conjugate to the 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.

It 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

Fig. 49.

1)1 !• FRACTION EXPEI! I .M K NTs

6/

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 Schwendener'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 grating. Fig. 51. — Diffraction image at back

of lens without eye-piece.'

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

The first 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. 52. Fig. 53.

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

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

F 2

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.

Fig. 54. Fig. 55.

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 the image
of the upper lines identical with that of the lower. On replacing
the eye-piece we see that it is so : the upper set of lines are 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, and are seen as in fig. 57.

Fig. 56. Fig. 57.

A case of apparent creation of structure similar in principle to
the foregoing, though more striking, is afforded by a network of
squares, such as fig. 58, having sides parallel to the page, which gives
the spectra shown in fig. 59, consisting of vertical rows for the
horizontal lines and horizontal rows for the vertical ones. But it
is readily seen that two 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 fch<
holds good we ought to find, on obstructing all the other spectra and

Fig. 58.

xc

0

fo

0

0

O

o\

0

0

0

O

0

V

Q

0

O

0

7

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 right angles to the diagonals in their place.

Fig. 60.

Fig. 61.

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

Fig. 62.

Fig. 63.

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

An object such as Pleurosigma angulatuwi, which gives six dif-

7o

VISION WITH THE COMPOUND MICROSCOPE

fraction 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
caf, and a third at right angles to g ad. 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 be a) will form, equilateral triangles and give
hexagonal markings. Or by stopping off all but gee (or b df) we
again have the spectra in the form of equilateral triangles ; but as
they are now further apart, the sides of the triangles in the two
cases being as \/ 3 : 1, the hexagons will be smaller and three times
as numerous. Their sides will also be arranged at a different angle
to those of the first set. The hexagons may also be entirely
obliterated by admitting only the spectra g c or gf or b f, &c. when
new lines will appear parallel at right angles or obliquely inclined
to the median line.

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

In practice, indeed, it has 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 mathema-
tical 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. Stephenson points out 1 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 be5) in the object itself. But let
a stop made of black paper, which entirely excludes the central beam
of light, be placed at the back of the objective and close to the pos-
terior lens ; in the stop let six 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
theory 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\ angulatum, as accessible to us with our present
numerical aperture ; and the diagram of the diffraction image, de-

1 Joum. B.M.S. vol. i. 1878, p. 180.

PLEUROSIG MA A NO ULA T I ' M

/I

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. angulatum, given in Plate X., where it will
be seen that the details shown in fig. G4 appear.

Let it be clearly understood that this does not pretend to be an
interpretation of the markings of the diatom ; it is only held by
Abbe to be an accurate indication by calculation of what image the
given ^ 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. angulatum 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. angulatum theory indicated the optical, but not necessarily the
structural existence 1 of smaller markings, shown in fig. 6 1. between
the circular spots. These had not been before seen by observers ; and
the mathematician who made the calculation (Dr. Eichhorn) had never
seen the diatom under the microscope; but when Mr. Stephenson
re-examined the object — stopping out the central beam as above
described and allowing the six spectra only to pass — he saw the
small markings, and showed them at a meeting of the Royal Micro-
scopical Society to many experts who were there. They were small
and faint, and no doubt purely optical; and, we learn from experiment,
may readily escape observation ; but by careful investigation they

1 Conf. Abbe's recent note, pp. 72 ct 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. We learn that dissimilar structures will
give identical microscopical images when the difference of their
diffractive effect is removed, and conversely similar structures 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 images of material forms, but only as signs of
material differences of composition of the particles composing the
object, so that nothing more can safely be inferred from the image
as presented to the eye than the presence in the object of such
structural peculiarities as will produce the specific diffraction pheno-
mena on which the images depend.1

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

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

For the same reason the diffraction fan of isolated corpuscles or
flay ell a 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, however minute they may
be ; 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 nagellum of Bacterium 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. 65. But we cannot pass over in tbis connection the
remarkable paper in the Journ. Quekett Club, ser. ii. vol. iv. on the ' Sub-stage
Condenser,' by Mr. Nelson. His photo-micrographs 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* PR< >I5I,FM 73

compute, which would give an exactly similar diffraction fan, but
abruptly broken off at the limit of the aperture. Theory shows' thai
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 niicroscope, resulting in the appearance of a thread
which- is the 'diffraction image.' But this latter is greater in widtli
than the actual thread or 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 different structure,
namely, of one the icJiole of ichose diffracted beams would (if it
physically existed) be represented by the utilised diffracted beams of
the structure in question.

At this point 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. angidatn.m
(tig. 6-1) 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 two points, viz. —

i. ' It is an exemplification of the general proposition that the
same object affords different images if different portions of the total
diffraction fan are admitted to the objective.

ii. ' The image in question shows to the observer what would be
the true aspect of that structure which will split up an incident beam
of light into six isolated maxima of second order of equal intensity,
suppressing totally the (central) maximum of the first order, as
fig. 65 ; a structure of such a particular and unusual diffraction effect
is theoretically possible, although it may be probably impossible to
realise it practically. Air. 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
whole. The images of such coarse objects will therefore be always
perfectly similar to the object, and the result of the interference
effect is the 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 expansion
(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. With very
minute structures, the diffraction fan
which can be admitted in air, and
even in water or balsam, is only a
greater or less centred 'portion of the
whole possible diffraction fan corre-
spon ding to those structures, and which
could be obtained if they were in a
medium of much shorter wave-length.
Under these circumstances no objec-
tive, however wide may be its aperture,
can yield a complete or strictly similar
image.'

It is at points of such extreme
delicacy and moment as this that the
diffraction hypothesis of Dr. Abbe is
so liable to misapprehension and mis-
interpretation, and a further note from
him relating to the dissimilarity of the
image in the case of incomplete admis-
sion of the diffraction pencil will be of
-Pig. 65. great value here.

i. ' In the case of regular periodic
structures (i.e. equidistant stripe, 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) is admitted to the
objective and at least one of the next diffracted rays, or, in other words,
one of the next maxima of second order. The range of dissimilarity

DIFFRACTION THEORY UNIVERSALLY A IM'LH'A 1>LK 75

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

' If not more than the said two 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 striae 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 pellucida, 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 central incident
beam or axial light may lose a greater or smaller portion of the
whole diffracted light if the angular expansion of the fan extends to
the aperture of the objective in use. But oblique illumination must
always involve a loss, and this less 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 wdiich 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 struct ures,
isolated elements of any shape, equally produce diffraction effects,
observed either by transmitted or reflected light, and being either
transparent, semitransparent, or opaque.

The value of a = n sin u indicates the number of rays which
an objective can admit ; the aperture equivalent measures the very
essence of microscopical performance. It measures the degree in
which a given objective is 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

76

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 twice
the number of undulations in an inch multiplied by the numerical
aperture.

To those who have studied this subject it will be seen that the
1 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 1 sine of half
the angle of aperture,' the proposition is only true with the addition
that the number of undulations be calculated from the wave-length
within the special medium to which the angle of 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/ti in the solar spectrum as being
sufficiently luminous for vision, we find the maximum — so far as
seeing is concerned — to be 118,000 to the inch (the object, in this
case, being in air) ; but as the non-luminous chemical rays remain
in the field after the departure of the visible spectrum, a photo-
graphic image of lines much closer together might be produced.

S

Fig. 66.

This important subject can scarcely 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.

1. 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 DIFFEACTION THEORY

77

axial pencil of incident rays. If this object is observed in air, tin-
radiation from every point of the object is, in consequence of tin-
diffraction effect, composed of an axial pencil S, fig. 66 (the direct
continuation of the incident rays), and a number of bent-off pencils,
■Sj, S2, . . . 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 reduced in the exact proportion of w, 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-in from air the first, or the first and second,
diffraction beams on every side of the axial pencil, another objective
with a more dense front medium of the refractive index, n, will be
capable of admitting, from within the dense medium, exactly the same
beams (no more and no less), if its angular semi-aperture, v, is less
than u in the proportion :

sin v : sin u = 1 : n,

or

n sin v = sm u,

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

For example, a diatom for which the distance of the striie is O'G
will give the first bent-off beam of green light (\ = '55 //) in air under
an angle of 66*5°. This will be just admitted by a dry objective of
133° angular aperture. In balsam (n = 1*5) the same pencil will
be deflected by 37'5° only, and would be admitted, therefore, by an
objective of not more than 75° balsam-angle. Again, if the distance
of the lines should be greater, as the wconrf deflected beam

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

78

VISION WITH THE COMPOUND MICROSCOPE

would be emitted in air under an angle of 66*5°, but in balsam the
third would attain the same obliquity. Whilst 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, therefore, that a balsam-angle of 75° denotes the same
aperture as the larger air-angle of 133°, and a balsam-angle of 133°
a much greater aperture than an air-angle of the same number of
degrees, and in general two apertures of different objectives must be
equal if the sines of the semi-angles are in the inverse ratio- of the
refractive index of the medium to which they relate — or, which is the
same thing, if the product of the refractive index multiplied by sine
of the angular semi-aperture (n sin u) 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

S

AIR, \. \ \

5

/ A

/ / A

1 COVER y '''

— <sr

( SLIDE

g

<

Fig. 08.

fig. 663 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. But

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

(Striue = 22 fx, wave-length \ = "55 /x, medium air n = 1.)

Sx = 14° 30'
So = 30° 0'
S5 = 48° 36'

54 = 90° 0'.

(Striae = 2-l2 u, wave-length A ='56 fx, medium balsam n = 1*5.)

Sx = 9° 30'
S2 = 19° 28'

55 = 30° 0'

54 = 41° 48'

55 = 50° 26'
Sc - 90° 0'.

HOMOGENEOl'S VERSUS DRY OUJKCTI VKS

79

according to the law of refraction, this group, on passing to air by
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 oi
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 by
mounting the object in a dense medium, for no ray which could be
taken in from the uncovered object is lost by the balsam-mounting.

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

A structure the distance of whose elements equals 2*2 fi emits in
balsam six distinct beams on each side of the direct beam, but in air
only four (see figs. 66, 67, and 68); the fifth and sixth are completely
lost in air. A dry objective of an angular aperture 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 1 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 diffraction theory of micro-
scopic vision ; and

3. That ' dry' objectives, so far as regards the perfect delineation
of very minute structures, can only be considered as representing an
imperfect phase of construction. When made by the best hands,
with every 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 WITH 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
them from a -jUinch to a ^-inch focus. We have seen, them
advance from dry to water immersion, and from this to oil ; from
^--inch, a ^--inch, and a -J^-inch of N.A. 0'95 each, and re-
spectively to water immersions of N.A. 1*04 and then to 'oil
immersions' or homogeneous lenses of N.A. 1*38 for the ^-inch
and -^-inch respectively, and ultimately by a -^-inch with N.A.
of 1*50 ; and from that we have progressed to the apochromatic
objectives with compensating eye-pieces.

Now the objectives with which the earlier work done by the
present editor and his colleague, Dr. Drysdale, was effected — to
which allusion is made only as being the instance with which we
have most practical familiarity — are still in our possession ; what
was revealed by them fifteen, twelve, or ten years ago we can
exactly repeat to-day ; and in the general features of the work — in
the broad characteristics of the life histories of the saprophytic
organisms, minute as they are, revision with objectives of N.A. 1*50
and other lenses of the best English and German 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 ^oVtfth ^°
the soVirth of an inch in long diameter was known to possess a
distinct nucleus ; the long diameter of this was from the yfith to
the x.-th of the diameter of the whole body of the organism. In the
observations of ten to fifteen 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, initiated
by the organism as a whole. But by lenses of N.A. 1*50 and the
finest apochromatic objectives of Zeiss, 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 as 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 lower jioicers, 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

8l

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 the
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 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 wThole 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 Avork must be supplemented by the use of objectives
of great aperture. The details and relations of minute structure
must be studied in one field, and their general origin and sequences
in another. The latter will be 1 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 -
scopist ; 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 within 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.

G

82

VISION WITH THE COMPOUND MICROSCOPE

It will be" ' empty amplification,5 because there is nothing in the
image which requires so much power for distinct recognition. If
the power 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 line lines, whose
interspaces are one micron,1 is just possible, it is fruitless labour to
increase the amplification beyond what we know to be 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.

The circumstances on which what has been called ' penetration '
in objectives is dependent will be shortly considered ; 2 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.' 1

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 effective-
ness of these powers — an excess in such a case is a real damage.' 5

Moreover, in biological w ork - -constant application of the instru-
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 cor-
rection. If it be not necessary to encounter the possible difficulties
these things involve, to do so is to lose valuable moments. These
difficulties, of course, are diminished by the use of homogeneous, and
especially apochromatic objectives, but even with these they are
not, in practice, done away where the best results are sought.

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

It has been suggested that all objectives be made of relatively
wide apertures, and that they be 'stopped down' by diaphragms
when the work of 1 lower apertures ' lias to be done. But this is

1 A micron is /jl = t^oo mm- Vide Journ. Il.M.S. 1888, pp. 502 and 520 ; and
Nature, vol. xxxviii. pp. 221, 244. 2 See p. 83.

3 Abbe's explanation of the reason of the non-stereoscopic perception of these is
given (see pp. 98 et scq.).

4 ' The Relation of Aperture to Power,' Journ. It. M.S. series ii. vol. ii. p. 304.

* Ibid.

PENETRATING POWEK IN OBlECTIVEH

83

not a suggestion that commends itself to the working biologist. It
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 aperturea)
which will be found on the following page. As already stated, the
resolving powers are exactly proportional to the numerical apertures,
and the expressions for this latter will allow the resolving power of
different objectives to be compared, not only if the medium be the
same in each, but also if it be different.

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

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

The delineation of solid objects by a system of lenses is, by
virtue of the most general laws of optical delineation, subject to a
pecular disproportion in amplification: The linear amplification of
the depth-dimension is, when both the object and the image are in
the same medium (air), found to be always equal to the square 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

1 ' The Relation of Aperture to Power,' Join 11. It. M.S. series ii. vol. ii. p. 301).

G 2

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— 73

I 7^

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as p

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g

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t-i

ss

VISION WITH THE COMPOUND MICROSCOPE

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

The visual space, which at one adjustment of the focus is plainly
visible, is made up of two parts, the limits of which as regards the
depth 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
image-distance. For example, a moderately short-sighted eye sees
distinctly at 150 mm. as its shortest distance, and at .'>00 mm. as
its longest distance ; then the numerical equivalent of the extent
of accommodation would be equal to mm. ; the calculation for
an object in air would give a depth of vision by accommodation
amounting to

2-08 mm. with 10 times amplification

023 „ 30

002 „ 100

0-0023 „ 300

0-00021 „ 1000

0-00002 „ 3000

PRINCIPLES OF STEREOSCOPIC VISION

89

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

Secondly, the perception of depth is assisted by the insensi-
bility of the eye to small defects in the union of the rays in the optic
image, and therefore to small circles of confusion in the visual image.
Transverse sections of the object which are a little above and below
the exact focal adjustment are seen without prejudicial effects. The
total effect so obtained is the so-called penetration 07' depth of focus
of an objective. This may be determined numerically by defining
the allowable magnitude of the circles of confusion in the micro-
scopical image by the visual angle under which they appear to the eye.
It is found that one minute of arc denotes the limit of sharply
defined vision, two to three minutes for fairly distinct vision, and live
to six minutes the limits of vision only just tolerable. This being
determined, the focal depth depends only on tin4 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 •*>', the aperture angle of the image-
forming pencils to be G0° in air : the depth of focus of an object in
<"/• will then be —

0 073 mm. for 10 times amplification
0-024 „ :'»<» „
0*0078 „ 100
0O024 „ 800
000073 ,, 1000
000O24 „ 30OO

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

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

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 factor of no moment, and we have vision largely, indeed almost
wholly, dependent on depth of focus.

go

VISION WITH THE COMPOUND MICROSCOPE

The following table shows the total depth of vision from ten to
3,000 times :—

dification

Diameter of
Field

Accommoda-
tion Depth.

Focal Deptli

Depth of Vision,
Accommodation
Depth, and
Focal Depth

mm.

mm.

mm.

mm.

10

25-0

2-G8

0073

2-153

30

8-3

023

0024

0-251

100

2-5

0-02

0-0073

0-0273

300

0-83

00023

00021

0-0017

1000

0-25

0-00021

0-00073

0-00091

3000

0-083

0-00002

0-00024

0-00023

Diameter of
Field

11-6
1

~32?7
1

Trfaj
1

lT^G
1

2(df
1

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 A'alue in extending the indirect recognition of
space 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 more 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 smallesl natural objects with the same certainty as he is
accustomed to see with the naked eye the objects with which it is
concerned. This is a laruje advantage in the general scientific use
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
form 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
use of both cijrs 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

STEREOSCOPIC BINOCULAK VISION

91

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 tirst 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
<>t' 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 on looking at the object with the two eyes
conjointly, there is no confusion between the images, nor does the
mind dwell (.11 cither of them singly ; but from the blending of the
two a conception is gained of a solid projecting body, such as could
only be otherwise acquired by the sense of touch. Now if, instead
of looking at the solid object itself, we look with the right and left
eyes respectively at pictures of the object, corresponding to those
which would be formed by it on the retime of the two eyes if it were
] .laced at a moderate distance in front of them, ami 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 ^'heatstone or in the popular modification long subse-
quently introduced by Sir D. Brewster— simply serves to bring to
the two eyes, either by reflexion from mirrors or by refraction
through prisms or lenses, the two dissimilar pictures which would
accurately represent the solid object as seen by the two eyes respec-
tively, these being thrown 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 correctly
taken) is the precise counterpart. Thus in tig. 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

1 It hos 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

small square in the centre, but the four sides sloping equally to-
wards 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-
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

Fig. 69.

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 jtseudoscope, 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 stereosco/tic
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 the
two perspective projections formed of it by the right and left eyes
respectively ; whilst by ]>sendoscopic vision we mean that 4 conver-
sion of relief ' which is produced by the combination of two reversed
perspective projections, whether these be obtained directly from the
object (as by the pseudoscope) or from 'crossed' pictures (as in the
stereoscope). It is by no means every solid object, however, or every
pair of stereoscopic pictures which can become the subject of this
conversion. The degree of facility with which the ' converted ' form
can be apprehended by the mind appears to have great influence on
the readiness with whinh 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

CARPENTER'S V. ABBE'S VIEW OE STEREOSCOPIC VISION 93

once, there are others which resist such conversion with more or less,
of persistence.1

Now it is easily shown theoretically that the picture of any
projecting object seen through the microscope with only the right
hand half of an objective having an even moderate angle of aperture,
must differ sensibly from the picture of the same object received
through the left hand of the same objective : and, further, that the
difference between such pictures must increase with the angular
aperture of the objective. This difference may be practically made
apparent by adapting a ' stop ' to the objective in such a manner as
to cover either the right or the left half of its aperture, and then by
carefully tracing the outline of the object as seen through each half.
Bat 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 reliefs bringing out on a large scale
the solid form of the object from which they were taken. What is
needed, therefore, to give the true stereoscopic power to the micro-
scope is a means of so bisecting the cone of rays transmitted by the
objective that of its two lateral halves one shall be transmitted to
the right and the other to the left eye. If, however, the image thus
formed by the right half of the objective of a compound microscope
were seen by the right eye, and that formed by the left half were
seen by the h>ft eye, the resultant conception would be not stereo-
scopic but pteudoscopiCf the projecting parts being made to appear
receding, and vice versa. The reason of this is, that as the microscope
itself reverses the picture, the rays proceeding through the right and
the left hand halves of the objective must be made to cross to the
left and the right eyes respectively, in order to correspond with the
direct view of the object from the two sides ; for if this second
reversal does not take place, the effect of the first reversal of the
images produced by the microscope exactly corresponds with that
produced by the 'crossing ' of the pictures in the stereoscope, 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.

In contradistinetion to tit is 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
rnison 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

1 For a fuller discussion of this subject see the Author's Mental Fliynologijy
%% 1G8-170.

94

VISION WITH THE COMPOUND MICROSCOPE

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 are still presented.

Professor Abbe demonstrates 1 that in an aplanatic system pencils
of different obliquities yield identical images of every plane object,
or of a single layer of a solid object. 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 the
binocular microscope and vision with the unaided eye.

An object, not quite flat, as a curved diatom, when observed with
an objective of wide aperture will present points of great indistinct-
ness. This lias 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 explic-
able 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, we 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
aperture, good delineation with these must be confined to thinner
objects than can be successfully employed with objectives of narrow
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 per-

s

1 Jott/rn. B.M.8. series ii. vol. iv. pp. 21-24.

ABBE OX STEREOSCOPIC VISION

95

ceptible at different planes, the out-of -focus layers must appear con-
fused and no vision of depth would be possible.

Xow stereoscope vision requires, as shown by Dr. Carpenter, that
the delineating pencils shall be so divided that one portion of the
admitted cone of light is conducted to one eye and another portion to
the other eye. If this division of the image is effected in a symmetrical
way, the cross section of, e.g. a circle must be reduced to two semicircles
representing one of these two arrangements seen in O and P, tig. 70.

Fig. 70.

Dr. Abbe's theory is that the only condition necessary for ortho-
topic effect in any binocular system is that these semicircles or
their equivalents should be depicted according to diagram 0, fig. 70,
and for p» udoscopic effect according to diagram Pin the same figure ;
and he demonstrates that all other circumstances, such, e.g. as the
crossing of the images, are wholly immaterial.

Orthoseopic vision is always obtained when 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 orthoseopic
vision is not to be obtained in a binocular with non-erecting eye-pieces
unless the axes of the two halves of the admitted cone cross each
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
ordinary vision. Clearly, then, it is the perspective projections of
this image which require to be compared to the projections of solid
objects in ordinary vision, in respect to which the criteria of ortho-
seopic 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 ;
and 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 pencils 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 Nachet binocular (pp. 98, 99) crossing over^s required
because the inversion of the pencils is not changed by two reflexions.
If the delineating pencils have been reflected an even number of times

Fig. 71

BINOCULAR MICROSCOPES

97

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 bin -
ocular 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 New Orleans. It was devised in 1851 and constructed in
1852, and a description of its nature and its genesis was given by
him in the second volume of the first series of the ' Quarterly Journal
of ^Microscopical Science ' in the year 185 1. 1

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. 72.

It will be seen that the pencil of rays emerging from the back
lens of the combination 7 is divided into two, each half passing re-
spectively into the right and left prisms ; the path of the rays is
.indicated at a, 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 travel-
ling 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 con-
nected by racks, one acting-
above and the other below the
same pinion, so that right- and
left-handed movements are
communicated by turning the
pinion.

This instrument could only
be used in a vertical position,
as shown in the figure (71) ;
to obviate this considerable

drawback Riddell mounted two right-angled prisms in brass caps,
which could be slipped over the eye-pieces. This arrangement in-
verts the image in both planes, and it is seen through the instru-
ment as in nature.

This system of binocular excited much interest in England im-

Fig. 73. — Arrangement of prisms in Nachet's
stereoscopic binocular microscope.

1 P. \\

98 VISION WITH THE COMPOUND MICROSCOPE

mediately after its publication, and Mr. Wenham in London and
MM. Nachet, of Paris, soon suggested and devised a variety of

binocular systems.

Nachet's Binocular.— One of these (not now, we have reason to

believe, advocated or employed by its inventor) was that devised by
MM. Sachet, constructed "on the method shown in fig. 73. The
cone of rays issuing from the back lens of the objective meets the
flat surface of a prism (})) placed above it. whose section is an equi-
lateral triangle, and is divided by reflexion within this prism into
two lateral halves, which cross each other in its interior. The rays
ab that form the right half of the cone, impinging very obliquely on
the internal face of the prism, suiter total reflexion, emerging through
its left side perpendicularly to its surface, ami therefore undergoing
no refraction ; whilst the rays a! h\ forming the left half of the cone,
are reflected in like manner towards the right. Each of these
pencils is received by a lateral prism, which again changes its direc-
tion, so as to render it parallel to its original course, and thus the
two halves^/; and a b' of the original pencil arc completely separated
from each other, the former being received im<> the left hand bod)
of the microscope (rig. 73), and the latter into ii^ right hand body.
These two bodies are parallel ; and. by means of an adjusting -m row
at their base, which alters the distance between the central and the
lateral prisms, they can be separated from or approximated towards
each other, so that the difference between their axes can be brought
into exact coincidence with the distance between the a\e> of the
eyes of the individual observer. This instrument gives true • stereo-
scopic' projection to the conjoint image formed bj the mental fusion
of the two distinct pictures, and with loin powers of moderate
angular aperture its performance is highlj satisfactory There ai r,
however, certain drawbacks to its general utility. First, even raj
of each pencil sutlers two reflexions, and ha-, to pass through four
surfaces: this necessarily involves a considerable loss ut* li-hi. with a
further liability to the impairment of the image b) the unalleet
want of exactness in the form of either of the prisms. Secondly, the
mechanical arrangements requisite for varying the distance of the
bodies involve an additional liability to derangement in the adjust
ment of the prisms. Thirdly, the instrument can onh be used tor its
own special purpose; so that the observer must also be provided
with an ordinary single-bodied microscope for the examination of
objects unsuited to the powers of bis binocular. Fourthly, tl i < ' pa i.i I
ielism of the bodies involves parallelism of theaxesof the ol lerveVi
eyes, the maintenance of which for an) length of tin e i fatiguing.

Wenham's Stereoscopic Binocular. Ml these ... ,,.
overcome in the admirable arrangement devised by the ingenuity of
Mr. Wenham, in whose binocular the .one of ra\ , ... ,1,,,- ' up
wards from the objective is divided b) the interpo ition <•»' a pri m
of the peculiar form shown in fig, 7 1. bo placed in the tube which
carries the objective (figs. 75, 76, a), as only to interrupt ons h ilt.

a c, ot the cone, the other half, <> A, going on contini Ij to the eye

piece of the principal or right-hand body, I;, in the axis of which the
objective is placed. The interrupted half of the cone (flfi 74 70 a)

WENHAM'S BINOCULAR PRISM

99

on its entrance into the prism, is scarcely subjected to any refraction,
since its axial ray is perpendicular to the surface it meets; bul
within the prism it is subjected to two reflexions at b and c, which
send it forth again obliquely in the line
d towards the eye-piece of the secondary
or left-hand body (tig. 75, L) ; and since
at its emergence its axial ray is again
perpendicular 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 right
half of the objective, and have been sub- pIGi 74 —"Wenham's prism,
jected 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 in-
dividuals is made by drawing out or pushing in the eye-pieces, which

• L li

Fig. 75. Fig. 76.

Wenham's stereoscopic binocular microscope.

are moved consentaneously by means of a milled-head, as shown in
tig. 76. Now, although it may be objected to Mr. Wenham's method
(1) that, as the rays which pass through the prism and are obliquely
reflected into the secondary body traverse a longer distance than

h 2

100

VISION WITH THE COMPOUND MICEOSCOPE

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 ; 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 [\) that a slight differ-
ence in the size of the two pictures is no bar to their perfect com-
bination ; and (2) that if one of the pictures be good, the full effect
of relief is given to the image, even though the other picture be
faint and imperfect, provided that the outlines of the latter are
sufficiently distinct to represent its perspective projection. Hence
if, instead of the two equally half-good pictures %\ Inch are obtainable
by MM. Nachet's original construction, we had in Mr. Wenham's
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. Wenhain s arrangement is
less considerable than that of boiA pictures
in the original arrangement of M M. Nachct ;
so that the optical performance of the Wen-
ham binocular IS in every w iv superior. It

has, in addition, these further advantages
over the preceding : Fust, tin' ^ivater com-
fort in using it (especially for some length
of time together), which results from the
convergence of the ;i \< >s i >f t he eves it 1 heir
usual angle for moderately near objects;
secondly, thai this hinocular arrangement
does not necessitate a sperial instrument,
but may be applied toan\ iiiierosenpe which
is capable of carrying the weight of the
secondary body, tin* prism being so lived in
a movable frame that it may in a moment
be taken out of the tulie or replaced there
in, so that when it has been removed the
principal body acts iii even respect as an
ordinary microscope, t he .nine c >ne of rays
passing uninterruptedly into it ; and thirdly, thai the - implicit \ of
its construction renders its derangement almost Impossible '

Stephenson's Binocular. A tiev Form ol t< pic b] rular

has been introduced by Mr, Stephenson,5 which ha m tain dis
tinctive features, and at the time Mi. Stephen on devised n he was

entirely unaware that any part of the method be emploved had I n

used by another. He had, however, independent!} conceived Rid
dell's device for dividing the beam aa o pari of hi irer) ingenious
instrument. This he discovered and acknowledged . houl lliree

* i T^AuSor cannot allow this opportunitj to m r itboui i tprei lug hi* w&nm
of the liberality with which Mr. Wenham freel) presented ... th, publ' thii ....
portant invention, by which, there can be no doubt, U, n,.k<ht huv„ I ell pro
? if lie had chosen to retain the exclusive right to it.
Monthly Microscopical Journal, vol. iv. ,1870;, p. 01, and vol. vii. (1873), p. M7.

Pig. 77— Eiddell's binocular
prisms, as applied by Mr.
Stephenson.

STEPHENSON'S BINOCULA R

IOI

Fig. 78.

years after the full description and completion of his binocular.1
The cone of rays passing upwards from the object-glass meets a pair
of prisms (A A, fig. 77) fixed in the tube of the microscope imme-
diately above the posterior combination of the objective, so as to catch
the light-rays on their emergence from it ; these it divides into two
halves and behaves as described in the Riddell prisms, which, in fact
they are. As the cone of rays is equally divided by the two prisms,
and its two halves are similarly acted on, the two pictures are equally
illuminated, and of the same size ; while the close approximation 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 arrangement, the observer looks
through them with more comfort. But Mr. Ste-
phenson's ingenious arrangement is liable to the
great drawback of not being convertible (like Mi*.
Wenham's) into an ordinary monocular by the
withdrawal of a prism, so that the use of this form
of it will be probably restricted to those who desire
to work with a binocular when employing high
powers. In order to avoid slight errors arising
from the impinging of the central ray of the cone, at its emergence
from the objective, against the double edge of the prism-combination,
Mr. Stephenson has devised a special form of sub-stage condenser
(also made by Mr. Browning), which causes the illuminating rays to
issue from the object in two
separate pencils, which will
strike the surfaces of the two
prisms. This consists of two
deep cylindrical lenses A and
B, fig. 78, whose focal lengths
are as 2*3 to 1, having their
curved faces opposed to each
other, as shown in section at
C ; the larger and less convex
being placed with its plane side
downwards, so as to receive
light from the mirror, or (which
is preferable) direct from a
lamp. Under this combination
slides a movable stop, with two circular openings, as shown in fig.
79. The lamp being placed in front of the instrument, the two
apertures admit similar pencils of light from it, so that each eye
receives a completely equal illumination, and no confusion can occur
from the impinging of the rays on the lower edges of the prisms.
With this arrangement the Podura markings are shown as figured
by the late Richard Beck, while the curvatures of the scale come
out with the distinctness peculiar to binocular vision.

1 Monthly Microscopical Journal, vol. x. p. 41.

Fig. 79.

102

VISION WITH THE COMPOUND MICROSCOPE

1! IIS

•••l.^'-H^^X .

, ;--c-\ • . <} . ; .IB. ■

Fig. 80.

-Stephenson's erecting
prism.

But one of the greatest advantages attendant on Mr. Stephen-
son's construction is its capability of being combined with an
erecting arrangement, which renders it applicable to purposes tor
which 'the Wenham binocular cannot be conveniently used. By
the interposition of a plane silvered mirror, or (still better) of a
reflecting prism (fig. 80), above the tube containing the binocular

prisms, each halt ot tne cone of rays is
so deflected that its image i^ reversed
vertically] the rays entering the prism
through the surface V P>, being reflected
by the surface A B, so as to pass out
again by the surface A C in the direc-
tion of the dotted lines. Tims the right
and the left half -cones are directs I respec-
tively into the right and tin4 left bodies,
which are inclined at a convenient
angle, as shown in 6g, 81 : so that —
the stage being horizontal the instru-
ment becomes a most useful compound
dissecting microscope, and as thus ar-
ranged by Swift, wit h w ell adjusted rest s
for the hands, lias but tew equals for the
purposes of minute dissections and delicate mounting operations ;
indeed, the value of the erecting binocular consists in its applic-
ability *to the picking out of Very minute objects., such as /)ittti>mtt,

Polycystina, or FovaminifiBfti,
and to the prosecution of

minute dissections, especially
when these have to U» carried on
in fluid. N«» i»ne \\ ho lias only
thus worked unmorutarfy can
appreciate the guidance derivable

from hi norit/or \ isjnu whetl 01108
t he habit < >t w ork ing with it lias

been formed.

Tollcs' Binocular Eye-piece.

Am ingenious eye piece has hecii

constructed bi Ddr.xollesj Boston!
U.S.A.), which, tttted into the

body of a monocular mil tosco|k*,

converts it into an erecting itereo
soopic binocular. This nm\ ersion
is effected Uy t lie intei posit ion

of a system of prisms similar to t li.it originally devised by MM.
Nachet, but made on a larger scale, between an 'erector1 (n
semblmg that used in the eye-piece of a day-telescope) and a pail
of ordinary Huyghenian eye-piece,, the ,, 'tr<tl or dividing prism
being placed at or near the plane of the secondary image formed b)
the erector, while the two eye-pieces are placed immediatelj above
*wo. lateral prisms, and the combination thu making that
division m the pencils forming the secondary image which in the

Fig. 81. — Stephenson's erecting
binocular.

ABBE'S BINOCULAR EYE-PIECE

103

!Nachet binocular it makes in the pencils emerging from the objective.
As 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 AVenham 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

Fig. 82. — Abbe's binocular eye-piece.

Abbe"s Stereoscopic Eye-piece. — Fig. 82 represents this in-
strument in section. The body A A' contains three prisms of
crown glass, a, b, and b' . The two eye-pieces B, B' are let into the
top plate, the former being fixed, whilst the latter has a lateral
sliding movement; the bottom plate carries the tube C for inserting
the eye-piece into the microscope tube like an ordinary eye-piece.

The two prisms a and b are united so as to form a thick plate
with parallel sides, their continuity, however, being broken by an

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

io4

VISION WITH THE COMPOUND MICROSCOPE

exceedingly thin stratum of air— less than O'Ol mm.— inclined to
the axis at an angle of 38-5°. The cone of rays from the objective
is divided into two parts, one being transmitted and the other
reflected. The transmitted rays pass through o. b without deviation,
and form an image of the object in the axial eye-piece ft. The rays
reflected at the angle shown in the figure pass through the second
surface of the prism b (upon which they are incident at right angles),
and emerging at an inclination of 13° with the horizontal are
totally reflected into the eye-piece ft' at an angle of 90° by the
hypothenuse surface of the right-angled equilateral prism b\ the
axis of which also makes an angle of 13° with the axis of the
microscope.

Adjustment for different distances between the eyes is effected
by the screw D, which moves the eye-piece ft, together with the
prism b', in a parallel direction. The tube- of the eye pieces can
also be drawn out if greater separation i> required.

The eye-pieces have the usual two lenses, but are of special eon
struction in order to equalise the length of the direct axis and the
doubly reflected axis, and in spite of tin- inequality obtain sharply
defined images of equal amplification with the same focus.

Stereoscopic vision is obtained by halving the cones of rays
above the eye-pieces. This is effected by stopping otl' half of t lu-
real image of the objective opening formed aho\e the eye pieces at
the so-called 1 eye-point ' ft or ft', which represents the common
cross-section of all the pencils emerging from the eyepiece. A cap
with a semicircular diaphragm is fitted t<> the eye piece (shown in
the figure over ft ), the straight edge of which is exactly in the
optic axis of the eye-piece, and can be raised <»r lowered 1»\ screwing
so as to obtain a uniform bisect ion of the coin - <>t rays rrom every
point of the field.

The height of the diaphragm is regulated once lor all t*«»r the
same length of the microscope- tube l»\ finding the position tor which
the aperture-image (which on withdrawing the eye tVom the eye-
piece is visible as a bright circle above it) shows no parallax against

the straight edge of the diaphragm, i,e. BO thai <>n ving the eye

laterally the image always appears fco adhere tn the edge.

In addition to the above caps with diaphragms the instrument
is supplied with ordinary caps with circular aperture . al in ft-
They taper slightly and simply slide into the eye piere, so that
they can be readily changed. The special feature of the instrument
is 1st, that it is capable of being used \\ i 1 1 1 the highest powers ; and
2ndly, that it is not necessary t«. eo\ er up half of Bach of the eye piece
tubes, thus losing half the total amount of light. It i lumoient if
one only (the lateral one) is half obscured, leaving the other tree.
As the normal division of light between the two tube i two thirds
(in the axial) and one-third (in the lateral), the total lo of light il
reduced to one-sixth.

The field of view in the axial eve piece in thia arrangement in
any case necessarily appears brighter than that of the lateral OH
seen with the same eye, and in regard to thia Dr. Abbe re-
marks that the difference between the brightness of the two fields
in birocular observation 'is not only no defect, but, on the contrary,

USE OF THE BINOCULAR

105

a decided advantage.' For experience has long proved that, to
obtain a good stereoscopic effect, it is only necessary that one image
should be as perfect and clear as possible, whilst the other may,
without appreciable disadvantage, be of sensibly less perfection.

It might therefore be anticipated that this would apply (as in
fact it does) in the same way to difference of luminosity. Moreover,
an additional fact must be taken into account — that the two eyes,
especially of microscopists, always show unequal sensibility to light
as the result of constant unequal use. The less used eve, whose
acuteness of vision is always less than that of the one more fre-
quently exercised, shows a greater sensibility to light, and the
difference is so considerable that the less luminous image of the
lateral eye-piece, when viewed with the less exercised (generally the
left) eye, seems even brighter than the other when viewed with the
exercised eye. The unequal division of the light is therefore a
welcome element, as it serves to equalise this physiological difference.
The observer has only to take care that the less used eye is applied to
the lateral eye-piece ; and 3rdly, the ingenious arrangement whereby,
by simply turning the caps with the diaphragms, orthoscopic or
pseudoscopic effect can be produced instantaneously at pleasure. It
is more particularly available for tubes of short length, for which
the AVenham prism is inapplicable.

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 upon the unequal inclination of the pencils which
form the two images to the plane of the preparation, or the axis of
the microscope. By uniform halving of the pencils — whether 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 ray-
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
plane of the preparation, and the deflected image by one whose axis
is inclined about a fourth of the angle of aperture.

With low powers, which allow of a relatively considerable
depth-perspective, the slight difference of inclination, which remains
in the latter case, is quite sufficient to produce a very marked dif-
ference 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
increase of the angle of aperture, so long as ordinary central illumi-
nation 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 cannot embrace more without the
clearness of the microscopic image being affected and the focal

IOO

VISION WITH THE COMPOUND MICROSCOPE

r

Fig. 83.

depth also being unnecessarily decreased. But as those parrs of the
preparation which especially allow of solid conception are always
formed by direct transmitted rays in observation with transmitted
light, it follows that under these circumstances the difference of the
two images is founded, not on the whole aperture-angle of the ob-
jective, but on the much smaller angle of the incident and directly
'transmitted pencils, which only allow of relatively small differences
of inclination of the image-forming rays to the preparation. It is

evident, however, that when objectives
of short focus and correspondingly largo
ancle are used, a considerably greater
differentiation of the two images w ith re-
spect to parallax can be produced if, in
place of one axial illuminating pencil, two
pencils are used oppositely inclined to the
^^^^^—^^^a^^^^^— axis in such a way that each of the
^| W images is produced by one of the pencils.

•1y This kind of double illumination, though

^■■■■^■■■■■■■■■^ it cannot be obtained by the simple

mirror, can be easily produced by using
with tin1 condenser a diaphragm with two
openings (fig. 83), placed in the diaphragm stage under the eon
denser. We then have it in our power to use, at pleasure, pencils
of narrower or wider aperture and of greater or lr-> nieli nation
towards the axis by making the openings of different width and
different distance apart.

With diaphragms of this form (which can easily be made «»ut of
card-board) the larger aperture angles of high power <>bjeeti\e-> mas-
he made use of to intensify the stereoscopic effect without employing
wide pencils, which are prejudicial both as diminishing tie- clearness
of the image and the focal dt p! h.

Of course with this method of illuminat ion both eye pieces must
be half covered in order that one image may receive light only from
one of the two illuminating .•ours, and the other only

f I from the other. The division of light in both the aperture
Q Q images will then be as shown in tig. *4 ; and it is evident
J that in this case the bright i ass (.t' i he image for both eyes

Fig. «4. together is exactly the bi ■ as would be given b) one

of the two cones alone without any Covering,
The method of illumination here referred t«» which «7M origin
ally recommended by Mr. Stephenson for his binocular micro cope
has, in fact, proved itself to be by tar the bed srhen it is a question
of using higher powers than about auO tin m s. |i ,„.,•■• ,u ik iv,, aires
very well corrected and properly adjusted objective if thesharpni
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 OOtBT
vation presents within a small depth a sufficient I \ characteristic
structure.

Non-Stereoscopic Binoculars. The great comfort vhich is ex
penenced by the microscopist from the conjoint use of both eyes lias

POWELL AND LE ALAND'S HIGKH-POWEB KENOOULAE 107

Fig. 85.

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 surface of a refracting medium a part of it is
reflected without entering that medium at all. In the
place usually occupied by the Wenham prism, they
interpose 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 passes
into the principal body with very little change in its
course, whilst another portion is reflected from its sur-
face into a rectangular prism so placed as to direct it
obliquely upwards into the secondary body (fig. 80).
Although there is a decided difference in brightness be-
tween the two images, that formed by the reflected rays
being the fainter, yet there is marvellously little loss of
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, for securing
binocular vision with the highest powers. We have used the latter
of these with perfect satisfaction, but all that is required is at the
disposal of the student in the arrangement of Powell and Lealand.

To those who have used these forms of binocular habitually it
has been a frequent source of surprise and perplexity that, although
theoretically such a form as that of Powell and Lealand's is non-
stereoscopic, yet objects studied with high powers have appeared as
if in relief, and the effect upon the mind of stereoscopic vision has
been distinctly manifest. The Editor was conscious
of this for many years in the use of the Powell and
Lealand form, with even the -^th 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,2 is due to Abbe. Since (fig. 86)
when 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
flat. But if the eye-pieces be racked down, so as
to be nearer together, the centres of the pupils fall
upon the outer halves of the Ramsden circles, and we
have the conditions of orthoscopic effect ; while if they be racked up
so as to be more separated, the centres of the pupils fall on the inner
halves, and we have pseudoscopic effect.

1 Transactions of the Microsc. Soc. N.S. vol. xiv. (18GG), p. 105.

2 Journ. R. M. S. ser. ii, vol. ii. p. 271.

Fig. 80.

io8

VISION WITH THE COMPOUND MICROSCOPE

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 tker

principles of theoretical nw ap-
plied optics as they relate to the
microscope, we believe ire shall
serve the higher interests of
microscopy, and the wants or de-
sires of the more advanced micro-
scopical experts, it we endeavour
to present in a form either devoid
of technicality or with inevitable
technicalities explained »' gsmeral
outline and then an application
of the famous dioptric investiga-
tions or' (,'onss, an eminent Ger
man mat hemat irian, who amongst
many other brilliant labours in

applied mathematics expounded
//,, laws "f t/" infraction of'/i>/ht
in the < <'»■ of a co-a.cial si/tttem of
spherical surfaces^ having tnsuia
tlr' various refractive indices ///'"//

I- tin > i) tlo ni.

Although the assumptions
Upon n\ hi. h t he formula' of < itlUSI

rest are not coincident w it li the
conditions presented bj the lens

Combinations which are employed
in the construe! ion of modern
object i\ es <»i' great apart are. t be
results, nevertheless, furnish fin
admirable presentation of the
pat h of • lie ra\ s and t he posit ion-
of cardinal points, even in the
microscope ;i - w e know and use it

We remember I bs I the micro
Bcope is largely used in England
and Americs by men u bo can

only empl<>\ it m i bfir more OF

less brief recessions from profei
Bional and commercial pursuitS|
bat who often empl< »\ it wit h an
thusiasm and intelligent purpose.
Much scientific work may be done
by such men, and it will promote
the accomplishment of this, in our judgment, if the frequentl) ex
pressed desire be met which will enable Buch Btudent to understand!

1 This figure is greatly exaggerated for the wake of cImTU I

DIOPTRIC INVESTIGATION P>Y GAUSS

IO

In a general but thoroughly intelligent 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 exposition
of the dioptric system of Gauss, with a following example worked
• out in full and with every step made clear, will be of real and
practical value. Without some intelligible understanding of the
ultimate principles of the microscope no results 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 RN, SN' (fig. 87) be the spherical surfaces of a lens of
density greater than air, and let P R kS p be the course of a ray of
light passing through it; C, C, the centres of the spherical surfaces.

Let PR, RS be produced to meet the perpendiculars through
O and C in A and A'.

Let C E = r, C'S = r',1 fx = index of refraction out of air into
the medium. NN'=rf, the thickness of the lens. N R = />,
N' S = b' . These may be considered as straight lines.

Let the equation to P R be y — b = m (x — 0 N) . , (1)

R S „ y - b = ml (x — ON) . . (2)

-or, y-bf = m' (x - ON'). . (3)

Bp „ y-b' = m"(x-OW) . . (4)

" From (2) and (3)

b' -b = m' (O W - 0 N) = ml W N = m' d . . (5)
Now sin C R A = /x . sin C R B ;

or, ? ~ . sin C A R = u . . sin C B R.

OR C R

Now C A and C B are the values of y in equations (1) and (2)
when x = O C ;

.-. CA = H m (O C - 0 N) = b + m r ■
and similarly C B = b + m! r ;

(b + m r) sin C A R = /x (b + m' r) sin C B R.

Now CAR, GBR do not in general differ much from each
other, so that for a first approximation we may consider them to be
equal.

.*. b + m r = ix (b + m' r\ i.e. u m! = m —J^ . . b.

r

Let ft — 1 =±u.. then /x m' = m — b u . . . (6)

r

Similarly, sin C'SB'=/t. sin G S'A' ; •

or, C- B' . sin C.B'S = C~ . sin O A' S ;

G S G; S

1 If either of the curvatures be turned in the opposite direction the sign of the
corresponding r must be changed.

I IO

VISION WITH THE COMPOUND MICROSCOPE

and, as before,

C B' = b' + m" r', C A' = V + m' r' from equations (4) and ($) :
.*. as before we may take

y + m" 7-' = fx (&' '•')• or pm' = m

r

Let = then p m' = m" — b i>' . . . (7)

r'

From (5) and (6) V =b + - = ;' ~ - ) + - •

7 , / */ l»\ Ml ff »'

„ this and (7) jm = fi m -f & « ^1 J +

and from (6) =w-6u + 0»i [ I - +

-(i+t)+*(- -"-";"')•

Assume

^ _ /t i ^M _ ' i + ^ " = / ^ '' " = /•

then l>' z= (i h -\- It in 'i , » ; i ,

;7|/ , where ol — A A = 1 . . (8Y

Now let X, Y be the coordinates of P, the point from which tin-
ray of light proceeds ;

then by (1) b = Y - m (X - ON) j

substituting in (8) b' = y Y + m (h 1 » N

m" = h Y + m (/ - A X -ON);

whence

m" - AY v t>, j Vl A -?(X-0 N )

772, = I = '/ l + ( //' — AY)

Z-jfc(X-ON)' ' /_A(X-oN)

Now substituting in (i> the equation i<« the refracted ray

becomes

^ V / - / (X - o N )/

k / — / ( X — ()N)j

or by (8)

* Z-A(X-ON) k fU*(X-ON)J ' (

First: If X be taken such thai I — / (X — ON) a I, i.r.

X = O N - = O E, su ] ,| n .Si • ;

A

then when

x=OW - h + ff. -7 1 = ON' + 1 "i«OE', luppote,
2/ = Y, or P and p are equally distant from the axi

DIOPTRIC INVESTIGATION BY GAUSS

l I I

Also, if Y = 0, y = 0 ; or if a ray proceed from E, it will after

refraction pass through E'. Also in == ,. =zm" that is

I — k (X — ON) ' '

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

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

du'

O E = O N - 1 1 = 0 N +
/«■

= OX +

du'

it — a — clu to'

/x (u' — a ) — du a

OE'=ON'+_

1 - 9 ._

k

OX +

OX' +

d It

d n

vJ — u — duu'

— u) — duu'

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

b = — in, and the equation to the incident ray becomes
y + - m = m (x — O X), or y = m ( x — O X — ^ ;

1 +

du'

.'. when y = 0, x = O X + = O X + _

k u' — u — du it'

= OF, suppose.

If m = 0, or the ray be parallel to the axis before refraction, we
have from (8)

V = g b = 'f in", and the equation to the refracted ray becomes
y - | m" = m" (x - Q N'X or y = m" (x - 0 X' + |) ;

1 -

.-. when y = 0, x = O X - ■' = 0 X'

h

— 0 F', suppose.
F and F' are called the ' focal points.'

O F = O X + - t. ± d )
fx (ii ' — it) — a a u

fJL — d u

du

u' — it — d T v.'

OF' = OX-

fx (it/ — u) — d u u'

112 VISION WITH THE COMPOUND MICEOSCOPE

The focal distance -/=OF~OE = OE' - O.F

n —1

/.t (uf — it) — d u it k

Similarly, it may be shown that if there be two lenses, and sub-
script numbers refer to the first and second lens respectively, while
IE, E', E, F' refer to the entire system, and if

o = OE2-OE/,

- 7? = ft (UV — u\) - di u\ ui*

V2 = — — r = fi2 (U-2 — Ul) — d-1 U.2 11%,

OE = OE1+ >,]hv-2

OE' = 0 E/

Hi vi + H\ v% +c v\Vi
ft%V\

ftV\ + ft V2 + ^ V'l

OF = OE, + M/'. + *«») )

OF

jU2 17, + jU, V.2 + V2

We are now prepared to work out an example of the Gauss system
"by tracing a ray through two or more lenses on an axis, showing how
any conjugate may be found through two or more lenses on that axis.1

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. 88, or an axis x y are given. No. 1 is
a double convex of crown -| inch thick, the refractive index /x being

1 Remembering our object, and the assumed conditions of some for whom we
write, we do not hesitate to preface this with the following notes to remind the
reader of the sense attached to certain mathematical expressions.

x means infinity. A plane surface of a lens is considered a spherical surface of
-an infinite radius. Any number divided by x =0; any number divided by 0 — » ;
any number multiplied by 0 — 0. oo plus, or minus, or multiplied by any number is
still x .

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

In the multiplication or division of like signs the result is always plus ; 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 terms

with a minus sign; subtract the less from the greater and affix the sign of the
reater. Example :

+ 3- 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 :

- 3

+_2

- 5

An example occurs in the annexed equations (x) and (xi), p. 114, of — — = + ,
hut then the + is changed into a — by the negative sign in front of the fraction.
In (xii), p. 114, however, there being a + in front of the fraction, the result remains
.positive.

EXAMPLE AETER GAUSS

"3

-3, the radius of the surface A is | and that of B 1 inch. Xo. 2 lens
is a plano-concave of flint 1\T inch thick, the refractive index fi being
4, the radius of the surface C is §, and the surface D is plane. The
-distance between the lenses, that is, from B to C measured on the
axis, is J inch. The problem is to find the conjugate focus of anv
given point V.

In order to accomplish this two points have hrst to be found with
regard to each lens. These points are called principal points (see
PP, Q Q in fig. 88), When the radii of curvature r and r', rf,
the thickness, and // , /<.,, the refractive indices of the respective
lenses,1 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 formulae to lens 1 the distance of
P from the vertex A can be determined — see p. 116 (i) — similarly P'
from B — p. 116 (ii). In the same way the points Q Q' from C and I)
in lens 2 can be measured off— (v) (vi), p. 117.

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. 116 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 B, being measured from the vertex to its centre or from right to
left, is — . Similarly with the concave surface, C being measured
from l ight to left is — .

In both the examples before us the points PP', Q Q' fall inside
their respective lenses, but it does not follow that they will do so in
•every instance. Tn some forms of menisci, for example, they will fall
outside the lens altogether.

With regard to the focus of the lens it follows the same rule ;
thus, /* in lens 1 is measured to the left from P, and f to the right
from P' ; similarly in lens 2, f" is measured to the right from Q,
and /"' to the left from Q'.

Having determined the focal length of each lens, the distance
between the right hand principal point of the first lens P' and the
left hand principal point of the second lens Q must next be found. It
manifestly is the distance of B from P' + the distance B C between
the lenses, Q being at the point C. Therefore

P Q = -21 + -25 = -46 =3 2.

When these three data have been obtained — that is, the focal
length of each lens, and the distance between them — we are in a
position to apply the formulae (ix) and (x), p. 118, to find the principal
points E and E' of the combination.

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 /x 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. .

I

II4 VISION WITH THE COMPOUND MICROSCOPE

In selecting the value of the focus to be put into the equations-
for both lenses, the last must be taken, that is, in lens 1 (iv), or
+ •947, and in lens 2 (viii), or -1;875.

It will be noticed that the value ot E being negative, it will be
measured -314 inch to the left from P. Similarly, E is measured
•622 inch to the left from Q'. .

cb also is 1-28 to the left from E, and <f> 1*28 to the right

£r°These four points, E, E' and p, <p', are called the cardinal
points of the combination.

Here it must be observed that in this work it has been necessary
for want of space to restrict the problem to dry Lenses, that is. to
those cases where the ray emerges from the combination into air, the
same medium in which it was travelling on immergence. It is on
that account that the values of and q> are the s uae.

Havino- now obtained the four cardinal points, wo may at onoe
proceed to find the conjugate ot x.

Let x equal the distance of the point .r from tin- focal plane jfe
and y the distance of its conjugate from <f>'. Then by formula (xiii)

xy = <£2, and as x = 1 inch, y = 1 ^ 1 = 1*638 I.

This numerically determines the position of the conjugate plane.
If the rays incident on the combination are parallel, then X = t .

and y = ^- = 0, which means that y is eoinoidenl with </>'.

00

The following is the graphic method ot' finding t lie conjugate ot'
V. From V, fig. 88, draw a line parallel to the axis to meet K', and
from the point where it meets K draw a line through N, tin- point
where cf>' cuts the axis, to W .

From V draw another line through M, tin- point where 0 ruts
the axis, to meet E, and from the point w here it meets K drawn
line parallel to the axis, cutting the other line in W . \\ will l>e tie
conjugate of V, which was required.

If it is required to find the con jugate ot' a ray passing througli
three lenses on an axis, two of the Lenses must he combined and
their four cardinal points ton ml.

The principal points and the focal length of the third leni must
then be calculated, and then combined in their turn by formula- (ix),

(x), (xi), and (xii), p. 1 18, w it h t he cardinal points of t he double com
bination. Sis taken as the distance of the lirst principal point of
the combination, nearest the third len>, to the second principal point
of the lens, nearest the combination. A t'redi sri . >\ . .i id i nal point'
is determined in this manner for the three Lenses,

So also with four lenses; the cardinal points ot each pair being
found, they are combined by the same formula-, and new cardinal
points for the whole combi i iat ion of four Lenses are obtained. Sum
larly, the cardinal points of five, six, or any number of lenses can
be found and the conjugate of any point Localised

Finally, no one need be discouraged by the appearance of tlx
length of the calculation ; the example is given in full, so that any

A PKACTICAL EXAMPLE AFTEE

OAUSS

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
simple, and the denomina-
tors, of the four equations
are all alike ; so, too, in
the equations for No. 2 and
in those for both lenses.
Further, f is the same as
f, f" as /", and <£' as <£.
Hence the problem is much
shorter that it looks.

If the conjugate of a
point on the axis is only
required, and if the prin-
cipal points and foci of
each lens have been de-
termined, it will not be
necessary to enter into the
further calculation to find
E, E', and <£, the cardi-
nal points of the combina-
tion.

The method of proce-
dure is as follows : If x is
the given point, its dis-
tance from f, the focus of
lens No. 1, must first be
measured. Call this dis-
tance x. Then the distance
of o its conjugate from the
other focus, f\ supposing
lens No. 2 to be removed,
can be found by formula

o x = f2,

O = J-

X

P = -897, x= 1-65 ;

This is the distance from
/' to o.

As the distance from
x to f is positive, the dis-
tance between f and o is
also positive ; so o is to the
right of/'.

i 2

Il6 VISION WITH THE COMPOUND MICROSCOPE

Before 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, ami
o = oo ; 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 ;
o would then be measured off to the left of/", and the conjugate
would be virtual. This means that there will be no real image,
because the rays will be divergent on the/' side of the lens, as if
they had come from some focus on the f side of the lens. But t«>
return. The point o having been found to be the conjugate of c,
due to the sole influence of No. 1 lens, we have next to measure the
distance between o and /", and. by applying the same formula, find
the distance of its conjugate from/' . owing t<> the exclusive effect
of No. 2 lens now replaced. Thi> distance Of may be found tints :

P' o = P'/' + f o = -947 + -543 = 1-49 ;
P'/" = P'B + B C + Qf" » *21 -f -25+ 1 2-335 ;

pyv/ _ p/0 =0f" _ 2335 - 1-49 = '845,

Calling this distance 0, then, by formula y '. weshall 6nd

the distance of y from /"", which we shall call >i. u = 1 =

== 4*16, which is positive ; therefore y lies H6 inches from / to the
right hand, y is therefore the conjugate of «\ duo to the influence
of both lenses 1 and 2. Similarly, the conjugate of urn point on the
■axis may be found through any number of lenses.

Lens JSTo. 1 : Data. — Radius A = ► = r ; radius B = — I = /• ;

f°chf,f'; thickness = - = d\ fi = ? ■ l* = principal point mea-
sured from A ; V = principal point inmsurrd from B
3 3

u = nrl = H l-*-u'-?r 1 oil 1 -

r 3 3 ' r — 1 2

4

(tf - tt) = | (_ 1 - |) = _ \ , ,/ ,,„> m 1 x J x - 1 = _ ' :

2 \ 2 3/ J i> :i 2

^ (u' — u) — duu' = — ' + 1 = _ 1 1 1 — _ 1 .58a .

I (J 12 '

I 1

P = A+ . - | . g J _ 8

^(yfc' — u) — du u' i 5

19

•»

= A + 158 (i)

1 2

P' = B + rfw ... 2 _ 3 D I

/x [u' — w) — duu iv | 9 l g

A PRACTICAL EXAMPLE AFTEE GAUSS 117

3

/*=P+ = P + — p _ 18

J fX (u' - u) - duu' _ 19 19

12

= P - '947 (iii)

3

/' = ¥' — " — _ = p - ~JL = p + 15

f.i{ii' — u) — duu 19 19

~~ 12

= P + '947 (iv)

9

Lens JVo. 2 : Data. — Radius 0 = — = r ; radius D = co = r' :

0

1 8

foci, f", f" ; thickness = ~ = d ; li = ^ ; Q = principal point
measured from C ; Q' = principal point measured from D.

8-l 8-l
/. - 1 . .5 \ , . , 8 ■ y_ /n - 1 _ 5 _ _ Q ,

r _ 9J " 15 ? ™ r' a.

8

(u' - u) == 5 |0 + BJ = — ; duu' > ^ x - — x 0 = 0;
„ (V _ „) _ M it' = 64 _ 0 = 64 = -853 :

Q = C + ^ -7 = c + -2r=c+o . . (v)

/.i (u — u) — duu 04

7~5

1 _ A

Q'=D + ^ ? = D+IL-.J-JL5=.D-.L

li (u' — u) — duu' 64 16

75

= I) - '0625 (vi)

8

f'=Q + £ -j , = Q + ~ = Q + \- = Q + 1-875 (vii)

75

8

/" = <*- -n Y—r-~' = q,-u = Qf-^

H [u — u) — d u u o4 o

75

= Q' _ 1-875 .... (viii)
Both Lenses.— Distance apart = B C = \ = '25 ; P'Q = «21 4- -25

4:

= -46 = a ; /= focus of No. 1 lens = -947 ; f = focus of No. 2
lens = - 1-875.

Il8 VISION WITH THE COMPOUND MICROSCOPE

^ - r +/+/' -1 ~~ -917 - 1-875 - -46 - 1*388

= P - 314 (ix)

E'-O'- V -0'- '46 X ~ 1>875

= Q' - ~" '8f2=Q' - '621 . . . (x)
^ - 1-388 v

e6-E- = E — *947 X " 1,875

' f+f'-l -947 - 1-875 - -46

=e-_;s=e-1-8 • • • (xo

*'=E' + ff -=E'+- "95 X " ^
^ ; -947 - 1-875 - -46

xy = <l>2; 2/ = .— = — . . . . (xiii)

ii9

CHAPTER III
'the history and evolution 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 on its mechanical and
optical sides, by a thoughtful perusal of these details. It will do
more to enable the student to infer what a good microscope should
I>e 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 were 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 criticised,
-with the result that all such passages can be explained without in-
volving this assumption.

We learn from Pliny the elder and others, that crystal globes
^filled with water were employed for cauterisation by focussing the

1 Cantor Lectures on the Microscope, 1886, p. 1.

THE HISTORY AND EVOLUTION OF THE MICBOSCOPE

sun's rays as a burning-glass, and that these wore used to produce-
ignition ; but there is no trace of suggestion that these retracting
globes could act as magnifying instruments.

Seneca (< Quest. Nat.' i. 6, § 5) states, however, that ' letters
though small and indistinct are seen enlarged and more distinct
through a globe of glass filled with water.' He also states that
4 fruit&appears larger when seen immersed in a vase of glass.* But
he only concludes from this that all objects seen through water
appear larger than they are.

In like manner it" could be shown that Archimedes, Ptolemy,
and others had no knowledge of the principles on which retraction
took place at curved surfaces.

Nor is there any ancient mention of spectacles or other aids to
vision. Optical phenomena were treated of : Aristotle ami the Greek
physician Alexander dealt with myopy and presbyopy ; Plutarch
treated of myopy, and Pliny on the sight. But DO allusion is made
to even the most simple optical aids : nor is there any reference to
any such instruments by any Greek or Roman physician or author.
In the fifth century of the Christian era the Greek physician Actius
says that myopy is incurable : and similarly in the thirteenth
century another Greek 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 tin' invention of spectacles,
they are frequently referred to in medical treatises and other works.

If we turn to the works of ancient artist^ ur find amongst their
cut gems some works which reveal extreme minuteness "t detail and
delicacy of execution, and some have contended that these could
only have been executed by means of lrn ^. v. I » n t it is the opinion
of experts that there is no engraved work in our national collection
in the gem department that could not have been engraved bj I
qualified modern engraver by means of unaided vision ; and in
reference to some verv minute writ in-' which it was stated by 1 M i 1 1 \
that Cicero saw, Solinusand Plutan h, as well as Pliny, allude bo I heae
marvels of workmanship for the purpose of proving that lOmfl men
are naturally endowed with power- of virion quite exceptional in
their excellence, no attempt being made to explain their minute
details as the result of using magnify ing Lenses,

These and many other instances in which reference («> lenses
must have been made had they existed <>r been known are eon
elusive; for it is inconceivable that even simple dioptric lenses, to
say nothing of spectacles, microscopes and telescopes, could have
been known to the ancients without reference to them having been
made by many writers, and especially bv such men as Galen and
Pliny.

The earliest known reference to the Invention of pectaclei i
found in a manuscript dating from Florence in 1299, in which tin-
writer says,' I find myself so pressed bj age thai I can neither
read nor write without those glasses they call spectacles, lately in-
vented, to the great advantage of poor old men when then Bight
grows weak.'1 Giordano da Rivalto in 1305 says thai the invention

1 Smith's Optics, Canmrid-c, IT.".*, 2 vols. ii. pn, |:t.

A 'LENS' FROM SARGON'S PALACE

I 2 I

of spectacles dates back 'twenty years,' which would be about 1285.
It is now known that they were invented by Salvino d'Armato degli
Armati, a Florentine, who died in 1317. He kept the secret for
profit, but it was discovered and published before his death. But
there is a singular .evidence that a lens used for the purpose of
magnification was in existence as early as between 1513 and 1520
for at that time Raphael painted a portrait of Pope Leo X. which
is in the Palazzo Pitti, Florence. In this picture the Pope is drawn
holding a hand magnifier, evidently intended to examine carefully
the pages of a book open before him. But no instruments com-
parable to the modern telescope and microscope arose earlier than
the beginning of the seventeenth century and the closing years of
the sixteenth 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
excavations of Sargon's Palace at
Ximroud, and which Sir David
Brewster believed was a lens de-
signed for the purpose of magnify-
ing. 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. 89
and 90; but Mr. Mayall gives
strong and clear reasons for con-
cluding that its lenticular cha-
racter as a dioptric instrument
has certainly not been made out.
There are cloudy striae 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 J surface is not smooth, but produced by a large number
of irregular facets, making the curvature quite unfit for optical
purposes. In truth, it may be fairly taken as established that there
is no evidence of any kind to justify us in believing that lenses
for optical purposes were known or used before the invention of
spectacles.

From the simple spectacle-lens, the transition to lenses of shorter
and shorter focus, and ultimately to the combination of lenses into
a compound form, would be — in such an age as that in which the
invention of spectacles arose — only a matter of time. But it is
almost impossible to fix the exact date of the production of the first
microscope, as distinguished from a mere magnifying lens.

There is nevertheless a consent on the part of those best able
to judge that it must have been between 1590 and 1609 ; while it is.
probable (but by no means certain) that Hans and Zacharias Janssen,

Fig. 90— An Assyrian 'lens' (?).

122 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

.spectacle makers, of Middleburg, Holland, were the inventors. But
it would appear that the earliest microscope was constructed for
observing objects by reflected light only.

At the Loan Collection of Scientilie Instruments in London in
1876 an old microscope, which had been found. at Middleburg, was
shown, which, Professor Harting considered, might possibly have
been made by the Janssens. It is drawn in tie-. 91, and consists of
a combination of a convex object-lens and a convex eye-
lens, which form was not published as an actual con-
struction until 1646 by Fontana. which, as Mr. Mayall
points out, does not harmonise with the assumption
that this instrument was constructed by one of the
Janssens.

It is strictly a compound microscope, and the dis-
tance between the lenses can ho regulated by two
draw-tubes. There are three diaphragms, and the Bye-
lens lies in a wood cell, and is held there by a wire ring
sprung in. The object-lens, is loose in the actual
instrument, but was originally fixed in a similar way

Fig. 91.

< Janssen's '
compound
microscope.

to b.

It cannot be an easy ta^k if it be even a pos-
sible one — to definitely determine upon the actual indi-
vidual or individuals by whom the compound micro-
scope was first invented. Recently some valuable
evidence has been adduced claiming its sole invention
for Galileo. In a memoir published in 18881 Pro-
fessor G. Govi, who has made the question a Bubjecl <>t' large and
continuous research, certainly adduces evidence <>t a kind nol easily
waived.

Huyghens and, following him, many others assign the invention
of the compound microscope to Cornelius Drebbel, i Dutchman, in
the year 1621 ; but it has been suggested thai he derived his Irnon
ledge from Zacharias Janssen <>r his father, Hans Janssen, spectacle
makers, in Holland, about the year 1590; while Fontana, fl N « i
politan, claimed the discovery f< >r himself in I '» 1 s. It is Baid that
the Janssens presented the first microscope to Charles Albert, Arch
duke of Austria ; and Sir D. Brewster states, in his 'Treatise on the
Microscope,' that one of their microscopes which they presented to
Prince Maurice was in 1617 in the possession of Cornelius Drebbel,
then Mathematician to the Court of James [., where 'he made
microscopes and passed them off as his own invention.'

Nevertheless we are told by Viviani, an [talian mathematician,
in his ' Life of Galileo, 'that 'thisgreal man was led i<» the discovery
of the microscope from that of the telescope/ and thai * in 1612 In-
sent one to Sigismund, King of Poland.5

We now receive evidence through the researches of Govi that
the invention was solely due to Galileo in the year 1610. Professor
Govi understands by 'simple microscope ' an instrument 1 consisting
of a single lens or mirror,' and by 'compound microscope ' <>..«• 'con

1 Atti B. Acad. Sci. Fis. Nat. Napoli, vol. ii. Mfffea ii. 'II microacopio oompotto

mventato da Galileo,' Joiim. B.M.S. Pt. IV. Ihhm, p. .-,71.

DID GALILEO INVENT THE COMPOUND MICBOSCOPE? 1 23

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

Tn 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 nunc admirabilis huius perspicilli perfectiones explanare
no conabor : sensus ipse iudex est integerrimus circa obiectum pro-
prium. Quid quod eminus mille passus et ultra cum neque videre
iudicares obiectum, adhibito perspicillo, statim certo cognoscas, esse
hunc Socratem Sophronici hlium venientem, sed tempus nos docebit
■et quotidians nouarum rerum detectiones quam egregie perspicillum
suo fungatur munere, nam in hoc tota omnis instrumenti sita est
pulchritude

' Audiueram, paucis ante diebus authorem ipsum Excellentissimo
D. Cremonino purpurato philosopho varia narrantera scitu dignissima
et inter cetera quomodo ille minimorum animantium organa motus,
et sensus ex perspicillo ad vnguem distinguat ; in particulari autem
de quodem insecto quod utrumque habet oculum membrana crassius-
cula vestitum, qua? tamen septe foraminibus ad instar larva? ferrea?
militis cataphracti terebrata, viam praebet speciebus visibilium. En
tibi [so says AVodderborn to Horky] nouum argumentum, quod per-
spicillum per concentrationem radiorum multiplicet obiectii ; sed
audi prius quid tibi dicturus sum : in caeteris animalibus eiusdem
magnitudinis, vel minoris, quorum etiam aliqua splendidiores habent
oculos, gemini tantum apparent cum suis superciliis aliisque partibus
annexis.'

To this Govi adds : —

k I have wished to quote this passage of Wodderborn textuallv,
so that the honour of ha vino- been the first to obtain from the Dutch
telescope a compound microscoi>e should remain with Galileo, which
the later called occhialino, and that the glory of having reduced the
KepCerian 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 AVodderborn s account, published in 1610 (the
•date of the dedication to Henry Wotton, English Ambassador at
"Venice, is October 16, 1610), which reads thus ; —

124 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

< I will not now attempt to explain all the perfections of this,
wonderful occhiale ; our sense alone is a safe judge of the things
which concern it. But what more can I say of it than that by
pointing a glass to an object more than a thousand paces off, which
does not even seem alive, you immediately recognise it to he
Socrates, son of Sophronicus, who is approaching ? But time and
the daily discoveries of new things will teach us how admirably the
<>-lass does its work, for in that alone lies all the beauty of that
instrument.

' I heard a few days back the author himself (Galileo) narrate to
the Most Excellent Signor Cremonius various things most desirable
to be known, and amongst others in what manner he perfectly dis-
tinguishes with his telescope the organs of motion and of the senses
in the smaller animals ; and especially in a certain insect which has
each eye covered by a rather thick membrane, which, however, per-
forated with seven holes, like the visor of a warrior, allows it sight.
Here hast thou a new proof that the glass concentrating its rays
enlarges the object ; but mind what I am about to tell thee, viz. in
the other animals of the same size and even smaller, some of which
have nevertheless brighter eyes, these appear only double with their
eyebrows and the other adjacent parts.3

After reading this document Govi judges that it is impossible to
refuse Galileo the credit of the in vent ion of a r<>m/>t>n mt microdcop*
in 1610, and the application of it to examine some very minute
animals; and if he himself neither then nor for man? yean 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 Qalileo after these lir->t
experiments quite forgot the microscope, for in preparing the
' Saggiatore ' between the end of 1619 and the middle of October,
1622, he spoke thus to Lotario Sarsi Begensano (anagram <>t Oratio
Grassi Salonense)

' I might tell Sarsi something new if anything nan could be bold
him. Let him take any substance whatever, be it stone, or wood,
or metal, and holding it in the sun examine it attentively, and he
will see all the colours distributed in the most minute partiolea, and
if he will make use of a telescope arranged so that our can Bee very
near objects, he will see far more distinctly what I >a\.'

It will not therefore be surprising if, in 1624 (according to
some letters from Rome, written by (im.lamo AJeandro to the
famous M. cle Peiresc), two microscopes of Bluffier, or rather Drebbel,
having been sent to the Cardinal of B. Susanna, who at first did DOl
know how to use them, they were shown to Galileo, who was then in
Rome, and he, as soon as he saw them, explained their use, u
Aleandro writes to Peiresc on May 24, adding, 'Galileo told me
that he had invented an occhiale which magnifies things an much
as 50,000 times, so that one sees a My as large at ■ hen.

This assertion of Galileo, that he had invented a telescope which
magnified 50,000 times, so that a fly appears as bin „ „ I,,,,
must, without doubt, be referred to tin- y.-ar Hi 10, and from the
measure given of the amplification by the soli, lit \ or volume the

GALILEO'S ' OCCHIALE '

125

linear amplification (as it is usually expressed now) would have
been equal to something less than the cubic root of 50,000, that is'
.about 36, and that is pretty fairly the relative size of a fly and'
a hen.

Aleanclro's letter of May 24 (1624) does not state at what time
Galileo saw the telescope and explained the use of it, but another
letter of Faber's to Cesi, amongst the autograph letters in the
possession of D. B. Boncompagni, says (May 11) : 'I was yesterday
evening at the house of our Signor Galileo, who lives near the
Madnlena ; he gave the Cardinal di Zoller a magnificent eye-glass
for the Duke of Bavaria. I saw a fly which Signor Galileo him-
self showed me. I was astounded, and told Signor Galileo that lie
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 occhiale mentioned by Signor Galileo, which makes Hies
like hens, is of his own invention, of which he made also a copy
for Archduke Albert of pious memory, which used to be placed on
the ground, where a fly would be seen the size of a hen, and the
instrument was of no greater height than an ordinary dining-room
table.' Which description answers far better to a Dutch tele-
scope used as a microscope, in the same way exactly as Galileo
had used it, rather than to a microscope with two convex
lenses.

One cannot find any further particulars concerning Galileo's
occhialini (so he had christened them in the year 1624), either in
Bartholomew Imperiali's letter of September 5, 1624, in which he
thanks Galileo for having given him one in every way perfect, or in
that of Galileo to Cesi of September 23, 1624, accompanying the
gift of an occhialino, or in Federico Cesi's answer of October 26, or
in a letter of Bartholomeo Balbi to Galileo of October 25, 1624,
which speaks of the longing with which Balbi is awaiting £ the little
occhiale of the new invention,' or in that of Galileo to Cesar Marsili
of December 17 in the same year, in which Galileo says to the
learned Boloimese 'that he would have sent him an occlt ialino 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

126 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

Florence on September 23, 1624. more than three months after his
departure from Rome : —

'I send your Excellency an occhiafino, by which to see close the
smallest things, which I hope may give you no small pleasure and
entertainment, as it does me. I have been long in sending it, because
I could not perfect it before, having experienced some difficulty in
finding the way of cutting the glasses perfectly. The object must
be placed on the movable circle which is at the base, and moved to
see it all, for that which one sees at one look is but a small pan.
And because the distance between the lens and the object must be
most exact, in looking at objects which have relict one must be able
to move the glass nearer or further, according as one is looking at
this or that part ; therefore the little tube is made movable on its stand
or guide, as we may wish to call it. It must also be used in very
bright, clear weather, or even in the sun itself, remembering that the
object must be sufficiently illuminated. I have contemplated verj
many animals with infinite admiration, amongst which the flea is
most 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 feel upwards,
But your Excellency will have the opportunity of observing thousands
and thousands of other details of the most curious kind, of which 1
beg you to give me account. In fact, one may contemplate endlessly
the greatness of Nature, and how subtilely she works, and with \\ hat
unspeakable diligence. — P.S. The little tube is in two pieces, and
you may lengthen it or shorten it at pleasure.'

It would be very strange, knowing Galileo's character, that in
1624, and after the attacks made on him for ha\ ing perhaps a little
too much allowed the Dutch telescope to be considered his invent ion,
he should have been induced to imitate Ihebbel's glass with the t\\.»
convex lenses, and have wished to make them passasniso^ n invention,
whilst he had always used, and continued to use to the end oi hifl days,
telescopes with a convex and a concave lens without showing that
he had read or in the least appreciated the proposal mad.- I»y Kepler,
ever since 1611, to use two convex glasses in order to have telescopes
with a large field and more powerful and convenient.

In any case it is impossible to form a decided opinion on such a
matter, the data failing ; but the very fan that from 1624 onwards
Galileo thought no more of the ocr/u,,/i (probably because he found
it less powerful and less useful than the **(•<•// mi v < >f l irebbel ), as he
had not occupied himself with :t or had scarcely remembered it from
the year 1610 to 1624, seems sufficient to show t hat t he occhialino.
like the microscope of 1610, was a small Dutch telescope with two
lenses, one convex and one concave, and not a ivilu.nl Keplerian
telescope like that invented by Drebbel in 1621.

The name of microscope, like that of telescope, originated with
the Academy of the Lincei, and it wasGiovanni Faberwho invented
it as shown by a letter of his to Cesi, written April 13, I ''»'_'">, and
which is amongst the Lincei letters in the pofi - ion of I >. B. Boil
compagm. Here is the passage in Faber's letter : -

'I only wish to say this more to your Excellency, that is, that

GALILEO THE INVENTOR OF THE MICROSCOPE IN 1610 12

you will glance only at what I have written concerning the new in-
ventions of Signor Galileo ; if I have not put in everything, or ii
anything ought to he left unsaid, do as best 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 •
Lyceum gave to the first the name of telescope, so they have wished
to give a convenient name to this also, and rightly so, because they
are the first in Home who had one. As soon as Signor Rikio'&
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, 162-3. Your Excellency's most
humble servant, Giovanni Faber (Lynceo).'

The Abbe Rezzi, in a work of his on the invention of the micro-
scope, thought that he might conclude from the passage of
Wodderborn, reproduced above, that Galileo did not invent the com-
pound microscope, but gave a convenient form to the simple micro-
scope, and in this way as good as invented it, for the Latin word used
by Wodderborn, perspicillum, 1 signified at that time, it is clear,' Rezzi
says, ' 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 I, 1611.
If, therefore, the word microscope had not yet been invented, and
if the telescope, or the occhiale, as it was then called, was by all
named perspicillum, one cannot see why Wodderborn's perspicilhtm
cannot have been a cannocchiale (telescope) smaller than the usual
ones, so that it could easily be used to look at near objects, but yet
a cannocchiale with two lenses, one convex and one concave, like the
others, and, therefore, a real compound microscope, although not
mentioned by that name either by Wodderborn or others. And,
besides that, how could it be that Wodderborn beginning to treat
' admirabilis huius perspicilli,' that is, of the telescope in the first
line, should then have called perspicillum a single lens in the eleventh
line of the same page 1 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 Yenturi.

It thus appears as in the highest degree probable that Galileo, »
in 1610, was the inventor of the compound microscope ; it was \
subsequently invented, or introduced, and zealously adopted in
Holland ; and when Dutch invention penetrated into Italy in 1624
Galileo attempted a reclamation of his invention (which was undoubt-
edly distinct from that of Drebbel) ; but as these were not warmly
seconded and responded to abroad he allowed the whole thing to pass.
Nevertheless the facts Govi gives are as interesting as they are
important.

In regard to the discovery of the simple lens Govi points out

128 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

that after the year 1000, minds having reopened to hope and in-
tellects to study, there began to dawn some light of science, so that
in 1276 a Franciscan monk, Roger Bacon, of Ilchester, in his ' Opus
Ma-jus,' dedicated and presented by him to Clement IV.. could show
many marvellous things, and amongst these the efficacy of crystal
lenses, in order to show things larger, and in this wise he says make
of them 'an instrument useful to old men and those whose sight is
weakened, who in such a way will be able to see the letters suf-
ficiently enlarged, however small they are.' As long as no documents
anterior to him are discovered, Roger Bacon may be considered the
first inventor of convergent lenses, and therefore of the simple micro-
scope, however small the enlargement by his lenses may have been.

As, however, that man of rare genius, the initiator of experi-
mental physics, had brought on himself the hatred of his contempo-
raries, they kept him for many years in prison, then -hut 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 destruction, ami so the invention
-of lenses, or the knowledge of their use to enlarge images and to
alleviate the infirmities of sight, remained unknow n or forgotten in
the pages of the famous 4 Opus Majus,' which only Came to light in
1733 by the care of Samuel Jebb, a learned English doctor.

A Florentine, byname Salvino degli Armati, at the end of fche
-thirteenth century (? 1280) (in Bacon's lifetime), had therefore the
glory of inventing spectacles, and it was a monk of Pisa, Alexander

Spina, who suddenly 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 tin*
use of converging lenses for long-sighted
people, and of diverging lenses for short
sight, whilst the English monk had only

spoken of the lenses for long sight, and

perhaps they added to this first invention

the capability of varying the focal lengths of
the lenses according to need, and t he other of
fixing them on to the visor <>f a cap to keep
them firm in front of the eves, or to fasten

them into two circles made of metal, OT of
bone joined by a small elast ic bridge o\ er ! he
nose. However- it may be, the discovery of

spectacles, or, as it may be called, of the
simple microscope^ may be equally divided

between Roger Bacon and Salvino degli

Armati, leaving especially to the latter the

Fig. ^.-Descartes' simple inVentio11 °* Spectacles,
microscope with reflector. The earliest know n illustration Of ■

simple microscope is given by Descartes in
his 'Dioptrique' in 1637: fig. \Y1 reproduces it. It is practically
identical with one devised bvLieberkuhn a century after and shown

' GALILEO'S ' AND CAMPANI'S MICROSCOPES

I29

on p. 138. 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 impracticable

and could never have existed
save as a suggestion. But he
appears to have been the first
to publish figures and descrip-
tions for grinding and polishing
lenses.

In the Museo di Fisica there

Fig. 94. — Campani's microscope
(? 1686).

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 by
Galileo. They are shown in
fig. 93, 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 L642 ; and there is a specially interesting compound

K

Fig. 93.— Galileo's microscopes.

130 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

microscope, by Giuseppe Campani, which was published first in 1 6S6,
which is presented in fig. 94 ; its close similarity to 1 Galileo micro-
scopes' is plainly apparent, making it still more improbable that
these could be given a date prior to 1642.

In a journal of the travels of OVI. de Monconys, published in
1665, there is a description of his microscope which is of much
interest. He states that the distance from the object to the first
lens is one inch and a half ; the focus of the first lens is one inch j
the distance from the first lens to the second is fifteen
inches ; the focus of the second lens, one inch and a half ;
distance from the second to the third, one inch and
eight lines ; the focus of the third lens, one inch and
eight lines ; and the distance from the eye to the third
lens, eight lines.

This would form the data of a practical com-
pound microscope with a field lens ; and as Mon
conys had this instrument made in 16 GO by the
'son-in-law of Viselius,' it becomes probable in it
very high degree that to him must be attributed
the earliest device of a microscope with <> ji> Id-
lens.

In 1665 Hooke published his 'Micro-
graphia,' giving an account and a figure of
his compound microscope. He adopted
the field-lens employed by Monconv- and
gives details as to the mode and object

Ftg 95.— Hookc's compound microscope | L665)b

DIVINI'S COMPOUND MICROSCOPE

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.'

Eig. 95 is a reproduction of the ori-
ginal drawing, and the general design ap-
pears 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) was
a small ball fitted into a kind of socket F,
made in the side of the brass ring G,
through which the small end of the tube
was screwed, by means of which contri-
vance I could place and fix the tube in
whatsoever posture I desired (which for
many observations was exceedingly neces •
sary), and adjusted it most exactly to any
object.'

It need hardly be remarked that, useful
as the ball-and-socket joint is for many
purposes in microscopy, it is not advan-
tageously 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 1668 a description was published
in the 'Griornale dei Letterati' of a com-
pound microscope by Eustachio Divini,
which Eabri had previously commended.
It was stated to be about 16£- inches
high, and adjustable to four different Fig. 96.— Dirini's compound
lengths by draw-tubes, giving a range of microscope.

it

not hesitate to refer its origin to Divini.

I32 THE HISTORY AND EVOLUTION- OF THE MICROSCOPE

magnification from 41 to 148 diameters, Instead of the usual bi-
convex eye-lens, two plano-convex lenses were applied with then-
convex surfaces in contact, by which lie claimed to obtain a much
flatter field Mr Mayall found in the Mnseo Copermcano at Rome
a microscope answering so closely to this description that ho does

He made the sketch of
■ it given in liix. 96.
But the optical con-
struction had been
tampered with and
could not be esti-
mated.

Che'rubin d'Orleans
published, in 1671, a
treatise containing a
design for a micro-
scope, of which tig.
97 is an illustration.
The scrolls were of
ebony, firmly at-
tached to the base
and to the collar
encircling the fixed
central portion of t he

body-tube. An ex-
terior sliding tube
carried the eve piece

above on the fixed

tube, and a similar
sliding tube carried

t he object - lens below,

these sliding tubes

serving to focus the
image and regulate
( w it hineertain limits)

t he magnificat Loo.

lie also suggested a

screw arrangement to
he applied beneath

the stage for focus
sing. I le de\ ised, or
recoin nded, sevo

pal combinationi of

lenses for t he opt ieal
part of the micro
scope, and refers to combinations of three or four separate lenses,
by which objects could be seen erect, which lie considered 1 much to

be preferred.'

^ He also invented a binocular form of microscope and published
it in his work, ' La Vision Parfaite,' in 1077. It consisted of two
compound microscopes joined together in one etting, so as to he

Fig. 97. — Che'rubin d'Orlcans' compound microscope

EAKLY BINOCULAR MICROSCOPE

133

applicable to both eyes at once; a segment of each obiect-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 at the common focus. Mechanism was provided for
regulating the width of the axes to correspond with the observer's
eyes.

Fig. 98, showing the optical construction, is copied from the
original diagram (' La Vision Parfaite,' tab. i. fig. 2, p. 80).

A drawing of this binocular, as known to Zahn, was given in
the first edition of his ' Oculus Artificialis ' in 1685 (Fundamen III.
p. 233), and is reproduced in fig. 99.

In 1672 Sir Isaac Newton communicated to' the Royal Society

I34 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

a note and diagram for a reflecting microscope : we have, however,
no evidence that it was ever constructed. But in lb , 3 Leeuwenhoek
began to send to the Royal Society his microscopical discoveries.
Nothing was known of the construction of his instruments, except
that they were simple microscopes, even down to so late a period as
1709 We know, however, that his microscopes were mechanically
rouo-h, and that opticallv they consisted of simple bi convex lenses,
with worked surfaces mounted between two plates of thin metal
with minute apertures through which the object were directly seen.
At his death Leeuwenhoek bequeathed a cabinet of twenty-six
of his microscopes to the Royal Society: unhappily, they have
mysteriously disappeared. But Mr. Mayall was enabled to Hgure

one lodged in the museum
of the Utrecht University,
which is given in tigs. 100
and 101 in full size. It is
seen on both sides. The lens
is seen in the upper third of
the plate, it baa ■ .[-inch
t'oais. The object is held
in front of the lens, on the

point of a short rod, with
screw arrangements for ad-
justing the object under the
Lens.

Many modifications of
this and the preceding in-
struments arc found with
some early English forms,
but no important construc-
tive or optical modification
immediately presents itself.
Hut some ingenious arrange-
incuts are found in the
simple microscopes de\ iscd
DJ Mlissclienbroek in the
ea rlj \ ea rs < >\ the eighteenth
century.

Grindl figured a microscope in his ' Micrographia Nova 1 in 1687,
in which optical modifications arise. Divini bad, as was staled,
combined two plano-convex lei ises, with (heir convex surfaces facing,
to form an eye-piece: this idea was carried further in 1668 by a
London optician, who used two pairs of these Lenses; Grind! did this
also, but in addition he used two similar (hut smaller) lenses in the
same manner as an objective. The form of the mioroscope itself
was copied from that of Cherubin d'Orlean (fig. (.'7), hut, wm
modified by the application of an external screw.

In 1691 Bonanni modified preceding arrangements by devising
a means of clipping the object between two plates pressed away from
the object-lens by a spiral spring, the focussing being then effected
by a 'screw barrel.'

Fig. 100. fto. 101.

Leeuwenhoek's microscope.

HARTSOEKER'S MICROSCOPE

135

This system of focussing was employed in a more practical form
by Hartsoeker in 1694 and was adopted by Wilson in 1702. It
became a very popular form for the microscope in the eighteenth
century.

We are indebted to Bonanni also for originating a horizontal
form of microscope, which is interesting and which, in a drawing of
the instrument, is shown to possess a sub-stage compound condenser
fitted with focussing arrangements for illuminating transparent
objects.

In Hartsoeker's microscope ' the lens-carrier A B, fig. 102 (on
which the cell P, containing the lens, is screwed), screws into the
body O C, Q D at O Q ; the thin brass plates E and F fit within the
body, the portions cut out allowing them to slide on the short pillars
O C and Q D, and the spiral spring pressing them towards C D ; the
object- slides, or an animalcule ca.ge GH (hinged at a b to allow the

Fig. 102. — Hartsoeker's simple microscope (1694).

lid G to fit into H, enclosing the objects between strips of talc),
slide between the plates E and F when in position, and the "screw
barrel " I K fits into the screw-socket C D and regulates the focus-
sing ; a condensing lens, N", fits on a second " screw-barrel," L M,
which is applied in the screw socket of I K. This arrangement of
the condenser is better than the plan adopted by Wilson, as it allows
the illumination to be focussecl 011 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.'

In Harris's 'Lexicon Technicum ' (1704, 2 vols, fol.), under the
word microscope, Marshall's compound microscope (fig. 103) is de-
scribed and figured. Several important innovations in microscopical
construction were here embodied. (1) A fine-adjustment screw F

Ioiix Marshall's

New Invented

DoubleMicroscope,

For Viewing the

Cine ul ATI on oF the Blood

Made S^Sold bv him af flieArchimedes &3
Golden Spectacles in Lmkaxe S<ree(.

fLa. ok, M*r

y

Fig. 103.

HERTEL'S MICROSCOPE

137

Fig. 104— Hertel's microscope (1716).

138 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

is connected with the sliding socket E, supporting the arm D, in
which the body-tube is screwed ; the focussing could thus be con-
trolled in a far more effective manner than by any system previously
applied to a large microscope. The previous systems involved the
direct movement of the body-tube either by rotating in a screw -
socket (as in Hooke's) or by sliding in a cylindrical socket (as in
Divini's and Cherubin's) ; in a few instances the object was moved
in relation to the object-lens, but all these plans were more or less
defective, especially with microscopes of large dimensions. Marshall's
system was a distinct mechanical improvement, for the object could
now be viewed during the actual process of focussing, as the
would remain steadily in the field. (2) A fork. N X, is here applied
with a thumb-screw clamp, O, on the pillar itself. (3) Hooke's ball-
and-socket joint, which was applied to the arm I, is here shifted to
the lower end of the pillar, where it would give the movements of
inclination to the whole microscope instead of to the body-tube only,
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 condenser. From the singular position of
the candle beneath the condenser, we may infer, without doubt) that
the mirror was still unknown as a microscopioal accessory in
England.

In fact, in no microscope up to this time has there been any
trace of, or reference to, a mirror ; but in L716 Hertel employed it
and introduced some other considerable modifications The general
appearance of the instrument as originally figured by I [ertel 18 given
in fig. 104. Not only have we the mirror below the stage, but also
above the stage a concave metal mirror reflecting light through 0
condenser on the object, while the stage is movable on n pillar
support. The body-tube is hinged and i^ inclined 1>\ ;i screw sector
mechanism.

In 1738 Dr. N. Lieberkulm devised, what had been employed
in principle by Descartes a century before,1 the instrumenl that has

ever since been known by his
name, and w hich is st ill of con-
siderable value to the micro*

BOOpifit. Kig. 1 0.~> is a reproduc-
tion from the earliest drawing

known of Lieberkuhn'i mioro-
scope. A A 1 11 concuN e mirror
of silver ; Prom it a form t Ik- lighl

Fig. lOS.-Lieberkiihn's'Sscope. '? f£°m " a fooU n"

the object ( '. riie mirror ih

pierced in the centre at B and the Lens, or object -lavs, is inserted
and adjusted, the eye being placed behind in the direction l> at any
point the single lens or a combination mighl require.

'4 P°cket Reflecting Microscope' was figured by Benjamin
Martin in his ' Micrographia Nova ' in 1742, having the interesting
feature of a micrometer eye-piece depending on e sore* with 1

1 See pp. 128-i).

QUEKETT'S MICROSCOPE

139

certain number of threads to the inch, and by which accurate mea-
surements could be taken. It was called a reflecting microscope
because it had a mirror fitted into its cylindrical base ; but it was,
in reality, a compound refracting form, and appears to have a good
claim to have been the original from whence the modern ' drum '
microscopes were taken.

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* applied rack-and-
pinion focussing adjust-
ments, to the compound
microscope he added in-
clining movements to
the pillar carrying the
stage and mirror, and
he furnished the stage
with rectangular move-
ments.

It was to this maker
that the late Professor
Quekett was indebted
for an early microscope,
of which he evidently to
the last thought highly,
and which he subse-
quently gave to the
Royal Microscopical So-
ciety. A drawing of this
instrument is given
fig. 106,

in

and should be
described in Quekett's
own words. He says :
' It stands about two
feet in height, and is
supported on a tripod
base, A; the central part
or stem, B, is of tri-
angular figure, having a
rack at the back, upon
which the stage, O, and
frame, D, supporting the
mirror, E, are capable
of being moved up or
down. The compound

Fig. 10G. — Martin's large universal microscope as used

by Quekett.

body, F, is three inches in diameter ; it is composed of two tubes,
the inner of which contains the eye-piece, and can be raised or de-
pressed by rack and pinion, so as to increase or diminish the mag-
nifying power. At the base of the triangular bar is a cradle joint,
G, by which the instrument can be inclined by turning the screw-
head, H [connected with an endless screw acting upon a worm-

THE HISTORY AND EVOLUTION OF THE MICROSCOPE

wheell The arm, I, supporting the compound body, is supplied With
a rack and pinion, K, by which it can be moved backwards and tor-
wards, and a joint is placed below it, upon which the body can be
turned into a horizontal position ; another bar carrying a stage and
mirror can be attached by the screw, L N, so as to convert it into a
horizontal microscope. The stage, 0, is provided with all the usual
apparatus for clamping objects, and a condenser can be applied to its
under surface ; the stage itself may be removed, the arm. 1\ sup-
porting it, turned round on the pivot C. and another stage ol
exquisite workmanship placed in its stead, the under surface of
which is shown at Q.

'This stage is strictly a micrometer one, having reel angular
movements and a fine adjustment, the movements being accom-
plished by fine-threaded screws, the milled heads of which are
graduated. The mirror, E, is a double one. and can be raised or
depressed by rack and pinion ; it is also capable of removal, and an
apparatus for holding large opaque objects such as minerals, can bo
substituted for it. The accessory instruments are very numerous
and amongst the more remarkable may he mentioned a tube, M, con-
taining a speculum, which can take the place of the tube, K, and 30
form a reflecting microscope. The apparatus for holding animalcules
or other live objects, which is represented at B, U well as a plate ol
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
this microscope are twenty-four in uumber ; the\ vary in focal
length from four inches to one-tenth of an inch ; ton of them ate
supplied with Lieberkiilms. A small arm, capable of carrj ing single
lenses, can be 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 ,',,'h bo /..th of an
inch. The performance of all the lenses is excellent, unci QO paini
appear to have been spared in their construction. There are
numerous other pieces of accessory apparatu all remarkable for the
beauty of their workmanship.' 1

Cuff designed and made a microscope, which Baker figured and
described in his * Employment for the Microscope ' in I7."».">, which
possessed several conveniences and improvements. Not the least
of these is that which gives greater delicacy to the fine ailju I
ment than is found in any preceding model. It was subsequently
further improved by the addition of a cradle joint *i the bottom of
the pillar by Adams. Cuff also designed a simple form of micrometer.

There were three designs of microscopes by George Adams, of
London, in 1746 and 1771, which 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 ' Micrographia [llustrata,' and is reproduced
m fig. 107.

, In this instrument Adams claims to have embodied a numher of
1 ' £ Practical Treatise on the Use of the Microscope, 8rd od. London, Is:,:,, hv«>,

the Variable Mickoscopb

By George A.d.san.sjy?6o,jF7&t Slreet^LONDOu^

Fig. 107

142 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

improvements on all previous constructions. He applied 'two eye-
glasses at A, a third near B, and a fourth in the conical parr between
B and C ' by which he increased 1 the field of view and of light ' ;
draw-tubes were at A and B, by which these lenses could be separated
more or less. He also arranged the object -lenses, or -buttons, a
and b, to be combined; seven 'buttons' were provided, 'also six
silver specula [' Lieberkuhns '] highly polished, each having a magni-
fier adapted to the focus of its concavity, one of which is represented
at e,' and the 'buttons' could also be used with 1 any one of these
specula ' by means of the adapter, (7.

The body-tube, ABC, with its arm, F (in which it screwed at/),
and stem attachment with the fine adjustment were clearly modified
from a design which Cuff originated. The large ivory head, I, actuated
a pinion and rack for raising or depressing the body-attachment on
the stem. The stage and mirror were adjustable on the stem. The
large ratchet-wheel controlled by the pinion-handle, S, gave the
required inclination to the stem.

Nos. 1 and 2 were ivory and glass k sliders ' for objects, to be
applied in the spring-stage No. 3 fitting at T ; the 1 hollow at K | No.
3] is to receive the glass tube Xo. 10/ No. i was a diaphragm tit ting
in the lower end of No. 3, ' to exclude some part of the lighi which
is reflected from the mirror Q.' The forceps, N«>. •">. could be placed
'in one of the small holes near the extremities of the stage, OX in the
socket, R, at the end of the chain of balls No. ('».' No. 6 was an arm
composed of a series of ball-and-socket joints similar t<> the Bystem
employed by Musschenbroek, by Joblnt , and by Lyonet, and was in-
tended to be applied at W, when tin- stage was removed. No. 7 was
a box of ivory in which discs of talc ami brasfl rings Wi re packed ;
No. 8, a hand-magnifier ; No. 9, a sliding .inn lens carrier fitting on
Z, when the instrument was required to be U8ed as a simple micro-
scope ; No. 11, a rod of wire with spiral at the end for pu king up
soft objects from bottles &c. ; and NO. 1'-', an Lvory disc, black 0D
one side and white on the other, fitting at T, to carry opaque objects.

To use the instrument as a simple microscope the body tube,
ABC, was removed from the ring, F ; the lens currier, No. 9, was
placed on Z, and a lens with reflector, K, screwed in the ring, fl ;
the ball-and-socket arm, No. G, was applied a1 Wt by the pari X,
and the object held by either of the EOrceps could DC turned and
viewed as desired. For dissections &C the Btage could be screwed
on at F, and a glass plate applied at T.

One of the best examples of this design thai Mr. Ma vail has seen
we learn has a nose-piece with a slide carrying three objectives one
of the first arrangement of ' triple nose piece,' or, inde< a, of changing
nose-piece for objectives (as distinguished from simple lens-earners)
that has been met with.

A microscope devised by Dellebarre w as made the subject <>l* ■
special report to the 'Academic des Sciences' in June 1 777, hut
there is nothing in it deserving special consideration b comparison
with contemporary or even anterior forms as bearing upon the evo-
lution of the microscope as we now know it. In fact, Up i«» tin- lime
when achromatism exerted so powerful an influence upon the form

JONES'S MICROSCOPE

H3

and construction of the instrument, there is no microscope that calls
for further consideration save one — by an English maker named
J ones — it was called J ones's ' Most Approved Compound Microscope
and Apparatus,' and although, in principle, it does not differ from
Adams's instrument, tig. 107, it yet presented differences of detail.
Its date was 1798,
and is seen in fig. 108,
which is taken from
the original figure in
Adams's ' Essays on
the Microscope.'

The base is a fold-
ing tripod, and the
stem inclines upon a
-compass-joint on the
top of the pillar.
Mr. Mayall justly re-
marks that this was
the best system de-
vised up to this date.
The arm carrying the
body-tube can be ro-
tated on the top of
the limb E, and is
also provided with a
rack and pinion D. An
extra carrier, W, is
provided for special
purposes pivoting at
S, so that objects will
remain in the optic
axis though the stao-e
be moved in arc.
There are also clips
provided for the
stage. There is a con-
denser at IT, which
slides on the stem by
the socket it. 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 ^ inch focus, which on the removal of the
lens-disc can be screwed into the nose-piece.

There were also designed some interesting forms of reflecting
microscope, to the details of which we can afford no space, their in-
fluence having been of no value in the development of the microscope
.as we know it. There was a reflecting microscope suggested by Sir

Jones's most im proved compound
micro s cop e and ap para tvs.

Fig. 108

I44 THE HISTORY AND EVOLUTION OF THE MICEOSCOPE

Isaac Newton in 1672, and one was devised on the principle of the
Gregorian Telescope by Barker in 1736 ; another of the L assegraiman
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 bght-beams is
oiven in ft" 109. It was for examining transparent objects and Was
similar to &the Cassegrainian telescope, but with an extra Ion- eye-
piece tube to permit the focussing by movement of the eve-lens.
The object was placed at IN ; 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
used as a microscope. ,

Even without a condenser there are good images attainable With

Fig. 109. — Smith's n il.' tin^ mi. r. • < , .].,■ 17;;-.

this instrument, but with the condenser it would be of course, im-
proved.

We have not followed in any detail the forms of simple micro
scopes as they presented themselves, but in I7-V> a form was made
by Cuff that can only be regarded as the precursor of the molt com
plete and perfect of our simple dissecting microscopes : it show n
in fig. 110. A disc of plane glass, C, or a concave, M, was applied,
on the stage of which dissections &C. could be made ; a mirror, [,
was fitted in a gimbal with a stem sliding in a socket in the pillar ;
the lens-carrier, F, alone, or with Ldeberkuhn, P, BOrewed 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 ol
the pillar for focussing &c. The pillar screwed 00 the lid of the
box, within which the instrument was packed \\ it h sundry accessories.

It was to the discovery of achromatism as applied to microscopic
object-glasses that we must attribute the strictly scientific Vftloi and

THE EISE OF ACHROMATISM

145

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, bo 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. 110. — Ellis's aquatic microscope (1755L

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 German by Kliigel in 1778, but no
result of these discussions of the theory of achromatism can be
discovered earlier than 1791, when Francois Beeldsnvder made an
achromatic objective which was presented byHartingto the museum
of the University of Utrecht ; but it was far from satisfactory. It

L

I46 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

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

C Chevalier tells us 1 that between 1800 and 1810 M. C harles, of
the < Institute Paris, made small achromatic lenses : but they were
too imperfect to be of real service. In 1811 Fvaunhotcr made
achromatic doublets with no great success : and m 1823-4 an achro-
matic microscope was made by the Messrs. Chevalier, with tour
doublet lenses arranged according to a plan devised by SeUique.
Their < Microscope d'Euler ' followed, and in 182 < Anna constructed a

horizontal unci oscope
on achromatic princi-
ples, 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, three,
or four plano-convex
achromat icdoubletsoi
similar foci, one above

the other, to increase
the power and aper-
ture vvas Sellii|ue in
1 823, it is nOW known
t hat this had been an-
ticipated by Marzoli

(ch. v. p. ;;o-_>). Sel
lique^s plan was car-
ried into execul ion by
t In- Mi'nsin. ( Ihei alier,

The instrument cm
bodying tliis plan is
show n in fig, 111.

In b report to t he
Academic Etoyale des
Sciences, the well
k m iwn ma t hemal ician

GFresnel says, concerning this microscope, t hat in comparing the ob
jectives with those of one of Adams's best non achromatic instruments
— that up to a magnification of two hundred times Selliquc's was
decidedly superior; but beyond that magnification there was no
superiority in the achromatic form, and In- preferred Adams'l form
for prolonged observations because it gave a larger field than
Sellique's.

The mechanism of this microscope was similar to the English
model of Jones, shown at fig. 1 0<S. The focussing was by rack and
pinion acting on the stage, the j)in ion remaining stationary and not
travelling on the rack. Two draw-tubes, A and II, were applied

Fig. 111. — Sellique's achromatic microscope I ls-J:!-l

Des MicroscojJes, Paris, lbiil), p. 86

MODEL STANDS FOR ACHROMATIC OR.JECTJVKS

within the body-tube, C, the upper one having a biconcave lens, S,
at the lower end, serving as an amplifier, which was probably the first
application of a ' Barlow lens ' to a 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 1200 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 mi-
croscope beside that of
Sellique. It was made
by Tully the optician, of
London, who at Dr.
Goring's instance had
been working at the
achromatising of the mi-
croscope. Sellique's is a
manifest modification of
one of the best forms as
made by Adams, Jones,
or Dollond. Tully, aid-
ed by Joseph J. Lister,
discovered that great ac-
curacy of workmanship
and complete steadiness
in the stand were need-
ful for achromatic mi-
croscopes, and to this
end he adopted struts,
such as were used in
telescopes, connecting
the body-tube with the
base. The instrument is
shown in fig. 112. He also provided mechanical movements to the
stage, but no fine adjustment was applied. There was a sub-stage
provided with a rotating disc of graduated diaphragms. This micro-
scope was made in all probability about the year 1826.

As compared with single lenses of equal power, from which so
much light was inevitably stopped out by the small diaphragm that

l 2

Fig. 112. — Tully's achromatic microscope.

148 THE HLSTOKY AND EVOLUTION OF THE MICROSCOPE

it was needful to use in 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
probably not long after 1824, and bearing much resemblance to that
of Sellique, is shown in fig. 113. 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 Sellique'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 show n
in it was the application of a
right-angled prism immediately
above the objective to defied
the rays through the horisonta]
body tube. The object-glasses
were composed of three lenses
Superposed, each having a focus

of three lines and ■ greatly in-
creased aperture. It had also
extra eye pieces by moans of
which the amplification could
be increased.

Meantime tin- subject <>|
achromatism was engaging the
attention of the most d is tin
guished English mathemati-
fig. H8.-0. Chevalier's achromatic jians. Sir John Sersehel, Sir

microscope. George (then Professor) Aii\.

Professor liailow, Mr. (\>d-
dington, and several others, worked with some vigour at the subject.
Also, for some years, Joseph J. Lister had been earns Hy working
experimentally and mathematically on the same subject, and he

discovered certain properties in an achromatic oombinati whic h

were of importance, although they had not been before observed.1
In 1829 a paper from Lister was received and published by the
Royal Society,2and putting the principles it laid down into practice,
Lister was enabled to obtain a combination of Leu ei capable of
transmitting a pencil of 50° with a large corrected field. This paper
and its results exerted a very powerful influence on the immediate

improvement of English achromatic object glat e and for d ■

1 Vide Objectives, Ch. V. p. 304. - Tram. Boy, Boe, toe ISM.

150 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

permanent basis of advancement for the microscope, not only in
its optical, but also indirectly in its mechanical construction and

refinements.

For convenience, at
this point we may ad-
vance a little in order to
complete our brief out-
line of the mechanical
application of achroma-
tism 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 haying ap-
plied Lister s principles
with great success, he
discovered, as we have

already pointed out in
Ch. T.,1 that by covering

the object under exami-
nat icn \ya a thin film of
glass or talc the correc-
tions were disturbed it
they had been adapted to

an uncovered object ; and
we ha \ e seen thai it was
in L837 that Etoss de-
vised a simple means of
correct ing this. I Ee wm
an indefal igable w orker
in the interests of the
Advancement « »t the me-
chanical as well as the

optieal side of the mi

crt iscope. Pig, I I I pre-
sents an early form of

one of I toss's earliest mi
Cr< >SCO] »fs, from a n c\ t a nt

example, * hich is ;i form
issued under Pritchard's

name. The stage is
act uate< I in diagonal di-
reet ions on eit her side of
t he stem, and its general

Fig. 115.— Pritchard's microscope with 'Continental
fine adjustment.

form coincides with one which Mr. Mayall assigns to Andre*

Pritchard, which fig. 115 illustrates.

•P. 20.

It has the Bame kind of Btace

A ROSS' 'JACKSON' MODEL

movement and general construction, 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 G. Oberhiiuser,
or it may have even preceded it. It is interesting to note, however,
that the sub-stage arrangements 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 Trans-
actions of that date.1

The Ross form cannot be inclined, nor can the Pritchard ; and
'the fine adjustment in the former is effected by means of a long
screw passing up the pillar and acting 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 above the
tripod.'

The stage has supports,
evidently to enable dissection
to be effected without flexure
by the weight or pressure of
the hands. Rectangular me-
chanical movements are em-
ployed acting diagonally on
either side of the stem by
rather fine screws, so that the
motions are slow ; which is
a desideratum not always
found even in our best in-
struments to-day.

It was A. Ross, however,
who successfully applied the
long-lever system of fine ad-
justment acting on the nose-
piece, and to which we shall
again refer,2 since it is the
most perfect mode of accom-
plishing fine adjustment that
we now know of, and in its most
perfect form is used in the
microscope of the highest class
(that by Messrs. Powell and
Lealand) made in the present
day.

But A. Ross at an early period worked out a ' J apkson ' form of
microscope, with the limb supporting the body-tube. He applied
' Trans. Boy. Soc. 1S29. 2 P- 101

Fig. HO. — A Ross microscope.

Fig. 117. — H. Powell's microscope (lull).

PRINCIPAL MODERN STANDS

153

a fine adjustment in this to act upon the nose-piece only, which, as
we shall subsequently see, is a very inferior method. This instru-
ment is shown in fig. 116. Ross tried various modifications of this
fine adjustment and model, but from about 1841 he worked only at
the long-lever method as applied to the nose-piece through the 1 cross
arm ' and brought it to a high state of perfection. But the full possi-
bilities of this method, as concerned its sensitiveness, were never
utilised by Ross, and were subsequently pressed into service by Powell.

In 18-11 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 the instrument at
fig. 117), but the principle was by no means an advance in the
direction of the wants of the modern microscopist. An adjustment
which gave the object a movable place between the condenser and
the objective was subversive of all ease in manipulation, and would
have made some of the finer results of modern microscopy almost
impossible. Our illumination needs, as we shall subsequently see,
centring and focussing with the accuracy of the lens itself, but by
this arrangement improved focussing of the image would involve
inevitable derangement of the illumination.

Smith and Beck also made a fine instrument which embodied
the 'Jackson' limb and fine adjustment. It is illustrated in fig.
118, being the first model made by this firm in this form, and it
has many features of interest from the point of view of our present
requirements. But after we have once secured steadiness, the
crucial points in a microscope are the quality of the fine adjustment,
and the delicacy, 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 opti-
cians. We have sought no opticians' aid ; we have carefully exa-
mined all the forms that lay any just claim to presenting an instru-
ment 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 respon-
sibility for these judgments. Haying sought for twenty years the
best that could be produced in microscopes and objectives, our judg-
ment 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
estimate of the value of such an instrument, whether the form be
illustrated in these pages or found in the catalogues of the makers.

With this object before us we shall facilitate its attainment by
at once considering what are the essentials of a good microscope.

FlG. 11H. — Smitli iiiid Heck's micros; ope.

THE 'BODY' OF THE MICKOSCOpE

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 micrometry1 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 have
a high centre of gravity.

Now it is possible to secure steadiness by (1) weight or (2)
design. The Continental method has invariably been weight. The
pillar of the instrument is fixed in 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.

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 draw-tube. In a first-class instrument this latter should always,
be provided with a rack-and-pinion motion, and should have a scale
of from two to three inches, divided into tenths or millimetres. This
enables the operator the more accurately to adjust apochromatic objec-
tives so sensitive, for their best action, to accurate adjustment of tube-
length. In fact, it is always important to remember that objectives
are corrected for a special tube-length ; that is to say, for the forma-
tion of the image at a certain definite distance.

There are, however, two kinds of tube-length, (1) an optical and
(2) a mechanical.

The optical tube-length is measured from the posterior principal
focus of the objective to the anterior principal focus of the eye-piece.

The mechanical tube-length should be measured from the top of
the tube into which the eye-piece fits, and upon which the bearings-
of the eye-piece rest to the end of the nose-piece into which the
objective is screwed.

Unfortunately different makers estimate tube-length differently,,
and take different points from which to make their measurements.
Looking at the matter broadly, there are two estimates for tube-
length in practical use : these are the English and the Continental.

What was formerly known as the English standard tube had an

. 1 Chapter IV.

156 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

optical length for high and moderate power objectives of Um inches ;
with low powers, however, it was less. The mechanical tube-length
was 8 J inches.

Professor Abbe, in constructing his apochromatic objectives for
the English body, has taken the mechanical tube-length at 9-8
inches = 250 mm. ; and the optical tube-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 k draw-
tube ' becomes of great practical value.

The tube-length of the Continental mechanical tube is 6*3 inches
= 160 mm., and the optical tube-length is 7'08 inches = I SO mm.

The question has been asked, 'Which is the Letter of these two
differing tube-lengths ? ' So far as the image in the instrument is
concerned, there is not much difference. It is of little importance
whether the initial magnifying power of an objective be increased
by a slightly lower eye-piece used at a longer distance or a slightly
deeper (higher) eye-piece at a shorter distance. Bui it is of practical
importance to note that a 'small diffi r> nee of tufa -h ngth products a
greater effect on adjustment with a short body than with a long otta
The principal difference, however, between the long and the short
body as affording a datum for their respective values i-> thai when
a short body is used by a person having normal accommodation of
sight, the stage of the microscope cannot be Been unless the head is
removed from the eye-piece, whereas with the long body the eye
need not be taken from the eye-piece a' all. as the stage can he seen
with the unused eye.

« III. Arrangements for focussing sta ud next in order of import
ance. Every microscope of the first class is provided with two
arrangements for focussing, one a eoarst adjuBtni rtf, actum rapidly,
and the other a fine adjustment, which should act with great delicacy
and precision. A good 'coarse adjustment ' or primary movable
part of the instrument is of great importance. The first requisite LI
that the body or movable part should mme easily, smoothly, but
without 'shake' in the groove or slot or whatever else it slides.
We have found in practice that a bar shaped like a truncated prism
sliding in a suitable groove acts best and longest. But a bar planed
true and placed in a groove ploughed to suit it is not enough. The
inevitable friction determines wear, and this brings with it a fatal
'shake.' All such grooves, which are usually V shaped, should be
cut and sprung on one side, so that by ' tightening up' the V's l»y
means of screws the bar or limb is again firmly gripped. Further,
the bar should not 'bear' for its whole length along the groove, hut,
only on points at either end an< I in the middle Powell introduced
these prime essentials to a good 'coarse adjustment' half ;> oentury
ago; yet what thousands of instruments in which these principles
have not been applied have been, by shear friction wear, soon
changed into useless brass since then ! But instruments made by
this firm are as good after thirty years' use as I hey were v. hen new.

Frequently bad workmanship is concealed by the free employment
ot what is known as 'optician's grease' and an over tightening o! the

FOCUSSIN( J A URAXGEM EVES

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 oli' the ' optician's
grease ' with petroleum from both bar and groove, oil with wateh-
maker's oil, and replace the bar in the groove, and before refixing
the pinion see if it slides smoothly and without lateral shake.

What has been said about the ' springing ' of the bar in this special
instance applies equally to all moving parts, in stage and sub-stage
movements, and wherever constant friction is incurred ; equally
applicable, too, is the lubricant we suggest. An instrument left
unused in its native ' grease ' for twelve months becomes so immobile
in most of its parts by the hardening of its ' normal ' lubricant that
motion becomes a peril to its future if persisted in in that condition.

If a 'coarse adjustment' be what it should be, all lower powers
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.

Rack-and-pinion work for microscopes has been greatly improved
by the adoption of the suggestion of Mr. John Mayall, jun., to em-

SWIFT

Fig. 119. — ' Stepped ' diagonal rack-
work for coarse adjustment.

^§1

Fig. 120. — Less complex forms.

ploy a ' stepped ' diagonal rack -work, which greatly increases the
smoothness of the motion. Fig. 119 shows the arrangement as first
applied with three racks, the teeth of each part being set out of line
to the extent of one-third their pitch, and the spiral pinions being
fitted to correspond with the racks. The effect is similar to what
would be obtained by pitching the teeth of a single rack three times
as finely, but at the same time retaining the strength due to the
coarser rack.

On account of difficulty in the workmanship the racks were sub-
sequently reduced to two, as in fig. 120, which still retains considerable
advantage over the ordinary form.

There is a defect in either microscope or microscopist if the
* fine adjustment ' is resorted to before the object is focussed into
clear view, even with the highest powers.

The Fine Adjustment. — This part of the modem microscope
possesses an importance not easily exaggerated, and deficiency or
bad principle in the construction of this makes not only inferior,

158 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

but for critical purposes absolutely useless what are otherwise
instruments of excellent workmanship and real value.

There arc two kinds of fine adjustment usually employed : —
i. Those which simply move the nose-piece which receives the

■objective. ,

' ii. Those which move the whole body, or the whole body including

the coarse adjustment.

Every construction of the second class has proved impracticable,
and even pernicious. It inevitably breaks down just as the purchaser,
by practice, begins to realise the value of perfect action. With a
large experience of stands of every class, we are obliged to say that
generally with one or two years of icork it loses whatever value it
•at first possessed.

To this broad statement there are possibly two exceptions, inven-
tions still sub judice, viz. Swift's side lever and Campbell's differential
screw, to which we shall subsequently refer.

It is, however, upon this model, with all its radical and glaring
imperfections, that the majority of Continental microscopes are built.

A screw of an extremely fine thread, and therefore of extremely
shallow incision — a micrometer screw in fact /ms to hear the ux aro/
lifting and lowering the entire weight of the body, with its coarse ad-
justment, lenses, and so forth ; while the sole obji ct of the adjustment
should be to give a delicate, almost imperceptible, motion to the
object-glass cdone. It needs no great experience bo foresee tin4 inevi-
table result ; the screw loses its power to art, and something incom
j)arably worse than a tolerable coarse adjustment is left in its place.

Yet it is the Continental model that has become the darling of
English laboratories, and that still receives the appreciation of pro
fessors and their students. True they answer in the main the
]3urposes sought — the exigencies of a limited course of practical in
struction. But how many of those who receive it are the medical
men of the future, and to whom a microscope not of necessity a
costly one — of the right construction would be of increasing value
through a lifetime 1

Almost any instrument, however inferior, could he employed
successfully with a J-inch objective <»t' ' low angle ' (to give it what
has been called 'the needful penetration ' for histological subjects !)

to obtain an image corresponding to a figure in a text I k of, Bay,

a Malpighian corpuscle, or a sect ion of kidney, brain, or spinal cord.
The quality of a fine adjustment is never tested l>y these means,
for, 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 book , a t reat ise,
or a course of lectures; without doubt it is a subsidiary purpOM ;
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 potentiality
(without costliness) in the mechanical and optical character of the
microscopes commended and approved.

A low-priced student's microscope of good workmanship and

IMPERFECT MODERN MODELS

1 59

perfect design could easily be devised if the demand for it arose.
Indeed, during the past few years a certain class of students' micro-
scopes have been improved greatly ; this has been a concomitant of
the science of bacteriology, which has compelled the use of the sub-
stage condenser. We have said enough of the value of this instru-
ment in a succeeding chapter, but until recent years histologists
did not use it because it was not used in Germany or with German
instruments ! Its present use, nevertheless, has had the effect
of improving the definition obtained by the objectives used by students
generally. Some who perceive this, endeavour to attribute it to the
improvement effected
in modern objectives,
but this is not the
case ; the objectives
in many cases are not
even new, and until
the introduction of the
Jena glass 1 the ordi-
nary students' objec-
tives wrere not really
so good as the English
objectives of thirty-
live years ago. But it
could easily be shown
that one of these early
objectives, used as it
always was with a con-
denser, 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 de-
formed by the adop-
tion of this radical

during the last ten Fig. 121— Koss-Jackson model,

years it has been ap-
plied to some of the most imposing and expensive instruments made
in England and America on what is known as the ' J ackson ' 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. 121, where it will be seen that the 'limb,' which is swung be-
tween the pillars, and which carries the body-tubes and the fine

1 Vide Chapter I.

l6o THE HISTORY AND EVOLUTION OF THE MICROSCOPE

adjustment, is in one solid piece. If nothing were sacrificed this
would be a boon. Formerly, this model was supplied with a fine
adjustment which only moved the nose-piece, but on a principle
which we shall see was wrong, and from its imperfections it was
abandoned, and the solid Jackson arm was cut, and the whole body
and its coarse adjustment was pivoted on the lever of the tine ad-
justment. Thus its normal virtue (a solid limb) was sacrificed, and
'a 'fine adjustment,' doomed to failure, was giver, to it.

A complex roller, a wedge, and a differential screw have in turn

been since employed to re-
deem this instrument from
the failure that had over-
taken it. Partially, or com-
pletely, each has failed. The
differential screw certainly
comes theoretic-ally nearest
to success with this form of
instrument. Hut at the out-
set this is tin* case Only where
it wholly abandons the lifting

and lowering of the body-tube

<fcc, by tli** at-l i«>n of a Mine
adjustment, ' and its motion is

only brought into operation
upon the e<juivalen1 oxe nose-

piece. The form specially
adopted tor this instrument
was devised l»y lM\ Hugo

Schroeder, and is a remark*

able arrangement, worthy of
being understood. 1 1 is illus
hated in figs. 122 and 128*

The nose piece, A, is at -
taehed to a tube which is
fitted to slide accurately in
adjustable bearings in the
body I Elbe, I ». The nose-
piece tube has :t short pro-
ject ing arm, < hy means of
whieh it is pressed upwards
by a strong spiral spring mounted in a cylindrical box, L, outside the
lower end of the body-tube. The arm, C, is moved against the Bpring
by the differential-screw mechanism (with milled head, which is
gimballed on a bracket, E, attached to the upper pari of the bod} tube.

The differential-screw mechanism consists <•! a steel rod, V (con
nected with D), which has two screw threads at the lower end, one
working in a thread cut in the end of the inner tube, G, and the
other in the block, H, which is soldered within the sheath, J.
When the milled head is turned to the left, the block and with H
the sheath — moves downwards, while the rod itself, carrviufl the
block and sheath, moves upwards. As the SCreWE are cut retp*C

Fig. 122.

Fig. L28.

Schroeder's fine adjustment for the Jai b on
model.

FIXE ADJUSTMENTS

tively to forty-five and fifty-two threads to the inch, the resultant
motion is equivalent to the difference between the motion of the two
screws — that is, to the motion of a screw of nearly 33o threads to
the inch.

The end of the sheath is tipped with a small sphere, K, of "
polished steel, while the projecting arm of the nose-piece tube,
against which the end works, has a corresponding concave bed of
polished agate.

The great fault in this design is in its mod? of adoption of what
is otherwise the highly valuable principle of the ' differential screw.'
As a fine adjustment it undoubtedly consists of too many parts or
separate pieces. It is a kind of pencil-case which extends and con-
tracts by means of a differential screw acting on the nose-piece.
Then its mode of attachment to the body makes vibration inevi-
table ; the micrometer screw is in an extremely bad position, giving
the utmost leverage for vibratory action in using it, and at the
same time, on account of its height from the table, must be worked
by the arm entirely without rest. Complexity of parts and infelicity
of arrangement make this device not only very costly, but by no
means desirable.

We may now direct our attention to the former of the two divisions
into which we have separated the various kinds of fine adjustment, viz.
that in which the nose-piece only is controlled by the adjustment screw.

An early form of this was employed by Andrew Ross. It was
applied to a microscope having a bar movement, the general form of
which — so far as the bar and the application of the fine adjustment
are concerned — is seen in fig. 131. It consisted of a lever of the second
order inserted ivithin the bar, and actuated by a micrometer screw
with a milled head at one end, the fulcrum being at the other, and
the nose-piece between them. This served admirably in the days of
low-angled objectives ; but there were two faults belonging to it :
one was that the tube of the nose-piece had not a sufficient 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 adopted by Powell. His instrument
also had a bar movement; but the bar being of relatively great
length, he employed a lever of the first order, the micrometer-screw
being at one end, the nose-piece at the other, and the fulcrum between
them. The ratio of the arms of the lever Avas 2:1; consequently,
as the screw has fifty threads to the inch, a complete revolution of
the screw causes a movement of the objective of T-Joth of an inch.
The position of the screw is immediately behind the pivot on which
the bar turns, and this precludes the possibility of the impartation
of vibration to the body ; and, as the nose-piece tube is very long,
and only bears on three points at either end, this adjustment is the
steadiest, the smoothest, and the most reliable for all objectives of
any of the several devices wThich have come before us during the last
twenty years. In fact this fine adjustment has held an unrivalled
position for the past forty years (fig. 129).

The fine adjustment that was employed as its rival on the earlier

M

1 62 THE HISTOEY AND EVOLUTION OF THE MICROSCOPE

forms of the Jackson model was known as the slwrt-side lever and
it was sometimes employed in the commoner bar-movement micro-
scopes. Its position and character will be seen on the right-hand
side of the body of the Beck-Jackson model tig. 118. In the light
of what we now 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
steadiness of movement does not belong to it. It is only that it
was concurrent with the belief in 'low angles.' and consequent -pene-
tration,' in objectives (with which no critical work could be done),
that it is possible to account for the toleration for so long in num-
bers of English microscopes of this wholly inefficient adjustment.

Swift's r> rtical side lever is one
of the new forms of tine adjust-
ment worthy of careful trial ; it
has in it elements of great merit.
It can, however, only be applied to
the Jackson model, and promises to
redeem that instrument from what

must otherwise have proved its
extinction as a tirst -class micro-

scope.

Th? fj rst form of (his adjust'
merit was sound iii principle and
ingenious in construction. It is
difficult bo know why the in\entor

and patentee lias abandoned it

for aitot her, t lit- \ ;i iuc of w hich

as a modification lias vet to be
est ablished.

The early form employed by

Swift avoided what had lieen a,

sheer necessity of all successful
line adjust ment b of t his t \ pe, \ i/..
the accuracy ami perfection of the

titt in-.;' of t In- nose piece t uhe. This

was done, as Bhown in ftg, 124, by

attaching a Vertical prism shaped
bar, A, to the nose piece, and slid

ing this in V grooves in a bos at
t he back <>t' the body. A horizontal
micrometer screw with a milled
head, K, acts on a vertical bent
lexer, D, on which a stud, E, fixed
to the prism bar bean.

There is also an adjust tnent tor

Fig. 124.— Swift's patent fine adjustment, tightening up the prism bar in the
£, ? V-grooves, Bli. Side shake and

loss of time' are impossible with this form of adjustment ; while the
power to 'tighten up' by means of the capstan headed screw a

FINE ADJUSTMENTS

enables wear and tear to be compensated. It is obvious that tin-
slowness of the motion is here controlled by three factors : (1) the
length of the lever, D; (2) the distance of the lifting-stud, E, from
the pivot or fulcrum ; and the pitch of the screw-thread on F.

Manifestly, where a side-lever tine adjustment such as this is
employed it should be placed on the left-hand side of the operator :
we can readily focus with the left hand, and leave the rig] it hand
free for moving the slip and effecting other adjustments. Anibi-

adjustment screw to the left hand.

dexterity is -not at present a common gift, and to have the right
hand free is important. This was pointed out by Mr. Nelson when
this fine adjustment was first introduced, and he had a student's
microscope constructed with the micrometer milled head on the
left side, as in fig. 12o. It is manifest, however, that it would
greatly improve this adjustment if the screw-pinion were carried
right through and a milled head placed on both the right and the
left sides of the body.

51 2

1 64 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

On what principle the makers have returned to the former plan
of niacin- the head on the right hand of the operator is not easily
explained ; but this is the case, and it involves an awkwardness and
inelegance in the manipulation of these instruments greatly to be

regretted

irnTiWuiiiiiiiiiiiiiiiiiiiiiiTi

m

The later form of this fine adjustment consists m reverting to
the plan— which we have seen such strong reason to condemn— of
throwing upon the fine-adjustment screw the movement of the entire
body. A Jackson limb fitted with this movement has two slides-
one for coarse adjustment, which moves the entire body, including
the fine-adjustment box; in front of this is a second box or ex-
cavation for the fine-adjustment movement of the body only. The
mechanism is shown in fig. 126 ; it is precisely similar to that in
ficr. 124, with the exception that the stud on which the lever bears
is°fixed to the body-slide instead of to the nose-piece-slide.

By this very simple mechanism the fine adjustment is applied on
the front of the coarse adjustment, and acts on the whole body-tube
and not merely on the nose-piece.

There remains one other form of this adjustment to be con-
sidered ; it is a form of differential sen w devised by the Rev. J.
Campbell, of Fetlar, Shetland. Its object is to supplant the direct-
action screw, where the form of the microscope may appear to make
that a necessity. This has been the case with the Continental
model. It was applied by its inventor to a microscope made

by himself, and was brought before the Quekett
Club by Mr. E. M . Nelson.

It is very simple, and IB made by cutting
two threads in the micrometer screw. Fig, 127
Avill illustrate the exact method. I> 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. Bis the fixed socket forming part of the limb
of the microscope, ami II is the travelling socket
connected with the support of the body-tube.
The revolution of l> causes the screw thread S
to move Up and down in \\ at the rate of twenty

turns to the inch, whilst thescrevi thread T causes
the travelling socket II bo move in the reverse
direction at the rat«- of" twents li\e turns to the
inch. The combined effect, I herefore, of t urning J>
twenty revolutions isto raise or lower T and with
it the body tube ith of an inch, or ^...th of an inch for each revolu
tion. The spiralspring below H keeps the hearings in do e contact.

Of course any desired speed can be attained by profier combine
tion of the threads : thus 32 and :>U would give
for each revolution, and 31 and 30 would give

This screw has provided for the Continental model w ha t Swift's
vertical lever has done for the Jackson model j Mr. Baker, of
Holborn, has adopted it and w ith very satisfactory re nil ; for it
has passed through that most crucial of tests for a fine adjustment.

Fig. 1-27.— Campbell's
differential screw
fine adjustment.

, ' ,,t h of an inch
t h of an inch.

THE STAGE OF THE MICROSCOPE

its employment in photo-micrography, with excellent results ; and
Ave hope that it may become the general line adjustment for this
form of microscope in place of the old form of direct-acting screw.

From the foregoing we learn that there are three types of micro-
scope models for which a suitable fine adjustment has been found.

i. The bar movement model, for which Powell's first order of
lever is the perfect method.

ii. The Jackson model, for which Swift's vertical side lever is
the best form known.

iii. The Continental model, for which Campbell's differentia]
screw is the most smooth and delicate device yet suggested.

IV. The stage of the microscope will next call for considera-
tion. What is known as a mechanical stage must be a part of every
tirst-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 effected by rack and 'pinion and screw. The
employment of levers, cams, and that class of stage-gear is in practice,
for critical purposes, a mere mechanical mockery. Better trust to
and educate the fingers to move the object than be beguiled by any
such practically tormenting delusions. They are simply impossible
as accompaniments of a first-class microscope.

The principle upon which alone a perfect mechanical stage can
be constructed, so as to work smoothly without ; loss of time,' and
endure constant use without failure, must be the employment of
prism-shaped plates sliding in sprung V-shaped grooves, and bearing
only on four points.

We may test the mechanical quality of the movements of a stage,
as in the case of the coarse adjustment, by removing the parts, denn-
ing them, and replacing them, when they should work smoothly and
without shake. Where the sliding parts are tightened into easily
titting and merely ploughed grooves by pressing the pinion into the
rack, the desirable result of smooth working and instant responsive-
ness of sliding plates to milled heads will not present itself.

But besides the perfect action of the sliding parts, a perfect
mechanical stage should have equal speed of motion vertically and
horizontally. A common fault is that the speed of 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, and the rack it works has twenty-four teeth to the inch, then
three turns of the milled head (and pinion) would cause one inch of
movement to the stage. In order, therefore, to get the same rate of
movement in the lateral motion the screw should be so pitched as
only to move the stage through an inch with three revolutions of the
milled head.

It is most desirable that the pinions should be fixed, not movable
with the movements of the stage, and the milled le ads carrying the
respective parts should be as near to each other as possible. The best
form is that of Tun-ell's, where one (a screw) is hollow, and the other
(a pinion) passes through it ; this permits both to be turned at the

1 66 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

2.
3.

same time with one hand, giving a diagonal motion, as well as the
separate rectangular ones, and gives great facility for instantly pro-
ducing any motion required without removing the hand from its
position ; a most desirable attribute of a stage when the rapid move-
ments 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 form of mechanical stage was devised by Mr. Tolles, and
has been adopted in this country. We regret to say that, for work-
ing purposes, it is a most undesirable form : flexure is inevitable and
•steadiness is impossible. Its character will be understood from
fig. 128.

It has three principal errors, viz. :
1. Plates so thin that they lack rigidity.

The upper plate is only supported on one side.
The Turrell milled heads are placed vertically on the top of the

stage, a position in
which their value is
reduced to its lowest
in actual work.

We have pointed
out in Chapter IV.
that the stage plates
' >f a mechanical stage
should be suitably
graduated to Jtun-
dn dt hs of <t n inch as
n \ftitdi r,' and the
principle on which
they should be con-
structed and cm
I »lo\ ed is given under

t hat head in detail.
( >n the upper stags

jilntr t here should be
a ledge for ths slip to
rest upon and a st<>]>
at ths left liiititl side
hi yond wh ieh if can -
not be pushed, Thil

should be removable-,

Fig. 128. — The Tolles mechanical stage adopted
by Boss.

but capable of being replaced with absolute precision as <<> position.

The aperture in the stage should always be large, at least two
inches in diameter. There ought always to be space enough above
the ordinary slip when it is in position to permit of the easy inset*
tion of the index finger, for by its proper use, focussing with the
highest powers may be greatly facilitated. The object il to raise or
lower the slip, as the objective approaches the object, so as to dil
cover how nearly it may be to contact with the front lens of B high
power in approaching focus. The focal distance should always be
felt and not sought with the eye.

QUALITIES NEEDFUI, IN A STAGE

167

Let it be supposed that we are using a dry object-glass with a full
aperture, and consequently short working distance. With the riirhl
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 linger 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 off 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 Jth of an inch, but after this there must be a cautious but
steady advance. The body may be racked down until by gentle
movement the slip is found to touch the front of the objective ;
then proceed cautiously by delicately lifting the slip from time to
time, the ability to do so proving 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
focus an object in the field with a ^L-inch objective in ten or
twelve seconds.

If a perfect mechanical stage cannot be obtained, take no middle
course, have a firm, 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
mechanical stage, and for the same reason.

Mr. Nelson suggested a stage of large size, which should have a
1£ or 1| inch aperture bored in it, and then have the intervening
brass between it and the front taken away, so that the stage
assumes a horse-shoe form. This is thoroughly efficient, and the
principle is seen in fig. 125.

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 also
adopted the mechanical stage ; the form adopted by Zeiss is figured
in the accompanying illustration (fig. 128 a).

It must, however, be noted that the usual Continental model
adopts a small stage with a %-ineh 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, 1 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

1 68 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

made instruments the pinion engages the rack so lightly that this
rapid motion may easily be given to it. In others the pinion can
be disengaged and rapid 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.

o

o

Fig. 12H a. — Zeiss's mechanical stage.

and is found in some high-class instruments ; hut t his is not needful,
for all that is really required is to rotate an objed without losing
?t. In fact exact centring would have to be readjusted for every
separate objective if it were needed. Bui any Blight departure
from the axial centre can be much more readily met by bringing
the object into centre by the mechanics I stage.

There are three movements in every microscope which should be
graduated', these are (1) the milled head of the fine-adjustment
screw ; (2) the extension draw-tube carrying bhe eye-piece; and (3)

THE SUB-STA&E — ITS REQUIREMENTS

169

the rotation of the stage. Divided arcs are imposing, and to tin*
multitude look ' scientific ' ; but in practice they are superfluous
in the most complete instrument beyond those indicated.

There is a simple form of super-stage now employed 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 bearings
shall be glass and the friction reduced to a minimum.

Such a super-stage can be made to work with remarkable smooth-
ness, *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 convenient
points of hold-fast for the hands, and consequently is more manage-
able. Against its employment is the fact : 1st, that the slide is
clipped into a rigid position ; and 2ndly, that the aperture is too
small to admit of the employment of the ringer in moving the slide
to assist in rapid focussing. But these are defects which might
certainly be overcome.

Y. The sub-stag'e is scarcely second in importance in a first-
class microscope to the stage itself. It is intended to receive and
enable us to use in the most efficient manner the optical and other
apparatus employed to illuminate the objects suitably with the
various powers found needful. Upon this much of the finest
critical work with the modern microscope depends.

To accomplish this a good sub-stage must have rectangular
movements, and a rack-and-pinion focussing adjustment.

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 remain 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 be in right lines :
motion in arcs whose tangents intersect at right angles are quite as
efficient. A steady, even, reliable motion that will enable a centre
to be found is all that is required.

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: Xelson 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 ;i
slot in the same way. In fact, any simple device for focussing the
condenser more slowly than the rack-work will do, pushing the con-

170 THE HISTOEY AND EVOLUTION OF THE MICROSCOPE

denser up to, or causing it to recede from, the under surface of the
slide. But no means should be employed for this end which will
imperil the absolute firmness of the sub-stage, or else more will be
lost than can be gained. The arrangement in Powell and Lealand's
sub-stage is shown in tig. 130, p. 175.

It is almost a matter of compulsion to refer here to a recent
arrangement known as a swinging sub-stage, which is, as its name
implies, a sub-stage so arranged as to be capable of being moved
laterally out of the axis in an arc which has the object on the stage
for its centre.

The sole purpose of this is to secure oblique illumination, which
practically, at the time the swinging sub-stage was devised, meant
obtaining a more 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 that this alone was taken to be 'oblique illumination/
But whatever sends oblique light through an object into the
objective is an oblique illuminator. Two condensers may have
numerical apertures of 14 and 1*5 respectively : a stop behind the
back lens in each has a narrow sector cut out. representing the con-
ditions of the so-called 'oblique illuminators'; by the former we
get an oil angle of 131° 10', by the latter a similar angle of 101° *J."> .
These sectors of the cone of light of 67° 5' and 80° 41' respec-
tively are in every sense "oblique illuminators, 1 and the one more
oblique than the other.

Whether or not it is needful or best to use such a Bector Ls
scarcely an open question; it is manifesl that by taking the stop
with its sector away from each condenser and sending in the <-<>m plete
cone of light formed by the condenser, we are still using oblique
illuminators, but the obliquity is in nil azimuths.

There can be no doubt that a large aperture in a condenser
provides the microscopist with far greater wealth of resource than an
oblique illuminator in one azimuth can ever gi\e him. A condenser
with an oil angle of 161° 23' is much more valuable than even the
semi-angle obtained by a mere section of a luminous cone. The
power to utilise the entire cone Ls ;i gain <>t I lie highest order.

It will be manifest to all that we wan1 concentration as well ai

obliquity. In the catadioptric illuminator of Mr. Stephenson, the
one defect, as in discussing it we point out, is, in our judgment, want
of concentration, due to the length of focus of its concave reflector.

Ordinary concentration depends upon the /,ou>i r <>f th> condenser.
If it is required to concentrate the lighl from the edge of the flame
of a paraffin lamp upon an Amphipleu < < r, II ',,<■',< I,,, the condenser
must be at least a Jth inch or /.th inch in power, which w ill give an
image of the flame nearly the same size as the object. The amount
of light which is concentrated upon that object will of OOUrse depend
upon the aperture of the condenser. An oblique cone of great in
tensity is here what is needed ; the illuminating cone should be
equal and conjugate to that which exists between the object and the
objective.

Now it is certain that this condition cannot be met by an 'oblique

' OJJLIQl'E ' ILLUMINATION

illuminator ' of the kind commonly understood by that name ; to gel
immersion contact, which is of course a sine qua non, we must
employ a hemispherical button — or one greater than a hemisphere —
placed in immersion contact with the under surface of the slide.
This may be illuminated by a beam from a dry combination, made
oblique by the sub-stage being swung out of the axis. Granted that
the angle is attained which can be got with a condenser of great
aperture, we manifestly obtain only a portion, and an attenuated
and small portion, of the light given in every, or at will any, azimuth
by the condenser.

Theoretically perfect illumination of an objective, for example,,
a gth 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
required to be illuminated. The cone of illumination would precisely
equal that taken up by the objective, and would be of maximum
intensity.

Now these conditions are more nearly approached by a high-class
achromatic condenser of great aperture and of homogeneous 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 done tuith 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 rather a defect in the
microscope — 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 *uh-
stage should also be provided with a raek-and-pinion rotary motion :
this 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.

VI. The mirror is also an indispensable part of a complete
microscope. In a first-class stand it should be plane and concave
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 conceive. This mirror, from its curve,
has a focus, a point in which the reflected rays all meet ; and the
mirror should not be fixed, but so mounted that it may be focussed
on the object.

The plane mirror is sometimes found to give several reflexions of
a lamp flame at one time. It is due to some unexplained molecular
condition of the glass, and is undesirable for the purpose ; but it may
be obviated or altogether overcome by rotating the mirror in its cell
until a certain point is reached where all the images will be super-

1/2 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

imposed. All mirrors should be so mounted as to admit of this
rotation,

The present Editor is greatly in favour of the employment of a
rectangular prism cut with care and precision. We get by this
means total reflexion and no double reflexions ; and he believes
that finer images can be obtained by its means than with the plane
mirror. It may be mounted in the place of the plane mirror— that
is to say, the concave mirror may be as usual in its cell— and in the
other cell, which would have received the plane mirror, the rectan-
gular prism may be mounted and be capable of rotation as the plane
mirror would have been.

It should, however, be noted that this applies only when the
light is required to be reflected at an exact right angle. It is of the
greatest service when the microscope is of necessity used in a rigidly
upright position. .

If it be used for angles other than right angles, there will be
refraction as well as reflexion ; and as the necessary decomposition
of the light into a spectrum will accompany the refraction, care must
be exercised to see that the rays emerging from the prism are at
right angles to those incident to it, and that the areas of the square
faces of the prism 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 cloud illuminator^
that is, a disc of plaster of Paris, or opal glass with a polished
surface. But a disc of finely ground glass dropped in to the diaphragm-
holder of the condenser will give a precisely similar result.

Mr. A. Michael has, however, pointed out the curious tart that an
opalescent mirror becomes an inexpensive and excellent substitute
for a polarising 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 loss
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 th«- microscope <»t' any
given maker from whose catalogue he proposes to elect, ami can
discover by comparison its incidntc, or <>//,, rn-ise " if/, the type
given here to which it corresponds.

Beginning with the highest types we place first on the list Powell
and Zealand's No. I. This instrument may claim a seniority <»\n-
all the foremost instruments, because for nearly forty years i1 has
practically remained the same. All its principal features were
brought to their present perfection nearly forty years ago, while all
other microscopes during this period have been redesigned and
materially altered over and overagain. This is nosmall commends
tion, for during that period, as the reader so well knows, the aper-
tures of objectives have been enormously enlarged, and with this
has come a great increase of focal sensibility. As a result the

THE HISTORY AND EVOLUTION OF THE MICROSCOPE

majority of the microscopes of forty years ago are absolutely useless
for the objectives of to-day, but the focussing- and stage movements
of Powell and Lealand's microscope still hold the first place.

Fig. 129 represents the instrument in its monocular form. The
foot of the stand is a tripod in one casting ; it has an extended base
•of 7 X 9 inches, forming at once the steadiest and the lightest foot
of any existing' microscope. The feet are plugged with cork, and
when the body is in a horizontal position the optic axis is (as it
should be) 10 inches from the table.

The coarse adjustment is effected by a bar, consisting of a mas-
.sive gun-metal truncated prism in form, which bears only on a
narrow part at the angles. It extends sufficiently to focus a 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 Jackson model
many different devices have been tried, the latest being the placing
of the stage pinions in a vertical position above the stage (tig. 128,
p. 166), which is an unquestionable error.

The rotation of the stage in the Powell and Lealand model is by
means of a milled head most conveniently placed, and the divided
circle is on a plate of silver.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 lex er tor the tine adjust-
ment (p. 161). 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 body may be, with great ease, entirely r< movt djrom the arm ;
this makes the use of the binocular or monocular body or of a short
or long body a matter of choice, whileit gives access for cleaning and
other purposes to the nose-piece tube, as well as for the inst i l ion
and focussing of the lens used with an apertometer,a or an analysing
prism. So also it is of service in low-power photo-micrography.

We have already referred to the stage of this instrumenl ;
but it may be briefly stated that it is large, lias complete rotation,
it has one inch of rectangular motion, being graduated to the , ,'M,th
inch for a tinder. There is the saw, speed in the vortical 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 -crew in either direc
tion, as well as a rotary movement by pinion. The coarse adjust
fluent is by rack-work, and a fine adjustment is added when desired.

1 This is now made of platinum if desired, and thus tarnisli is obviated.

2 Chapter V. p. 337.

POWELL AND LEA LAND'S BEST STAND

175

Fig. 130 illustrates this stage, showing its under side in order to
enable the fine adjustment to be seen.

The vertical and upper horizontal milled heads are centring
screws acting at right angles to each other, while the diagonal screw to
the left is the milled head, which
causes the stage to rotate, and it
will be seen that at the end of
the screw turned by A is a milled
head controlling a screw spindle
terminating in a steel cone, B.
On rotating A, B turns and with
a very slow motion forces up (or
releases, as the case may be) a
pin, C, inserted in the base plate,
E, of the sub-stage. This motion
of C carries with it the con-
denser. At right angles to, and
forming part of E at the back
an inner sliding plate works
against a spring at the upper
end between bearings F at each

side which are fixed upon the FlG 130._poweii and LeaWs sub-stage
usual racked slide, 1), 01 the sub- with fine adjustment,

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 ^ inch — the difference in radius between the smaller and
larger ends of the steel cone.

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.
The present Editor has had one of these microscopes in constant, and
often prolonged and continuous, use for over twenty years, and the
most delicate work can be done with it to-day. It is nowhere
defective, and the instrument has only once been - tightened up ' in
some parts. Even in such small details as the springing of the sliding
clip — the very best clip that can be used — the pivots of the mirror,
and the carefully sprung conditions of all cylinders intended to receive
apparatus, all are done with care and conscientiousness.

An instrument of this kind may be made to appear perfect to
the eye, but at the same time may lack some most important elements
as a finished instrument. But this is an instrument of the highest
order as such, and at the same time a very fine specimen of highly
finished brass work.

A note must 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 Huyghenian 2-inch eye-piece, having the
largest field-glass possible. The size of this field-glass depends 011
two factors.

i;6 THE HISTORY AND EVOLUTION OF THE MICKOSCOPE

1. The distance between the centres of the eyes.

2. The mechanical tube-length.

Tn order that the binocular may suit persons with 'narrow

Fig. 131.— The original Ross model.

centres' to their eyes, the distance between them should not In-
greater than 2-i- inches. The mechanical tube-length is 8| inches for

R< )SS' ORIGINAL MICRi >S< !( >PE

the standard tube. Wlien the eye-pieces were 'home' in their
places in the tubes they just touched each other, the inner sides of
the binocular tubes being cut away; so under the above conditions
a larger held than is thus obtained is simply impossible. The size
of the field-glass determines the size of the eye-piece, and that was
made to fix the diameter of the body-tube.

Very wisely these makers made the tube of the sub-stage the
same size, so as to have one gauge of tubing throughout. This
allows a Kellner or other eye-piece to be used as a condenser, thus
reducing the number of adapters.

Lately this firm have altered their sub-stage tube to a gauge of
1-^ inch, as recommended by the Royal Microscopical Society. This
involves an adapter where the sub-stage apparatus was adapted to
the old gauge, or when an eye-piece is used as a condenser; for 1},
inch is an impossible size for a binocular body-tube.

The Itoss ?nodeI, in its completest form as left by Andrew Koss
(though not now made to
so large an extent as
formerly), deserves study.

It was a bar move-
ment, with a foot formed
of a triangular plate to
which were bolted two
parallel upright plates to
carry the trunnions of
the microscope. This
base was defective in not
being sufficiently ex-,
tended to carry so large
an instrument, with its
centre of gravity so high.
This instrument is illus
trated in fig. 131. The
coarse - adjustment bar
was rectangular, which
is theoretically inferior
to a prism, but it is well
sprung and works with
great smoothness. The
fine adjustment is a lever
of the second order, with
the milled head in the
middle of the bar, which
involves tremor, and the
tube of the nose-piece is
short, making shake pos-
sible. Pig. 13_\ — Ross-Jackson model.

The stage movements
are of unequal speed, th© lateral movement being slower than the
vertical. There is no finder, and the rotation of the stage is but

partial.

The sub-stage and mirror are good.

It was a commanding

N

178 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

instrument in its day, and was of excellent workmanship and finish ;
but it was not equal to the strain of critical work with immersion objec-
tives of great aperture. Nevertheless the defects of this stand could
have been readily corrected. With a more extended base, a better
arrangement of the fine adjustment, a mechanical stage constructed
on better principles, and the rotation made complete and concen-
tric —which it was not— this would have been, even for our present
requirements, an admirable instrument.

This important firm were otherwise advised, however : and, in-
stead of correcting the errors of the instrument whose history they
had made, they designed an entirely new model in which a J ackson
limb was substituted for the bar movement. Fig. 132 illustrates this,
form of the instrument, from which it will be seen that the foot also
was changed for the worse by being cast in one piece ; the base was
not sufficiently extended, and the hinder part of the foot was too
large, so that it sometimes rocked on four points, because the hinder
part was too wide — a flat surface, in fact. A true tripod will stand
firm on an uneven table, but this form will not. It is a form fre-
quently used by various niakers now, and is known as the ' bent claw.'
It is a bad design, and may be, as it has been, easily thrown over
laterally.

The introduction of the Jackson limb brought its inevitable
troubles — notably, with the fine adjustment — to which we have fully
referred under that head. But in the Ross- J ackson model the fine-
adjustment screw was placed behind the body (as the figure shows),
which was an improvement ; still 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 Zentmayer, known subsequently as the Ross-Zentinayer
model. This was the Ross-Jackson instrument with a 'swinging
sub-stage.' This instrument is illustrated in tig. I .">.">. It will he
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.

We have already assessed the value of a swinging sub-stage, and
found that in our judgment it is at best redundant. 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 he done with a slotted
stop at the back of the condenser. This elaborate appendage is there
fore without justification. Yet in the impatience for large illumi-
nating apertures, which were not at that time prodded In/ condensers^
this phase of illumination was carried to a still greater and more
elaborate development in the production of a conn it/ric microscope.
This was a Ross-Wenham, known as the rail in I microscope.

In the early days of this instrument, when no achromatic
condenser exceeding 170° in air was to be obtained, in some very
difficult researches needing all the great advantages that come
from great aperture, the present Editor was able, with much
labour, to get results with this instrument not otherwise attain-

1 P. 169 et seq.

180 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

able but which were none the less almost counterbalanced by the
deficiency of its line adjustment. Nevertheless since the advent of
. i chromatic and apochromatic condensers with oil contact all this is
changed.

F13. 1M. Fig. ia:>.

centre, and in addition to this it has a swinging sub-stage.

The optic axis of this instrument is capable of being rotated in
three planes at right angles to one another with the object as a

This

will be seen and understood
from the illustrations given in
tigs. 134-137.

Concerning the line adjust-
ment of this as a .Jackson model
primarily we have already
written.

Another h', nihil/ form of the
first clci88 is the No. 1 of Messrs.
R. and J. lieck. The early an-
cestor of it was shown on page
154, but it has undergone im-
portant changes as it is now
presented (fig, L38). It is a
Jackson model, the foot being a
good tripod, and the trunnions
on pillars (as fig. L38 illus-
trates). It has a short lever
fine adjustment, L, acting on a
nun-able nose-piece, and placed
in front of the body so that the
body and coarse adjustment are carried by the fine-adjustment screw.
The stage has a rotation, but not complete. The stage aperture is
not so large as it should be; on a pivot attached to the limb the

Fig. 130.

BECK'S FIRST-CLASS MICROSCOPE

1 8 1

entire stage can be rotated, so as to be set at any inclination, thus
complying with the temporary need so elaborately met by the
' radial ' form ; and the angle can be recorded on the divided plate, R.
But the stage may be inverted so that an object may be placed on
its under part. The sub-stage proper has no rectangular centring-
movements, but these are supplied on a separate adapter. There
are no specialities in the character of the swinging apparatus, save
that the whole of this part of the stand can be raised or lowered on a
dovetailed fitting in the optical axis of the instrument by the lever,
Z, in order to raise or depress the centre of the arc of the swing to
keep it concentric with the object, and by that means to compensate
for various thicknesses of slip. The sub-stage apparatus is focussed
by the milled head W. The entire bar is attached to an arc, J,
working in the circular fitting, Y, and rotated by means of the milled
head X — the amount of rotation being recorded bv a graduated circle
— and the sub-stage can be swung above the stage on either side
for dark-ground illumination. The mirror fits in a tail-piece, V,
or it can be fitted on to the swinging sub-stage, U. There is an

Fig. 137.

elaborate arrangement for the rotation of the entire instrument on
its foot, which is graduated at B. It is not easy to see the practical
purpose of this, as the object is not in the axis of rotation ; but it is
provided in order that ' the microscope may be turned round with-
out being lifted from the table, and the amount of such rotation
registered/

The workmanship of these makers is of the highest order, and
many of their pieces of apparatus have great structural merit. This
their leading microscope is an imposing piece of tine work ; but the
model we have seen has defects against which the finest work-
manship and ingenuity of device strive without satisfactory result.

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 Jackson similar to Beck's. The principal difference was that
the foot was of the 'bent claw" form. We have already seen that
by their invention of the vertical lever fine adjustment (tigs. 124 and
126) Swift and Son have made possible a useful future for the Jack-
son limb ; and their model of this form, with the exception of an in-
complete stage rotation, has the stage and sub-stage movements ; the

Fig. 138.— E. and J. Bock's No. 1 stand.

Fig. 139. — Zentmayer's microscope

1 84 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

latter, however, depends for its action upon a loose ring opposed on one
side by a spring which is counteracted at will by screws, so altering
the position of the centre of the sub-stage. It is not the best form
for so important a part of the instrument.

All the movable parts of Swift's instruments are sprung on Powell
and Lealand's method, and the movements are smooth and sound.

There have been many stands devised by American opticians
during the past twelve or fourteen years, but they have been based
u | ion one or other of the great English models, and the modifications,,
whether for good or evil, have been adopted into the recent modifi-
cations of the older English types, and have been 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. The fine adjustment of this instrument was
niost defective. Tolles, again, who wholly deserves the very high
reputation he attained, made an instrument in which he mounted
the stage on a disc, as is now the case with the Beck model (fig. 138).
Near the edge of this disc the sub-stage is made to travel in a groove
carrying the condenser, or dry combination, in an arc round the
object as a centre. This was only another elaboration of the same
swinging sub-stage.

In later constructions of this form, Tolles first used the mechanical
stage actuated by two pinions vertical to the surface of the stage
and subsequently adapted by Ross (fig. 128). The fine adjustment
in this instrument had the fatal defects characteristic of its form.

Bulloch, another American maker of note, made some modifica-
tions in the Zentmayer model, but they were in the interests of the
swinging sub-stage, and, although no doubt ingenious, must pass
with this transient form of the microscope

An illustration of the leading form of Zentmaver's microscopes
is seen in fig. 139.

It will be noted that, as in the case of the Ross form of it (fig.
133) its chief characteristic — no longer, if ever, a merit— is its swing-
ing sub-stage. But this has the claim of being the first modern in
strument to respond to the cry for swinging sub-stage, and certainly
no better response has subsequently been made.

In the stage on the complete instrument is the ingenious ar-
rangement of a glass super-stage, which has been so freely adopted
in England on a certain class of instrument, and, in the absence of
a complete mechanical stage, is the only substitute to be tolerated.

But another stage was mads with this instrument, shown in fig.
140, with, however, some modifications in detail. This is not distinct
from English forms of stage of long standing.

A modification of this stand was devised by Bulloch, seen in
tig. 141. It presents no special point, save the employment of a
Gillett condenser with the diaphragm drum above the /nisrs !

A later development of this form of instrument is given by the
same maker some years later, and shown in fig. 142; but the chief
difference consists in the adoption of a stage in which the milled
heads stand upon the stage, which is the reverse of an advance.

WALE'S TRA VEKSLVi ARC Foil LLYIR

An instrument made by Bausch and Lomb, and known as their
professional microsco]w, is illustrated in fig. 143. It is on the same
general plan, but the mirror and sub-stage bars can be moved inde
penclently of each other, or simultaneously when the arm on the
mirror is placed in a recess in the sub-stage bar.

A speciality claimed for this form is a ' frictionless fine adjust-'
ment ' ; but it is one of the many which have the intolerable burden
of lifting the entire body of the instrument to secure the delicate-
focal adjustment of the object-glass ; and, although highly ingenious,
is therefore, to our mind, wholly imperfect for the end in view.

Mr. George Wale, of America, devised a plan of some merit for
certain classes of microscopes. The ' limb ' which carries the bod \
and the stage, instead of being swung by pivots — as ordinarily — on
the two lateral supports (so that the balance of the microscope is
greatly altered when it is much inclined), has a circular groove cut

Fig. 140. — Zentmayer's stage.

on either side, into which fits a circular rid<;e cast on the inner side
of each support, as shown in fig. 144. The two supports, each having
its own fore-foot, are cast separately (in iron), so as to meet to form
the hinder foot, where they are held together by a strong pin ; while
by turning the milled head on the right support the two are drawn
together by a screw, which thus regulates the pressure made by the
two ridges that work into the two grooves on the limb. When this
pressure is moderate, nothing can be more satisfactory than either
the smoothness of the inclining movement or the balancing of the
instrument in all positions j while, by a slight tightening of the
screw, it can be firmly fixed either horizontally, vertically, or at any
inclination. The ' coarse ' adjustment is made by a smooth-working
rack ; but the fine adjustment is made to carry the whole weight of

[86 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

the body and the coarse adjustment. This modification has also
been adapted by Swift and Son to some of their many instruments.

Fig. 141.— Bulloch's stand.

We are now prepared to consider second-class microscopes,

which, for practical purposes, are simply smaller forms of the first
class, with some of the elaborate work omitted. Tims the sta«j<>

o

Fig. 142.— Bulloch's new Congress stand.

188 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

rotation is usually without a rack-and-pinion movement, while stand
and stage are smaller. But in the best instruments the body-tube is

Fig. 143.— Bausch and Lomb's professional stand.

SECOND-CLASS STANDS

.the same length, and they receive the same objectives and sub-stage
iittings as No. 1.

As a model toe prefer Beck's ' small first-class ' stand to their Large
instrument. It has an excellent single pillar tripocl foot, TurrelPs
.arrangement of milled heads (like Powell and Lealand's stands) for

Fig. 144. — Mr. George Wale's instrument with new form of limb.

the mechanical movements of the stage, and rectangular movements
for the sub-stage. This is a fine instrument, and would be admirable
in every way with a more perfect line adjustment. Powell and
Lealand's second-class instrument is, in all essential particulars, buflt
upon the principle of their No. 1 stand, though less elaborate.

In instruments of the second class there is by no means so great

190 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

a variety, there being not so great a demand for these as there is for
those of the third class.

It will be useful to discuss the relative merit of the third in
contrast with the first class ; but before doing so it will be well to
examine some of the best instruments presented to the purchaser as
belonging to this class.

Third-class Microscopes. — Powell and Lealand make one having
a quality of work not second even to their large stand. It greatly
resembles their second-class instrument. It is illustrated in fig. 145.
The tube length is the same, but the stage and the foot are smaller
than in the second-class 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 way a defect. All
the movements and adjustments are otherwise as in No. 1.

As a rule, third-class microscopes are without mechanical stages ;
in this respect Powell and Lealand's is an important exception,
because it has a stage provided with the most perfect mechanism that
can be employed.

Beck's third-class microscope is shown in fig. 146. It has a good
flat tripod foot with a single pillar. The Jackson model is used, but
a peculiar fine adjustment is employed, the lever being placed below
the stage, the screw being placed immediately behind the pillar which
supports the limb, and Avhere 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 supports the body limb
and coarse adjustment. In fact, save in its fine adjustment, this
form approximates somewhat to the Continental model. The fine-
adjustment lever is rather short, but it will be found to be steadier
and slower than the direct-acting screw.

The stage is plain, without mechanical movements ; but it has a
movable glass stage over the principal stage ; to this the slip is
clipped and the whole super-stage of glass is moved with much 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 third-class microscope in its most suitable form
dates from about the time of the vertical lever fine adjustment,
patented by that firm. It was first made from the designs of Mr.
E. M. Nelson, and it presented three distinctive features : —

(1) The milled head of the fine adjustment was placed on the
left-hand side of the limb.

(2) The stage was of a horse-shoe form, the aperture being
entirely cut out to the front of the stage ; and

(3) The body-tube, which was of standard size, viz. 8| inches,
was made in two pieces which not only secured portability, but also
permitted the use of both long and short tubes.

This instrument is illustrated in fig. 125. It was also possessed
of a cheaply made and fairly good centring sub-stage, to carry
Powell and Lealand's dry achromatic combination lilted with a turn-

THIRD-CLASS STAN US

191

out rotary arm to carry stops. The sub-stage was made by adapi ing
Swift's centring nose-piece, and providing it with a rack-and-pinion
focussing arrangement, as illustrated in fig. 147. There was also a
graduated stage-plate and sliding bar, a plan devised by Mr. Lewis

Fig. 145.

Wright for a finder. The eye-pieces were provided with rings, like
Powell and Lealand's, outside the tube to govern the depth which
each should slide into the draw-tube, by which means the diaphragm
is in the same place whatever the depth of the eye-piece employed,

Fig. 11C. — Messrs. R. and J. Beck's third-class micrc scope.

A GOOD STUDENTS M ](']{< )SCOPE

193

and it was constructed to do critical work with the highest
powers.

Another form of this instrument has recently been introduced by
the firm of Chas. Baker, of Holborn, London. It arose in a sugges-
tion by Mr. Nelson, that this form
should be adapted to the Campbell
differential screw fine adjustment,
making a good quality third-class
microscope. It should be noted that
the differential screw permits of slow
action being obtained by means of
coarse threads ; it is therefore very
strong. In the ordinary Continental
form of direct-acting fine-adjustment
screw, if the motion is slow, the thread
must be fine. Hence in forms where
the fine adjustment is made to lift the
body, the differential screw is of great
value. Fig. 148 illustrates this admirable instrument. It has a true
tripod foot of the Powell and Lealand type, a plain horse-shoe stage
with a sliding bar, a condenser focussing by a spiral action, and a rack-
and-pinion coarse adjustment to the body, with draw-tube arranged
to work with apochromatic objectives made for either long or short
tube.

Fig. 147. — Centring nose-piece
used as sub-stage.

Fig. 148. — Nelson's form.

This instrument has the peculiarity that the stage is H inch
below the trunnions, which has the effect of lowering the centre of
gravity when the instrument is in a vertical or nearly vertical posi-

o

THE HISTORY AND EVOLUTION OF THE MICROSCOPE

tion. When horizontal the optic axis is 8J inches from the table.
The ends of the feet are plugged with cork.

It proved on testing that the Campbell differential screw was
equal to the most critical work, and could be used in photo-micro-
oraphy. As a result several additions were made, such as raek-and-
pinion focussing and rectangular movements to the sub-stage and a
rack- work arrangement to the draw- tube. Subsequently a larger
and heavier instrument was made, having a ^-inch more of horizontal
height. In this model the milled head of the differential screw is
placed below the arm, instead of above it, which is an improvement
for photo-micrographic purposes, 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 sometimes put to
this instrument, but those which we have seen have not given the
aperture sufficient dimensions for modern focussing.

This instrument in its complete form as devised by Baker is
shown in rig. 149. The stage has changed its form ; but if the
aperture be kept large enough this may be fully counterbalanced by
the rotation given to it, and with the Campbell screw fitted behind
the mirror for the fine adjustment of the condenser is a very attrac-
tive and useful microscope, and may be safely recommended to the
amateur and the student.

There is not sufficient rack-work to focus the 70 mm. objective of
Zeiss, but the nose-piece unscrews and the objective is held inside
the tube by an adapter.

This larger form of the instrument is placed amongst microscopes
of the third class because it is unprovided with a mechanical stage,
but a supplementary and removable mechanical stage, devised by
Mr. John Mayall and made by Baker, and Swift, and also by Zeiss,
of Jena, can easily be added, as it is in the figure (149). This acts
fairly well, and is a useful appendage for more delicate stage work.

Fourth-class Microscopes.— Tin sse should have a rack-and-pinion
coarse adjustment and a direct-acting differential-screw tine adjust-
ment, a plain stage and, if possible, a sliding tube sub-stage. Beck's
' Economic ' is of this class, as is also the ' Star ' microscope by the same
makers. The former has the flat tripod, as tig. I •">(), representing this
instrument, shows, and it may be obtained furnished with a glass
super-stage. The present Editor can speak highly of this instrument
for elementary class work, and especially with a glass stage. The
' Star' microscope is also a very remarkable instrument, sufficiently
so to justify us in departing from a rule to point out that with two
eye-pieces, two objectives — a ?y-inch and a £-inch — a, sub-stage con-
denser, with an iris diaphragm, with the whole placed in a. cabinet,
is sold for 51. 15s.

An instrument of the same class is made by Swift and Son ; it is
shown in fig. 151 — a very common model, having rack and-pinion
coarse adjustment, direct-acting screw fine adjustment, and plain
stage on a ' bent claw ' foot. When first introduced these instru-
ments were little better than the Hartnack instruments made in
England, with inclining bodies on a claw foot, instead of a solid mass
of metal. But they have been greatly improved in the past few years.

INEXPENSIVE MH 'R< )S( '( >I'Ks

'95

. Microscopes are made also like the last, without rack-and-pinion
coarse adjustment, by many makers, which might be classed as fifth-

Fig. 149. — Baker's modification of Nelson's form.

class instruments, while some are yet made without a fine adjust"
merit ; and it is a little notable that the best of this class 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
only a tine adjustment with a sliding tube for the coarse adjustment.

o 2

TORY AND EVOLUTION OF THE MICROSCOPE

Fig. 150. — Beck's economic microscope.

and is supplied with a beautifully made sliding Ledge, which wi'l]
move easily and firmly with pressure from one side only.

INEXPENSIVE MICROS!'* >I'KS

^J7

The stage is fastened to the upper side of two brackets which are
cast in one piece with the limb ; on the under side of these brackets
there is another plate which holds the sub- stage tube.

This instrument is supplied with large plane and concave mirrors ;
and considering that it constitutes a sixth class of microscope has
very much in its favour as a secondary instrument for the work-table.
Like all these makers' instruments the feet are plugged with cork

Fig. 151. — Swift's fourth-class microscope.

and we know of some of these microscopes that have been in use
for forty years, and are still the trusted 'journeymen ' instruments
of mounters and other workers of various orders in many depart-
ments of microscopy.

The Messrs. Beck also make a microscope of this kind called a
'histological dissecting microscope.' It is illustrated in tig. 152 ; the
body is removable, and ' loups ' or simple lenses can be used instead.

198 TEE HISTORY AND EVOLUTION OF THE MICEOSCOPE

The two modes of use are seen in the figure. In this case, however,
the body does not incline ; but in simplicity and price for an in-
strument of this class it deserves commendation.

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 investigations, and are desirable 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 by Powell and Lealand. As opened for use it is illustrated
in fig. 153 ; 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 unscrews, and the whole
lies in a very small space, giving
at the same time fittings in the
cabinet for lenses, condensers, 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

Fig. 152. — Beck's histological and dissecting microscope.

has rack-and-pinion movements and rectangular sector centring,
while all the apparatus provided with the largesi instrument can be
employed with it, We have used this instrument for delicate and
critical work for fifteen years and there is no falling oil" in its quality ;
and when packed with the additional apparai us required the ease is
12 x 7 x 3 inches.

Swift and Son subsequently made an instrument on similar lines.
The tripod and stage are packed practically as Was Powell and Lea
land's, but the stage in this case is plain. It carries a very con-
venient achromatic condenser, to which we call attention in its proper

PORTABLE MICROSCOPES

199

place ; but. its fine adjustment is so unsteady that it cannot be used
with high powers or for critical work.

This instrument 'set up' is seen in fig. 154, and in its packed

Fig. 153. — Powell and Lealand's portable microscope.

condition it is illustrated in tig. 155. The case in which this instru-
ment is packed is 10 J x 6^ x 3 ^ inches.

The Messrs. Beck were the next in order of time to manufacture
-an instrument of this kind. This is illustrated in tig. 156. It differs

THE HISTORY AND EVOLUTION OE THE MICROSCOPE

from the two preceding forms in being a Jackson model. The bin-
ocular body and the coarse adjustment have to be lifted and lowered
by the fine adjustment whenever it is used. The stage is plain, but it
rotates, and the sab-stage has no centring gear. The instrument
packs into a box 104- x 7£ X 3 J inches. There is a condenser
specially made for this instrument. _

Mr.'Housselet has designed an admirable little instrument of port-
able form but of the sixth class. It is binocular. The tripod folds :
the stage is plain, with a sliding ledge. The condenser focusses by

Fig. 154. — Swift's portable microscope,

means of a spiral tube, within which an inner tube slides, carrying
stops, diaphragms, etc. The mirror is jointed so as to be vised above
the stage; and as its focus is only 1J inch can be used as a side
reflector. It is also arranged so that eye-pieces with large field glasses
may be employed. It packs in a box 10^ x 5i x 3^ and weighs six
pounds complete.

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 arranged for

DISSECTING MICROSC< >PE8

20 r

use with compound lenses has been devised by employing the bin-
ocular of Mr. Stephenson. This instrument is illustrated in fig. 1">7.
It is made by Swift and Son. The stage is a large, flat table, with
special rests for the arms. The objective and binocular part of the
body remain vertical and focus vertically by a rack-and-pinion-
coarse adjustment, there being no fine adjustment. The bodies
above the binocular prisms are suitably inclined, mirrors being placed
inside them to reflect the image. This reflexion also causes the
erection of the image, which is valuable to the majority engaged in
inse'ct dissection or the dissection of very delicate and minute organ-
isms or organs.

We have now to consider tJie most primitive stands adopted for
simple microscopes. That in the form of a bull s eye stand is the

Fig. 155.

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 of a
circular brass foot, wherein is screwed a short tubular pillar (tig. 1 58),
which is ' sprung ' at its upper end, so as to grasp a second tube,
also 'sprung,' by the drawing out of which the pillar may be elon-
gated by about three inches. This carries at its upper end a jointed
socket, through which a square bar about 3£ inches long slides rather
stiffly : and one end of this bar carries another joint, to which is
attached a ring for holding the lenses. By lengthening or shortening
the pillar, by varying the angle which the square bar makes with its
summit, and by sliding that bar through the socket, almost any posi-

202 THE HISTORY ASD EVOLUTION OF THE MICROSCOPE

tion and elevation may be given to the lens that can be required for
t^pTrposes to which it maybe most usefully applied, care bemg

ffARE

Fig. 156. — Beck's portable microscope.

taken in all instances that the ring which carries the lens should (by
•means of its joint) be placed horizontally. At A is seen the position

SIMPLE LENS OR LOUP-HOLDERS

203

which adapts it best for picking out minute shells or for other similar
manipulations, the sand or dredgings to l»e examined being spread
upon a piece of black paper and raised upon a book, a box, or
some other support to such a height that when the lens is adjusted
thereto the eye may be applied to it continuously without unnecessa ry
fatigue. It will be found advantageous that the foot of the micro-
scope 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

Fig. 157. — Swift's Stephenson's binocular-, arranged for dissecting purposes.

to bring a fresh portion of the matters to be examined into view ;
and, generally speaking, it will be found convenient to place it on
the opposite side of the object, rather than on the same side with
the observer. At b is shown the position in which it may be most
conveniently set for the dissection of objects contained in a plate or
trough, the sides of which, being higher than the lens, would prevent
the use of any magnifier mounted on a horizontal arm. The powers
usually supplied with this instrument are one of an inch focus, and
a second of either a half or a quarter of an inch. By unscrewing

204 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

the pillar, the whole is made to pack into a small, flat case, the extreme
portability of which is a great recommendation. There is, however,
a form of mounting for this instrument, which was devised by
Quekett, which has superior advantages. In the form illustrated
we are obliged to mechanically arrange the horizontally of the lens,
which, of course, is important, In Quekett's form the loup or lens-
is so hung in a ring that
it has a pendulous motion,
and with every change in
the position or angle of
the bar, the lens, by the
action of gravity, becomes
perfectly horizontal. ' This
is by far the best form of
mounting. Although the
uses of this little instru-
ment are greatly limited
by its want of stage, mir-
ror, etc. yet, for the class
of purposes to which it is
suited, it has advantages
over perhaps every other
form that has been de-
vised. AY here, on t he
other hand, portability
may be altogether sacri-
ficed, and the instrument
is to be adapted to the
making of large dissec-
tions under a low magni-
fying power, some such
fori ii as is represented in
fig. 159 — constructed by
Messrs. Baker on the basis
of thatdevised by Professor
Huxley for the use of his
Practical Class at South
Kensington — will be
found decidedly prefer-
able. The framework of
the instrument is solidly
constructed in mahogany,
all its surfaces being
blackened, and is so ar-
ranged as to give two up-
rights for the support of the stage and two oblique rests for the
hands. ^ Close to the summit of each of these uprights is a groove
into which the stage-plate slides ; and this may be either a square
of moderately thick glass or a plate of ebonite having a central
perforation into which a disc of the same material may be fitted,
so as to lie flush with its surface, one of those being readily

Fig. 158.

DISSECTING 1 XSTR I ' M K N TS

205

substituted for the other, as may best suit the use to be made
of it. The lens is carried 011 an arm working on a racked stem,
which is raised or lowered by a milled-head pinion attached to a
pillar at the further right-hand corner of the stage. The length
of the rack is sufficient to allow the arm to be adjusted to any
focal distance between two inches and a quarter of an inch. But as
the height of the pillar is not sufficient to allow the use of a lens of
three 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
unscrewing 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
is tixed to the wooden basis of the instrument, and places for the

Fig. 159. — Laboratory dissecting microscope.

lenses are made in grooves beneath the hand-supports. The ad-
vantages of this general design have now been satisfactorily demon-
strated by the large use that has been made of it ; but the details
of its construction (such as the height and slope to be given to the
hand-rests) may be easily adapted to individual requirements.

A very portable simple microscope stand has been designed by
Mr. Ward to take a Steinheil x 10 aplanatic loup. It consists of
a circular table on three legs ; it has a mirror, one side of which is
arranged for white cloud illumination ; it also has a bull's-eye for
opaque. The focussing is performed by a sliding arm, and the whole
packs into the remarkably small area of a case whose outside dimen-
sions are 3 J X 2 J X 1 inches.

But the very best form of dissecting microscope for loups or simple
lenses which we believe to be at present constructed is made by Zeiss.
We illustrate this form, fig. 1G0. It has a large firm stage 4 inches
square and \h inches from the table, to which wooden arm-rests can

DISSECTING MICROSCOPES

207

be attached, or i:ot, as may be desired. The stage has a Large
opening, 3 x 3-| inches, into which can be placed either a flat brai
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 be turned aside. The arm which is focussed by an excel
lent spiral rack-work adjustment carries either a Zeiss dissecting
microscope, which with and without its concave eye-lens yields six.
different powers, varying from 15 to 100 diameters, or the arm will
receive the very fine Zeiss- Steinheil loups.

The instrument is provided with a large plane and concave,
mirror on a jointed arm. The utility of this simple microscope is
very great, and we do not hesitate to pronounce it the best thing
of its class we have ever seen.

A substantial and elaborate form of dissecting microscope, devised

Fig. 161. — Beck's binocular dissecting microscope.

by the late Mr. R. Beck, is represented in fig. 161. From the angles
of a square mahogany base there rise four strong brass pillars, which
support at a height of four inches a brass plate 6^ inches square, having
a central aperture of one inch across ; upon this rests a circular brass
plate, of which the diameter is equal to the side of the preceding,
and which is attached to it by a revolving fitting that surrounds the
central aperture, and can be tightened by a large milled head
beneath ; whilst above this is a third plate, which slides easily ovei
the second, being held down upon it by springs which allow a move-
ment of H inch in any direction. The top plate lias an aperture
of H inch for the reception of various glasses and troughs suitable
for containing objects for dissection ; and into it can also be fitted
a spring-holder, suitable to receive and secure a glass slide of the
ordinary size. By turning the large circular plate the object under
observation may be easily made to rotate, without disturbing its

208 THE HISTORY AND EVOLUTION OF THE MICKOSCOPE

relation to the optical portions of the instrument ; whilst a travers-
ing movement may be given to it in any direction by acting upon
the smaller plate. The "left-hand back pillar contains a triangular
bar with rack-and-pinion movement for focal adjustment, which
carries the horizontal arm for the support of the lenses ; this arm
can be turned away towards the left side, but it is provided with
a stop which checks it in the opposite direction, when the lens is
exactly over the centre of the stage-aperture. Beneath this aperture
is a concave mirror, which, when not in use, lies in a recess in the
mahogany base, so as to leave the space beneath the stage entirely
free to receive a box containing apparatus ; whilst from the right-
hand back corner there can be raised a stem carrying a side condens-
ing lens, with a ball-and-socket movement. In addition to the single
lenses and special combinations ordinarily used for the purposes of
dissection, a binocular arrangement was devised by Mr. R. Beck, on
the principle applied by MM. Nachet, about the same date, in their
stereo-pseudoscopic microscope. Adopting Mr. AVenham's method
of allowing half the cone of rays to proceed to one eye without inter-
ruption, he caused the other half to be intercepted by a pair of
prisms, and to be by them transmitted to the other eye. But we
find its utility to be practically limited by the narrowness of its
field of view, by its deficiency of light and of magnifying power, and
by the inconvenience of the manner in which the eyes have to be
applied to it.

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 mi 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 thai of many <>t' the leading-
English and American microsropists, of i ihe form of microscope known
as the Continental model; and we cherish strong hopes in the in-
terests of the science of microscopy that so enterprising and eminent
a firm as that of Zeiss, of Jena, will bring oul a model that will
comport more completely 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, that
we are indebted for the splendid perfection to which the optica] 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 abso-
lutely indispensable appendages, and finally from an instrument wit h
a perfectly plain stage with ' clips ' into what is now a stage with me
•chanical movements — we can but hope that these concessions to what,
lias belonged to the best English models for over fori y years may lead

A CRITICISM OF THE ' (ONTI MENTAL' STAND 209

to an entire reconstruction of the stand— a wholly new model in-
tended to meet all the requirements of modern high-class work in all
■departments, and with a hue adjustment of the most refined class.
We cannot doubt, if this were so, that the same genius which has
so nobly elevated the optical requirements of the instrument would
•act with equal success on its construction and mechanism.
i §|jWe 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 Ave 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.

At the same time we do not blind ourselves to the fact that an
English market for the ' Hartnack * model has had very much to
do with the perpetuation of the errors which that form contains.

The reason of this it is not difficult to trace. The inductive
method advanced but slowly, in practice, upon the professional
activities and even the professional training of medical men. The
country which was the home of Bacon and Xewton, and Harvevand
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 centurv.
Medicine was absolutely unaffected by Bacon until the latter half of
the seventeenth century. It was not until the early years of this cen-
tury that the modern school of medicine began its beneficent career.
But at that time the microscope — one of the most powerful instru-
ments which can be thought of in the application of experimental
and deductive methods to the science of medicine — was looked upon
and treated by the faculty as a philosophical toy, a mere plaything
for the rich dilettanti. But in spite of this the microscope was
brought gradually to a high state of perfection, and by the end of
the first third of the century was remarkably advanced as a practical
instrument, all its essentials being more or less completely developed.
Meanwhile, on the Continent, the microscope was regarded by the
Faculty as a scientific instrument of great and increasing value,
being used to good purpose in making important discoveries in
anatomy, histology, and biology generally.

This was gradually re<disrd 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, were easily accessible to our medical men in their own
country — because it was on the Continent that the investigations
referred to had been made — it was nothing less than the Continental,
microscope that was sought after and obtained. Because early

p

2IO THE HISTORY AND EVOLUTION OF THE MICROSCOPE

observations of a histological character (and therefore of a nature to
lie beyond the sphere of the lay amateur) had been successfully made

with a certain form of microscope

on the Continent, it was practi-
cally argued that this must be
the most suitable instrument for
such a purpose ; but this was an
inference made without know-
ledge of or reference to the well-
known English models.

Let us carefully examine thi&
instrument. The typical form
was that made by Hartnack.
Seen in its best primitive state,
we have it in one of Zeiss' in-
struments represented in hg. 162.
It is a non-inclining instrument
with a short tube on a narrow
horse-shoe foot, in which steadi-
ness is obtained by sheer weight.
It has a sliding tube as a coarse
adjustment, and a direct-acting
screw for the fine adjustment.
The stage is small, and the aper-
ture 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 im-
mediately below the stage, and a
concave mirror.

A sub-staye condenser was
rarely used, because up to a com-
paratively late date (1874) it was
regarded by many on the Con-
tinent as a mere elegant play
thing ; its true value was not
perceived.

On this model all the micro-
scopes of the firm of Zeiss, of
Jena, are constructed, as they
are used almost exclusively on
the Continent, and art; regarded
in many of the universities and
medical schools, both here and
in America, as possessing all tli<-
qualities required for the besl

biological research.

Fig. 1C2.

If we examine
these instruments
1885, we are impressed

the finest of
made up to

as we

CKITICIS3I OF THE 'CONTINENTAL' STAND

2 I r

always are, with the beauty and care of the workmanship and finish
of this firm; but there is the same heavy horse- shoe foot, steady
enough while the instrument is non- inclining, only needlessly hea\ \ ,
requiring common ingenuity alone to get equal steadiness with
one-fourth the weight. But since this instrument 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 ; 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 another foot to which the microscope is clamped.

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. That this is seen to be a disadvantage would appear
certain, because the photographic microscope model of Zeiss has a
larger body-tube ; and in their recent ' Appendix ' to their latest
catalogue they admit that for certain purposes other stands made by
them, ' owing to the limited diameter of their tubes, cut off the
held ; ' a significant fact for those who would narrow the English body.

At this present date out of eighteen models ten are made with
inclining bodies, and three have 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. This is a manifest conformity of the primitive
model to the English type.

The direct-acting screw, only slightly modified, obtains universally
in these models. We have already plainly said that this is not suf-
ficiently delicate in its action for critical work with an apochromatic
objective of 1*4 or 1*5 numerical aperture, especially as a micrometer
screw with a necessarily delicate thread is bound to carry the com-
bined weight of the body, limb, coarse adjustment, and the opposing
spring ; that it will wear loose under the stress of constant work is
inevitable, and thus its utility must be wholly gone.

The 1889 model has a new form of fine adjustment, the alteration
being that the micrometer screw acts on a hardened steel point ; tins
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 pro-
longed use without injurious wear.

The stage of this instrument, in common with all built on the same
model, has three fundamental errors of design : — -

i. The stage is so narrow that the edges of the 3x1 slips are, in
some Continental stands, allowed to project over the edges. Messrs.
Zeiss have profitably departed from this fault, by giving to 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

p ?.

212 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

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 justifi-
cation 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, but
its value is great, and it is an indispensable adjunct to practical
work.

Messrs. Zeiss have introduced a substitute for a rotary stage.;
they have done this by making the body and limb solid with the stage,
so that the whole rotates together.

Practically there is only one point in favour of such a movement,
and that is, that the object remains exactly in the same position in
regard to the field. But against this arrangement there is —

1. The liability of throwing the optic axis above the stage out of
centre with that below the stage, and this though the workmanship
be, as it is, of the highest order.

'2. The rotation of a microscope object for ordinary examination
is really unimportant, as there can be no top or bottom to it. Even
for oblique illumination it is not required, as it is always easier to
rotate the illuminating pencil. The only instances in which rotation
of the object is important are : (a) When the object is polarised, ami
then it is a distinct disadvantage not to be able to rotate the object
independently of the body which carries the analyser. In short, the
stage rotating independently of the body would be preferable because,
if it is required to rotate the object on a dark polarised field, the
polarising and analysing prisms can be set at the proper angles, and
then the object rotated without disturbing the relative positions of
the prisms.

But this cannot be done with the arrangement of the Zeiss model,
which rotates body and stage.

(/3) For yhoto-microgra phie purposes. — In this ease in the Zeiss
stand the head of the fine-adjustment screw is geared to the focussing
rod; so, manifestly, rotation of the body becomes impossible.

Thus, by adopting rotation in the form chosen, the highest ends
for which the microscope stage should 1 revolve cannot be accomplished.

Hie sub-stage is often quite wanting in the common Continental
forms. This was true of theHartnack stands, with ra; e exceptions ;
the Nachet instruments were provided with an elementary form.

As we have seen, until quite recent times, the condenser was
regarded on the Continent as a superfluous, if not a foolish a pplia nee,
but that prejudice has been killed by the light thrown on the whole
question by (1) the chromatic (1873), and now (2) the achromatic
condenser of Abbe. But even a compound condenser was in use in
England in the year 1691, and the best work in England since the
invention of achromatism has never been done without one.

In the mounting of the Abbe condenser every possible ingenuity
has been displayed to make it do its work without a sub-stage ; but
a permanent centring and focussing sub-stage into which this optical
arrangement could, amongst others, fit, might be made for half the
labour, ingenuity, and cost. But rather than this, we have the con-

HOW TO PURCHASE A MICROSCOPE

213

denser made to slide on the tail-piece, and to be jammed with a
screw.

It has therefore neither centring nor focussing (/ear, but, striking
as it may appear, a diaphragm, which cannot be used with, and is
no part of, the condenser, is supplied with meclianical centring and
rack-work focussing movements ! That is to say, the delicate centre
of an optical combination may take care of 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, 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 ; so that whilst the iris diaphragm or a rotating wheel of
diaphragms would meet all the case of the racking and centring of
these, there is nothing in the best stands of what is doubtless the
largest and most enlightened house for the manufacture of micro-
scopes in the world, to supply the indispensable needs of the modern
condenser.

We observe with pleasure advances in every direction to which
we call attention to defects. The more recent instruments are
marvels of ingenuity ; we present, in fig. 163, the latest and finest
form of Zeiss's best microscope.

There is no fault in the workmanship, it is the best possible.
The design only is 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, we can but
believe that the advantages of these improvements will make plain
the greater advantage that would accrue from an entirely new model.

The Purchase of a Microscope.- A desire to possess a good but
not costly microscope is extremely common, but as a rule the intend-
ing purchaser has little knowledge of the instrument and does not
profess to know what are the indispensable parts of such an apparatus,
nor what parts may, in the interests of economy and his special ob-
ject, 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 parts
of a microscope will commend itself to all workers of large and broad
experience : —

1. A coarse adjustment by rack and pinion.

2. A sub-stage.

3. A fine adjustment.

4. Mechanical movements to sub-stage, i.e. focussing and centring.

5. Mechanical stage.

"WHAT IS DESIRABLE IN A MICROSCOPE

215

6. Rack-work to draw- tube.

7. Finder to stage.

8. Plain rotary stage.

9. Graduation and rack-work to rotary stage.

10. Fine adjustment to sub-stage.

11. Rotary sub -stage.

1 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 prefered before a microscope with a rack-and-
pinion coarse adjustment, a fine adjustment, but no sub-stage. Or,
a microscope with a coarse adjustment by rack and pinion, a sub-
stage, and a fine adjustment is to be preferred before one with the
same coarse adjustment and a mechanical stage movement, but no
•sub-stage or fine adjustment ; and so on. The last item is of least
importance, and the importance of all the others is in the order of
their numeration.

Another matter of some significance to the tiro is the relative
value from the point of view of time consumed and, therefore, of
prime cost, in producing the several kinds of microscopes. The
!N"o. 1 stands of half-a-dozen makers may be near the same cost, but
may nevertheless have involved the consumption of very different
quantities of the highest class of skilled labour in their production.

Manifestly the first thing to be looked at in a microscope making
any pretensions to quality is the character of the tcorkmanship ; and
this should carry with it the question, how much machine, and how
much hand work and fitting, there is in it. Arcs graduated on silver,
for example, are very attractive, and with many are most impressive ;
but they are simply machine work and quite inexpensive.

In the two great types of models, the bar movement and the
Jackson limb, the bar movement involves more than double the
actual hand fitting ; while a fine adjustment with a movable nose-
piece takes twice the fitting of one in which the whole body is moved
by the fine-adjustment screw. In the same way a mechanical stage
which is made of machine-planed plates, sliding in a machine-ploughed
groove, is much less costly in time and quality of labour than a hand-
made sprung stage. So a sub-stage having a movable ring pressed
by two screws against a spring has very far less work, and work of
<pl lower class, than one with a true rectangular centring movement.

It will follow, then, that a Jackson-limbed microscope with no
movable nose-piece, with a machine-made mechanical stage, and a
movable ring for sub-stage, will not have involved more, perhaps;,
than a third of the skilled work which must be expended on a well-
made instrument of the same size with a bar 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 tiro will be warned by this not to purchase a pretentious
instrument with a bar movement and mechanical stage for, say, V.

But if a low-priced instrument is to be pwrchased, if, as is almost,
certain, it be a Jackson model, see that it has a rack-work coarse
adjustment, eschew the short-lever nose-piece, and have a differential

216 THE HISTORY iVSD EVOLUTION OF THE MICROSCOPE

screw fine adjustment, a large plain stage, and an elementary
centring sub stage. Such an instrument should be obtained for

hi 10*. „ , _ . . .

Although not frequently used, it would be doing our work im-
perfectly not to refer to a special form of microscope devised for
chemical purposes. This is an inverted microscope originally con-
structed by MM. Nachet on the plan devised by Dr. J. Lawrence
Smith, of Louisiana, U.S.A., for the purpose of viewing objects from
their under side when heat or reagents are being applied to them, 1
has lately been improved by its constructor with a special view to

Fig. — Nachet's chemical microscope.

meeting the requirements of observers engaged in the 'cultivation''
of the minute organisms which act as ferments. The general
arrangement of this instrument is shown in fig. L64. On the table
which forms its base there rests a box containing a glass mirror,
silvered on its upper surface, which is placed at such an angle as to
reflect the light-rays received through the inverted objective mount < ■< I
on the top of the box into the body fixed into its oblique face. < >vei
the objective is placed the stage, above which again is the mirror
for reflecting light downwards through the object placed upon it.
The focal adjustment is made in the first place by means of a sliding
tube which carries the objective, and then by the micrometer screw,

1 This idea was suggested at nearly the same time by Dr. Leeson, and was
carried out in an instrument constructed for him by Messrs. Smith and Beck.

ADAPTABLE CHEMICAL AM) ORDINARY STANDS 217

V, which raises or lowers the stage. The platform on which tin-
optical apparatus rests can be moved in rectangular directions by the
two milled heads, O, T, and is furnished with two graduated scales
by means of which it may be brought with exactness into any posi-
tion previously recorded, so that any point of the object may be
immediately re-found — an arrangement of special value in cultivation
experiments. On the stage is a circular glass cell, C, for holding
the fluid to be examined; in the bottom of this is an aperture which
is closed by a piece of thin cover-glass well cemented round its
edgQS, thus allowing the use of high magnifying powers having a
very short focus ; while its top is ground flat, so that a cover of thin
plate glass may be closely fitted to it by the intervention of a little

Fig. 165. — Bauscli and Lomb's laboratory microscope, used for chemical work.

grease or glycerine, the whole being secured in its place by three
small uprights. The cell is furnished also with two small glass
taps, R, R, with which indiarubber tubes are connected. By this
cell — which may be made to serve as a moist, a warm, and a gas-
chamber — experiments on the rarefaction and compression of air,
and on the absorption of gases, can be made with great facility.
For 'cultivation' experiments smaller cells are provided, which are
attached to brass plates so arranged as to have a lixed position on
the stage.

The Bausch and Lomb Optical Company have now combined the
above with the ordinary vertical form of n tie rosea p>\ the principle in-
volved being (they believe) entirely new. This form of instrument
is particularly adapted for chemical investigations, for the reason

2l8 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

tli at crystals may be studied as they lie in their natural position in
any depth of fluid, and the head is sufficiently distant from the stage
not to inhale any fumes.

Further than this, it is valuable in the examination of diatomacese
and other objects in water which are heavier than it, and therefore
.sink to the bottom; also in the moist histological preparations, as

Fig. 166. — The same instrument changed into an ordinary form.

they adhere to the surface of the slide, and are therefore in one
plane. It is also an excellent dissecting microscope, as it is partially
erecting, offers no hindrance to manipulation with any power, and
makes it convenient to observe the object directly. There are two
forms, the 'Laboratory' and the 'University.' The Laboratory
microscope, when used as an inverted instrument, is shown in fig. 165.

CHEMICAL MICKOSCO I ' i;s

219

The mirror-bar swings on an axis in the plane of the stage to any
point above or below it. The mirror and sub-stage are adjustable
on the mirror-bar.

The sub- stage, carries a revolving diaphragm, and is fixed on a
pivot, so that it will swing in and out of the optic axis, allowing
the polariser to be attached and ready for instant use. On the slide
is the arm, to the lower side of which is fastened the prism box.

On the upper horizontal surface of this is the nose-piece, with an
extra adapter for high powers, and in the oblique surface is a screw-
socket for the body-tube.

To transform the instrument into an ordinary microscope (fig. 166),
the tube is unscrewed, the milled head at the front of the arm
loosened, which releases the prism box, and the arm is swung on its

r

Fig. 167. — The ' University ' microscope as a chemical instrument.

axis from between the pillars into an upright position. The tube is
now attached to the opposite side of the nose-piece, and after the
stage-clips are reversed it is ready for work.

The 'University' microscope (figs. 167 and 168) is in its general
construction similar to the preceding, except that the (single) pillar
•and the arm are not japanned, but are of brass, and that the
instrument swings on an axis which is the same as that of the
mirror-bar. The stage consists of a glass plate mounted in a brass
ring.

The prism used for inversion is that suggested by Mr. J. Law-
rence Smith in 1851, having four faces, with angles of 57°, 150°,
48°, and 105°, the rays being twice totally reflected.

Tank microscopes (also called aquarium microscopes) have, for

220 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

certain kinds of work, a value of their own. They may be used
witli low powers outside the glass or above the water; or the
object-glass may be protected by a water-tight tube outside it, and
with a disc of glass fixed (also water-tight) into that end of the

tube which stands be-
low the front lens of
the objective, at a
proper distance for
the focus, may then
be plunged into the
aquarium. Indeed, the
tube of the instrument
may be so protected
as to work for some
depth, and have some
range in the water of
a o-ood-sized tank.

A beautiful instru-
ment of this class has
been devised by Mr.
J. AY. Stephenson for
the examination of
living objects in an
aquarium. A brass
bar is laid across the
aquarium, as shown in
the woodcut (tig. 10!)).
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 nulled head. The
milled head at the

side, by pressing on a

loose plate, fastens the
bar securely to the
aquarium.

Between the ends

of (he bar slides an
n rm 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 this cylinder is screwed the body-tube of the microscope
with eye-piece and objective, which are thus protected from the
water of the aquarium. The microscope is focussed by rack and

Fig. 168.

The 'University' microscope fixed for upright use.

AQUARIUM MICROSCOPES

2 2 1

pinion (milled head just below the eye-piece), and in addition the
objective is screwed to a draw-tube, so that its posit ion in the cylinder
may be approximately regulated.

The arm of the socket is hinged to allow of the microscope being
inclined in a plane parallel to the sides of the aquarium. The lower
milled head clamps the hinge at any desired inclination.

The socket also rotates on the arm, so that the microscope can be
inclined in a plane parallel to the front of the aquarium. Thus any
point of the aquarium can be reached.

A very convenient form for some classes of botanical work with
very low powers, and also for aquarium work of a general kind, was

Fig. 1G9.

devised by Ross and Co. It may be either monocular or binocular,
and has a rack movement on a horizontal bar, giving it considerable
range; and a rack focal movement on the upright bar and on
the body, with an additional screw movement in a direction to and
from the observer, so that all the principal movements arc arranged
in its construction. Its general character will be understood by the
illustration given in fig. 170. It is extremely useful in the general
study of small tanks.

Mr. C. Collins's aquarium microscope (fig. 171) differs from all
other forms in that it is applied to the side of the aquarium itself.
This is accomplished by making use of a simple pneumatic apparatus.
The head of the ' sucker ' is shown on the left of the drawing, with

222 THE HISTOEY AND EVOLUTION OF THE MICEOSCOPE

an indiarubber ring surrounding a central piston. The ring is applied
to the glass surface of the aquarium, and the air is exhausted by
screwing round the head of the piston seen on the right. Two turns
are sufficient to fasten the sucker securely. The rod to which the
support of the body-tube is attached passes through the sucker-arm,
and can be clamped at any height desired.

Professor E. Schultze has designed and Messrs. Klonne and Miiller
have made the microscope, fig. 172, for the observation of small aqua-
tic organisms in an aquarium specially constructed for the purpose.
There are three parts : (1) the stand, the greater part of which is
nickel-plated ; ('2) the aquarium ; (3) the illuminating mirror.

Fig. 170.

The stand consists essentially of a microscope-tube, which i.s
supported in a horizontal position upon a tripod in such a way that
it can be moved in three different directions by rack and pinion.
The column of the tripod carries a rack and pinion, by which the
tube is moved vertically. On the tube which carries the rack is a slid-
ing piece with a second rack for the horizontal movement from right
to left ; upon this slide the microscope is fixed in a horizontal posi-
tion, and can be moved backwards and forwards in a tube provided
with rack and pinion. There are therefore three movements — verti-
cal, horizontal-lateral, and horizontal-sagittal— so that the organism

AQUARIUM MICKOSC< >PES

223

observed can be followed by the tube as it moves upon the glass
wall of the aquarium.

The aquarium consists of a stand with a frame which carries the
aquarium proper, 10 cm. in breadth and height and 10 mm. in
thickness ; this may be replaced by others. The frame is made of
brass lacquered black. The aquarium itself consists of a horseshoe-
shaped piece of glass, both sides of which are closed by plates o€
cover-glass, leaving the upper end open. It is thus possible to observe
an organism upon either of the two thin sides with an objective gi\ in g
a linear amplification of 200 to 300. To screen off the superfluous lighl
and the numerous reflexions in the aquarium, the frame carries a
diaphragm arrangement which can be applied on either side at
pleasure. This consists of a sliding plate which moves the two
horizontal guides ; it is divided into three parts, and has an oblong.

Fig. 171. — Collins's aquarium microscope, applied to the aquarium by a 'sucker.'

opening in one of the divisions. In this opening a thin plate slides.,
and can be clamped at any point. In this plate again is a circular
aperture, which can be closed to a greater or less extent by various
diaphragms kept in position by a small spring.

If an animal or other small organism is on the upper left-hand
corner of the side turned towards the microscope, the sliding plate
is first moved so that the vertical longitudinal opening lies in the
left-hand third, the small plate is then set so that its opening lies in
the upper third. If, on the other hand, the animal is on the right-
hand side, the larger sliding plate is moved so that the longitudinal
opening lies on the right, and if the animal is towards the bottom,
the small slide with its opening is moved downwards. The two
sliding plates are now so directed that light may be thrown by the
mirror through the aquarium and upon the animal on the front side.
The aperture can be further reduced by diaphragms.

224 THE HISTORY AND EVOLUTION OF THE MICROSCOPE

The mirror is concave, 10 cm. in diameter, and fixed upon its
stand with a ball-and-socket joint so that it can be adjusted in any
position.

As an adjunct, and admirable aid to the student of the tank and
pond, as well as a simple and easy means by which specific forms of
microscopic life maybe be found and readily taken, we call attention
to the tank microscope of Mr. C. Rousselet. It is illustrated in fig.
173 and scarcely needs further description.

One of Zeiss's Steinheil f loups ' or aplanatic lenses, to which we
have 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.

Fig. 172. — Professor E. Schultze's aquarium microscope.

of the tank, and the lens is focussed by means of a rack and
pinion, arranged upon the body of the clamp, as seen upon the
left-hand corner of the figure. The following points will recom-
mend themselves to those who are in the habit of looking at their
captures with the pocket lens in the ordinary way :

When an object of interest is found, it can be followed with
the greatest ease and taken up with a pipette, both hands being
free for this operation.

1 We prefer to have a stand or ' rest' for the tank, and on one side of tliis 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 L 1 1 < - radl
carrying the ' loup.'

AQI'AK1L'.M MICROSCOPES

225

It so frequently happens th.it 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.

Microscopes have been
arranged in many ways
to facilitate class demon-
stration in microscopic-
work, but we have seen
none that is more simple,
efficient, and inexpensive
than that suggested by
Dr. Beale. The instru-
ment is made by attaching
its outer tube on a wooden
support to a horizontal
board, which also carries
a small lamp attached to
it in the required position
(tig. 174). The object

having been fixed in its place, and the coarse adjustment made by
sliding the body in the outer tube, these parts may then be immov-
ably secured, nothing being left movable except the eye-tube, by
sliding which in or out the tine adjustment may be effected. Thus

Fig. 173. — Ronsselet's aquarium microscope.

Fig. 174.

the whole apparatus may be passed from hand to hand with the
greatest facility, and without any probability of disarrangement,
and every observer may readily 1 focus ' for himself, without any risk
of injuring the object.

226

CHAPTER IV

ACCESSORY APPARATUS

Tins chapter on apparatus accessory to the microscope might be
easily made to occupy the whole of the space we propose to devote to
tiie entire remainder of the book ■ the ingenuity of successive micro-
scopists and the variety of conditions presented by successive improve-
ments in the microscope itself have given origin to such a variety
of appliances and accessory apparatus that it would be futile in a
practical handbook to attempt to figure and describe. We propose,
therefore, only to describe, and to explain the mode of successfully
employing, the essential and the best accessories now in use, neglect-
ing, or only incidentally referring to, those which are either sup-
planted, or which present modifications either not important in them-
selves, or accounted for by the fact of their production by different
opticians.

I. Micrometers and Methods of measuring minute Objects.— It

is of the utmost importance to be able with accuracy, and as much
simplicity as possible, to measure the objects or parts of objects that
are visible to us through the microscope.

The simplest mode of doing this is to project the magnified
image of the object by any of the methods described under ' Camera
Lucida and Drawing ' (p. 233). If 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 k stage mi-
crometer,5 which is simply a slip of thin glass ruled to any desired
scale, such as tenths, hundredths, thousandths of an inch and up-
wards. 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 k?ioivn scale, both being magnified to the same ex-
tent. The amount of magnification in no way affects the problem.
Thus, if the drawn picture of a certain object exactly fills the in-
terval between the drawing representing the -01 inch, the object
measures the *G1 inch, and whether we arc employing a magnifying
power of a hundred or a thousand diameters is not a factor that
enters into our determination of the size of the object. In fact, all
drawings of microscopic objects are rendered much more practically
valuable by having the magnified scale placed beneath them, so that
measurements may at any time be made.

In favour of the above method of micro-measurement, it will be
noted (l)that no extra apparatus is required, (2) that it is extremely
simple, and (3) that it is accurate.

The most efficient piece of apparatus for micro measurement is

MICROMETER EYE-PIECE

without doubt the screw-micrometer eye-piece ; it was invented
'by Ramsden for telescopes, and if well constructed is a most valu-
able adjunct to the microscope. It is made by stretching across tin;
field of an eye-piece two extremely tine 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 pass by an index as the milled
head is turned : it is seen in fig. 175, 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 having notches at its edge, whose
distance corresponds to that of the threads of the screw, every fifth
notch being made deeper than the rest to make the work of enume-
ration easier. In the original Ramsden eye-piece one filament was
stationary, the object being brought into such a position that one
of its edges appeared to touch the fixed wire, the other wire being
unoved by the micrometer screw until it appears to lie in contact

Fig. 175. — The micrometer eye-piece.

<with the other edge of the object ; the number of entire divisions on
the scale then shows 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 shows the value of the fraction
■of a turn that may have been made in addition. Usually a screw
with 100 threads to the inch is employed, which gives to each divi-
sion in the scale in the eye-piece the value of y^oth 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 wire that Ramsden made
stationary. There is no advantage in this plan, and it involves
needless complexity in calculation. The best method, there can be
no doubt, is the one employed by Mr. Nelson, 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 convenient but important, because the
-magnification is not uniform throughout the field. If the power

o 2

228

ACCESSOEY APPARATUS

employed is high, in order to effect the span of the great magnifica-
tion one wire (the fixed central one) will be in the middle of the
field the other at the margin, and the comparison will not be true-
on account of the unequal magnification of the eye-piece through-
out the held, whereas if the wire be placed rive notches on one side,,
both measurements are brought more within the centre of the held.

Messrs. Zeiss now make a Eamsden micrometer eye-jnece. It is-
provided with a glass plate with crossed lines, which together with
the eye-piece are carried across the image formed by the objective-

Fig. 176.

by means of the measuring screw, so that the adjustment always;
remains in the centre of the field of view.

Fig. 176 illustrates this instrument, complete and in longitudinal*
section.

Each division on the edge of the drum corresponds to 0*002 nun.
Whole turns are counted on a numbered scale seen in the visual!
held, and the image may be measured up to 8 nun.

A modification of this instrument, facilitating both accuracy and
simplicity, has recently been devised by Mr, Nelson, of which we*
think highly, and of which Ave give an illustration in fig. 177.

This screw micrometer eye-piece differs from those of the old
form mainly in two respects : first, the optical part is compensated ;.
secondly, the micrometer part with both webs can be made to*
traverse en bloc the field of the eye-piece by screw motion.

More particularly speaking, the instrument consists of two parts :
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.

NELSONS SCREW MICROMETER EYE-PIECE

22<J

The flat inner box lias a screw attached to it which enffiuges 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.

Fig. 177.— Nelson's new form of screw micrometer
eye-piece.

Immediately below the web is an iris diaphragm. This permits
a diaphragm to be used suitable to the power of the eye-piece
employed. A guiding line at right angles to the webs lias been
added. Care must be taken to observe that when the movable
web coincides precisely with the fixed web, the indicator on the
graduated head stands at zero. If this is not the case, the finger
screw must be loosed, which will liberate the graduated head, and
then it can be placed in its proper position and fixed. This is of
universal application to all screw micrometers.

Four points are gained by this arrangement : —

(1) The compensating eye-piece yields far better definition 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 webs
equidistant from the centre of the field, and thus eliminate errors
due to distortion.

(•1) The preceding advantage is secured without sacrificing the
benefit of a fixed zero web.

To use the screio micrometer with success it should not be inserted,
.as the custom has been, like an ordinary eye-piece into the tube of
the microscope, but it should have a Jinn stand quite independently,
preventing actual contact with the body-tube.

Plate II. gives the mode of its employment, the illustration being

230

ACCESSORY APPARATUS

made from a photograph by Mr. Nelson. The micrometer eye-piece,
it will be seen, is fitted into a stand wholly independent of the
microscope. This consists of a strong upright, fitted into a massive
tripod or circular foot. The foot in either case only rests on three
points; the upright is capable of telescopic extension by a clamping
tube ; a short tube which takes the eye-piece is fixed to this upright
by a 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 iri the
drawing. It is well to leave from -|th to ^ 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 recorrected; but 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 ine-
vitably 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-^^th of an inch by means
of a stage micrometer in the focus of the objective: this wTas replaced
by a mounted specimen of Amph ij>l<'u ra )><>11 ucida, and he has counted
ninety-six lines in the TTj~jQth of an inch by making the movable wire
pass successively over them until the fixed wire was reached. By
similar means the Editor has measured single objects less than the
iQ^th of an inch.

It will have been premised by the careful reader that the stage
micrometer must be used in every set of measurements ; at least we
Avould 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
uncorrected and accuracy of tube-length forgotten or ignored. The
correction of an objective and the tube-length ought to vary with
every object, and therefore a comparison of the stage-micrometer
and the screw-micrometer should be made with every set of measure-
ments.

Moreover, the majority of stage micrometers exhibit very con-
siderable discrepancies in the several intervals between the lines;
it is well in the interests of accuracy to take tjhe screw value of each
under a high power, find the value of the average, and then note the
particular space or spaces that may be in agreement with (he average
and always use it. An illustration will make this clear.

TO OBTAIN THE VALUE OF A MICROMETER INTERVAL 23 1

Zeiss provides a stage micrometer of 1 mm. divided into '1 and
•01 The following are the actual values obtained for eac h of the '05
divisions, viz. — g ^

8-37
8-38
8-38
8-36
8-36
8-58
8-33
8-31
8-47
8-33
8-33
8-38
8-44
8-38
8-40
8-37
8-40
8-25
8-38
20)16-760

8*38 mean value.

In this instance it will be 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 object
being known, the size of the object may be found by simple propor-
tion ; thus, viz. if 8*38 is the screw-micrometer value for •05 mm.
and 6-45 that for a certain object, the size of the object is

(i) 8-38 : 6-45 :: -05 : xmm.;

x = _- - = -Oobo 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

(ii) 8-38 : 6-45:: — 5 : jcinch:

x = = . = •001515 inch.

8-38 8-38

If the stage-micrometer is ruled in fractions of English inches,
then suppose the screw-micrometer value for y^^th inch = 4"_)">7,
and that for the object = 6*45 as before.

(iii) 4-257 : 6*45 :: -001 : x inch ;

6-45 x -001 nnrr. ,
x = = *001ol.) inch.

4-257

1 In the number given for screw value the whole number stands for a complete
revolution or number of revolutions of the screw-head, and the decimal, the portion
of a revolution read off beyond tliis.

ACCESSORY APPARATUS

[f the answer is required in metrical measurement, then as 1
inch = 25*4 mm.

(iv

4-257 : 6'45 :: (-001 x 25*4) : a* mm. ;
6-45 x -0254 -1638

x

= -0385 mm.

4-257 • 4-257
In tli is connection it will he as well to give two examples of
scale comparison which are sometimes required. Thus you have a
certain interval on a metrical stage micrometer which you know to
l)c accurate, and you wish to compare an English stage micrometer
with this scale in order to find out which particular interval of
inch agrees with it. Suppose -05 mm. = 8-38 screw value as above,
then all that is necessary is to find the point to which the screw
micrometer 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) -05 mm. : -0254 mm. :: 8*38 : x screw value ;

1 (HI ()

X

•0254x8-38

•05

- = 4*257 screw value.

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) -0254 mm. : '05 mm. :: 4*257 : x screw value ;
•05 x 4-257

r0254

8*38 screw value for *05 mm.

A

A cheap substitute for the screw-micrometer has been devised by
Mr. G. Jackson. It consists in having a transparent arbitrary scale
inserted into an ordinary Huyghenian eye-piece in the focus of the

eye-lens, so that it
w ill be in the same
plane as the magni-
fied image of the
object to be mea-
sured. It is seen in
fig. 178. The method
of using it is precisely
similar to that of the
screw micrometer ; the

B

value of -xqYu) inch or
mm., as the case

JijUliAdLiLtikii

may be, is found in
terms of the arbitrary
scale. The value of
the object in terms of
the same scale is also
found, and compari-
son made accord-
ingly. All that need

be done is to substitute the terms of the arbitrary scale for screw
values m the preceding examples, and they will meet the case.

The arbitrary scale should be capable of movement by a screw,

Fig. 178.— Jackson's eye-piece micrometer.

ESTIMATING THE EDGES <)E MINI TE OIMKCTS

233

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 lube- or
camera-length, the magnified image of the stage micrometer is pro-
jected on the ground glass : this is spanned by means of a pair of
spring dividers. The negative film is then scratched by these
dividers. Then you are in a position to make the most accurate
measurement the microscope is capable of yielding.

It is exceedingly important, when performing micrornetrie
measurements, to remember that the precise edges of all objects in
the microscope are never seen. Consequently it is impossible to ascer-
tain 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 series
•of experiments : —

1. The focus and adjustment to be chosen may be termed that of
the ' black dot ' (see Elimination of errors of interpretation, p. 356) ;
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 ; (6)
by using as large an illuminating cone as possible.

3. Measure from the inner edge of the inner diffraction band to
the inner edge of the inner diffraction band on the opposite side.

4. But if the diameter of a hole be required, then the measure-
ment must be made from the outer edge of the outer black diffractive
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 jmrticular 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 which project the microscopical image on to the surface
provided for the drawing.

2. Those which project the pencil and paper into the field of the
microscope.

234

ACCESSORY APPARATUS

zontal position.

Fig. 179.

We shall describe what we consider the most practical forms of

each. 7-7

In point of antiquity Wollaston's camera lucida claims the post
of honour ; but to use it the microscope must be placed in a hori-
Its general form is shown in fig. 179. The rays-
on leaving the eye-piece, above which it
is fixed by a collar, enter a prism, and
after two internal reflexions pass up-
wards to the eye of the observer. It is
easy to see a projection of the micro-
scopic image with this instrument, but it
is when we desire at the same time to
see the paper and the Angers holding the
pencil that the difficulty begins. The
eye has to be held in such a position that
the edge of the prism bisects the pupil,,
so that one-half of the pupil receives the
microscopic image and the other half the images of the paper and
the hand employed in drawing. If this bisection is not equal, too
much of one image is seen at the expense of the other. This was in
some sense supposed to be compensated by the use of lenses, as seen
in the figure ; but the 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
right to be turned to the left, and vice verm. This is an advantage
the value of which we shall subsequently see.

A simple camera was made by Soemmering by means of a small
mirror or circular reflector, which is placed in the path of the
emergent pencil at an angle of 45° to the optic axis, thus reflecting
rays from the image upwards. The instrument is seen in tig. 180'
and slides on to the eye-piece. The reflector must
be smaller than the pupil of the eye, because it is.
through the peripheral portion of the pupil that the
rays, not stopped out by the mirror, come from the
paper and pencil. Hence, as in the case of Wol-
iaston's camera, the pupil of the eye must be kept
perfectly centred to the small reflector. As there
is but one reflexion, the image is inverted but not
transposed. To see the outline of the image as it is in the micro-
scope, the drawing must be made upon tracing paper, and inverted,
looking at it as a transparency from the wrong side.

There is considerable variety in the experience of different
microscopists as to the facility with which these two instruments
can be used. The difference in all probability depends on tin
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 first surface, of the glass
reflects the magnified image upwards to the eye, the paper and

Fig. 180.— Simple
camera.

CAMERjE lucid a-:

235

pencil being seen through the glass. In its simplest form it is seen
in fig. 181. The glass is tinted to lender the second reflexion from
the internal surface of the glass inoperative. The reflexion of the
image is identical with that of Soemmering's.

Fig. 182sho\vsatitting adopted byliaust-h and Lomb for the neutral
tint camera. It is made
of vulcanite, and the half
ring to which the frame
holding the neutral tint
glass is fixed, fits 011 the
cap* of the eye-piece, and
with sufficient grip.

Amongst Hie carru rce
lucidce which project tit*-
image of tic- paper <ni>l
pencil into the microscope
tube is first that devised
by Amici, and adapted to
the horizontal microscope
by Chevalier. The eye
looks through the micro-
scope at the object (as in

Fig. 181. — Beale's camera. Fic;. 1S2.— Bausch and Lomb's fitting for Beale's

neutral tint camera lucida.

the ordinary view of it), instead of looking at its projection upon the
paper, the image of the tracingpoint being projected upon the field — an
arrangement which is in many
respects more advantageous. This
is effected by combining a per-
forated silver-on-glass mirror
with a reflecting prism : and its
action will be understood by the
accompanying diagram (tig. 183).
The lay a b proceeding from the
object, after emerging from the
eye-piece of the microscope,
passes through the central per-
foration 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 upwards from the
tracingpoint, enters the prism P,
is reflected from its inclined sur-
face 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.

Fig. 183.

236

A < !CESSOEY APPARATUS

The Editor has used with great facility and success a camera devised
by Dr. Hugo Schroder and produced by Messrs. Ross. It is figured
at L84, and consists of a combination of a right-angled prism (tig.
L85) A B (J and a rhomboidal prism DEFG, so arranged that when
adjusted very nearly in contact (i.e. separated by only a thin stratum
of air) the faces B C and D E are parallel, and consequently between
DE and BE' they act together as a thick parallel plate of glass
through which the drawing paper and pencil can be seen. The
rhomboidal prism is so constructed that when the face G Fis 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 DG. At J a part of the ray is
reflected to the eye by ordinary reflexion in the direction of J K,
and a part transmitted to J' on the face A C of the right-angled
prism. Of the latter a portion is also reflected to Iv by ordinary
reflexion at J'. The hypothenuse face A C is cut at such an angle
that the reflexion from J7 coincides with that from J at the eye
point K, thus utilising the secondary reflexion to strengthen the

K

Fig. 184. — Schroder's Fig. 185. — Diagram explaining Schroder's camera lueida.

camera lueida.

luminosity of the image. The angle G is arranged so that the
extreme marginal ray H' from the field of the B eye-piece strikes
upon DGat a point just beyond the angle of total reflexion, the
diffraction-bands at the limiting angle being faintly discernible at
this edge of the field. This angle gives the greatest amount of light
by ordinary reflexion, short of total reflexion.

In use, the microscope should be inclined at an angle of 45°, and
the image focussed through the eye-piece as usual ; the camera is
then placed in position on the eye-piece, and pushed down until the
image of the object is fully and well seen. The drawing paper must
be fixed upon a table on a level with the stage immediately under
the camera. The observer will then see the microscopical image pro-
jected on the paper, and the fingers carrying the pencil point will be
clearly in view, the whole pupil of the eye being available for both
images, the diaphragm on the instrument being considerably larger
than the pupil. The eye may be removed as often as required, and
if all is allowed to remain without alteration, the drawing may be
left and recommenced, without the slightest shifting of the image.

Tf a vertical position of the microscope be needful, this may be

ABBE'S CAMERA LI ( IDA

237

done by inclining the table and drawing paper to an angle of 45°
either in front or at the side of the microscope. Kor accurate-
drawing, in all azimuths, the drawing paper should of course coincide
with the plane of the optical image.

This camera may be used with a hand-magnifler, or with simple •
lenses used for dissection and other purposes.

Professor Abbe has also devised an instrument which we
have used with complete success. The accompanying drawing
(tig. 186) will at once show the simplicity of its action. The
image of the paper and pencil coming, say, in a vertical direction
(S.,, tig. 186) is reflected by a large mirror 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 trans-
mits 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

Fig. 186.

pencil with which it is to be drawn are seen coincidentally 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 puipose 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 tiro 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,
the determining of the magnifying power of objectives. It is manifest
that the distance between the paper and the eye of the observer
cannot be so readily determined in this case as in those forms of the
instrument where the image of the paper and pencil is seen direct.

With one or other of the foregoing contrivances, everyone may
learn to draw an outline of the microscopic image ; and it is ex-
tremely desirable for the sake of accuracy that every representation
of an object should be based on such a delineation. Some persons
will use one instrument more readily, some another, the fact being.

ACCESSORY APPARATUS

that there is a sorb 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 pos-
sible, until the tracing shall have been completed. It is essential to
keep in view that the proportion between the size of the tracing and
it hat 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 lamp,
seen to the left of Plate III., which illustrates the correct method of
using the camera lucida ; this lamp is simple and is capable of being
raised or lowered, fitted with a paper shade, for a great deal of the
success attendant on the use of the camera depends on the relative
illumination of the microscopic image on the one side and of the paper
and fingers and pencil of the executant on the other. It is not a matter
to be determined by rules ; personal equation, sometimes idiosyncrasy,
determines how the light shall be regulated. Many finished micro-
draughtsmen use a feeble light in the image, and a strong light on
the hand and paper, and others equally successful manipulate in
the precisely reverse way. But upon the adjustment of the respec-
tive sources of light to the personal comfort of the draughtsman
will depend his success.

Care must be exercised in this work in the case of critical images.
These must not be sacrificed either by racking the condenser into or
out of focus, or by reducing its angle by a diaphragm. If the in-
tensity of the light has to be reduced, it must be done by the inter-
position of glass screens, and this is beautifully provided in Abbe's
camera. The illustration of how the various apparatus for the use of
the camera lucida should be disposed, given in Plate III., may be pro-
fitably 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
coloured glass as may be found needful, and the lamp illuminating
the paper and pencil, and carefully shaded above, is also seen at the
eye-piece end of the body-tube. Often, if the image is too bright,
we find that bringing the lamp down to illuminate the paper more
intensely suffices. If not, use screens ; the illuminating cone must
not be tampered with.

III. The determination of magnifying power is an important
and independent branch of this subject. For this ourpose, and for

HOW TO DETERMINE MAGNIFYING POWER 239

the reason given above, Beale's neutral tint camera 1 is eminently
suitable — indeed, is the best. We can easily and accuratelv measure
the path of the ray from the paper to the eye. What is necessary is
to project the image of a stage micrometer on to an accurat <
placed ten inches from the eye-lens of the eye-piece. There must be
complete accuracy in this matter.

We can best show how absolute magnifying power is thus deter
mined by an example.

Suppose that the magnified image of two T()1(7T)ths of an inch
divisions of the stage micrometer spans y^ths of an inch on a rule
placed as required ; then

(i) -002 inch : '8 inch :: 1 inch : x power ;

x = ^ * ^ = 400 diameters ;
•002

for it is obvious that under these conditions one inch bears the same
proportion to the magnifying power that y^^ths of an inch bears to
Ts0ths 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/,w mm. when
projected covers T^ inch ; then, as there are 25*4 mm. in one inch

(ii) *02 mm. : (*8 inch x 25-4) :: 1 : x power ;

•8 x 25-4 x 1
x = — — = 1016 diameters.

If the reverse is the case, viz. that you have an English stage
micrometer and a metrical scale, then, if the magnified image of
two y-oVo lncn spans 18 mm.,

(iii) -002 inch : :: 1 : x ;
v ; 25-4 '

x = ^ * ^ = 354*3 diameters.

The above results indicate the combined magnifying power of the
objective and eye-piece taken at a distance of ten inches. The arbi-
trary distance of ten inches is selected as being the accommodation
distance for normal vision.

The magnifying power, however, is very different in the case of 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 inches.
To do this he must cause the objective conjugate focus to 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 brings
also the virtual image of the eye-lens nearer.

1 Page 235.

240

ACCESSORY APPARATUS

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
foeussed by ordinary or myopic sight. This is in harmony with
Abbe's demonstration (pp. 25, 26, fig. 28) 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.

To find the precise initial power of any lens, or to find the exact
multiplying power of any eye-piece, is not so easy. A laborious,
calculation, involving the knowledge of the distances, thickness, and
refractive indices of the lenses, is required. But a very approximate
determination, sufficiently accurate for all practical purposes, may be
easily made, especially if one has a photo-micrographic camera at
hand. The principle is as follows.

Select a lens of medium power— a ^-inch is very suitable. Now
with the microscope in a horizontal position, and with a powerful;
illumination, project the image of the stage micrometer onto a screen
distant five feet, measured from the back lens of the objective. If no-
photo-micrographic camera is at hand, it will be necessary to perform
the experiment in a darkened room shading the illuminating source.
Divide the magnifying power thus obtained by 6 ; the quotient will
give the initial power of the lens at ten inches to a 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 J, it is, in practice, found to be very near the back
lens of the objective. So by taking a long distance, such as five feet,
the error introduced by a small displacement of the posterior prin-
cipal focus does not materially amount to much.

There is a further error introduced by the approximation of the
objective to the stage micrometer in order to focus the conjugate at
such a distance, but this is small. We can see, therefore, that this
error tends to slightly increase the initial magnifying power.

The initial power of the \ being found, and its combined magni-
fying power, with a given eye-piece, being known, the combined
power divided by the initial power gives the multiplying power of
the eye-piece. Care must be of course taken to notice the tube-
length 2 when the combined power is measured. The initial power
of any other lens may be found by dividing the combined power of

1 English Mechanic, vol. xlvi. No. 1185. Article on measurements of magnifying
power of microscope objectives by E. M. Nelson.

2 Ibid, vol. xxxviii. No. 981, ' Optical Tube-length,' by Frank Crisp.

ANCIENT 4 NOSE-P] KCKS '

-Mr

-that lens with the eye-piece, whose multiplying power has been
determined, by the multiplying power of that eye-piece.1

Nose-pieces. — The term nose-piece primarily means thai pari oi
a microscope into which the objective screws, but the term is also
applied to various pieces of apparatus which can be fitted between the
nose-piece of the microscope and the objective. There are, for instance,
.rotating, calotte, centring, changing, and analysing nose-pieces.

Nose-pieces, although thought to be so, are not a modern idea ;
our predecessors of a century ago employed similar means. Mr.
Crisp has recently acquired a microscope, which possesses a double
arm, at the end of which is a cell for
receiving different lenses. This cell fits
over the end of the nose-piece, and so
keeps the several objectives which may
be inserted in position. It dates, in all
probability, from the end of the seven-
teenth or the early part of the eighteenth
century.

But in the early days of the micro-
scope rotating discs of objectives, as shown
in fig. 187 (or, perhaps, older still, a long
dovetailed slide of objectives, such as fig.
188 shows), were frequently employed.

It is continually desirable to be able
to .substitute one objective for another with as little expenditure of

Fig. 1ST. — Rotating disc of ob-
jectives.

Fig. 188. — Sliding plate of objectives.

time and trouble as possible, so as to be able to examine under a
higher magnifying power the de-
tails of an object of which a
general view has been obtained by
means of a lower ; or to use the
lower for the purpose of finding
a minute object (such as a parti-
cular diatom in the midst of a
slideful) which we wish to sub-
mit to higher amplification. This
was conveniently effected by the
nose-piece of Mr. C. Brooke,
which, being screwed into the
object end of the body of the
microscope, carries two objectives,
either of which may be brought
into position by turning the arm
189.

IG. 180. — Brooke's
nose-piece as made
by Swift.

on a pivot. This is shown

in

fig-

1 English Mechanic, vol. xlvi. No. 1178, ' Measurement of Power,' by E. If, Nelson.

R

242

ACCESSORY APPARATUS

The most generally useful of all nose-pieces now in use are the
rotating forms which enable one to carry two, three, or four objectives
on the microscope at one time, and by mere rotation each is succes-
sively brought central to the optic axis, seen in figs. 190, 191, 192,
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 focusseci. This objection has been entirely
removed by the introduction of the bent form by Messrs. Powell and
Lealand, and others, shown in figs. 189-192. There can be no doubt-
that for ordinary dry lens work some such device is imperative..
Home, however, who do a very large amount of microscopical work
prefer to use two microscopes • the one a third or fourth class micro-
scope, with only a coarse adj ustment and a 1-inch objective and mirror,,
the other having a coarse and fine adjustment and a J-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?

Fig. 191. Fig. li)'2.

weight it throws upon the fine adjustment. As this subject is fully-
treated of under the heading of ' Microscope ' no more will be said at
present than that a double nose-piece is to be preferred to a triple,,
and a quadruple need not be entertained for a delicate instrument
unless it is required to find out in how short a time a fine adjustment
may be ruined ; for let it be noted that a 2-inch, 1-inch, ^-inch, and'
^-inch objective of English make weigh together 8.} oz. without any
nose-piece.

For the proper use of a rotating nose-piece the length of the
objective mounts should be so arranged that when the objective is-
changed little focal adjustment will be necessary.

An excellent calotte nose-piece for four objectives is made by
Zeiss ; this is so arranged that only the optical portion of the objec-
tive is screwed into the nose-piece. This plan much lightens it, so-
that the nose-piece and the four lenses weigh 3| oz., or only 1 oz.
more than an English ^-inch with a screw collar, and ^ oz. more than;
an English -J -inch of wide angle.

A centring nose-piece has been made with the view of placing

N< ISE-PIECES

243

any objective centra] 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. Xelson, as we have seen (fig. 147), pointed out, at
a time when the sub-stage was costly, that such a nose-piece turned
upside down, with a turn-out rotating ring for stops &c. fitted below,
made a very efficient rectangular centring sub-stage 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 changing nose-piece 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, &c. Unless this is done
you have no guarantee that the axis
of the objective is parallel to that of
the body. Therefore all those appli-
ances which merely grip the objec-
tive, or an adapter screwed on to the
objective, are simply of no value.
Secondly, the appliance, whatever it
is, should be light.

Kachet's changing nose-piece
which fulfils none of these conditions
cannot be called good. The nose-
piece is large and heavy, even for the
small objective it is intended to take,
the screws of which are ^ only in dia-
meter 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 1qq y ; .. UAl i- r

. ' . . bio. l!)o. — Zeiss s sluling-objective

SUCll a device would be Simply lm- changer, with objective in position.

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

B 2

7

244

ACCESSORY APPARATUS

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 objec-
tives to be centred to one another ;
and the second is, that they have
adapters to equalise the length of the
objectives, so when a change of objec-
tives is made little change of focal
adjustment is required. Figs. 193,
194 show the nature of this arrange-
ment. In Kelson's chaiwino- nose-
piece a small ring with three studs is
screwed on to the objective ; a nose-
piece is screwed on the microscope
having three slots and three inclined
planes. Therefoi'e, by placing the

Pig. 194. — The objective detached
from the body-slide.

studs into the slots and giving the

objective a quarter of a turn, the studs
run up the inclined planes, thus causing the flanges to face up tightly.

Mr. Nelson has pointed out a far better and simpler method
which dispenses with all extra apparatus.

Three portions of the thread in the nose-piece of the microscope
itself are cut away, and also three portions on the screw of the objective.
Those portions where the thread is left on the objective pass through
those spaces in the nose-piece where it has been cut away. The
screw engages just as if the whole screw were there and the objec-
tive faces up in the usual manner. This plan in no way injures
either the microscope or the objectives for use in the ordinary way ;
thus uncut objectives will screw into the nose-piece, and cut objec-
tives will screw into an uncut nose-piece. This plan is similar to
that employed in closing the breach of guns, and it was seeing one
of them in 1882 which suggested to Mr. Nelson to adapt the same
principle to the microscope. Subsequently it lias been found that
in 1869 Mr. James Vogan had proposed much the same plan, only
cutting away two portions instead of three ; it is curious that such
an excellent idea was allowed to drop.

An analysing nose-piece is that which carries a Nicolas analysing
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.

SUGGESTIONS AS TO FINDERS

245

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 only consider 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 pre-
pared stop. For many years Messrs. Powell and Lealand have supplied
their Xo. 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
centres it, the differences in latitude and longitude may be noted, and
will give the constants for the correction, which must be added to
or subtracted from the figures given by the sender.

Mr. Xelson has made some very practical suggestions touching
the improvement of finders. He suggests, what we heartily accord
with —

1. That the stage-stop shall be always on the left hand of the
stage.

2. That the zero of the horizontal graduation shall be on the left
hand of the scale.

3. That the zero of the vertical graduation shall be on the top of
the scale.

4. That when the finder is placed to 0, 0, a spot marked on the

246

ACCESSORY APPARATUS

bottom edge of a 3 x 1 inch brass template two inches from the stop
shall be in the optic axis of the instrument, In other words, the
latitude and longitude of the centre of a 3 x 1 inch glass slip shall be
50, 50.

5. That the divisions shall be in T±j>th of an inch, and the scales
one inch long.

If these very simple suggestions were adopted generally, an object
found on one microscope could be easily found on any other. This,
like the ' society's screw ' for object-glasses and a universal sub-stage
fitting, deserves, in the interests of international microscopy, the
consideration of opticians.

In practical ' logging f the use of a hand lens will enable the ob-
server to read by estimation very accurately ; half a division can be
very approximately judged of, and this is as close as will be required
with the highest powers. We have found, for very delicate work,
that we could log with advantage between the divisions thus : say
* long. 41 ' ; but if slightly over but not an estimated half, 1 41 + ' ; if
half, ' 41-^ ' ; if more than this but less than 42, it is logged ' — 42.'
.For logging purposes the lens we recommend is one of Zeiss' ' loups,'
magnifying six diameters. They are admirable instruments, and
are furnished with a handle, which may be used or not at the will of
the worker.

The other finder we desire to consider is called after its inventor,
and is known as ' Maltwood's finder.' 1

It consists of a micro-photograph, one square inch in size, divided
into 2500 little squares, so that each is xo*h inch square. Each
square contains two numbers, one indicating the latitude and one the
longitude. To log any object the slide containing the object must be
removed and the slip holding the micro-photograph substituted for it,
then the figure in the square which most nearly agrees with the
centre of the field is noted. Of course, both the object and the
Maltwood finder must be carefully made to abut against the stop.

There are two drawbacks to this finder.

1. The divisions are not fine enough, so that it is only suitable
for low powers.

2. The removal of the slide, and its substitution by the Maltwood
finder, renders 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.

Dr. J. Pantocsek describes a finder,2 which appears to have some
advantage not possessed by that of Maltwood, which he considers in
comparison 1 time wasting ' and ' minute.'

Two lines are drawn on the stage at right angles, intersecting in
the optic axis ; these are marked 0. Lines a millimetre apart are
drawn parallel to those on the upper half and the left half of the
finder, thus giving horizontal lines in the right upper quadrant,
vertical lines in the left lower quadrant, and squares in the left
upper one. Each ten of the lines is marked as shown in fig. 195.

1 Trans, of the Micro. Soc. new series, vol. vi. 1858, p. 59.
Zeitschr.f. Wiss. Mikr. vol. v. 1888, pp. 39-42; J. B. M. S. 1889, p. 121.

THE IBIS DIAPHRAGM

247

When the object is in the field note is taken of the two lines on
which the left and upper sides of the slide rest ; thus, 42 11.

Diaphragms. — There are three kinds of diaphragms in
First, the commonest form is that of a rotating disc of several ap»-r-

Fig. 105. — Dr. Pantocsek's finder.

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, in its new form, made with sixteen shutters, has been brought
to great perfection by Mr.
Baker, and is also beautifully
made by Zeiss, as shown in
rig. 196. 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 it does not Ire in the
path of the cone, but at its
apex, and will not cut the cone
unless it be exceedingly small.
A very small diaphragm aper-
ture is objectionable, as it is
liable to introduce diffrac-
tional effects. Therefore it is FlG> 190 _z,iss s Iris diaphragm,
better to use a larger aper-
ture further away from the stage than a pin-hole near the stage.
When a diaphragm is used in connection with a condenser, it
should be placed just behind the back lens, and never above the
front lens. Calotte diaphragms placed close under the stage, and

248

ACCESSORY APPARATUS

which have been much in use lately both here and on the Continent,,
are a mistake.1

A very good way of cutting down a cone from a mirror is to have
the diaphragm fitted 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 have now been brought to such perfection,
and are so comparatively inexpensive, that they have superseded for
general work and ordinary purposes all others ; but whatever dia-
phragm is used it should work easily. Iris diaphragms work some-
times so stiffly that the microscope may be moved before the diaphragm.
So, too, with the diaphragm wheels, some require a pair of pliers
before they can be rotated. This is easily accounted for when we
examine the way in which they are fixed. The usual method is to
screw the wheel to the under side of the metal stage. Now, if there
are neither washers nor a shoulder to the screw, it is more than
probable that when the diaphragm is rotated it will screw up and
jamb. The purchaser may easily observe a matter of this kind.

Condensers for Sub-stage Illumination.2 — This condenser is an
absolutely indispensable part of a complete microscope. Its value
cannot be over-rated, for the ability of the best lenses to do their
best work, even in the most skilful hands, is determined by it.
Perfection in the corrections of object-glasses is indispensable ; but
those who suppose and affirm that this is all that we need — that the
objective is the microscope — cannot understand the nature of modern
critical work. The importance of it could not have been realised in
the sense in which we know it in the earlier dates of the history of
the instrument ; but at as early a period as 1G91 we pointed out
(p. 135) that a drawing of Bonanni's horizontal microscope showed
the presence of a compound condenser. It is, in fact, of some
interest to note how our modern condensers gradually arose.

The microscope that amongst the older forms (1094) appears
most efficient and suited for the examination of objects by trans-
mitted light was that of Hartsoeker, p. 135, fig. 102. It will be re-
membered that it not only was furnished 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,7 devised in 1829, was a decided improvement in all respects.
It consisted of two plano-convex lenses ; but this was again improved

* Quekett, Micro. Journ. vol. iv. p. 121 et seq.

- The word 'condenser' throughout this work is applied to optical appliances for
tne sub-stage; what is known as the ' bull's-eye ' is not called a 'condenser.'

EARLY USE OF THE ( '< )NI)ENSEK

249.

by Pritchard, who altered the lens distances and placed a diaphragm
between the lenses. When the object was illuminated with a con-
denser this formed what was the best dioptric microscope of pre-
achromatic times.

Good results, within certain limits, may be obtained by means of
the best Pritchard doublets. With a -j^th inch the surface of a
strong Podura scale may be seen as a surface symmetrically scored
or engraved, but the Editor has never himself been able to reveal the
'exclamation ' marks ; and as this is the experience of other efficient
experts, it may be taken that no resolution of these was accomplished
in pre-achromatic days ; these lenses, in fact, overlapped the discovery
of achromatism.

But the practical results of the use of achromatic lenses soon
led the most experienced men in its 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 1S31 ad-
vocated an achromatic condenser in these remarkable words, viz. 1 1
have no hesitation in saying that the apparatus for illumination
requires to be as perfect as the apparatus for vision, and on this account
I would recommend that the iUumi/nating lens should be ferfeetty
free from ch romatic and sjtherical 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 and experienced experts, has fully
con tinned.

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 (p. 135) ; 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 ob-
ject might be seen at all.

In the condenser used by Smith in his catoptric microscope
(p. 144) 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 detinition is given throughout, free from
all ^rottenness' of outline or detail — and an i uncritical ' or imperfect
image could be made. It was not, apparently, at the time it was
first used considered to be so important as we now know it to be ;
and it is probable that the mode of focussing the light upon the
object by its means was to direct the instrument to the sky with
one hand and to use the biconvex condenser with the other. In
1837 Sir D. Brewster writes of it with appreciation, saying that
* it performs wonderfully well, though both the specula have their
polish considerably injured. It shows the lines on some of the test
objects with very considerable sharpness.'

No advance was made on this condenser for nearly a century.
In 1829 Wollaston recommends the focussing of the image of tic
diaphragm by means of a plano-convex lens of j of an inch focus
upon the object, and Goring in 1832 says concerning it : 1 There is no

250

ACCESSOEY APPARATUS

modification of daylight illumination superior to that invented by
Dr. Wollaston.' But Sir D. Brewster objected to this, contending
that the source of light itself should be focussed upon the object. He
preferred a Herscheleian doublet placed in the optic axis of the
microscope. 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 J of an inch focus, the method of. focussing the diaphragm was as
good as any other, because the diaphragm was placed at a distance
from the lens of at least five times its focus, so that the difference
between diaphragm focus, and ' white cloud ' focus, or the focussing
of the image of a white cloud upon the object, was not very great.
But Brewster was writing of a candle-flame when he insisted on the
bringing of the condenser to a focus on the object, and in this he
was, beyond all cavil, right.

In 1839 Andrew Boss gave some rules for the illumination of
objects in the ' Penny Cyclopedia.' These were —

1. That the illuminating cone should equal the aperture of the
objective, and no more.

2. With daylight, a white cloud being in focus, the object was
to be placed nearly at the apex of the cone. The object was seen
better sometimes above, and sometimes below the apex of the cone.

3. With lamplight a bull's-eye is to be used to parallelise the
rays, so that they maybe similar to those coming from a white cloud.

Of the old forms of condenser, that devised by Mr. Gillett was,

there can be no doubt, the
best. It was achromatic, and
had an aperture of 80°. Fig.
197 illustrates it. It was
fitted with a rotating ring of
diaphragms placed close behind
the lens combination. This
was formed, as the figure
shows, by a eonieal ring witli
apertures and stops, and on
account of the large num-
ber of apertures and stops
it would admit, which, pro-
vided they are carefully ' cen-
tred,' are of great value in
practical work, as well as from
the fact that they are so placed
as not to interfere with the
stage, makes this arrangement
of diaphragms and stops an excellent one, and it is not clear why it
has fallen into disuse.

It had been the custom to recommend the use of this instrument
racked either within or without its focus. Carpenter employed it
without and Quekett within, and one or other of these methods was
general. But in the use of good achromatic condensers with high-

Fig. 197.— Gillett's condenser, from ' Hogj
on the Microscope.'

WHAT IS ESSENTIAL IN A CONDENSER

25I

power work it soon became manifest to practical workers that it is
only when, as Sir David Brewster pointed out, the source of light is
focussed by the condenser on the object that a really critical image was
to be obtained. And Mr. Nelson readily demonstrated this fact even
with the condenser Gillett had devised.

The next condenser of any moment is a most valuable one, and
constitutes one of the great modern improvements of the microscope*.
It was an achromatic condenser of 170° devised and manufactured
by Messrs. Powell and Lealand. We have used this instrument for
twenty-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. 198 illustrates this apparatus. The optical combina-
tion is a ith of an inch power, and it is
therefore more suitable for objectives tdfij^fe
from a jtlt of an inch and upwards ;
but by removing the front lens it may ^&!9mr
be used with objectives as low as one j^^^^^^^^^Sk

Having given to this condenser so fegpi^fcBy ^^^S^^P

high a place amongst even those of our

immediate times, it may be well to

specify what the requirements are ^ ,!' B"'ifr"~"~-'

which a condenser employed in critical ^ ,«« t» n -. t 1 -»
, .,11-1 7 77 Fig. 198. — Powell and Lealand s

work with high powers should meet. condenser.

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 ravs 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 work
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 combine a large aperture with a 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 ;
(b) 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 (d) transmits the largest ' solid ' cone free from spherical
aberration ; (e) that it has the greatest working distance ; (/)
that its chromatic aberrations are perfectly balanced. In the pos-

1 This is one of the many expressions which are inevitable to the practical use of
apparatus; it is simply convenient, and means a full cone of light, a cone with none
of its rays stopped out.

252

ACCESSORY APPARATUS

session of these three essential qualities it has 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
for low powers. When the highest class of work has to be done it
is needful to have condensers suited to the power of the objective used.

A low-power condenser of much merit is made by Swift and Son;,
it begins, in its relation to low powers, where the condenser of Powell
and Lealand leaves off. It consists of two doublets with a single front,
and is much lower in both power and aperture than that of the latter
makers ■ but by sliding off the front cap into which the front lens is
burnished both power and aperture may be further reduced. It is
achromatic, and is a practical and useful instrument capable of adap-
tation to any microscope. Fig..
199 is a general illustration of

this appliance.

A condenser having con-
siderable value, and specially
adapted to lenses of low power,
and up to those of \ inch
focus, has just been constructed
and placed in our hands by
Messrs. Powell and Lealand.
It was made in response to the
earnest suggestion of several
leading microscopists, and in
many respects fully answers-
its purpose. It is achromatic,
has a numerical aperture of
*83, with an aplanatic aperture
for dark-ground illumination is possessed of the highest
Its power is a |- inch, and will prove a most useful

Fig

9

199. — Swift's condenser.

of *5, and
qualities.

adjunct to the photo-micrographer, since it M ill enable him to get a
large image of the source of light on the object; but its aberrations
are not so perfectly balanced as we could desire.

It is possessed of a new feature so far as the condenser of these
makers is concerned, having permanently placed beneath the
optical arrangement an iris diaphragrh^ and in addition the con-
denser mount is supplied with a series of diaphragms and stops which
are placed in a turn-out-arm carrier ; this provides the worker with
facility as well as accuracy of method, since both of these can be
used under the same adjustment. The aperture of the cone trans-
mitted by the condenser with each diaphragm is engraved upon that
diaphragm, and with the stops for dark ground; the aperture of the ob-
jective with which the stop will yield a dark ground is also engraved
on it. This embodies the recommendations we have made below.
We give an illustration which is self-explanatory of this appa-
200.

Before the introduction of the homogeneous system, and the
production of such great apertures by Powell and Lealand as.

ratus, fig

ACHROMATIC CONDENSER OF LARCH APERTURE

^53

;a 1*5 in a (lth, a T\,th, and a
transmitted by Powell's dry
large as could be utilised. But
with aperturas such as these, and
the subsequent 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 on the same system.

Jf.th of an inch focus, the cone
achromatic condenser

was as

Fig. 200. — Powell and Lealand's new low-
power condenser.

This combination consists 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 lias been brought still more recently to a very high state of per-

FlG. 201.

Fig. 202.

0 ®® *

Powell and Lealand's high-power achromatic condenser.

fection, having an aperture of 1*40. It will work through a mount-
ing slip of "07, and for aperture and working distance is, like its
dry predecessor, quite unapproached.

We present a general view of this instrument in figs. 201 and
202, but it will be found that other stops than those illustrated will
be required, some of these being of little or no value ; while the
stops made with rings may be made much less expensively of the
same form, but without the outer rings, having merely three points
•or arms to rest upon the edge of the socket which receives them.

254

ACCESSORY APPARATUS

In its original form it had the great disadvantage that the
focussing had to be wholly disarranged to change a diaphragm, but
it occurred to the present Editor that this could be obviated by a
sliding tube carrying one ring which received the diaphragms.
Originally this was fixed at B, fig. 202, the socket for the diaphragms
being turned out from that position as in B, fig. 201. By placing
this on a sliding tube we can slide the diaphragm carrier down,
quite within reach of eye and hand, as at A B, fig. 201, without in
the least disturbing the optical combination ; and when a diaphragm
has been removed and another replaced in B, fig. 201, is turned in,
and the whole is slidden up into position by means of A, as shown
in fig. 202.

As these sheets are passing through the press, Messrs. Powell
and Lealand have placed in our hands an entirely new condenser,
strictly apochromatic, employing a fluorite lens in the combination,
and presenting features in the highest degree desirable. AYe find its
N.A. to be 0*95, its focal length long enough for a thick slip, its-
aplanatic aperture '9. It will be of great value in critical work.

It is essential for ideal illumination with transmitted light (1) that
the illuminating axial cone should be approximately equal to the aper-
ture of the objective used ; (2) that the object should be placed at the
apex of this cone.

If an objective breaks down with this ideal illumination, which
is very probable, we must be content to sacrifice the ideal ; or, as
is also exceedingly probable, the object under examination lacks
contrast, the ideal method must be modified. But if we have a
suitable object, and a perfect objective, it is the strong conviction
of some leading experts that, as we increase the cone in aperture, we
increase the perfect rendering of the image, until the point is reached
where the cone from the condenser is equal to the aperture of the
objective, and, whatever be the object used, it is advisable not to
exceed this. With the most perfect objectives of the present day,
we find in practice that the best results are obtained when a cone of
light is used, which, on the removal of the eye-piece, is found to
occupy three-quarters of the area of the back lens of the objective.

ISTo condenser is sufficiently free from spherical aberration to
transmit a cone equal to its own aperture. Condensers are all more
or less under-corrected, and consequently focus t heir central rays at
a greater distance than their marginal rays. If we rack up the con-
denser so that the marginal rays are focussed on the object, the focus
of the rays which pass through the centre w ill be beyond the object.

It is well 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 or down the intensity of the illumination is rapidly diminished.
But in the case of a condenser with great aperture, if it be racked,
up the marginal rays will have their full intensity, while those which
passed through the central portion of the condenser will have a
diminished intensity.

The extent to which this will take place will be wholly dependent

DISCOVERY OF EFFECTIVE APERTURE OF CONDENSER 255

on the amount of under-correction present in the condenser. Tn
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 mere annular illumination, which is not a
desirable 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 cbne we are using.

We may measure the total aperture of a condenser just as we
do that of an objective, viz. by means of Abbe's apertometer. 1 But
the effective aperture cannot be measured in that way ; that is to say,
the aperture of the largest aplanatic cone, or cone free from spherical
aberration the condenser is capable of giving, cannot be so discovered.

To do this place the condenser in the sub-stage and an objective
on the nose-piece, focus 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. Now move the object just out of the
field, remove the eye-piece and examine the back of the objective,
and if the aperture of the aplanatic illuminating cone is greater than
that of the objective it will show the back lens to be full of light
(fig. 203). Therefore, if the aperture of the objective is -5, we know
that the aplanatic illuminating cone cannot be less than *5, If now

Fig. 203. Fig. 204. Fig. 205. Fig. 200.

we close the diaphragm so that the image of it just appears at the
back of the objective, we are able to determine the aperture of the
illuminating cone with that given opening in the diaphragm ; thus
in fig. 204 it is a trifle less than -5 N. A.

In a similar manner the apertures of the other diaphragm open-
ings can be determined.

Now let the diaphragm be opened to the full aperture, and an
objective with a wider aperture, say *95, be used. It will perhaps
be found that before we are able to fill the back of the objective
with light by racking up the condenser two black spots will be
formed on either side the middle of the disc. When we reach
the disc of light that is largest (fig. 205), any further racking up
causes the appearance shown in fig. 206. The last point before tic
appearance of the black spots indicates the largest aplanatic apertn r>
of the condenser, and is the limit of the condenser for critical work.2

There are many other condensers of more or less merit and use-

1 Chapter V.

3 ' The Back of the Objective and the Condenser,' E. ME. Nelson, Eng. Mcch.
vol. xlviii. No. 1234.

ACCESSORY APPARATUS

fulness, but we must confine our consideration to those that will
prove of greatest value for their several purposes.

A condenser known as the 'Webster' was first made in 1865,
and is still a very useful one for low powers. It is the same as
that made by Swift,1 but without the middle combination. Its
an<de is less, and its range is not so extensive ; but its chief
commendation in possessing these qualities is that, having one com-
bination less than Swift's, it is of necessity lower in price, and on
that account will be welcome to some workers.

In its present form it reverses its primary construction. It is
now made with a double front and a single back, instead of a single
front and a double back.

An achromatic condenser which has been very largely used in
England and America, and which has secured a great deal of com-
mendation, is that of Professor Abbe. The optical productions of Abbe
are too well known and too valuable, as a rule, to make it needful to
be other than perfectly frank concerning so important a piece of appa-
ratus 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
fact that it has been employed by those chiefly who, previously
ignorant of the value of any condenser, have at once perceived the
enhanced 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 has been so long and pertinaciously taught that the ' correct'
histological microscope must be of the Hartnack type, and that it
should be used with narrow-angled dry lenses, perhaps a -Ith-inch
focus, and no illumination but that afforded by a small concave
mirror, the focal point of which is extremely doubtful or unknown,
and in practice wholly disregarded. No 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 an
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 other high-class achromatic condensers^ but of
images obtained without any sub-stage optical arrangements at all,
placed in contrast with the results obtained by using this condenser
against the same objective when used without its aid. But that even
these images are entirely inferior to the images obtained by the
higher order of achromatic condensers we only require the practical
testimony of Professor Abbe to prove; for he has since produced <n>
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

1 Page 252, fig. 199.

IMPOBTANCE OF AN A I'LANATIC OONDENSEB 257

two lower lenses form a Herscheleian doublet. This combination is
shown in fig. 207, and the general form of the instrument as app]i<<]
to Zeiss's own microscopes is shown in fig. 208.

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
^10th-inch. Its aplanatic aperture is
therefore only *5. Now, whilst it is a
gain of no inconsiderable character to
have an achromatised condenser, and
one which, with greater or less enthu-
siasm, all workers admit, yet the point

Fig. 207. — Optical arrangement
of Abbe's chromatic condenser.

Fig. 208. — Abbe's chromatic condenser as applied to the Zeiss microscopes.

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

s

258

ACCESSORY APPARATUS

Zealand's dry achromatic (fig. 198) with the top removed is id this
respect as efficient as this form of Abbe's.

This condenser can be used either dry or homogeneously ; but
of course with objectives of greater aperture than 1-0 the base of
the slide should always be in oil-contact with the condenser.

It gives the principal modifications from direct to oblique illu-
mination with transmitted light by changing and moving a set of
diaphragms placed in a movable fitting and the diaphragm may be
moved eccentrically to the optical axis of the condenser by moving the
milled head. It gives dark-ground illumination with objectives of
:5 N.A. ; for such illumination, in fact, it is, perhaps, the best illu-
minator extant, and shows objects on a dark ground with sparkling
brilliancy, and may be used with polarised light.

A chromatic condenser, somewhat similar in construction to this,
and of low price, is made by Messrs. Powell and Lealand, but it is
of much higher power, so that the distance between the foci for
the central and peripheral rays is not so great, and on this account

it yields a somewhat larger
aplanatic cone. This in-
strument with its dia-
phragms is shown in fig. 209.
It is more convenient in
form, and can be handled
and adjusted with greater
facility, than that of Abbe.
The size of their respective
back lenses is significant in
t his regard, that of Powell's
being inch, and that of
Abbe's being 1T3(T inch.

The diaphragms (tig.
2 1 0, A) have a central aper-
ture, for the purpose of
centring, and the move-
ment is made by means of
an outer sliding tube, S, with
a slot at the top in which
the arm A fits, and another
arm, B, is placed at thelower
end so as to give ready command of the rotation. This plan allows of
the use of one or two oblique pencils incident 90° apart in azimuth.
The condenser thus mounted is only intended as an oblique illuminator.
It forms one of the best of the very cheap condensers when it is
mounted in a plain tube mount with a ledge to hold the diaphragms.
I) is the optical part of the condenser placed immediately above the
diaphragms and in oil-immersion contact with the base of the slide.
The circular diaphragm is fixed into the inner tube attached to tlx
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
m each. ^ It will be seen that when the diaphragms are placed
together in this manner the movement of the arm will produce the
changes in the light as above mentioned.

Fig. 209.— Powell and Lealand's
chromatic oil condenser.

Pig. lild.

QUALITY OF ABBE'S ACHROMATIC CONDENSES

259

As we intimated above, Professor Abbe has now produced an
achromatic condenser, ostensibly for use in high-power photographic
work, but in fact of much more general utility. It consists of a single
front with two double backs, and it projects a sharp and perfectly
achromatic image of the source of light in the plane of the object.
Its power is low, being J, -inch focus, and it has a total aperture of 1 -0.
Its great superiority over the chromatic form is that it transmits a
much larger aplanatic cone than that ; for whereas the former gave

Fig. 211. — Abbe's achromatic condenser.

only an aplanatic cone of *5, this instrument yields a similar cone of
•65. Like its predecessor it is large and heavy ; and, with great defer-
ence 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 5 of an inch is utilised
when it is transmitting its largest cone. The instrument is repre-
sented in fiff. 211, but a very excellent modification in fitting it to
English microscopes has been made by Mr. Charles Baker, the
optician, which is shown in fig. 212, where it will be seen that,
the fitting for diaphragms is
conveniently placed, and an
iris diaphragm can be used with
great ease below this. This
1 turn-out ' arm carries a disc
of metal to receive the dia-
phragms, stops, lire. Over this
is fitted a ring into which screw
adapters, which will allow other
condensers to be used on the
one mechanism.

The metal disc should have
a central aperture as large as
the largest back lens of anv of
the combinations to be used with
the mount. It should be thick enough to receive two stops or dia-
phragms at a time. This power to alter a diaphragm or stop so as

s a

Fig. 212.— Baker's fitting for Abbe's achro-
matic condenser used in English micro-
scopes.

260

ACCESSORY APPARATUS

to secure any required arrangement of apertures and stops without
in the least disturbing any of the adjustments of the condenser is a
practical gain of a very valuable kind.

Diaphragms should be marked with the numerical aperture they
yield, and stops should be marked with the numerical aperture of
the cone they cut out. Empirical numbers are misleading and
valueless. This special marking need not involve two sets of dia-
phragms with two condenser combinations, one for high and the
other for low powers; the different numerical apertures for each
may be marked on either side of the diaphragm or stop. Memory
cannot fail if we make the lower side of the diaphragm indicate the
apertures for the lower-power condenser, and vice versa.

We may note that for dark-ground work stops should be placed
close to the back lens of the condenser, and in the case of a dia-
phragm— which is less important — an inch of distance should not
be exceeded.

The iris diaphragm is for general purposes more convenient than
the usual circular plate, but it has the drawback of being incapable
of setting to any exact size. A delicate point in an image, caught
with a 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.

Messrs. Beck provide a condenser with a movable top, carrying
front lenses of different power central with the backs. Its character
will be readily seen in the illustration given in fig. 213. This com-
bines a high-, low-, or medium-immersion
or dry condenser in one piece of appa-
ratus. The first lens when brought over
the back combination has a low angle,
and is intended for use without fluid for
histological subjects. The next is a full-
aperture lens, with which, by revolving
the diaphragm, the angle can be varied
from 180° downwards. The third lens,
with full aperture of diaphragm, has an
angle of 110° in glass = 1*25 N.A., and
is truncated, cutting out the central rays.
213.— Beck's variable con- The fourth lens lias also an aperture of

1*25, and is similar to No. 3, but the
periphery is painted over so as to allow pencils only at right angles
to pass. Ingenious as this arrangement is, it is likely to interfere
with the corrections; and as the aperture is not exceptionally great,
it calls here for no special notice.

It may be of service to those who are unable or indisposed to
spend considerable sums upon condensers to state that an excellent

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) 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

MODE OF USING- THE CONDENSER

achromatic condenser can be made by placing a Zeiss 'aplanatische
Lupen' on Steinheil's formula in the sub-stage.1 These are made in
two different powers, viz. 1 inch and 1^ inch, and we can fully
testify to their being the most useful hand-lenses for ordinary work
that can be employed. Great 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 felt. 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 excellent 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 in-
struments, 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. p. 169); 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, stops, glasses, Are.
Centring gear is not necessary with students' and elementary micro-
scopes. The slight displacements due to varying centres of different
objectives will with such microscopes prove of no moment if the
sub-stage is once for all carefully fixed centrally in the axis.

What wre require to do is to centre the image of the lamp flame,
as seen with a low-power lens through the condenser, so that it
stands in the middle of the field. This can be done by moving the
lamp or the mirror, and until this is satisfactory the best results
cannot be obtained. To obviate the inconvenience of having to re-
move the combination in order to alter a diaphragm 2 or stop, in
this simple mount an internal sliding tube may be used, something-
like that described in the latest form of the apochromatic condenser
of Powell and Lealand. 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 may be added
with advantage to this kind of mount, acting like a pocket pencil.
For students' and elementary microscopes — now so often and so
unwisely without condensers — this is a most inexpensive and most
convenient arrangement.

inappreciable in the case of a photographic lens. (2) It is in small apertures such
as are seldom used in photographic lenses where the difficulty arises in the case of
the microscope. (3) It is in the small apertures that the iris fails to respond to
the movement of the lever.

1 Journ. Boy. Micro. Soc. series ii. vol. iii. on Zeiss' loup. E. INI. Nelson.

2 In the technical language or usage of microscopists a diaphragm means a hole
or aperture; thus a ' large diaphragm' means that the opening in the diaphragm
plate, disc, or iris is large. A 1 stop ' is an opaque disc stopping out central rays.

262 ACCESSORY APPARATUS

An epitome of its principal points may be of service.

1 . A sub-stage tube fixed centrally to the body of the microscope.

2. A spiral slotted tube to push into 1.

3. A tube carrying the optical combination of the condenser
sliding into 2, with a pin moving in the spiral slot.

4. A long tube carrying the diaphragm and slots sliding into 3.

5. A cell carrying coloured glasses sliding into the bottom of 4.

Condensers require special mounting for use with the polariscope.
Then at least two ' turn-out ' rotating rings are required to hold
selenites. Swift makes an ingenious multum in parvo mount for
employing, amongst other things, the condenser with the polariscope,
to which we call attention in describing the polariscope. But we
know of no plan equal to that found in the best stand of Powell and
Lealand. The sub-stage has a double ring, one placed concentrically
within the other. The inner one revolves by a milled head and
receives the usual sub-stage apparatus. The outer one receives a
mount of three selenites which revolve, and are placed on ' turn-out'
arms. On the upper part of this mount of selenites is a screw, which
receives the optical combination of their 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 &c. Now from the under part of the sub-
stage into the inner and revolving ring is fitted the polariser, and
this leaves little to be desired in practice.

We would advise the microscopist to avoid condenser mounts
which carry their own centring movements apart from the sub-
stage. It is with regret that we find that this plan has been adopted
in Abbe's new achromatic condenser. It is manifestly better to fit
the rectangular movements to the sub- stage, and then they become
available for all the apparatus employed with the sub-stage. A plan
which requires that each piece of sub-stage apparatus which needs
centring should be provided with separate fittings for this purpose
can have nothing to recommend it.

We give on the adjoining page a list presenting the most import-
ant features of the most important condensers, which we believe
will be of service to the student and worker.

The aplanatic aperture given in the table means the N.A. of the
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 long as the image seen at the back of
the object-glass when the eye-piece is removed (the condenser and
flame being centred to the optic axis of the objective, and the source
of light focussed by the condenser on the object) presents an un-
broken disc of light.

The moment, however, the disc breaks, that is, black spots appear
in it, or its 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 has been exceeded.

The limit given in the table is for the edge of the flame as a
source of light. When, however, a single point of light in the axis
is the source, the condenser will be much more sensitive, and a lower

RELATIVE PROPERTIES OF KNOWN CONDENSERS 263

value for the aplanatic aperture than that given in the table will ho
obtained. But as a single point of light is seldom, if ever, practically
used in microscopy, it was deemed better to place in the table a
practical rather than a theoretical and probably truer result.

It has been stated that the best dark grounds are obtained when
a stop is used which is of just a sufficient size to give a suitable dark
field and no more.

When such a stop has been chosen, and excellent results are ob-
tained with, say, balsam-mounted objects, and then 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.

Total

Aplanatic

Conuenser

ancrture

aperture
N.A.

Power

X.A.

1.

Powell and Lealand's dry achromatic (1857)

•99

•8

1

5

2.

„ ,, top lens removed .

•5

1

3.

„ „ bottom lens only

•24

§

4.

Swift's achromatic

•92

•5

4
10

5.

,, „ top lens removed

•22

1

6. Abbe's chromatic (3 lenses) (1873)

1*36

•5

1

3

7.

„ „ top lens removed

•3

£

8.

Powell and Lealand s chromatic (Abbe's for-

mula) .

1-3

•7

s
±

9.

Powell and Lealand's oil achromatic (188(5) .

1-4

11

10.

„ „ used dry ....

1-0

1

i

6

11.

„ „ top lens removed

4

12.

Abbe's achromatic (1888) . . -

•98

•65

1°
2

13.

„ ,, top lens removed

•28

1

14.

Powell and Lealand's low-power achromatic

(1889)

•83

"5

1

15.

Powell and Lealand's Apochromatic (1891) .
Zeiss's 1 aplanatische Lupen ' large field

•95

•9

4
10

16.

(Steinheil formula) . . .

•32

1

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 wdiolly 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 Powell
and Lealand's oil-achromatic condenser. But not to ignore those
who may still desire it in. its older form, we may say that it is found
in its very best form in Stephenson's catadioptric illuminator. By
its means a greater obliquity can be obtained than can be utilised
even by our modern objectives. The general form of this instrument
is seen in fig. 214, but the principle of its construction and operation
will be understood by the diagram given in fig. 215. The illuminator

264

ACCESSOEY APPARATUS

consists of a segment, C, D, E (cut from the edge inwards), of a,
plano-convex lens of crown glass of 1-inch radius of curvature, the
lens having a diameter of 1*2 inch, and therefore a thickness of
0-2 inch, the upper surface being black. The segment has a length
of 0*56 inch and a depth of 05 inch, and is therefore almost a
square. The curved surface is silvered as in the catoptric lens
described in 1879. It is cemented to a rectangular piece of flint
glass, A, B, C, E, the refractive index of which is 1-652. The
thickness of the flint being 0'34 inch makes the total thickness,
when the two are cemented together, rather more than half an inch.

The object of the flint glass is twofold. In the first place, it
disposes of three-fourths of the spherical aberration of a concave
mirror of these dimensions, and enables one to use light which is
practically parallel ; and, in the second place, it has the very obvious
one of securing greater aperture, which is the primary consideration.

Fro. 214. — Stephenson's catadioptric
illuminator.

As fig. 215 shows, rays which enter the Hint glass horizontally are
reflected at the silvered surface of the crown-glass segment, and
apertures are thus obtained ranging from 0*77 to 1*G44, N.A. in
flint and 1*512 N.A. in crown.

To obtain the smaller apertures, the plane surface of the under
part of the flint is utilised by cementing to it a segment, F (rather
more than half), of a plano-convex lens of radius 0*25 inch, with
a focus in crown O04 inch above the upper surface of the flint,
and, therefore, at the upper surface of a slide having a thickness of
0*04 inch. In order that the rays may be received from the
mirror beneath (and not horizontally) a small right-angle reflecting
prism, G, is placed with one of its sides opposite to and parallel with
the receiving side of the flint-glass.

The aperture of the illuminator is only limited by the refractive

STEPHENSON'S ILLUMINATOR OF GREAT APERTURE 265

index of the flint-glass used. If it is desired to have a larger aper
ture than 1644 N.A., it can be obtained by using a flint-glass of
higher refractive index than 1*652. The aperture is always within
half per cent, of the refractive index of the glass (on a slide of the
same material 0*03 inch thick), so that if glass were used of the
refractive index 1*721 the aperture of the illuminator would be
1*712 N. A., and so on to any extent with increasing densities. But
when we use glass beyond about 1*65 we get into difficulties, as very
dense glass is always more or less coloured, which, by quenching the
more refrangible rays of the spectrum, has a tendency to diminish
the effective aperture. It is, moreover, difficult to work, and in-
volves the use of some fluid at least as refractive as itself. With
flint of 1*65 we have monobromide of naphthaline to connect the lens
.and the slide, and the glass is practically colourless and easy to-
work ; above all, we have an aperture which exceeds by 8 per cent,
a balsam angle of 180°. To connect the ordinary crown-glass with
the illuminator, the homogeneous medium (oil or otherwise) used with
the objective can always be employed, but with flint-glass slides a
more refractive fluid, such as oil of cassia or monobromide of
naphthaline, is necessary.

The oil of cassia if pure will suffice for an aperture of, say, 1*62
N.A., and, in the opinion of most people, it has a less unpleasant
smell ; but for the full aperture of 1*644 this is insufficient, and
monobromide, or some equally refractive medium, must be employed.
Such solutions of biniodide of mercury, although inodorous, are ob-
jectionable for this purpose, from their chemical action, and for other
reasons. The light obtained by this ingenious instrument is free
from colour, since it is obtained by reflexion, but in actual use we
find its weakness to lie in the fact that its light lacks intensity.

Of all the older oblique illuminators Wenham's reflex illuminator
was probably the best in its day, although not easy to manage, and
its results can now, of course, be surpassed by the best condenser.
It is composed of a glass cylinder (fig. 216, a) half an inch long, and
four-tenths of an inch in diameter ; one side of which, starting from
the bottom edge, is worked to a polished face at an angle of 64° with
the base. The top of the cylinder is polished flat, whilst its lower
surface is convex, being polished to a radius of ^ of an inch ; close
beneath this last is set a plano-convex lens of 1^-inch focus ; and
the combination is set eccentrically in a fitting, i i, adapted to be*
received into the sub -stage. The parallel rays, fff, reflected up
into it from the mirror, are made to converge, by the convex surfaces
at the base of the cylinder, at such an angle that if their course
Avere continued through glass they would meet at the point h, above
the glass slide c ; but by impinging on the inclined polished surface,
they are reflected to the flat segmental top, from which again they
would be reflected obliquely downwards so as to meet in the point
/>, but for its being brought into 1 immersion-contact ' with the under
side of the slide. Passing upwards through the slide, they meet in
a point, g, a little above its upper surface, in the optic axis of the
microscope, to which point the object must be brought ; and by
giving rotation either to the object or to the illuminator it may
be illuminated from every azimuth. For convenience of centring, a

266

ACCESSORY APPARATUS

1 //

black half-cylinder, e, is so fixed by the side of the cylinder
that if a dot upon its upper surface be brought into the centre of the
field of view of a low-power objective, its focus, g, will lie m the
optic axis. Some skill and practice are required to use this apparatus
to advantage, but fit will amply repay the trouble of master-
in"- its difficulties. It is best suited to thin, flat objects ; with those

that are thick and irregular
distortion is unavoidable.
Although specially designed
as a ' black-ground ' illu-
minator, it may also be
made useful in the resolu-
tion of difficult test-objects
by transmitted light, the
illuminator being lowered
until a coloured spectrum
appears in the field, the
rays of which bring out
their markings with re-
markable distinctness. For
use with either of these
arrangements for - black -
ground ' illumination, it is
better that the objects
should be mounted ' dry,'
especially when they are
to be viewed under 'im-
mersion ' objectives, bal-
sam-mounted objects being
thus seen better with dry-
front objectives.

This was followed by
' disc ' and ' button ' illu-
minators. Mr. Wenham
devised the simple illumi-
nator shown in fig. 217. This consists of a semicircular disc of
glass (somewhat resembling the half of a button), of half an inch
in diameter, the sides of which are flattened, while the circular edge
is rounded and well polished to a transverse radius of x^th of an inch.
This concentrates the light thrown upon any part of its circumference
A -g on. to an object mounted on a slide of the usual

n^g. thickness with whose under side it is brought
||| into immersion-contact by the intervention of
^ either water, glycerine, or a more refractive oil.

Fig. 217.— Wenham's A i \ 1 xi

disc illuminator. As xt should 0e so fitted to the microscope as to
illuminate the object from any azimuth, it should
have its flat sides grasped in a clip, which may either be mounted
on the sub-stage or attached to the under side of the stage, in either
case having its diametric section brought up to the under surface of
the object-slide. By giving rotation to the object, the illuminator
remaining fixed, the illuminating beam may be made to cross the

Fig. 216. — Wenham's reflex illuminator.

Pil RA 130LIC J lAA'M I XATORS

267

former in any direction that is fitted to bring out its markings; then
followed small hemispherical lenses of various foci attached by oil or
other fluid to the base of the slide, and caused to receive oblique rays
from the mirror, or a low-power objective, arranged by ' swinging '
.sub-stage to give various angles uponthis lens. The difficulty with all
these was (1 ) that a lens of different focus was needed for different
powers ; which, however, was reduced to its minimum by the use of,
say, three lenses of different foci, greater than hemispheres, and so
mounted on the rack that takes the sub-stage as to control or alter
the position of the focal point by racking up or down * and (2) it is
not always possible to get sufficient intensity of light by their means.
It was the arrival of Abbe's chromatic condenser that put an end,
for all practical purposes, to such devices. When it was employed
writh oil great obliquity with far greater intensity was the immediate
result, and this, as we have seen, was surpassed by Powell's. This
particular quality of oblique illumination was still further advanced
hj Powell's ' truncated '' for m of their latest condenser, which constitutes
one of the most oblique illuminators extant ; and that too with
abundance of intense light. It is, of course, very chromatic. But
we can obtain all desirable obliquity with all needful intensity witli
the Powell and Lealand achromatic oil-condenser with a slot, and that)
quite without colour. Hence it must be looked upon as the best
oblique illuminator at present made. We have written enough to
make the grounds of our strongly expressed judgment clear from a
practical point of view, that, for the purpose of obtaining critical
images, and getting the best results from object-glasses, ' swinging '
sub-stages and concentric microscopes are needless, and may be
mischievous elaborations. 1

To give completeness to this part of our subject it is needful to
refer to the spot-lexs and the paraboloid, although they have
practically lost their importance to the microscopist.

A spot lens is a condenser with a permanent axial stop fixed in
it to cut off the central rays for the purpose of obtaining a dark
ground upon which the illuminated object lies. Obviously it is not
.a commendable accessory ; it is a condenser, but having % fixed stop
it can only be used for this mode of illumination ; and, in this form,
it can only be used critically with objectives of a certain definite
aperture, because, in order to obtain a good dark-ground illumination
the stop ought to be suited to the aperture of the objective employed.
In other words, a spot lens only does in a limited and imperfect
way for certain object-glasses what a good condenser with proper
stops will do for all with which the spot lens can be employed. The
fact is that the best recent condensers render needless much costly
sub-stage apparatus, and, therefore, the younger microscopists may
well afford a fine condenser at once.

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 glass that reflects to its focus the rays
which fall upon its internal surface. A diagrammatic section of

1 Chapter III. p. 1G9.

268 ACCESSORY APPARATUS

this instrument, showing the course of the rays through it, is given*
in fig. 218, the shaded portion representing the paraboloid.1 The-
parallel rays r r' r" (fig. 219), 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 angles ; and if"

Fig. 219.

the least of these, G F H, be greater than the angle of aperture of
the object-glass, none of them can enter it. The stop S is attached
to a stem of wire, which passes vertically through the paraboloid
and terminates in a knob beneath, as shown in fig. 218 ; and by
means of this it may be pushed upwards so as to cut off 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 ^-inch objective, although it does give excellent results with

1 A parabolic illuminator was first devised by Mr. Wenham, who, however,
employed a silver speculum for the purpose. About the same time Mr. Shadbolt
devised an annular condenser of glass for the same purpose (sen Trans. Microsr.
Soc. ser. i. vol. iii. 1852, pp. 85, 132;. The two principles are combined in the glass-
paraboloid.

EDMUNDS'S PARABOLOID

269

-vrery low powers such as 1^-inch, 2-inch, and .'5-inch objectivei
-when employed to illuminate large objects such as whole insed
because this instrument gives more diffusion of light over the whole
•of a large object than a condenser does.

There is another paraboloid which, for completeness, we musl
briefly consider. In order to adapt the paraboloid for dark-ground
illumination under objectives of wide angle of aperture, Mr.
Wenham 1 long since constructed a flat-topped paraboloid, fitted to
reflect only rays of such extreme obliquity that they would not
pass out of the flat surface of the paraboloid into the under surface
-of the* slide unless a film of either water or of some liquid of higher
.refractive index (such as turpentine or oil of cloves) were interposed
between them. When thus enabled to enter the slide these rays pass
on until they meet the cover, from which (in the case of dry-front
objectives) they are reflected downwards upon the surface of the
object, giving it a bright illumination on a perfectly dark field. The
special value of this instrument, however, not being then under-
stood, it was not constructed for sale ; the same principle having
been more recently taken up by Dr. Edmunds — an immersion-para-
boloid was specially devised by him for use with immersion-
objectives of wide aperture. But it utterly fails for this purpose.
The object has to be mounted on the slip dry, and therefore the
value of the immersion-objective at once is sacrificed, because the
air-space between the object and the cover— which is essential to
this illuminator — destroys homogeneity and reduces the objective
practically to an air-lens. It was believed that Amphipleura
■pellucida was resolved by its means with an oil-objective, but this
has not been, and we believe cannot be, established. By good and
careful manipulation we have obtained faint resolution of A. pellucida
with a dry lens, but an immersion lens used dry is so completely
out of correction that it will resolve nothing.

Polarising Apparatus. — In order to examine transparent objects
by polarised light, it is necessary to employ some means of polarising
the rays before they pass through the object, and to apply to them,
in some part of their course between the object and the eye, »an
analysing medium. These two requirements may be provided for
in different modes. The polariser may be either a bundle of plates
of thin glass, used in place of the mirror, and polarising the rays by
reflexion ; or it may be a ' single image ' or ' Nicol ' prism of Iceland
spar, which is so constructed as to transmit only one of the two
rays into which a beam of ordinary light is made to divaricate by
passing through this substance. Of these two methods the ' Nicol '
prism is the one generally preferred, the objection to the reflecting
polariser being that it cannot be made to rotate. This polarising
prism is usually fixed in a tube (fig. 220, A), furnished with a
large milled head at the bottom, by which it is made to rotate
in a collar which screws into the sub-stage fitting. For the
analyser a second ' Nicol ' prism is usually employed ; and this,
fixed in a short tube, may be fitted either into a collar interposed
between the lower end of the body and the objective, or into a cap
1 Trans. Microsc. Sue. N.S. vol. iv. 1S5G.

2yo

ACCESSORY APPARATUS

placed over the eye-piece (fig. 220, B), in the stead of the ordinary
eye-piece cap. The former arrangement, which is specially adapted'

B for use with the binocular

microscope, has the advan-
tage of not limiting the
field, but it stops a good
deal of light ; while in
the latter the

image

is

Fig. 220.

A, Fitting of polarising B, Fitting of analysing

prism in sub-stage. prism above eye-piece.

brighter, but a good deal
of the margin of the field
is cut off; and it is
always important that the
polarising prism should
be large, so as not to act
as a diaphragm to the con-
denser, thus cutting off the

light when it is used ; for the polarising apparatus may be worked
in combination either with the achromatic condenser, by which means-
it may be employed with high-power objectives; or as a ' dark-ground '
illuminator, which shows many objects — such as the horny polyparies
of zoophytes — gorgeously projected in colours upon a dark field.

For bringing out certain effects of colour by the use of polarised
light it is, as already stated, desirable to interpose a plate of selenite

between the polariser and the object ;
and it is advantageous that this should
be made to revolve. A very convenient
mode of 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. In order
to obtain the greatest variety of colora-
tion with different objects, films of
selenite of different thicknesses should
be employed ; and this may be accom-
plished by substituting one for another

Fig. 221.

. the revolving collar. A still greater

variety may be obtained by mounting
three films, which separately give three different colours, in collars
revolving in a frame resembling that in which hand-magnifiers are
usually mounted, this frame being fitted into the sub-stage in such
a manner that either a single selenite, or any combination of two
selenites, or all three together, may be brought into the optic axis;
above the polarising prism (fig. 221). As many as thirteen different
tints may thus be obtained. When the construction of the micro-
scope does not readily admit of the connection of the selenite plate
with the polarising prism, it is convenient to make use of a plate of
brass (fig. 222) somewhat larger than the glass slides in whiclr
objects are ordinarily mounted, with a ledge near one edge for
the slide to rest against and a large circular aperture into whichi
a glass is fitted, having a film of selenite cemented to it ; this.
* selenite stage ' or object -carrier being laid upon the stage of the

*

SELENITES

271

Fig. 222.

microscope, the slide containing the object is placed u\>o\i it, and,
by an ingenious modification contrived by Dr. Leeson, the ring into,
which the selenite plate is
fitted being made movable,
one plate may be substituted
for another, whilst rotation
may be given to the ring by
means of a tangent-screw
fitted into the brass plate.
The variety of tints given
by a selenite film under
polarised light is so greatly increased by the interposition of su
rotating him of mica, that two selenites — red and blue — with a mica
film, are found to give the
entire series of colours ob-
tainable from any number
of selenite films, either
separately or in combina-
tion with each other.

The compact appa-
ratus made by Swift as a
general sub-stao-e illumi-
nator is useful and com-
mendable, and is capable
of adaptation to most
English microscopes. It
is shown in h> 223. The
special advantage of this
condenser lies in its hav-
ing the polarising prism,
the selenite- and mica-
films, the black-ground
and oblique-light stops,
and the moderator, all
brought close under the
back lens of the achro-
matic ; whilst it combines
in itself all the most im-
portant appliances which
the sub-stage of a good
moderate microscope can
require.

The use of monochro-
matic Jvjht is frequently
desirable in microscopic
work, especially blue light,
although of less moment

than in pre-achromatic days. The usual method of obtaining
coloured light is to pass sunlight through coloured glass, or through
a coloured solution, such as the ammonio-sulphate of copper ; but this
is a most imperfect and unsatisfactory method, and does not m\e

Fig. 223.

-Swift's illuminating and polarising*
apparatus.

ACCESSORY APPARATUS

monochromatic light. The Messrs. Zeiss have constructed an
apparatus (after Hartnack) for obtaining monochromatic light m
a true form, which is illustrated in fig. 224.

By means of two prisms, P1, P2, of strong dispersive power a
spectrum of considerable length is projected upon the object from
beneath, so that with high powers the entire field is illuminated with a
near approach to monochromatic light. The light enters the instru-
ment through the slit Sp, which is adjustable m width by the screw
S2, and passes through the prisms and the lens at O, forming a spec-
trum at Bpk, where the object on the stage is supposed to be situated.

By moving the slit by the screw S1 the spectrum is caused to
pass over the* object, the different colours following in succession.
The instrument may be used for low powers with ordinary daylight,

Fig. 224.

but for high powers sunlight must be employed. Moreover in practice
it is needful to use a bull's-eye to focus the light on the slit Bp. But
this instrument lacks the higher qualities required of it oil account of
the low angle of the combination 0 which acts as a condenser. We
really want a pencil of monochromatic Light, either parallel or slightly
divergent, of such a size as to,//// the back lens of a high-class condenser.
If such a condenser as Powell's dry achromatic or later homogeneous
achromatic of large aperture could be adapted to this instrument
(which is no doubt possible) it might be of service to microscopists.

Sorby-Browning Micro-spectroscope.1 — When the solar ray is
decomposed into a coloured spectrum by a prism of sufficient disper-

1 For general information on the spectroscope and its usesthe student is referred to
Professor Roscoe's Lectures on Spectrum Analusi s, or t ho translation of Dr. Scliollen's
Spectrum Analysis, and How to Use the Spectroscope, by Mr. John Browning,

NATURE OF THE MICRO-SPKCTROSCOPK

273

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 J .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, &c. 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
allowing rays of other colours to pass through. Again, when ordinary
light is made to pass through coloured bodies (solid, liquid, or
gaseous), or is reflected from their surfaces, so as to affect the eye
with the sensation of colour, its spectrum is commonly found to ex-
hibit absorption bands, which differ from the Fraunhofer lines, not
only in their greater breadth, but in being more or less nebulous, or
cloudy, so that they cannot be resolved into distinct lines by magni-
fication, while too much dispersion thins them out to indistinctness.
Now, it is by the character of these bands, and by their position in
the spectrum, that the colours of different substances can be most ac-
curately and scientifically compared, many colours whose impressions
on the eye are so similar that they cannot be distinguished being
readily discriminated by their spectra. The purpose of the micro-
spectroscope 1 is to apply the spec-
troscopic test to very minute quan-
tities 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-
structed as follows (fig. 225) :
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. 226, F F) interposed between three
of crown (C C C), in such a manner
that the emergent rays, r r, which
have been separated by dispersion, leave the prisms much in the same
direction as the immergent ray entered it. Below the eye-glass,

* We do not make the change, lest complications should arise ; hut we think it
would he more harmonious with analogy to call this instrument the apectro-fMCroscope.

Fig. 225. — Micro-spectroscope.

Fig. 226.

274

ACCESSOKY APPARATUS

in the place of the ordinary stop, is a diaphragm with a narrow slit,
which limits the admission of light (fig. 225) ; this can be adjusted in
vertical position by the milled head, H, whilst the breadth of the slit is
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 covers one-half of this slit, and reflects 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 only
requisite to reflect light into this
aperture from the small mirror,
I, carried at the side ; whilst for
the production of the spectrum of
any substance through which the
light reflected from this mirror
can be transmitted, it is only
necessary to place the slide carry-
ing the section or crystalline film,
or the tube containing the solu-
tion, in 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 produced by the object
viewed through the body of the
microscope, so that the two can
be exactly compared.

The exact position of the ab-
sorption bands is as important as
that of the Fraunhofer lines ;
and some of the most conspicuous
of the latter afford fixed points

Fig. 227.-Bright-line spectro-micrometer. of reference, provided the same

spectroscope 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 micrometer, shown in fig. 227. At B is a
small mirror by which light from the lamp employed can be re-
flected through E D to the lens C, which, by means of a perforated
stop, forms a bright pointed image on the surface of the upper
prism, whence it is reflected to the eye of the observer. The rotation
of a wheel worked by the milled head, M, carries this bright point

THE USE OF THE MJCRO-SI,ECTROS( :< )I>E

^75

over the spectrum, and the exact amount of motion may be read off
to ToJuoth incn 011 tne graduated circle of the wheel. To use this
apparatus, the Fraunhofer lines must be viewed by sending bright
daylight through the spectroscope, and the positions of the princip
lines carefully measured, the reading on the micrometer-wheel being
noted down. A spectrum map may then be drawn on cardboard, on
a scale of equal parts, and the lines marked on it, as shown in the
upper half of hg. 228. 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 hrst hold it up to
the sky on a clear day, without the intervention of the microscope,
and note the effects of opening and closing the slit by rotating the
screw, C (fig. 225) ; the lines can only be well seen when the slit is
reduced to a narrow opening. The screw H diminishes the length
of the slit, and causes the spectrum to be seen as a broad or a narrow
ribbon. The screw E (or in some patterns two small sliding-knobs)
regulates the quantity of light admitted through the square aperture
seen between the points of the springs, D D. Water tinged with

Fig. 228. — Upper half, map of solar spectrum, showing Fraunhofer lines. Lower
half, absorption spectrum, showing position of bands in relation to lines.

port wine, madder, and blood are good fluids with which to com-
mence this study of absorption bands.2 As each colour varies in
refrangibility, the focus must be adjusted by the screw B, fig. 225,
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 removed by
withdrawing the tube containing them ; the slit should then be
opened wide, and the object, or part of it, brought into the centre of
the field ; the vertical and horizontal slits can then be partly shut
so as to enclose it ; and if the prisms are then replaced, and a
suitable objective employed, the required spectrum will be seen,

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. (See Jouj-u. of Boij. M/crosc. Sue. vol. i. 1878, p. 320,
and vol. ii. 1879, p. 81.)

2 A series of specimens, in small tubes, for the study of absorption-spectra, is
kept on sale by Mr. Browning; and the directions given in his How to Work with
the Mioro-spectroscopc should be carefully attended to.

T 2

276

ACCESSORY APPARATUS

unaffected by adjacent objects. For ordinary observations objectives,
of from two inches to f -inch focus will be found most suitable ; but
for very minute quantities of material a higher power must be

employed.

single

Even a
red blood-corpuscle may be
made to show the character-
istic absorption bands re-
presented (after Professor
Stokes) in fig. 229.1

For the study of coloured
liquids in test-tubes or small
cells, the binocular spectrum
microscope, described by Dr.
Sorby in the ' Proceedings of
the Royal Society,' No. 92,
1867, p. 33, is extremely con-
venient.

The spectral ocular by
Zeiss is another and a very
perfect form of the micro-*
spectroscope. Thisis an opinion
expressed by Dr. Sorby and
other experts, and it is mani-
fest in the character of the
instrument. Fig. 230 represents a sectional view of the instrument.
It will be seen that the lower part is an ordinary eye-piece with its.

Fig. 229.

Fig. 230.

Fig. 281,

two lenses, but in place of the ordinary diaphragm there is a slity
adjustable in length and breadth, shown in fig. 231. ]>y studying

m ] For further information on 'The Spectrum Method of Detecting Blood,' see an
important paper by Dr. Sorby in Monthly Microsc. Join 'H. vol. vi. 1871, p. 'J.

THE USE OF THE ZUICRO-SPECTPOSCOPE

277

this figure the method of adjustment, with two screws, F and IE
and the projecting lever, which carries a reflecting prism, can be
readily understood. The upper part of the instrument swings
about the pivot, K. so that by opening the slit the eye-piece can be
used for focussing an object, the slit being the diaphragm. The
upper portion contains the prisms, and also a scale in the tube, N,
which is illuminated by the mirror, O. 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. 230, 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.

The Method of Using the Micro-spectroscope.— The objects to be
investigated are of two sorts, liquid and solid. Colouring substances
as chlorophyll, the colouring matter of hair, blood, lire, will frequently
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 dark 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 bands, depends naturally upon
the time during which the material is in the extracting medium, as
well as on the quantity of the material.

Commonly also a solution of less concentration will give the same
intensity of spectrum if a 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,
with two parallel glass plates, is very useful. For exact investigations,
however, the trough-flask is preferable. It is a flask whose two
sides, back and front, are parallel, furnished with a carefully fitted
ground-glass stopper. It should be filled quite full of the solution
and then laid with its broad side on the stage. It is especially in-
dispensable when we wish to study the combination spectrum of two
solutions. In that case two flasks are filled each with a different
solution and both laid upon the stage one upon the other. For the
purpose of examining small quantities of any liquid, a sufficient
depth being obtained with very little material, vertical glass tubes
attached to horizontal plates are used, as proposed by Mr. Sorby and
shown in fig. 232. The narrow tubes are made of various lengths
from sections of barometer tubing, in order to present different

278

ACCESSOEY APPARATUS

thicknesses of the contained fluid, the broad tube being higher on.
one side than the other, and thus constituting a wedge-shaped cell,
which, when filled and closed by a thin cover-glass, will present a
varying thickness of fluid for study and comparison^ If the object
to be investigated is not a solution but a preparation of the kind
which we commonly employ in microscopic inquiries, we must first
of all bring it into the focus of the objective system. To do this we
must first remove the tube bearing the prisms, open the slit somewhat,
and use the apparatus as a simple ocular. If one has to deal with a
small object which would not entirely fill the slit, but so that rays
of light might come in by it and disturb the spectrum, he should turn
the comparison prism so as to shut up some of the slit, without,. how-
ever, 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 necessary. On the other hand, should the object consist
of a number of single minute grains, which would cause to be drawn
across the spectrum, in the direction of its length, perpendicular to
the Fraunhofer lines, a like number of dark lines, one must adjust
the microscope so that the 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 likewise throwing the object somewhat out of focus.

Fig. 232.

Illumination by Reflexion. — Objects of almost every description
will require at times to be examined and studied by what is called
reflected 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 illumination,' which, how-
ever, 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 examined 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, l»ut it is manifestly not
' opaque illumination.' The designation of this method of illumina-
tion 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 5 (which is nowhere in this work called a 'condenser ' ; this would,
as it often has done, lead to confusion) ; it is enough to designate it as
we have done. It is a plano-convex lens of short focus, two or three
inches in diameter, mounted upon a separate stand, in such a manner
as to permit of its being placed in a great variety of positions. The
mounting shown in fig. 233 is one of the best that can be adopted :

THE BULL'S-EYE AND HOW To I SK IT

279

the frame which carries the lens is borne at the bottom upon a swivel
joint, which allows it to be turned in any azimuth ; whilst i( may
be inclined at any angle to the horizon, by the revolution of the
horizontal tube to which it is attached, around the other horizontal
tube which j^rojects from the stem; by the sliding of one of these
tubes within the other, again, the horizontal arm may be lengthened
or shortened ; the lens may be secured in any position (as its weight
is apt to drag it down when it is inclined, unless the tubes be made
to work, the one into the other, more stiffly than is convenient) by
means of a tightening collar milled at its edges • and finally the
horizontal arm is attached to
a spring socket which slides up
and down upon a vertical stem.

The plane side of the
bull's-eye should be turned
towards the object. Some
microscopists like to have
their bull's-eye attached to
some part of the microscope ;
but if this is done, care must
be taken to attach it to a fixed
part of the microscope, and
not to either the 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
trunk or foot of the microscope. 1

The optical effect of such a
bull's-eye differs according to

Fig. 233.— The bull's-eye.

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 t owards pa rallel or towards the least diverging
rays, consequently, when used by daylight, its plane side should be
turned towards the object, and the same position should be given to it
when it is used for 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

1 Rousselet's bull's-eye attachment to lamp is convenient and inexpensive. — Journ.
Quekett Club, 1890.

28o

ACCESSOKY APPARATUS

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. J ames 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 lower
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 Avedge of light. The ad-
justment is best made by first placing a slip of white card on the
stage, and, when this is well illuminated, substituting the object-
slide for it, making the final adjustment while the object is being-
viewed under the microscope. No difficulty is experienced in getting
good results with powers of from 200 to 300 diameters, but higher
powers 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. Neither of these plans w ill answer for other than low
powers, where there is plenty of room for the light to pass between
the objective and the object. The ingenious use of the bull's-eye
employed by Mr. James Smith, as detailed above, increases the pos-
sibility of magnification, but it needs practice and care.

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 place* I 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 su rprisi i ig 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 be-
fore homogeneous lenses, helped us over many difficulties of detail. It
was the first illumination to actually resolve the Amphipleura pellucida.
It could be very easily obtained with a student's microscope provided
1 See Journ. Boy. Microsc. Hoc. vol. iii. 1kh(), p. 898

SIDE REFLECTOR — PARABOLIC SPECULUM 28 r

with Nelson's open stage,1 for on this the bull's-eye could he placed
-against the edge of the slip without any special apparatus or fitting.

Another and popular method of 'opaque illumination' is by
means of a specialised form of mirror, generally of polished silver,
called a side reflector, and fixed as in the case of the bull's-eye,
and for the same reasons, to an immovable part of the microscope.

The manner of employing this 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 premitotic

Fig. 234. Fig. 235.

speculum is commonly employed mounted either on the objective,
as in Beck's form, tig. 234, or on an adapter, as in Crouch's,
shown in fig. 235, where a collar is interposed between the lower
end of the body of the microscope and the objective seen at A.
This is not a commendable plan, for it increases the distance between
the objective and the Wenham binocular prism ; and as the bin-
ocular is specially suited for the kind of object usually examined
with this speculum, this increased distance acting detrimentally on
the behaviour of the binocular prisms, and causing the available
racking distance for the focus of objectives of very low power to be
.shortened by the width of such collar, is to be avoided.

The best plan without doubt is to attach the speculum to a fixed
part of the stand, as is done in the Powell and Lealand, the Ross,
and the Beck stands.

A modification of the parabolic reflector teas devised by Dr. Sorby,
and has proved to be very useful in certain investigations, such as
the microscopic structure of metals. It consists of a parabolic
reflector, in the centre of which, in a semi-cylindrical tube open in

1 Chapter II. p. 193, fig. 14S.

282

ACCESSORY APPARATUS

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. 236 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 readily used, as the plane reflector is attached to an

arm so that it can be swung out of the
way when not required, as shown in
the figure.

Dr. Sorby was able to get results in
the examination of polished sections of,
steel not otherwise attainable.

ISTo opaque illumination, however,
has yet surpassed the venerable
Lieberkiihn ; the best experts freely admit that the finest critical
images to be obtained by this method of illumination are secured
by the Lieberkiihn. This mode of illuminating opaque objects is
by means of a small concave speculum reflecting directly down upon
them to a focus the light reflected up to it from the mirror ; it was
formerly much in use, but is now comparatively seldom employed.
This concave speculum, termed a 'Lieberkiihn' from the celebrated
microscopist who invented it, is made to fit 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

Fig. 236. — Sorby's modification of
the parabolic reflector.

Fig. 237.

its light from a mirror beneath (fig. 237, A), the object must be so
mounted as only to stop out the central port ion 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.

UEBERKUHN — HOAV TO MOUNT OIUKOTS I-OK IT

It has two manifest drawbacks : the first one, that of requiring n
separate Lieberkiihn for each object ice, is a difficulty which in the
nature of things cannot be overcome. The radius of the Lieberkiihn
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 compensate it on
the tube which slides the Lieberkiihn on to the nose of the objective.

The second drawback has reference to the special way in which
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.

3. A stop of paper or varnish should never be placed behind an
object.

Let every opaque mount be also a transparent one, since it is
often most useful to examine an opaque object afterwards by trans-
mitted light. The stop should always be a separate one ; this may
be a disc on a pin held in the sub-stage, or, what is still simpler, a
piece of moderately thick ' cover ' glass, cut to the 3x1 inch size,
or rather shorter, should have a small disc of Brunswick black put
on it centrally on the ' turn-table ' 1 and this may be placed under
the slide when 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 be examined either with the Lieberkiihn or by directly
transmitted light, and of course by having a larger stop the same
object may 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 thickress of
slip and diameter of cover-glass were adopted. For the thickness
of the slip (the o^th of an inch would prove most suitable, and for
the diameter of the cover- o-lass 4 of an inch would be most com e-
nient, 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 cover-glasses — such as those T(jths of an inch in
diameter — are to be wholly condemned. They do not allow the con-
ditions required by modern microscopy, being adverse to the employ-
ment of oil-immersion lenses in anything like the most efficient way.

Lieberkuhns can be used with objectives as high as ^ of an

1 Chapter VII.

284

ACCESSORY APPARATUS

inch focus of '77 N. A. For higher powers than this a perfectly flat
speculum may replace the conical form, being illuminated by a con-
denser with a stop, and racked up well within its focus. The
oblique annular ring of light falls on the flat speculum, and is

then reflected on the object.

Illumination for Lieberkuhn may be
either from the flat of the lamp-flame,
reflected by the plane mirror, or by the
edge of the flame, the rays being rendered
parallel by a bull's-eye, and reflected from
the plane mirror to the Lieberkiihn.

Very often this kind of illumination
is so intense that it requires modifying.
It is useful for this and many other pur-
poses in illuminating objects to have a
small frame, mounted like an ordinary
buWs-eye stand, into which a plate of blue
glass may be inserted and placed betiveen
the light and the mirror. Two inches by
four inches is a very useful size for such
a screen, and two or three plates may
be kept of various depths of colour, so
that they may be changed as circum-
stances require.

Messrs. Beck make a small and useful
light-modifier, which can be also used
for this purpose, which is illustrated in
tig. 238 ; but what is required is not merely
a tinted glass, but a combination of dif-
ferent tints resulting in a correct blue not
otherwise attainable.

There is one other kind of reflected
illumination employed, produced by the
vertical illuminator, which, although it
has been in use for some years, has re-
ceived an accession of value from the em-
ployment of immersion lenses. It is pro-
duced by a device which secures vertical
Fig. 238.— Beck's light-modifier, illumination invented by Professor H. L.

Smith, of Geneva, U.S.A.
The principle of this illuminator is to employ the objective as
its own illuminator. This may be done in several ways.

1. That of Professor Smith is to place a speculum with an aper-
ture in it in the body of the microscope at an angle of 45° to the
optic axis ; opposite this speculum is an aperture in the tube for
light to enter. It will be understood by figs. 239 and 240, which re-
present a longitudinal section of the nose-piece at C of the vertical
illuminator at e, and of part of the objective at d. a is the aperture
for admitting the light to the speculum b. The path of the beam is
depicted, and it will be seen that on being reflected from the specu-
lum it passes through the combination of lenses, making the objective

VERTICAL ILLUMINATOR — HOW To I'SK IT

285

a ' condenser,' and is brought to a focus on the object, which is tin-
focus of the objective. The rays now proceed from the illuminated
object once more, through the objective upwards, and pass through
the central aperture in the speculum to the eye-piece and the eye.

2. Tolles, instead of this, places a small right-angled prism just
at the back of the front lens of the objective. But this only gives
oblique illumination in one azimuth, and from experience we are
obliged to express dissatisfaction with its performance.

3. Messrs. R. and J. Beck, in place of a speculum pierced,
employ a disc of cover-glass. The cover-glass is mounted on a
pin, B, fig. 240, in order that it may be rotated and oblique light
obtained by the milled head, / A, fig. 240. We believe that it would
be far better in practice to fix the cover-glass at the angle of 45°.
But it will be seen that by this plan total reflexion may be obtained
in one direction and transmission to the eye in another.

Fig. 239. Fig. 240.

4. Powell and Lealand's method is to fix a piece of glass, work"/
flat, at an angle of 45° to the optic axis, with a rotating diaphragm
in front of the aperture admitting the light.

To use these instruments the edge of the lamp flame should be
placed in front of the reflector, so that the rays may be reflected on
to the back lens of the objective in a line parallel to the optic axis.
The distance from the lamp to the reflector must exactly equal the
distance from the reflector to the diaphragm of the eye-piece in a
positive eye-piece, or the eye-lens of a negative eye-piece, otherwise
the rays will not be focussed on the object.

This illumination is only suitable for objects mounted dry on the
cover, and with immersion lenses. No good result was ever obtained
until the immersion lenses were brought into use.

Of all the light which is caused to pass out of the front lens of
the objective, through the oil and into the cover-glass, that which
has an obliquity less than the critical angle of glass (41°) passes

286

ACCESSORY APPARATUS

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
from the under surface of the cover-glass, 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 objective be examined, it will be seen that all
that portion of the back of the objective whose aperture exceeds 1-0
is brilliantly illuminated. This annulus represents, and is produced by,
the excess of aperture beyond the equivalent angle of 180°, of which
it is also a measure. The internal dark space is of the exact diameter
of that of a dry objective of the same focus, and is the maximum
space which it can itself utilise on a dry object by transmitted light.

By means of this instrument carefully used, the most stubborn
frustules of difficult tests and the most crucial lined objects have
been resolved ; while 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 invisible with the vertical illumi-
nator. 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
1*1 N.A. will have a narrow annulus, and that of 1*4 or 1*5 a broad
and still broader one. In this way, by practice, a fair approximation
of the aperture of an objective may be obtained.

It is not the absolute size of the annulus, but the relation of the
size of the annulus to that of the whole back, that must be estimated.
Thus a ^th of N.A. 1*2 will have as broad an annulus as a -^.jth of
1-4 N.A., but the diameter of the back of the -^th is, of course, much
larger than that of the -yjth, and this involves the
necessity of a relative comparison.

In examining objects with those higher powers
which focus extremely close to the covering glass
the slightest inadvertence is likely to lead to a
fracture of the glass, and perhaps to the destruction
of a valuable slide. This is a serious matter with
Moller's diatom type slide, or Nobert's test lines,
or with many others that are expensive or perhaps
impossible to replace. To remove this source of
danger, Mr. Stephenson- contrired the safety stage,
shown in fig. 241. The frame on which the slide
Fig. 241. carrying the object rests is hinged at its upper part,
and kept in its true position by slight springs,
which give way directly the slide is pressed by the objective. It is
found that springs firm enough to ensure the steadiness required
for high powers may yet be sufficiently flexible to give way before
very thin glass is endangered, and a glance at the st.age shows if it is
made to deviate from the normal position in which its upper and
lower edges are parallel.

STAGE A PITTANCES

287

Fig. 242. — Stage-forceps.

Fig. 243.— Stage-forceps.

Useful as this piece of apparatus may be, it is nevertheless quite
unnecessary if the front of the stage of the microscope be either open
or large enough to permit of the use of the finger on the edge of- the
slide, when it is easy to discover by touch whether or when the
front of the lens is in actual contact with the cover of the object.

Appliances for the practical Study of living and other Objects
with the Microscope, — Stage-forceps and Vice.- For bringing under
the object-glass in different positions such small opaque objects as
can be conveniently held in a pair of forceps, the stage-forceps (fig.
242) supplied with most microscopes affords a ready means. These
are mounted by means of a joint upon a pin, which fits into a hole
either in the corner of the stage itself or in the object-platform ; the
object is inserted by press-
ing the pin that projects
from one of the blades,
whereby it is separated
from the other ; and the
blades close again by their
own elasticity, so as to
retain the object when
the pressure is withdrawn.
By sliding the wire stem
which bears the forceps
through its socket, and by
moving that socket verti-
cally upon its joint, and
the joint horizontally
upon the pin, the object
may be brought into the
field precisely in the posi-
tion required ; and it may
be turned round and
round, so that all sides of
it may be examined, by
simply giving a twisting
movement to the wire
stem. The other ex-
tremity of the stem often bears a small brass box filled with cork,
and perforated with holes in its side, seen in fig. 243 ; 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 &xs. may be attached,
whereon objects are mounted for being viewed with the Lieberkuhn.
This method of mounting was formerly much in vogue, but has been
less employed of late, since the Lieberkuhn has unfortunately fallen
into comparative disuse. The forceps in fig. 244 is also often of
great practical value, and is adjusted for holding by a screw.
That which is known as the stage-vice, for the purpose of holding
small hard bodies, such as minerals, apt to be jerked out by the
angular motion of the blades of the forceps, or very delicate substances
that will not bear rough compression, is very useful, and

Fig. 244.

Three-pronged forceps, screw adjustment.

Fig. 245.— The stage-vice.

is seen

in fig. 245.

The stage-vice fits into a plate, as is the case with Beck's

288

ACCESSORY APPARATUS

disc-bolder, fig. 246, 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 perma-
nently attached to discs, no means is comparable to the disc-holder
of Mr. R. Beck (fig. 246) in regard to the facility it affords for pre-
senting them in every variety of position. The object being attached
by gum (having a small quantity of glycerine mixed with it) or by
gold size to the surface of a small blackened metallic disc, this is
fitted by a short stem projecting from its under surface into a
cylindrical holder ; and the holder carrying the disc can be made to
rotate around a vertical axis by turning the milled head on the
right, which acts on it by means of a small chain that works through
the horizontal tubular stem ; whilst it can be made to incline to one
side or to the other, until its plane becomes vertical, by turning the
whole movement on the horizontal axis of its cylindrical 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

away, by inserting their
stems into a plate per-
forated with holes. Several
such plates, with inter-
vening guards to prevent
them from coming into too
close apposition, may be
packed into a small box.
To the value of this little
piece of apparatus the Author can bear the strongest testimony from
his own experience, having found his study of the Foraminifera
greatly facilitated by it.

Glass Stage-plate. — Every microscope should be furnished with a
piece of plate glass, about 3^ in. by 2 in., to one margin of which a
narrow strip of glass is cemented, so as to form a ledge. This is
extremely useful, both for laying objects upon (the ledge preventing
them — together with their covers, if used — from sliding down when
the microscope is inclined), and for preserving the stage from injury
by the spilling of sea-water or other saline or corrosive liquids, when
such are in use. Such a plate not only serves for the examination
of transparent, but also of opaque objects ; for if the condensing
lens be 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
minute aquatic organisms, and of ' cultivating ' such as develop and

1 A small pair of forceps adapted to take up minute objects may be fitted into-
the cylindrical bolder in place of a disc.

Fig. 246.— Beck's disc-bolder.

GROWING SLIDES

289

multiply themselves in particular fluids. One of the simplest and
most effective, that of Mr. Botterill, represented in fig. 1^17, con i t ,
of a slip of ebonite, three inches by one, with a central aperture of
three-fourths of an inch at its under side ; this aperture is reduced
by a projecting shoulder, whereon is cemented a disc of thin glass,
which thus forms the bottom of a cell hollowed in the thickness of
the ebonite slide. On each side of this central cell a small lateral cell
communicating with it, and about a fourth of an inch in diameter, is
drilled out to the same depth ; this serves for the reception of a supply

Fig. 247.

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 off from the others — either by the use, from time to
time, of the small glass syringe to be hereafter described, or by
threads so arranged as to produce a continuous drip into one and
from the other — a constantly renewed supply is furnished to the
central cell, which it
enters on one side, C
and leaves on the
other, by capillary at-
traction.

Dr. Lewis' and
Dr. Maddox's growing
slide is shown in fig.
248. Two semicircles L
of asphalte varnish Fig. 24s.

are brushed on the

slide, one being rather larger than the other, so that the ends of one
half-circle might overlap 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 was placed in the centre, upon which a few sporea
were sown, a cover-glass being placed over it, which adhered to the
semi-dried varnish. The slide was placed under a bell-glass, kept
damp by being lined with moist blotting-paper.

Dr. Maddoxs growing slide will be understood from the annexed
sketch, fig. 249. The shaded parts are pieces of tinfoil fastened with

u

290

ACCESSORY APPAEATUS

shellac glue to a -lass slide. The minute fungi or spores to be
crown are placed on a glass cover large enough to cover the tmtoil,
with a droplet of the fluid required. This, after examination to see
that no extraneous matter is introduced, is placed over the tinfoil,
mid the edges fastened with wax softened with oil, leaving free the
spaces, X X, for entrance of air. Growing slides of this description
could be made cheaply with thin glass instead of tinfoil

Dallinger and Drysdale s Moxst
Stage for Continuous Observations.
It is needful in working out the
life histories of minute forms to
be able to keep the organisms in a
normal and undisturbed condition
for sometimes weeks at a time :
only a small drop of fluid containing
the organism can be under observa-
tion, and this, without proper pro-
vision, 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
a plain glass stage, fig. 250, 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.250, 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 circular

Fig. '249. — Maddox's growing stage.

Fig. 250. — Dallinger and Drysdale's moist continuous growing stage.

aperture, b, is cut through it, and a thin piece of good glass, c, d,c, j\
is fixed over the under surface of it with Canada balsam : this may
be as thin as the condenser may require. At the end of the arm
«, which extends some distance beyond the stage to the right of the
reader, but when the arrangement is set up on the microscope to the
left of the operator, a brass socket with a ring attached is fixed with
marine glue. It is marked in the drawing g, g, g. The object of
this ring is to hold a glass vessel, fig. 251, about 1| or 2 inches

(GROWING STAGE K<>K CONTINUOUS WORK

291

Fig. 251.

deep. It simply drops in, and the top, «, being slightly larger than
the ring, ff, tig- 250, it is prevented from slipping through.

Let us suppose the stage to be in its position on the microscope,
and the vessel, rig. 251, inserted in this manner into fig. 250. A
piece of good, new linen is now cut to the shape drawn in fi«\ 254
the part a being long enough to reach to the end of the glass stage
and then at b bent over, leaving the part in the
vessel, rig. 251, which is inserted into g, fig. 250.
Its position is indicated in rig. 250, by the dotted
lines, %h, It, &C4 But before it is laid upon the
stage a circular aperture, d, rig. 254 is cut out, Which
must be much larger in diameter than the covering
glass which it is intended to use. We therefore
employ small covers.

The glass with the flap of linen in it is now rilled
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, rig.
250, and the covering glass, i, must be laid on. It will be seen that
there is a broad, clear space between the covering glass and the linen.
We now 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 con-
tinual renewal, than it can be from the film of
fluid.

Indeed, the moisture in the chamber is so
great under favourable circumstances that it Fig. 25-2.

rather increases than allows a diminution of

the film of fluid. The manner in which we effect this is simple. A
piece of glass tubing, about 1^ inch in diameter, is cut to about f of
an inch in length. At one end of this a piece of thin sheet caou-
tchouc is firmly stretched, and a small hole is made in its centre.
Fig. 252 gives a drawing of it ; a is the piece of glass tubing, h is
the stretched elastic film, which is securely tied on by means of a
groove in the glass at d, and e is the aperture. The bottom edge,
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 covering glass. The object-glass is now
racked down through the small hide, c, fig. 252, and adjusted to focus.
The caoutchouc should be thin enough to afford no impediment to
the action of the fine adjustment, when it will be seen that it clasps
the object-glass by its elasticity at the aperture ; and the gentile
pressure forces the under edge of the chamber upon the linen, so
that little or no air is admitted, while if the under edge of the
-chamber be carefully ground it will suffer the stage, linen and all,
to move under it when the milled heads for working the mechanical
=stage are in action. ,

u a

292

ACCESSORY APPARATUS

A drawing of the apparatus in working order is given in perpen-
dicular section at fig. 253. The parts a, a in this figure represent
the glass stage corresponding to a, a, fig. 250 ; b in both figures
stands for the round aperture in the thick glass ; b, in fig. 253, cor
responds to the thin glass which covers this aperture, marked c, d,

e, f in fig. 250, but in the
form of this device now used
by the Editor the thin fflass-
floor is cemented to the
bottom of the plate glass, a, a,
thus making a cell equal to the
thickness of the whole staoe.
The linen is marked in dotted
lines in both figures : d, fio-.
253, represents the covering-
glass, i, in fig. 250 ; e,e, fig.
253, is the piece of glass
tubing, shown in fig. 252;/',
tig. 253, is the stretched caou-
tchouc seen at b in fig. 252, with the object-glass g3 penetrating
and tightly filling up the aperture c in the figure, thus forming the
moist chamber, ch, ch, by enclosing parts h, It. fig. 253, 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. 253, sunk as a cell,
by the attachment of the thin glass floor to the under side of the
stage, as described above, this annular flap of linen overhangs, but
does not lie upon, the floor on which the drop of fluid with its living
inhabitants is placed. This is a great security against accidental1
flooding.

It will be seen that the instrument must be horizontal ; but there
is no inconvenience arising from this it' it be placed on a sufficiently
low support, and it will be found in practice that it may be worked!

Fig. 253.

Fk.. -j:,4.

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. Bollinger's Thermostatic Stage for Continuous Observations:
at High Temperatures. — It frequently happens that either for the pur-
pose of experiment, or the study of special organisms, the student

WARM CONTINUOUS MOIST STAGE

293

needs a similar continuous stage to the above, but one in which
varying temperatures may be obtained and kept at any point st.it i,-
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. 255. At A, << b are two grooved pieces of solid
metal which permit the stage to slide on to the stage of an ordinary
microscope, and partake of the mechanical movements effected by
the milled heads ; B is a vessel for water with a thermometer <t,
of sufficient delicacy for indicating the temperature ; b is a mer-
curial regulator, carefully made, but of the usual pattern ; c brings

Fi<;. >2,-)5.

'the gas from the main ; d conveys as much of the gas as is allowed
1o escape from between the top of the mercury, and the bottom of
the gas delivery tube to the burner e. The regulation of this
apparatus so as to obtain a static temperature, as is well known, is
a matter of detail depending chiefly on the careful use of the mer-
•curial screw-plug f, and the height and intensity of the burner e.
A temperature quite as accurate as is needed can be obtained for
the purpose required.

The stage A is placed in position on the instrument, and two
openings in this hollow stage at cd (A) are connected with two
similar openings in the water vessel, viz. g It (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 be seen a small cylinder of glass;

294

ACCESSORY APPARATUS

this is ground at the end placed on the stage, and covered with a
sort of drumhead of india-rubber at the upper end. By examining
C with a lens it will be seen that a cell is counter-sunk into the-
upper plate of the hollow stage at el', 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
under surfaces of this glass aperture. A glass cup is placed in the
jacketed receptacle / (A and C), and this also is filled with water.
A piece of linen is now laid on the stage (A, g) with an aperture cut in
its centre slightly less than the counter-sunk cell in which the glass-
disc e" is fixed, and a flap from it is allowed to fall over into the glass
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 annulus of the linen is enclosed within
the cylinder. The drop of fluid to be examined is placed on the small!
circular glass plate and covered with the thinnest glass, the drum-
head cylinder is placed in position, the point of a high-power lens,
is gently forced upon the top of the india-rubber through a small
aperture, thus forcing the lower ground surface of the cylinder upon
the linen, and making the space within the closed cylinder practi-
cally air-tight, but still admitting of capillary action in the linen..
Thus the enclosed air becomes saturated.

By complete circulation the water in the vessel e (A) is but
slightly below that within the jacket of the stage, and thus the-
vapour as well as the stage arc 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 "Boy. Micro. Soc.''
vol. vii. ser. ii. pp. 299-316.

The Live-box and Com-
pressors. What is now so-
well known even to the tiro-
as the ' live-box ' was origin-
ally devised by Tully, and it
was afterwards improved bv
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
Fig. 25G. it into the top of the tube,

where it formed the floor of
this 'animalcule cage': this prevented the draining oft' of the
water at the edge by capillary attraction. But in that form a

LIVE-BOXES AND COMPRESSORS

295

condenser cannot be used successfully with it, and therefore a dark
ground cannot be employed. But as it is Rotiferse and Infusoria
generally that constitute the raison d'Stre 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 lost much of its value.

The whole piece of apparatus is seen at A, tig. 2T>6. A section of
it is seen at B. A modification was subsequently made by Swift,
which is seen at C, where the glass floor-plate is seen to be burnished
into the brass plate, and the cover-glass is made to slide down upon
it on %a tube. But in this still greater difficulty arose, when the
focus of the lens was shorter than the length of the tube ; then,
indeed, the motion of the * box ' was limited to the difference in
diameter between the cell in which the glasses were sunk and the
object-glass — in fact only low powers could be employed with it.

Mr. Rousselet has overcome this difficulty by a device which is
shown in fig. 257.

In this the glass plate bevelled for the floor is somewhat reduced
in diameter, but the outer ring is enlarged sufficiently to allow any
high power to focus to the very edge of this glass floor. An object
lying anywhere over the floor can be reached by the condenser from

Fig. 257.

below, and by both high and low powers from above, and when well
made it acts well as a compressor. A drop of water so small that
a rotifer maybe unable to swim out of the field of -view of a ]-
inch objective can be readily arranged with it ; and a little practice
enables the operator to employ it for many useful purposes in the
study of ' pond life.'

The compressor or compressorium is a more elaborate device,
somewhat after the same kind, but arranged to give the operator
more accurate control over the amount of pressure to which the
object is subjected. Mr. Rousselet has constructed one of very
efficient form. 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 glass< s, 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.

An extremely handy r< u< rsible compressor is seen in fig. 258 : it is
the device of Mr. W. Rowland. It is made of german silver, and

296

ACCESSORY APPARATUS

the rings have two glass covers cemented to them ; the lower plate
is} attached to a rod turning in a pocket, while the upper one pivots
on a milled head which clamps if required or releases it when
required for more easy cleaning. Varying pressures of the cover-
glasses are obtained by turning the milled head in the centre of the
plate, and the socket fits in the stage in the same position as the
stage forceps <kc.

It is sometimes required, in very delicate work on very minute

Fig. 258. — Mr. Rowland's reversible compressor.

organisms, to compress them ver}T slowly and with great care while
we are examining them with high powers. The cover-glass is of
necessity thin, and the pressure from the minute dimensions of the
object must be very considerable in order to act upon the organism at
all. For this purpose we have found a somewhat heavy compressor
made by Powell and Lealand of great value. A strong brass plate
forms its base, and a lever arm carrying the cover-glass of the
compressor is raised and lowered witli a tine screw. The cover-glass
may be cemented on to two or three separate fronts which screw
on,^and may carry cover-glass of different thicknesses. The cover,
to prevent flexure, is small, but will work easily with any of the
modern high-power lenses. The whole body of the ring to which
the cover-glass is attached is very solid and strong. A glass slip of

Fig. 259. — Professor Delay 's parallel compressor.

the ordinary size slips in, and is clamped by a spring on the base,
and upon this the cover-glass is brought down in compression.
An object the 5000th of an inch in thickness can be readily
held and compressed by its means, but the fronts must be
turned with great care, and the cover-glasses used for cementing on
must be much larger than the aperture in the ring, so that a large
surface may be cemented.

HOW TO EMPLOY THE ZOOPHYTE TROUGH 29

Recently Professor Y. Delate has devised a very admirable form of
-compressor for the most delicate observations (figs. 2o9, 260) in which
-the pressure is effected by the action of a screw on an inclined plane
A, and working against the spring R. When the screw is turned
on one side, the upper part of the compressor can be raised on the

Fig. 260. — Professor Delage's parallel compressor.

pivots B B', as shown in fig. 260. The frame holding the upper
plate has a gimbal motion on the pivot D (and the corresponding
•one on the opposite side), and the frame can be detached by press-
ing the pin C and the corresponding one on the opposite side,
causing the frame-holder to spring open slightly. The two glasses
being oblong and lying crossed, it is easy to add a drop of liquid
during compression. The compressor can be reversed, and in that
•case rests on the three small pillars, which are high enough to allow
the milled head of the screw to clear the stage.

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. 261, being formed of slips
of glass, and has a loose horizon-
tal plate of glass equal to the
inside length of the trough, so
that it may be moved freely
Avithin 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 edfte touching the inside

of the front glass, a small ivory pIG< 261.

wedge is inserted between the

front glass of the trough and the upper part of the loose ver-
tical plate, which it serves to press backwards ; but this pressure

298

ACCESSORY APPARATUS

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
movin* 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 evident 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 convenience for observation.

By very little contrivance these troughs with their contents may-
be kept, when not under examination, in much larger aquaria,
obtaining the advantage of aeration and coolness.

Mr. Botterill devised a trough which is made of two plates of
vulcanite or metal which screw together, and between them are two
plates of glass, of the proper size, of any desired thickness, kept
apart by half a ring of vulcanised indiarubber, the whole being-
screwed tightly enough together by three milled heads to prevent
leakage. But leakage or the fracture of glasses is not uncommon
with this otherwise convenient form.

An excellent, though shallow, trough was made by Mr. C. G..
Dunning, which we illustrate in tig. 262. The lower plate or trough
proper is made of metal, 3 inches long by H wide and about TL
thick, with an oval or oblong perforation in the centre, and the-
under side is recessed, as shown in fig. 262, 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 pre-
vent 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 (5g. 262,0)
is formed of a piece of
tli in brass, rather shorter
than the trough, but
about the same width ;
it lias 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 cornel's, and at the two top corners are formed a
couple of projecting ears. In order to use this apparatus it must be
laid fiat upon the table, and filled quite full of water. The object

1 Watch- spring or other elastic metal should not be used on account, of oxidation.

Fig. 2(52.

EXAMINING INFUSi >KIA

299

to be examined is then placed in the cell, and may be properly
arranged therein ; the cover is then lowered gently down, the two
notches at the bottom edges being lirst placed against the pins : in tin's
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 im-
portant qualities : first, it does not leak ; second, it is not readily
broken without gross carelessness. The shallowness maybe over
come by placing an ebonite plate with the required aperture bet ween
the two mounted glasses.

Infusoria, minute Algse, &c. however, can be 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 4 cells ' hereafter to
be described, and as, when the cells are tilled with fluid, their glass
covers will adhere by capillary attraction, provided the superfluous
moisture that surrounds their edges be removed by blotting-paper,
they will remain in place when the microscope is inclined. An
annular cell, that may be used either as a 'live-box ; or as a 'grow -
ing slide,' has lately been devised by Mr. Weber (U.S.A.). It is a
slip of plate-glass, of the usual size and ordinary thickness, out of
which a circular 'cell' of ^-inch diameter is ground, in such a
manner that its bottom is convex instead of concave, its shallowest
part being in the centre and the deepest round the margin. A
small drop of the fluid to be examined being placed upon the central,
convexity (the highest part of which should be almost flush with the
general 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-on 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 be maintained for some days, or,
if the two should be balanced (as in an aquarium), for some week .

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 tig. 263,
but of somewhat larger dimensions. These were formerly designated
as 'fishing tubes,' the purpose for which they were originally de-
vised having been the fishing out of water-fleas, aquatic insect-
larva3, 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

300

ACCESSOEY APPARATUS

A

B C

1

into a large glass cell.

applicable, of course, to the selection of minute plants ; and they
may be turned to many other no less useful purposes, some of which
will be specified hereafter. When it is desired to secure an object
which can be seen either with the eye alone or with a magnifying
glass, one of these tubes is passed down into the liquid, its upper
orifice having been previously closed by the forefinger, until its lower
•orifice is immediately above the object ; the finger being then re-
moved, the liquid suddenly rises into the tube, prob-
ably carrying the object up with it ; and if this is
seen to be the case, by putting the linger again on
the top of the tube, its contents remain in it when
the tube is lifted out, and may be deposited on -a slip
of glass, or on the lower disc of the aquatic box, or,
if too copious for either receptacle, may be discharged

In thus fishing in jars for
any but minute objects, it will be
generally found convenient to employ
the open-mouthed tube C ; those with
smaller orifices, A, B, being employed
for ' fishing ' for animalcules etc. in
small bottles or tubes, or for selecting
minute objects from the cell into which
the water taken up by the tube C has
been discharged. It will be found very
convenient to have the tops of these
last blown into small funnels, which
shall be covered with thin sheet india-
rubber, or topped with indiarubber
nipples, which by compression and ex
pansion can then be
the greatest nicety.

In dealing with minute aquatic
objects, and in a great variety of other
manipulations, a small glass syringe of
the pattern represented in fig. 264, and
of about double the dimensions, will be
found extremely convenient. When
this is firmly held between the fore and
GlaBS middle lingers, and the thumb is in-
serted into the ring at the summit of
the piston-rod, such complete command
ris 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 t he simple microscope,
if necessary) from amongst a number in the same drop, and trans-
ferred to a separate slip. A set of such syringes, with points drawn
to different degrees of fineness, and bent to different curvatures,
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

regulated with

u

Pig. 268.— Dip-
ping tubes.

Fig. 264

syringe

PROTECTION FOR THE COARSE ADJUSTMENT

30I

that if a dipping tube with a glass bulb have an indiarubber hollow
ball or teat attached to the top of it, it will act, for the majority of
purposes, as well as a syringe.

Forceps. — Another instrument so indispensable tothemicroscopist
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 convenient is represented in fig. 265
of something less than the actual size. As the forceps, in marine re-
searches, have continually to be plunged into sea-water, it is better
that they should be made of brass or of german silver than of steel,
since the latter rusts far more readily ; and as they are not intended
(like dissecting forceps) to take a firm grasp of the object, but merely
to hold it, they may be made very light, and their spring portion

r Fig. 2G5.

slender. As it is essential, 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. 266 : this ring has two pins and
a screw projecting inwards. When the screw
is withdrawn the rings can be slipped over
the milled heads of the coarse adjustment,
and by screwing the small screw 1 home ' the
ring cannot be withdrawn ; but as they are
loose upon the milled heads, the latter can-
not 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 bv this firm ; and Messrs. Beck
have made these rings with slight modifica-
tions more recently. They are the most Fll. 266.— Powell and Lea-
efficient means of counteracting the danger land s protecting ring for
incident on public exhibition of delicate course adjustment,
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 co afford
facilities for the preparation and mounting of objects will be de-
scribed in a future chapter (Chapter VI.).

302

CHAPTER V

OBJECTIVES, EYE-PIECES, THE APEBTOMETEB .

| i is manifest that everything in the form and construction as well
as in fche 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- achromatic lenses, and reflecting
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 modem 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. May all discovered a reference to this
effort to make achromatic lenses, and through the courtesy of the
President of the Athenaeum of Brescia discovered that Marzoli was
^an amateur optician, and 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
preserved,' and was generously presented in 1890 to the Royal Micro-
scopical Society of London. With it was forwarded the 4 Processo
Verbale,' 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 Hint presented to the object ; and if this was a part of
the intended construction, of which there appears small room for
•doubt, Marzoli preceded Chevalier in this, as we shall subsequently
■see, very practical improvement.

1 Journ. Boy. Mic. Soc. 1890, p. 420.

MAK/OLl'S ACHROMATIC OJUECTIVE

303

It has been, however, customary to accredit the first practicable
attempts to achromatise object-glasses to M. Selligues. In this
optician was the first known in France to superimpose two, three,
•or four achromatised plano-convex * doublets,' that is to say, pairs
of lenses. These objectives had their convex surfaces presented to
the object, which gave them four times as much spherical aberration
as would have been the case had their positions been reversed,1 and,
as we have just seen, Marzoli reversed them. This necessitated an
excessive reduction of the apertures, which, nevertheless, still too
manifestly displayed an obtrusive aberration. Still the conception
of an achromatised combination had been embodied in an initial
manner. In 1825 M. Chevalier perceived the exact nature of the
mistake made by M. Selligues, and made the lenses of less focal
length and more achromatic, and inverted them, placing the plane
side of the Hint towards the object.

It is somewhat important, as it is interesting, to note that the
:idea of the superposition of a combination of lenses did not originate
from theoretical considerations of the optical principles involved.
It is scarcely conceivable that where there was manifest ignorance
of the position of a plano-convex lens for least spherical aberration
(a principle now thoroughly understood) there could have been in-
sight enough either to detect the presence of the two aplanatic 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 prob-
ability, applies to Marzoli's work as to that of Chevalier.

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. Tullv, in this country,
without any knowledge of what was being done on the Continent,
made an achromatic objective in 182 i. This was a single combina-
tion, being an achromatic uncemented triplet. It was, in fact, a
miniature telescope object-glass, and is illus-
trated in fig. '2i') 7. Two lenses made on this
principle by Tullv, having j4)T and fa 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 FlG 267.— TuUVa achro-
•equal to that of the matic triples.

Subsequently a doublet was placed in front
of a similar triplet of somewhat shorter focus, forming a double
combination objective of 38° aperture. This was pronounced to be
a great advance upon all preceding combinations, even those which
had been produced upon the Continent.

A note of Lister's at this time upon the objectives of Chevalier
is of interest. He found them much stopped down, and, in one

1 Chapter I.

304

OBJECTIVES, EYE -PIECES, THE APERTOMETER

instance, ho opened the stop and improved the effect. Lister says :
'The French optician knows nothing of the value of aperture, but
he has show n us that fine performance is not confined to triple ob-
jectives ; 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 penetrating 1

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.
l/t 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,
1827, and brought with him specimens of his
work, which produced a most favourable im-
pression, and subsequently he made an objective
of ^-inch focus.

Meantime, in this country, Mr. Lister brought
about an important epoch in the evolution of the
achromatic object-glass by the discovery 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 aber-
rations of one doublet may be neutralised by a
second.

As the basis of a microscopic 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 aberration 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. 268, 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, f, d, e, g, issuing
from the radiant point, f, h e being a perpendicular to the convex

1 ' Penetrating ' meant ' resolving ' power in those days ; he alludes, therefore, to
increase of aperture.

Fig. 268. — The two
aplanatic foci of an
optical combination.

EFFECTS OF LISTER'S DISCOVERY ON ACHROMATISM 305

surface, and i d to the plane one — under these circumstances tin-
angle of ('mergence, y e /1, much exceeds that of incidence, f 'I i,
being probably almost three times as great.

If the radiant is now made to approach the glass, SO that the
course of the ray, f tie g, shall be more divergent from the axis, as
the angles of incidence and emergence become more nearly equal
to each other, the spherical aberration produced by the two will be
found to bear a less proportion to the opposing error of the
single correcting curve acb ; 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 diver-
gence of the ray, till it will exceed that of emergence, which has in
the meanwhile been diminishing, and at length the spherical error
produced by them will recover its original proportion to the oppo-
site error of the curve of correction. When f has reached this point
f" (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 f 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 sonic
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, Lister taught himself
the art of lens-grinding, and he made an objective whose front was
a meniscus pair, with a triple middle combination, and the back a
plano-convex doublet. He declared this to be the best lens of its
immediate time, and it had a 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.

Andreto Ross began their manufacture in 1831. He was followed
by Hugh Powell in 183-4, and in 1839 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

x

306 OBJECTIVES, EYE-PIECES, THE APEBTOMETEK

English makers, and undoubtedly carried the palm both here andom
the Continent for the excellence of his objectives. <<

1 inch L4° two doublets, 1832
L8° single triple, 1833.
55° three pairs, 1834. -
60° „ „ T.18S6.

||o ^ triple front and two double backs ^xg^^j- Lister's formula.

I

l

4
t

10
1

|

44°
63°
74°

5>

, 1842.

Examples of these old lenses are extant and in perfect preserva-
tion,, 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 i-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. 269.
The foci of these three pairs are in the proportion of
1:2:3. In 1837 this maker had so completely
■pic 269 —A 1 in eorrected the errors of spherical and chromatic aber-
combiiiation4 by ration that the circumstance of covering an object
Andrew Boss. with a plate of the thinnest glass was found to dis-
turb the corrections ; that is to say, the correc-
tions were so relatively perfect that if the combination were adapted
to an uncovered object, covering the object with the thinnest glass
introduced 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 Hoss
applied this correction by
mounting the front len&
of an objective in a tube
which slid over another
tube carrying the two
other pairs. There was
a small guide-pin in an
L-shaped slot to limit the
amount of movement ; for
uncovered objects, the front
combination was drawn
out and the pin was turned
into the foot L ; and for
covered objects the combinations were closed together to their limit.
Subsequently this arrangement was modified by the introduction

1 Vide Chapter I.

Uncovered.
Covered.

Fig. 270. — Section of adjusting object-glass.

the modern use of collar coklkction

307

q£, a screw arrangement, as in fig. _!70. The front pair of leu ea i
fixed into a tube (A) which slides over an interior tube (I>) by which
the other two pairs are held ; and it is drawn up or down by means
of a collar (C), which works in a furrow cut in the inner tube, and
upon a screw-thread cut in the outer, so that its revolution in the
plane to which it is fixed by the one tube gives a vertical movement
to the other. In one part of the outer tube an oblong 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
engVaved, 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 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 f covered,' it indicates that the front lens
has been brought into such proximity with the other two as to
produce an ' under correction ' in the objective, fitted to neutralise
the ' over- correction ' produced by the interposition of a glass cover
of extremest thickness.

This method of collar correction served the purposes of micro-
scopy for upwards of thirty years, but when more critical investiga-
tions were undertaken and objectives had more aperture given to them
it was found that the method had two
great faults.

The first was that the * covered ' and
1 uncovered ' marks were too crude. To
remedy this, the screw collar was
graduated into fifty divisions so that
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 correcting
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 object in the arrangement
as shown in fig. 270a enables the
front lens to maintain a fixed position,
while the correctional collar acts on the
posterior combinations only.

On the Continent it has been the
practice to graduate the correctional collar in terms of the thic kness
of the cover-glass in decimals of a millimetre. Thus if a cover-glass
& x 2

Fig. 270a. — Present collar
correction.

308 objectives, eye-pieces, the apektometek

be 0-18 mm. thick, the correctional collar should be set to the
division marked 0*18.

I ii England, on the contrary, the divisions are entirely empirical, so
that the operator has to discover for himself the proper adjustment.
It is not to be supposed, however, that the English method is un-
scientific, for when an operator becomes expert he would never for
an instant think of adjusting by any other indication 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 surrender 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 dis-
turbed, 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 practical
optics at that time.

This subject of under- and over-correction is one of large import-
ance^ and it may be well at this point to enable the tiro 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 peripheral
portion of the lens will be found by experiment with a card to be
brought to a focus at a point on the axis nearer the lens than those
passing through the centre. This is under-correction. 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, it will be sur-
rounded by a coma, and even the portion of the flame which is in
focus will lack brightness. But with the convex side towards the
flame it will be found that in the image on the card the coma is
greatly reduced, and the image of the flame brightened. The rea-
son for this is, as already stated, that the spherical aberration is
four times as great when the convex side of the lens is towards the
card.

The practice of these simple tests will be most instructive to
those unfamiliar with the optical principles on which an objective is
constructed. They make plain that an over-corrected lens is one
which brings its peripheral rays to a longer focus than its central.
But a cover-glass produces over- correction, therefore the means
employed to neutralise the error is by the under- correction of the
objective. If, however, the objective employed should be unpro-

MICROSCOPES BY THREE LEADING MAKERS IN 1811 309

vided 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 1S.°>7, we find thai n
further improvement was made by Lister, who employed a triple
front combination. This consisted of two crown plano-convexes 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. 271, which is drawn
from an early -^-inch objective by Andrew Ross,
having bayonet-catch correction adjustment. In
1842 a -J-inch of 44°, a ^-inch of 63°, and a {--inch
of 74° were made upon the same lines.

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, represent- T-ii?7 combination
ing with moral certainty the very best work of by A. Ros 3.
the several makers ; they are consequently valu-
able as reliable specimens of the best work of the period.

The objectives supplied by James Smith have the peculiarity of
being separating lenses.

The lowest power is about l^-inch focus. 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 T^-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
with a full cone, and the field is much curved.

There is also a separating 1^-inch and J|-inch which is good,
while the -f^-inch and the ^-inch may be considered fair.

The lenses supplied by Andrew Ross are a good 2 -inch and a
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 }, \, -|,
yVinch fairly good. The apertures of the -J- and the ^.-inch are
of course very low.

On the whole it may be said that the corrections are well
balanced in the lower lenses, and the apertures moderate ; but when
we come to the higher powers it is the deficiency of aperture that
becomes so oppressively apparent. In 1844 Amici made a 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 Hint succeeded

1 Under-correction is also known as ' positive aberration* ; over-correction as
' negative aberration.'

<3IQ : OBJECTIVES, EYE-PIECES', THE ' APEKTOMETEft' I

in extending the aperture of a ^-inch' objective to 8'5°, or -68
N.A. ; and a /.-inch objective to 135°, or -93 K.A. Of this
■latter it was affirmed that it was ' the largest angular pencil that
could bo passed through a microscope object-glass.'

In LSfiO object-g lassos were made with a triple back combination ;
these were attributed to Lister ; but it is also affirmed that they
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.
272, 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

Fig. 272. A triple- middle, and triple back combinations. It may be

back combina- noticed that Tully's objective had a triple back,
f1? Amid 9)1Stel* ^ut it was not the 1 result of intended construction ;

,'. it was a fortunate combination the real value of
which was neither understood nor appreciated, and as a consequence
its existence was evanescent.

In this same year Wenham produced another modification of the
achromatic objective of considerable value, but more to the manu-
facturer than the user of the microscope. It consisted of a single
front ; the combination is seen in fig. 273, which it will be seen is a
simpler construction, but this did not affect in the least the price
of the objectives produced. Subsequently, how-
ever, the form was adopted on the Continent for
low-priced objectives, which led to a reduction of
the cost of English objectives of the same con-
struction.

Manifestly, the single front lessened the risk
of technical errors, but we have never been able
yet to find a single front dry achromatic objective
Fig. 273.— A single- which has shown any superiority over a similar

front combination „ • i • i /■ ■

by Wenham one possessing a triple iront.

The single front employed with two combina-
tions at the back was the form in which the celebrated ivater-
immersion objectives of Powell and Lealand were made. It was by
one of these that the strise on Amphipleura pellucida 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 ; but it was an advance the optical principles of which
were certainly not at the time understood.

In Paris, Prazmowski and Hartnack brought these objectives to
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-

THE IN1-LI KXCE OF THE DIFFRACTION THEORY } I j

immersion system touched its highest point, apertures as great as
1 23 were reached ; and in America, Spencer, Tolles, and Wales
produced some extremely fine lenses of large aperture.

During the year 1869 Wenham experimented with and sug-
gested 1 the employment of a duplex front ; that is so 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. 274, 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 im-
mersion of 110° balsam angle, which passed
into the possession of the Army Medical ' "b^Wen?
Museum at Washington. There can be little ham 1869.
doubt but this objective would have produced

■a much deeper impression but for the fact that it was in advance of
its immediate time.

Tolles, as we have hinted above, used the duplex front in the
construction of some of his immersion objectives, and was followed
in this by the best English makers, and in the case of a celebrated
-^-inch, purchased by Mr. Crisp, Tolles was able to reach a balsam
.angle of 96°.

At the time that the water-immersion lenses were being con-
structed by rival opticians with increasing perfection the great
theory of Professor Abbe concerning microscopic vision, the import-
ance of diffraction spectra, and the relation of aperture to power 1
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 T>Vinch> was n°t uncommon to
find apertures less than 1*2, while objectives of J^, jnr, 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.

This 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 powrer with a large aperture.

Thus a ^-inch of 0'65 N.A. will be far more expensive and prob-
ably not as well corrected as ]t of 0*7 N.A. The i-inch objective,

1 Monthly Micro. Journ. vol. i. p. 172.

312 OBJECTIVES, EYE-PIECES, THE APEKTOMETEK

even if a good one, is sure to exhibit spherical aberration, while
the I of low aperture will show many minute objects with con-
siderable 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
construction 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 were
soon manifest; for in 1878 the homogeneous system of immersion
objectives2 was introduced as a logical outcome of the diffraction,
theory of microscopic vision. A formula for a ^-inch objective on
this system was prepared by Abbe, to whom, we learn from himself,
it had been suggested by Mr. J. W. Stephenson, of the Royal
Microscopical Society.3 It has been already shown 4 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 TL-inch, and subsequently in the same year a i-inch
objective, each with a duplex front to work in soft balsam, and with
a 1ST. A. of 1-27. These objectives were examined by the late Dr.

5 tide Chapter II. 2 j^.

5 P. 27; also Joum. Boy. Microsc. Soc. vol. ii. 1879, p. 257. 4 Chapter I.

THE EXCLUSION OF THE SECONDARY SPECTRUM 3 1 3

Woodward, of the Army Medical Department, New York, and with
that examination were allowed to drop. For Tulles as an original
deviser of a practical homogeneous system this was unfortunate ; for
the actual introduction of the system in a form capable of universal
application, and worked out in all its details in an entirely inde-
pendent manner, we are wholly indebted to Abbe.

The principle was not, nevertheless, so readily and warmly
adopted in England on its first introduction
as , might have been anticipated. This arose
partly, however, from the fact that water im-
mersions had been brought to so high a point
of excellence by Messrs. Powell and Lealand that
the early homogeneous objectives were not pos-
sessed of more aperture, and were not sensibly
superior to the best immersions made in England.

The homogeneous objectives were made with
duplex fronts and two double backs. A general
diagram of their mode of construction is given in Fl<?' 275.— Combina-

°atrK tion for homoge-

tlg. I/O. neous' immersion

So long as crown glass was employed in their by Abbe,
manufacture, and the anterior front lens was a
hemisphere, it appeared that N.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 N.A.

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 glass, 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
making the front of flint. By this means they obtained apertures
which have not as yet been equalled by any other makers, reaching
in a -J, a -j1^, and a a N.A. of 1*50, out of a theoretically possible
aperture of 1*52. Professor Abbe has since, it is true, made an
objective with a numerical aperture of 1*63, but this requires the
objects to be mounted and studied in a medium of corresponding
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.

We have already pointed out in detail 1 that it was the great
defect of the ordinary crown and flint achromatics that two colours
only could be combined and that the other colours caused out-of
focus images, which appealed 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 m>t possible in the
flint and crown achromatic to combine two colours in all the zones

1 CHaptei I.

314 OBJECTIVES, EYE-PIECES, THE APERTOMETER

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 dispersive
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 requirements 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-
lations of a most elaborate and exhaustive kind made by Dr. Abbe,
objectives are made by Zeiss which not only combine three parts of
the spectrum instead of two, as formerly, but which are also aplanatic
for two colours instead of for one. This higher stage of achromatism
Abbe has called apochromatism.

A general plan of the construction of an apochromatic objective
as made by Zeiss is shown in fig. 276, 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 chro-
matic 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
Pig 2 6 D' seen the optician can do in the manufacture of

apochr^attc^om- g^assj we mav hope that an equivalent of this
bination. mineral in all optical qualities may be discovered.

The medium for mounting and immersion
contact has, of course, to be of a corresponding refractive and
dispersive index in all objectives 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 qud non
in the case of the new objective just made by the house of Zeiss,
and a specimen of which has been generously given by the Firm
to the Royal Microscopical Society. This glass has a numerical
1 Chapter I. 2 chapter II.

COMPARATIVE ACTION OF A POCIIKOMAT1C & OTHER LENS IIS 315

<iperture of 1'63; it is too e;irly to criticise its qualities, but in
u subsequent chapter on the present state of our knowledge us to
the ultimute structure of diutorns, we ure enabled to present the
results of some of the photo-micrographs produced by its mean-.
But it may be noted thut very much will depend upon the N.A. of
the illuminating cone which cun be employed with it — not theoreti-
cally, but practically.

On the whole, and for the purposes of pructicul biological inves-
tigation, it is to the dry upochromatics 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 cleur under-
standing of the difference of action of chromutic, uchromutic, and
iipochromutic lenses, we venture to present a diagrammatic illustru-

/. 2. 5.

Fig. 277.

tion, which, while not strictly accurute, will curry with it no error,
us a populur illustration of this importunt subject.

In fig. 277, 1, 2, 3, we huve representutions, us truly us they cun
be drawn, of zones of equal light ; thut is to suy, the peripheral zone
will trunsmit un umount of light equul to thut given either by the
intermediute zone or the centrul circle. Let them therefore be
nulled equilucent zones.

If we ussign u numerical vulue for the visuul intensity of the
whole spectrum, suy 100, mude up of the following purts, viz.

Red . . . . . . . . .15

Orange yellow ....... 40

Yellow green 30

Blue . . . . . . . .15

then if in any one of the equilucent zones the whole spectrum is
brought to u focus, we shull huve for thut zone 100 us its effect ive value.

But the entire object-glass is divided, as in the diagram, into
"three equilucent zones ; consequently 300 will represent the vulue of
the whole lens, provided the whole of the spectrum is brought to the
sume focus.

By referring to the diugrums we see thut in a non-achromatic
lens (tig. 277, 3) we shall get only 40, because only one part of the
spectrum is brought to the focus in its intermediate zone ; and as
sphericul uberrution cuuses the light which pusses through the other

316 OBJECTIVES, EYE-PIECES, THE APEKTOMETER

zones to be brought to other foci, they for all practical purposes might
be stopped out.

In the achromatic lens we have (fig. 277, 1) in the intermediate
zone two parts of the spectrum combined, as 40 + 30 = 70, and one
in each of the other zones is also brought to the same focus, say 30
in the outer zone, and 40 in the centre circle. The result is that
the \\ hole achromatic lens gives a total of light on the principle stated
above of 30 + 70 + 40 = 140. In the apochromatic system, how-
ever (fig. 277, 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
without 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, whilst the vitreous compounds with which Abbe's
beautiful objectives are constructed are now accessible to all opticians,
and whilst without these Abbe's objectives could never have been
constructed, yet it does not by any means follow that because an
objective is made with the Abbe-Schott glass it is therefore apo-
chromatic ; the secondary spectrum must be removed, and the spherico-
chromatic aberration balanced, or it is ' apochromatic ' only by mis-
nomer. It is another feature of these objectives, which it is import-
ant to note, that they are so constructed that the upper focal 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 1 that in the wide-aperture objective
of high power there is an outstanding error which there are 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 powers in
order that the same over-corrected eye-pieces might be available for
use with them.

It appears worthy of note in this relation that one of the best
forms for the combination of three lenses is that 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-

1 Chapter II.

THE CHARACTER OF ZEISS'S APOCHEOMATIC8 317

ing the apochromatic lens construction (fig. 276) 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.

It is another distinctive feature of the 3 mm. objective that it
has a triplex front; thus Zeiss's 3 mm. (= 1-inch focus) has tin-
errors from three uncorrected lenses balanced by two triple backs,
i.e. nine lenses taken together.

The foci of the set of apochromatic lenses now made by Z< riss
are integral divisions of what may be termed a unit lens of 24 mm. ;
24 he chooses as a means of avoiding the inconveniences inseparable
from the use of the decimal system.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 -J inch, each has an aperture
of *3 ; a 12 mm. and 8 mm. = English I inch, and ^ inch, have
■each an aperture of '65 ; while a 6 mm. and a 4 mm. = i inch and
^ inch, have both an aperture of '95.

There are also water-immersions; a 2*5 mm. s inch, with
N.A., 1'25, and two oil-immersions respectively 3 mm. and 2 mm.
= ^ inch and rV inch, both being made either with 1*3 or
1-4 N.A.

Apart from these, intended to be used for photographic pur-
poses without an eye-piece, is a 70 mm. = a 3-inch, also a 35 mm.
or H-inch objective.

With the exception of the 6 mm., 4 mm., and 2*5 mm. objectives
which have the screw-collar adjustment, this series have rigid mounts,
correction being secured by alteration of the tube-length.

The performance of these lenses, as they are now made, is of the
"very highest order. They present to the most experienced eye unsur-
passed images. They are 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 designations as powers. The
^V-inch is such, and not a -^-inch designated -J-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 a ^-inch has an initial power of '2 1 .
But modern achromatics of fair aperture are always greatly in
excess of their designated power; ^ are nearly -l -inch. A ',-inch
over 40° has an initial power of 25, and is a ^.-inch ; inch
objectives are in reality ^-inch ; and {-inch objectives of 90° and
upwards have initial powers of 50 instead of 40, which they should
have, so that they are in reality Hhs ; some in fact — by uo means
uncommon — have an initial power of GO, and are actually £th-inch
objectives.

1 Although the foci of the lenses are expressed iu integers, with the single excep-
tion of the water immersion 2*5 mm., there are inconvenient decimal fractions in the
initial magnifying power of all the series except those of 2 5 and 2 mm. focus.

3 18 OBJECTIVES, EYE-PIECES, THE APERTOMETER

This is explicable enough from the maker's point of view ; it is
far easier to put power into an object-glass than aperture. It is
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
N.A. and 12 mm. of -65 N.A. may be considered as lenses of the
very highest order ; the relation of their aperture to their power is
such that everything which a keen and trained eye is capable of
taking cognisance of is resolved when the objective is yielding a
magnification equal to twelve times its initial power ; for this purpose
an objective must have 0*26 N.A. for each hundred diameters of
combined magnification. Under these conditions an object is seen
in the most perfect manner possible.

It may be well for the student to prove this, which may be
readily done.

Take a suitable object, such as a well-prepared proboscis of
a blow-fly, and examine it under critical illumination with the
24 mm. '3 N.A. (= 1 inch) objective, and a 12 compensating eye-
piece. Note with close attention every particular of the image :
the resolution of the points of the minute hairs, the form of the
edges of the cut suctorial tubes, the extent of the surface taken into
the ' field,' and the relation of all the parts to the whole.

Now change the objective for the 16 mm. *3 N.A. (= -f, but
with the same aperture). Nothing more is to be seen ; the most
dexterous manipulation cannot bring out a single fresh detail ; the
resolution is in no sense carried farther ; the cut suctorial tubes were
in fact, in our judgment, better seen with a lower power, while with
it all of course a smaller extent of the object occupies the ' field.'

It can in fact be scarcely doubted that the picture presented
by the § is a distinct retrogression in every sense compared with
that presented by the 1 inch when both are equally well made
and have equal apertures, viz. *3. But beyond all this, whatever
may be done by the 16 mm. *3 N.A. can be accomplished in an
equally satisfactory manner by removing the 12 eye-piece and re-
placing 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. (= -|-inch), and an A Zeiss
achromatic of -20 N.A. (== frds inch) using a 12 eye-piece. Those
who may still retain some conviction as to the value of ' low-angled
glasses to secure penetration ' can want no further evidence than
such a simple experiment affords of its entire fallacy.

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

HISTOLOGICAL ADVANTAGE OF LA KG F APFRTIKF 319

accepted statement that Lenrvangled glasses are the most suitable
for histological purposes. The assumption is founded on the fuel
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
cap be clearly seen. A sharp outline is nowhere manifest, because
the image of one cell is confused with the outlines of others upon
which it is superposed. We have seen that there is no perspective
proper in a microscopic image ; therefore it is better to use high
apertures in objectives, and obtain a clear view of one plane at one
time and train the mind to appreciate perspective by means of focal
adjustment.

It will be admitted that no clear idea of what an endothelium
cell is can be obtained from fig. 1 .

But fig. 8 (frontispiece) represents the same structure slightly
less magnified (x 138) 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 traced, and a sharp definition is given to
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 N.A. = §rds 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 N.A.),
the former image is found to be dim and ill-defined. The muscular
sheath is so ill-defined that it would not be noticed at all if it had
not been clearly revealed by the objective of wider aperture. But,
on the other hand, the objective of 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 1 pene-
trating power ' varies inversely as the numerical aperture, it also
varies inversely as the square of the power.

Now, from what we know of histological teaching in this country,
we do not hesitate to say that a histologist would not have attempted
to examine the above object with even a Zeiss A objective. He
would have advised the use of ' the ^-inch,' of perhaps -Go aperture ;
but by so doing he would have secured only one-third of the pene-
trating power quel aperture and one-seventh of the penetrating power
qud 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 power than is needful.
A ^-inch where a ^-inch would answer involves loss in many ways,

1 Chapter I.

320 OBJECTIVES, EYE-PIECES, THE APEKTOMETEK

and would never jbe resorted to if the aperture of the lenses employed
ivere as (jreat as the power used legitimately permitted. 1

A given structure to be seen at all must have a given aperture ;
to obtain this, as objectives now made for laboratory purposes run,
they are obliged to use too high a power. The result is that in seek-
ing to avoid what is accounted the loss of ' penetrating power ' at
an inverse ratio to the aperture, it is forgotten that we are losing it
inversely as the 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. -65 N.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 J-inch focus
of 80° (almost certainly a -fa 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
diatom — one indeed that was considered critical until that with the
apochromatic was seen. But in comparison it is dull and yellowish.
From which it follows that an exceptionally fine achromatic -^T-
inch of 60° or -5 N.A. will not suffer comparison of the image it yields
with that of an apochromatic ^-inch 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 -g-inch (4 mm. *95 N.A.), and more
recently ^-inch. 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. A remarkable instance of this
is provided by the production of two objectives by E. Leitz, of
Germany ; they have lately come into our hands ; they are but
semi-apochromatic. The one is low, having an initial power of 14,
with an aperture of 30° ; the other is practically a ^-inch of '88 N.A.
The low power has surpassed every achromatic of its kind we have met
with, and the higher power can, without hesitancy, be spoken of as an
exceedingly good glass ; nevertheless the price of these two objectives
is together less than the price charged for the lower power, if made
in England on achromatic principles, would certainly be ! Yet
Reichert has even surpassed this, and we feel that we shall be doing
a great service to students of small means in calling their attention
to the following remarkable and low-priced objectives : Leitz No. 2

1 Chapter II.

Till-; KYIM'IKCH

321

{I'D N.A. -2 ; Leitz No. 3 (less than Ijrds) N.A. -26 ; Reichert No. 6
-ath) N.A. -81; Reichert 18b foth) N.A. L-24. The Reichert
No. G is a lens whose low price is astonishing, when the perfection
of the performance of ;ill that we have seen is taken into account,
for it is the rival of even true apochroinatics. This fact is of im -
portance to the medical student and to the opticians generally. By
apochromatised objectives of the highest order the work of present,
and future microscopy will be done — that is inevitable. To thoroughly
understand 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 diver-
ging 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
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 //osilive eye-piece we can obtain a virtual iniaye of an object by
using it as a simple microscope, because its focus is exterior to itself.
This is not easy with the negative eye-jriece, because its focus is within
itself.

The eye-piece in common use is negative, and is generally known
as Huyghens', and sometimes as Campani's. Hooke appears to have
been the first (16G5) to have applied the field-lens to the eye-lens of
the microscope, although there is a high probability that such a lens
was first used by Monconys ; but how far he was indebted for this
to the compound eye-piece attributed to Huyghens cannot now be
determined.

This instrument as commonly used 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. 3 : 1, and the distance
between them being equal to half the sum of their focal lengths, a
diaphragm being placed in the focus of the eye-lens. The general
form of a Huyghenian eye-piece is shown in longitudinal section in
fig. 278. This makes a very convenient form of eye-piece of 5 and
10 magnifying power ; but when the power much exceeds this hist
amount, the eye-lens becomes of deep curvature and short focus, so
that the eye must be placed uncomfortably near the eye-lens. This,
however, is its chief defect, and it may fairly be considered the best
ordinary eye-piece.

Perhaps the best form of the Huyghenian eye-piece is that de-
vised by Sir G. B. Airy : its field-glass is a meniscus with radii
11:4 the convexity towards the object-glass. The eye-lens is a
crossed convex G : 1 the natter side to the eye, the (list a nee be-
tween them being twice the focus of the eye-lens, focus of field -

Y

OBJECTIVES, EYE-PIECES, THE APERTOMETER

lens being three times that of the eye-lens, the diaphragm being in>
focus.1

Another negative eye-piece is that known as the Keiiner, or
orthoscope. This consists of a bi-convex field-glass, and an achromatic,
doublet meniscus (bi-convex and bi-concave) eye-lens. A vertical
section of one so constructed is seen in fig. 279. These eye-pieces
usually magnify ten times, and the advantage they are supposed to-
give consists in a large field of view ; but they are not good in practice
for this very reason, they take in a field of view greater than the-

Fig. 278. — Huyghenian eye-piece.

Fig. 279. — Kellner eye-piece.

objective can stand, and as a rule even the centre of the field will not
bear comparison in sharpness with the Huyghenian form.

It is a suggestion of Mr. Nelson's that a crossed convex 6 : 1
field-lens and a meniscus and concave-convex doublet eye-lens might
work well for this form of eye-piece.

Positive Eye-pieces. — In the early compound microscopes the
eye-pieces were all positive ; that is to say, they consisted of a single
bi-convex eye-lens and no field-glass. The definition with this must

have been most imperfect ; the addition of a
field-lens, though it were a bi-convex, not in:
the correct ratio of focus, nor the theoretically
best distance, must have been considered a
great advance.

In this way matters rested, however, until
the theoretically perfect Huyghenian form was
devisyd. Nothing has yet displaced this com-
bination or successfully altered its formula.
Object-glasses have been used as eye-pieces and
all forms of loujis or simple microscopic lenses have been employed
for the same purpose. Solid eye-pieces have also been used both
in England and America, but with no results that surpassed a well-
made Huyghenian combination ; but the best form of all of the
combinations which have been tried by us as positive single eye-
pieces are the Steinheil triple loups ; a section of one of these is-

1 It is a curious fact that in practice the usual formula for the Huyghenian eye-
piece is radius of field-lens twice that of eye-lens, and the distance between them,
equal to half the sum of their foci.

Fig. 280.

COMPENSATING K VK-I'I K( T,S

323

seen in fig. 280. But a positive eye-piece was devised by li nn <l< n,
consisting of two plano-convex lenses of equal foci ; tin; distance is to
be equal to two-thirds the focal length of one. The diaphragm w ill
of course be exterior.

Abbe's Compensating Eye-pieces. — We have already given a
general description of the nature and action in connection with the
apochromatic objectives of this form of eye-piece.1 In the section
above on objectives we have referred to the fact that these eye-piece ,
are over- corrected ; this may be easily seen by observing the colour
at the edge of the diaphragm, which is an orange yellow. If we
compare this with the colour in the same position with a Iluyghenian
eye-piece, this will be blue, being seen through the simple uncorrected
eye-lens.

There are three kinds of compensating eye-piece as deigned by
Abbe. These are

1. Searcher eye-pieces

2. Working ,,

3. Projection ,,

1. The searcher forms are negatives of very low power, intended
only for the purpose of finding an object ; they consist of a single
field-lens and a doublet eye-lens.

The working forms are both positive and negative. The 4 power
is a negative form resembling the searcher in that for the short
tube ; the eye-piece for the long tube has a triplet eye-lens ; but
the remainder, viz. 8, 12, 18, and 27, when first introduced, were all
positives ; they were subsequently, however, changed for negatives.
Having used both, we are glad to learn that the positives are being
again introduced. It may be convenient to have the 8 a negative
like the 4, but with regard to the 12, 18, 27, it is important that
they should be positives.

These positive forms are on a totally new plan, being composed
of a triple with a single plano-convex over it ; the diaphragm is,
of course, exterior to the lens. With these the definition is of the
finest quality throughout the field ; they present the admirable
condition that with the deeper powers the proper position of the eye
is further from the eye-lens, which makes it as easy to use an eye-
piece of as great a power as 18 or 27 as one of 4 or 8.

The field of these eye-pieces has, as we believe, been very wisely
limited to five or six inches. The attempt on the part of English opti
cians to give to our eye-pieces fields reaching eighteen inches is an error.
A microscopic objective with the lowest aperture has the field greatly
in excess of any other optical instrument ; and to deal with such
eccentric pencils as must be engaged by an eye-piece with a field
of eighteen inches is a strain not justified by 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

» Chapter I., p. 38.

Y 2

OBJECTIVES, EYE-PIECES, THE APERTOMETEK

fractions. But without further elaboration it may be well to say
that 12 is the most generally useful eye-piece, and if only one com-
pensating eye-piece is to be selected, there can be no question, from
a practical point of view, but this is the best to employ. 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 expression 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 secured by the series of Abbe now in use.

It may be well to give further emphasis to the fact that this con-
struction of eyepiece is not only essential to the proper work of
<ipochromatic objectives, but they greatly enhance the images given
by ordinary achromatic lenses ; and it may be noted that the 8, 1 2,
and 18 eye-pieces for the short tube are identical with 12, 18, 27 for
the long tube.

The projection eye-piece is mainly intended for photo-micro-
graphy, but it is also useful for drawing and exhibition purposes. It
is a negative, with a single field-lens and a triple projection-lens. The
projection-lens is fitted with a spiral focussing arrangement in order
that the diaphragm which limits the field may be focussed on to the
screen or paper. The field of this eye-piece is small, but its definition
is exquisitely sharp.

It may not be generally known that good photo -micrographs can
be obtained by projection with the ordinary compensating working
eye-pieces, but this is a fact worthy of note.

It will perhaps be of practical utility if we append a table indi-
cating the focus of the compensating eye-pieces when used with
the long and the short body.

Focus of Eye-pieces for long Body.

Power .

2

4

8

12

18 •

27

Focus in mm.

135

67-5

337

22-5

15

10

,, inches .

5-3

2-6

1-3

•88

•59

•39

Focus of 'Eye-pieces for short Body.

Power .

1

2

4

6

8

12

18

Focus in mm.

180

90

45

30

22-5

15

10

,, inches .

7-08

3-54

1-77

1-18

•88

•59

•39

Projection Eye-pieces— 2 for short and 3 for long bodies =90 mm. or 3-54
inches ; 4 for short and G for long bodies = 45 mm. or 1-77 in.

Special Eye-pieces.— The most important of these, the micrometer
eye-piece, we_ have already considered, so far as its application to
micrometry is concerned.1 Its optical character may be properly
considered here. If it is a negative eye-piece the micrometer is placed
m the focus of the eye-lens, but if a positive combination it is placed

1 Chapter IV.

H()W TO TEST OWKCTIVKS

325

in the focus of the eye-piece itself. The Ramsden form described
above is thoroughly suited for this purpose, but a negative form is
generally employed, the micrometer being placed inside the eye-piece
in the diaphragm, i.e. the focus of the eye-lens.

In order that the micrometer may be 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 14-inch focus j 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 §-inch focus.

Spectroscopic, polarising1, goniometer, and binocular eye-pieces
are each treated under their respective subjects.

The index eye-piece is one which has a pointer placed at the dia-
phragm, so constructed that it can be turned in or out of the field,
and is used to point to the position of an object. It is of limited
utility, for with low powers it is scarcely needed ; we can readily
indicate by description, to another, the position of an object in a
held every object of which is in view. With a high power, on the
other hand, it is very difficult to use such a ' finger.'

A better plan when the magnification is great is to have a series
of diaphragms of different apertures to drop into the eye-piece and
diminish the field of view. This not only makes the object to be
pointed out more easily accessible to the eye, but — as we have bv
many years of observation proved — it aids in close observation upon
minute objects by cutting off a large area of light without altering
the intensity of what remains, and so makes close observation more
easy.

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 strise or lines with oblique light the effect is much
strengthened by placing a KieoVs analysing prism over the eye-
piece.

Testing- Object-glasses. — It will have been noted by the attentive
reader that many of the more important qualities of objectives
are determined by the principles of their construction, and become
in fact questions simply of the quality of the workmanship involved
in producing the optical and mechanical parts of the object-glass.

The quality of the workmanship may be tested by technical
means described below, and by that subtii 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. Bat, 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 Cup to the

326 OBJECTIVES, EYE-PIECES, THE APERTOMETER

most powerful that can be obtained) are at hand. Nothing tests
the quality of an objective so uncompromisingly as a deep eye-piece.
For brilliancy of image a moderate power of eye-piece is of course
best ; but the capacity of the object-glass is clearly commensurate
with its ability to endure high eye-pieces without loss of 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 the image 5 yielded by these objectives ' down.'

This mode of testing is of course to a large extent subjective, or
at least is controlled by incommunicable judgments. It is most
important therefore to have a mode of judgment that shall be acces-
sible to the beginner and the interested amateur. Dr. Abbe has
proposed a method which is at least accessible to all.

In ordinary practice microscope objectives, if tested at all by
their possessors, are simply subjected to a comparison of perform-
ance with other lenses tried upon the same ' test objects.'

The relative excellence of the image seen through each lens may,
however, depend in a great part upon fortunate illumination, and
not a little upon the experience and manipulative skill of the ob-
server ; besides which any trustworthy estimate of the performance
of the lens under examination involves the consideration of a suitable
test-object, as well as the magnifying power and aperture of the
objective. It is knowing what is meant by a ' critical image,' and
being able to discover whether or not a given objective will yield it.
Clearly all tests of optical instruments, which are not capable o£
numerical expression, must be comparative. Magnifying power can
be measured numerically ; it is not comparative. In the same way
resolving power is mathematically measurable ; so is penetrating
power. But definition and brilliancy of image, and evidence of
centring, can have no numerical expression ; they are consequently
comparative.

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 should 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
observer ; 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 illumination
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

TKSTIN(J OIUKCT IVKS

327

optical delineation, a fair comparison can only be drawn between
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 there-
fore ought to be obtained.

. On the other hand, extrav agant 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 thai
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 amplification, in
whose mind separation of detail means analysis of structure, and
optically void interspaces prove the non-existence of anything which
he does not see.

As much time is often lost by frequent repetition of these com-
petitive examinations (which, after all, lead to no better result than

that the observer finds or fancies that one lens 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
conditions of good performance as will enable him to make the best
use of his intrument, 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
4 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 : —

328 OBJECTIVES, EYE-PIECES, THE APEETOMETEE

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-
tius 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 tracts 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 T\r 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 T\r inch clear between its own
inner margin and the centre of the field, as in fig. 281, where

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

Fig. 281. Fig. 282 should occupy a zone on the opposite side of

it, between the ^ and inch (measured
from the centre); and the third the peripheral zone on the same
side as the first in fig. 282.

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
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 dispersion 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 co-

AIIJ1KS MODE OF 'IK STING

3^J

incident images in other parts of the field. It is t<» he 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 objec-
tive— and to make these as plain as possible is a sine qud 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 Professoi
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 prevents the rays passing through the
holes in the diaphragm from being focussed on the object -the
silvered plate of lines — at the same time. In the 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. 281, 282 ;
the card is then fixed in its place below the condenser. We are
strongly, however, inclined to believe, partly from experiment, that
better results would be obtained by putting sections of annular Blits
at the back of the objective. If the condenser be fitted so as to
revolve round the axis of the instrument, and also carry with it
the ring or tube to which the card diaphragm is fixed, the pencils
of light admitted through the holes will, by simply turning the eon-
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 J-inch
diameter, in which circles of card may be dropped, the card being-
pierced with holes of different sizes according to the directions given
above. We doubt, however, if any sub-stage will revolve with
sufficient accuracy for so delicate a test.

OBJECTIVES, EYE-PIECES, THE APEETOMETEK

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
below the lens. In the cardboard, holes of various sizes, and at
various distances from each other, may be pierced according to
pleasure. By simply passing the slip through the tube, the pencils
of light admitted through the holes (which form images of these
holes in the upper focal plane of the objective) are made to traverse
the field of view, and by rotating the sub-stage the whole face of the
lens is swept, and thus searched in any direction required. But here,
again, the spherical aberration of an uncorrected condenser would,
with an objective of large aperture, cause the oblique pencils
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 provided 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
& 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 be made to sweep round in
the field, and thus permit each part of the central or peripheral zones
to be brought into play. Against the accurate value of this, again,
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 corrected.

Fig. 283.

The test-plate consists of a series of cover-glasses, ranging in
thickness from 0'09 mm. to 0*24 mm., silvered on the under surface,
and cemented side by side on a slide, the thickness of each being
marked on the silver film. 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

THE TEST-I'LATE

331

power and widest aperture. The test-plate in its natural size is seen
in fig. 2815, and one of the circles enlarged is seen in fig. 28 1.

To examine an objective of large aperture, the discs must be
focussed in succession, observing in each case
the quality of the image in the centre of the
field, and the variation produced by using alter- A
nately central and very oblique illumination. km

When the objective is perfectly corrected ■
for.spherical aberration for the particular thick-
ness of cover-glass under examination, the out-
lines of the lines in the centre of the field will
be perfectly sharp by oblique illumination, and
without any nebulous doubling or indistinctness
of the minute irregularities of the edges. If, after exactly adjusting
the objective for oblique light, central illumination is used, no altera-
tion 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 of
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 central as with oblique light,
then the spherical correction of the objective is more or less im-
perfect.

Nebulous doubling with oblique illumination indicates over-cor-
rection of the marginal zone ; indistinctness of the edges without
marked nebulosity indicates under-correction of this zone ; an
alteration of the focus for oblique and central illumination (that is,
a 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
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
on account of its spherical aberration. We want at least his
achromatic condenser. For the examination of ordinary immersion
objectives, the apertures of which are, as a rule, greater than 180°
in arc (l'OO A.N.), 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 ob-
jectives are used. We may add, as a matter of experience, that

332 OBJECTIVES, EYE -PIECES, THE APERTOMETER

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 corrections 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 0*01 or O0*2 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.

Indistinctness of the outlines towards the borders of the field of
view arises, as a rule, from unequal magnification of the different
zones of the objective ; colour-bands in the peripheral portion (with
good colour- correction in the middle) are always caused by unequal
magnification of the different coloured images. Imperfections of this
kind, improperly called ' curvature of the field,' are shown to a greater
or less extent in the best objectives, when their aperture is considerable.

Testing an objective does not mean seeing the most delicate
points in an object ; it rather means the manner in which an object
of some size is defined.

A test for low powers up to -J- of 80° or N. A. *65 is an object on
a dark ground. Nothing is so sensitive. One of the Polycistince
because it takes light well, is good. For higher powers a coarse
diatom, a Triceratium Jimbriatum, is excellent ; for unless an
objective is well corrected the image will be fringed and surrounded
with scattered 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
which the Abbe method of testing does not cover. But if we place
a sensitive object in a certain direction, and when the best adjust^
ments have given the best image rotate that object through an angle
of 90°, only a well-centred objective will give an unaltered image
throughout. If not well-centred it will at certain parts grow
fainter or sharper. The most useful image for this purpose with
medium powers is a hair of Polyxenus lagurus 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

OIUKCTS FOR LKNS-TKSTJNO. A PERT* >.M ITER

333

be strongly marked, and must be in optical contact with the OOVer-
glass; this maybe tested by means of an oil-immersion and the
4 vertical illuminator' (p. 281). b

The objectives of widest aperture are not readily tested because •
there is no condenser sufficiently aplanatic to do it exhaustively.
The best that can be done is to take a diatom, such as a Coscinodis-
cus, in balsam with strong 'secondaries' (Plate I. figs. .3 and 1), with
the largest aplanatic cone that can be obtained, which at present
can.be best accomplished with Powell and Lealand's achromatic
condenser of 1*4 N.A. It must be a good objective indeed that
does not show signs of breaking down under this strain ; and there
is extreme susceptibility to cover correction to which close attention
must be paid. An illuminating cone of N. A. 1 '0 is probably just below
the point of over-strain with the best lenses at present at our disposal.

Testing lenses, therefore, resolves itself into two 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 /V '//.o-nus lay a r».<.

3. Centring for high powers by means of podura scale.

4. Definition Coscinodiscus asteromphalus with wide-angled cone
obtaining sharp, brilliant, and clear view of ' secondaries.'

The apertometer, as its name implies, is an instrument for mea-
suring the aperture of a microscopic objective. As correct ideas of
aperture have only obtained during the past few years, it may be
inferred that apertometers constructed before the definition of aper-
ture was given and accepted were crude and practically useless.

The controversy on the 1 aperture question,' which Mas in full
operation some eighteen years since, is not an altogether satisfactory
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 devised by P. B. Tolles, of
America, which accurately measured the true aperture of an objec-
tive. 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 ajxrhire, making-
it, as we have seen, the equivalent of the mathematical expression
71 sine u, n being the refractive index of the medium and a half
the angle of aperture.1

The application of this formula to, and its general bearing upon,
the diffraction theory of microscopic vision has been given in its
proper place J but as the aim of this manual is thoroughly practical
we shall be pardoned for even a small measure of repetition in
endeavouring to explain the use of this formula in such a manner
that only a knowledge of simple arithmetic will be required to enable
the student to work out any of the problems which are likely to
arise in his practical work.

1 A knowledge of the meaning of the trigonometrical expression ' sine ' is not
necessary in solving any of the following questions. As the values ure nil found in
tables it is only necessary to caution those who are unacquainted with the use of
mathematical tables to see that they have the ' natural sine ' and not the ' log sine.'

OBJECTIVES, EYE-PIECES, THE APE RTOME TE R

We can best accomplish this by illustration.

i. If a certain dry objective has an angular aperture of 60°, what
is its N.A. ? (i.e. numerical aperture).

All that is needful is to find the value of n sine u ; in this case
n = the refractive index of the medium, which is air, is 1 ; and %iT
which is half of 60°, = 30° opposite 30° in a table of natural sines,1 is
r> ; sine u, therefore, = -5, which multiplied by 1 gives -5 as the N.A.
of a dry objective having 60° of angular aperture.

ii. What is the N.A. of a water-immersion whose angular
aperture == 44° ?

n here = 1"33, the refractive index of water ; and u, or half 44°,
is 22°. Sine 22° from tables = '375, which multiplied by T33 = 5
(nearly), which is the N.A. required.

iii. What is the N.A. of an oil-immersion objective having-
38^° of angular aperture 1

n the refractive index of oil, which is equal to that of crown
glass, is 1*52 ; u = 19 J and sine u from tables = *329, which multi-
plied by 1*52= -5.

Thus it is seen that a dry objective of 60°, a water-immersion of
44°, and an oil-immersion of 38^° all have the same N.A. of *5.

It will be well, perhaps, to give the converse of this method.

iv. If a dry objective is '5 N.A., what is its angular aperture ?

•5

Here because n sine u = '5, sine u— — ; the objective being

dry n = 1, therefore sine u = *5. Opposite "5 in the table of
natural sines is 30° ; hence u = 30°. But as u is half the angular
aperture of the objective, 2it or 60° = the angular aperture required.

v. What is the angular aperture of a water-immersion objective
whose N.A. = -5 ?

Here n = 1*33, n sine u = "5 ; sine u = L = = -376 ;

n 1"33

u = 22° (nearly) from tables of sines ; 2u = 44°, the angle re-
quired.

vi. What is the angular aperture of an oil-immersion objective
of -5 N.A. ?

•5 *5

Here n= 1'52, n sine u = *5, sine u = — = - - = -329 :

n l-52

u = 19^° (by tables of sines) ; and 2u — 38-^, the angle required.

We may yet further by a simple illustration explain the use of
n sine u.

In the accompanying diagram, fig. 285, let n' represent a vessel
of glass ; let the line A be perpendicular to the surface of the water
C D ; suppose now that a pencil of light impinges on the surface of
the water at the point where the 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
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

1 Vide Appendix A to this volume,

SIMPLE ILLUSTRATION OF THE USE OF N SINK U 33 >

equal to n' sine u' on the water side. Thus on the air side // = 1,
II = 30°, and by the tables of sines sine .'50° = ~) ; consequently on
the .air side we have n sine u = *5.

On the water side n' = 1*33, and uf is to be found. J Jul as

which (as

... . , n sine //

n sine u = n sine u, sine u = —j—

n

= = 376
l :v;>

the tables show) is the natural sine of an angle of 22° ( nearly) ; eon
sequently u' — 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 S° further aicau
from the perpendicular, and so make an angle of 30° with it.

Now if we suppose that these
pencils of light revolve round il><
perpendicular, cones would be dt
scribed, 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 remain-
ing 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*.")2.

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 sine u

u' = n sine u: sine vi =

t)V

, -rig =329, which (..

the tables) is the natural sine of
19j°. It follows that the pencil
has been bent in the cedar oil 10^°
out of its original course, and a
cone of G0° in air becomes a cone of
38^° in cedar oil or crown glass.

Finallv, it is instructive to
note the result when an incident
pencil in air makes an angle of
90° with the perpendicular ; rtsine
u becomes unity, and >/ in water
48:,l°, in oil 41° (nearly) ; conse-

quently a cone of either 97.}° in water, or 82j 0 in oil or crown glass,
is the exact equivalent of the whole hemispherical radiant in air. In

336 OBJECTIVES, EYE-PIECES, THE APEKTOMETEK

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 equivalent to a water-immersion of 97-J-°, and an oil-immersion of
82 J ° angular aperture.

The last problem that need occupy us is to find the , angular
aperture of an oil-immersion which shall be equivalent to a water-
Lmmersiorj of 180° angular aperture.

vii. On the water side n == l*33j u = 90°, sine 90° = 1, n sine u.

= 1-33. On the oil side ri = 1*52 and u' has to be found.

, yisineit 1*33 , a-,0

As n' sine u' = n sine u, sine u' = = = *o/ 0; u = bl

{nearly) by the tables; 2u' = 122° (nearly), the angle required.

It thus appears (1) that dry and immersion objectives having
different angular apertures, if of the same equivalent aperture, are
designated by the same term. Thus objectives of 60° in air, or 44°
in water, or 38^° in oil, have identically the same aperture, and are
known by the same designation of -5 N.A.

(2) The penetrating power of any objective is proportional to

1

. 5 and its illuminating power to (1ST. A.)2 Therefore, if we double
!N". A..

the 1ST. A. we halve the penetrating power, and increase the illumi-
nating power four times.

In comparing the penetrating and illuminating powers of objec-
tives, however, care must be taken to avoid a popular error, by
making them between objectives of different foci.

It cannot, for example, be said that a ^-inch objective of '8 N.A.
lias half the penetrating power of a ^-inch of *4 N.A. Neither can
it be said that it has four times the illuminating power. What is
meant is that a l-inch of *8 N.A. has half the penetrating and four
times the illuminating power of a ^-inch objective of *4 N.A.

But because penetrating and illuminating powers diminish as
*che square of the foci, a -^-inch objective of -6 N.A. has four times
the illuminating and nearly four times the penetrating power of a
^-inch of -6 N.A.

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 1 here
that the resolving power of an objective is directly proportional to
its numerical aperture. If we double the N.A. we also double the
resolving power ; and this not simply with objectives of the same
foci, as in the case of penetrating and illuminating powers. Thus it
is not only true that a -Jr-inch objective of -6 N.A. resolves twice as
many lines to the inch as a ^-inch of *3 N.A., but so also does a
^■-inch of 1*4 N.A. resolve twice, and only twice, as many as a
^-inch of -7 N.A.

Within certain limits, then, the advantage lies with long foci of

1 Chapter I.

THE USE OF THE APERTOMETEH

337

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 evideni that

the' employment of the microscope as an instrument of precision i.

hugely 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 lias been greatly enriched by his
having introduced a thoroughly simple and useful apertometer. This
involves the same principle as that of Tolles, but it is carried out in
a simpler manner.

Abbe's instrument is presented in fig. 286. 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 bV, 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.

Fig. '28(5. — Abbe's apertometer.

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 lamp,
is assumed to be in front and on both sides. Suppose the lens to
be measured is a dry ^-ineh ; 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
supplied with the apertometer is screwed into the end of the draw-
tube. This lens with the eye-piece in the draw-tube forms a low-
power compound microscope. This is now inserted into the body-
tube, and the back lens of the objective whose aperture we desire
to measure is brought into focus. In the image of the back lens
will be seen stretched across, as it were, the image of the circular
part of the apertometer. It will appear as a bright band, because
the light which enters normally at the surface is reflected by the
bevelled part of the chord in a vertical direction, so that in reality
a fan of ISO0 in air is formed. There are two sliding screens seen

z

338 OBJECTIVES, EYE-PIECES, THE APEETOMETEE

on either side of the figure of the apertometer ; they slide on the
vertical circular portion of the instrument. The images of these
screens can be seen in the image of the bright band. These screens
should now be moved so that their edges just touch the periphery of
the bach lens. They act, as it were, as a diaphragm to cut the fan
and reduce it, so that its angle just equals the aperture of the objec-
tive and no more.

This angle is now determined by the arc of glass between tho
screens ; thus we get an angle in glass the exact equivalent of the
aperture of the objective. As the numerical apertures of these arcs
are engraved on the apertometer they can be read off 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 requi ?-es. In that case the edges,
of the screen refuse to touch the periphery.

On the whole we have found that a far better way of employing-
this instrument is to use it in connection with a graduated rotary
stage, 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 stage will
cause the image of the flame to travel towards the edge of the
aperture ; rotation is continued until the image of the flame is half
extinguished by the edge of the aperture, 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 ^-inch as well as its angular aperture in air.1

(i) As before, N.A. = n sine u, and n sine n = n' sine u' ;
which means that the aperture on the air side is equal to the aperture
on the glass side ; n = 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) ; n sine u' = 1*015 X sine 30°
= 1*615 x *5 = *8 = n sine w=the N.A. required.

(ii) Again, to find the angidar aperture or 2u. As before, n sine ?*

, i n • n' sine u' 1*615 x *5 Q kqo •

=n sine u and sine u — = — =*8 ; u = 53

n 1

nearly (by the tables) ; 2u = 106°, which is the angle required.

1 Vide p. 2 ct seq.

THK I'SK <>F TIIK A I'KRTOMETEB

339

(iii) If it be a water-iunw rsi<m, we have to deal with, suppose
the mean angle = 45° = u' ; sine 45° = -707 (by the tables) ; n
= 1*33 ; and u = 1*615.

n sine u = n' sine w' = 1*615 X '707 = 1*14, the N.A. required.

r \ k ■ • n' sine 1*615 X*707 Q£i

(iv) Again, sine u = . = — — = *oo ; u — D9{

n 1 '-Vo

(by the tables); and 2u = lli^.1,0, the angle required.

(v) In the case of an oil -immersion, suppose the mean angle
= G0° = u ; sine 60° = -866 (by the tables) ; n = 1*52 ; nf = 1*615 ;
n sine it = n' sine u' = 1*615 X '800 = 1*4, which is the N.A.
required.

z .v » . . n' sine ?t' l*615x,866 no

(vi) Again, sine u = — = - = -92.

n = G7° (by the tables), 2u = 134°, the angle required.

It is manifest that if the refractive index of the apertometer
equals that of the oil of cedar, the mean angle measured is the semi-
angle of aperture of the objective, and its sine multiplied by that
refractive index is the numerical aperture.

This will be found the more 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.

34Q

CHAPTER VI

PRACTICAL MICROSCOPY: 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 prac-
tical skill to a particular scientific end.

But a ' microscoyncal 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 in-
strument 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

MICKOSCOI'ISTS- WORK-TABLES

.341

knowledge to biological and other investigations is entered QDOH in
the subsequent chapters of the hook.

To begin his work with success it 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 ;i
space commanding, if possible, a north aspect, ami which can be
arranged to readily exclude the daylight and command complete
darkness.

The first requirement will be a suitable tal>l> .

This should be thoroughly J"''", and it should be rectangular in
shape. A round table, if small especially, is most undesirable, as ii
otters 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 no way interferes with the
usefulness of the table. In the same way the right-hand corner
may be used for the cabinet of objects which is being worked, or the
apparatus needful for use.

The most important part of the table — that is, the middle, from
front to back — should be kept quite clear for the purposes of mani-
pulation, and a sufficient space should be kept clear on either side of
the instrument for resting the arms, and no loose pieces of apparatus
should ever be deposited within those spaces. This soon becomes a
habit in practice, for experience teaches — sometimes painfully by the
unwitting destruction of a more or less valuable appliance.

The spaces to the right beyond that left for the arm of the
operator may be used for the work immediately in hand- especially
for a second and simpler microscope. An instrument with only a
coarse adjustment and a 1-inch or a &-inch objective will suffice, or
a good dissectin g-stand Avill 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 flat cylindrical glass shade with a knob on
the top. The stand should be suitably arranged to hold two eve
pieces, three objectives, one condenser, a buttle of cedar-oil (fitted
with a suitable pointed dipper), and a box containing the c mdenser
stops. This is a most useful arrangement for such a table : and it
need not have a diameter greater than nine inches.

Thr sizr/nr tin tn/> of sttcli a f<(/>/>> should be I.1, x 3 feet, and as

342

PRACTICAL MICROSCOPY

no work, such as mounting or dissecting, maybe supposed to be done
at this table, it is well to cover the surface with morocco, that
being very pleasant and suitable to work upon.

It should be remembered that for a full-sized microscope a depth
of three feet is required for comfortable work. When the micro-
scope is set up for drawing, 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 height 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 par-
ticular 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 il-
lustration (tig. 287), with
the appended references,
will make quite clear the
character of the table
which we recommend, as
well as the mode of using
it.

The table above de-
scribed is supposed to be employed wholly for general purposes of
observation or research on wholly or partially mounted objects.
But the microscopist who aims at more than this will require an
arrangement 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 essen-
tials, especially if the work done is a mere occasional occupation ;
but where anything like continuity or periodical regularity of
occupation with such work is intended, it 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 of the jeweller's bench — serves admirably. A rough sugges-

1 Chapter IV. p. 238.

Fig. 287.— Microscopist's table.
(Scale, h 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 ;
10. Bull's-eye stand ; 11. Light-modifier.

LABORATORY TABLE l'<>R MIOROSCOPIST

34.S

tion of this is given in fig. 2SN, which presents tin- plan of tin* top
-of the table. The whole area beneath should be unoccupied, but ai
A and J> 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
scaljtels, needles j scissors, forceps, pipettes, life-slides, Arc. in the upper
one; and fliers, cutting pliers, small si tears, files oi various coarse
nesses and finenesses, ifec. in the other ; on the A side a single
drawer containing slips, covers of various thicknesses, bone, tin,
glass, and other cells of all (assorted) sizes, watclt-ylusses, staining
enps or slabs, lifters (if used), saw with fine teeth, hones of various
shapes, pewter plate for grinding and polishing glass, Arc, platinum
capsule, camera lucida, three ' No. '2 ' sable brushes (water-
colour), &c.

A B

Fig. 888. — Dissecting and* mounting table.

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-
ment which most moderately ingenious manipulators could accom-
plish each of the articles in the drawers has a fixed place, there will
be no difficulty in finding by touch what is wanted.

The table top may be of pitch pine stained black, or, still better,
some very hard wood finished smoothly, but 'grey.'

The space in the figure immediately in front of the 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
levelled, so that it may slide firmly into a prepared space cut into
the surface of the table and occupy this space, the surface being
exactly level with the surface of the table. This plate of glass should
be made black on its under side, so as to present a uniform black
surface. This is often of great value in certain kinds of work.
Equally useful is a purely white unabsorbent surface, and a slab of
white porcelain may be easily obtained of the same size and be made
to fit exactly into the same place.

In using tiiis table tor dissection the arms have complete rest.

344

PRACTICAL MICROSCOPY

and 1 m tlie 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 1 to-
answer this purpose admirably.

3 is a small vessel of spirit (dilute) for use with the section knife.
I 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.

5 is a nest of apertures in which to place partly mounted objects,
to protect them from dust, while the balsam, dammar, &c. may be
hardening on the cover so as to be in a suitable state for final mount-
ing. A slide may go over the sloping front of this and wholly ex-
clude dust.

6 is a stand of cements, varnishes, &c. such as are needful ; and

7 is a turn-table.

For the work of dissection, when the subject requires reflected
lia'ht, one of the desiderata is a mode of illumination at once conve-

nient and intense. Mr. Frank
B. Cheshire, F.L.S. &c, whose
work on ' Bees and Bee-keep-
ing ' is a proof of knowledge
and practice of minute anatomy,,
adopts an old plan which we
Fig. 289.— Mode of illumination for have always found admirable.

dissection. It is illustrated in fig. 289.

Bays of light from a lamp are
parallelised by a bull's-eye full upon an Abraham's prism and
focussed upon the object. The prism may be mounted on a long
many-jointed arm and is of most varied usefulness. 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 a plainy
firm table 4 feet 6 inches x 3 feet in area, of a suitable height for
the worker, should be fitted as follows, viz. : if fig. 290 represent the
rough plan of the table, 1 and 2 are gas fittings attached to the main
to supply blowpipe, Hansen's burner, <kc.

4 is a small tube of metal attached to the water main, with a
tap, and bent in the form of an inverted fl, with the attached leg of
the f] the longer. This affords a pleasant stream of water for wash-
ing dissections &c. ; and if the open end be made ivith a screw, and
have a suitably made piece of tubing fitted, to screw onto it, this latter-
may be attached to cm indiarubber tube, at the other end of which we
may fasten fine glass nozzles, which will act as wash bottles of the finest
lore, and serve with the finest dissecting work.

5 is a glass trough for waste, with a perforated aperture, 6, con-
nected with a waste-pipe, through which the waste water &c. flows;
innocuously away.

3 represents the position of a Thoma microtome, and A B are two
well-framed flat slides, which may, be drawn out eighteen inches, or
1 Journ. B.M.S. new series, 1887, p. 682.

AITLIANCKS FOR MICH* >S( 'OPICAL LABORATORY

345

puslmd fully in. They are found at times to be of greal service,

where the space is somewhat confined.

This table may befitted
on one side (the left) at
least with a set of drawers
and shelves for receiving
various apparatus and
materials, with larger
quantities of stains, and re-
agents, hardening, mace-
rating and other materials;
while if a door covers the
whole, the inner side of
this may be readily fitted
to receive drop-bottles 1
containing all the stains,
reagents, and similar ma-
terials in constant use.
If these be labelled with
paper labels sat ura ted in a
solution of solid paraffin in

turpentine, and after the turpentine has evaporated firmly fixed on the
bottle, they are very permanent, and, indeed, better than anything
we have tried save where the name of the contents is enamelled or
engraved on the bottle.

It has been already
pointed out that there
are conditions of re-
search in which the
microscope has to be in
a constantly vertical
position. This was the
case with the researches
on the saprophytic
organisms made con-
jointly by the present
Editor and Dr. J. J.
Drysdale.2 It must
always be the case
where certain forms of

Fig. 290.— Laboratory tal>le for microscopical
work.

stages

Fio. 291. — Tripod for using microscope in an

Upright position.

continuous life

are employed for prolonged or continuous observations on the deve-
lopment of the minuter forms of life.

In such cases the table is quite unsuitable, and special stands
have to be employed that from their form give greal 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.

J Chapter VII. p. 44T..

- Monthly Micro. Journ. VOlS. X. to xviii. ; Jonrn. 7P..V..S'. vol. iii. p. 1 vol. v.
series ii. p. 177 ; vol. vi. p. l!»i> ; vol. vii. p. ls."> ; vol. viii. p. 177.

34^

PEA CTICAL MICROSCOPY

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. 291 is an outline of the construction. The three legs of the
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.
P> 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 an arrangement at the two sides,

Fig. 292. — Using the microscope in an upright position.

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
using this arrangement is seen in fig. 292. In that case, however,
the whole is employed for the making of a camera lucida drawing with
a ^ -inch objective. But the position of the basal tripod, the micro-
scope upon it, the position of the lamp (partly seen in the immediate
foreground 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 intensity of which we can fully rely on. Daylight

LAM] s

347

Juts certain (juaHt 'n s that involve ad raid 'ayes at times, and under Ipecial
circumstances, in its employment, hut th is is flu- exception ratlu r than
the ride. What is needed is a well-made lamp with a tlat flame
this we should be able to control witli great ease as to height and

distance from the microscope. Nothing is equal practically to a
1-inch or a 1-inch paraffin lamp ; this gives the whitest light artiti
•cially accessible save the higher intensities of the incandescent elect He
light. But there is nothing at present accessible to the student of
this kind. Tin; employment of the ejhje of the name of a well made
paraffin lamp used with
good 'oil ' has no present
rival. Gas is much yel-
lower, and not so easy in
employment.

To get the best form
of microscopical lam}) is a
matter of some import-
ance. We call the atten-
tion of the reader to the
best simple form of lamp
which will accomplish
every purpose. This is a
model arranged by Mr.
Nelson, and the drawing
of which is given in tig.
293. The lamp burns
paraffin and has an ordi-
nary .\-inch wick burner.
The reservoir is rectangu-
lar and flat, .H x 4 x 1£ ;
it serves three distinct
purposes : 1st, it will hold
sufficient oil to burn for a
whole day ; 2nd, permits
the lamp to be lowered
near the table ; 3rd, radi-
ates the heat conducted
by the metal chimney and
prevents the oil boiling.
The lamp is placed at one
angle of the reservoir to Fig. 2ns.— Lamp devised by Mr. E. M. Nelson,
enable the flame to be

placed very near the stage of the microscope, which is exceedingly
useful with some kinds of illumination, especially with reflected light,
with the higher powers, and for Powell and Lea land's super-stage
condenser.

The hole for tilling the reservoir is placed at the diagonal corner
for convenience. The chimney is metal with an ordinary \\ 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 he only 1 \ inch

348

PRACTICAL MICROSCOPY

Ion- • length of chimney should be 7 inches. Chimney should be-
lead black inside. This chimney serves four purposes : 1st, image
of tme s not distorted by stri/ and specks common to ordmary

Fig. '2(.)4.

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

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.

LAMPS

349

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 ecu
trallv with the lamp flame. The arm is slotted so that tin- bull s-cyc
may be focussed to the flame J it can be tixed by a clamping screw.
The bull's-eye may also be elevated Off depressed and tixed 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 position simply by rotation of the arm. There should
be a groove in the pillar with a steadying pin on the lamp to prevent
rotation during elevation or depression.

The form of the clamping screw is important ; it should be at the
upper part of the tube, and not at the lower, as shown in the ligure
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 (fig. lHJ-r>), 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 lamn is made bv Messrs. R. and J. Beck. We illus-
t rate it in fig. 294.

The base, A, consists of a heavy ring, into which a square brass
rod, B, is screwed. The square rod carries a socket, C, with an arm,
D, to which the lamp is attached.

On each side of the burner, and attached to the arm, D, is an
upright rod, G, to one of which the chimney is fixed, independent <>f
the reservoir of the lamp, thus enabling the observer to revolve the
burner and reservoir, and obtain either the edge or the flat side of
the flame without altering the position of the chimney. The
chimney, F, is made of thin brass, with two openings opposite to
each other, into which slide 3 x 1 glass slips of either white, blue,
or opal glass, the latter serving as a reflector; but we do not con-
sider 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, G, to which it is
attached by the pins, H, placed level with the middle of the flame :
to this semicircle is tixed a dovetailed. bar, L, carrying a sliding fitting,
O, which bears a Herschel bull's-eye, P. This is complex and there-
fore 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 usewith the microscope in an upright position, when prolonged
investigations have to take place, the lamp becomes even of more
importance than under ordinary eircunistances. The present Kditor
devised a somewhat elaborate apparatus of this kind, which he
always employs in this kind of observation.1 But the essential part
of it is only an arrangement by whic h a milled-head movement of

1 Monthly Micro. Jvurm vol. xv. p. 103.

35o

PRACTICAL MICROSCOPY

the entire lamp may take place to the right or the left of the ob-
server, as well as a similar power to elevate or depress the position,
of the flame. When the microscope is fixed, and the rectangular
prism for illumination (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 lamp. A very simple form of this lamp'
lias been made for the Editor by Mr. Charles Baker, of Holborn :
it is seen in fig. 295, 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 en-
graving) 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 arrange-
ment by means of which the
bull's-eye (with a catch fixing
its focus from the flame) were
so affixed as to be carried up
and down and to right and left
with the lamp.

When the microscope is
fixed in its upright position,
and the prism is arranged to
give direct and not oblique re-
flexion, the lamp flame, by
means of a card, is arranged
as nearly right for the re-
flexion of the image of the
flame into the centre of the
field as may be, and then a
little movement in one or both
milled heads will bring it ac-
curately into the field.
FlG 2g5 We may arrange the micro-

scope for ordinary transmitted
light, that is, for light caused to pass through the object into the
object-glass, by placing it upon the table, arranged as already
directed ; the instrument is then sloped to the required position,
and a condenser, suitable to the power to be employed,1 is put into
the sub-stage. The lamp is now put into the right position, with a
bull's-eye, on the left of the observer. The condenser is then, as
described below (p. 351), to be 'centred'; when the objective

1 Vide Chapter IV. p. 24B.

THE USE OF THE BULL'S-EYE

351

may be changed as desired, and the eye piece altered t<» suit. liui
it should be carefully noted, that if apochromatie powers air being
used, there must be accurate adjustment of the tube length if the
bett results are to be obtained; and with any serious increase of

the power of the objective a condenser of higher aperture and shorter

focus must be used.

Often, however, as good or better results may be obtained with-
out the employment of the mirror at all, the light being sent directly
through the condenser from the lamp flame. The mode of ananin-

ment 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 screens of coloured glass is placed between
the lamp flame and the con-
denser (sub-stage).

By this means the light
is sent into the condenser
and upon the object, and is
then treated as is the case

D

(for centring) when the Fl(; 29G._Edge of lamp fllime in centre imi]
mirror is used. focus of bull s-eye.

The first step in the
direction of efficiency in the use of the microscope is to understand
the principles of ID minat'um, and a knowledge of the various effects
produced by the bull's-eye lies on the threshold of this.

Having given details as to the forms of lamp which are of most
service, we assume that a paraffin lamp with '-inch wick is used.

Tf we place the edge of this flame (E, fig. 29G) in the centre and
exact focus of the bull's-eye B, A shows the effect of doing so.

Tf a piece of card were held in the path of the rays proceeding
from B, the picture as shown at
A would not be seen — instead of Jj
it an enlarged and inverted image
of the flame. The image at A is
obtained by placing the eye in
the rays and by looking directly

at the bull's-eye. II'

The light is so intense that it
is more pleasant to take the held /7
lens of a 2-inch eye-piece and
place it in the path of the rays
focussing the image of the bull s- Flt; o,,7._AlttMV(, reiatiou. between lamp
eye on a card. It should be flame and bull's-eye.

noticed with care that the

diameter of the disc A depends upon the diameter of the bull's
eye B ; but the intensity of the light in A depends OH the focal
length of 1). The shorter the focus, the more intense will be the
light,

We are here assuming throughout that the field lens i» at a fixed
distance from the bull's-eve B.

o w

352

PRACTICAL MICROSCOPY

we g

But if we move the flame, E— still central— within the focus of B,
ei the result show., in I), fig. 297. But by moving E without
, he focus of B we get the picture H, while K is the picture when E is

focussed 6w« not centred. . ,1 £ i •

\ common error, one repeatedly met with, is that ot placing a
concave mirror 0 (fig. 208) so that the flame E is in its pnwci;*^
facwa 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 curvature 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 in illumi-
nation, it must be placed
so that E is not at its

Fig. 298.

-Result of placing flame in principal focus
of concave mirror.

principal focus, but at
its centre of curvature.

The bull's-eye gives
an illustration of what
is of wider application.

Fig. 299.— Mode of obtaining critical image.

The method of obtaining a critical image with a condenser by means
of transmitted light is shown in fig. 299. E is the edge of the flame,
S represents the sub-stage condenser, and F the object. F is thus

the focal conjugate of E,
£ ™ and F and E are in the

principal axis of S ; that
is to say, these are the re-
lations which exist when
a condenser is focussed on
and centred to an object.
Let this be understood as
the law, and there can be
but little difficulty remain-
ing in getting the best
results from a condenser.

Fig. 300 illustrates
another method of getting
the same result. We may
illuminate a condenser
with light direct from the
flame, as in fig. 299, or we
may interpose the mirror,
as in fig. 300. 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 will,
on the whole, be prefer-
able.

Nothing can be of more moment to the beginner than to under-
stand the practical use of the condenser, We must direct the student

Fig. 300.-

- Another method of getting critical
image.

HOW T<> OBTAIN A CRITICAL l .M IGE

353

I 'i<;. :>0l. — Condenser and ohject-
tflass with the same aperture.

T

Xow only a
at T in the

to what has been stated concerning it in Chapter IV. Hut th,.
following should be carefully considered. Ing. .">0I show! a sub-
stage condenser, S, and an objective, O, /?
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 full of light, as at R. The
same thing, but with the aperture of
the condenser cut down by a stop, is seen in fig. .'502.
part of the back of the objective is tilled with light, as
same illustration.

Now it does not follow, because
the back lens of the objective is full of
light, as in fig. 301, that therefore ///'
field ought to be full of light. The
Meld only shows the bright image of the
edf/e of the jbuiK', and it is in that alone
that o crificot jnetnre can l>< fun n<l '. If
the condenser be racked either within
or without the focus, the whole field
mill become ill n mi noted, but at the same time a far smaller portion
of the objective will be utilised. On removing the eye-piece and
examining the back lens of the objective, pictures like D, H, fig. 297,
will be seen — D when within, and H when without the focus.

The condition represented in fig. 301 at 11 and O is the severest
test which can can be applied to the microscopic objective ; that is
to say, to fill the whole objective with light and so test the marginal
otid central />ortions at the same time.

o

Fig. :>02. — The same, with tho
aperture of the condenser cut
down.

FlG. B03. — Illumination for 1 diffused daylight. '

Even to obtain the state of illumination known as 1 diffused day-
light' with the simple mirror when no condenser is used is frequently
done in a most inaccurate manner. The correct method of doing
this is shown in fig. 303. F is the plain1 of the object. C is the con
cave mirror, the mirror being placed at the distance of its principal
focus from the object. Hut the manner in which it is usuallv done,
from want of thought or knowledge, or both, is shown in tig. 304,

354

PRACTICAL MICROSCOPY

where it is manifest that there is a total disregard of the true focal
point of the mirror and its incidence on the plane of the object.
From the impracticability of this diagram as a representation of a
working plan of illumination, we may see at once the importance of

Fig. 804. — Erroneous method of arrangement for ' diffused daylight.'

Iiaving the mirror fixed upon a sliding tube, so that its focal point
may be adjusted.

It is also important here to note that in daylight illumination

Fig. 305.— Light from the open sky falls upon the mirror in all directions.

a plane mirror gives a cone of illumination, as in fig. 305, when there
is ample sky-room ; but a window acts as a limiting diaphragm.

In regard to the parallelism of the direct solar rays there is of
course no question. But the parallelism of that portion of the solar

Lid IIT HKKLK(TKI) To A I'ocrs FROM THE OPEN SKY 355

light which goes to form the firmament in our own higher atmo-
sphere is so completely broken up by refraction and reflexion
amongst the subtil particles of this higher atmosphere that the rays
which constitute our daylight fall from every point of the visible
heavens (though with greatly diminished intensity;. That is to lay,
we have at disposal a light source extending over 180°, while the sun
itself extends orer a risuul uw/le <>f hut half u rfeyree. Being thus
surrounded by an illimitable and self-luminou.s 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
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
becomes equally clear that in order to strike the object the light
must always fall obliquely mi tin* mirror.

Then it follows from what has been said that the lijjit falling
from the open sky upon a mirror falls in all conceivable directions.
Thus fig. 305 shows the lines 1 to 7, including an angle of 'M)°. Tf
nothing intervene the light of that sky surface must fall upon the
mirror, a b, and be reflected on O. The intermediate rays, % 4,
5, 6, form the conreryiiu/ ill uminatiny pencil^ with of course an in-
finity of others filling up the spaces between.

In other words, every point of a minor is a radiant of a whole
hemisphere, and this is equally true whether the mirror be plane,
concave, or convex, so long as they are exposed to a boundless sky.
Therefore a plane, concave, or convex mirror will give a cone of
illumination of which the object is its apex, no matter what the in-
clination or distance of the mirror. The amde 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 prac-
tice ; therefore it practically does
make a difference whether the
plane or concave mirror is used,
and whetherthe latter is focussed
on the object or not.

The dotted lines in fig, 306
show rays falling on six different
points on a plane mirror: the

continuous lines show the re- Fig. 808.— With the open aky, light ifl
flexions of these rays on the oh- forussed at all points,

ject. The heavy lines from

either cxtremitv of the mirror t». the object show the maximum
angle of cone that mirror will give in that particular position.

The influence of a limitation (as by means of a window) should

356

PRACTICAL MICROSCOPY

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 examination of the back of the lens of the objective when the
eve-piece is removed. Fig. 307 shows the back of the objective when
bhe plane mirror is used, and fig. 301, R, when the concave mirror
is used, as in fig. 303. The beginner should study these experiments
by repeating them.

The correct method of illuminating to obtain a dark ground with
the object luminous is shown in fig. 308. This may also be used
when, by means of that shown by fig. 299, there is not enough of
the field illuminated even when the flat of the flame is used. Of

Fig. 308. — Illumination for dark ground (with
stop beneath the condenser).

regard-

Fig. 307. — Image at the back of the
objective when daylight and a plane
mirror are used,

course it will be understood
that for the dark-ground re-
sult a suitable stop is inserted
beneath the sub-stage con-
denser.

It has been shown by
many illustrations on many subjects that certain results in critical
work can be obtained with the bull's-eye which are not so accessible
without its use. But Mr. T< F. Smith has made this clear
ins' 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 increase
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. 299 or 300, 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 aper-
ture are used, because illuminating
cones up to "8 N. A. can be obtained
- " ffl with good condensers by the

method shown in fig. 299. But
when the microscope is of necessity
used upright the rectangular prism
or the plane mirror must be used,
fig. 300.

The arrangement at fig. 308 is sometimes useful for photo-
micrography when it is otherwise impossible to illuminate the whole
field. But in ordinary cases it is better to contract the field than
use a bull's-eye, as it invariably impairs the definition.

Fig. 309.

-Same result with concave
mirror.

j|(»W TO SET UP l>.\KK-GROl'NI) I L M M I. NATION

357

In regard to tins last figure it will be understood that (as before)
E represents the edge of t he flame, l> the bull's-eye, M the minor, 0
the condenser under the Stage, and F the plane of tlx- object.

The same result as the above may be obtained by the concave
minor (as shown in fig; 309) instead ol tin- bull - eve. Imt this
is a veiy difficult arrangement, yielding the best results Only With
great application and care.

Hut the supmiii' fully of Using a coiie^VB mirror and fl bulFt-eyc
is shown in fig. .'510, where C is the concave minor and (as before) 8
the* sub-stage condenser ; t his secures a result — as will be seen by the
relation of the light to the condenser (S) which is as far froiu what
is sou gilt and desirable as it can well be. While another lesson of
great importance may be Learnt from fig. .'Ill, which illustrates
the error of not lairing the edge of the Home K in the principal foeut

of the hulVs-t'ift: B. The rays converge on the condenser Sj SO thai
it will become in all probability impossible to focus it on the
object. This is a lateral lesson on the value of having the bull's-
eye fixed to the Lamp, SO that both maybe moved together; and
there sJioiihl be a notch in the slot or arm which carries the
lmll's-eye to denote when the flame of the lamp is in its principal

f( )CUS.

Fig. 810. — Absurdity of using a bull's-eye Fig. 311. — Absurdity of using a bull's-
and a concave mirror. eye with the edge of the lamp Maine

not 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 theirdetails 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 tig. .'i08.

Let an object such as a triceratium (diatom) be taken, and sup-
pose that the objective employed is a ^ - inch of '28 N.A. We must
first adjust the lamp and bull's-eye, as in tig. "JIM), and get the edge
of the lamp flame extended to a disc as at A.

Now let a small aperture be put into the condenser and a tri-
ceratium on the stage and the ^-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
assumed — and it should now be arranged as to height, SO that the
rays from the bulTs-eve should fall fairly on the plane mirror, this

358

PRACTICAL MICROSCOPY

latter being inclined so as to reflect the beam on the back of the
sub-stage conde riser.

Now with any kind of light, focus, and place in the centre of the
field, the triceratium, as in fig. 312 ; then rack the condenser until
the small aperture in its diaphragm comes into focus ; centre this,
to the triceratium, as in fig. 313. Rack the condenser closer up
until the bull's-eye is in focus, as in fig. 314.

I lore it, happens that the bull's-eye is not in the centre, and it is
not uniformly filled with light, as in fig. 296, A, 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 less filled with light,

Fig. 312. Fig. 318. Fig. 314. Fig. 315.

and may or may not be more nearly centred. In this case we have
next to centre the image of the bull's-eye to the triceratium by
moving the mirror, as in fig. 315.

But it will be noticed that this centring of the image of the
bull's-eye does not rectify the diffusion of the light. This will be at
once done by moving the lamp with attached bull's-eye ; this motion
requires to be a kind of rotation in azimuth round the wick as an axis.
The relative positions of the lamp and bull's-eye must on no account
be altered, and it is understood that the lamp was adjusted to the
picture A in fig. 296 by inspection and without the microscope. A
very slight movement in azimuth, however, is enough to effect the
desired end (fig. 316), and all that now remains is to open the full aper-
ture 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 remarkable 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 aberra-
tion the condenser will probably have to be racked
up slightly to obtain the greatest intensity of light.

In fig. 316 the expanded edge of the flame
covers the triceratium. When the whole aperture
of the condenser is opened the size of that disc ivill
not be altered, its intensity only will be increased.
When the stop is placed at the back of the con-
denser, 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 objects,

EXAMINING THE KNOWN \NI> THE UNKNOWN

359

bring the lamp Dearer the mirror* The size of the disc of Light

depends on three things : —

a. The diameter of tlie bull's-eye.

/?. The length of the path of the rays from the bull's-eye to the

sub-stage oondenser.

y. The magnifying power of the condenser.

If a and y are constants, the only way of varying the size of the
dark field is by (3.

, I n the same way the intensity (fftfa light in the disc depends on
three things.

A. The initial intensity of the illumination.

I>. The angular aperture of the bull's-eye.

C. The angular aperture of the sub-stage condenser.

If the student will thoroughly and praet ieally 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 two kinds of microscopical work — one, the more usual
and comparatively easy, is the examination of an object to see
something which M known. The other is the examination of an
object in search of the unknown. Thus some blood may be ex-
amined for the purpose of finding a white corpuscle. It matters
little what 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 white corpuscle. It
is quite immaterial as to whether the observer had ever seen one or
not ; so long as he possesses the knowledge that there is such a thing,
the finding of it, even under unfavourable conditions, will be an
easy task.

But if the observer has not that knowledge, he may examine
blood many times, under favourable conditions, and yet not notice
the presence of a white corpuscle, and that, too, with one immediately
in the centre of the field ; this, moreover, is a large object.

It is only those in the habit of searching for new things who can
appreciate the enormous difficulty in first recognising a new point.
Therefore when critical work is undertaken care 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 is required is thai the lamps
should have shades and be placed at such a height that the direct
rays do not enter the observer's eve.

If these precautions are taken, several hours continued work
may be carried on without any injurious effect.

Some observers use only the left eve, some the right, others the
right or left indiscriminately.

360

PKACTICAL MICROSCOPY

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. But in continuous work extending over
many months of long daily observation it is a great gain to use the
binocular, especially with high powers. The effect of years of work
with optical instruments on those possessed with strong normal
sight seems to be an increase in the denning perception accompanied
with 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 blue 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 yellow-
ness is a sign of imperfect achromatism in an objective. We 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
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 neither blue glass nor amnion io-
cupric solution will yield monochromatic illumination. If the light
passing through these media be examined by the spectroscope, red,
yellow, and green will be seen. Some specimens of blue glass pass
more red than others. Out of a number of samples the best result
was obtained by using two thicknesses of cobalt ' pot ' glass, which
gave two bands in the red. If a greater thickness were used the
light became too dim. The ammonio-cupric solution merely dims
down the whole spectrum. When a sufficient thickness of solution
is used to cut out all the red light, then the light is too enfeebled to
be of any use visually, and also, what one would not expect,
photographic exposure is greatly prolonged.

It would seem that true monochromatic illumination obtained by
absorbing media does not exist. Neither does the manipulation of
the source of illumination by means of chemical substances
apparently give satisfactory results.

Coloured light derived from a polariscope and a selenite is not
monochromatic.

There are two ways of obtaining true monochromatic illumi-

MONOCHROMATIC LIGHT — DAYLIGHT

nation. One which would be extremely simple would In- to con-
dense a powerful beam on a diffraction grat ing ruled mm ;i speculum.
The costliness of the apparatus is a bar to this method.

Equally perfect monochromatic illumination can be obtained by
prismatic dispersion. Hartnack designed such an apparatus,1 which
was an ordinary spect roscope.

But it never came into popular use because; with any ordinary
artificial light, such as that from a paraffin lamp, the light was ex-
cessively dim, although it is powerful enough with an electric arc.
The only use it has served is to give oblique illumination, with sun-
light from a heliostat. The following is a method of approximating
to monochromatic illumination used by Mr. Nelson which answers
admirably with an ordinary .',-inch wick paraffin lamp.

He places the edge of the flame in the principal focus of a

lens known as a Wray 5 x 4 RR, working at In the parallel

beam from this lens and close to it he places an equilateral prism of
dense flint set at minimum deviation. Close to the prism he places

another Wray 5 X 4 RR, working at i- . If a cardboard screen

be held at the principal focus of this lens, there will be seen three
images of the edge of the (lame, red, green, and blue, brilliant ly illu-
minated. A slit ^ 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 condenser. For visual
work green is the best, but for photographic work blue would be
chosen unless orthochromatic work required a colour lower down the
spectrum.

For critical vork, such as test inc/ lenses or forcing out the greatest
resolution with the widest-angled oil-immersion lenses, daylight
illumination is inadmissible.

When daylight illumination is used a northern aspect, or at least
one away from direct sunlight, is to be preferred.

It is a good plan where it is possible, to 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 right angle. A screen may be placed parallel
to the window which just allows the mirror of the microscope to
project beyond it. This cuts off 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-

1 Chapter IV. j>. 27'2, fig. 22 1.

362 PRACTICAL MICROSCOPY

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; even with
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 neces-
sary ; but for an object such as a single diatom the lamp flame will
usually be large enough.

In examining diatoms or other objects, such at 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. Professor Abbe does not advise their employment as
in any way final ; he says that ' the resulting image produced by
means of a broad illuminating beam is always a mixture of a multi-
tude of partial images which are more or less different and dissimilar
from the object itself ; ' and he does not conceive that there is any
ground for expectation ' that this mixture should come nearer to a
strictly correct projection of the object . . . than the image which
is projected by a 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 the
images they present. This is the more a necessity since Mr. Nelson
has 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 images
with the coarser diatoms. But with wide cones he has proved that
these false images cannot be produced ; and that when the true image
is reached by a wide cone, the image is not altered by any change of
focus, but simply fades in and out of focus ' as a daisy under a 4-
inch objective.'

Mr. Nelson has photographed all these results, and we have seen
them demonstrated. When theory and practice are thus at variance
we must pause for further light.

If it is required to accentuate a known structure, such as the per-
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 gives
on slight focal alterations a variety of patterns like a kaleidoscope ;
with a wide-angled cone a single structure gives a single focus, i.e.
it goes completely out of focus on focal alteration. When a large-
angled and a wide-angled objective are used a change of pattern only
occurs when the structure is fine. This practical observation has its
value and must not be forgotten.

To properly display objects under a microscope is to a certain ex-

How TO DISPLAY <>1UK<TS MICROSCOPICALLY

tent an art, for it not only demands dexterity in tlie manipulation
of the instrument and its appliances, hut it also requires knowledge
of what sort of illumination is best suited to the particular obji
For learning the manipulation of the instrument no class of objed
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 -,4n of SO , a good
binocular, and a dark-ground illumination will give a fine cM'ect. This
is ijot 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 bo
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,
are seen very well when mounted dry on rover by means of a ^ -inch
objective ami a lAelterki'thn ; the bull's-eye and the plane mirror should
be used. Some objects are so transparent, or become so transparent
in the medium in which they are mounted, that they will not bear a
large illuminating cone, the brightness of the illumination destroying
the contrast. It will illustrate this when we recall that dirt on an
eye-piece which is quite invisible in a strong light becomes im-
mediately apparent in a feeble light. Thus animalcules require a
small cone of illumination when they are being examined, particularly
with a {-inch objective ; for a general view of 1 pond life ' a H-inch
objective with a dark-ground illumination, employing a binocular, is
very suitable. Stained bacteria in tissue are best seen with a large
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 tigs. 2 and 3 in the frontispiece. Fig. •"> is
a critical image magnified olO 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 t<>
insect hail's, but grow directly from a delicate membrane.

This photograph was taken with an apochromatie J of '95 N.A.
and No. \\ projection eye-piece ; and it was illuminated with a largo
solid cone of 'Go N.A. from an achromatic condenser.

F'ig. 2 is an uncritical image, with all the conditions as above,

1 Directions for the use of Abbe's illuiniimt in*,' apparatus — a leaflet issued by
Carl Zeiss, 1888.

364

PRACTICAL MICROSCOPY

save that a cone of small angle, i.e. of 0*1, was used for illumina-
tion.

The first alteration which thrusts itself upon the eye is the
doubling of the hairs, which are in the least degree out of focus.
But 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 the rather a distinct loss is incurred, by making
the illuminating cone larger than 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 attributed 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 base. In reality the 'stiff and
longish bristle ' is an extremely long and delicate filament, totally
unlike a bristle, being not tapered but of nearly uniform thickness.
The ' minute spines ' are in reality very curious hairs, and, as far as
we at present know, unlike any others. They are delicate, lambent,
bulbous hairs. What they most resemble are the tentacles of a sea-
anemone, and there are two tubes discoverable which are important
and comparatively large objects. There appears to be considerable
probability that this interesting object upon the last ring of the body
of the flea, and known as its 'pygidium,' acts as an auditory instru-
ment.1 In the examination of ordinary stained histological and
pathological sections by transmitted light, unless some very delicate
point is sought, the condenser 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.
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 -| and ^ and one of *6 N.A.
for the -\ and jr. 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 importance, as more than nine- tenths

1 Micros. Journ. April 24, 1885, ' Pygidium of Flea' (E. M. Nelson).

THE USE OF LARGE ILLUMINATING CONEN

3«5

of all microscopic objects are examined by means of transmitted
light.

Let us now note the effect of large cones on the simplest obj. < t
A microscope is set up having an achromatic condenser with an irii
diaphragm ; let three good wide-angled objectives be chosen, say I

inch, a .',-inch, and J -inch dry. Let the object be the one we have
already studied to some extent in this relation, viz. one of the stiff
hairs on the maxillary palpus of the blow-fly's tongue ; place the 1-
inch on the nose-piece, open the full aperture of the condenser and get
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 w ill appear as a different object, its outline being perfectly elear
and sharp. If the eye-piece is removed about two-thirds of the ob-
jective back will be full of light. Now, without disturbing any of the
ad justments, replace the 1-inch by the and itwill be found that 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
the back of the objective, two-thirds of it will be found full of light, and
so on with the \. We call the attention of the student to these facts
as having a direct bearing upon the question of the comparative effects
of large and small illuminating cones, and with no idea of offering
opposing opinions to those of Professor Abbe, we have no 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
01 f the aperture of the illuminating cone to that of the objective cone.
Apochromatic objectives behave precisely as achromatic objectives
in this respect. Of course, if the hair becomes pale and indistincton
the opening of the iris, it shows that then4 is uncorrected spherical
aberration in the objective ; another objective must therefore be used :
that paleness has nothing whatever to do with the glaze or false light
mentioned above.

In photo-micror/raphs of bacterid one frequently sees a white halo
round them. This in our hands has always resulted from the photo-
micrograph having been taken with too small a cone of illumination.
Photo-micrography with ;i small cone is quite easy, as great contrast
can be secured. With a large cone the difficulties begin ditliculties
of adjustment, ditliculties 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 micrometry
it is essential that the edges of the object should be defined : COnSG
quently a large cone must then be employed.

For the examination of Polycystines, Foraminifera, decs binocular
is useful ; illumination may be by a Lieherkuhn if mounted dry, and
by dark ground by a condenser if mounted in haUam. Parts of
insects should be usually examined with dark-ground illumination ;

366

PRACTICAL MICROSCOPY

whole insects are seen best with the Lieberkiihn, and the binocular
should be used for both.

Home of this class of objects are best seen under double illumina-
tion ; that is, a dark ground with a condenser and light thrown
from ((hove with a silver side-reflector, as the Lieberkiihn cannot be
used in conjunction with an achromatic condenser. It is a good
plan with low-power Lieberkiihn work to interpose between the slip
and the ledge a strip of plain glass h -inch wide : this prevents the
ledge stopping oat light from the Lieberkiihn when it is larger in dia-
meter than the slip.

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 lamp light it is
as well to combine a piece of blue glass with it. With objectives of
greater angle than -6 N.A. it is usually difficult to get satisfactory
illumination with a dark ground. The best that can be done is to
use an oil-immersion condenser with a suitable stop : this will give
a good dark ground up to '65 N.A., but it will fail if the object is
dry on the cover. Generally speaking, the only way of accomplishing
this with objectives of wider aperture is to reduce the aperture of
the objective by a stop placed at the back.

When a condenser is united by a film of oil to a slip, if the slip
is thin, the oil invariably runs down when the condenser is focussed.

The following is a method by which this may
be entirely prevented. A piece of thick
cover-glass about "02 inch, and 1 inch square,
has a strip of thicker glass ^-inch broad,
cemented by shellac to one edge. This piece
of glass is oiled to the slip, the ledge being-
hooked over the top of the slide ; this not
only prevents its slipping down, but also keeps
the oil from creeping out at the bottom, which
would be the case if the two edges of the
glass coincided.1 This is illustrated in fig.
317.

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 sub-
ject. Here a direct practical presentation of
the matter may be of service to the student.

A normal unaided human eye can divide
_o..i_ inch at ten inches. Consequently a
microscope with a power of 200 should be capable of showing-
structure as fine as tattoo inch. Now as this power can be made
up by a ^-inch objective and a 1-inch eye-piece, it follows that
sufficient aperture ought to be given to the ^-inch to enable it to
resolve 50,000 lines per inch. This 2 will be -52 N.A. The inch

1 Q. M. C. Journal, November 1885.

2 In reality it will require more, because an axial cone is assumed to be used
instead of an oblique beam.

Thin slip of glass with
ledge to place glass slip
with oil contact, so as
to vary the thickness of

I a slide.

L |lllll|llillllil|lir:^

.Slide in situ on thin slip
with ledge.

Fig. 317.

Sl'MMARY OF Ql ALITJEa OF OBJBCTXVES-^THEIB TE8T8 j6?
objective should has e half this aperture, and the \ double, and the

1 four times as much, if perfect vision is required ; in other VTOldfl,
'26 N.A. for every 100 diameters.1 These i deals ha\ e (as u <• have

before indicated) been realised, notably by the Zeiss apochromatic .
the 1-inch and the i-inch 2 resolving everything capable of being ap

preciated by the eye when the I '_' compensating eve piece fa used.

The J-inch is also a near approach to the ideal, as ii has been vety
wisely kept a dry lens. The £ with a (5 eye-piece also attains the
idcaj. This relation of aperture to power is very significant, and
should be carefully pondered by those who still desire low apertures
as only perfect form of objectives.

It is as well to mention that objectives may be arranged in two
series — one the 2, 1, [„ \, and the Other 1 ;;, .J, One of

these series will form a complete battery, as it is unnecessary to
have objectives differing from the next in the series by less than
double the power.

The most usual combination is perhaps the 1 and the j of one
series, or the § and the ('; of the other. Of these two preference
might rather be given to the last. The only exception would be the
addition of a 1^-inch for pond life.

Eye-pieces should also double the power thus ; ."3, 10, and 20
(uncompensated), or (>, 1 2, and 27 (compensated), the most useful of
the three being the 10 (uncompensated) and the 12 (compensated).
As there is 110 6 -power compensated eye-piece for the long tube, a -1
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.
."). Kesolving power.

4. Penetrating power

5. Illuminating power.
(>. Flatness of field.

7. 1 defining power.

1. Mdgnifyirif/ potrer. No test is required, as the initial magni-
fying power can be directly measured.

2. Aperture or N.A. can be directly measured ; no test is there-
fore necessary.

."). "Resolving power, being directly proportional to N.A., is syno-
nymous with it. No test is needful because N.A. can be measured.
1. PenetrntiiKj pott er is the reciprocal of the resolving power of

. No test needed.

N . A .

f> Illumhitttiny potrer is the square of the numerical aperture
(N.A.)2. No test is necessary.

1 English Mechanic, vol. xxxviii. 1888, No. 97!).— E. M. Nelson.

2 This lens, with an 8 compensating eye-piece, will resolve a Plcuroricjnin
angulatum with an axial cone: this is the lowest power with whieh it lui> . n r DQQD
done.

368

PRACTICAL MICROSCOPY

6. Flatness of field is, in the strict meaning of the term, an
optical impossibility. The best thing therefore is to contract the
visible field as is done in the compensating eye-pieces. (Tests: For
low powers a micro-photograph; for medium and high powers a stage
micrometer.

7. Defining 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
their optic axes, (ii.) the parallelism of their planes, (iii.) the setting
of their planes at right angles to the optic axis.

Defining power can only be tested by a critical image. The
following is a list of suitable objects of which a critical image is to
be obtained, using a solid axial cone of illumination equal to at least
three-fourths of the aperture of the objective.

Very low powers (3-, 2-, and 1^-inch). — Wing of Agrion pul- •
chellum £ (dragon-fly).

Low powers (1 and §). — Proboscis of blow-fly.

Medium powers (|, J, and low-angled }). — Minute hairs on
proboscis of blow-fly ; hair of pencil-tail (Polyxenus lagurus) ;
diatoms on a dark ground. This last is a most sensitive test ; unless
the objective is good there is sure to be false light.

Medium powers (with wide aperture). — Pleurosigma formosum ;
Navicula lyra in balsam or styrax; Pleurosigma angulatum dry on
cover ; bacteria and micrococci stained.

High powers (wide apertures and oil-immersion J and y1^.- — The
secondary structure of diatoms, especially the fracture through the
perforations. Navicula rhomboides from cherry field in balsam or
styrax ; bacteria and micrococci stained.

Test with a 10 or 12 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 rolling the milled head of the fine 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
the first and easiest thing mastered by the tiro.

Beyond the correct and judicious use of the microscope and all
its appliances, there is the matter of the elimination of errors of 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

KKROKS OK lNTKRI'KKTATJO.N

■which it presents to him when examined in the various modes now
specified will necessarily depend in a great decree upon his previotl
experience in microscopic observat ion and upon liis k now lei lg€ 1 >i
the class of bodies to which the particular Bp6Cimei] may belong*
Not only are observations of any kind liable to certain fallacies
arising out of the previous notions whicli the observer may entertain
in regard to t he constitution of the objects or the nature; of t he 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 any scientific investigation were fully made known
it would generally appeal- that the stability and completeness of the
•conclusions finally 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 maybe proved to be fallacious at some future time, per-
haps even by our own more extended and careful researches. The
suspension of the judgment whenever there seems room for doubt is
a lesson inculcated by all those philosophers who have gained the
highest repute for practical wisdom ; and it is one which the miero-
scopist cannot too soon learn or too constantly practise. Besides these
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 adjitfifmrnit are not at all uncommon amongst mieroscopists,
and some of the most serious arise from the use of small cones of
illumination. With lenses of high power, and 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
more or less indistinct, but is often wrongly represented. The in-
distinctness of outline will sometimes present the appearance of a
pellucid boi-der, which like the ditl'raction-band may be mistaken
for actual substance. But the most common error Is that which is
produced by the reversal of the lights and shadow s resulting from the
refractive powers of the object itself ; thus, the bi-concavity of the
blood-discs of human (and other mammalian) blood occasions their

B U

37o

PRACTICAL MICROSCOPY

centres to appear dark when in the focus of the microscope, through
the divergence of the rays 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, or 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 on experienced microscopists, 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 lacunai and canaliculi of bone, which were long supposed
to be solid corpuscles with radiating filaments of peculiar opacity,
instead of being, as is now universally admitted, minute chambers
with diverging passages excavated in the solid osseous substance.
When Canada balsam fills up the excavations, being nearly of the
same refractive power with 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-
hubbies, 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 inquiry will be in regard to
their meaning.

Although no experienced micrdscopist could now be led astray
by such obvious fallacies as those alluded to, it is necessary to
notice them as warnings to those who have still to go through the
same education. The best method of learning to appreciate the
class of appearances in question is the comparison of the aspect of
globules of oil in water with that of globules of water in oil, or of
bubbles of air in water or Canada balsam. This comparison may
be very readily made by shaking up some oil with water to which
a little gum has been added, so as to form an emulsion, or by
simply 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

1 Monthly Microscojrical Journal, vol. v. 1872, p. 14.

STUDIES JN INTERPRETATION

371

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

of the focus of an air-bubble in water and Canada balsam, and of ;i
fat-globule in water, may be thus illustrated, viz. a diaphragm of
about rj of a mm. being placed at a distance of 5 mm. beneath the
stage, and the concave mirror exactly centred.

Air-bubble8 in water. — No. 1 (fig. 318) represents the dihVrcni
appearances of an air-bubble in water. On focussing the objective

Fig. 818. — Air-bubbles in (1) water, {'!) Canada balsam; (:\) fat-globules in water.

to the middle of the bubble (B), the centre of the image is seen to be.
very bright — brighter than the rest of the Held. It is surrounded by
a greyish zone, and a somewhat broad black ring interrupted by one
or more brighter circles. Hound the black ring are again one or
more concentric circles (of diffraction), brighter than the field.

On focussing to the bottom of the bubble (A) the central white
circle diminishes and becomes bl ighter : its margin is sharper, and
it is surrounded by a very broad black ring, which has on its
periphery one or more diffraction c ircles.

When the objective is foOUSSed to the upper surface of the

1! B I

.372

PRACTICAL MICROSCOPY

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-bubbles in Canada Balsam. — Canada balsam being of a
higher refractive index than water, the limiting angle instead of
being 48° 35' is 41° only, so that the rays which are incident much
less obliquely on the surface of separation undergo total reflexion,
and it will be only those rays which face very close to the lower
pole of the bubble that will reach the eye, and the black marginal
zone will therefore be much larger. This is shown in fig. 319, 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 (C) 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. 318, 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 objective 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
the centre.

These appearances are the converse of those presented by the
air-bubble. That, as we saw, has a black ring and a white centre,
which are the sharper as the objective is approached to the lower
pole of the bubble. The fat-globule has, however, a dark ring
which is the broader, and a centre which is the sharper, according
as the objective is brought nearer to the upper pole.

These considerations, apart from their enabling us to distinguish
between air-bubbles 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
which is sharper and smaller, and a black ring which is larger when
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.

INTERPRETING MOVEMEN r

373

A fat-globlllc, indeed, seems to he comp< »scd of ;i series of cmi-
centric layers like a grain of starch. With blue light these fringes
are also multiplitied, but are closer together and finer, so that thej
are 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 ease
they are not exaggerated by monochromatic light ; but if, on the
contrary, they are found to be doubled, or quadrupled, with this
light, we may be certain that they are diffraction fringes.

]>ut 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 movement as such, and especially concerning
the movement exhibited by certain very minute particles of matter in
a state of suspension in Jin ids. 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 the focillo, 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. J>ut it was discovered in IS 27, by Dr. Robert
Brown, that inorganic substances in a state of tine 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 contents
of the fluid cavities in quartz in the oldest rocks. These have
probably retained their dancing motion for aeons. 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 discoverer,
as Brownian movement, but now more generally called perf* sis.

The movement is chiefly of an oscillotori/ nature, but the particles
also rotate backwards and forwards on their axes, and gradually (if
persistently watched) cltawje 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 (he
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.

The movement of the smallest particles in pedesis is always the
most active, while in the majority of cases particles greater than the
.-, (Mloth of an inch are wholly inactive. A drop of common ink
which has been exposed to the air for some weeks, or a drop of tine
clay (such as the prepared hiolin used by photographers) shaken up
with water, is recommended by Professor .Jevons,1 w ho has recently
studied this subject, as showing the movement (which he designates
1 Quarterly Journal of Micro. Science, N.S. vol. viii. Is7f\ p. 17*2.

374

PRACTIC AJ a M I.CHOSCOPY

pedesis) extremely well. But none of the particles he has examined
are so active as those of pumice-stone that has been ground up in an
agate mortar; for these are seen under the microscope to leap and
swarm with an incessant quivering movement, so rapid that it is
impossible to follow the course of a particle which probably changes
its direction of motion fifteen or twenty times in a second. The
distance through which a particle moves at any one bound is usually
less than -rJm)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 off
from all possibility of evaporation ; and it has been known to con-
tinue for many years in a small quantity of fluid enclosed between
two glasses in an air-tight case ; and for the same reason it can
scarcely be connected with the chemical change. But the observa-
tions of Professor Jevons (loc. cit.) show that it is greatly affected
by the admixture of 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 J evons 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 aggregate
and sink, so that the liquid clears itself.1

Pedetic motion depends on, that is, is affected by —

1. The size of the particles.

2. The specific gravity 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. The nature of the liquid. No liquid stops pedesis, but liquids
which have a chemical action on the substance do. This action may
be very slow, still it tends to agglomerate the particles. For in-
stance, barium sulphate, when precipitated from the cold solution,
takes a long time to settle ; whereas, when warm and in presence
of hydrochloric acid, agglomeration soon occurs. Iron precipitated
as hydrate in presence of salts of ammonium, and mud in salt water,
are other instances. The motion does not cease, but the particles
adhere together and move very slowly.

But besides the right appreciation of the nature of pedesis, there
is the utmost caution required in the interpretation of the rapidity of
movement, and kind of movement which living and motile forms effect.

The observation of the phenomena of motion under the microscope 2
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

1 See also the Rev. J. Delsaulx 'On the Thermo-dynamic Origin of the Brownian
Motions ' in Monthly Journ. of Microsc. Sci. vol. xviii. 1877.

2 Das MikrosJtop, Naegeli and Schwendener, p. 258 (Eng. edit.).

MOVEMENT I N MICROSCOPIC OBJECTS

375

a metre in a hour. It must not, therefore, be forgotten lli.il ili«*
rapidity of motion of microscopical objects LB only an apparent one.
and that its accurate estimation is only possible by taking as our
standard the actual ratio between time and spare. If we widi, for
the sake of exact comparison, to estimate 1 1 1 < - magnitude of t In- 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 tin; 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, appeal- exactly as if the
movement were a true serpentine one. As an example of an appear-
ance of this nature, we may mention the alleged serpentine motion
of Spirillum and 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 thU category belong, for instance, the supposed oscillations of
the 08ciUariccei 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 cast's, if the object
is very small and the contents homogeneous, it is
quite impossible. The slight variations from cylin-
drical or spherical form, as they occur in each cell,
are therefore just sufficient to admit of our perceiv-
ing whether any rotation does take place. The
discovery of the direction of the rotation is only
possible when tixed 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, dec. : we must be able to distinguish
clearly which are the sides of the windings tinned
towards or turned away from us.

If the course of the windings is very irregular,
as in tig. 319, a little practice and care are needed
to distinguish a spiral line as such in small ob-

• . mj • • i • -i -iii 1 '"• •>1;, — A spiral

JOCtS. IJie microscopical image might easily lead in motion.

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, BS they

originate from the epidermis cells of many seeds, were thus inter

376

PRACTICAL MICROSCOPY

pretecl, although here and there by the side of the irregular spirals
quite regular ones are also observed.

Moreover, it must not be forgotten that in the microscopical
image a spiral line always appears wound in the same manner as
when seen with the naked eye, while in a mirror (the inversion being
only a half one) a right-handed screw is obviously represented as
left-handed, and conversely. If, therefore, the microscopical image
is observed in a mirror, as in drawing with the Sommering mirror,
or if the image-forming pencils are anywhere turned aside by a
single reflexion, a similar inversion takes place from right-handed to
left-handed, and this inversion is again cancelled by a second reflex-
ion, in some microscopes. 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 appearance of the
structure will be double the fineness of the actual structure which is
causing the interference.1

Upon this law there appears to depend a number of possible fal-
lacies, errors which may arise from either its misapprehension or misin-
terpretation. At least these appear to us, from a practical point of
view, to be of sufficient importance to need either caution or a
fuller exposition of the great law of Abbe in regard to them.

If, for example, figs. 320, 321, and 322 may be taken to represent
a 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. 321, then when/ the grat-
ing is focussed at P the smetww of
the first order only will be brought
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 P7 the diffraction elements of the second order only are
brought to the focal conjugate ; consequently by the hypothesis the
image will have 50,000 holes per linear inch, or double that of the
original. In other words, placing a grating at the longer focus of an
over-corrected objective is apparently tantamount to cutting out the
diffraction spectra of the first order by a stop at the back of the-
objective.

1 See Chapter II.

Fig. 320. Fig. 821.

INTERPRETATION AND THE N.A. TABLE

377

Tlie effect of this is to give an impression that there i* a strong
grating with 25,000 holes per Linear inch ; and over it nnotJur grat-
ing with 50,000 holes per linear inch. The raising the focus " u

to bring P to P' necessarily gives the idea of t Ik- fine structure being-
gape rim posed on tlie coarse. Therefore; the inicioscopist should
beware whenever he notices a structure of double fineness over
another one lest lie has a condition of things similar to fig. 321. 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. 32 1 ;
next by means of the draw-tube increase the dis-
tance 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 ob-
tained as in fig. 322. 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 tnl»s j,/oc the 50,000 grating m
a mere <l iff met ion ghost.

A precisely similar condition of things exists with an under-
corrected objective, only in that case the false finer grating will
appear below the original coarse grating, and to increase the distance
between them the draw-tube must be shortened.1

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. 323 gives six illustrations of the back of an objective (the
eye-piece being removed) of '$3 N.A., or 112° in air : I) stands for

Fi<;. :v>->.

O

dioptric beam; 1 for diffraction spectrum of the Qrsl order; 2 for
diffraction spectrum of the second order.

1 It is well to note here that we hnve seen a photo-micrograph hy Mr. Comheio/
a diatom surface which is uneven. In those part* in correct focus the structure ia
single, hut in the parts where the focus is withdrawn it is douhled.

378

PRACTICAL MICROSCOPY

When the back of an objective of -83 N.A. shows an arrange-
ment as in

No. I then, although the structure will be invisible,

Lt cannot be coarser than . . . 40,000 per inch.
No. 2 „ „ „ 80,000

NO. 3 I ben i he st ructure does not differ greatly from 40,000
No. 4 „ „ „ 80,000

No. 6 „ „ „ 20,000

No. 6 „ „ „ 40,000

[t will be understood by the student that the preservation of the
microscope 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 the lenses employed with care ;
and all processes involving the use of the vapours of volatile acids,
or which develop sulphuretted hydrogen, chlorine, &c. must never
take place in a room in which a microscope of any value is placed.

Dry elder-pith and Japanese paper are by some workers sug-
gested for cleaning the front lenses of homogeneous objectives ; but
while these are excellent, especially the former, we rind nothing-
better than the simple cambric we suggest.

Two or three good chamois leathers should be kept by the
worker for specific purposes and not interchanged. Cleanliness, care,
delicacy of touch, and a purpose to be accurate in all that he does or
seeks to do, are essentials of the successful microscopist.

It may be noted that dust on the eye-piece can be detected in a
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.

379

CHAPTER VI]
PJ&EPABATION, MOUNTING, AND COLLBQXION OF OBJECTS

UNDBB this head it is intended to give an account of those materials,
'Instruments, and appliances of various kinds, w hich have been found
most serviceable to microscopists engaged in general biological re-
searcli, and to describe the; most approved methods of employing
them in the preparation and mounting of objects for the display of
the minute st ructures thus brought to our knowledge. Not only is
it of the greatest advantage that the discoveries made by microscopic
research Bhould -as far as possible — be embodied (so to speak) in
* preparations,* which shall enable them to be studied by everyone
who may desire to do so, but it is now universally admitted thai
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. Jt 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
<>f peculiar interest ; and as the histological student can lind 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 Applian< k>.

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 inch.. For objects too large to be
mounted on these the size of •*> in. by 1 in. may be adopted. Such
slips may be purchased, accurately cut to size, and ground at the
edges, for so little more than the cost of the glass that few persons
to whom time is an object would trouble themselves to prepare
them ; it being only when glass slides of some unusual dimensions
are required, or when it is desired t<> construct ' built-up cells,' that
a facility in cutting glass with a glazier's diamond becomes useful.
The glass slides prepared for use should be free in mi \ eins, air-bubbles,
or other flaws, at least in the central pari OH which the object is
placed ; and any whose defects render them unsuitable for ordinary
purposes should be selected and laid aside for 0868 to which the

380 PREPARATION, MOUNTING-, AND COLLECTION OF OBJECTS

working microscopist will find no difficulty in putting them. As
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. Great 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 afterwards 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 sodre,
finishing with clean water.

Thin Glass. — The older microscopists were obliged to employ thin
laminae 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, down to the ^^-(7th 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 low
powers ; the second, which should not exceed -005 in. in thickness,
for objects to be viewed with medium powers ; and the third, which
ought never to exceed *004 in. in thickness, for objects which either
require or may be capable of being used with hi<jh 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

LEVEE OF CONTACT

handle safely, and is most liable t<> tract are from accidents of i ari< m

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 t lius 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 pur] o>e
very well; but Ross's lever of contort (fig. 324), 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 ) (HH.tM °f an inch, so that the entire arc measures the
- \ , 1 1 1 of an inch ; at the other end is a pivot on which moves a Long and

Fig. 324. — Ross's lever of contact.

•delicate lever of steel, whose extremity points to the graduated arc,
whilst it has very near its pivot a sort of projecting tooth, which
bears it against a vertical plate of steel that is screwed to the
horizontal table. The piece of thin glass to be measured being in-
serted between the vertical plate and the projecting tooth of the
lever, its thickness in thousandths of an inch is given by the number
on the graduated arc to which the extremity of the lever points.
Thus, if the number be 8, the thickness of the glass is -008, or the TJ . 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
size or nature it is impossible to apply high powers, it is 1 tetter 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 by
Zeiss, of Jena, and illustrated in fig. .'»L)\ It will be seen that the
measurement is effected by a clip projecting from a box, between the
jaws of which the cover to be measured is placed ; the reading is
given by an indicator moving over a divided circle on the upper face
of the box. The divisions show hundredths of a millimetre, and the
instrument measures to upwards of 5 mm.

3<S2 PREPARATION, MOUNTING-, AND COLLECTION OF OBJECTS

Tt 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 thick-
ness of the covers it
contains. What is then
required is a simple ap-
paratus for cleaning the
delicate covers with the
least risk of breakage.
This can be well - accom-
plished by having two
blocks of boxwood,
shaped so as to be easily
held one in each hand,
turned with perfect true-
ness 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 and covers by hand, finishing should be done
with old fine cambric handkerchiefs. These should not be washed
with soap, but with common soda and hot water, plenty of the latter
being subsequently employed to get rid of every trace of the alkali.
But when dry these cloths must not be ' ironed ' or smoothed in any
way, the ' rough-dry ' surface acting admirably for wiping delicate
glass.

Varnishes and Cements. — There are three very distinct purposes
for which cements which possess the power of holding firmly to glass,
and of resisting not merely water but other preservative liquids,,
are required by the microscopist, these being (1) the attachment of
the glass covers to the slides or cells containing the object, (2) the
formation of thin 1 cells ' of cement only, and (3) the attachment of
the 1 glass plate ' or ' tube-cells ' to the slides. The two former of
these purposes are answered by liquid cements or varnishes, which,
may be applied without heat ; the last requires a solid cement of
greater tenacity, which can only be used in the melted state. Among
the many such cements that have been recommended by different
workers, two or three will be selected by the worker for general
purposes, and perhaps three or four for special purposes, and the re-
mainder will be in practice neglected. We do not hesitate to say

Fig. 325. — Zeiss's cover-glass tester.

VARNISHES AND CEMENTS

383

that the two cements on which the most complete trust may I j« - re-
posed M i1 j<(p(vnner*t gold rize and l»<lVa cement. This opinion is the

result of over twenty years of special observation.

A good von t'tsh may easily, in a general way, he tested \ when it i
throughly hard and old, if scraped otl' it comes away in shreds ; unsafe
varnishes break under- the scraper in flakes and dust. To those who
put up valuable preparations and objects of value the ri^k should
never be run of using a new and unknown varnish Of cement.
Neither appearance nor facility nor cheapness in use should for one
moment weigh against a varnish Or cement of known and tested
\\ 1 nth.

Jojki ,1 icr's gold-size may be obtained from the colour shops. I'
may be used for closing-in mounted objects of almost any descript ion.
It takes a peculiarly firm hold of glass, and when dry it becomes
extremely tough without brittleness. When new it is very liquid
and 'runs' rather too 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 l ings 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.

Atphalte Varnish. — This is a black vamish made by dissolving-
half a drachm of caoutchouc in mineral naphtha, and then adding 4 <>z.
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 Authors experience leads him to recommend that
it should only be employed either for making shallow i cement cells'
or for finishing oft" 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 Al>i> s Imlsa men and PiftUB
canadensis j 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 Arc. Although fresh, soft balsam may be hardened by heating
it on the slide to which the object is to be attached, yet it may be
preferably hardened > n ma886 by exposing it in a shallow vesstl to

384 P RE PA RATION, MOUNTING, AND COLLECTION OF OBJECTS

the prolonged but moderate heat of an oven, until so much of its
volatile oil has been driven off' 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
>e fcransf erred 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.

Brunswick black is a very useful cement, obtainable at the op-
tician's as prepared for the use of microscopists. It is one of the best
■cements for the purpose of ringing mounts, and it may be recom-
mended for turning cells. We have already stated that we do
not, as a rule, recommend opaque or black ground mounting ; but if
this is desired or needful no better ' ground ' can be obtained than
foy 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 hon>?y 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.
It 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 with homo-
geneous lenses. It is a sure protection against the otherwise in-
jurious action of the cedar oil. Hollis's liquid glue may also be
employed with confidence for this purpose.

Sealing-wax varnish, which is made by digesting powdered
sealing-wax at a gentle heat in alcohol, should never be used as a
cement ; it is serviceable only as a varnish, and resists cedar oil.

Venice turpentine is the liquid resinous exudation of Abies larix.
It must be dissolved in enough alcohol to filter readily, and after
filtering must be placed in an evaporating dish, and by means of a
sand-bath must be reduced by evaporation one-fourth.

This cement is used for closing glycerin mounts. Square covers
are used, and we find it best to edge the cover with glycerin jelly.
A piece of copper wire of No. 10 to No. 12 gauge is taken, and one
end of it is bent just the length of one of the sides of the cover at
right angles to the length of the wire. This end is now heated in a
spirit lamp, plunged into the cement, which adheres in fair quantity,
and is instantly brought down upon the slide and the margin of the

cnLnl'KKI) VAKNISHKS — hi;Y MOUNTING

cover. The fluid turpentine distributes it. self evenly along theCOVeV
and slide and hardens at once. We have no long experience of it,
but from souk; of its characteristics we are inclined in l« li< \<- it will
prove a useful cement for this purpose.

Marine (flue, 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 qualit [es
of this substance are made for the several purposes to which it is ap-
pliedj and the one most suitable to the wants of the micrOSCOpist
IB 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
or metal rings which thus form 'cells 'for the reception of objects
to be mounted in fluid, no 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 block varnish of this kind is made by working up very
finely powdered lamp-black with gold-size. For redt 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 forcuminifi ro,
parts of insects, <tc), 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 arabic
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 bv
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 theobjeel to be mounted cdryJ
(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
prevent the varnish applied round its border from running in.
"Where a somewhat deeper cell is required, 1Y<>i. EL L Smith

c c

386 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

(.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
a glass slide, and the cover, with the object dried upon it, laid on the
ring, it is to be held in its place by the forceps or spring-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 5 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 (p. 391). 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 positionis 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
laid on. It will be found convenient to make a considerable number
of such cells at once, and to keep a stock of them ready prepared for
use. If the surface of any ring should not be sufficiently level for a
covering glass to lie flat upon it, a slight rubbing upon a piece of
fine emery paper laid upon a flat table (the ring being held down-
wards) will make it so.

Bing-cells. — For mounting objects of greater thickness it is
desirable to use cells made by cementing rings, either of glass or metal
to the glass slides, with marine glue. Glass rings of any size, dia-
meter, thickness, and breadth are made by cutting transverse sections
of thick-walled tubes, the surfaces of these sections being ground
flat and parallel. Not only may round cells (fig. 326, A, B) of vari-
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 quad

M<>r.\TIN<; IN ( KLLS

3*7

A

rangular, or square, or oblong (0, D). Por intermediate thickw ■

between cement-cells and glass ring-cells, the Kditor has found no
kind more convenient than the rings stamped out of tin, of rariou
thicknesses. These, after being cemented to the slides, should have
their surfaces made perfectly flat by rubbing on a piece of tine ^rit
or a corundum-tile, and then smoothed on a Water-of-Avr stone :
to such surfaces the glass covers will he found to adhere with great
tenacity. The ebonite and hone-cells are cheap, and also easy of
manipulation. They are specially useful for dry mount-.

The glass slides and cells which are to he 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 preferable. 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 com-
pletely covered with liquefied
glue, the cell is to be taken
up with a paii- of forceps,
turned over, and deposited in
its proper place on the slide ;
and it is then to be firmly
pressed down with a stick
(such as the handle of the
needle), or with a piece of
flat wood, so as to squeeze out
any superfluous glue from
beneath. If any air-bubbles
should be seen between the
cell and the slide, these •
should if possible be got rid of by pressure, or by slightly moving
the cell from side to side ; but if their presence results, as is some-
times the case, from deficiency of cement at that point, the eel]
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 cement ing has been satis-
factorily accomplished, the slides should be allowed to cool gradually
in order to secure the firm adhesion of the glue , and thi^ j> readily
accomplished, in the first instance, by pushing each, as it is finished,

o o S

c

I)

Fie. 82(5. — (iluss ring-cells.

388 PKEPARATION, MOUNTING, AND COLLECTION OF OBJECTS

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 prevent them from cooling too fast. Before they
are quite cold, the superfluous glue should be scraped from the glass
with a small chisel or awl, and the surface should then be carefully
cleansed with a solution of potash, which may be rubbed upon it
with a piece of rag covering a stick shaped like a chisel. The cells,
should next be washed with a hard brush and soap and water, and
may be finally cleansed by rubbing with a little weak spirit and a
soft cloth. In cases in which appearance is not of much consequence,
and especially in those in which the cell is to be used for mounting-
large opaque objects, it is decidedly preferable not to scrape off the
glue too closely round the edges of attachment, as the 'hold' is
much firmer, and the probability of the penetration 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 recommended
that all cells which require marine glue cementing be purchased
from the dealers in microscopic apparatus, and it is well to note that
all cells cemented with marine glue should be well 'payed,5 as the

nautical expression is,,
or well surrounded with
shellac varnish, or gold-
size as indicated by the
nature of the enclosed
fluid. Many media, saline .
fluids especially, work
their way between the
cell and the slide, and at
length destroy the marine

A

B

C

glue.

Plate-glass Cells. —

Where large shallow cells
with flat bottoms are re-
quired (as for mounting
zoophytes, small medusa',
&c.), they may be made
by drilling holes in pieces
of plate-glass of various .
sizes, shapes, and thick-
nesses (fig. 327, A), which
are then cemented to glass slides with marine glue. By dr illing two •
holes at a suitable distance, and cutting out the piece between them,
any required elongation of the cavity may be obtained (B, C, D).

Sunk-cells. — This name is given to round or oval hollows, exca-
vated by grinding in the substance of glass slides, which for this

Fig. 327.— Plate-glass cells.

GROUND-OUT AND BUILT-UP CELLS

389

purpose, should be thicker than ordinary. Tin y arc shown in ti_r.
328, A, B, C. Such cells have the advantage not only of comparative
cheapness, but also of durability, as they are not liable to injury by.
a sudden jar, such as sometimes causes tin; detachment of a cemented
plate or rin^. For ob-

jects whose shape adapts
them to the form and
depth of the cavity, such
cells will be found very
convenient. It naturally
suggests itself as an ob-
jection to the use of such
cells that the concavity
of their bottom must so
deflect the light-rays as
to distort or obscure the
image ; but as the cavity
is filler I either with water
some other liquid of

Fig. 328. — Plate-<rlass sunk-cells.

or

higher refractive power,
the deflection is so slight
as to be prac tically in-
operative. Before mount-
ing 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 up of separate pieces of
glass cemented together. Large shallow cells, suitable for mounting
zoophytes or similar flat objects, may be easily constructed after the
following method : A piece of plate-glass, of a thickness that shall
give the desired depth to the cell, is to be cut to the dimensions of
its outside wall ; and a strip is then to be cut off with the diamond
from each of its edges, of
such breadth as shall leave
the interior piece equal in its
dimensions to the cavity of
the cell that is desired.
This piece being rejected,
the four strips are then to
be cemented upon the glass
slide in their original posi-
tion, so that the diamond-
cuts shall tit 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 construc-
tion of large deep cells of this kind, as shown in tig. .*>LMJ, A, B, how-

Fig. 890. — Built-up cells.

390 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

.ever, requires a nicety of workmanship which few amateurs possess,
and the expenditure of more time than microscopists generally have
to spare ; and as it is consequently preferable to obtain them ready-
made, directions for making them need not be here given.

Wooden Slides for Opaque Objects. — Such 'dry 'objects as/ora-
minifera, the capsules of mosses, parts of insects, 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
serves as a protective ' cell.' Let a number of slips of mahogany or
cedar be provided, each of the 3-inch by 1-inch size, and of any
thickness that may be found convenient, with a corres23onding
number of slips of card of the same dimensions, and of pieces of
dead-h\&ck. 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 ' cell ' will thus
be formed for the reception of the object, as we see in fig. 330, the
depth of which will be determined by the thickness of the slide, and

the diameter by the size of the
perforation ; and it will be found
convenient to provide slides of
various thicknesses, with apertures
of different sizes. The cell should
Fig. 330.— Slip made of wood. always be deep enough for its

wall to rise above the object ; but,
on the other hand, it should not be too deep for its walls to inter-
fere 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,
' bed ' to this by gum thickened with starch. If, on the other
hand, it should be desired to mount the object edgeways (as when
the mouth of a foraminifer is to be brought into view), the side
of the object may be attached with a little gum to the wall of the
cell. The complete protection thus given to the object is the
great recommendation of this method. But this is by no means
its only convenience. It allows the slides not only 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
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

1 It will be found a very convenient plan to prepare a large number of such slides
at once, and this may be done in a marvellously short time if the slips of card have
been previously cut to the exact size in a bookbinder's press. The slides, when put.
"together, should be placed in pairs, back to back, and every pair should have each
-of its ends embraced by a spring-press (fig. 33G) until dry.

FINISHING

Fie. :■»:>!. —SlmdboU's tuni-tuhlc

it should be desired to pack these covered slide-, together, if is only
necessary tO Interpose guards of card somewhat thicker than the
glass covers.

Turn-table. — This simple instrument (fig. 331), devised by Mr.
Shadbolt, is almost indispensable to the microscopic who desire* to
preserve preparations that are mounted in any 'medium1 beneath

circular covers; since it not only serves for the making of those
' cement-cells ' in which thin, transparent objects can he besl
mOUllted in any kind of 1 medium/ but also enables him to ;ipply

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 in-
strument is that the
cover-glass, not the
slide, should be 'cen-
tred ' ; which can be
readily done, if several
concentric circles have
been turned on the
rotating - table, by

making the cover-glass correspond with the one having its own
diameter. A number of ingenious modifications have been devised
in this simple instrument with a view to secure exact centring.
The most practicable and inexpensive of these is an application of
Mr. E. H. Griffith's device shown in its improved form in tig. .'$32.

The centre of the table marked
with circles has a straight spring-
attached to it beneath. The slide,
being placed between the two pins
A and B in this centre, is partially
rotated against the spring and
pushed forward, when the spring
keeps it between the two pins and
a third fixed pin, D, at the upper
side of the slide, centring it per-
fectly for widt h. 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

Fig. 882.— Griffith's born-table,

is a countersunk decent ring
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 short bar with which the decentring
wheel may be turned, forcing the pin against the slide, pushing it
as far out of centre as may be desired. Another improvement is in
making the end-pin a screw, which may be turned down out of the
w ay it' desired.

Mounting Plate and Water-bath. Whenever heal has to be

392 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

applied either in the cementing of cells or in the mounting of ob-
jects, it is desirable that the slide should not be exposed direct to
the name, 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
supplied by a plate of metal ; and the Author's experience leads him
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 by raising or lowering the ring any desired amount of heat
may be imparted to it by the lamp or gas -flame beneath. The

Fig. 333. — Apparatus for preparing mounting media, paraffin, &c. for imbedding

by beat.

advantage of a plate of this size and thickness consists in the
gradational 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 plates that one may be cooling while the other is being heated.

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
snp 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

WATER-BATH

— SPRING PRESSES

heated air between the main surface of the heated brass and tlial of
the glass, giving more facility for rapid and delicate heating! This
may he either as separate 'table' or a plate titted to a retort itatuL

Beyond this, however, heat of various kinds, dry and moi-i. of
variable but determinate temperatures, will be required for various
purposes, especially for melting the various mounting media, such as
gelatin, agar agar, and also, as we shall shortly 060, for the
preparation of imbedding waxes for section cutting and a variety
of other purposes. One of the many pieces of apparatus wlm-li
have been devised to combine as large a number of the requirements
of the mounter in one construction as can be conveniently done
was devised by Dr. P. flayer and his colleagues. It is illustrated
in fig, 333.

W is the bath ; Z the tube by which it is filled with water ; 1,
2, 3, 4 are glass tubes j a is a pot for melting and clarifying the
paraffin, and which may be replaced by others for other needful
purposes ; h and r are half-cylinders with handles for imbedding ; / is
a thermometer bent at a right angle \ the horizontal Leg ends in the
air-bath, which 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 the water-bath. It
serves well for evaporating chloroform Arc. ; t is the thermometer for
the water-bath ; R is a Reichert 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 n mica chimney. There is a small independent and
removable water-bath, r, fitted with water by means of rubber tubes
attached to lateral openings. Tt is supplied with a thermometer, t,
is warmed on the platform, F, and is intended chiefly for fixing
objects which are small in the right position in the bedding or wax,
usually known as 'orienting' objects, under a simple lens or dis-
secting 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 8lidt -
forceps, seen in fig. ."W4, will be found extremely convenient. This,

Fa;. :'.:') i. — Slide-forceps.

by its elasticity, atfbrds a scenic 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, atlbrds a level support to the foroepe,
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 (tig. 335),
sold at a cheap rate by dealers in microscopic apparatus, will be

394 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

found extremely convenient. Or if a stronger pressure be required,,
recourse may be had to a simple spring-press made by a slight
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. 336). One of these
convenient little implements may also be easily made to serve the
pur-pose of a slide-forceps by cutting back the upper edge of the

Fig. 885. — Spring-clip. Fig. 886. — Spring-press.

clip, and filing the lower to such a plane that when it rests on its
flat side it shall hold the slide parallel to the surface of the table, as
in fig. 334.

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 directly applied to the slide from beneath ; and it
is attached by a stout wire to a handle shown in fig. 337. 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
the centre of the slide-plate, and is terminated by an ivory knob ;
this wire is pressed upwards by a spring beneath it, whilst, on the

Fig. 887. — Smith's mounting instrument.

«

other hand, it is made to approximate the lower by a milled head
turning on a screw, so as to bring its ivory knob to bear with greater
or less force on the covering glass. The special use of this arrange-
ment will be explained hereafter.

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

ARRANGEMENTS FOR DISSECTJXCi

395

sizo and depth of the vessel should he proport ioned to t be diincn lion
of the object to be dissected ; since, for t lie ready access of 1 lie hands
and dissecting instruments, it is convenient that the object should
neither be far from its walls nor lie under any threat depth of water.
Where there is no occasion that the bottom of the 76816] should be
transparent, no kind of dissecting trough is more; convenient than
that which everyone may readily make for himself, of any dimen
sion he may desire, by taking a piece of sheet gutta-percha of adequate

Fig. 338. — Swift's Stephenson binocular dissecting microscope.

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 •
ground, which assists the observer in distinguishing delicate mem-
branes, fibres, Arc. 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-

396 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

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 intro-
duced a little of the liquid wherein the dissection is being carried
on. Where the body under dissection is so transparent that more
advantage is gained by transmitting light through it than by looking
at it as an opaque .object, the trough should have a glass bottom ;
and for this purpose, unless the body be of unusual size, some of the
glass cells already described (figs. 327-328) 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 more
generally serviceable than the erecting binocular form of stand as
recently modified for dissecting purposes by Swift. It is an instru-
ment which combines conveniences and supplies wants which only
a worker at dissection could have known. It is illustrated in fig.
338, and will be thoroughly suitable for all the work in which it will
b>e 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, &c. ; the fine
instruments used in operations upon the eye, however, will commonly

be found most suitable. A
pair of delicate scissors,
curved to one side, is ex-
tremely convenient for cut-
ting open tubular parts ;
Fig. 389.— Spring scissors. these should have their

points blunted, but other
scissors should have fine points. A pair of very fine-pointed scissors
(fig. 339), one leg of which is fixed in a light handle, and the other
kept apart from it by a spring, so as to close by the pressure of the

1 These may be recommended as useful in a great variety of manipulations which
are best performed under a low magnifying power, with the conjoint use of both
eyes. Where a high power is needed, recourse may be advantageously had to
Messrs. Beck's 3-inch achromatic binocular magnifier, which is constructed on the
same principle, allowing the object to be brought very near the eyes, without
"requiring any uncomfortable convergence of their axes.

DISSECTING [NSTRU2M KNTS

397

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. ^\
pair of small straight forceps with fine points, and another pair of
curved forceps, will be found useful in addition to the ordinary
dissecting forceps.

Of all the instruments contrived for delicate dissections, however,
none are more serviceable than those which the microscopist may
make -for himself out of ordinary needles. These should be fixed in
light wooden handles (the cedar sticks used for camel-hair pencils,
or the handles of steel penholders, or small porcupine 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 be done by their mere tearing action ; but if it be desired
to use them as Gutting 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 he done upon a hone. A
needle with its point bent to a right angle, or nearly so, is often use-
ful ; and this may be shaped by simply heating the point in a lamp
or candle, giving to it the required turn with a pair of pliers, and
then hardening the point again by re-heating it and plunging it into
cold water or tallow.

Section-cutting. — The young microscopist will do well to practise
the cutting of thin sections of soft vegetable and animal substances
with a sharp razor ; considerable
practice is needed, however, to
make effectual use of it, and
some individuals acquire a degree
of dexterity which others never
succeed in attaining. The making Fl(i 840.-Curved scissors foi cutting thin
of hand-sections will be greatly sections,
facilitated by the previous use

of the hardening and imbedding processes to be hereafter described ;
but the best of them cannot be supposed to approach in quality good
sections cut by a microtome. For the preliminary examination of
any soft structure, such a pair of scissors as is represented in tig.
.'UO will often be found very useful ; since, owing to the curvature
of the blades, 1 the two extremities of a section taken from a flat

■"ihiHllllilll m.i.k,

Fie. 841.

surface will generally be found to thin away, although the middle
of it may be too thick to exhibit any structure ; bul the cutting
instrument seen in tig. .*H1 is still more serviceable: it cuts
with precision and becomes with its firm spring, as it were, part of

1 It is difficult to convey by a drawing the idea of tho real curvature of tlii-
instrument, the blades of which, when it is held in front view, curve, not t<> either
side, but towards the observer, these scissors bein;_r, as the French instrument-
makers say, courbta sur If plat.

393 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

Fig. 342. — Simple microtome.

the operator's fingers. In fact, the only instruments which we have
found essential, with the exception of ophthalmic needles in very
line work, are shown in the two figures 340 and 341 . The two-bladed
knife contrived by Professor Valentin was formerly much used for

cutting microscopic sections
of soft tissues ; but as such
sections can be cut far more
effectively by the methods
to be presently described, a
mere mention of this instru-
ment will here suffice.

Microtome, — There is a
large class of substances, of
moderate hardness, both
animal and vegetable, of
which extremely thin and
uniform slices can be made
by a sharp cutting instru-
ment, if they be properly
held and supported, and the
thickness of the section be
regulated by a mechanical
contrivance ; such are, in particular, the stems and roots of plants,
and the horns, hoofs, cartilages, and similarly firm structures of

animals. Various costly ma-
chines have been devised for
this purpose, some of them
characterised by great in-
genuity of contrivance and
beauty of workmanship ; but
most 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 be
firmly attached by means of
a T-shaped piece of wood
(fig. 342) 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. 343. This in-
strument essentially consists
of an upright hollow cylinder
of brass, with a kind of piston
which is pushed from below upwards by a fine-threaded or ' micro-
meter ' 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

Fig. 343. — Microtome.

SECTIQN«-CUTTER H

399

form its cutting bed. At one side is seen a small nulled head, H blcfa
acts upon a 'binding screw,' whose extremity project! into iIm-
cavity of the cylinder, and serves to compress and steady anything
that it holds. For this is now generally substituted a pair of screw .
working through the side of the cylinder, instead of one as in fig. 343.
A cylindric al stem of wood, a piece of horn, whalebone, cartilage. »vc
is to be fitted to the interior of the cylinder, so as to project a little
above its top, and is to be steadied by the ' binding screw ' : it is then
to be cut to a level by means of a sharp knife or razor laid flat upon
the table. The large milled head is next to be moved through such
a portion of a turn as may very slightly elevate the substance to be
cut, so as to make it project in an almost insensible degree above tin-
table, and this projecting part is to be sliced oft* with a knife pre-
viously dipped in water or, preferably, methylated spirit and water in
equal parts. For many purposes an ordinary razor will answer
sufficiently well ; but thinner and more uniform sections can be cut
by a special knife, having its edge parallel to its back, its sides
slightly concave, and its back with a uniform thickness of rather
less than :}--inch. Such a knife should be four or five inches long, and
-jjj-inch broad, and should be set in a firm handle about four inches
long. The motion given to its edge should be a combination of
<lrav:iiuj and jhvss'uhj. (It will be generally found that better
sections are made by working the knife from the operator than
totrcinU him). AVhen one slice has been thus taken off', it should be
removed from the blade by dipping it into spirit and water, or la-
the use of a camel-hair brush; the milled head should be again
advanced, and another section taken, and so on. Different sub-
stances will be found both to bear and to require different degrees
of thickness, and the amount that suits each can only be found by
trial. It is advantageous to have the large milled head graduated,
and furnished with a fixed index, so that this amount having been
once determined, the screw shall be so turned as to always produce
the exact elevation required. 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 vertical 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
described, the small section of the latter being carefully taken off"
the knife, or floated away from it, on each occasion, to prevent it
from being lost among the lamellae 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 be 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.1

The modern art of section-cutting, as practised by the most
accomplished experts, with the most complete of the many almost
perfect recent microtomes, is one of the most refined and beautiful

1 Vide Elder-pith.

400 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

with which the scientific mind can concern itself. The cutting,,
staining, and mounting of the most delicate organic tissues In
almost every conceivable state has thrown a light upon histological
a nd pathological matters, the present" and prospective value of which
we can scarcely estimate too highly • while some of the profoundest
and most interesting questions of biology are opening themselves to-
renewed research by its means.

Throughout this chapter we only seek to give the possessor of a
good microscope a fair outline of the 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. 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. We shall there-
fore 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 consideration 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 thin sections
are not the supreme purpose of microtomes. Good sections, treated
with success from beginning to end, are the first consideration. The
tenuity of a section must be guided by the character of a tissue.
Nevertheless nothing is gained by cutting a tissue thicker than is
necessary to hold its elements together. When we have this, and no
more, we can with certain success employ the highest aperture it
will admit of, and penetrate all its substance as fully as with a low
power and narrow aperture, and with vastly more satisfactory results.

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 embedding agents contract greatly,
without distorting, the anatomical elements, and sections much
thinner than would be normally required to completely disclose
what is sought may be often successfully made in tissues so prepared.

It is none the less true that a mere race for extreme attenuation
in sections is in every sense undesirable ; and for extremely thin
sections — say the g oVo^n °^ an incm in thickness, or less — only small
sections should be attempted.

Here it may be advisable to state that the standard unit in
microscopy as accepted by the Council of the Royal Microscopical
Society,1 is the ^o^thof a millimetre, which is indicated by the sign
H, being known as a micron.

1 Joum. Boy. Microsc. Sec. ser. ii. vol. vii. pp. 502, 526; Nat. xxxviii. p. 221,

THE THOMA MICROTOME

40 1

The choice of microtomes, English, Continental, and American, i
very large, and high merit is characteristie of many. liut one of
these, devised by Thoma and made l>y Jung, of Heidelberg, entered
the field early, having from the first been based on thoroughly
sound, practical principle! ; and as a result it has been BUSCeptible
of and has lent itself to every improvement Suggested by the
advancing refinements of this beautiful art of microtomy. Ju its
latest form we describe and illustrate it, satisfied that it will in
an almost perfect manner meet the general wants of the biologist'
laboratory.

This microtome is based upon the model of Rivet ; 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, S
and (), fig. 344. These are fastened to the bottom plate by screws.
S supports the knife-slide, MS, which rests at three points on a planed
and polished track ; whilst on the side of the knife-slide two other

Fie.. 844. — Jung's Thoma microtome.

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 <> 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, ( )S, which rests in its place exactly as does the
knife holder, MS.

This plate also bears the scale T//, which, by means of a vernier

on the object-holder, enables the thickness of the section to be read

off.

Hie bottom plate is at once a base and a receiver for the drip-
ping spirit, oil, Arc.

For fasten tut 1 the knife a thumb-screw, ( \ tig. 34 L, serves : but in
the instrument designed by the Zoological Station, Naples, this is

!> I>

402 PEE PA RATION, MOUNTING, AND COLLECTION OF OBJECTS

replaced by a single head-screw, E, fig. 345, which is provided with
holes and tightened by means of a lever ; and to give greater freedom
to the use of the knife there are several holes drilled and tapped into
which this screw fits.

The knives, of the form A, fig. 344, are generally screwed directly
to the knife-slide, and are capable of the best adjustment for the
most delicate and largest sections to be made in the direction of its
length.

The knife, however, is also made up >on another model, E, fig. 345 ; it
then has a special holder a, and is secured in conical apertures by the
screws b, b\ and firmly held ; and as b or bl is screwed farther, the
edge may be adjusted towards a horizontal position.

For deep objects requiring considerable length to cut from, there
are plates provided for elevating the knives and the knife-holders.

Fig. 345. — The Thoma microtome with special knife.

The exigencies of section- cutting have given rise to a great variety
of section-holders in this instrument. The simplest is seen in OS,
fig. 344, which is a pair of jaws clamped by screws and fixed upon
the pivot Sz by the milled head a. At n is the vernier, which indi-
cates the position on the mm. scale, Th, and t is an agate highly
polished upon which the micrometer screw m works to drive forward
the object-carrier, OS.

The Zoological Station at Naples employs a holder specially de-
signed for use with paraffin ; the object is soldered with paraffin on
to the cylinder, b y ,fig. 345. This may be shifted vertically and
horizontally by means of the small screw u, and it is fastened by
means of the milled head, m. By the spring n it may be displaced
over 90°, and as great an inclination can be taken in a plane perpen-
dicular to this by the supporting metal frames by means of the

THE TIln.MA MICROTOME

403

spring i>. In this way every desired inclination of tin- object to the
knife can be readily secured.

Fig. 34G presents the same object holder, but instead of tin-
cylinder a simple pair of jaws with the screw rn to secure objects of-

Flo. 846. — Object-holder with jaws.

every variety. A cylinder-holder as in fig. 349 can be ['laced in
these jaws from which the benefits of the Neapolitan holder can be
secured. -But tig. 346 shows a still greater improvement which can be
applied to both object-holders, viz. a perpendicular displacement by
means qf a coy and spring governing the height of the mass from
which the sections are to be cut.

The elevator in this case is supported on one side by the prism P,
and on the other by the rod C ; these are joined by the bridge &,

St

Fig. 847. — Object-holder movable about two horizontal axes at right angles

to each other.

to which a cogged bar is fastened, into which a spring catches, which
is moved by the lever V, allowing a perpendicular displacement of the
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.

D i> 2

404 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

An object-holder movable about two horizontal axes situated
perpendicularly to each other is seen in fig. 347. These positions
are fixed by the milled heads b\ b ;.e shows the jaws for holding the
object, into which, however, cylinders like fig. 349 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 St, the lower part of which is
furnished with hinges ; on the hinge the screw Y 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 move-
ment is read off by a scale under S.

Eig. 348 presents an object-holder intended to analyse by diversified
section objects which are wedged or fan-shaped in form on a fixed
axis, but may be applied to other purposes.

Fig. 848. — Object-holder for analysis by diversified section.

B is a prism-shaped, semicircularly bent bar, moving in the slot
FF1; at b and bl the jaws occupy the position common to those of
the ordinary form.

On the circumference of B a spiral is cut, which becomes slightly
visible at g ; into this spiral a screw passes at H, which is turned by
the milled head S, which can alter the position of the arc to the
horizontal to the extent of 1 mm .; and the amount of the change of
position can be read oft' 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
fixed axis of it lies in this plane, it will only be required that
the screw S be brought 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
jaws is represented in fig. 349 : by is the cylinder, G the compressing
screw for it, the mass W being held in the jaws.

The object slide with its vernier may be slidden up the in-
cline ; but it is much more accurate to control its movement with the

THE TH0J1A MICROTOME

405

micrometer-screw. The point of it in fig. 345, t, works on the polished
plane of an agate cone. The thread in which the sere* works U
held firmly in its place by the milled head W in 8ch, It may stoeteh
up as far as 0, being refastened by \V.

The screw 1,1 is so DUt that a single rotation moves the elide on

t&Pif mm., which in the inclination of the plane of 1 :20 gives

Fig. 34l>. — Cylinder for use with jaws.

an elevation of the object of y1;";,, mm. The barrel or drum, K,
situated on the axis of the screw, is divided into fifteen parts ; eon
sequently the interval of each division corresponds to an elevation of
1 (mYo 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 brought into action or not at the option of the operator.

Fig. 350. — Freezing apparatus for the Thoma mierotome.

Besides these object-holders a freezing apparatus can be added
which is simply placed on the object-slide as shown in tig. :>.">0.

The freezing is effected by ether-spray. A specially favourable
effect is obtained if the cylinder rj is mica and not glass. A layer of
water freezes in from thirty to thirty -five seconds.

In tig. 351 is shown a similar arrangement as an independent.

406 PBEPARATION, MOUNTING, AND COLLECTION OF OBJECTS'

instrument. A is the plate on which the preparation is laid, g the
mica cylinder, and B the under-setting of it in which the ether tubes
for the production of the spray are fixed. No. 1 tube is from
the bellows ; No. 2 carries air to the ether bottle ■ No. 3 is for the
spray point of the ether bottle itself ; and No. 4 is the overflow for
excess of ether.

The glass plate G serves as the knife-rest ; b is divided in order
to determine the thickness of the sections (1 division = ¥ i_ mm.) ; C
is the micrometer-screw which raises the object ; R is the screw
which fixes the clamp to a table ; D is the knife commonly used ;
and E a stilet for clearing the spray points without enlarging the
openings.

Fig. 350 is the same instrument capable of being used with the

microtome instead of sejDarately.

Fig. 351. — Independent freezing apparatus.

An arrangement of this machine for cutting large objects has
also been devised which is illustrated in fig. 352.

The knife is to be placed considerably higher in front than
behind, in order to lessen the pressure on the objects. In order to
satisfy all demands, the knife-rest is adjustable.

The knife is so arranged that the whole length of blade can be
used, and then the screw c is fairly tightly screwed down. As strong-
knives, even of a length of 36 cm., easily give, a knife-support has
been constructed : this is fastened by the screw c' to the carrier.
The support is arranged parallel with the back of the knife M ; if
the extremity n be slightly pressed backwards, so that it touches the
knife, it is then fixed in this position by the screw o (scarcely evident
in the illustration).

This done, the spirit-vessel can be arranged in a position
which will not interfere with the free movement of the knife. In

408 PKEPAKATION, MOUNTING-, AND COLLECTION OF OBJECTS

order that a stream of spirit may follow the knife over the object,
the following arrangement is adopted. The spirit-vessel Bp turns
round an axis on the column h ; to it is joined the arm L, which
carries in front the fine tube r (connected with U'), and also the rod
]) ; the latter is movable perpendicularly, and to its lower end a
bridge or grip with two small rollers i and i' is fastened. The rod
p 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 Sr, the whole apparatus is so arranged that, when the knife-
carrier is in motion, no other friction occurs than that of the rollers
on the strip 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 06, by means of the screw K, is firmly grasped in the fangs
of the object-carrier ; the correct direction for the position of the
knife is given to its surface by the screws at f and/*,, and then
the axes of the fangs are tightened up by the levers q and q' . If
the height of the object is not quite correct, adjustment is made by
the screw m. By turning the screws S, S, the holder is fixed.

V is a wheel with cranked axle Ezt;, and this by means of a cat-
gut band moves the knife.

For the rapid production of ribbons of sections, however, the
instrument par excellence is the Cambridge rocking microtome. It is
illustrated in fig. 353.

The principle is the employment of a rotary instead of a sliding
movement of the parts. Two uprights are cast on the baseplate,
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
be made to slide backwards or forwards, so as to bring the imbedded
object near to the razor ready for adjusting. 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 bear-
ings of the pivot are V-shaped grooves, which themselves form part
of another pivoted system.

Immediately under the first pair of V's is another pair of inverted
V's, which rest on a rod fixed to two uprights cast on the base-plate.
A horizontal arm projects at right angles to the plane of the two
sets of V's, the whole being parts of the same casting. On the end
of the horizontal arm is a boss with a hole in it, through which a
screw passes freely. The bottom of 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

THE ROCKING MICKoToMH \cyj

in the boss. The bottom of the screw rests on ;i pin fixed in the
base-plate.

It will he seen that the effect of turning th<* xtcw is to raise Or

4IO PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

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 6^ 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.

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 milliDg 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 wdieel 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 of a
turn ; and hence, since the screw has 25 threads to the inch, the
thickness of the sections cut can be varied from a minimum,
depending on the perfection with which the razor is sharpened, to a

5 111 ...

maximum of — of - of , or of a turn. The practical mmi-

32 25 6|' 1000 1

mum thickness obtainable with a good razor is approximately

inch. The value of the teeth on the milled wheel are as follows : —

1 tooth of the milled wheel =£^§00 in. = '000(!25 mm.

2 teeth „ „ = ggL^ in. = -001250 mm.
4 „ „ „ =sm6s in. = -0026 mm.

16 „ „ „ =2^>o m« = '01 mm-

The movement of the Lever which carries the imbedded object is.
effected by a string attached to one end of the lever. This 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 feed takes place, the string is
pulled, the imbedded object is raised past the razor, and the spring
is stretched. When the arm is allowed to move back, the 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
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 an important adjustment, as it causes the razor to com-
mence 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

KTHKK FREEZING MICROTOMES

instrument : (1 ) the price is low, one-sixth that of the original form.
(2) Less skill is required from the operator, for the endless silk band
is superseded, and the troublesome and difficult operat ion of lifting
the first sections from the razor on to the silk hand is entirely avoided.
The ribbon of sections now falls of its own weight direct from the
razor on to a piece of paper or glass slide placed to receive t hem, and
by occasionally moving the paper forward any length of ribbon can be
obtained. (3) The razor is fixed at what has in practice been found
the most advantageous inclination and angle for cutting j and thus
an unnecessary adjustment and waste of time are avoided. (4) The
imbedded object is with great ease and quickness brought up to or
away from the edge of the razor ; first, for large amounts by sliding
backwards or forwards the brass tube on the cast-iron lever, then
for smaller amounts by turning round the screw, when the paw] is
out of gear, by means of a small milled head placed on the top for
this purpose. (5) 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.

But it is needful also to describe one or more of the best in-
struments designed specially for cutting sections by congelation,
or freezing of the imbedding mass. Dr. R. A. Hayes designed an
ether freezing microtome with the object of affording to those who
have occasional need to cut sections of tissues for pathological
investigations Are. the means of doing so quickly, conveniently, and

Fk;. 354. — Dr. Hayes's ether freezing microtome.

accurately. It is illustrated in fig. 354. It is very compact, solidly
constructed, and simple in plan. It freezes rapidly, and permits
sections of large surface to be made with precision : sections 1 in.
x S in. having been cut by it without difficulty.

It consists of a solid cast-iron base, A, 10 in. x I.1, 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 sidesof which
slides a heavy metal block, 1>, on the flat top of which the razor i->
secured (any ordinary razor can be used), the tang being grasped

412 PEEPAKATION, MOUNTING-, AND COLLECTION OF OBJECTS

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 fro.

The freezing-chamber is formed by a short vulcanite cylinder, D,
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, D, and rising from
the base, E, is an ordinary spray, the air and ether being supplied
through tubes, g and H, passing outside, through the base. There
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 0*01 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, D, having been carefully rubbed clean and well oiled, the
razor is clamped at any desired angle, the bottle, K, is filled with ether
(good, dry methylated ether 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 working by the hand-pump, M ; in a short
time the tissue will be frozen quite through, and if a number of sec-
tions are required an occasional stroke or two of the 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 stroke 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 as
they are cut.

Another most serviceable and admirable, because inexpensive and
efficient, microtome, especially for freezing purposes, was devised by
Mr. Cathcart ; and it is now presented in a simplified and improved
condition. The instrument is illustrated in fig. 355.

In this form the clamping arrangements are much more perfect
than in the old form ; the principal screw and its milled head are
larger and more convenient ; the freezing-plate is circular, and is
provided with an arrangement for preventing the ether with which
the freezing is effected from reaching the upper side of the plate ; and
the instrument is now so modified that it can be used for ordinary
imbedding as well as freezing.

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-

ETIIEIi FIJEK/INfi MICKOTo.MF

413

part a finor movement to the screw. The relation between thepiti h
of the screw and the circnmferenoe <»f its head ia such that if the

Fig. 35.").' — Cathcart's freezing microtome.

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 is
so much reduced that the conducting
surface is not Greater than in the old
microtome. The arrangement for cut-
ting imbedded sections consists of a
tube which fits the principal well of the
microtome, and within which tits a hinged part similar to an ordinary
vice. With the instrument arc provided the means of preparing
paraffin blocks for imbedding sections.

When it is intended to use the microtome for imbedding, the
ether spray, spray-bellows, and ether-bottle should be removed, and

Fig. 8.V'.
IL>luVr fox Cathcart's microtome.

.414 PREPARATION, MOUNTING-, AND COLLECTION OF OBJECTS

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. 356, 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 backwards.

Mr. Cathcart recommends in freezing with this instrument that
a few drops of mucilage (1 part gum to 3 parts water) be placed on
the zinc plate, and that a piece of the tissue be cut, of about a quarter
of an inch in thickness, and pressed into the gum ; the ether-bottle,
filled with anhydrous methylated ether, is taken and the spray points
pushed into their socket. All spirit must of course have been pre-
viously removed by soaking for a night in water. The tissue should
afterwards be soaked in gum for a like time before being cut. The
operator must now work the spray-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.

Preparation and Mounting of Objects.

Imbedding Processes. — The preparation of soft organic substances
for section-cutting by ' imbedding ' may be made in two modes, the
choice between which will depend upon the consistency of the sub-
stance. If (1) it be compact, like a piece of liver or kidney, it only
needs to be surrounded by the imbedding mass, which will afford it
as a wliole the requisite support. But if (2) it be partly occupied,
like a piece of lung, by interstitial cavities, it must be 'penetrated by
the imbedding substance, so that every part may be duly supported.

The former is simple imbedding ; and it may be readily effected
by immersing the object to be cut in some such substance as molten
wax, which on becoming cold will acquire a consistency, neither too
brittle nor too soft, which will permit of thin slices being cut. But
in the second class of casus, where it is necessary to fill all natural
cavities and interstices with the imbedding material, so that the
most delicate organ may retain its tissues, and even its separate cells
in situ in each section, it is much more complex.

It may be effected by a similar process of infiltration as is
employed in simple imbedding, only made more complete by the
previous preparation of the tissue, employing materials in which to
previously soak it which are solvent of the imbedding material,
and which will therefore secure more thorough infiltration. Or
the same thing may be accomplished by a process of evaporation.
A substance may be used which in a fluid state is capable of pene-
trating the most delicate cavities of a tissue, which after the evapo-
ration of the solvent will leave the imbedded object with a consistency
which will admit of cutting.

For simple imbedding the use of carrot or pith will suffice ; in
using the latter, when the cylinder of pith has been cut longitu-
dinally, and a cavity has to be made to receive the object, the cavity
should be made by pressure ; a blunt ivory point will suffice to effect
this. If the cells of pith be cut out, we lose the firmness of enclosure
that is always the result of having obtained the cavity by pressure,

#

i.mi:km>i.v; 4,5

because when the object is enclosed between the two half -cylinder,
of pith and plugged (if necessary) to the shape or size of tin- micro-
tome-well or holder, the pith should then be moistened with alcohol,
or other suitable fluid, and the compressed pith-cells driven together
to make space for the tissue tend by capillarity to regain their form
and position as the alcohol penetrates them ; but this could not have
been the case had the cavity been cut out. In this way the object en
closed is firmly held in all directions, and may be readily divided into
sections.

For simple imbedding in wax or paraffin it is necessary to provide
a cylinder of metal, or a box or tray of cardboard or paper, into
which to pour the melted composition for imbedding. Hut a Mill
better plan is to employ sets of two pieces of type-metal, cast in
rectangular form of various heights and capable of being placed together
as in fig. 356 A ; in this way a suitable box is formed, and the end of
the shorter arm being triangularly enlarged outwards, it is closed
sufficiently to retain the wax. Placed in this way, with the short
arms nearer to or farther from each other, as a less or greater imbed-
ding mass is required,
they are set on a plate
of class which has been
wetted with glycerin
and gently warmed.
The melted paraffin or
composition is now
poured into this mould
until a certain desired
level of the wax is
reached, and when the
wax has cooled to a con-
sistency capable of receiving and retaining in its place the mass
to be imbedded, it is placed in a desired position on the surface of the
cool wax and the remainder of the mould is filled up with melted wax.

It often happens that it is most desirable to place an object or a
tissue in a special position in the imbedding mass, so that sections in
a given direction may be secured. In this case we have to use some
method of 'orienting ' the object, and in some cases this requires the
use of at least a fixed hand lens or even a microscope. It is
frequently a very difficult matter .to properly orient very small
objects, such as spherical eggs, so that sections may pass through any
desired plane. In working on the embryology of the common
shrimp, Mr. J. S. Kingsley found the following process very conve-
nient. Impregnation with paraffin is accomplished in the usual way,
and then the eggs (in numbers) in melted paraffin are placed in a
shallow watch-crystal. They immediately sink to the bottom, and
then the whole is allowed to cool. The crystal, glass upwards, is
now placed on the stage, and the eggs examined under a lens.

In this way one can readily see exactly how any egg lies, and
then with a knife it may be cut out with the surrounding paraffin,
and in such a way that it can readily be fastened to the block in any
desired position After all which have been dropped in a suitable

416 PKEPAKATION, MOUNTING, AND COLLECTION OF OBJECTS

position are thus cut out, the paraffin is again melted, and after
stilling the eggs the cutting is continued as before.

A more general method is to take a common flat medicine bottle,
as in fig. 357, 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 wTater 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 re-
ceptacle 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
jjaraffin congeals rapidly, and may be easily re-
moved 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 the Thoma form of microtome
where the object is held in jaws, the imbedding
mass must of course be cast a suitable shape,
and placed directly in the jaws, or be cemented
to slices of cork which may be placed in contact
with the jaws.

After sections have been cut and are placed
upon the thin cover for mounting, it is very
desirable to be able easily to free the sections
from the imbedding mass that may be clinging
to it. This may be done by means of turpen-
tine, creosote, xylol, or oil of cloves. But in
simple imbedding, where the interpenetration of
the cavities of the object is not the special aim, a still more efficient
method is to prepare the object before imbedding it by covering it
with a film of some substance which prevents the immediate contact
of the imbedding mass with the object, and which can be even more
easily removed than the paraffin. This may be done with collodion,
into which the prepared tissue is plunged for a short time and taken
out and allowed to evaporate. The oil of cloves used for clearing the
section will dissolve it, and the cast of wax will fall away.

Imbedding Masses. — These may be procured ready prepared for
two or three temperatures, made up according to the formuke of
some of the most experienced biologists. The composition of the
imbedding mass is of large importance. The temperature of the
laboratory must determine the melting-point of the paraffin ; by

Fig. 357. — Arran^
merit for the orienta-
tion of objects in
paraffin.

IMI5KI)]>I Vi DKLICATE TISSUES

417

mixing paraffins of various melting points we can obtain accurately
a wax of the consistency we need for laboratories of different tem-
peratures, or for the .same Laboratory at different periods of 1 be year.

If we take, for example, a palatini with a melt ing-poinl of I |\
and melt two parts of this with one part of a paraffin having a
melting-point of 110° F., we shall have a mass that will cul satisf y
torily in a temperature of 70° F. ; while the paraffin with the lower
melting-point will serve well for a temperature of 60° F. We
rarely need to work in lower or higher temperatures than these.
But by mixing equal parts by weight of pure; vaselin and the
paraffin melting at 136° F., an imbedding mass that will cut well at
55° F. can be obtained.

Another imbedding mass which is extremely useful maybe made
by melting together equal parts by weight of white wax and olive
oil. This will work well from 60° F. to 65° F. For a higher tem-
perature than this add a little more wax ; for a lower temperature, a
little more oil.

In order to prepare delicate tissues etc. by complete impregnation
with paraffin before imbedding, it is needful to use a suitable solvent.
This may be chloroform, creosote, oil of cloves, or oil of turpentine.

Before imbedding, the material to be imbedded would have been
hardened and otherwise prepared ; the mode in which this may be
done will be described subsequently in this chapter. Let it be
assumed that the object is taken immediately from alcohol of full
strength. From this it is to be transferred to creosote, oil of cloves,
or chloroform, oil of turpentine answering sufficiently well for larger
bodies. When this has completely replaced the spirit, the body is to
be immersed for some little time in a hot saturated solution of
paraffin in oil of turpentine. When it has lain sufficiently long in
this to be thoroughly penetrated, it is to be immersed in the melted
paraffin, which should not be more heated than is necessary to keep
it quite liquid ; and it should be moved about in this for some little
time, an occasional gentle squeeze being given to it with the forceps
so that the solution may be replaced as completely as possible by the
liquefied paraffin. When hardened by cooling, the substance thus
prepared may be 'imbedded ' in the moulds in the manner already
described, the coating with gum being of course omitted. When
the paraffin has perfectly solidified, the mould is to be lifted off its
glass bottom, and the block taken out is then ready for cutting.
In using the section-knife, care should be taken to keep it constantly
wetted with methylated spirit, and it is desirable th at each section
should be removed from it before another is taken, except of course
where the connexion of each section with the preceding is sought
to be maintained, when sections united successively together in
1 ribbons' can be readily effected, all that is needful being the right
quality in the imbedding material involving the adhesion of each to
the other as they are successively cut, and a section smoother on the
edge of the cutting blade.

Celloidiii as on imbedding mass. — This is a preparation of pure
pyroxylin. It may be obtained in cakes of a tough, pliant nature,
having the consistency suitable for section-cutting. It is soluble in

I l

41 8 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

ether and alcohol. Small pieces of the celloidin tablets are dissolved
readily in equal parts of absolute alcohol and ether • the properly
hardened tissue, having been dehydrated in strong alcohol, is re-
moved to a mixture of equal parts of alcohol and ether, where it
must remain two or three hours, and longer if convenient. It is then
transferred to a thin solution of celloidin in a well-stoppered bottle
or other tightly closed vessel, where it should remain ten or twelve
hours at least, and can only be benefited by a very prolonged bath.
A much thicker solution of celloidin must be prepared and at hand.
Take out the organ or tissue from the thin solution, and suffer the
celloidin to evaporate until a film is formed. This is preferably done
on a piece of leather. Surround it now with the thick celloidin, and
wait until again a film has formed, and then the whole is thrown
into strong alcohol, where it may remain until it is desired to cut
sections.

Sections are best cut with the razor moistened with alcohol, and
they may be floated into the same fluid.

To mount, it is not needful to remove the celloidin if we employ
glycerin, glycerin jelly, or Farrant's medium (q.v.) ; but if we desire
to mount in balsam the section should be dehydrated with 95 per
cent, of alcohol, and cleared with oil of origanum or oil of clove.

Imbedding by Congelation or Freezing. — From the amount of
water they contain, perfectly fresh tissues may be frozen without
being enclosed in any medium, and as readily cut ; but the inevitable
formation of ice crystals cuts and tears delicate anatomical elements.
A fluid mass that will freeze without crystallising, and when frozen
permits the action of the knife, is what is needed. Gum arabic is pre-
cisely suited in this case. The substance to be cut (which may either
be fresh, or have been hardened by some of the processes to be here-
after described, must be thoroughly penetrated by a thick solution of
gum. If the substance to be cut has been immersed in alcohol, this
must be completely removed in the first instance by immersion in
water for from six to twenty-four hours, according to the size of the
mass ; for the gum will not penetrate any part which is still
alcoholised. The substance should be then immersed in the gum-
solution for from twelve to twenty-four hours before it is frozen,
in order that every part may be permeated by the gum, and no water
be left to form crystals of ice.

With the ether-spray microtome, which is simpler and easier to
work than the ice-freezing instrument, the freezing is produced by
the rapid evaporation of the liquid injected into the freezing- chamber.
The substance to be cut is to be introduced into the well, as soon
as the gum begins to harden at its periphery, and should be held in
place until fixed by the advancing congelation. In cutting the
sections no wetting of the knife is necessary, as it is kept suf-
ficiently wetted by the thawing gum. The sections should be placed
in methylated spirit diluted with twice its volume of water ; and this
soon not only dissolves out the gum, but removes any air-bubbles the
section may contain. If the section is to be at once mounted (which
should always be done if it be very delicate and liable to be spoiled
by manipulation), it should be placed on the slide before it has thawed

CKLLOIDIX FOR CONGELATION

and washed by forming around it a little pool of dilute spirit, w 1 1 i < • 1 1
may be readily changed two or three times by the glass syringe.
Sections cut by the freezing process may for the most part be mounted
in glycerin jelly, for which no other preparation will be needed than
the use (if desired) of the staining process hereafter to be described.
But if, for the sake of rendering the sections more transparent,
mounting them in Canada balsam or dammar is preferred, they must
be treated first with strong spirit, then with absolute alcohol, and
then with either oil of cloves or oil of turpentine. It is claimed by
Dr. Rutherford, as the special advantage of the freezing process,
that 'delicate organs, such as the retina, the embryo, villi of the in-
testines, lung, trachea with its ciliated epithelium, may all he readily
cut without fear of their being destroyed by the imbedding agents.'
When imbedded in paraffin, very delicate structures are more liable
to damage, the villi of the intestine, for instance, being often de-
nuded of their epithelium, and sometimes themselves torn.

Celloidin as a Congelation Mass.— Dr. A. Hill, of Downing
College,Cambridge, has for years employed celloidin as a freezing sub-
stance with beautiful results. A similar process, differing somewhat
in details, was, however, published by J. W. Barrett (' Journ, Anat.
and Phys.' No. 7, 1885). Dr Hill has at the Editor's request favoured
him with the following description of his method, and some of his
exquisite mountings of brain tissue accomplished by its means.

As originally recommended the celloidin is set in 80 per cent,
alcohol and cut under spirit, but under these circumstances it presents
a tough, resilient, and disagreeable substance to the razor, whereas
there is no substance or tissue more satisfactory to cut than celloidin
saturated with water and frozen. For this method pieces of tissue
are placed after appropriate hardening in absolute alcohol. The al-
cohol is replaced once or twice by a mixture of absolute alcohol three
parts, ether one part. Into the bottle containing the tissue a piece of
Schering's celloidin is then dropped, next day another piece, and so
on until the solution of celloidin flows with difficulty. To obtain
this result requires about a fortnight, and the process may of course
be cut short by using a strong solution of celloidin in the first instance ;
but the longer process amply repays for the trouble by ensuringcom-
plete penetration of the celloidin into the tissue and yielding when
set a homogeneous mass not liable to curl up when cut. Tissue and
celloidin solution are then poured out together into a shallow basin,
which is covered by a glass plate. Evaporation takes place slowly,
and in three to four days a tirm, uniform mass is obtained. A block
of celloidin with the tissue imbedded is cut out and thrown into
water for two hours, at the end of which time it may be placed in
gum and ether frozen and cut at once or preserved for an indefinite
period in aseptic gum or spirit. The celloidin may be set much
more expeditiously by pouring it into a paper boat which placed
in a vessel of chloroform. In this way the celloidin is set in a very
short time with, as far as can be estimated, no loss of bulk.

The sections when cut and stained must be dehydrated in strong
Spirit, immersed for a few moments in absolute alcohol, and cleared
in oil of thyme, oil of bergamot, or turpentine and creosote (ten parts

420 PKEPAKATION, MOUNTING, AND COLLECTION OF OBJECTS

to one part heated together), but not in oil of cloves, the celloidin
being soluble in the latter reagent.

Grinding and Polishing Sections of Hard Substances.— Sub-
stances which are too hard to be 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. General directions for
making such preparations will be here given ;l but those special de-
tails 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,2 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-
laneous 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 dia-
mond-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, re-
sinous 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

1 The following directions do not apply to siliceous substances, as sections of these
can only be prepared by those who possess a regular lapidary's apparatus, and have
been specially instructed in the use of it.

2 Joum. QueheM Microsc. Club, vol. vi. 1880, p. 83.

POLISHINC (iKOl'NI) SI-XTIONS

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
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 sup-
port one another.1

The mode in which the operation is then to be proceeded with,
depends upon whether tin; 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 1 stone, or
on a hone or 'Turkey' stone, or on an 'Arkansas' stone ; the titst 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

1 Thus, in making horizontal and vertical sections of Foraminifrra, as it would
be impossible to slice them through, they must be laid close together in ft bed of
hardened Canada balsam on a slip of glass, in suc h 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. YVallich i.lnn.
of Nat. Hist. July 1S(51, 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 hy the same means; when tin \ have
been ground down as far as may be desired, the slide is gradually heated just suf-
ficiently to allow of the detachment of the mica-film and the specimens it carries ; and
a clean slide with a thin layer of hardened balsam having been pre pan d, the mica-
film is transferred to it with the ground surface downwards. When its adhesion is
complete, the grinding may be proceeded with; and as the mica-tihn will yield to the
stone without the least difficulty, the specimens, now reversed in position, may be
reduced to requisite thinness.

- As the flatness of the polished surface is a matter of the first importance, that
of the stones themselves should he tested from time to time; and whenever they are
found to have been rubbed down on any one part more than on another, they should
be flattened on a paving-stone with fine sand, or on the lead-plate with emery.

422 rKEPARATION, MOUNTING, AND COLLECTION OF OBJECTS

ready yielding to the thumb-nail, it should be heated a little more,
care being taken not to make it boil so as to form bubbles ; if too
hard, which will be shown by its chipping, it should be re-melted
and diluted with more fluid balsam, and then set aside to cool as
before. When it is found to be of the right consistence, the section
should be laid upon its surface with the polished side downwards ;.
the slip of glass is next to be gradually warmed until the balsam is
softened, special care being taken to avoid the formation of bubbles ;
and the section is then to be gently pressed down upon the liquefied
balsam, the pressure being at first applied rather on one side than
over its whole area, so as to drive the superfluous balsam in a sort
of wave towards the other side, and an equable pressure being finally
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 be 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 off, to melt a little fresh bal-
sam 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 off 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
contact 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 thinness
which is most suitable for the display 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
oan only be displayed in its highest perfection, when a very little

VAKIOCS HARD SKCTJONS

423

more reduction would destroy the section altogether; and every
microscopist who lias 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 be limited, it is
advisable to stop short as soon as a <j<md section has heen made, and
to lay it aside — ' letting well alone' — whilst the attempt is being
Blade 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 has been consumed; the Jirst section,
however, still remaining, whereas, if the first, like every subsequent
section, be sacrificed in the attempt to obtain perfection, no trace
will be left 1 to show what once has been.' In judging of the
appearance of a section in this stage under the microscope, it is to
be remembered that its transparence will subsequently be consider-
ably increased by mounting in Canada balsam: this is particularly
the case with fossils to which a deep hue has been given by the
infiltration of some colouring matter, and with any substances
whose particles have a molecular 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 may be
made of structures, in which (as1 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
( lanada 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, impenetrated 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

1 See Koch in Zooloqisclicr Aiizri;/. Hd. i. p. 86, Tin- Author, having scon
(by the kindness of Mr. rl. N. Moselevi sonic sections of corals prepared by this
process, 'can testify to its complete success.

424 PREPARATION, MOUNTING-, AND COLLECTION OE OBJECTS

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
<*i 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 re-
quired 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 finer file. When this cannot safely be carried farther, the
section must be rubbed down upon that one of the fine stones already
mentioned which is found best to suit it ; as long as the section is
tolerably thick, the finger may be used to press and move it ; ' but as
soon as the finger itself begins to come into contact with the stone,
it must be guarded by a flat slice of cork, or by a piece of gutta-percha
a little larger than the object. Under either of these, the section
may be rubbed down to the desired thinness ; but even the most
careful working on the finest-grained stone will leave its surface
covered with scratches, which not only 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
on both sides of the section, until all the marks of the scratches left
by the stone shall have been rubbed out, when the specimen will be
fit for mounting 1 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
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.

Fig. 358. — Hand machine for cutting hard sections.

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.

SECTION' (TTTINC MACINNK

The first (fig. 3oS) is a hand machine. The specimen is cemented
to the carrier, O, which is movable on tlie axis, h, and can also be
rotated in two directions. The object is pressed by the weight,*?,
against the steel disc, which is revolved by the wheel, r, acting mi
a smaller toothed wheel on the axis of d.

The second (fig. i5o9) is intended to be worked by the foot. The
parts a, h, c, and <i arc the same as before. The wheel and treadle at
/and g work the pulley, c, by which the steel disc, d, is revolved • h

Fhi. 359. — Treadle machine for cutting hard sections.

is part of the cover for the disc, to prevent the emery flying about.
A box beneath also catches the powder that falls.

(This arrangement is also supplied with tig. .">~<S, though not shown
in the woodcut.) A second wheel at /, with a cord passing over ky
actuates a vertical spindle, I, which rotates a horizontal cast-iron plate
at )n for polishing.

Decalcification. When it La desired to examine the structure of
the organic matrix in which the calcareous salts are deposited that
give hardness to many animal and to a few vegetable structures (such
.as the true corallines), these salts must be dissolved away by the

426 PEEPAKATION, MOUNTING, AND COLLECTION OF OBJECTS

action of some mineral acid, which may be either nitric or hydro-
chloric. 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 lime is in the state of carbonate (as, for
example, in the skeletons of echino'derms), the body to be decalcified
should be placed in a glass jar or wide-mouthed bottle holding from
4 to 6 oz. of water, and the acid should be added drop by drop, until
the disengagement of air-bubbles shows that it is taking effect ;
and the solvent process should be allowed to take place very gradually,
more acid being added as required. When, on the other hand, much
of the lime is in the state of phosphate, as in bones and teeth, the
strength of the acid solvent must be increased ; and for the hardening
of the softer parts of the organic matrix, it is desirable that chromic
acid should be used. In the case of small bones, or delicate portions
of large (such as the cochlea of the ear), a per cent, solution of
chromic acid will itself serve as the solvent ; but larger masses
require either nitric or hydrochloric acid in addition, to the extent
of 2 per cent, of the former or 5 per cent, of the latter. By some
the chromic and the nitric or muriatic acid are mixed 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 lamellae. Acid solvents may also be em-
ployed 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 Foraminifera) ; or for getting rid
of them entirely, so as to bring into complete view any ' internal cast '
which may have been formed by the silicification of its originally soft
contents. It has been in this mode, even more than by the cutting
of thin sections, that the structure of Eozoon canadeiise 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 ' in-
ternal cast ' may be altogether dissolved away.

Bush 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 on a 95 per
cent, solution of alcohol for three days ; they must then be placed in

VARIOtS PREPARATIONS FOR Mol'NTINO

42/

the nitric acid, wliicli must be changed daily for eight days. They
must not remain after the decalcification is complete, or staining will
ensue. On removal the bones must be washed for a couple of hours
in running water and placed again in 05 per cent, alcohol, and in ,t
few days placed again in fresh alcohol.

Desilicification. — It is desirable t<> he 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 tin-
action of the acid used taking place on the sides of the vessel. The
subject to be cleared of its silica is placed in alcohol in the coated
vessel, and hydrofluoric acid is added drop hydro]). As the mucous
membranes are fiercely attacked by this acid, great care must be ex-
ercised in its use ; but small sponges and other similar siliceous ob-
jects 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 re-
quired, 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 alg(e, fungi, or other succulent cryptogams. But
the woody 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 be applied to hard-coated seeds, the 'stones
of fruit, 'vegetable ivory,' and other like substances. Some vegetable
substances, on the other hand, are too soft to be cut sufficiently thin
without previous hardening, either by allowing them to lose some of
their moisture by evaporation, or by drawing it out by steeping them
in spirit. Either treatment answers very well with such substances
as that which forms the tuber of the potato, sections of which dis-
play the starch-grains in situ. Where, on the other hand, it is de-
sired to preserve colour, spirit must not be used ; and recourse may
be had to gum-imbedding, which is particularly serviceable where
the substance is penetrated by air-cavities, as is the case with the
stem of the rush, the thick leaves of the water-lily &c. But where
the staining processs is to be employed, the substance should be pre-
viously bleached by the action of chlorine (preferably by Labarraque's
chlorinated soda), and then treated with alcohol for a few hours.

Hardening Agents. — The methods of imbedding and freezing
have become so perfect, and have such universal application, that this
process of hardening, which at an early period in the art of niiem
tomy was of so much importance, has fallen into a matter of second-
rate imp< >rtance.

Hardening processes may act either by coagulating the albumen
of the tissues, without entering into chemical combination therewith,
as is the case with alcohol, nitric acid, and picric acid, or they may
produce the results effected by their employment by entering in some

428 PKEPARATION, MOUNTING-, AND COLLECTION OF OBJECTS

manner into chemical combination with the components of the tissues.
To this latter group belong osmic acid, chromic acid, and chloride of
palladium. The latter group produces effects in the tissues that do
not lend themselves to successful staining, although they admit of
delicate sections.

Alcohol. — This should be used strong. Most operators advise,
for certain purposes at least, absolute alcohol ; but Mr. Arthur Cole,
whose experience is very large, says that this should not be used for
hardening purposes, either with animal or vegetable tissues. He
affirms that methylated spirit should always be employed, for the
' water in alcohol renders it useless for hardening.' Large quantities
of spirit should be employed in proportion to the object, and many
weeks may be required to harden large specimens.

Picric acid is used for the same purposes as chromic acid ; its
hardening power is not so great, but it does not shrivel the tissues
as much, its action is more rapid, and it may be advantageously
used where 'decalcification' is necessary. As it is but slightly
soluble in water, a cold-water solution must be saturated ; and the
quantity of liquid should be large in proportion to that of the sub-
stance to be acted on. Picric acid is used, in combination with
carmine or anilin blue, as a staining material.

Osmic Acid. — This agent is one of peculiar value to the micro-
scopist whose studies lie among the lower forms of animal and
vegetable life ; as its application immediately kills them, without
producing any retraction or shrinking of their parts, and not only
preserves their tissues, but brings out differences in those which
might otherwise escape observation. It is sold in the solid state
in sealed tubes, and is most conveniently kept as. a 1 per cent,
solution in distilled water. The solution should be preserved in
well-stoppered bottles secluded from the light ; and should be used
with great caution, as it gives forth a pungent vapour which is very
irritating to the eyes and nostrils. It is recommended by Dr.
Pelletan,1 M. Certes,2 and M. Raphael Blanchard 3 f or fixing and
preserving animalcules {Infusoria and Rotifera), Desmidieos, Dia-
tomaceo3, Bacteria, Vibriones, &c. ; by Dr. Yignal 4 for Noctilnca ;
by Mr. T. J effrey Parker 5 for Entomostraca and other small
Crustacea ; and it has been successfully used also in the preparation
of insect structures. To the histologist its special value lies in its
blackening of fatty matters and the medullary substance of nerve-
fibres. And the embryologist finds it of peculiar value in giving
firmness and distinctness to the delicate textures with which he has
to deal. Various degrees of dilution of the 1 per cent, solution will
be needed for these different purposes. Mr. Parker further states
(loc. cit.) that he has found this agent very serviceable in the
preparation of delicate vegetable structures. 'The acid seems to
be taken up by each granule of the protoplasm, and these to be

1 Joum. of Boy. Microsc. Soc. vol. i. 1878, p. 189.

2 Ibid. vol. ii. 1879, p. 331 ; and Comptes Bendus, 1879, p. 433.
5 Ibid. vol. ii. 1879, p. 463.

4 Robin's Archives de Physiologie, tome xiv. 1878, p. 586.

5 Joum. of Boy. Microsc. Soc. vol. ii. 1879, p. 381.

OSMIC ACID VAI'oli;

429

decomposed, giving to the granule the characteristic grey colour,
thus «at the same time both hardening and staining.' A mixture of
9 parts of a £ per cent, solution of chromic acid with one pari
of a 1 per cent, solution of osmic acid answers for many purpo e
better than osmic acid alone, the brittlenesss produced by its use
being completely avoided. After being subjected to this agent, the
specimens should be treated with 30 per cent, alcohol, gradually
increased in strength to absolute.

There is now no doubt that osmic acid is best employed as
vapour, wherever it is possible to expose the tissue to its action.
The tissues are pinned on a cork fitting a wide-mouthed bottle in
which is contained a small quantity of the acid either in a solid or
dilute form, and should be kept there until the tissues begin to
turn brown. Without any intermediate process such tissues may be
stained at once with picro-carmine (q. v.)

Osmic acid stains all fatty structures opaquely black, and should
not be employed where these are present.

Professor J. N. Langley, of Trinity College, Cambridge, has
employed osmic acid vapour in the preparation of certain mucous
glands with very tine results. As we have seen, the use of osmic
acid vapour to fix and harden tissues has been some time in use ;
but its employment in the case of mucous glands preserved for the
purpose of microscopic sections is due to Professor Langley.

By this means (1) the mucous granules can be preserved in a
more or less spherical form, (2) they are stained with most reagents
far more deeply than any other part of the tissue. The success of
the method depends much upon the detail, and apparently to some
extent upon the gland. Professor Langley's best results have been
obtained with the orbital gland of the dog. The following is the
method, viz. a tine needle with a silk thread is passed through a
small lobule of the fresh gland of an animal which has been killed
by bleeding whilst under the influence of an aiuesthetic, or which
has been killed by decapitation ; the lobule is cut out, lowered by the
silk thread passing through it into a bottle half full of 2 per cent,
osmic acid. When the lobule is a short distance from the surface of
the fluid, the stopper is inserted, which, as it clamps the thread,
keeps the piece of gland from touching the fluid. In a day the tissue,
now hardened throughout, is washed for a few minutes with water,
placed in 30 per cent, alcohol and in 50 percent, alcohol for about a
quarter of an hour, in 75 per cent, alcohol and in (J.~> per cent, alcohol
for about half an hour, in absolute alcohol for one to two hours, in
benzol for half an hour to an hour, and then imbedded in hard
paraftin. Serial sections are cut, and then either (a) fixed on a glass
slide with egg albumin, stained with methyl-blue ami mounted
in Canada balsam, or (b) treated with benzol or turpentine to
dissolve the paraffin, passed through alcohols of various strengths
to water and stained with a strong aqueous solution of methyl-
blue, and finally — after the usual treatment— mounted in Canada
balsam.

In all other methods of hardening mucous glands which I have
tried, the mucous granules are more or less altered, being either

430 PREPARATION, MOUNTING, AND COLLECTION OE OBJECTS

swollen up to form a clear mass, or being shrunken to irregular, small
clumps. By the method given above the mucous granules in the
stained specimen are very obvious, and further in a stimulated gland
many of the alveoli show an inner zone of granules stained deep blue
and an outer zone stained of a brownish tint. Even in the unsuc-
cessful preparations, in which the granules are not separately
distinguishable, the mucous portion of the cell is unusually well
marked in stained specimens.

In the mucous cells of the mucous membrane of various lower
vertebrates in which granules can be made out in the fresh state,
the granules can often be preserved by the vapour of osmic acid.
For certain special points in other glands, particularly in the parotid,
the method is useful, but this together with further details with
regard to its employment on mucous cells I propose to deal with
after making further observations.

Bichromate of jjotass, in a 2 per cent, watery solution, may be
used where very slow and prolonged hardening is required. With
the addition of 1 per cent, of sulphate of soda, it constitutes Mutter's
fluid, which may be conveniently used to harden large pieces that
may be left in it for several weeks ; the fluid should be changed
every other day for a fortnight, and weekly for the second fortnight.
The hardened substance, after being well washed, is to be treated
with spirit, as in the preceding case.

The best formula for this fluid is 2 oz. bichromate potass, 1 oz.
sodium sulphide to one Winchester quart of water.

Staining Processes. — Much and increasing attention has been
given of late years to the use of agents, which, either by simply
•dyeing or by chemically acting on organic substances in different
modes and degrees, serve to differentiate the different parts of organs
or tissues of complex structure, and to render more distinct such
delicate features in preparations mounted in transparent media as
might otherwise escape notice. One of the chief ends of staining
animal tissues is to obtain a clear stain of the nuclear parts — the
nuclei and their surrounding cell-protoplasm — as distinct from the
non-nucleated parts of the same tissue. The agents which merely
dye the tissues are for the most part colouring matters of vegetable
or animal origin ; those which act upon them chemically are
mineral substances. The staining processes may be used either
before or after section- cutting, according to circumstances. Where
the substance is in mass, and is not readily penetrable by the
staining fluid (which is especially liable to be the case where it has
been hardened in chromic acid), it is generally better to stain the
sections after cutting, if they hold sufficiently well together to bear
being transferred from one fluid to another ; and if the substance
is to be imbedded in gum, and cut with the freezing microtome, it
is generally preferable to stain the sections after they have been
cut, as the processes necessary for the removal of the gum would be
likely also to remove the dye. But where the substance to be cut
has to be penetrated by wax or paraffin, it is better that the stain-
ing should be effected in the first instance. As a general rule, it is
better that where the substance is to be stained en masse the

STAINING

431

staining fluid should be weak and its action slow, because in that
mode the stain is more equably diffused. When, on the other- hand,
the process is made use of with thin sections, it is convenient that
the action should be more rapid, and the staining fluid may therefore
be stronger ; but unless its operation be carefully watched, so as to
be stopped at the right stage, the whole tissue may be deeply dyed,
and the value of the selective staining altogether lost.

1^ will generally be found convenient to carry on the staining of
thin sections either in watch -glasses or in small shallow glass or
porcelain vessels ; but care must be taken not to place many sections
together so as to lie one upon another, as this will prevent the
staining from being uniform. Very useful receivers for sections
that are to undergo staining are the solid glass blocks, now procur-
able at very small cost at the optician's, which are about 14 inch
square and ^ inch thick, and they have a shallow, saucer-like
excavation on their upper surface into which sections may be
placed, a flat glass cover being laid over the whole. Small delicate
sections may often be stained upon the glass slides if they be
mounted upon the slide, but where it can be effected at all, it is far
preferable to mount upon the cover-glass. This section being placed
and fixed upon the cover by processes shortly to be described, the
paraffin is removed by turpentine, creosote or other means, and the
whole placed, and kept in gentle motion, in 95 per cent, of alcohol ;
for a short time it is then held in 75 per cent, alcohol, and at once a
drop of the stain dissolved in 75 per cent, alcohol is placed upon the
section. It should then be put in a chamber where the moisture in
the surrounding atmosphere should make the evaporation of the drop
or so of stain very slow, or even impossible ; here it should remain
for from twelve to twenty-four hours. It should now be taken and
a jet from a small wash-bottle containing 75 percent, alcohol should
be made to stream upon the slip, washing away the loose staining
fluid thoroughly, after which dehydrate in strong alcohol and clear
in cedar oil, mounting in xylol balsam (q,vX

If, however, a watery stain has been used, after getting rid of the
paraffin by turpentine, and washing this oft* with alcohol, we dip
and waive the cover in water ; this washes off the watery stain ;
then alcohol of increasing strengths is used until dehydration is
effected as above.

If it be inconvenient or dangerous to manipulate the cover-glass
by itself, it may be attached, for the purposes of manipulation, to a
narrow, long slip of glass, on its upper surface, by a delicate point of
wax.

It is customary to recommend the use of 'section lifters' in order
to raise the delicate sections out of the fluid in which they finally are
placed into the position in which they are to be mounted. For very
large sections they are probably essential ; but from personal ex-
perience, supported by the most accomplished histological mount en
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. 360, where it will be seen that one end of the ' lifter '
is perforated, for the purpose of drainage, and the other is plain. It

432 PRE P AK ATI ON, MOUNTING-, AND COLLECTION OF OBJECTS

must be manifest that the less we have to manipulate such delicate
sections as we are now considering, the better ; to get a section on
and off the ' lifter ; is a needless process. We should, as stated above,

mount on the cover-glass, and this
cover should be the only lifter em-
ployed.

Although it will involve a
slight displacement in the con-
templated order, we will, on ac-
count of its practical usefulness in
this place, describe how this should
be done.

The cover must be carefully
cleaned, and properly selected as
to size and tenuity. By means
of a needle or the handle of an
Fig. 360. ivory dissecting-knife the turpen-

tine in which the section is resting-
prior to mounting is gently disturbed, in a good-sized vessel or saucer,
until the section desired is in its proper position on the cover. Now
lay the cover, section upwards, on fresh blotting-paper, to take off'
the superfluous turpentine from the free side of the cover, and then
hold the edge of the slip at an angle, more or less acute, with the
section towards the blotting-paper, but never suffering the former
to touch the lattter • when this has removed the superfluous turpen-
tine from the section, lay the cover section upwards, on a glass slip,
put on (say) the benzole balsam until it stands in an evenly diffused
mound covering the section, and lay it aside absolutely protected
from dust for twenty -four hours in order that the benzole may
evaporate.

Now take it out, place upon the centre of the section one small
drop of fresh benzole balsam, and turn the cover over on to a warm
slip, being careful to have guides to the position on the slip on which
it should be fixed ; and in an hour or so we may clean off superfluous
balsam and finish the slide.

To those who mount much this will prove the quicker plan, as,
for fine results, it is undoubtedly the better.

It would be impossible in this treatise even to attempt to enu-
merate the principal stains now employed in the botanical and
zoological laboratories where histology is pursued.

It will be the utmost we can find space for if we indicate those
which we, from experience, have found most useful. 1

Hcematoxylin. — We believe this to be, in spite of some of its
defects, one of the best, if not on the whole the best, of all stains.
It is well known that it is the active principle in the extract of log-
wood. It is not readily or easily extracted ; but it may be obtained
in the market prepared.

An aqueous solution of it may be made in either of the following
ways, viz. —

1 For fuller information see the MicrotomisVs Vade Mecum, B, Lee ; Cole's
Studies; Practical Histology, Fearnley; Botanical Micro- chemistry, Poulson.

LOGWOOD STAINING

433

(<») Dissolve '35 grm. hematoxylin in LO gems, water. Dissolve
3 grms. alum in 30 gnus, water. To tlx- h;> matoxylin solution add
ft few drops of tho solution of alum. It makes a beautiful viol<-t
fluid which stains nuclei a deep blue.

(/>) Take 00 grms. of dried extract of hematoxylin, 180 grms. of
powdered alum, and work them thoroughly together with a pestle
and mortal-, adding by degrees 300 c.c. of distilled water. Mix the
whole carefully and then filter, after which add 20 c.c of absolute
alcohol. It should be kept some time, well stoppered, and in a cool
place before being used.

An alcoholic solution may be prepared by making the three fol-
lowing saturated solutions, viz. (1) calcic chloride in 70 per cent,
alcohol, (2) powdered alum in the same, and (3) hematoxylin in
absolute alcohol. Mix one part of the calcic chloride solution with
eight parts of the alum solution, and add th»' hematoxylin solution
drop by drop until a dee}) purple colour is attained. The colour
becomes richer by time.

To stain with either of these solutions take a few minims of the
v>ne chosen and place in a dram of distilled water, then filter into a
glass block as described above. Be provided with a 5 per cent,
solution of sodium bicarbonate, and if the tisNiie has been hardened
in any chromium or acid medium it must be placed in the bicarbonate
before being put in the stain. In fact, tissues or sections must,
always be cleared of all trace of acid before being put in the stain ;
when this is accomplished they must be washed in distilled water at
from 30° to 40° C. Place the sections now in the hollow of the
block containing the filtered solution of hamiatoxylin for a period
which may vary from five minutes to many hours ; in the latter case
the vessel containing the stain should be kept in a moist chamber.

From what has been already stated it will be understood that,
after staining, all sections to be mounted in balsam must be dehy-
drated, or deprived of all water, which may be readily effected by
thirty minutes in methylated spirit ; they must then be cleared in < il
of cloves until the section sinks in the oil, w hen it is transferred to
turpentine and mounted.

Tf a logwood staining has been carried to excess it may be greatly
reduced, and indeed brought to a desirable intensity, by being placed
from a few seconds to a few minutes in the following solution, viz.

1 per cent, hydrochloric acid in distilled water . 1 part
Absolute alcohol ...... 2 parts

This acts in the same way with tissues over- stained with carmine.
Logwood stains vegetable sections with extreme delicacy and great
beauty.

Dr. A. Hill, of Downing College, Cambridge, has employed with
beautiful results an original method for applying Weigert's hema-
toxylin stain to nerve-cells, and to him the Editor is indebted for these
otherwise unpublished details. As ordinarily used it is considered
to be one of the virtues of Weigert's well-known staining method
that the medullated nerve-fibres are stained a deep violet, while the
grey matter through which they run merely takes on a brownish

f f

434 PREPARATION, MOUNTING-, AND COLLECTION OF OBJECTS

tint. The object of Pal's modification of Weigert's stain is to»
render the grey matter still fainter in colour. Dr. Hill has shown,.,
however, that it is possible to render nerve-cells and non-medullated
fibres extremely susceptible to the stain by treating them before-
hand with carmine alum or simply alum solution. To produce a
successful result Weigert's staining method must be somewhat
modified and the steps of the process about to be described rigidly
followed : (1) Pieces of brain or spinal cord must be placed for six
weeks in bichromate of ammonia to harden (2 to 2^ per cent.)_
(2) The bichromate is washed out with abundance of 30 per cent,
spirit, changed daily until the solution takes no colour from the
tissue. (3) The tissue is further hardened in strong alcohol..
(4) Pieces of tissue of convenient size are placed for an hour in water
to remove the spirit and then for two days in carmine alum solution.
The best way to make carmine alum solution, which is a most useful
stain for many purposes, is to place in a shallow dish carmine,
potash, alum, and distilled water, boiling the mixture for three hours,
the water being restored to its original level from time to time as it
evaporates. Both carmine and alum should be in excess, i.e. in
larger quantity than the water will dissolve. When cold the solu-
tion is decanted and filtered. (5) The tissue is then cut into-
sections which are placed for twenty-four hours in a solution of
acetate of coj)per half saturated ; for convenience a saturated solution,
is kept and mixed with an equal volume of distilled water before
use. (6) From the acetate of copper the sections are placed in the
hematoxylin mixture for- eight hours, the mixture being maintained
at a temperature of 40° C. circ. At the end of this time the
sections are stained black and covered with a precipitate of hema-
toxylin. (7) The sections are then decolourised to exactly the right
degree in a solution of ferricyanide of potassium. It is well to.
have a basin of water at hand into which the sections are placed
from time to time to ascertain the progress of the decolonisation. If
the sections are imbedded in celloidin it may always be safely
assumed that the matrix of the tissue and the imbedding celloidin
will be decolourised at the same time.

This method gives extremely good results when applied to the
cerebellum or to groups of large nerve-cells such as constitute the
nucleus of the third and other motor cranial nerves ; a photo-micro-
graph of a section of the cerebellum of a lamb prepared by Dr. Hill
is given at fig. 4- of the frontispiece. It was photographed X 77
diams. with a 1-inch apochromatic objective of N.A. 3, and sufficiently
illustrates the value of Dr. Hill's method.

Weigert's hematoxylin solution : —

Crystallised hematoxylin . . 1 gramme.
Dissolved in absolute alcohol .... 10 c.c.
Add distilled water .... 90 c.c.

Lei the mixture come to the boil.

Weifferfs decolourising mixture : —

Ferricyanide of potassium . . 2\ grammes..

Borax 2 grammes.

Water ... .200 c.c.

STAINING

4.35

This mixture is too strong for Dr. Hill's method. It usually
works well if diluted with an equal volume <>t* water.

Carmine. — This was one of the firsl dyes employed for staining
purposes; and its value was specially insisted on by Dr. Beale, as
enabling li villi;- protoplasm (l>y!iim designated 'germinal matter,' or
' bioplasm') to be distinguished from any kind of 'formed material.'
It has a special affinity for cell-nuclei (protoplasts) and the axial
cylinders of white nerve-fibres ; and thus, if the substance to be
stained be only left in the carmine fluid long enough for it to dye
these substances, they are strikingly differentiated from all others.
It is essential that the fluid should have a slight alkaline reaction.
It is very difficult to get perfectly good nuclear stains with carmine
in the case of tissues that have been treated with chromic and other
acids. Tissues hardened with picric acid or alcohol stain beautifully
with it. The presence of too much alkali is injurious j the want of
it, on the other hand, causes the dye to act on the tissues generally,
and thus negatives its differentiating effect. Dr. Beale directs it to
be prepared as follows : — Ten grains of carmine in small fragments
are to be placed in a test-tube, and half a drachm of strong liquor
ammonia* added ; by agitation and the heat of a spirit-lamp the
carmine is soon dissolved, and the liquid, after boiling for a few-
seconds, is to be allowed to cool. After the lapse of an hour, much
of the excess of ammonia w ill have escaped ; and the solution is then
to be mixed with 2 oz. of distilled water, 2 oz. of pure glycerin, and
!j oz. of alcohol. The whole may be passed through a filter, or after
being allowed to stand for some time the perfectly clear supernatant
fluid maybe poured off and kept for use. If, after long keeping,
a little of the carmine should be deposited through the escape i £
the ammonia, the addition of a drop or two of liquor ammonia'
will redissolve it. Carmine is used as a general stain in ' double
staining' ; and a suitable fluid for this purpose is made by mixing
thirty grains of carmine with two drachms of borax, and four fluid
ounces of water, and pouring off the clear supernatant fluid. To
Jix the stain of carmine, the section should be immersed for a few
minutes in a mixture of five drops of glacial acetic acid and one ounce
of water.

To stain sections with this, first wash in distilled water, then in
a 1 per cent, solution of sodium bicarbonate, and once more in
distilled water. Transfer now to the carmine fluid, and leave from
one minute onwards until the stain is sufficient, then place in
methylated spirit and wash carefully. The sections may now be
bottled in spirit until required for mounting.

In botanical sections it will be found that carmine colours most
vegetable albuminoids, while starch and cellulose take it up slightly
or not at all.

Boradc Solution Carmine— Strasburger used a borax solution of
this stain for the study of the embvro-sac. It was prepared as
follows : Four parts borax are dissolved in fifty-six parts distilled
water. To this one part of carmine is added. One volume of this
solution is diluted in two volumes absolute alcohol and filtered.
This stains the nucleus with great clearness.

r r 2

436 PREPAKATION, MOUNTING, AND COLLECTION OF OBJECTS

Mr. A. Cole has effected some extremely beautiful vegetable
•stains with another preparation of borax carmine.

i. Take 10 grains of borax and dissolve in 1 oz. of distilled water
and add 4 drachms of glycerin and "4 drachms of alcohol.

ii. Take 10 grains of carmine and dissolve in 20 minims of liq.
amnion, fort, and 30 minims of distilled water in a test-tube supplied
with gentle heat. Let it cool, then thoroughly mix i. and ii. ; filter
and keep in a well-stoppered bottle.

Next make a saturated solution of iodin green in alcohol.
Bleach the sections in chlorinated soda solution. Wash or
twenty-four hours in frequently changed water ; stain in the carmine
solution ten to thirty minutes ; place now in methylated spirit until
no more colour comes out. Wash in clean spirit ; add enough of
the iodin green solution to render this spirit bright green. Here
the sections may remain until needed for mounting ; then they must
be taken out, washed in clean spirit, after which clear in oil of cloves
and mount in balsam.

Picro-carminate of ammonia, known as picro -carmine, is a very
excellent staining material, which is applicable to a great variety of
purposes. Being somewhat difficult to prepare, it is best purchased
ready for use (from Martindale, New Cavendish Street). Use a 2
per cent, filtered solution and let the section remain in from half an
hour to twelve or more hours ; wash rapidly in water and mount in
Warrants' solution, glycerin, or balsam. This dye, used alone, pro-
duces a double staining, the nuclei fixing upon the carmine, while the
other tissues are coloured yellow by the picric acid. If the sections
be placed in methylated spirit, they may be kept without loss of colour
and may be afterwards subjected to other processes. If placed in
water the picric acid stain is removed, while the carmine is left.

Magenta has nearly the same selective staining property as car-
mine, and is useful in the examination of specimens for which rapid
action and sharp definition are required. But, like other anilin
^dyes, it is liable to fade, and should, therefore, not be employed for
permanent preparations. Ordinary magenta fluid may be prepared
by dissolving 1^ grain of magenta crystals in 7 fl. oz. of distilled
water, and adding \ n- oz- °f rectified spirit. The colour of a section
stained with this may be preserved for some time by immersing it
in a ^ per cent, watery solution of corrosive sublimate.

Eosin, which dyes the tissues generally of a beautiful garnet-red
colour, should be used in a strong watery solution, and the sections
must be well washed in water after staining. Its chief use is in
' double staining.'

For bine and green staining the various anilin dyes are princi-
pally used. They are, for the most part, however, rather fugitive in
their effects, not forming durable combinations with the tissues they
stain. Some of them are soluble in water, others only in spirit ;
and the selection between the dyes of these two classes will have to
be guided by the mode in which the preparations are treated. These
dyes are for the most part best fixed by benzole ; and as the sections
treated with this fluid may be at once mounted in Canada balsam,
there is greater probability of their colours being preserved. Be-
sides blue and green, the anilin series furnishes a deep rich brown^

STAINING BACTEKU

437

known ;is Bismarck's brown | and a blue black, which baa been re
commended for staining nerve-cells.

A good blue stain (tending to purple) is also given by the sub-,
stance termed indigo carmine, which is particularly recommended
for sections of the brain and spinal cord that have been hardened
in chromic acid. A saturated solution of the powder in distilled
water having been prepared, this may either boused with the ad-
dition of about 4 per cent, of oxalic acid, or, if an alcoholic fluid bo
preferred, methylated spirit may be added to the aqueous solution,
the mixture being filtered to remove any colouring matter thai may
have been precipitated. If sections thus stained have an excess of
colour, this may be removed by the action of a saturated solution of
oxalic acid in alcohol.

A beautiful green hue is given by treating wit 1 1 a saturated
solution of picric acid in water sections previously stained with
anilin blue, or the two agents may be used t ogether, four or five parts
of a saturated solution of the latter being added to a saturated

aqueous solution of the former. This picro-anilin, it is believed,
may be relied on for permanence, and it acts well in double staining
with picro-carmine.

Two chemical agents, nitrate of silver and chloride of gold, are
much used by histologists for bringing out particular tissues, the
former being especially valuable for the staining of the epithelium
cells, the latter for staining nerve-cells, connective-tissue corpuscles,
tendon-cells, and cartilage-cells. The most advantageous use of these
can only be made by the careful observance of the directions which
will be found in treatises on practical histology.

Molybdate of ammonia is recommended as affording a cool, blue-
grey or neutral-tint general stain, which affords a pleasant 'ground '
to parts strongly coloured by bright, selective stains.

Staining Bacteria. — Ft is needful to employ somewhat special-
ised methods for staining the saprophytic, pathogenic, and other
schizomycetes. Some of these stain admirably, but others, especially
the somewhat larger forms, are much altered, and 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.
From each take out with a pipette a small quantity, and transfer to
a carefully prepared and well-filtered decoction of veal in a small
glass vessel, at the temperature of the respective putrefactions ; leave
this for half an hour. Then with a fine pipette take out a minute
drop from each vessel and diffuse each drop upon a cover-glass ; let
evaporation go on in a warm room for twenty minutes, then ti\ the

film of saprophytes by means of fairly strong osmic acid vapour
(p. 4l,(J) ; float the cover with the surface of bacteria downwards on a
vessel of violet of methylanilin 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 BOVera] vessels and in a
moist, growing cell examine the li\ ing forms, and compare these with
your dried and stained preparations.

P>y another method, which will apply also to the bacillus of"

438 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

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.
Mix 1 c.c. of concentrated solution of methyl-blue in alcohol, 0*2 c.c.
of 10 per cent, solution of potash and 200 c.c. of distilled water. Into
this put the cover with its surface of bacteria 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 methyl-blue
from all but the bacteria. Finish with alcohol and oil of cloves,
and mount in balsam.

For the same purpose Professor Heneage Gibbes gives a method
which has proved of great value. Take of rosanilin hydrochloride 2
grins, methyl blue 1 grm. ; rub them up in a glass mortar. Then
dissolve anilin oil, 3 c.c, in rectified spirit, 15 c.c. ; add the spirit
slowly to the stains until all is dissolved, then slowly add distilled
water, 15 c.c. Keep in a stoppered bottle.

In the usual way dry the sputum &c. on a cover-glass ; 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 Bacteria in Tissues. — To 100 parts of solution of caustic
potash of 1 : 10,000 add 30 parts of saturated alcoholic solution of
methyl-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.

Double and Multiple Staining. — It is needful to allude to this
mode of staining tissues, because during the last ten years it has re-
ceived much attention, and also because of its apparent potentiality
as an aid to histological research, and the extreme beauty of the pre-
parations that may be made by its means. But in point of fact, as a
means of investigation, it is of little value. It differentiates tissues,
but not in a manner that will make any further knowledge of them at
all possible. For class and popular purposes it will obtain ; but it has,
so far as can be at present seen, no future for the investigator.

Very beautiful effects are doubtless produced by the simultaneous
or successive action of two or three staining fluids, which will re-
spectively pick out (so to speak) the parts of a section for which
they have special affinities. Thus, if a section through the base of
the tongue of a cat or dog be stained with picro-carmine, rosein,
and iodin green, the muscle-fibres will take the first, the connective
tissue and protoplasm of cells will be coloured by the second, while
the third will lay hold of the nuclei in the superficial epithelium,
serous glands, and non-striated muscle in the vessels ; and, further,
the mucous glands will show a purple formed by the combined action
of the red and green (Gibbes). 1 A very striking contrast of the like
kind is shown in the double staining of the frond of a fern with log-
wood and anilin blue, the sori taking the latter, and standing out

1 See his Practical Histology, Chapter V.; and his paper in Joum. of Boy,
Microsc. Soc. vol. iii. 1880, p. 390.

DIFFERENTIAL ST.USINCi

A \9

brilliantly on the; general surface linked l>y tin- former. The Bffectl
produced by using one stain after the other air generally much
better than those obtained by simultaneous staining. The selective
action of a second stain is not. prevented by a previous genera]
Staining ; for the dye which gives the latter seems to be more weakly
held by the parts which take the former, so as to be (as it wore) di-
placed by it. Thus, it" a section of a stem be stained throughout by
a solution of eosin (2 grains to 1 oz.), and be then placed, after
washing in strong alcohol, in a /,-grain solution of Nicholson's
blue made neutral, the blue will in no long time entirely drive out
the red ; but by carefully watching tin; process, it will be seen that
the different tissues will change colour in different times, the softer
cells giving up their red and taking in the; blue more quickly than tin;
harder; so that, by stopping the process at the right point (which
must be determined by taking out a section, dipping it in alcohol,
and examining it under the microscope), the two kinds of cells are
beautifully differentiated by their colouring.1 The best effects are
usually produced by carmine and indigo carmine, logwood and picro-
carmine, carmine or logwood and anilin blue or anilin green. 1 Jut
very much lias yet to be learnt on this subject ; and the further
investigation of it will be likely to produce results that will amply
repaythe time and labour bestowed.

It will be enough to give a practic al illustration of some of tin;
methods. Make a dilute solution of picro-carmine (about ten drops
to a watch-glass of water). Stain in it for about half an hour, wash
out for an hour in water acidulated with a few drops of acetic or
picric acid, and then double stain either with rose in and anilin blue,
or with anilin violet and anilin blue, or with anilin violet and a a din
green) or with rosein and anilin green.

A process of differential staining of bacillus tuberculosis which
was devised by MM. Pittionand Roux was presented recently (1S^9)
to the Societe de Medecine de Lyon, and has met with high com-
mendation. It requires three solutions : —

A. Ten parts of fuchsin dissolved in 100 parts of absolute alcohol.

B. Three parts of liquid ammonia dissolved in 100 parts of distilled
water.

C. Alcohol oO parts, water 30 parts, nitric acid 20 parts, anilin
greet] to saturation. In preparing this solution dissolve the green
in the alcohol, add the water, and lastly the acid.

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. ( >ne
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 tine rose-red upon a pale

green ground.

Chemical Testing. It is often requisite, alike in biological and

1 See Jutini. of Hoy. Microsc. Sue. vol. iii. 1SS0, y>. QM.

440 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS'

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 bottles, pp. 446-7, with a tine-pointed
nozzle, as the most convenient instrument. One of its advantages is
the very precise regulation of the quantity of the test to be deposited
which can be obtained by the dexterous use of it ; whilst another
consists in the power of withdrawing any excess. Care must be
taken in using it to avoid the contact of the test-liquid with the
packing of the piston. Whatever method is employed, great care
should be taken to avoid carrying away from the slide to which the
test-liquid is applied any loose particles which may lie upon it, and
which may be thus transferred to some other object, to the great
perplexity of the microscopist. For testing inorganic substances the
ordinary chemical reagents are of course to be employed ; but certain
special tests are required in biological investigation, the following
being those most frequently required : —

a. Solution of iodin in water (1 gr. of iodin, 3 grs. of iodide of;
potassium, 1 oz. of distilled Avater) turns starch blue and cellulose
brown ; it also gives an intense brown to albuminous substances.

ft. Chlor-iodide of zinc is made by dissolving zinc in pure hydro-
chloric acid, evaporating to the density of sulphuric acid, in contact
with metallic zinc, and adding as much potassic iodide as the solution,
will take up. Finally saturate with iodin crystals.

This is extremely useful for the detection of pure cellulose. The-
zinc chloride converts cellulose into amyloid, which is then turned
blue by free iodin. AVood-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 arid (one of acid to two or three parts of*
water) gives to cellulose that has been previously dyed with iodin
a blue or purple hue : also, when mixed with a solution of sugar, it;
gives a rose-red hue, more <>r less deep, with nitrogenous substances
and witli bile (Pettenkofer's test).

Sulphuric acid causes starch grains to swell and similarly affects
cellulose.

€. Concentrated nitric acid srives to albuminous substances an
Intense yellow.

£. Acid nitrate of mercury (Millon's test) colours albuminous
substances red.

rj. Acetic Acid, which should be kept both concentrated and diluted
with from three to five parts of water, is very useful to the animal

WHAT PRESERVATIVE MEDIA ARE SUITABLE 441

histologic from its power of dissolving, or ;it least o£ reducing fco >n« li ;i

state of transparence t hat t hey can no Longer be disl Lnguished, certain

kinds of membranous and fibrous tissues, so that other parts (especially
nuclei) are brought more strongly into view.

9. Ether dissolves resins, fats, and oils ; hut it will not act on
these through membranes penetrated with watery fluid. For the
same purpose chloroform, benzol, oil of turpentine, and carbon bisul-
phide are used.

1: Alcohol dissolves resins and some volatile oils, but it does qo1
act on ordinary oils and fats. It coagulates albuminous matters,
and consequently renders more opaque such textures as contain them.
The opacity, however, may be removed by the addition of a small
quantity of soda.

Preservative Media. — We have now fco 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 tro itspo rent objects. Abroad distinction may
be in the first place laid down between resinous and ctqueous pre-
servative media ; to the former belong only Canada balsam and
dammar, whilst the latter include all the mixtures 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 advantage-
ously imparted to it. Sections of substances which have been not
only imbedded in, but penetrated by paraffin, wax, or cacao-butter
and have been stained (if desired) previously to cutting, are, as a ride,
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 dur-
ability of this method of mounting makes it preferable in all ca>es to
which it is suitable, the exception being where it renders a very
thin section too transparent, which is specially liable to happen with
dammar. When it is desired to mount in either of these media
sections of structures that have been imbedded in gum or gelatin,
these substances must first be completely dissolved out by steeping
in water ; the sections must then be 'dehydrated' by subjecting
them to mixtures of spirit and water progressively increased in
strength to absolute alcohol ; and after this has been etlected they
are to be transferred to turpentine, and thence to benzole balsam.
Tn this process much of the staining is apt to be lost : so that stained
sections are often more advantageously mounted in some of those
aqueous preparations of glycerin which approach the resinous media
in transparence and permanence. When Canada balsam was tir^t
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 con-
stituent was driven Off by heat in the process of mounting (bubbles

442 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

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 dis-
solving the resin thus obtained either in benzole or chloroform, but
far preferably the former, the solution being made of such viscidity
as will allow it to ' run 5 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 also of great
value, being by some mounters preferred to any other. It is made of
equal volumes of xylol and balsam. The natural balsam, however,
may be preferably used (with care to avoid the liberation of bubbles
by overheating) in mounting sections already cemented to the slides
by hardened balsam, and also for mounting the chitinous textures
of insects, which it has a peculiar power of rendering transparent, and
which seem to be penetrated by it more thoroughly than they are
by the artificially prepared solution. The solution of dammar in
benzole is very convenient to work with, and hardens quickly.

The following are the principal aqueous media whose value has
been best tested by general and protracted experience : —

a. Fresh specimens of minute protophytes can often be very well
preserved in distilled water saturated with camphor, the complete
exclusion of air serving both to check their living actions and to pre-
vent 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.

/?. Aqueous Solution of Carbolic Acid. — Even the very small quan-
tity of this agent which cold water will take up has a powerful pre-
servative effect, and the solution may be advantageously employed
for mounting preparations of many delicate structures, both animal
and vegetable.

y. The same may be said of salicylic acid, which has been very
successfully employed for delicate preparations in the small proportion
that will dissolve in cold water. For coarser structures a stronger
solution is preferable ; and this may be made by combining with the
acid a small quantity either of borax dissolved in glycerin or of
acetate of potass.

8. Where the preservation of minute histological detail is not
so much desired as the exhibition of larger structural features of
objects to be viewed by reflected light nothing is better than dilute
spirit, the proportion most generally serviceable being one of alcohol
to four or five of water, and an even weaker mixture serving to pre-
vent further change in tissues already hardened by strong alcohol.

e. Salt solution 0'75 per cent, sodium chloride in water.

£. Fruit juice, wliite of an egg. — Simply filter.

7). Syrup in which is dissolved 1 to -r) per cent, of chloral hydrate,
or 1 per cent, of carbolic acid.

0. CJdorcd Hydrate. — A 5 per cent, solution in water, or 12 grains

v.Ufiors pkkskkvativk .mki»; \

443

-chloral hydrate to 1 fluid ounce of camphor water. Mount in strong
glycerin jelly.)

l. Con'osirf Sublimate aial Salt. ( so bichloride of mercury one
part ; common salt two parts, water 200 parts. This is very useful.
It preserves well muscular fibre, nerves, and epithelia, and with very
slight change is preservative of spermatic fluid.

k. (in ni and Syrup. — Gum-mucilage (15. P.) five parts, syrup three
parts. Add 5 grains of pure carbolic acid to each ounce of the medium.

B".P. GrU i n - i n u c i 1 age 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 dis-
tilled water and boiling.

X. The glycerin jelly prepared after the manner of Mr. Lawrence
may be strongly recommended as suitable for a great variety of ob-
jects, animal as well as vegetable, subject to the cautions already
given : — ' Take any quantity of Nelson's gelatin, and let it soak for
two or three hours in cold water, pour 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 efts. 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).' 1 A
small quantity of absolute phenol may be added to it with advantage.
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.

fx. For objects which would be injured by the small amount of
heat required to liquefy the last-mentioned medium, the (jlycriit and
gum medium of Mr. Farrants 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 \egetaMe tissues, and in
most cases increases their transparence.

1 Avery pore fflyoeiil) jelly, of which the Author lui* ui;i<i«- cousideruhlc use, is
prepared hy Mr. Riiinuiu-rtoii, chemist, Bradford, Yorkshire.

444 PREPARATION, MOUNTING-, AND COLLECTION OF OBJECTS

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 sur-
faces, and should never be employed, therefore, in the mounting of"
objects to be viewed by reflected light, although many objects-
mounted in the media to be presently specified are beautifully
shown by ' dark-ground ' illumination.

1. A mixture of one part of glycerin and two parts of
camphor-water may be used for the preservation of many vegetable
structures.

2. For preserving soft and delicate 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-water the most suitable preservative.

3. For preserving minute vegetable preparations the following
method, devised by Hantzsch, is said to be peculiarly efficient : A mix-
ture is made of three parts of pure alcohol, two parts of distilled water,
and one part of glycerin ; and the object, laid in a cement-cell, is
to be covered with a drop of this liquid, and then put aside under abell-
glass. The alcohol and water soon evaporate, so that the glycerin
alone is left ; and another drop of the liquid is then to be added,
and a second evaporation permitted, the process being repeated, if
necessary, until enough glycerin is left to fill the cell, which is
then to be covered and closed in the usual mode.1

Canada balsam is one of the most universally employed mounting
media ; very old hard balsam should be dissolved in enough pure
benzole to make a thin solution, which should be carefully filtered.

Dammar. — Dissolve gum-dammar with heat in a mixture of
equal parts of benzole and turpentine, and evaporate to a syrupy
consistency. This is pleasant to use, but treacherous. The prepa-
rations often subsequently 'closed.'

Gum Styrax. — This is a resin which must be disolved in benzole,
choloroform, or ether. It should have the consistency of olive oil ;
all the benzole must be evaporated before putting the cover on the

1 See the Kev. W. W. Spicer's Hand y -hook to the Collection and Preparation
of Freshwater and Marine Algce, tic. pp. 57-51). ' Nothing,' says Mr. Spicer, 'can
exceed the beauty of the preparations of Desmidiacece prepared after Herr Hantzsch's
method, the form of the plant and the colouring of the endochrome having under-
gone no change whatever.'

MOUNTING MEDIA WITH HIGH REFRAl riVE INDICES 445

slip; its refractive; index is said to be then I '583, its value it in
the mounting of diatoms, where .1 marked difference between the

refractive index of the siliceous frustules ami the medium in which
they are mounted facilitates t he discovery of obscure del ails. There i ,
a marked increase of visibility in proportion .is the mounting medium
has a refractive index higher than the object (diatom) mounted.

Now the refractive index of the silex of diatoms is 1*43. But
Canada balsam is 1*52: hence the 'index of visibility ' in obscure
markings is i), while styrax by comparison is 15.

J/ottohromidf, of iiaphtludin is .-mother 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 no medium of such high value as that suggested and
-very successfully employed by -Mr. J. \V. Stephenson, viz.

Pho$phorus. — 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 on account of its spontaneous combustion in
air, and the severe nature of the burns it inflicts. But it is with
slight practice by no means an unmanageable medium.

To prepare it, take a 2-drachm bottle with no contraction for the
neck. Make a cylinder of wood that will just 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
<m 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 ail- 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
insttiutly 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 (p. 382)
used warm and allowed to cool. It is now a still' jelly. Lay the

446 PREPARATION, MOUNTING-, AND COLLECTION OF OBJECTS

cover in its place, with the diatoms downwards, touching the ring at
one side, but raised by a fine wire on the side next the operator. A
pipette may also be used made of glass tubing an eighth of an inch in
external diameter, drawn to a fine point at one end, and somewhat
enlarged at the other, and to which an indiarubber cap or nipple is
fastened air- tight. 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 finally
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 the specimens fromi
time to time, so as to judge the condition of each.

1 11 dealing with the small quantities of fluid
media required in mounting microscopic ob-
jects, it is essential for the operator to be pro-
vided 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 diluted
alcohol or the carbolic acid solution, is in con-
tinual use, it will be found very convenient to
keep it in the small dropping-bottle represented
Fig. 861. in fig. 361. The stopper is perforated, and is.

Dropping-bottle. elongated below into a fine tube, whilst it ex-
pands 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 reap-
plied, some of the residual air will be forced out, so that by again
immersing the end of the tube, and removing the pressure, more
fluid will enter. This operation may be repeated as often as may
be necessary, until the bulb is entirely filled ; and when it is thus,
charged with fluid, as much or as little as may be needed is then

BOTTLES FOB USE WITH REAGENTS

447

readily expelled from it by the pressure of the finger on tin (--over,
the bulb being always refilled if care be t;iken to immerse tlx- Lowei
end of the tube before; the pressure is withdrawn. We speak from
large experience of the value of this little implement, which is
very clean, simple, and useful. But the small pipettes now used so
commonly for filling the stylograph it pens, fitted into the centre of a
cork and placed in any wide-mouthed bottle, will be found to be,
though less elegant, equally useful and much less costly.

Solutions of Canada balsam and gum-dammar in volatile fluids
are best kept in wide-mouthed <-(f/>/>rr/ 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. Great care should be
taken to keep the inside of the cap and the part of the neck of the
jar on which it fits quite clean^ so as to prevent the fixation of the
neck by the adhesion between these two surfaces. Should such ad-
hesion take place, the cautious application of the heat of a spirit-
lamp will 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 de
posited.

A bottle for use
with reagents, enabling
the operator to pour
out only the quantity
he desires, is invalu-
able. Small capped
and stoppered bottles,
the stoppers of which
are tubes, and the wel 1
fitting caps of which
prevent evaporation,

are very valuable for aqueous and thin fluids. We illustrate this
bottle in tig. 3G2. 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 German bottles, shown in fig.
363, containing about 30 grammes, in which two deep grooves are
cut on opposite sides of the stopper, 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 a in ti<;. 363 ; the air travels down
this groove, and by inverting the bottle the fluid enters the other
groove of the stopper and finds its way to a third groove cut in the
inside of the neck and extending to the lip. The figure shows the
bottle complete.

Mounting Thin Sections. The thin sections em by the micro

Fig. 862. — Expan-
sion drop-bottle.

Fig. 868.
German drop-bottle.

448 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

tome, or membranes obtained by dissection, do not require to be
placed in cells when mounted in any viscid medium • since its tena-
city will serve to keep off injurious pressure by the cover-glass.
When the preparation has been previously immersed in aqueous
liquids, and is to be mounted in glycerin, glycerin jelly, or
Farrants' medium, the best mode of placing it on the cover is to float
it in a saucer or shallow capsule of water, to place the cover beneath
it, and, when the object lies in a suitable position above it, to raise
the cover cautiously, holding the object in place by a needle, until it
is entirely out of the water ; and the small quantity of liquid still
surrounding the object is to be carefully drawn 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 mount-
ing microscope, for the purpose of improving (if desirable) its
disposition on the slide, and of removing any foreign particles that
may be accidentally present. A drop of the medium (liquefied, if
necessary, by a gentle warmth) is then to be placed upon it, and
another drop placed on the slip and allowed to spread out. The
cover being then taken up with a pair of forceps must be inverted
over the object, and brought to touch the slide 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 be raised,
but a little may be deposited at its edge, whence it will soon be drawn
in by capillary attraction, especially if a gentle warmth be applied
to the slide. It will then be 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 first
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 be
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. S. 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-

MOUNTING

449

table, and the preparation 'sealed' by a ring cither of dammar or
of Bell's cement, which should be carried a little over the edge of the
cover, and outside the margin of the ring of glycerin jelly. Tin
'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 oti' with a coat of Hollis's glue, which is not attacked by
cedar oil. Until the cover has been perfectly secured, a slide carrying
a glycerin preparation should never l>e placed in an inclined posi-
tion,* as its cover will be almost sure to slide by its own weight. If
glycerin jelly or Farrant's medium lias been employed, less caution
need be used, as the cover-glass, after a few day*' etting, will adhere
with sufficient firmness to resist displacement. The superfluous
medium having been removed by the cautious use of a knife, the slide
and the margin of the cover may be completely cleansed by 8 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 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 resinous medium, it must have been previously pre-
pared for this in the modes already described, which will present it to
the mounter either in turpentine or some other essential oil, or in
alcohol. From either of these it may be transferred to the cover in
the manner already described.

Mounting Objects in 'Natural' Balsam.— Although it Lb pre-
ferable for histological purposes to employ a solution of hardened
balsam, yet as there are many objects for mounting for which the use
of the 1 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 they should be mounted without being detached,
unless they have become clogged with the abraded particles, and require
to be cleansed out — as is sometimes the case with sections of the
shells, spines, itc. of echinoderms, when the balsam by which they
have been cemented is too soft. If the detachment of a specimen be
desirable, it may be loosened by heat, and lifted off with a camel's-
hair brash 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 then falls oti' of itself by the
solution of its cement. It may be thoroughly cleansed by boiling it
in methylated spirit, and afterwards laid upon a piece of blotting-
paper to diy, after which it may be mounted in fresh balsam on a
slide, just as it' it had remained attached. The slide having been
warmed on the water-bath lid, a sufficient miantitvot balsam should
be dropped on the object, and care should be taken that this, if pre-
viously loosened, should be thoroughly penetrated by it. It any air-
bubbles arise, they should be broken with the needle-point. Tin1.

G G

450 PKEPAKATION, MOUNTING, AND COLLECTION OF OBJECTS

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 be turned over and letdown on the object in the manner
already described. If this operation be performed over the water-
bath, instead 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 accumulate 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 may thus be mounted without a single air-bubble.
(The Author has thus mounted sections of JEozoon three inches
square.) In mounting minute balsam objects, such as diatoms,
2? oly cystine?, sponge-spicules, and the beautiful minute spines of ojihiu-
rida, no better plan can be adopted than to arrange these objects
carefully upon the cover, either by ' scattering ■ or ' arrangement/
and then to drop on to the whole cover and its arranged objects as
much balsam as the cover will receive without overflow ; this should
stand free from dust for some hours, after which the partly hardened
balsam may receive a small drop of fresh balsam, and being placed
upon the slip in proper position, may by the use of gentle heat be
pressed finally into position. When the chitinous textures of insects
are to be 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 forceps is to be plunged in
it, and the cover (balsamed as before) let down upon it. It is with
objects of this class that the spring-clip and the spring-press 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 of gasteropods) 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. Carbolic
acid has been recommended by Dr. Ralph1 as most efficient in
drawing out water from specimens to be mounted in balsam or
dammar, which afterwards readily take its place. Sections of horns,
hoofs, &c. which afford most beautiful objects for the polariscope, are
best mounted in natural balsam, which has a remarkable power of
increasing their transparence. It is better to set aside in a warm
place the slides which have been thus mounted before attempting
to clean off the superfluous 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 well 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-

1 See the account of Dr. Ralph's method in Journ. Boy. Microsc. Soc. vol. iii. .

1880, p. 858.

MOUNTING

451

tion is not such as to cause the cover to he drawn to the irl.t— -dide
hy capillary attraction, or whenever the cover is s< ,,sihli/ 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
tin; 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 heneath the cover, which process is pretty sure to
continue when it may have once commenced. When cement-cell-
are employed for this purpose, care must he taken that the surface
of the ring is perfectly fiat, so that when the cover glass is laid on,
no 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 be examined on the
dissecting microscope that its entire freedom from foreign particles
and from air-bubbles may be assured, and that its disposition may
be corrected if necessary. The cover should then be laid on very
cautiously, so as not to displace the object ; which in this case is
best done by keeping the drop highest in the centre, and keeping
the cover parallel to the slide whilst it is being lowered, so as to
expel the superfluous fluid all round. This being taken up by
the syringe, the cement ring and the margin of the cover are to be
dried with blotting-paper, especial care being taken to avoid drawing
oft' too much liquid, which will cause the gold-size to run in. It is
generally best to apply the lirst coat of gold-size thin, with a very
small and flexible brush worked with the hand ; this will dry suf-
ficiently 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 thickly, being then applied so
as to raise the ring to the level of the surface of the cover. As ex-
perience shows that preparations thus mounted, which have re-
mained 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. Knock,1 who puts a metallic
ring of angular section (T) round the outside of the ctll, slightly
overlapping the cover-glass and enclosing the rim made good w it h
cement : this proves perfect.

Mounting of Objects in Deep Cells. — The object - w hich require
deep cells are, as a rule, such as are to be viewed by reflected light,
and are usually of sufficient size and substance to allow of air being

entangled in their tissues. This ua especially liable to occur where

they have undergone the process of decalcification, w hich w ill very
probably leave behind it bubbles of carbonic acid. For the extrac-
tion of such bubbles theuseof an air-pump is commonly recommended :
but the Author has seldom found this answer t hepurpi »>e sat isfactorily,

1 Quekctt Jottrn. second scries, vol. i. p. 40.

o o 2

452 PEEPAKATI ON, MOUNTING, AND COLLECTION OF OBJECT

and is much' disposed to place confidence in a method lately recom-
mended—steeping the specimen in a stoppered jar filled. "wi*tti f i oshly^
boiled water, which 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 size exactly
suitable to that of the ring, of whose breadth it should cover about
two-thirds, leaving an adequate margin uncovered for the attachment
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 can
be easily made perfectly flat by grinding with water, first on a piece
of grit, and then on a Water-of-Ayr stone, these stones having been
previously reduced to a plane surface, or still better with a good flat
file. If glass rings are not found to be ' true,' they must be ground
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 blot-
ting-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
minutiaB 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 as 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

PERMANENT LA]', ELS

453

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
have been successfully disposed of, the cell may be 'sealed' and
1 ringed ' in the manner already described.

Importance of Cleanliness. — The success of the result of any of
the foregoing operations is greatly detracted from if, in consequence
of the adhesion of foreign substances to the glasses whereon the
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, <fcc, 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 j and especially to be contaminated by particles of dust
floating through the air, the access of which may be readily prevented
by proper precautions. It is desirable to have at hand a well-closed
cupboard furnished with shelves, or a cabinet of well-fitted drawers,
or a number of bell-glasses upon a flat table, for the purpose of
securing glasses, objects, &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 microscopist be likely to make
of them. Great care ought, of course, to be taken that the media
employed for mounting should be freed by effectual nitration 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 <>n which
the number is placed should be as permanent and immovable as the
slip itself. We know of cabinets in which only numbers are
marked on slides, and all details are recorded in 'the book.' We
do not advise this ; but all who keep cabinets know how in the course
of years paper labels become displaced and lost, and in many
instances the value of slides is greatly diminished.

454 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

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 obj ect.

The present Editor has found the following plan to be hitherto,
after fourteen 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.
Gn 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
Faber pencil marked HHHH. 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 &c.

Now when all this is written take thin covers, cut respectively
1 x f inch and 1 x h inch, and by means of benzole 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 manage-
ment 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 consecutively.
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. Now a number of notebooks
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.

COLLECTING

155

Thus a diatomist would have probably a thick ledger for his diatom
collection, whereas an entomologist would have a thin notebook for
his diatoms and a thick ledger for his insects, and so on. The note-
books might be distinguished from one another by a letter of the
alphabet.

In the event of a second notebook being required for the same
subject or class of objects, it might be identified by doubling the
letter ; thus, D D. Now a large huh y notebook will be required in
whicfi one line is given to each slide. This notebook contains
merely the number of the slide and the letter and page of the special
notebook wherein all about the slide will be found. Thus : —

G49, F 127.

This means that in notebook F on page 127 we shall find an
account of slide No. G49.

On turning to notebook F we find (say) that the subject is
geology. The following will be a facsimile of the page : —

Slide No. G 49 127

Section of porphyry from Peterhead, Aug. 188G. — The quartz
crystals in this section have minute cavities containing a liquid C02.
In each cavity there is a bubble ; some of these bubbles are ex-
tremely minute, and exhibit rapid Brownian movement. A good
example of which is —

No. 2 (referring to a second microscope when used), 46-51.

A large bubble with no Brownian movement.

No. 2 (microscope), 44-47.

Section too thick for oil immersion.

Best seen dry £ "95 N. A. deep eye-piece ; condenser aperture
•G N.A.

At the back of each notebook there is an alphabetical index.
In this instance if we look up porphyry we shall find 127, and if
we look up quartz (cavities in) we shall find 127, and if we look up
carbonic acid (in quartz) we shall find 127, and if we look up bubbles
(in quartz) we shall find 127.

By this means the collector can find a slide if he know the
subject, and also the subject if he have the slide.

This is the only scientific method we know of dealing with a
microscopical collection ; it is one of the greatest practical mistakes to
make the cabinet its own index. It always ends in supreme confu-
sion. 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 + 1 inches ; but all are not, some geological
and mineralogical sections, sections of coal, Arc. are often much
larger. Many objects, again, are in deeper cells than the ordinary
cabinet drawer or slide-box will admit of ; all this may he 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 spec ial
series of mounts. 1

1 It will be understood that there are ninny forms of cabinet which space ]>iv\ents
our describing; they are made suitable for the pocket, for postal transmission, «fcc.f
and may be readily seen at the opticians'.

456 PREPABATION, MOUNTING, AND COLLECTION OF OBJECTS

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
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 ivhere
to look and ivhat 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, ifec. 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 pond-
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
attached to the submerged plants, the bottle is to be plunged into
the water with its mouth downwards, carried into the situation in
which it is desired that it should be filled, and then suddenly turned
with its mouth upwards. . By unscrewing the bottle from the collar,
and screwing on its cover, the contents may be securely preserved.
The net should be a bag of fine muslin, which may be simply sewn
to a ring of stout wire. But it is desirable for many purposes that
the muslin should be made removable ; and this may be provided
for by the substitution of a wooden hoop grooved on its outside, for
the wire ring • the muslin being strained upon it by a ring of

COLLECTING i

457

vulcanised indiarubber, which lies in the groove, and which may be
readily slipped off and on, so as to allow a fresh piece of muslin to In-
put in the place of th.it which has been last used. At the end of the
muslin bag is tied a small rimmed tube bottle of thin clear glfl
three inches long by one inch in diameter. In this, objects can be
fairly seen. The collector should also be furnished with a number
of bottles, into which he may transfer tin; samples thus obtained,
and none are so convenient as the screw-topped bottles made in all
sizes by (he York Glass Company. It is well that the bottles should
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 apparatus for collecting. 1 It is
made by cutting a U-shaped piece out of a Hat and solid piece of india-
rubber, about G inches long by 2j 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 Hat 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 tilled 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 Hat bottle
one can see at a glance almost everything the dip contains, and
every object can be examined with the pocket lens with ease.

For collecting purposes the objects sought in pond or stream are
divisible into free-swimming, and attached or fixed to water plant s &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 tilled 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, rot if era, ivc. are best found with the Hat 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 Vorticella) ; Epistylis, Zoothamium, and Carchesium, the
trumpet-shaped Stentors, the crown Rotifer Stephanoceros, the
tubes of Melicerta, Lyminas, the various Polyzoa, also Hydra,
and many more, can at once be seen with the naked eye, when

1 Q.yi. Journ. sit. ii. vol. ii. p. .">">.

458 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS

present, and in this way the good branches can be selected. Some
creatures, however, such as the beautiful floscules, cannot be seen
easily, even with the lens, not so much on account of their small
size, as of the perfect transparency of their bodies. Experience
will soon teach one how to see which branches are likely to prove
prolific. As a general rule, old-looking but still sound and green
branches will be the best. The Water Milfoil (Myriophyllum) is
decidedly the best of water plants to examine and collect, on account
of the ease with which its leaves can subsequently be placed under the
microscope. Anacharis is much more difficult of manipulation, and
I mostly only take it now to aerate my aquaria.

In placing a weed in the flat bottle, do not put in more than one
branch at a time, otherwise the branches will only obscure each 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 Avith a green soup
made by crushing some anacharis, or other green weed, in a small
mortar in a little water, which is then filtered through muslin.
They can be seen to feed on this under the microscope, their tiny
stomachs soon becoming filled with little balls of chlorophyll.

Under favourable conditions Melicerta, Stephanoceros, the Flos-
cules, and also Asplanchna, and other forms, breed and multiply in
the aquarium, and can then be preserved for a considerable time.

A little mud taken from a pond in winter or early spring, and
put in a tank at home, will often produce an unexpected number
and variety of rotifers and infusoria, which are hatched from
winter eggs and dormant germs.1

There must of course be a balance in every tank between the
animal and vegetable life, or aeration must be artificially maintained,
So also food must be obtainable by the organisms, however small.
But experience alone is the perfect teacher in this matter.

The same general method is to be followed in the collection of
such marine forms of vegetable and animal life as inhabit the
neighbourhood of the shore, and can be reached by the pond-stick.
But there are many which need to be brought up from the bottom
by means of the dredge, and many others which swim freely
through the waters of the ocean, and are only to be captured by the
toiv-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

1 On some Methods of Collecting and Keeping Pond Life for the Microscope,'
from the Trans. Middlesex Nat. Hist. Sue.

COLLKCTINC

459

use of the latter for the purposes of the microscopic require!
.special management. The net should be of fine muslin, firmly sews
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, or it may be fixed to a stick BO held in
the hand as to project from the sob' of the boat. In either ease the
uet should be taken in from time to time, and held up to allow tie-
water it contains to drain through it ; and should then be tinned
inside out and moved about in a bucket of water carried in tie-
boat, so that any minute organisms adhering to it may be washed
off before it is again immersed. It is by this simple method thai
marine on i male ides, the living forms of Jiadioloria, the smaller
Medosoids (with their allies Beroe and Cydippe), JSfoctiluca, the
free-swimming larva* of Kelt / io>di<rmota some of the most curious
of the Tunicata, the larva? of Moffnsco, TurbeUa/rtOj and Annelida^
some curious adult forms of these classes, EntomostracO) and the
larva? of higher Crostorco, are obtained by the naturalist ; and
the great increase in our knowledge of these forms which lias been
gained within recent years is mainly due to the assiduous use
which 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 rehinod'-r/n forum),
it is essential that the boat should be rowed so slowly that the net
may move gently through the water, so as to avoid crushing its soft
contents against its sides. Those of tinner structure (such as the
Ento most 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
steamer, in much more rapid motion.1 When this method is em-
ployed, it will be found advantageous to make the net of conical
form, and to attach to its deepest part a wide-mouthed bottle,
which may be prevented from sinking too deeply by suspending it
from a cork float : into this bottle many of the minute animals
caught by the net will be carried by the current produced by the
motion of the vessel through the water, and they will be thus
removed from liability to injury. It will also be useful to attach to
the ring an inner net, the cone of which, more obtuse than that of
the outer, is cut off at some little distance from the apex ; this
serves as a kind of valve, to prevent objects once caught from being
washed out again. The net is to be drawn in from time to time,
and the bottle to be thrust up through the hole in the inner cone ;
and its contents being transferred to a screw-capped bottle for
examination, the net may be again immersed. This form of net,
however, is less suitable for the most delicate objects than the simple
stick-net used in the manner just described. The microscopist on
a visit to the seaside, who prefers a quiet row in tranquil waters
to the trouble (and occasional mtdaise) of dredging, will lind 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, hut at various depths heneath it. heing attached to a line
which was made to hang vertically in the water hy 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.

460

CHAPTER VIII

MICBOSCOPIC FOBMS OF VEGETABLE LIFE — THALL OPHYTES

Those who desire to make themselves familiar with microscopic
appearances, and to acquire dexterity in microscopic manipulation,
cannot do better than educate themselves for more difficult inquiries
by the study of those humblest types of vegetation which present
organic structure under its most elementary aspect. And such as
desire to search out the nature and conditions of living action will
find in the study of its simplest manifestations the best clue to the
analysis of those intricate and diversified combinations under which
it presents itself in the highest animal organisms. For it has now
been put beyond question that the fundamental phenomena of life
are identical in plants and in animals, and that the living substance
which exhibits them is of a nature essentially the same throughout
both kingdoms. The determination of this general fact, which forms
the basis of the science of Biology, is the most important result of
modern microscopic inquiry ; and the illustration of it will be kept
constantly in view, in the exposition now to be given of the chief
applications of the microscope to the study of those minute 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 ex-
hibited by organised structure. But we now know that all the
essential functions of life maybe carried on by minute 'jelly-specks,'
in whose apparently homogeneous semi-fluid substance nothing like
£ organisation ' can be detected ; and, further, that even in the very
highest organisms, which present us with the greatest variety of
' differentiated ' structures, the essential part of the life-work is done
by the same material — these structures merely furnishing the
mechanism (so to speak) through which its wonderful properties
exert themselves. Hence this substance,1 known in vegetable

1 Attention was drawn in 1835 by Dujardin (the French zoologist to whom we owe
the transfer of the Foraminifera from the highest to the lowest 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 substance, to which
he gave the name sarcode (rudimentary flesh). In 1851 the eminent botanist Von
Mohl showed that a similar substance forms the essential constituent of the cells of
plants, and termed it protoplasm (primitive plastic or organisable material). And in
1863 it was pointed out by Prof. Max Schultze, who had made a special study of the
rhizopod group, that the ' sarcode ' of animals and the ' protoplasm ' of plants are
identical. See his memoir Ueber das Protoplasma der Bhizopoden und Pflanzen-
zellen.

DIFFERENCES BETWEEN I 'LA NTS AM) ANIMALS 46 1

physiology as pnttofJasm, but often referred to by zoolo^i t ; ,1
sarcode, has been appropriately designated by lYofc.-^or I lux lev 'tin-
physical basis of life' In its typical state (such as it presents
amon<( r/i izojmds) it is a semi-fluid, tenacious, glairy substance,
resembling alike in aspect and in composition the albumen (or
uncoagulatcd '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
spontaneous movement, which shows itself in an extraordinary variety
of actions, sometimes slow and progressive, sometimes rapid, some-
times wave-like and continuous, and sometimes rhvthmical with
regular intervals of rest. W hen examined under a sufficiently high
magnifying power, multitudes of minute granules are usually seen to
be diffused through it, which have been termed 1 microsomes.'
Protoplasm, whether living or dead, has a great power of absorbing
water ; but the distinction between these two states is singularly
marked by its behaviour in regard to any colouring matter which the
water may contain. Thus, if living protoplasm be treated with a
solution of carmine, it will remain unstained so long as it retains
its vitality. But if the protoplasm be dead, the carmine will at once
ervade its whole substance, and stain it throughout with a colour
even more intense than that of the solution ; thus furnishing (as
w as 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 essentially consist 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 is
impossible, in the present state of our knowledge, to lay down any
definite line of demarcation between the two kingdoms ; since there
is no single character by which the animal or vegetable nature of
any organism can be tested. Probably the one which is most
generally applicable among those that most closely 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 organic comjiou nils already formed
which it takes (in some way or other) into the interior of its body ,
or, on the other, its possession of the power of producing the organic
compounds which it applies to the increase of its fabric, at the
expense of the inorganic clr,,i< at* with which it is supplied by air
and water. The former, though perhaps not an al/solutel is a </• n> ral
characteristic of the anima/ kingdom ; the latter, but for the exist-

462

MICROSCOPIC FOEMS OF VEGETABLE LIFE

ence of which animal life would be impossible, is certainly the
prominent attribute of the vegetable. We shall 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 their nourishment from the atmosphere or the water in
which they live, and, like them, are distinguished by their power of
decomposing carbonic acid (C02) under the influence of light —
setting free its oxygen, and combining its carbon with the elements
of water to form the carbohydrates (starch, cellulose, &c), 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 descrip-
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
character in the plant as in the animal, being little hair-like fila-
ments, termed cilia (from the Latin word cilium, an eye-lash), or
longer whip-like Jlagella, by whose rhythmical vibrations the body of
which they form part is propelled in definite directions. The
peculiar contractility of these organs seems to be an intensification
of that of the general protoplasmic substance, of which they are
special extensions.

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 C02 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 cryptogams, with the entire group of Fungi ;
which, however, in a large number of cases, depend rather for their
nutritive materials upon organic matter in a state of decomposition,
many of them having the power of promoting that process by their
zymotic (fermentative) action. Among animals, again, there are
several in whose tissues are found organic compounds, such as chloro-
phyll, starch, and cellulose, which are characteristically vegetable ;
but it has not yet been proved that they generate these compounds
for themselves by the decomposition of C02.

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 the
lowest and simplest forms of vegetation, consists of nothing else
than an aggregation of the bodies termed cells, every one of which
(save in the forms that lie near the border-ground between animal

the ye<;et.\i;u: cell

4C3

and vegetable lift;) has its little particle of protoplasm enclosed by a
suing of the substance termed cellulose — a non-nitrogenou 1 rah tance
identical in chemical composition with starch. The entire mass of
cells of which any vegetable organism is composed has been gene
rated from one ancestral cell by processes of multiplication to be
presently described ; and the difference between the fabrics of the
lowest and of the highest plants essentially consists in this, that whilst
the cells produced by the repeated multiplication of the ancestral
cell of the protophyte are all mere repetitions of it and of one another,
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 de-
veloped, which is made up of a number of distinct 'organs ' (stem,
leaves, roots, flowers, <fcc), 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 first definitely stated by
Schleiden, it is in the life-history of. the 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-
ar//f. This cell-wall is composed, as long as the cell is in a living state,
of the substance known as cellulose, one of the group of compounds
called 'carbohydrates,' and bearing the definite chemical composi-
tion C()H1()0-,. From a physical point of view it consists of particles
or micella of cellulose surrounded by water. With regard to the mode
of growth of the cell-wall, two hypotheses have been proposed : one,
that it is formed by apposition, that is, by the constant addition of
fresh layers to the inner surface of the cell- wall ; the other that it
increases by intussusception, or the intercalation of fresh particles of
cellulose between those already in existence. Our present state of
knowledge does not enable us to decide definitely between these two
theories.

The contents of the plant-cell, which may be collectively termed
the endopla&m (answering to the ' enclosure ' of rhizopods), or, when
strongly coloured throughout (as in many algce), t lie endochrome,
consist in the first place of an outer layer of protoplasmic substance
called the ectoplasm, primordial utricle, or parietal at rich . 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. 366), or by the influence of reagents which cause it to con
tract by drawing forth part of its contents (fig. 364, C). It is not
sharply defined on its internal face, but passes gradat tonally into the
inner mass of protoplasm, from which it is chiefly distinguishable by
the absence of granules ; and it is shown by the effects <>r" reagents
to have the albuminous composition of protoplasm. It may thus In-
regarded as the slightly condensed external film of the protoplasmic
layer with which its inner surface is in contact ; and it essentially

464 MICROSCOPIC FORMS OF VEGETABLE LIFE

corresponds with the 'ectosarc' of Amoeba or any other rhizopod.
The ' ectoplasm ' and ' cellulose wall ' can be readily distinguished
from each other by chemical tests ; and also by the action of car-
mine, 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, &c. ; the peculiar body termed the
' nucleus ' ; and chlorophyll corpuscles (enclosing starch granules),
oil particles, &c. In the young state of the cell the whole cavity
is occupied by the 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.
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 micro-
scopist of all its manifestations of vital activity. The nucleus is a
small body, usually of lenticular or subglobose form (fig. 364, A, a),
and of albuminous composition, that lies imbedded in protoplasmic
substance, either on the cell- wall or nearer the centre of the cavity.
It is not, however, constantly present even in the higher forms of
cell-structure ; for in those cells whose active life has been completed,
the nucleus is usually absent, having probably been resolved again
into the protoplasm from which it was originally formed. And in
the cells of some of the lower cryptogams it has not at present been
distinguished with certainty at any stage of their existence. Cells
containing a number of nuclei, or ' multi-nucleated cells,' are not un-
common. They occur, for example, in many algse, in the ' suspensor '
and ' embryo-sac ' of the ovule of phanerogams, and in the ' latici-
ferous ' tubes. Within the nucleus are often seen one or more small
distinct particles termed nucleoli (fig. 3fi4, A, b), which can be best dis-
tinguished by the strong coloration they receive from a twenty-four
hours' immersion in carmine, and subsequent washing in water
slightly acidulated with acetic acid. Though in some points the pre-
cise function of the nucleus is still unknown, there can be no doubt
of its peculiar relation to the vital activity of the cell ; for, in the
nucleated cells which exhibit ' cyclosis,3 it may be observed that
if the nucleus remains attached to the cell -wall, it constitutes a
centre from which the protoplasmic streams diverge, and to which
they return ; whilst if 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

CONTENTS OF THE CELL

465

presently described, t li.it tin* speciality of tin* nucleus as tin rrntri
the vital activity of the nil is most strongU 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 (!<),, 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 colour.-.,
which seem to be produced from it by chemical action. Starch (/rains
are always formed in the first instance in the interior of the chloro-
phyll corpuscles, and gradually increase in size until they take the
places of the corpuscles that produced them. So long as they con-
tinue 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 arc
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 limitary membrane, though the superficial
layer seems to have a tinner 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 gives
it the character of an 1 ectoplasm.' Such individualised masses of
protoplasm, destitute of a tme cell-wall, have sometimes been
termed primordial cells. The nucleus, as already mentioned, is
apparently sometimes absent from the cells of the lower proto-
phytes. It is an extremely curious feature in the cell life of certain
protophytes that they not only move like animalcules by cilia 01
ffagella, 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 t'ormafimi within the
parent -cell. The first stage of the former process consists in the
elongation and transverse constriction of the nucleus ; and this con-
striction becomes deeper and deeper, until the nucleus divides itself
into two halves (fig. 364, B, a, a'). These then separating from

11 11

466

MICROSCOPIC FORMS OF VEGETABLE LIFE

each other, the endoplasm of the parent-cell collects round the two
new centres, so as to divide itself into two distinct masses (C, o, a') ;

and by the investment of
these two secondary ' endo-
plasms ' with cellulose-
walls a complete pair of
new cells (I), a, a') is formed
within the cavity of the
parent- cell. The process
of free-cell formation which
is very common among
protophytes (being that
by which ' zoospores,' or
1 swarm -spores/ are com-
monly produced, is seen
among phanerogams in the
production of a number
of cells at once within the
cavity of the £ embryo-sac,'
which may itself be con-
sidered as a distended
parent-cell. The endo-
plasm, in the former of
these cases, instead of
itself into two
up

Fig. 364. — Binary subdivision of cells 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 halves, usually breaks

into numerous

dividing

of nucleus into two halves, a and a' ; C, cell in
same stage, showing contraction of endoplasm
(produced by addition of water), into two sepa-
rate masses round the two segments of original
nucleus ; D, two complete cells within mother-
cell, divided by a partition.

segments
corresponding with one
another in size and form
(fig. 375), each of which, es-
caping from the parent-
cavity, becomes an inde-
pendent 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 en-
doplasm groups itself,
more or less completely,
round several centres,

each of which has its
Fig. 365-Successive stages of free-cell formation Qwn micleus formed
in embryo-sac of seed oi scarlet-runner : a, a, a, . . . '
completed cells, each having its proper cell-wall, Oy Subdivision OI tlie
nucleus, and endoplasm, lying in a protoplasmic nucleus of the parent-
mass, through which are dispersed nuclei and cells ceU .md thege secQn_
m various stages oi development. , „ . ,

dary cells, m various
stages of development, lie free within the cavity of the parent-cell,
imbedded in its residual endoplasm, each proceeding to complete

FORMATION OF N E W CELLS

Ac>7

itself as a cell by the formation of a limiting wall of cellulose
(fi#. 3(55). Now, in this second case, as the new brood of (•••Us
•continues to form part of the fabric in which it originated, its
production is clearly an act of growth ; and although, in the first
case, the setting free of the ' swarm -spores ' from the parent-cell calls
into existence a fresh brood of secondary organisms, this is no more
to be regarded in strictness as a ' new generation ' than is the put-
ting forth of a new set of leaf-buds by a tree — every one of thou,
when separated from its stock, developing itself under favourable
conditions into the likeness of that which produced it. As a 'new
generation ' in any phanerogamic plant has its origin in the fertilisa-
tion of a highly specialised 'germ-cell' (contained within the ovule)
by the contents of a 'sperm-cell' (the pollen-grain), so do we find,
among all save the lowest cryptoga/mSy a provision for the union of
the contents of two highly specialised cells, the 'germ-cells' being
fertilised by the access of motile protoplasmic bodies (antherozoids),
set free from the cavities of the 'sperm-cells' within which they
were developed. l>ut although the sexual process can be traced
downwards under this form into 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.'

Great attention has recently been paid by Strasburger (see his
work, ' Ueber Zellbildung und Zelltheilung ') and others to the con-
stitution of the endoplasm and to the processes connected with cell-
division. On both these subjects it is impossible hereto give more
than the barest outlines. Strasburger distinguishes between the
following differentiated parts of the protoplasm of the living cell : —
The protoplasm outside the nucleus he terms the 'cytoplasm' j the
portion which constitutes the nucleus is the ' nucleoplasm ' ; that
which enters into the composition of the chlorophyll corpuscles and
other allied substances is the ' chromatoplasm.' Each of these
three portions of protoplasm is composed of a hyaline matrix or

1 The term ' spore 'has been long used by cryptngamists to designate the minute re-
productive particles (such as those set free from the ' fructification ' of f« •riis. mosses
<fcc.) which were supposed — in the absence of all knowledge of t heir sexual relations —
to be the equivalents of the seeds of flowering plants. But it is now know n 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 fert ilised 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 MwBOdsphere, is the real representa-
tive °f tne '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 crypto-
gams, which have received a great variety of designations, are all to be regarded (as
will be presently explained) as equivalents of the leaf-buds of (lowering plants.

[The different interpretations placed upon the term ' spore ' and its deri\ at ives by dif-
ferent writers on cryptogamic botany present a great difficulty to the st udent. A differ-
ent terminology to 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 ot lierw ise be
necessary, it has been thought best, in the present edition, to retain l>r. (.'arpenter's
terminology, at all events until a greater agreement has been arrived at than is at present
the case. — Ed.]

h h '2

468

MICROSCOPIC FORMS OF VEGETABLE LIFE

' hyaloplasm ' and of imbedded granular structures or ' microsomes/
A distinct substance, known as 'nuclein,' absent from the cyto-
plasm, appears to enter into the composition of the nucleus. The
division of the nucleus may take place either directly, when the
process is known as ' fragmentation ; ; or indirectly. In the process
of indirect division, the protoplasm of which the nucleus is com-
posed undergoes a great variety of changes, in the course of which
it assumes the beautiful appearance known as the 'nuclear spindle,'

Fig. 866. —Division of the pollen-mother-cells of Fritillaria persica. (From Stras-
burger and Hillhouse's ' Practical Botany,' published by Sonnenschein.)

consisting of an equatorial disc, the ' nuclear plate,' and delicate
' spindle-fibres ' which converge towards the two poles of the spindle.
To follow out all these processes requires very high magnifying
powers of the microscope, great skill in manipulation, and the use of
very delicate staining reagents.

The older conception of the vegetable cell regarded it as a
completely closed vesicle, the endoplasm of which is entirely shut
off from contact with that of the adjacent cells. Recent observa-

I'MCKIiU'LAK ll.WTS

\(><J

tions require the modification of thiiooaception. Tt has be.-,, shown
that in many cases the cell-wall is perforated by very minute orifioea
through which excessively fine strings of protoplasm pas. from one
cell-cavity to another (tig. 367). Tliis mui 'inn it;/ of proto/tlns,,, has been
observed in some seaweeds and other alga?, in the endosperm of the
ovule, and in the pulvinus or motile organ of the leaves of the
Sensitive Plant, and is probably a widely extended phenomenon. In
the latter case it is undoubtedly connected with the remarkable
phenomenon of sensitiveness or irritability displayed by the leaves.

In the lowest forms of vegetation every single cell is not only
capable of living in a state; of isolation from the rest, but even
normally does so; and thus the plant may be said to be wnied-
lukur, every cell having an independent 'individuality.' There are
others, again, in which amorphous masses are made; up by the aggre-
gation of cells, which, though quite capable of living independently,
remain attached to each other by the mutual fusion (so to speak) of
their gelatinous invest-
ments ; and there are
others, moreover, in
which a definite adhe-
sion exists between the
cells, and in which
regular plant-like struc-
tures are thus formed,
notwithstanding that

a

I ■

Fig. 3<>7. — Continuity of protoplasm. (From Vines'*
' Physiology of Plants.' Cambridge University Press.)

every cell is but a repe-
tition of every other,
and is capable of living
independently if de-
tached, so as still to
answer to the designa-
tion of a ' unicellular '
or single-celled plant.
These different condi-
tions 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 independently ; but if, instead of under-
going this complete fission, they should be held together by the
intervening gelatinous envelope, a shapeless mass results from re-
peated subdivisions not taking place on any determinate plan :
and if, moreover, the binary subdivision should always take place
in one direction only, a long, narrow filament (fig. 374, D), or
if in two directions only, a broad, flat, leaf-like expansion ((J),
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 no disposition to separate from each other spontaneously.
-Still they correspond with those which are strictly unicellular, as to

470 MICROSCOPIC FORMS OF VEGETABLE LIFE

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 : alga?, 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 gradationally increasing com-
plexities of structure ; and these gradations show themselves espe-
cially in the provisions made for the generative process. Thus, in
some forms, a 'zygospore ' is produced by the fusion of the contents
of two cellss which neither present any sexual difference, the one
from the other, nor can be distinguished in any way from the rest.
In the next highest forms, while the ' conjugating ' cells are still
apparently undifferentiated from the rest of the structure, a sexual
difference shows itself between them ; the contents of one cell (male)
passing over into the cavity of the other (female), within which the
'zygospore 'is formed. The next stage in the ascent is the resolution of
the contents of the male cell into motile bodies ('antherozoids'), which,
escaping from it, move freely through the water, and find their way
to the female cell, whose contents, fertilised by coalescence with the
material they bring, form an ' oospore.' In the lower forms of this
stage, again, the generative cells are not distinguishable from the
rest until the contents begin to show their characteristically sexual
aspect ; but in the higher they are developed in special organs,,
constituting a true ' fructification.' This must, however, be dis-
tinguished from organs which, though commonly spoken of as the
' fructification,' have no real analogy with the generative apparatus
of flowering plants, their function being merely to give origin to
gonidial 1 cells or groups of cells, which simply mzdtiply the parent
stock, in the same manner that many flowering plants (such as the
potato) can be propagated by the artificial separation of their leaf-
buds. It frequently happens among cryptogams that this gonidial
fructification is by far the more conspicuous, the sexual fructifica-
tion being often so obscure that it cannot be detected at all without
great difficulty ; and we shall presently see that there are some
thallophytes in which the production of gonids seems to go on
indefinitely, no form of sexual generation having been detected

1 The term gonitis, originally applied to certain green cells in the lichen-crusts
that are capable, when detached, of reproducing the vegetative portion of the plant,
is used by some writers as a designation of the non-sexual spores of cryptogams
generally, which it is very important to discriminate from the generative ' obspheres.'
If possessed of motile powers, they are spoken of as ' zoospores,' or sometimes (on
account of the appearance they present when a number are set free at once) as
' swarm- spores.' In contradistinction to ' motile ' gonids or 'zoospores,' those which,
show no movement are often termed resting spores, or hypnospores ; but such may be
either sexual oiispheres or non- sexual gonids, the latter, like the former, often 'en-
cysting ' themselves in a firm envelope, and then remaining dormant for long periods,
of time.

DEVELOPMENT OF PALMOGiAE,\

471

in them. These general statements will now In- illustrated by
sketches of the life-history of some of those humble 1 hallophytt-s
which present the phenomena of cell division, conjugation, and
gonidial 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 PahnogUza macrococca (Ktz. ),' one of
those humble kinds of vegetation which spreads itself as a green
slime over damp stones, walls, etc. When this slime is examined
with the microscope, it is found to consist of a multitude of green
cells (fig. 36S, A), each surrounded by a gelatinous envelope ; the cell,
which does not seem to have any distinct membranous wall, is filled
with a granular ' endochrome,' consisting of green particles diffused
through odourless protoplasm ; and in the midst of this a nucleus
may sometimes be distinguished, but can always be brought into
view by tincture of iodine, which turns the 'endochrome' to a

Fig. 868. — Development of Pahnoglaa macrococca.

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 t be cell, as well as of
the nucleus (1) from each other, though they still remain in mutual
contact; in a yet later stage they are found detached from each
other (D), though still included within the same gelatinous envelope.
Each new cell then begins to secrete its own gelatinous envelope, so
that by its intervention the two are usually soon separated from
each other (E). Sometimes, however, this is not the case, the
process of subdivision being quickly repeated before there is time for

1 [Most of the species of K iit/.ing's genus Pal moijlu-a me now regurded us belonging
to the Dcsmidiacca-, and ure included under the genus Mrsotirnium. — Ed.]

472 MICROSCOPIC FORMS OF VEGETABLE LIFE

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 groivth,
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

Fig. 369.— Development of Protoeoccus jpluvialis.

like a particle of dust, yet resumes its vegetative activity whenever
placed in the conditions favourable to it. The conjugating process
commences by 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 new generation, speedily multiplying,
like the former ones, by binary subdivision. It is curious to observe
that during this conjugating process a production of oil particles
takes place in the cells ; these are at first small and distant, but

DEVELOPMENT OF [*ROTOCOC(TS

473

gradually become larger, and approximate more closely to each oth< r,
and at last coalesce so as to form oil-drops of various sizes, the green
granular matter disappearing; and the colour of tin- conj united
body changes, with the advance of this process, from green to a light
yellowish brown. When the zygospore begins to vegetate, OD the
other hand, a converse change occurs ; the oil-globules disappear,
and green granular matter takes their place. This is precisely wh.it
happens in the formation of the seed among some of the higher plant s :
for starchy substances an; transformed into oil, which is stored up in
the seed for the nutrition of the embryo, and is applied during germi-
nation to the purposes which are at other times answered by them.

If this (as seems probable) constitutes the entire life-cycle of
Palmoglcea, it affords no example of that curious ' motile ' stage
which is exhibited by most algal protophytes in some stage of their
existence, and which constitutes a large part of the life-history of the
minute unicellular organism now to be described, Protococcus plu-
vialis,1 fig. 369), 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 are some-
times red ; and their red form has received the distinguishing appel-
lation of ffasmatococcus (fig. 373). Very commonly the red colouring
matter forms only a central mass of greater or less size, having the
appearance of a nucleus (as shown at E, fig. 369) ; and sometimes it is
reduced to a single granular point, which has been described by
Professor Ehrenberg as the eye-sjtof 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

1 The Author hud under his own observation an extraordinary abundance of
what he now feels satisfied must have been this protophyte in an open rain-water
cistern which had been newly cleaned out. His notice was attracted to it by seeing
the surface of the water covered with a green froth whenever the sun shone upon
it. On examining a portion of this froth under the microscope he found that the
water was crowded with green cells in active motion; and although the only bodies
at all resembling them of which he could rind any description were the so-called
animalcules constituting the genus Chlami/do/nonas of Prof. Ehrenberg, and very
little was known at that time of the 4 motile ' conditions of plants of this descrip-
tion, yet of the vegetable nature of these bodies he could not entertain the smallest
doubt. They appeared in freshly collected rain-water, and could not. therefore, be
deriving their support from organic matter; under the influence of light they wen
obviously decomposing carbonic acid and liberating oxygen ; and this influence he
found to be essential to the continuance of their growth and development, which took
place entirely upon the vegetative plan. Not many days after the protophyte first
appeared in the water, a few wheel-animalcules presented themselves; these fed
greedily upon it, and increased so rapidly (the weather being very warm) that they
speedily bec ame almost as crowded as the cells of the I'rotocamts had been ; and it
was probably due in part to their voracity that the plant soon became less abundant,
and before long disappeared altogether. Had the Author been then aware of its
assumption of the 'still' condition, he might have found it at the bottom of the cis-
tern after it had ceased to present itself at the surface. The account of this plant
given above is derived from that of Dr. Colin, in the Xov<t Acta Acad. Xat. Curios.
(Bonn, 1850), torn, xxii., of which an abstract by Mr. (leorge Husk is contained in
she Botanical and Plnjsioloyical Memoirs, published by the Kay Society, for lR.r>8.

474 MICROSCOPIC POEMS OF VEGETABLE LIFE

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 the
surface of the colourless protoplasm is condensed into an ectoplasm,
which is surrounded by a tolerably firm cell-wall, consisting of cellu-
lose or of some modification of it. Outside this (as shown at A), when
the ' still ' cell is formed by a change in the condition of a cell that
has been previously 'motile,' we find another envelope, which seems
to be of the same nature, but which is separated by the interposition
of aqueous fluid ; this, however, may be altogether wanting. The
multiplication of the ' still ' cells by subdivision takes place as in
Palmogloea, the endoplasm first undergoing 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, sixteen new cells are successively
produced ; and these are sometimes set free by the complete dissolution
of the envelope of the original cell ; but they are more commonly
held together by its transformation into a gelatinous investment in
which they remain imbedded. Sometimes the endoplasm subdivides,
at once into four segments (as at D), of which every one forthwith*
acquires the character of an independent cell ; but this, although an
ordinary method of multiplication among the ' motile ' 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 zoospores
retain their motile powers, and develop themselves into the ordinary
'motile' cells ; others produce a firm cellulose envelope and become
c still ' cells ; and others (perhaps the majority) perish without any
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
flagella, 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 only by clear aqueous fluid, which are sometimes so
numerous as to take in a large part of the cavity of the cell, so thatr
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:

DEVELOPMENT OF PROTOCOCC1 B

•17 5

re-agents. The flagella 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 aet.ive lite .,i
the 'motile' cell the vibration of these flagella is so rapid that they
can be recognised only by the currents they produce in the water
through which the cells are quickly propelled ; but when the motion
becomes slacker, the flagella themselves arc readily distinguishable,
and they maybe 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 (I>), whereby a pair
of motile cells is produced (C), each resembling its single predecessor
in possessing the cellulose investment, the transparent beak, and the
vibratile flagella, before the dissolution of the original investment.
►Sometimes, again, the contents of the original cell undergo a seg-
mentation in the first instance into four divisions (D) ; which may
either become isolated by the dissolution of their envelope, and may
separate from each other in the condition of 1 free primordial cells '
(H), developing their cellulose investments at a future time, or
may acquire their cellulose investments (as in the preceding case)
before the solution of that of the original cell ; while sometimes, even
after the disappearance of this, and the formation of their own
independent investments, they remain attached to each other at their
beaked extremities, the primordial cells being connected with each
other by peduncular prolongations, and the whole compound body
having the form of a +. This quaternary segmentation appears to
be a more frequent mode of multiplication among the ' motile ' cells
than the subdivision into two, although, as we have seen, it is less
common in the 1 still ' condition. So also a primary segmentation of
the entire endochrome of the ' motile ' cells into eight, sixteen, or even
thirty-two parts, may take place (E, F), thus giving rise to as many
minute gonidial cells. These, when set free, and 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 flagella, thus passing into the 'still '
condition (A) ; and this last transformation may even take place
before they are set free from the envelope w ithin which they were
produced, so that they constitute a mulberry-like mass, which tills
the whole cavity of the original cell, and is kept in motion by
its flagella.

To what extent Protococcus is an autonomous organism is still
doubtful, but it appears to be more or less closely connected with
many forms of life which have been described, not merely as dis-
tinct species, but as distinct yen era of animalcules or <>f prot«»phytes
such as Chlmnydoiiumas, Enyloia, 7 'ra<-/ir/ni,mni(s, dyy?*, Go)iium,
Paudoriita, /!<>/ n/orysf is, l'c<'//<(, Syticryjifa, Montis, Astasia, A'**//", and
many others. Certain forms, such as the ' motile ' cells 1, K, L.
appear in a given infusion, at tirst exclusively and then principally .
they gradually diminish, become more and more rare, and Anally

476

MICKOSCOPIC FORMS OF VEGETABLE LIFE

disappear altogether, being replaced by the 'still' form. After
some time the number of the ' motile ' cells again increases, and
reaches, as before, an extraordinary amount ; and this alternation
may be repeated several times in the course of a few weeks. The
process of segmentation is often accomplished with great rapidity.
If a number of ' motile ' cells be transferred from a larger glass into a
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 does in the £ motile.5
What are the precise conditions luhich determine the transition
between the ' still ' and the ' motile ' states cannot yet be precisely
defined, but the influences of certain agencies can be predicted with
tolerable certainty. Thus it is only necessary to pour the water
containing these organisms from a smaller and deeper into a larger
and shallower vessel, in order at once to determine segmentation in
numerous cells ; a phenomenon which is observable also in many other
protophytes. The ' motile ' cells seem to be favourably 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, &c, they may present themselves where 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 of their
vital activity. They then very commonly become red throughout,
the red colouring substance extending itself from the centre towards
the circumference, and assuming an appearance like that of oil-

PROCESS OF CONJ1 GATION

477

drops ; and tlicse red cells, acquiring thick cell walls and ■ muootu
envelope, float in floeculent aggregations on the surface of tin* water.
This state seems to correspond with the ' r e s t i n g - s p o re s ' of other
protophytes ; and it may continue until warmth, air, and moisture
cause the development of the red cells into the ordinary 'still ' cells,
green matter being gradually produced, until the red substance forms
oidy the central part of the endoehrome. After this the cycle of
changes occurs which has been already described ; and the plant
may pass through a long series of these before it returns to the state
of the red thick-walled cell, in which it may again remain dormant
for an unlimited period. Kven this cycle, however, cannot be
regarded as completing the history of Profococcus, 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 Palmoglcea ; 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

Included under the family of ChrodcoccaceCB are a number of
minute microscopic organisms, exceedingly abundant in fresh water,
differing from the Protococcaceaj in not containing true chlorophyll
grains, the cell-cap 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, but
do not, in any case, produce zoospores. To this family belong the
genera C Irroococcus, Qlceocapsa, AjthaHocajM^Jferisino/tt <H(t, and many
others, the life-history of which is but very imperfectly known.

Resembling the Protococcacea; in the independence of their
individual cells are the two groups Dcsmhliacecv and Diatomacece,
which, in a systematic view, rank as subordinate divisions of the
family Conjugatae, their generative process being performed in the
same simple manner as that of Palmoglcea, But these two tribes
being of such special interest to the microscopist as to require sepa-
rate treatment, only that higher group, the Zygnemace(Ei will be
here noticed, in which the cells produced by binary subdivision
remain attached to each other, end to end, so as to form long
unbranched filaments (tig. 370), 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

1 In the above sketch the Author has presented the facts described by Or. Cohn
under the relation which they seemed to him naturally to bear, hut which differs from
that in which they will be found in the original memoir; and he is <;lad to be able
to state, from personal communication with its able author, that Dr. Colin's later
observations led him to adopt a view of the relationship of the 'still' and 'motile'
forms which is in essential accordance with hi- own.

478

MICROSCOPIC FORMS OF VEGETABLE LIFE

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. 370,
A) ; but as they advance towards the stage of conjugation, it
ordinarily arranges itself into regular spirals (B), a couple of star-
like discs in each cell, or a single plate running through it in an
axile direction. 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 cells pour their
endochromes into a dilatation of the passage that has been esta-

Fig. 370. — Various stages of the history of a Spirogyra : A, three cells, a, b, c, of a
young filament, of which b is undergoing division ; B, two filaments in the first
stage of conjugation, showing the spiral disposition of their endochromes and the
protuberances from the conjugating cells ; C, completion of the act of conjugation,
the endochromes of the cells of the filament a having entirely passed over to those
of filament b, in which the zygospores are formed.

Wished between them ; and it is there that they commingle so as to
form the ' zygospore.' But in the various species of Spirogyra (fig.
370, B), which are among the commonest and best known of Conju-
gate, the endochrome of one cell passes over entirely into the cavity
of the other ; and it is within the latter that the ' zygospore ' is
formed (C), the two endochromes coalescing into a simple mass,
around which a firm envelope gradually makes its appearance.
Further, it may be generally observed that all the cells of one
filament thus empty themselves, whilst all the cells of the other
filament become the recipients. Here, therefore, we seem to have a
foreshadowing of the sexual distinction of the generative cells into
' sperm -cells ' and 'germ-cells,' which we shall presently see in the
filamentous Confervacew. Conjugation between two adjacent cells of
the same individual also occurs in some species. Multiplication by
'zoospores 5 does not take place among the Conjugate.

VoLVnX (JLOUATHK

479

From the composite ' motile 'forms of Protoromis tin- transition
is easy to the group of Volvocineae, an assemblage of minute plants
of the greatest interest to the microseopist, on account I *« * t } i of the
animalcule-like activity of their movements and of the great beauty
and regularity of their forms. The most remarkable example of this
group is the well-known Vdvox gldbotOT (Plate VJj, 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 t he light, swimming through
the water which it inhabits. Its onward motion is usually of a roll-
ing kind ; but it sometimes glides smoothly along, without turning
on 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 liagella, so that the entire surface is beset with
these lashing filaments, to whose combined action its movements
are due. Within the external Bphere 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 fiagella. 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 singli
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, bv Pro-
fessor Ehrenberg, who, however, considered the.se 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 ;l and the
peculiarity of their history renders it desirable to describe it in some
< letail.

Each of the so-called 'monads ' (tig. 37 1 , Nos. 9, 1 1 ) is a somewhat
flask-shaped plant-cell, about -j^V^th of an inch in diameter, consist-
ing, as in the previous instances, of green chlorophyll granules diffused
through a colourless protoplasm, constituting an ' endochrome 1 (which
commonly includes also a red 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

1 Professor Stein, however, in his great work on tin- Infusoria i Or</<i nisni us Jrr
Infusiousthirrt; Abtheilung 111, Leipzig, lHTsi. still ranks the !'<»/ n>ci new among
the flagellate animalcules, to which they undoubt illy show a remarkable parallelism
in structure, the chief evidence of their vegetal le nature lying in their j-hysiuluyical

conformity to undoubted bhallophytea

48o

MICROSCOPIC FORMS OF VEGETABLE LIFE

beak or proboscis, from which proceed two flagella (fig. 371, No. 11) ;
and it is invested by a pellucid or hyaline envelope (No. 9, d) of con-
siderable 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

i 2 . -3

Fig. 371. — Structure of Volvox globator.

and that of the motile ' encysted ' cell of Protococcus phcvialis (fig.
369, K). There is not, in fact, any perceptible difference between
them, save that which arises from the regular aggregation, in Volvox,
of the cells which normally detach themselves from one another in
Protococcus. The presence of cellulose in the hyaline substance is
not indicated, in the ordinary condition of Volvox globator, by the

VOLVOX (,L()V,\T()ll

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 aurrnx. The flagella
and endoplasm, as in the motile forms of fntfttcorrtts, are tinged
a deep brown by iodine, with the exception of one or t wo 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 act ion of sulphuric,
acid and iodine. All these; reactions are characteristically wytable
in their nature. When the cell is approaching maturity, its endo-
plasm always exhibits one or more 1 vacuoles ' (tig. 37 1 ? No. 9, >/, a\ of a
spherical form, and usually about one-third of its own diameter;
and these 1 vacuoles' (which are the so-called 'stomachs' of Professor
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 tin;
dilatation is slow and gradual. This curious action ceases, however,
as the cell arrives at its full maturity 1 j 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 (b), which obviously consists, as in /Vo/o-
coccii8, of a peculiar modification of chlorophyll.

Each cell normally communicates with the cells in nearest
proximity with it by extensions of its own endochrome, which are
sometimes single and sometimes double (tig. 371, No. 5, 6, b) ; and these
connecting processes necessarily cross the lines of division between
their respective hyaline investments. The thickness of these pro-
cesses varies very considerably ; for sometimes they are broad bands,
and in other cases mere threads ; whilst they are occasionally want-
ing altogether. This difference seems partly to depend upon the
age of the individual, and partly upon the abundance of nutriment
which it obtains ; for, as we shall presently see, the connection is
most intimate at an early period, before the hyaline investments of
the cells have increased so much as to separate the masses of endo-
chrome to a distance from one another (tig. 371, Nos. '2, 3, 4) ; whilst
in a mature individual, in which the separation has taken place to
its full extent and the nutritive processes have become less active, the
masses of endochrome very commonly assume an angular form, and
the connecting processes are drawn out into threads (as seen in No.
5), or they retain their globular form, and the connecting processes
altogether disappear. The influence of reagents, or the infiltration
of water into the interior of the hyaline investment, will sometimes
cause the connecting processes (as in Protococcus) to be drawn back
into the central mass of endochrome ; and they will also retreat on
the mere rupture of the hyaline investment. From these circum-
stances it may be inferred that they are not enclosed in any definite

1 The existence of rhythmically contracting vacuoles in I'ulcox (though confirmed
by the observations of Prof. Stein) is denied by .Mr. Suville Kent [Manual of the
Infusoria, p. 47); but it may be fairly presumed that he has not looked for them at
the stage of development at which their action was witnessed by Mr. Busk.

1 I

t

482

MICROSCOPIC POEMS OP VEGETABLE LIPE

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
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. 371, No. 1) is com-
posed of an aggregation of somewhat angular masses of endochrome
■(b), separated by the interposition of hyaline substance and the
whole seems to be enclosed in a distinctly membranous envelope,
which is probably the distended hyaline investment of the original
cell, within which, as will presently appear, the entire aggrega-
tion originated. In the midst of the polygonal masses of endo-
chrome, one mass (a), rather larger than the rest, is seen to
present a circular form ; and this, as will presently appear, is the
originating cell of what is hereafter to become a new sphere.
The growing Volvox at first increases in size not only by the inter-
position of new hyaline substance between its component masses
of endochrome, but also by an increase in these masses themselves
(No. 2, a), which come into continuous connection with each other by
the coalescence of processes (b) which they severally put forth ; at
the same time an increase is observed in the size of the globular
cell (c), which is preliminary to its binary subdivision. A more
advanced stage of the same developmental process is seen in No. 3, in
which the connecting processes (a, a) are so much increased in size
as to establish a most intimate union between the masses of endo-
chrome, although the increase of the intervening hyaline substance
carries these masses apart from one another ; whilst the endochrome
of the central globular cell has undergone segmentation into two
halves. In the stage represented in No. 4 the masses of endochrome
have been still more widely separated by the interposition of hyaline
substance ; each has become furnished with its 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
clear view. The prolonged action of glycerin, moreover, will often
show that the boundary lines are double, being formed by the
coalescence of two contiguous cell-walls ; and they sometimes retreat

PLATE VI

VOLVOX (iLMUTnK

4*3

from each other so far tli.it the hexagonal areola' become roimdrd.
As the primary sphere approaches maturity, the large secondary
•germ-mass, or zoiis/joranye, whose origin has been traced from
the beginning, also advances in development, its contents under-
going multiplication by successive segmentations, so that we find it
to consist of eight, sixteen, thirty-two, sixty-four, and still more
numerous divisions, as shown in fig. 371, Nos. 6, 7, 8. Up to this
stage, at which the sphere first appears to become hollow, it is re-
tained within the hyaline envelope of the cell within which it has
been produced ; a similar envelope can be easily distinguished, as
shown in No. 10, just when the segmentation has been completed, and
at that stage theflagella pass into it, but do not extend beyond it ; and
even in the mature Vol vox it continues to form an investment around
the hyaline envelopes of the separate cells, as shown in the same figure
at No. 11. It seems to be by the adhesion of the hyaline investment
of the new sphere to that of the old that the secondary sphere re-
mains for a time attached to the interior wall of the primary ; at
what exact period, or in what precise manner, the separation between
the two takes place, has not yet been determined. At the time of the
separation the developmental process has generally advanced as far
as the stage represented in No. 1, the foundation of one or more tertiary
spheres being usually distinguishable in the enlargement of certain
of its cells.

The development and setting free of these composite ' zoo-
sporanges,' which is essentially a process of cell-subdivision or gemmi-
parous extension, is the ordinary mode of multiplication in Vol vox,
taking place at all times of the year, except when the sexual
generation (now to be described) is in progress. The mode in which
this process is here performed (for our knowledge of which we are
indebted to the persevering investigations of Professor Cohn) shows
a great advance upon the simple ' conjugation ' of two similar cells,
and closely resembles that which prevails not only among the higher
algae, but (under some form or other) through a large part of the
cryptogamic series. As autumn advances, the Vol vox spheres usuallv
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, remain
ing ' sterile.' Each sphere of Vol vox ;//(>/><> for (Plate VI, fig. 1) con-
tains both kinds of sexual cells, so that this species ranks as mono -
dons ; but V. aureus is dioecious, the 'sperm-cells ' and 'germ -cells '
occurring in separate spheres. Both kinds of ' sexual ' cells are at
first distinguishable from the ordinary 'sterile' cells by their larger
size (fig. 2, a), in this respect resembling ( zoosporanges 3 in an
early stage ; 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, take3
place not on the 'binary ' plan, but in such a manner that the en-
dochrome of the primary cell resolves itself into a cluster of very
peculiar secondary cells (tig. 1, <t, a\ fig. 5), each consisting of an
elongated 'body ' containing an orange-coloured endochrome with a

1 1 :>

484 MICROSCOPIC FORMS OF VEGETABLE LIFE

red corpuscle, and of a long, colourless beak, from the base of which
proceeds a pair of long fiagella (figs. 6, 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 anther vzoids, which show an active, inde-
pendent movement whilst still within the cavity of the primary cell
(fig. 1, a3) ; and finally escape by the giving way of its wall (a4),
cliff using themselves through the cavity of the Vol vox sphere. The
tjfirm-celte (fig. 1, b, b), 011 the other hand, continue to increase in
size without undergoing subdivision ; at first showing large vacuoles
in their protoplasm (b2, b2), but subsequently becoming filled with
dark-green endochrome. The form of the ' germ-cell ' gradually
changes from its original fiask-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 anthero-
zoids 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
Pahnogloea, to starch and a red or orange-coloured oil. As many
as forty of such ' oospores ' have been seen by Dr. Colin in a single-
sphere of Volvox, which thus acquires the peculiar appearance that
has been distinguished by Ehrenberg by a different specific name,
Volvox stellatus. Soon after the ' oospores ' reach maturity, the
parent-sphere breaks up, and the oospores fall to the bottom, where
they remain during the winter. Their further history has since
been traced out by Kirchner, who found that their germination
commenced in February with the liberation of the spherical ' endo-
spore ' 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-multi-
plication, 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 cha-
racteristic 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

1 The doctrine of the vegetable nature of Volvox, which had been suggested by
Siebold, Braun, and other German naturalists, was first distinctly enunciated by Prof.
Williamson, on the basis of the history of its development, in the Transact ions of
the Philosophical Society of Manchester, vol. ix.

[The most recent and detailed accounts of the development of the various forms of
Volvox are by Klein (PringsJieim's Jahrbiicher fur wissenschaftliche Botanih, vol.
xx. 1889, p. 133) and Overton (Botanisches Centralblatt, vol. xxxix. 1889), which do
not differ in any material point from the description given in the text. See also-
Bennett and Murray's Handbook of Cryptogamic Botany, p. 292. — Ed.]

VOLVOX; EUDORINA ; PAMBORINi

48S

moving mass of protoplasm that bears a strong resemblance to the
animal amaha, is affirmed by I)r. Hicks' to take place in Voir,,.,-,
under circumstances that leave no reasonable ground for that doubt
of its reality which has been raised in regard to the accounts of
similar phenomena occurring elsewhere. The endochrome-mass
of one of the ordinary cells increases to nearly double its usual size :
but, instead of undergoing binary subdivision so as to produce a
' zoosporange,' it loses its colour and its regularity of form, and
becomes an irregular mass of colourless protoplasm, containing a
number of brown or reddish-brown granules, and capable of alter-
ing its form by protruding or retracting any portion of its mem-
branous wall, exactly like a true Amnlxi. 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, pre-
cisely after the manner of an Amoho. After the 'amo-boid' 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 tran-
sition from the one condition to the other, and 011 the difficulty of
supposing that any such bodies could have entered the sphere para-
sitically from without— that the 'amoeboid ' is really the product of
the metamorphosis of a mass of vegetable protoplasm. This meta-
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 amoeboid bodies has not yet been
ascertained.2

In other organisms allied to Volvox and included in the family
Volvocineai we find a very interesting and instructive transition
between the various modes of multiplication already described. In
Eudori ittt, a common organism in still water, a sexual process, similar
to that in Volvox, has been observed. Tn 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 j
when they meet, their points at first come together, but gradually
their whole bodies coalesce, and a globular ' zygospore is thus formed
which germinates after a period of rest, reproducing by binary
subdivision the original sixteen-celled mulberry-like Pandorina.

1 Trans, of Microsc. Societyt n.s. vol. viii. 1HC»0, p. '.)'.); and Quart. Jo urn. of
Microec. Science, n.s. vol. ii. 1863, p. 96.

- A similar production of ' anuehoids ' has heen oli-i-n cd l>y Mr. Archer in Stepha-
nosplucra pluvialis, und is scarcely now to l>o considered an exceptional phenome-
non.

486 MICROSCOPIC FORMS OF VEGETABLE LIFE

We have here, therefore, a true process of ' conjugation ' between
motile protoplasm masses, each of which is in itself indistinguishable-
from a zoospore. A similar process takes place also in Conferva,
Ulothrix, Hydrodictyon, and a number of fresh-water algae (fig. 372).

Included by many writers under the general term PalmellaceSB
are a number of minute organisms of very simple structure, the
relationship of which to the Protococcaceoi 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
particles have little or no adhesion to each other ; or they may pre-
sent themselves (2) in the condition of an indefinite slimy film, or (3)
in that of a tolerably firm and definitely bounded membranous
' frond.' The first of these states we have seen to be characteristic
of Palmogloea 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 exten-
sive 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 Palmella. The
second is the condition of Palmella proper, of
which one species, P. cruenta, usually known
under the name of ' gory dew,' is common
on damp walls and in shady places, some-
times 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 Hamatococcus sanguineus
(fig. 373), which chiefly differs from Palmella in the partial persistence
of the walls of the parent-cells, so that the whole mass is subdivided
by partitions, which enclose a larger or smaller number of cells
originating in the subdivision of their contents. Besides increasing
in the ordinary mode of binary multiplication, the Palmella cells seem
occasionally to rupture and diffuse their granular contents through
the gelatinous stratum, aud thus to give origin to a whole cluster at
once, as seen at e, after the manner of other simple plants to be
presently described, save that these minute segments of the endo-
chrome, having no power of spontaneous motion, cannot be ranked
as 'zoospores.' The gelatinous masses of the Palmella are frequently
found to contain parasitic growths formed by the extension of other
plants through their substance ; but numerous branched filaments
sometimes present themselves, which, being traceable into absolute
continuity with the cells, must be considered as properly appertaining

Fig. 372. — A, conjugating
microzobspores of Ulo-
thrix ; B, megazob'spore
of Ulothrix, from Vines's
' Physiology of Plants.'

I'AMEIXACK.-K; I LVACEJS

487

to them. Sometimes these filaments radiate? in various directions from
a single central cell, and must at first bo considered as men; ex ten-
sions of this ; their extremities dilate, however, into new cells ; and
when these are fully formed, the tubular connections close up, and the
cells become detached from each other.1 Of the third condition we
have an example in the curious Palmodictyoii described by Kutzini:,
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 ' zoospores.' Tin;
alternation between the' motile' form and the 'still' or resting form,
which has been described as occurring in Proto<SOCCU8f 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

Fig. 878. — Hcrmatococcus sanguineus, in various stages of development: a, single
cells, inclosed in their mucous envelope; b,' r, cluster formed by subdivision of
parent-cell; <1, more numerous cluster, its component cells in various stages of
division ; r, large mass of young cells, formed by the subdivision of the parent
endochrome, and enclosed within a common mucous envelope.

of the plant and to prevent it from choking itself by overgrowth in
any one locality. It is very commonly by plants of this group that
the algal portions of lichens are formed.

Notwithstanding the very definite form and large size attained
by the fronds or leafy expansions <>r' the Ulvaceae, to which group
belong some of the most common grass-green seaweeds ('laver')
found on every coast, yet their essential structure differs but very
little from that of the preceding group ; and the principal advance
is shown in this, that the cells, when multiplied by binary sub-
division, not only remain in firm connection with each other, but
possess a very regular arrangement (in virtue of the determinate

1 This fact, first made public by Mr. Thwaites (Ann. of Sat. Hist., '2nd series,
vol. ii. 1H48, p. 8181, is one of fundamental importance in the determination of the
real character of this group.

MICROSCOPIC FORMS OF VEGETABLE LIFE

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. 374. The isolated
cells A, in which it originates, resembling in all points those of a
Protococcus oive rise by their successive subdivisions in determinate
directions to such regular clusters as those seen at B and C, or to
such converfoid 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 development),
the filament increases in
breadth as well as in length
(as seen at E), and finally

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becomes such a ' frond ' as
is shown at F, G-. In the
simple membranous expan-
sion or thai I us thus formed,
there is but little approach
to a differentiation of parts
in the formation of root, stem,
and leaf, such as the higher
algre present ; every portion
is the exact counterpart of
every other, and every por-
tion seems to take an equal
share in the operations of
growth and reproduction.
Each cell is very commonly
found to exhibit an imperfect
partitioning into four parts
preparatory to multiplication
by double bipartition, and
the entire frond usually
shows the groups of cells
arranged in clusters contain-
ing some multiple of four.
Besides this continuous increase of the individual frond, however,
we find in most species of Wva a provision for extending the plant
by the dispersion of 'zoospores.' The endochrome (fig. 375, a) sub-
divides into numerous segments (as at b and c), which at first are
seen to lie in close contact within the cell that contains them, then
begin to exhibit a kind of restless motion, and at last escape by
the bursting of the cell- wall, and they swim freely through the water
as zoospores (d) by means of their flagella, either two or four
belonging to each zoospore, with which they have become endowed
during the formation of the zoospores within their mother-cells. At

S&»* £#£o M \ _ up

Fig. 874. — Successive stages of development
of TJlva.

ULVACEJE

489

last, however, they come to rest, attach themselves to souk- fixed
point, and begin to grow into clusters or filaments («) in the manner
already described. The walls of the cells which have thus discharged
their endochrome remain as colourless spots on the frond ; sometimes
these are intermingled with 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. Tf the microscopist wli<>
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 1 swarm ' around their points of exit very much in
the manner that animalcules are often seen to do around particular

Fig. 375. — Formation of zoospores in Ulva latissima : a, portion of the ordinary
frond; b, cells in which the endochrome is beginning to break up into segments;
C, cells from the boundary between the coloured and colourless portions, some of
them containing zoospores, others being empty ; <l, flagellate zoospores, as in active
motion ; r, subsequent development of the zoospores.

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 little farther still thev are oh-
viously but masses of endochrome in the act of subdivision.1

More recent observation has brought out the interesting fact
that in Ulva and its allies there are two kinds of swarm-spore, a larger
kind, ■ megazoospores,' with four, and a smaller kind, 'microzodspores,'
with two cilia each (see tig. 372). Of these the mega zoospores germi-
nate directly, as above described, while the microzoospores have been

1 Such an observation the Author bad the good fortune to make in the year 1MJ,
when the emission of /.oospores from the / '(nunc, although it hail heen described
by the Swedish algologist Agardh, had not been seen ilu- believes) by any British
naturalist.

490 MICROSCOPIC FORMS OF VEGETABLE LIFE

observed to conjugate in pairs, producing zygospores, by the ger-
mination of which a new generation is produced. The two kinds of
zoospore may be produced on the same or on different individuals.

The Oscillariacese constitute another tribe of thallophytes 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 tine, 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 usually straight, and either each separate thread or a number
together are, in most of the genera, enclosed in a gelatinous sheath.
Some illustrations of these are seen on Plate VII. The contents of
the sheaths are imperfectly divided into cells by transverse divi-
sion ; small pieces of the threads, consisting of a few cells, occasion-
ally break off, round themselves off at both ends, move about with a
slow undulating motion, and finally develop into new threads ; these
portions are known as hormogones. The most abundant genus Oscil-
laria has been so named from the peculiar oscillating or waving motion
with which the threads are endowed. This consists of a creeping
motion in the direction of the length of the thread, now backwards,
now forwards, accompanied by a curvature of the thread and rotation
round its own axis. The cause of this motion is still a matter of
controversy. Professor Cohn 1 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 Spirulina 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 Oscillaria 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) rotation or flexion of the entire thread ; (5) sharp tremblings or
concussions ; and (6) a radiating arrangement 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 Rivulariacese and Scytonemacese (Pis. VII & VIII)
are exceedingly common organisms in stagnant water, resembling the
Oscillariacese in their blue-green colour, and in their reproduction by
means of ' hormogones.'

Nearly allied to the preceding is the family of Nostocacese, con-
sisting of distinctly beaded filaments, which, in the most familiar
genus Nostoc, lie in firmly gelatinous envelopes of definite outline

1 Arch. MiJcrosJc. Anatomie, 1867, p. 48.

2 Oesterreichische Bot. Zeitsch. 1880, p. 11.

5 See Bot. Centralblatt, vol. xii. 1882, p. 361.
4 Arch. Sci. Phys. et Nat. 1885, p. 164.

PLATE', vtl

N()ST( X 'ACH.K ; SJ PIK )NACK A-l

49 1

(tig. 37G). The filaments are usually simple, though sometimes
densely interwoven, and are almost always curved or twisted, often
taking a spiral direction. The masses of jelly in which they are im-
bedded are sometimes globular or nearly so, and sometimes extend
in more or less regular branches j they frequently attain a very con-
siderable 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, however, as they appear to be ; for
they shrink up into mere films in dry weather and expand again
with the first shower. Other species are not unfrequent among wet
moss or on the surface of damp rocks. Species of Anabfhna and
Ajthanizomenon, genera of Nostocacea*, constitute a large portion of
the bluish-green scum which i, floats on the surface of stagnant
water. Nostoc multiplies, like the Oscillariacea*, 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
1 hormogones,' similar to those of the Oscil-
lariacese. 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 sub-
division of its segments. By the repeti-
tion of this process a mass of new filaments
is produced, the parts of which are at first
confused, but afterwards become more dis-
tinctly separated by the interposition of
the gelatinous substance developed between
them. Besides the ordinary cells of the
beaded filaments, two other kinds are
known, both larger than the ordinary cells,
and called respectively heterocysts and
Spores. The function of the former is
unknown j the latter develop directly
into new individuals by division in the
transverse direction only, without any
sexual process.

Although many of the plants belong-
ing to the family Siphonaceae 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 thallophytes. They are inhabitants both of fresh
water and of the sea, and consist of very large tubular cells, which
often extend themselves into branches, so as to form an arborescent
frond. These branches, however, 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, named after a Genevese botanist, may be selected

Fig. 37(5. — Portion of gelatinous
frond of Nostoc.

492

MICEOSCOPIC FORMS OF VEGETABLE LIFE

as a particularly good illustration of this family, its history having
been pretty completely made out. Most of its species are inhabit-
ants of fresh water, but some are marine ; and they commonly
present themselves in the form of cushion- like masses, composed of
irregularly branching filaments, which, although they remain dis-
tinct, are densely tufted together and variously interwoven. Some
species commonly form dense green mats on damp soil in flower-
pots &c. The formation of motile gonids or ' zoospores ' may be
readily observed in these plants, the whole process usually occupying
but a very short time. The extremity of one of the filaments usually
swells up in the form of a club, and the endochrome accumulates in
it so as to give it a darker hue than the rest ; a separation of this
part from the remainder of the filament, by the interposition
of a transparent space, is next seen ; a new envelope is then formed
around the mass thus cut off ; and at last the membranous wall of the
investing tube gives way, and the ' zoospore ' 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 compressed, even to the extent of caus-
ing its endochrome to be discharged. 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 linger that the escape
of the zoospores generally takes place towards 8 a.m. ; to watch this
phenomenon, therefore, the plant should be gathered the day before,
and its tufts examined early in the morning. The same filament
may give off two or three zoospores successively.

Recent discoveries have shown that there exists in this humble
plant a true process of sexual generation, as was indeed long ago
suspected by Vaucher, though upon no sufficient grounds. 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. 377, A, a) ; and these have been shown by
Pringsheim to be ' antherids,' which produce ' antherozoids ' in their
interior ; whilst the capsule-like bodies (A, h) are ' oogones ' or
' archegones,' each containing a mass of endochrome which consti-
tutes an 1 oosphere ' that is destined to become, when fertilised, the
original cell of a new generation. The antherozoids (B, c, d)

SIPHONACEjE

493

when set free from the antherid a, swarm over the exterior of
the oiigone h, and have; been seen to penet rate its eavity through
an aperture in its wall, and to come into contact with the surface ot
its endochrome-niass, over which
they diffuse themselves ; there they
seem to undergo absorption, their
substance mingling itself with
that of the oospheie j and the
endophism of the ' oospore ' thus
formed, which had previously no
proper investment of its own,
soon begins to form an envelope
of cellulose (C, A), which increases
in thickness and strength, until
it has acquired such a density as
enables it to afford a firm pro-
tection to its contents. While
in VaucJieria the separate fila-
ments are so slender as to be
scarcely discernible to the naked
eye, the frond of other genera of
Siphonacee, mostly natives of
shallow seas in the warmer parts
of the globe, attains very large
dimensions. Thus in C odium it
is a spongy spherical or cylin-
drical floating mass, as much as
a foot in length ; in Caulerpa it
has the appearance of a branched
leaf springing from a stem, which
puts out roots from its under
side ; in Acetabuhi ria it takes a
mushroom-like form with a cap
or ' pilous,' a quarter of an inch
in diameter, divided into regular
chambers, at the summit of a
cylindrical stalk, 1 \ to 3 inches
in height. MunieT- Charles 1 be-
lieves that many fossils generally
regarded as Foramiirifera are in
reality the calcareous skeleton of
alga> belonging or nearly allied
to the Siphonacese.

The microscopist who wishes
to study the development of
zoospores, as well as several
other phenomena of this low type of vegetation, may advantageoush
have recourse to the little plant termed Achlya prolifera,-' which

1 Comptcs liouhifi, vol. lxw. 1S77, p. Si 1.

- This plant, though, as an inhabitant of water, formerly ranked anion;* Algtt, is
now generally regarded lelonging to the group of Fitmji on account of its

phases ot generative
process in Wturltrnd ses.sili.s: at A are
seen one of the 'horns' or antherids (a)
and one of the oogones (b), as yet un-
opened; at B the antherid is seen in
the act of emitting the antherozoids (el,
of which many enter the opening at the
apex of the oiigone, whilst others iih
which do not enter it display their ciliu
until they become motionless; at (.' the
orifice of the oiigone is closed again l>\
the formation of a cellulose coat around
its endochrome, thus constituting
oospore.

an

MICROSCOPIC FORMS OF VEGETABLE LIFE

grows parasitically upon the bodies of dead flies lying in the 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 partitions, extending
themselves in various directions. The tubes contain a colourless
slightly granular protoplasm, the particles of which are seen to move
slowly in streams along the walls, as in Chara, the currents occasion-
ally anastomosing with each other (fig. 378, C). Within about thirty-
six hours after the first appearance of the parasite on any body, the
protoplasm begins to accumulate 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 proto-
plasm continues for a time
to be distinguishable. Very
speedily, however, its endo-
plasm shows the appearance
of being broken up into a
laro;e number of distinct
masses, which are at first
in close contact with each
other, and with the walls
of the cell (fig. 378, 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
quite mature, they are set
free by the rupture of its
wall (B), to go forth, and,
after swarming about for a
time, to develop into tubi-
form cells resembling 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 dilatation, the cavity of which becomes shut oft*
by a transverse partition. Its contained endoplasm divides into two,
three, or four segments, each of which takes a globular form, and is

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, a fungus parasitic on the bodies of living fish, and
causing the very destructive disease to which salmon are liable.

Pig. 378. — Development of Achh/a jwolifera : A,
dilated extremity of a filament b, separated
from the rest by a partition a, and containing
zoospores in progress of formation ; B, end of
filament after the cell-wall has burst, and
setting free zoospores, a, b, c; C, portion of
filament, showing the course of the circulation
of granular protoplasm.

HYDRODICTYON

495

then fertilised by the penetration of an antheridial tube which comet

off from the filament a little below the partition. The 'oospore-

thus produced, escaping from the globular cavities, acquire linn en
velopes, and may remain unchanged for a long time even in water,
when no appropriate nidus exists for them ; hut will quickly germi
nate if a dead insect or other suitable object be thrown in.

One of the most curious forms of the lower algai is the ' water
net,' or Hydrodictyon utriculatum. which is torn id in fresh-water
pools* in the midland and southern counties of England. Tts frond
consists of a green open network of filaments, acquiring, when full
grown, a length of from four to six inches, and composed of ;i va»t
number of cylindrical tubular cells, which attain the length of four
lines or more, and adhere to each other by their rounded extremit ies,
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 f swarm -spores,' which
at a certain stage of their development are observed in active
motion in its interior, but come to rest in the course of 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 ' microzodspores ' 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. Quite recently
conjugation between these smaller ' zoospores ' has been observed by
Dodel-Port, taking 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 de-
scribed as ' hypnospores ' or 1 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 Protococcu8. 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, pre
sent the characters of ordinary 'zoospores,' each of them possessing
two flagella at its anterior semitransparent 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 polyhcdra, at the same time augment
ing 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

496 MICROSCOPIC FORMS OF VEGETABLE LIFE

cell-wall, on escaping from which it presents all the essential cha-
racters 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 emersion as 'zoospores,' measure no more than ^.^th
of an inch in length ; but in the course of a few hours they grow
to a length of from y^th 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 1 (fig. 379, A), or by a
notching more or less deep (fig. 380, B) ; but they differ in these two
important particulars — that the cells are not made up of two sym-

Fig. 879. — Various phases of development of Pediasirum granulatum.

metrical halves, and that they are always found in aggregation, which
is not, except in such genera as Scenedesmus which connect this group
with the Desmids, in linear series, but in the form of discoidal
fronds. In this tribe we meet with a form of multiplication by motile
' megazoospores ' which reminds us of the formation of the motile
spheres of Vohox, and which takes place in such a manner that the
resultant product may vary greatly in the number of its cells, and con-
sequently both in size and in form. Thus in Pediastrum granulatum
(fig. 379) the ' zoospores ' formed by the subdivision of theendochromeof
one cell, which may be four, eight, sixteen, thirty-two, or sixty-four in
number, escape from the parent- froud 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

PEDIASTRUM

497

to take place within a few hours previously from the cells a, A, r} f/} ,; j
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 Condi-
tions. 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 1 zoospores,'
nine of which have passed forth from its cavity, though still enve-
loped 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 emersion, is shown at 1> ; the
1 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 arrangement,
most commonly that seen at C, in which there is a single central
body surrounded by a circle of tive, 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 produc-
tion of its characteristic ' horns.' Within about four or five hours
after the escape of the 'zoospores ' the cluster has come to assume much
more of the distinctive aspect of the species, the marginal cells
having grown out into horns (E) ; still, however, they are not very
closely connected with each other, and between the cells of the
inner row considerable spaces yet intervene. It is in the course of
the second clay 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, however, the arrangement
of the interior cells does not follow the typical plan.1 The formation
of ' microzoospores ' has also been observed, which may possibly con-
jugate.

The varieties which present themselves, indeed, both as to the
number of cells m each cluster and the plan on which they are
disposed, 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. 3S0, D, under
the name of I't'diastriun /vrtttsttni, is in reality nothing else than a
young frond of P. grantUcUitm in the stage represented in ti lt. 579, E,
but consisting of thirty-two cells. On the other hand, in fig. 3^0, E,

1 See Prof. Braun on The PJirtunnrnon of Rejuvenescence in Nntitrr, published
by the Ruy Society in 1S.VI; and his subsequent memoir, Aljarum I'nicrllularittn
Ucncra nova aut /ninus co<j>iita, 1S35.

K K

498 MICROSCOPIC FORMS OF VEGETABLE LIFE

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
them presenting the two-horned shape. So, again, in fig. 380, B, and
C, are shown two varieties of Pediastrum Ehrenbergii, whose frond
is normally composed of sixteen cells ; whilst at A is figured a form
which is designated as P. tetras, but which may be strongly suspected
to be merely a four-celled variety of B and C. Many similar cases
might be cited ; and the Author would strongly urge those micro -
scopists who have the requisite time and opportunities, to apply
themselves to the determination of the real species of these 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 stock. The characters of such

D, P. pert us it m ; E, empty frond of P. granulatum.

varieties are diffused by the process of binary subdivision amongst
vast multitudes of so-called individuals. Thus it happens that, as
Mr. Balfs has remarked, 'one pool may abound with individuals of
Staurastrum dejectum or Arthrodesrnus incus having the mucro
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 any
particular character in all the specimens of one gathering is by no
means sufficient to entitle these to take rank as a distinct species j
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 part of those green threads

CONFERVACEJE

499

r 1

which are to be seen attached to stones, with their free ends float-
ing 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. 381). The plants of this family an; extremely
favourable subjects for the study of the method of cell-mul-
tiplication by binary snbdi rision. a
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 the subdivision
of the endochrome, and the inflexion
of the ectoplasm around it (fig. 381j
A, a) ; and thus there is gradually
formed a sort of hour-glass contraction
across the cavity of the parent-cell,
by which it is divided into two equal
halves (B). The two surfaces of the
infolded utricle produce a double layer
of cellulose membrane between them.
Sometimes, however, as in Cladophora
</h>merata (a common species), new cells
may originate as branches from any
part of the surface by a process of

budding, which, notwithstanding its Fig^ 381.— Pnn ,,(' « ,11-nniltiplica
difference of mode, agrees with that
just described in its essential character,
being the result of the subdivision 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 pro-
tuberance ; 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 Confervace:o are characterised by
the possession of a large number of nuclei. They are multiplied by

tion in Cladophura (jlumcrata. — A,
portion of filament with incom-
plete separation at a, and complete
partition at b; B, the separation
completed, a new cellulose partition
being formed at a ; C, formation of
additional layers of cellulose wall.

c, beneath the mucous investment,

d, and around the ectoplasm, <i,
which encloses the endochrome, b.

5oo

MICROSCOPIC FORMS OF VEGETABLE LIFE

zoospores, produced apparently indifferently from any cell of a fila-
ment, by free cell-formation. These zoospores are of two kinds, larger
or smaller ; the larger kind have either two or four cilia, and germi-
nate directly ; the smaller are biciliated, and conjugation between
them has been observed in a few instances.

7

Fig. 382. — Development and reproduction of Spliceroplea.

Nearly allied to the Confervacese is a very interesting plant in
which a true sexual mode of reproductionhas been observed, Sphsero-
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
1 Ann. des Sci. Nat. 4eme ser,, Bot., torn. v. 1850, p. 187.

SI'ILKKOI'LKA ANM LINA

501

the outer one is furnished wit h stellate prolongations (fig. 382, No. 1).

When it begins to vegetate its endoehrome breaks up tir.-a 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-t l ansparent 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 tin; separation
of the endoehrome of its two halves by the interposition of a vacuole
(No. 6), and in more advanced stages (Xos. 7, <s) a repetition of tie-
like interposition gives to the endoehrome that annular arrange-
ment from which the plant derives its specific name. This is seen
at No. 9, a, as it presents itself in the filaments of the adult plant ;
whilst at A, in the same figure, we see a sort of frothy appearance
which the endoehrome 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
endoehrome into definite masses (as seen at No. 10, a), which soon be-
come star-shaped (as seen at />), each one being contained within a
distinct compartment of the cell. In a somewhat more advanced
stage (as seen at No. 11, a), the masses of endoehrome begin to draw
themselves together again ; and they soon assume a globular or
ovoidal shape (/>), whilst at the same time definite openings (c) are
formed in their containing cell-wall. Through these openings the
i antherozoids ' developed within other cells gain admission, as
shown at No. 12, d ; and they become absorbed into the before-men-
tioned 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,
they precisely resemble the mature oospores which we have taken as
the starting-point in this curious history. Certain of the cells (as
in No. 14), instead of giving origin to oospores, have their annular
collections of endoehrome converted into 'antherozoids,' which, as
soon as they have disengaged themselves from the mucilaginous
sheath that envelopes them, move about rapidly in the cavity of their
containing cell (o, b) around the large vacuoles which occupy its
interior, and then make their escape through apertures (c, >/) which
form themselves in its wall, to find their way through similar aper-
tures into the interior of the 'oogones,' as already described. These
antherozoids are shown in \<». L5, 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 generathe process, the ordinary
filamentous cell developing oospheres on the one hand and anthero-

502 MICROSCOPIC FORMS OF VEGETABLE LIFE

zoids on the other, and in the simplicity of the means by which the
fecundating process is accomplished.

The (Edogoniacese resemble Confervacece in general aspect and
habit of life, but differ from them in some curious particulars. As
the component cells of the filaments extend themselves longitudin-
ally, new rings of cellulose are formed successively, and intercalated
into the cell-wall at its upper end, giving it a ringed appearance.
Only a single large zoospore is set free from each cell ; and its libera-
tion is accomplished by the almost complete fission of the wall of the
A B

Fig. 383.— A. Sexual generation of CEdogoniiim ciliatiim: 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 obgone in the act of being fertilised by the
entrance of an antherozoid set free from the interior of its androspore ; 2, free
antherozoids ; 3, mature oospore, still invested with the cell-membrane of the
parent-filament ; 4, portions of a filament bearing special cells, from one of which
an androspore is being set free ; 5, liberated androspore.

B, Branches of Chcetojjhora elegans, in the act of discharging ciliated zoospores,
which are seen, as in motion, on the right.

cell through one of these rings, a small part only remaining uncleft,
which serves as a kind of hinge whereby the two parts of the fila-
ment are prevented from being altogether separated. Sometimes
the zoospore does not completely extricate itself from the parent-cell ;
and it may begin to grow in this situation, the root-like processes
which it puts forth being extended into the cavity. The zoospores
are the largest known in any class of algse ; 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 (Edogoniaceca show a curious
departure from the ordinary type ; for whilst the ' oospheres ' are

(KlXMiOXIACK.K ; CILKToi'IK »KACEJE

503

formed within certain dilated cells of the ordinary filament nig. •"<v.'>,
A, No. 1), which may be termed 'oogones,' an<l are fertilised by tic-
penetration of 'anthero/oids' (Xo. li), these anthcrozoids arc not. in
the species, the immediate product of the; sperm-cells of the sain*- or
of another filament, l)ut 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 has come to an end, attaches
itself to the outer surface of an oogone, or of a cell in close proxi-
mity to an oogone, as shown at No. 1,6; it then develops into a
very small male plant, known as a 'dwarf-male,' consisting of two or
three cells ; the terminal of these cells is an antherid, from the aped
•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
same time an aperture is formed in the wall of the oogone by
which the antherozoid enters its cavity and fertilises its oosphere by
becoming absorbed into it. This mass then becomes an oospore ( Xo.'i),
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 be understood, and the inquiry is one so fraught
with physiological interest, and, from the facility of growing these
plants in aquaria, can be so easily pursued, that it may be hoped
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-
shaped processes (fig. 383, B). As in many preceding cases, these
plants multiply themselves by the conversion of the endochrome of
certain of their cells into 1 zoospores,' and these, when set free, are
seen to be furnished with either two or four cilia. ' Resting-
spores ' have also been seen in many species. One of the most
beautiful objects under the microscope is Drapamaldia glorru rata%
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 CkatophoraCea closely resembles that of
the ( 'onfervacece.

The Batrachospermese, whose name is indicative of the strong
resemblance which their beaded filaments bear to frog-spawn, are
now ranked as humble fresh- water forms of a far higher, chiefly
marine group of alga', the RKodotpertneCB, or red sea-weeds. But

504

MICKOSCOPIC FORMS OF VEGETABLE LIFE

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. 384). 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

radiating

Fig. 384.
Bairaeliospermum nioniliforme.

whorls of short,
branches, each of which is com
posed of rounded cells, arranged
in a bead-like row, and some-
times subdividing again into
two, or themselves giving off1
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 sub-
division this single cell becomes

converted into a beaded filament. Certain of these branches, how-
ever, instead of radiating from the main axis, grow downwards upon
it, so as to form a closely fitting investment that seems properly to
belong to it. Some of the radiating branches grow out into long
transparent bristles, like those of C1i<('tophoracece ; and within those
are produced ' antherozoids,' which, though not endowed with the
power of spontaneous movement, find their way to the oospheres
contained in other parts of the filaments ; and by the fertilisation
of the contents of these are produced the somewhat complicated
fructifications known as c cystocarps,' placed in the axils of the
branches (fig. 384).

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

1 Sirodot, Les Batrachospermees, fo. 1884 ; see also Comptcs Benclus, vol. lxxvi
1873, pp. 1216, 1335 ; vol. xci. 1880, p. 862 ; vol. xcii. 1881, p. 993.

CHAKACK.K

505

and his observations have since been confirmed by others. The

germinating spores of Hut ruchosj:/>rnu< ni put out, under certain
conditions, a kind of filament, known as a ' protoncme,' \s Inch
develops into a CJumtruitsiu, a non-sexual form of Hutiunhosju'riii n m ,
which can reproduce itself from generation to generation by simple
budding, or by means of non-sexual spores, without producing
sexual organs. Chuntrunsiu 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 Batru'-hns/iernium, bearing true sexual
organs, as above described. This may then go on reproducing itself,
or revert to the Chuntrunsiu form.

Among the highest of the AlgSB in regard to the complexity of
their generative apparatus, which contrasts strongly with the general
simplicity of their structure, is the family of Characese 1 ranked by
some botanists as a group of primary importance), some members
of which have received a large amount of attention from micro-
scopists on account of the interesting phenomena they exhibit.
These humble plants are for the most part inhabitants of fresh
waters, and are found rather in such as are still than in those
which are in motion ; a few species, however, may be met with in
ditches whose waters are rendered salt by communication with the
sea. They may be easily grown for the purposes of observation in
large glass jars exposed to the light, all th.it 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 (tig. 38o, 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 Churn 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 half-way between the nodes, their ends dovetailing
into one another. These investing tubes constitute what is termed
the 'cortex ' of Cham. 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 Chora, art'
a continuation of the cortical layer of the ' internode.' The branches
are altogether similar in structure to the primary axis, and termi-
nate 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-
ments they have gained their popular name of * stoneworts.' Tin
long tubiform cells of Nitella, and the terminal uncorticated cells of

«

506 MICKOSCOPIC FOKMS OF VEGETABLE LIEE

the branches of Chara, afford a very beautiful and instructive display
of the phenomenon of cyclosis, or rotation of protoplasm in their
interior. Each cell, in the healthy state, is lined by a layer of
chlorophyll grains, which cover every part, except two longitudinal
lines that remain nearly colourless (fig. 385, 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

A B

Fig. 385. — Nitella flexilis : A. Stem and brandies of the natural size : a, b, c, d, four
whorls of branches issuing from the stem ; e, f, subdivision of the branches.
B. Portion of the stem and branches enlarged : a, b, joints of stem ; e, d, whorls ;
e, f, new cells sprouting from the sides of the branches ; g, h, new cells sprouting
at the extremities of the branches.

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 par-
ticles, 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. 1 The produc-

1 This interesting phenomenon may be readily observed by taking a small por-
tion of the plant out of the water in which it is growing, and either placing it in

CI I A RACE. 'K

5^7

tion of new cells for the extension of the stem or branch* s, or for
the origination of new whorls, is not here accomplished by the
subdivision of the parent-cell, but takes place by the method of out-
growth (tig. 3S~), B, e, f\ {/, //), 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, <fcc. they not unfrequently develop 'bulbils/ which are
little cluster of cells, filled with starch, that sprout from the sides
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 plants
are reproduced, but they are peculiar among cryptogams in not
producing true ' spores,' either stationary or motile. The CharacecB
may be multiplied by artificial subdivision, the separated parts
continuing to growT under favourable circumstances, and gradually
developing themselves into the typical form.

The generative apparatus of Characea', consists of two sets of
bodies, both of which grow at the bases of the branches (fig. 386,
A, B), either on the same or on different individuals ; one set,
formerly known as 'globules,' are really antlurids \ whilst the
other, known as 1 nucules,' contain the oox pi teres, and are true
oogones or arclteyones. The 'globules,' which are nearly spherical,
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 to-
gether. From the centre of the inner face of each shield a cylindrical
cell termed a ' manubrium ' projects inwards nearly to the centre
of the sphere. The antherid is supported on a short flask-shaped
pedicel which also projects into the interior. At the apex of each
of the eight manubria is a roundish hyaline cell, called a ' capitulum,'
and at the apex of each capitulum six smaller cells or ' secondary
capitula.' From the centre of each of these secondary capitals
grow four long whip-shaped filaments (C), constituting the mass
already referred to. The number of these filaments in each antherid
is about 200, and each of these filaments divides by transverse
septa into from 100 to 200 small disc-shaped cells, which number
therefore from 20,000 to -10,000 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 << nthi round,
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

a large aquatic box or in the zoophyte-trough i p. "J'.>7 ' . or laying it on the glass
stage-plate and covering it with thin glass. A portion of Chara or Xitrllu placed
in the growing slide (pp. 2SS, '2S«.)) may he kept under observation for man} days
together.

508 MICROSCOPIC FORMS OF VEGETABLE LIFE

direction, by the lashing action of two long and very delicate cilia,
with which it is furnished. The exterior of the < nucule ' (A, B) is
formed by five or ten spirally twisted tubes that give it a very
peculiar aspect ; and these enclose a central sac containing proto-
plasm, oil, and starch-globules. 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.

-a. B

Fig. 386. — Generative organs of Chara fragilis : A, antherid or ' globule ' de-
veloped at the base of arcbegone 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 D, E, and F the successive stages of this
formation are seen ; and at G is shown the escape of the mature antherozoids, H.

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
oos2^here ; and through this canal the antherozoids make their way
down to perform the act of fertilisation by becoming absorbed into
the substance of the oosphere. Ultimately the nucule, which has
now become a hard black body, falls off, and the fertilised germ-

DESMIDIACEiE

509

•cell, or 'oospore/ gives origin to a new plant after the nucule
has remained dormant through the winter.1

Desmidiace^e and Diatomace^:.

Among those simple Algse 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 Desmidiacecv and the DiatomacecB.^ Both of them
were, ranked by Ehrenberg and many 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 Desmidiaceai 3 are minute plants of a bright green colour
growing in fresh water ; generally speaking, the cells are inde
pendent of each other (figs. 387-390) ; but sometimes those which
have been produced by binary subdivision from a single parent-
cell remain adherent one to another in linear series, so as to form a
filament (fig. 391 ; 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 1 sutural line,' which
is sometimes so decided as to have led to the belief that the cell
is really double (Plate VIII, figs. 2, 6), though in other cases
it is merely indicated by a slight notch ; the other 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. 387, 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. 391 ; Plate IX, fig. 6) ; but in

1 A full account of the Characece will be found in Prof. Sachs' Text-hook of
Botany, 2nd English edition, p. 292. Various observers have asserted that particles
of the protoplasmic contents of the cells of the Characece, when set free by the
rupture of their cells, may continue to live, move, and grow as independent rhizopods.
But the writer is disposed to think that the phenomena thus represented are rather
to be regarded as cases of parasitism, the decaying cells of Nitella having been found
by Cienkowski (Beitrage zur Kenntniss der Monaden, in Arch./. Mikr. Anat. bd. i
1865, p. 203) to be inhabited by minute, spindle-shaped, ciliated bodies, which seem
to correspond with the ' spores ' of the Myxomycetes, going through an amoeboid stage,
and then producing a, plasmode which, after undergoing a sort of encysting process,
finally ' breaks up ' into spindle-shaped particles resembling those found in the Nitella
cells.

2 [Whether the process which takes place in diatoms is a true sexual conjugation
is a point which cannot at present be regarded as fully determined. — Ed.]

5 Our first accurate knowledge of this group dates from the publication of Mr.
Ralfs' admirable monograph of the British Desmids in 1848. Later information in
regard to it will be found in the section contributed by Mr. W. Archer to the fourth
edition of Pritchard's Infusoria, and in Cooke's British Desmids, 1887.

5IO MICROSCOPIC FORMS OF VEGETABLE LIFE

other cases its existence is only indicated by its preventing the con-
tact of the cells. Klebs states 1 that in Desmids, as in the other
Conjugate?, 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 primordial 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, 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 2 describes this movement
as being of four kinds, viz. (1) a forward movement on the
surface, one end of the cell touching the bottom, while the other end
is more or less elevated, and oscillates backwards and forwards ;
(2) an elevation in a direction vertical to the substratum, the free
end making wide circular movements ; (3) a circular motion,
followed by an alternate sinking of the free end and elevation of
the other end ; and (4) an oblique elevation so that both ends
touch the bottom, lateral movements in this position, then an ele-
vation and circular motion of one end, and a sinking again to an
oblique or horizontal position. Klebs regards all these movements
as due to an exudation of mucilage, and the first two to the forma-
tion during the motions of a filament of mucilage by which the
desmid is temporarily attached to the bottom, and which gradually
lengthens. The movements of desmids are especially active when
they are in the process of dividing. Stahl found that, like the move-
ments of zoospores, they are affected by light, and always move
towards the light.

A ' cyclosis ' may be readily observed in many Desmidiacew,
and is particularly obvious along the convex and concave edges of
the cell of any vigorous specimen of Closterium, with a magnifying
power of 250 or 300 diameters (fig. 387, A, B). By careful focus-
sing the flow may be seen in broad streams over the whole surface
of the endrochrome ; 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 trans-
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),
and the globules it contains are kept in a sort of twisting movement
on the inner side (a) of the primordial utricle. Other currents are
seen apparently external to it, which form three or four distinct

1 Untersuchungen aus clem Bot. Inst. Tubingen, 1886, p. 333.

2 .Biologisckes Centralblatt, 1885, p. 353.

PLATE DC

Hest Stwnan cferomo.

Desmidiaceae.

DESMIDIACEiE

511

courses of particles, passing towards and away from c (as indicated
by the outer arrows). Another curious movement is often to be wit-
nessed in the interior of the cells of members of this family, which
has been described as ' the swarming of the granules,' from the ex-
traordinary resemblance which the mass of particles in active vibra-
tory motion bears to a swarm of bees. It is especially observable
in the hyaline terminal portions of the cells of species of Closterium,
as shown in fig. 387, 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 bean active form of the molecular movement
common to other minute particles freely suspended 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.

When the single cell has come to its full maturity it commonly
multiplies itself by binary subdivision ; but the plan on which this

Fig. 387. — Cyclosis in Closterium lunula: A, cell showing central separation at aT
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.

takes place is often peculiarly modified, so as to maintain the
symmetry characteristic of the tribe. In a cell of the simple
cylindrical form of those of Desmidium (fig. 391), little more is
necessary than the separation of the two halves at the sutural line,
and the formation of a partition between them by the infolding of
the primordial utricle ; in this manner, out of the lowest cell of the
filament A, a double cell, B, is produced. But it will be observed
that each of the simple cells has a bifid wart-like projection of the
cellulose wall on either side, and that the half of this projection,
which has been appropriated by each of the two new cells, is itself
becoming bifid, though not 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 re-
stored. In Closterium (fig. 387; Plate IX, fig. 2) the two halves of
the endochrome first retreat from one another at the sutural line, and
a constriction takes place round the cellulose wall : this constriction

512 MICROSCOPIC FORMS OF VEGETABLE LIFE

deepens until it becomes an hour-glass contraction, which proceeds
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 Staurastrum, the division taking place across the central con-
striction, and each half gradually acquiring the symmetry of the
original. In such forms as C osmarium, however, in which the cell
consists of two lobes united together by 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 is 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 narrow
neck ; and they progressively increase until they assume the appear-
ance of the half-segments of the original cell. In this state, there-
fore, the plant consists of a row of four segments, lying end to end,
the two old ones forming the extremes, and the two new ones (which
do not usually acquire the full size or the characteristic markings of
the original before the division occurs) occupying the intermediate
place. At last the central fission becomes complete, and two bi-
partite 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 den-
ticulata ; but as the small hyaline hemisphere, put forth in the
first instance from each half-cell (fig. 388, A), enlarges with the flow-
ing in of the endochrome, 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 (J?)
acquires the characteristic notched outline of its type, being only
distinguishable from the older half by its smaller size. The whole
of this process may take place within three hours and a half. In
Sphazrozosma the cells thus produced remain connected in rows
within a gelatinous sheath, like those of Desmidium (fig. 391) ;
and different stages of the process may commonly be observed in
the different parts of any one of the filaments thus formed. In any

DESMIDIACE^E

513

such filament it is obvious that the two oldest segments are found
at its opposite extremities, and that each subdivision of the inter-
mediate cells must carry them farther and farther from each other.
This is a very different mode of increase from that of the Confervacece,
in which commonly the terminal cell alone undergoes subdivision,
and is consequently the one last formed.

The sexual generative process in the Desmidiacere, which occurs
but rarely compared to that of binary division, always consists of
an act of ' conjugation.' It commences with the dehiscence of the

ABC

D E F

Fig. 388. — Successive stages of binary subdivision of Micrasterias denticulata.

firm external envelope of each of the. conjugating cells, so as to
separate it into two valves (fig. 389, C, D ; fig. 390, 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 be-
comes differentiated into three layers, of which the innermost and
outermost are colourless, while the middle one is firmer and brown.

1 In certain species of Closterium, as in many of the Diatomacece, the act of
conjugation gives origin to two zygospores.

L L

5H

MICROSCOPIC FORMS OF VEGETABLE LIFE

The outer surface is sometimes smooth, as in Closterium and its allies
(n>. 390 ; Plate IX, fig. 8) ; .but in Cosmarium it becomes granular,

tuberculated, or spinous (tig. 389,
D ; Plate VIII, figs. 1, 4), the spines
being sometimes simple and some-
times forked at their extremities.
The mode in which conjugation
takes place in the filamentous species
constituting the Desmidiece proper,
is, however, in many respects dif-
ferent. The filaments first separate
into their component joints, and
when two cells approach in conju-
gation, 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 connecting tube that unites
the cavities of the two cells (fig.
391, D, E). Through this tube the
entire endochrome of one cell passes
Totrytis "Armature cell^ B, empty over into the cavity of the other
cell- envelope ; C, transverse view; (D) ; and the two are commingled
zygospore with - empty cell-enve- gQ ^ to f()rm a gingle mags (E)? as

is the case in many of the Conju-
gatce. 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 y but the proper coats of the

zygospore gradually become
more distinct, and the en-
veloping cell-wall disap-
pears.

The subsequent history
of the zygospore has been
followed out in the case of
Cosmarium botrytis. After

Fig. 889.— Conjugation of Cosmarium

remaining at rest for a con-
siderable time, it germi-
nates by the bursting of
the two outer coats, the
protoplasmic contents es-
caping while still enclosed
in the innermost coat. In
this body the protoplsarn

Fig. 390— Conjugation of Closterium striolatum : and endochrome are already
A, ordinary cell; B, empty cell; C, two cells in divif-jed intQ twQ nalvey
conjugation, with zygospore. . . , '

J ° ' which contract somewhat,

and the whole becomes enveloped in a new cell- wall. A constric-
tion has, in the meantime, made its appearance between the two
halves, which are of somewhat unequal size, and thus the new
desmid is formed,

CLASSIFICATION OF DESAIJ 1)JA( E.F

515

The subdivision of this family into genenj, according to the
method of Mr. Palfs ('British Desmidieje ' ), 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 be-
come-separated by the com-
pletion of the fission. The
further division of the fila-
mentous group, in which
the zygospores are always
globular and smooth (PI. 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 con-
stricted nor furnished with
lateral teeth or projections ;
whilst in the other set (fig.
391 ; PI. IX, fig. 3) the length
and breadth of each joint are
nearly equal, and the joints
are more or less constricted,
or have lateral teeth or
projecting angles, or some
other figure ; and it is for
the most part upon the
variations in these last
particulars that the generic
•characters are based. The
solitary group presents a
similar basis for primary
division in the marked dif- Fig. 391.— Binary subdivision and conjugation of
ference in the proportions Desmidium cylindricufn : A, portion of filament,

of its cells, such elongated
forms as Closterium (figs.
387, 390 ; PI. IX, fig. 2), in

which the length is many times the breadth, being thus separated
from those in which, as in Micrasterias (fig. 388 : PI. IX, fig. 1), Cos-
marium (fig.389 ; PI. VIII, fig. 2), and Staurastrum (PI. VIII ; figs. 5,
6, 10), the breadth more nearly equals the length. In the former the
zygospores are smooth, whilst in the latter they are very commonly
spinous (PI. VIII, figs. 1, 4) and are sometimes quadrate. In this group
the chief secondary characters are derived from the degree of con-
striction between the two halves of the cell, the division of its margin
into segments by incisions more or less deep, and its extension into
teeth or spines.

The Desmidiacece are not found in running streams, unless the

surrounded by gelatinous envelope ; B, dividing
cell; C, single cell viewed transversely; D, two
cells in conjugation ; E, formation of zygospore.

5i6

MICROSCOPIC FORMS OF VEGETABLE LIFE

motion of the water be very slow, but are to be looked for in stand-
ing though not stagnant waters. Small shallow pools that do not
dry up in summer, especially in open, exposed situations, such as
boggy moors, are most productive. The larger and heavier species
commonly lie at the bottom of the pools, either spread out as a
thin, gelatinous stratum, or collected into finger-like tufts. By gently
passing the fingers beneath these they may be caused to rise towards
the surface of the water, and may then be lifted out by a tin box or
scoop. Other species form a slimy stratum floating on the surface of
bog-pools, or a greenish or dirty cloud upon the stems and leaves of
other aquatic plants ; and these also are best detached by passing
the hand beneath them, and ' stripping ' the plant between 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 quantities by the box or scoop and to separate them by
straining through a piece of linen. At first, nothing appears on the
linen but a mere stain or a little dirt ; but by the straining of
repeated quantities a considerable accumulation may be gradually
made. This should then be scraped off with a knife, and transferred
into bottles with fresh water. If what has been brought up by hand
be richly charged with these forms, it should be at once deposited in
a bottle ; this at first seems only to contain foul water ; but by
allowing it to remain undisturbed for a little time, the desmids will
sink to the bottom, and most of the water may then be poured off,
to be replaced by a fresh 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 constructed as to allow of the ready substitu-
tion 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, like the Desmidiacese, are simple cells, having
a firm, external coating, within which is included an ' endochrome '
whose superficial layer constitutes a 'primordial utricle,' but their
external coat is consolidated by silex, the presence 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 interpenetrated by the silex, since
a membrane bearing the characteristic surface-markings is found to

STRUCTURE OF DIATOMS

517

remain after its removal by hydrofluoric acid . The 1 endochrome '
of diatoms consists, as in other plants, of a viscid protoplasm, in
which float the granules of colouring matter. In the ordinary con-
dition 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 bright green, 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 is 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 protophy tes.
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 Diaiomacece,
such as Surirella biseriata, Nitzschia scalaris, and Gampylodiscus
spiralis, and by Prof essor Max Schultze in Coscinodiscus, Biddulphia,
and Rhizosolenia • but this movement has not the regularity so
remarkable in the preceding group.

Kiitzing has called these organisms Bacillariaeece from the genus
Bacillaria, as others have called them Diatomacew from Diatonic .
The latter name is preferable, as indicating the ease with which their
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. 403), in Isthmia (fig. 408), and in many
other diatoms ; in Biddulphia (fig. 396) 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 f rustule is a short cylinder, an aggregation of such cylinders,
end to end, must form a rounded filament, as in Melosira (fig. 398) ;
and, whatever may be the form of the sides of the frustules, if they
be parallel one to the other, a straight filament will be produced, as in
Achnanthes (fig. 412). 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 Jlabellata (fig.
401) ; or the cohesion maybe sufficient to occasion the band to wind
itself (as it were) round a central axis, and thus to form, not merely
:a complete circle, but a spiral of several turns, as in Meridion <-ircuJare
{fig. 399). Many diatoms, again, possess a stipe, or stalk-like ap-
pendage, by which aggregations of frustules are attached to other

MICROSCOPIC FORMS OF VEGETABLE LIFE

plants, or to stones, pieces of wood, &c. • and this may be a simple
foot-like appendage, as in Achnanthes longipes (fig. 412), or it may
be a composite plant-like structure, as in Licmophora (fig. 401 )?
Gomphonema (fig. 413), and Mastogloia (fig. 416). 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, uncon-
solidated by silex, analogous to the prolongations which have been
seen in the Desmidiacew, and to the filaments which sometimes con-
nect the cells of the Palmellacece. Some diatoms, again, have a
mucous or gelatinous investment, which may even be so substantial
that their frustules lie — as it were — in a bed of it, as in Mastogloia
(figs. 416 B, 417), or may form a sort of tubular sheath to them, as in
Schizonema (fig. 415). 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 completed, nor being in any way connected
either by a stipe, or by a gelatinous investment. This is the case,
for example, with Triceratium (fig. 393), Pleurosigma (Plate I,
figs. 1, 2), Actinocyclus, Actinoptychus (fig. 407), Arachnoidiscus
(Plate XII), C ampyl odiscus (fig. 405), Surirella (fig. 404), Coscino-
discus (Plate I, figs. 3, 4), Ileliopelta, and many others. The solitary
discoidal forms, however, when obtained in their living state, are
commonly found cohering to the surface of aquatic plants.

We have now to examine more minutely into the curious struc-
ture of the silicified casing which encloses every diatom-cell or
' 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 girdle, and
thus exactly represent a minute box which serves for the reproduc-
tion of the species. This process is known as the encystment, and is
not uncommon, especially amongst the Naviculacece, frustules being
frequently found amongst them open from the separation of the two
valves, showing the two rings covering each other, as the lid of a
box may cover a portion of the box itself.

It is thus not correct to designate the line shown in the front view
of the outer ring as the line of 1 suture,' since the suture is the line
of meeting bounding two surfaces placed on the same plane. The
form resulting, however, varies widely in different diatoms ; for
sometimes each valve is hemispherical, so that the cavity is globular ;
sometimes it is a smaller segment of a 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 any
modifications of the contour of the valves, which may be square,
triangular (fig. 393), heart-shaped (fig. 405, A), boat-shaped (fig. 404,
A), or very much elongated (fig. 400), and maybe furnished (though this

STRUCTURE OF DIATOMS

5*fJ

is rare among diatoms) with projecting outgrowths (figs. 409, 410).
Hence the shape presented by the frustule differs completely with
the aspect under which it is seen. In all instances, the frustule is
considered to present its ' front ' view when its line of meeting is
turned towards the eye, as in fig. 404, B, C ; whilst its ' side ' view
is seen when the centre of either valve is directly beneath the eye
(A). Although the two valves meet along the line of junction in
those newly formed frustules which have been just produced by
binary subdivision (as shown in fig. 396, A, e), yet as soon as they
begin i;o 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 by 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 apertures along the
so-called ' line of suture ' of the disc-shaped diatoms, and at the extre-
mities only of the elongated forms. Ehrenberg, followed by Kiitzing,
has interpreted as apertures or ostioles the central and terminal
nodules of the JYaviculacece, Cymbellecr, and similar forms ; but this
view is more generally regarded as incorrect. We have, in fact, no
positive demonstration of the existence of special apertures communi-
cating between the outside and the inside of the cell : and we are com-
pelled to have recourse, on this point, to hypothesis. It is, however,
certain that the diatom -cell is always composed of at least two valves,
between which the possibility of such a communication must
necessarily be admitted, or at least the existence of endosmotic and
exosmotic currents in the liquids. In the encysted forms we have
ascertained also the existence of an interval between the two rings,

1 [This refers to those diatoms in which the process of binary subdivision is pos-
sible ; but this, as will be seen presently, is not the case in many genera. — Ed.]

2 This was long since pointed out by Dr. Wallich in his important memoir on the
' Development and Structure of the Diatom-valve ' (Transact, of Microsc. Soc. n.s.
vol. viii. 1860, p. 129) ; but his observation seems not to have attracted the notice of
diatomists, until in 1877 he called attention to it in a more explicit manner (Montlih/
Microsc. Journ. vol. xvii. p. 61). The correctness of his statement has been con-
firmed by the distinguished American diatomist, Prof. W. Hamilton Smith ; but as it
has been called in question by Mr. J. D. Cox (American Journal of Microscopy,
vol. iii. 1878, p. 100), who asserts that mlsthmia there are three hoops — two attached
to the two valves, and the third overlapping them both at their line of junction — the
Author has himself made a very careful examination of a large series of specimens of
Isthmia and Biddulphia, the result of which has fully satisfied him of the correct-
ness of Dr. Wallich's original description.

520

MICROSCOPIC FORMS OF VEGETABLE LIFE

although it may be very minute ; while Navicula has been sometimes
seen with the values actually separated.

The nature of the delicate
markings with which almost
every diatom frustule is beset,
has been one of the most interest-
ing inquiries of the students of
these forms, since the introduc-
tion of the homogeneous, and
especially the apochromatic ob-
jectives ; and it cannot be
doubted that certain peculiari-
ties 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 abso-
Fig. 892. — Magnification of 'ultimate strnc- lute adhesion to any present

ture' of Coscinodiscus asieromvluiliis,ixo\\\ • + ■ ^ ,i, ■

a drawing by Messrs. Nelson and Karop. Interpretation of what IS now

('Quekett Journal,' vol. ii. ser. ii. p. 269.) held by some students of diatom

structure of no mean repute and
of unrivalled manipulative skill to be the absolute structure of some
of the larger forms.

Thus concerning the group Cpscinodiscece, representing the most
beautiful of the discoid forms of the whole group of Diatomacecr, we re-

Fig. 393. — Triceratium favus: A, side view ; B, front view.

present in fig. 3, Plate I, a micro-photographic image of C. asterom-
phalus magnified 110 diameters. But in fig. 392 the arwlo& of this
diatom are seen under great magnification with recent powers. It
is contended that the diatom, although consisting of a single siliceous
membrane, has a double structure, viz. coarse and fine areolations,
the latter within the former ; and there appears little reason to doubt
this. The coarse areolations are for the most part circular in outline,
and the intervening silex is thick. Inside these areolations is an ex-
tremely 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 wanting. In Plate I, fig.
4, we present a photo-micrograph of the same object magnified 2,000
diameters.

MINUTE STRUCTURE OF DIATOMS

521

In Isthmia nervosa, a side and front view of which are seen in
fig. 408, a similar construction is discoverable. In this diatom the
coarse areola tions are very large and the silex correspondingly
thick ; but the inner membrane is excessively thin and delicate. The
perforations are large and irregular in shape
around the margin, but small and circular in
the centre. In fig. 394 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 Kittonii ; a photo-micrograph of
this magnified 270 diameters is seen in PI. I,
fig. 5 • while a small portion of the centre of
■a kindred form, A. Sturtii, magnified 2,000

, . -, /. n • ,i i , Fig. 394. — Areolations m

times, is shown m ng. b m the same plate. isthmia nervosa.

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 abso-
lutely 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 in regular rows ; while the variety in the size and
arrangement of these particles shows that they are correlated with
the vital processes of the organisms, and afford characters for the
discrimination of the species. The nature of these granules, their
size, and the mode in which they are arranged, have from the earlier
days of microscopy 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 stria?,' these
being in truth simply the intervals which separate the boundaries of
the ' beads,7 apertures, or their equivalents, whatever they may ulti-
mately prove to be ; and this is clearly seen when they are observed
with objectives of sufficient numerical aperture and proportional
power. Pleurosigma angulatum is one of the most commonly em-
ployed 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, Amphipleura pellucida is extremely variable,
and is, as it were, the torment of microscope-makers and rival diatom
resolvers, who do not take into account the variability of this type,
forgetting, in fact, that one A. pellucida may be extremely fine, and
another, being in truth a varietal form, may be nearly as coarse as
Navicula rhomboides. The new apochromatic objectives, and the
compensating eye-pieces both for the eye and for projection, con-
structed by Zeiss, of Jena, have brought about such progress in
micrography that the image of P. angulatum appears to some minds
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

1 Note on the finer structure of certain diatoms, E. M. Nelson and G. C. Karop,
Journ. Quekett Club, vol. ii. ser. ii. p. 2(JG.

522 MICROSCOPIC FORMS OF VEGETABLE LIFE

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 j 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.
Haughton Gill has succeeded in filling up the ' dots ; or 'pearls' of the
Navicular and the secondary markings of the discoid and other forms,
so as to give evidence that the filling must be deposited in cavities.
It was clone by soaking clean diatoms in a solution of subnitrate of
mercury until their markings are filled with it ; then they are im-
mersed 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,asks, ' Would it have been possible to have seen these pearl-like ob-
jects isolated, if, instead of beads, we had had apertures or depres-
sions?' 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, we can ever be certain
as to our visual interpretation of these minute phenomena. On the
other hand, the areolated valves of Triceratium favus (fig. 393) present
a line of fracture which traverses indifferently the hexagonal areolse-
and the lines in relief which connect them.

Dr. Van Heurck has been able to employ the new lens made by
Abbe, having a numerical aperture of 1*63, upon his special subject,
the Diatomacece. He concludes that diatom valves consist of two
membranes or thin films and of an intermediate layer, the latter
being pierced with openings. The outer membrane is delicate, and
may be easily destroyed by acids, friction, and the several processes
of ' cleaning.' When the openings or apertures of this interior portion
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 has worked at this
subject for years, to say that he has long maintained this view, and
has presented skilful photo-micrographs in support of his contention.
In PL I, fig. 1, we have a photograph of his, showing the inside of a
valve of P. angulatum magnified 1,750 diameters, and exhibiting the
' postage s|amp ' 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
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 en-
abled to study these in Plate XI, of which a full description is given

Plate X.

Pleurosigma angulatum.

Magnified 4900 diams.
From a Photo -Micrograph by Dr. R. Zeiss taken
with the 2 m/m. Apochromatic Objective N. A. 1.30
and projection eye-piece 4.

Glassprint by Kuhl & Co., Frankfort on the Main.

REPRODUCTION OF DIATOMS

523

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 Nobert's nineteenth band.

Diatoms, like other organisms already described, are reproduced by
conjugation, and multiply by autofission or division. Reproduction
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 encysting of the
fruMide 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 right angles to its plane. Hence,
instead of finding, as a result of fission, a progressive diminution 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
Melosira subjlexilis (fig. 395, A) and M. varians (fig. 395, 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 on
the special conditions of evolution of the new frustule, by which
it is able to increase materially in size. This power of diatoms to
expand their siliceous coatings has therefore been denied by some,
who are induced to maintain this necessary consequence of the
division of encysted frustules, viz. the progressive decrease in size
of the young frustules, which would thus reach the smallest possible
dimensions. This has
led Pfitzer 1 to imagine
that when diatoms have
reached their smallest
possible dimensions by
repeated binary division,
the process of conjuga-
tion takes place between
them, resulting in the
formation of an auxo-
sjiore, capable of repro-
ducing two sporangial
frustules of considerably
larger size, which would
again give rise, by fission,
to a new series of dimi-
nishing frustules, until
these again reach their
minimum size. This
theory has, in the judg-
ment of Count Castra- Melosira suhflexilis. Melosira varians.
cane, deceived many bo-
tanists, from the idea that it was founded on actual observation, and
has at the same time been in harmony with the natural tendency to
generalisation, in attributing to the whole family of diatoms that
faculty of division which has been regarded as the universal property
of the vegetable cell. The unconfirmed auxospore theory rests on the
1 Untersuchungen iiber. Bau v. Entivickelung der Barilla) Hen, Svo, Bonn, 1371.

524 MICROSCOPIC FORMS OP VEGETABLE LIFE

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 silicious walls of diatoms are capable of
distension seems to result from the examples already given of Melosira
suhfiexilis and M. vnrian.% 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 sporangial frus-
tules of Orthosira Dickiei,1 where, in the chain of cylindrical frus-
tules of the same diameter, the sporangial frustule is dilated in its
equatorial axis, but much more so in its polar axis, pushing back the
base of the next cell and forcing 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 Tuffen West, Count
Castracane has been able to confirm by a magnificent preparation
of these diatoms in which are a number of sporangial 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 com-
mence 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-
tius reason this process cannot take place in those 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 Aster o-
larripra. That it is impossible for binary subdivision to take place
in these three classes of forms, is confirmed by the fact that, notwith-
standing 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
Diatomace^, it takes place on the same general plan as in the Des-
midiaceaj, 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
each end-valve, so that the two valves are separated by a band,
which progressively increases in breadth by addition to the free
edges of the hoops, as is well seen in fig. 396, A. In the newly
formed cell e, the two valves are in immediate apposition ; in d a
band intervenes ; in a this band has become much wider ; and in h
the increase has gone on until the original form of the cell is com-
pletely changed. At the same time the endochrome separates into two
halves * the nucleus also subdivides in the manner formerly shown

1 See Castracane, ' The Theory of the Reproduction of Diatoms,' A tti delV Accad.
Pontif. dei Nttovi Lincei, May 3i, 1874 ; and ' New Arguments to prove that Diatoms
are reproduced by means of germs,' ibid. March 19, 1876.

Plate XI

Fig. 1. Fig. 2. Fig. 3.

T)r. H. Va;V HeorcK phot. J. Maks, pliototyp

Test objects for the Microscope.

Objective by C. Zeiss, N.A. ],G0; Eye-piece 12. Monochromatic illumination by suuligbt.

REPRODUCTION OE DIATOMS

525

(fig. 368, G, H, I) ; and the primordial utricle folds in, first forming
a mere constriction, then an hour-glass contraction, and finally a
complete double partition, as in other instances. From each of its
adjacent surfaces a new silicious valve is formed, as shown at fig.
396, A, C, just as a new cellulose wall is generated in the subdivision
of other cells ; and this valve is usually the exact counterpart of the
one to which it is opposed, and forms with it a complete cell, so that
the original frustule is replaced by two frustules, each of which has
one old and one new valve, just as in Desmidiacece. Generally
speaking, the new valves are a little smaller than their predecessors ;
so that, after repeated subdivisions (as in chains of Isthmia), a
diminution of diameter becomes obvious.1 But sometimes the new
valves are a little larger than their predecessors ; so that, in the
filamentous species, there may be an increase sufficient to occasion a
gradual widening of the filament, although not perceptible except
when two contiguous frustulesare compared ; whilst, in the free forms,
frustules of different sizes
may be met with, of which
the larger are more numer-
ous than the smaller, the
increase in number having
taken place in geometrical
progression, whilst that of
size was uniform. It is not
always clear what becomes
of the 'hoop.' In Melosira
( fig. 395, A and B ), and
perhaps in the filamentous
species generally, the ' hoops '
appear to keep the new
frustules united together for
some time. This is at first
the case also in Biddulpliia
and Isthmia (fig. 408), in
which the continued connec-
tion of the two frustules by
its means gives rise to an
appearance of two complete
frustules having been de-
veloped within the original Fig. 896.— Biddulphia pulchella : A, chain of
(fig. 396, A, C) ; subse- cells in different states: a, full size; b, elon-
quently, however, the two gution preparatory to subdivision; c, forma-
„J , ' » , tion 01 two new cells; (/, e, young cells; B,

new trustules slip OUt 01 the end view; C, side view of a cell more highly
hoop, which then becomes magnified,
completely detached. The

same thing happens with many other diatoms, so that the 4 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

1 This conld not be explained on the hypothesis of the rigidity of the walls within
which fission takes place.

526

MICROSCOPIC FORMS OF VEGETABLE LIFE

whether 'it does hot become fused (as it were) into the gelatinous
envelope. During the healthy life of the diatom 1 the process of
binary division is continually being repeated ; and a very rapid multi-
plication of frustules thus takes place, all of which must be considered
to be repetitions of one and the same individual form. Hence it
may happen that myriads of frustules may be found in one locality,
uniformly distinguished by some peculiarity of form, size, or mark-
ing, which may yet have had the same remote origin as another col-
lection of frustules found in some different locality, and alike distin-
guished by some peculiarity of its own. For there is strong reason
to believe that such differences spring up among the progeny, of any
true generative act, and that when that progeny is dispersed by
currents into different localities, each will continue to multiply
its own special type so long as the process of binary division goes on.

We have seen that division is of the nature of multiplication,
and not of reproduction ; and that, where it does take place, it must
be regarded as the exception, and not as the rule. As respects
reproduction, Count Castracane, who has been an observer during
thirty years devoted to the study of diatoms, has had the oppor-
tunity of noting in what way the process differs in particular cases.
He contends that he has 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 vavians, and O'Meara in
Pleuvosigma Spencerii ; and in another case there were seen a
number of oval cysts of a species of Navicula easily recognisable.
The greater number of these were in a quiescent state ; but some
few were seen in motion by means of two flagelliform cilia ; so that
these larger or smaller cysts represented zygospores, and some of
them were shown to be zoozygospores. Castracane had the good
fortune to meet with a number of large and small oval cysts
imbedded in a gelatinous mass, all of them having in the centre two
similar corpuscles. From the condition of two greenish oblong
indistinct forms, these went on, by an easy transition, to manifest
themselves as naviculoid types, and at length developed into full-
grown frustules of Mastogloia. All this proves 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
sporules or gonids, which, after a period of repose or of incubation
inclosed within a cyst, or within a membranous frond, or within a
frustule, attain a condition for living an independent life and
reproducing in every respect the adult type of the mother-cell ; thus
the cyst, the membranous frond, or the frustule, performs the function
of a sporange. The Abbe 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,2 among the diatoms

1 This refers to those diatoms in which binary subdivision can take place.

2 See ' Observations on a Fossil Diatom in relation to the Process of Reproduction,'
Atti clelV Accad. Pontif. clei Nnovi Lincei, May 17, 1885.

PLATE XII.

Arachnoldiscus Japonicus.

CLASSIFICATION OF DIATOMS

of a marine deposit of the Miocene period, lie met with a perfect
frustule of Coscinodiscus punctatus, 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 has met
with other cases identical in character, sd 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.

"No one appears at present to have given attention to a circum-
stance described by the Abbe 1 in relation to a specimen of StriateLbi
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 always
exhibit the same arrangement of their endochrome. The attempt
has indeed been made to found the classification of diatoms on the
arrangement of the endochrome, according as it is present in the
form of plates or of granules ; thus distinguishing the placochromatic
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 Jlelosira
varians with its cell-cavity filled with endochrome, not in a condition
of unequal amorphous masses, but of uniform rounded corpuscles ;
and this demands particular attention, or at least gives good ground
for special research. A diligent examination instituted in these
cases has 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 with 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 attract the attention of some who are applying them-
selves 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 Rapladece 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 Pseudoraphidect ; while those in which
the valves have neither raphe nor its equivalent are called Crypto-
raphidece, or, better, Anaraphidece. While, therefore, in the present
state of our knowledge of diatoms, any classification can only be
regarded as provisional, we do not propose any innovation on this

1 See ' The Diatoms of the Coasts of Istria and Dalmatia,' Atti delV Accad. Pont if.
dei Nuovi Lincei, April 27 and May 25, 1873.

528 MICROSCOPIC FORMS OF VEGETABLE LIFE

point, although we ;ire disposed to accord our preference to that
suggested by H. L. Smith.

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 errone-
ously in the animal kingdom, although it affords no evidence of con-
sciousness. This power of movement, if not common to all diatoms,,
is very evident in those species .which are normally or accident-
ally free, and most conspicuously in oblong forms. In those also
which are stalked it has been noticed that if, from any cause, a
frustule becomes detached, it is 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 presented
without regard to its form. Hence those genera are not now
generally recognised which differ only in being enclosed in a mem-
branous frond, or in being stalked, especially since frustules contained,
for example, in Schizonema, 1 have been seen to escape from it, and
to be prevented from returning again to it in company with the
sister JVaviculce. Hence the genera Schizonema, Berkeley a, and
Dickiea must be reunited to JVavicula ; Cocconema, Endonema, and
Colletonenia to Cymbella ; and Homeocladia to Nitzscliia. 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 (tig. 400), in which the rod-like frustules are
seen to be continually gliding one along another, in a retrograde
direction, before they become detached — is found to be in general a
movement backwards and forwards in a straight line so far as they
meet with no impediment, while the intervention of obstacles
determines a passive change of direction. The cause of this move-
ment is uncertain ; but the most probable interpretation attributes
it to the action of the changes resulting from the nutrition of the
cell, which must necessarily absorb food in a liquid condition.
Taking account therefore of the relatively considerable quantity of
silex. necessary to the organisation of the diatom cell in proportion
to its minute dimensions, and bearing in mind, at the same time,
the incalculably small traces of silex in solution in the water, it
may be understood how active must be the exchange from the
exterior to the interior of the cell, and vice versa, and hence how
such an exchange must determine a continual change of position
backwards and forwards, through the reaction exercised on the
delicate floating frustules.

Conjugation, so far as is at present known, takes place among
the ordinary Diatomacece 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. 404), the
valves of two free and adjacent frustules separate from each other,
and the two endochromes (probably included in their primordial
utricles) are discharged ; these coalesce to form a single mass,,
which becomes enclosed in a gelatinous envelope ; and in due time

1 See Castracane, ' Observations on the Genera Homeocladia and Schizonema,' in
Atti delV Accad. Poniif. dei Nuovi Lincei, May 23, 1380

CONJUGATION OF DIATOMS

529

this 'zygospore' shapes itself into a frustule resembling that of its
parent, but of larger size. But in Epithemia (fig. 397, A, JJ), 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 sporangial 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 al-
most certain that the
contents of each zygo-
spore break up into a
brood of gonids, and
that it is from these
that the new genera-
tion originates. These
gonids, if each be sur-
rounded (as in many
other cases) by a distinct
cyst, may remain unde-
veloped for a consider-
able period ; and they
must augment consider-
ably in size before they
obtain the dimensions of
the parent frustule. It
is in this stage of the _ . . . . ■ ., .

,-, v-c Fig. 397.— Coniugation of Epithemia turcjida : A,

process that the modify- front view of single frustule . B, side view of the

ing influence of external same ; C, two frustules with their concave surfaces
agencies is most likely m close apposition; D, front view of one of the
° . /»i , ^ frustules, showing the separation of its valves ; h,

to exert its etiects ; and Fj side and front views after the formation of the
it may be easily con- zygospores,
ceived that (as in higher

plants and animals) this influence may give rise to various diversities
among the respective individuals of the same brood ; which diversi-
ties, as we have seen, will be transmitted to all the repetitions of each
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

1 See Annate of Natural History, vol. xx. ser. i. 1847, pp. 9, 343; and vol. i.
ser. ii. 1848, p. 1(51.

M M

530 MICROSCOPIC FORMS OF VEGETABLE LIFE

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 ' auxospores '

augment the size of
generation

as serving to

the cells which are to give origin to a new

takes place on a very different plan in some of those
filamentous types, such as Melosira (fig. 395, A, B), in which a strange
inequality presents itself in the diameters of the different cells of the
same filament, the larger ones being usually in various stages of
binary subdivision, by which they multiply themselves longitudinally.
According to the observations of Mr. Thwaites (loc. cit.), these also
are the products of a kind of conjugation between the adjacent cells
of the ordinary diameter, taking place before the completion of
their separation. He describes the endochrome of particular frus-

Fig. 398. — Self-conjugation (?) of Melosira italica (Aulacosira crenulata>
Thwaites): 1, simple filament; 2, filament developing auxospores; a, b, c, succes-
sive stages in the formation of auxospores ; auxospore-frustules in successive
stages, a, b, c, of multiplication.

tules, 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. 398, No. 2,
a, b, c) : around this a new envelope is developed, which may or
may not resemble that of the ordinary f rustules, but which remains
in continuity with them ; and this zygospore soon undergoes binary
subdivision (No. 3, a, b, c), the cells of the new series thus developed
presenting the character of those of the original filament (I), 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

1 See on this subject a valuable paper by Prof. W. Smith ' On the Determination
of Species in the Diatomacece,' in the Quart. Joum. of Microsc. Science, vol. hi.
1855, p. 130 ; a memoir by Prof. W. Gregory ' On Shape of Outline as a Specific
Character of Diatomacece, ' in Trans, of Microsc. Soc. 2nd series, vol. hi. 1855,
p. 10 ; and the Author's Presidential Address, in the same volume, pp. 44-50 ; ' On
Navicula crassinervis, Frustulia saxonica and N. rhomboides, as Test-objects,' by
W.H. Dallinger, Monthly Micro. Joum. 1876, vol. xvii. p. 1 ; also an Additional note
on the identity of these, by the same Author, ibid. p. 173.

MOVEMENTS OF DIATOMS

531

by a new formation of zygospores. Whether this formation par-
takes of the character of 1 conjugation ' (as supposed by Mr. Thwaites),
is still doubtful, some later observers regarding 4 auxospores ' as
simply enlarged forms of single cells.

Most of the diatoms which are not fixed by a stipe possess some
power of spontaneous movement ; and this is especially seen in those
whose frustules are of a long narrow form, such as that of the
Naviculm generally. The motion is of a peculiar kind, being usually
a series of jerks, which carry forward the frustule in the direction of
its 'length, and then carry it back through nearly the same path.
Sometimes, however, the motion is smooth and equable, and this is
especially the case with the curious Bacillaria paradoxa (fig. 400),
whose frustules slide over each other in one direction until they are
all but detached, and then slide as far in the opposite direction,
repeating this alternate movement at very regular intervals.1 In
either case the motion is obviously quite of a different nature from
that of beings possessed of a power of self-direction. ' An obstacle
in the path,' says Mr. W. Smith, 'is not avoided, but pushed aside ;
or, if it be sufficient to avert the onward course of the frustule, the
latter is detained for a time equal to that which it would have occu-
pied in its forward progression, and then retires from the impediment
as if it had accomplished its full course.' The character of the move-
ment is obviously similar to that of those motile forms of proto-
phytes which have been already described ; but it has not yet been
definitely traced to any organ of impulsion, and the cause of it is
still obscure. By Mr. W. Smith it is referred to forces operating
within the frustule, and originating in the vital operations of
growth &c. which may cause the surrounding fluid to be drawn in
through one set of apertures and expelled through the other.'2 ' If,'
as he remarks, ' the motion be produced by the exosmose taking place
alternately at one and the other extremity, while endosmose is
proceeding at the other, an alternating movement would be the
result in frustules of a linear form ; whilst in others of an elliptical
or orbicular outline, in which foramina exist along the entire line of
suture, the movements, if any, must be irregular or slowly lateral.
Such is precisely the case. The backward and forward movements
of the JSfaviculcE have been already described ; in Surirella (fig. 404)
and Campylodiscus (fig. 405) the motion never proceeds further than
a languid roll from one side to the other ; and in Gomphonema
(fig. 414), 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.'

1 This curious phenomenon the Author has himself repeatedly had the opportunity
of witnessing.

2 It has been objected to this view, by the authors of the Micro-graphic Dic-
tionary, that, if such were the case, the like movements would be frequently met
with in other minute unicellular organisms. But there are no other such organisms
in which the cell is almost entirely enclosed in an impermeable envelope, so that the
imbibition and expulsion of fluid are limited to a small number of definite points,,
instead of being allowed to take place equally (as in other unicellular organisms) over
the entire surface. See Mereschkowski in Journ. Boy. Microsc. Soc. vol. i. ser. ii.
1881, p. 102.

31 M 2

532 MICROSCOPIC FORMS OF VEGETABLE LIFE

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 pro-
portion of diatoms, and but little in any of them — shall have been
thoroughly followed out. The observations of Focke 1 render it
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 Diatomacece, the method of Professor Kiitzing, which

Fig. 399. — Meridion circulare. Fig. 400. — Bacillaria paradox a.

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.2 In each family the frustules may exist under
four conditions : (a) free, the binary division being entire, so that the
frustules separate as soon as the process has been completed ; (b}
stipitate, the frustules being implanted upon a common stem (fig.
401), which keeps them in mutual connection after they have them-
selves undergone a complete binary division ; (c) united in a filament,.

1 According to this observer {Ann. of Nat. Hist. 2nd series, vol. xv. 1855, p. 237)
Navicida bifrons forms, by the spontaneous fission of its internal substance, spherical
bodies, which, like gemmules, give rise to Surirella microcbra. These by conjuga-
tion produce N. sjplendida, which gives rise to N. bifrons by the same process. He is
only able to speak positively, however, as to the production of N. bifrons from N. splen-
dida • that of Surirella microcora from N. bifrons, and that of N. splendida from
Surirella microcora , being matters of inference from the phenomena witnessed by him.

2 The method of Kiitzing was the one followed, with some modification, by Mr. Ealfs
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.

DIATOMACEJE: EUXOTIEJE, MEKIDIEiE

533

which will be continuous (fig. 396 A, B,) if the cohesion extend to
the entire surfaces of the sides of the frustules, but may be a mere
zigzag chain (tig. 402) if the cohesion be limited to their angles ; (d)
aggregated into a frond (tig. 415), which consists of numerous frustules '
more or less regularly enclosed in a gelatinous investment.

Commencing with the last-named division (A), the first family
is that of Eunotiece, of which we have already seen a characteristic
example in Epithemia turgida (tig. 397). The essential characters
of this family consist in the more or less lunate form of the frustules
in the lateral view (fig. 397, B), and in the strire being continuous
across the valves without any interruption by a longitudinal line, In
the genus Eunotia the frustules are free ; in Epithemia they are very
commonly adherent by the flat or concave surface of the connecting
zone ; and in Himantidium they are usually united into ribbon-like
filaments. In the family Jleridiece 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 beautiful
Meridian circidare (fig.
399). Although this plant,
when gathered and placed
under the microscope, pre-
sents the appearance of
circles overlying one an-
other, it really grows in a
helicoid (screw-like) form,
making several continuous
turns. This diatom abounds
in many localities in this
country ; but there is none
in which it presents itself
in such rich luxuriance as
in the mountain -brooks
about West Point in the
United States, the bottoms of which, according to Professor Bailey,
4 are literally covered in the first warm days of spring with a ferru-
ginous-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 filamentous body
covered in this way is often very elegant.' The frustules of Mt ridion
are attached when young to a gelatinous cushion ; but this disappears
with the advance of age. In the family Licmophorece also the frustules

Fig. 401. — Licmoj)liora Jiabellata.

534

MICROSCOPIC FORMS OF VEGETABLE LIFE

are wedge-shaped ; in some genera they have transverse markings,
whilst in others these are dehcient ; but in most instances there are
to be observed two longitudinal suture-like lines on each valve (which,
have received the special designation of vittce) connecting their two
extremities. The newly formed part of the stipe in the genus.
Licmophora, instead of itself becoming double with each act of
binary division of the frustule, increases in breadth, while the frus-
tules themselves remain coherent, so that a beautiful fan-like arrange-
ment is produced (fig. 401). 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 flabellse or single frustules irregularly

scattered throughout the

entire
stalk.

length of the foot-
This beautiful plant

is marine, and

is attached
and zoo-

to seaweeds
phytes.

In the next family, that
of Fragilariece, the frus-
tules 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 transversely striated,
with a central nodule
when striae are present, they
run across the valves with-
out interruption. To this
family belongs the genus
Diatoma, which gives its
name to the entire group,
that name (which means
cutting through) being sug-
gested by the curious habit
of the genus, in which the
frustules, after division,
separate from each other
along their lines of junction, but remain connected at their angles,
so as to form zigzag chains (fig. 402). The valves of Diatoma, when
turned sideways (a), are seen to be strongly marked by transverse
stria*, which extend into the front view. The proportion between
the length and the breadth of each valve is found to ,vary so con-
siderably 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
streams, in which it is sometimes very abundant. The
Fragilaria is nearly allied to Diatoma, the difference

Fig. 402.

Fis

4°2

Fig. 402. — Diatoma vulgare : a, side view of
frustule ; b, frustule undergoing division.

Fig. 403. — Grammatophora serpentina : a, front
and side views of single frustule ; b, b, front and
end views of divided frustule ; c, frustule about
to undergo division; d, frustule completely
divided.

r unmng
genus

DIATOMACEJE : FRAGILARIEiE, SURIRELLECE S3S

B

C

between them consisting chiefly in the mode of adhesion of the
frustules, which in Fragihtria form long, straight filaments with
parallel sides ; the filaments, however, as the name of the genus im-
plies, 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
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 K. sigmoidea, which has been
used as a test object, but is not suitable for that purpose on account
of the extreme variability of its striation. Nearly allied to this is
the genus Bacittaria, so named from the elongated stafl'-like form of
its frustules ; its valves have a longitudinal punctated keel, and
their transverse stride are interrupted in the median line. The
principal species of this genus is the B. paradoxa, whose remarkable
movement has been already described. Owing to this displacement
of the frustules, its filaments seldom present themselves with straight
parallel sides, but nearly always in forms more or less oblique, such
as those represented in
fig. 400. This curious
object is an inhabitant
of salt or of brackish
water. Many of the
species formerly ranked
under this genus are now

genus
genera

referred to the
Diatoma. The
Nitzschia and Baxsillaria
have been associated by
Mr. Ealfs 1 with some
other genera which agree
with them in the bacillar
or staff-like form of the
frustules and in the pre-
sence of a longitudinal
keel, in the sub-family Kitzschieo\ which ranks as a section of
the Survrellece. Another sub-family, Synedrecv, consists 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 Surirellece proper the frustules are no longer bacillar,
and the breadth of the valves is usually (though not always) greater
than the front view. The distinctive character of the genus
Surirella, in addition to the presence of the supposed ' canaliculi,'
is derived from the longitudinal line down the centre of each valve
(fig. 404, A) and the prolongation of the margins into ' ala?.' Numerous
species are known, which are mostly of a somewhat ovate form,

i See Pritchard's Infusoria, 4th ed. p. 940. The genus Nitzschia was in the
first instance placed by Mr. Ralfs in the family Fragtlarieee, and the genus Bacil-
laria in the family Surirellece.

Fig. .404. — Surirella constricia : A, side view;
B, front view ; C, binary subdivision.

53^

MICROSCOPIC FORMS OF VEGETABLE LIFE

some being broader and others narrower than S. constricta ; the
greater part of them are inhabitants of fresh or brackish water,
though some few are marine ; and several occur in those infusorial
earths which seem to have been deposited at the bottoms of lakes,
such as that of the Mourne Mountains in Ireland (fig. 419, b, c, k).
In the genus Campylodixcus (fig. 405) the valves are so greatly in-
creased 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-
liculi ' are most developed, and it is consequently here that they may
be best studied ; and of there being here really costce, or internally
projecting ribs, no reasonable doubt can remain after examination
of them under the binocular microscope, especially with the ' black-
ground ' illumination. The form of the valves in most of the species
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

Fig. 405. — Campylocliscus costatus : A, front view; B, side view.

infusorial stratum discovered by Professor Ehrenberg at Soos, near
Ezer, in Bohemia, that the earth seems almost entirely composed of it.

The next family, Striatellece, forms a very distinct group, differ-
entiated from every other by having longitudinal costse on the
connecting portions of the frustules, these costoe 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
valves in the act of division, and on each repetition of such pro-
duction, being thus always definite in number ; whilst in other
cases the formation of the septa is continued after the production
of the valves, and is repeated an uncertain number of times before
the recurrence of a new valve-production, so that the annuli are
indefinite in number. In the curious Grammatophora serpentina
(fig. 403) 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

MATOMACEJE: STRIA TELLE iE, MELOSIK K . I \ 537

Grammatoplwra are very finely striated, and some species, as G.
subtilissima and G. marina, are used as test-objects. The frustules
in most of the genera of this family separate into zigzag chains, as
in Diatoma ; but in a few instances they cohere into a filament, and
still more rarely they are furnished with a stipe. The small 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 frustules, which appear in the front view as in Biddulphieas.
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
convexity of the valves and the breadth of the intervening hooped
band, the frustules may have the form either of thin discs, short
cylinders, bi-convex 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 Coscino-
discece, 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 differ-
ence, however, lies in this, that the frustules of the C oscinodiscece
are always free, whilst those of the Melosirece remain coherent into
filaments, which often so strongly resemble those of the simple
Confervacece as to be readily distinguishable only by the effect of
heat. Of these last the most important genus is Melosira (fig. 395).
Some of its species are marine, others fresh -water ; one of the latter,
M. ochracea, seems to grow best in boggy pools containing a fer-
ruginous 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. 419, a, a, d, d). The meaning of the remarkable
difference in the sizes and forms of the frustules of the same
filaments (fig. 395) has not yet been fully ascertained. The sides of
the valves are often marked with radiating stride (fig. 419, d, d) ;
and in some species they have toothed or serrated margins, by which
the frustules lock together. To this family belongs the genus Hyalo-
discus, 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

538

MICROSCOPIC FORMS OF VEGETABLE LIFE

valves in certain fossil deposits (fig. 418, a, a, a), 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 Melosira (fig. 395, B). The regularity of the
hexagonal areolation shown by its valves renders them beautiful
microscopic objects : in some species the areola are smallest near
the centre, and gradually increase in size towards the margin ; in
others a few of the central areolae are the largest, and the rest are
of nearly uniform size ; while in others, again, there are radiating
lines formed by areolae of a size different from the rest. Most of
the species are either marine or are inhabitants of brackish water ;
when living they are most commonly found adherent to sea-weeds
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

Fig. 406. — Structure of siliceous valve of Coscinodiscus oculus iriclis : 1, hexagonal
areola of inner or ' eye-spot ' layer; 2, areola of outer layer.

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
investigations of Mr. J. W. Stephenson 1 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
markings 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 suc-
ceeded in separating portions of the two layers, so that each could
be examined singly. He also mounted them in bisulphide of
carbon, the refractive index of which is high ; and also in a
solution of phosphorus in bisulphide of carbon, which has a still
higher refractive index. If we suppose a diatom to be marked with
convex depressions, they would act as concave lenses in air, which
is less refractive than their own silex ; but when such lenses are
immersed in bisulphide of carbon, or in the phosphorus solution,
they would be converted into convex lenses of the more refractive
substance, and have their action in air reversed. Analogous but

1 Monthly Microscojrical Journal, vol. x. 1878, p. 1.

DIATOMACEiE : COSCINODLSCEiK

539'

opposite changes must take place when convex diatom-lenses are
viewed first in air, and then in the more refractive media. Applying
these and others tests to Coscinodiscus oculus iridic, Mr. Stephen-
son considered both layers to be composed of hexagons, represented
in fig. 406, from drawings by Mr. Stewart. The upper layer is
much stronger and thicker than the lower one, and the framework
of its hexagons more readily exhibits its beaded appearance. The
lower layer is nearly transparent, and little conspicuous when seen
in bisulphide of carbon, except as shown in the figure, when the
framework of the hexagons and the rings in the midst of them
appear thickened 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 poly-
cystina. 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 protruded through them.

The genus Actinocydus 1 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 by
Professor Ehrenberg on minute differences presented by the rays as
to number and distribution ; but since scarcely two specimens can
be found in which 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 Asterol 'ampr a and Asteromphalus,
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 Astero-
lampra all the compartments are similar and equidistant and the rays
equal, whilst in Aster omplicd 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 umbilicus.
The eccentricity thus produced in the other rays has been made the
basis of another generic designation, Spatcmgidium ; but it may be
doubted whether this is founded on a valid distinction.2 These

1 The Author concurs with Mr. Ralfs in thinking it preferable to limit the genus
Actinocydus to the forms originally included in it by Ehrenberg, and to restore the
genus Actinojityclius of Ehrenberg, which had been improperly united with Actino-
cyclus by Professors Kiitzing and W. Smith.

2 See Greville in Quart. Journ. Microsc. Science, vol. vii. 1859, p. 158; and in
Trans. Microsc. Soc. vol. viii. n.s. 18G0, p. 102, and vol. x. 1862, p. 41 ; also.
Wallich in the same Transactions, vol. viii. 1860, p. 44.

54Q

MICROSCOPIC FORMS OF VEGETABLE LIFE

beautiful discs are for the most part obtainable from guano, and
from soundings in tropical and antarctic seas. From these we pass
on to the genus Actinoptychus (fig. 407), 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 (B), 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
difference of aspect which the different radial divisions present in
fig. 407 is simply due to the fact that one set is out of focus whilst
the other is in it, since the appearances are reversed by merely
altering the focal adjustment. The number of radial divisions has
been considered a character of sufficient importance to serve for the
distinction of species ; but this is probably subject to variation ;
since we not unfrequently meet with discs, of which one has (say)
■eight, and another ten such divisions, but which are precisely alike in

every other particular. The
valves of this genus also are
very abundant in the infusorial
earth of Richmond, Bermuda,
and Oran (fig. 418, b, b, 6), 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 sea-weeds
or zoophytes.

The Bermuda earth also contains the very beautiful form
which, though scarcely separable from Actinoptychus except by
its marginal spines, has received from Professor Ehrenberg the dis-
tinctive appellation of Heliopelta (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 areola? ; 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 areolation is
here displayed ; and it hence appears probable 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

1 It is stated by Mr. Stodder (Quart. Journ. Microsc. Science, vol. hi. n.s.
1863, p. 215) that not only has he seen, in broken specimens, the inner granulated
plate projecting beyond the outer, but that he has found the inner plate altogether
separated from the outer. The Author is indebted to this gentleman for pointing
out that his figure represents the inner surface of the valve.

Fig. 407. — Actinoptychus undulatus:
A, side view ; B, front view.

DIATOMACEJE : COSCIXODLSCEiE, BIDDULPHIEiE 54 r

dark centre, appear to be solid areolations of silex not traversed
by markings, as in many other diatoms ; they are apparently
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
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 sea-weeds from different parts of the world
(especially to a species employed by the Japanese in making soup),
is the Arachnoid he us (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 \- or -i-inch objective ; or by look-
ing at a valve as an opaque object (either by the parabolic illu-
minator, or by the Lieberkiihn, or by a side light) with a T*r
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 illumination.2

This family is connected with the succeeding by the small group
Wupodiscece, the members of which agree with the Coscinodiseeee in
the general character of their discoid frustules, and with the Bid-
did 'phiece in having areolar processes on their lateral surfaces. In
the beautiful Aulacodiscus these areolations are situated near the
margin, and are connected with bands radiating from the centre : tho
surface also is frequently inflated in a manner that reminds us of
Actinoptychus. These forms are for the most part obtained from
guano.

The members of the next family, Biddulphiece, 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

1 Trans. ITicrosc. Society, 1st series, vol. iii. p. 49.

2 These valves afford admirable objects for showing the ' cor.vers:on of relief ' in
Nachet's stereo-pseudoscopic microscope (p. 97).

542 MICROSCOPIC FORMS OF VEGETABLE LIFE

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 Biddulphia (fig. 396) 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 zigzag chain. They are marked externally by ribbings which seem
to be indicative of internal costce. partially subdividing the cavity.
Nearly allied to this is the beautiful genus Isthmia (fig. 408), in
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

sea-weeds 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 frustules (as at
6, fig. 408), until they have separated from
each other, but, after such separation, re-
mains for a time round one of the frustules,
so as to give it a truncated appearance
(a, a).

The family Angidiferce, distinguished
by the angular form of its valves in their
lateral aspect, is in many respects closely
allied to the preceding ; but in the com-
parative flattening of their valves its
members more resemble the Cosci?iodiscece
and Uupodiscece. Of this family we have
a characteristic example in the genus Tri-
ceratium, of which striking form a con-
siderable number of species are met with
in the Bermuda and other infusorial
earths, while others are inhabitants of the
Fig. 408.— Isthmia nervosa, existing ocean and of tidal rivers. The

T. favus (fig. 393), which is one of the
largest and most regularly marked of any of these, occurs in the mud
of the Thames and in various other estuaries on our own coast ; it
has been found, also, on the surface of large sea-shells from various
parts of the world, such as those of Hipp>op>us and Haliotis, before
they have been cleaned ; and it presents itself likewise in the infu-
sorial 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 eleva-
tions. 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 quadrangular and even
pentagonal forms, these being marked as varieties by their exact
•correspondence in sculpture, colour, &c. with the normal triangular

DlATOMACEJE : AXGULIFEE AZ, CILETOCEEE M

543

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 Triceratium from those included in
the genus Amphitetras, 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 Chcetocerece, some
of the filamentous types of which seem also allied to the Melosirew.
The peculiar distinction of this group consists in the presence of
tubular 1 awns,' frequently proceeding from the connecting hoop,
sometimes spinous and serrated, and often of great length (fig. 409) ;

Fig. 409. — Chcetoceros Wighamii : a, front Fig. 410. — Bacteriastrum

view, and b, side view of frustule ; c, side furcatum.
view of connecting hoop and awns ;
d, entire filament.

by the interlacing of which the frustules are united into filaments
whose continuity, however, is easily broken. In the genus Bacterias-
trum (fig. 410) 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 Bhizosolenia, of which several species are distinguished by the
extraordinary length of the frustule (which may be from six to
twenty times its breadth), giving it the aspect of a filament (fig. 411),
by a transverse annulation that imparts to this filament a jointed
appearance, and by the termination of the frustule at each end in a

1 See Mr. Brightwell's excellent memoirs ' On the Genus Triceratium ' in Quari.
Journ. Microsc. Science, vol. i. 1853, p. 245 ; vol. iv. 1856, p. 272 ; vol. vi. 1858,
p. 153; also Wallich in the same Journal, vol. iv. 1858, p. 242; and Greville in
Trans. Microsc. Soc. n.s. vol. ix. 1861, pp. 43, 69.

544 MICROSCOPIC FORMS OF VEGETABLE LIFE

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?,
Holothurise, and other marine animals.1

The second principal division (B) of the Diatomacece consists, it
will be remembered, of those in which the frustules have a median
longitudinal line and a central nodule. In the first of the families
which it includes, that of Cocconeidew, the central nodule is obscure

Fig. 411. — Bhizo- Fig. 412. — Achnanthes Fig. 413. — Gomphonema gemi-
solenia imbri- longipes : a, b, c, d, e, natiim : its frustules connected by

cata. frustules in different a dichotomous stipe,

stages of binary divi-
sion.

or altogether wanting on one of the valves, which is distinguished
as 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 adherent
to each other. The frustules in this genus are frequently invested by
a membranous envelope which forms a border to them ; but this

1 See Brightwell in Quart. Journ. Microsc. Science, vol. iv. 1856, p. 105 ;
vol. vi. 1858, p. 93 ; Wallich in Trans. Microsc. Soc. n.s. vol. viii. 1800, p. 48 ;
and West, in the same, p. 151.

DIATOMACEJE : ACHXAXTHE^E, GO MPHOXEME.E

seems to belong to the immature state, subsequently disappearing
more or less completely.

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
(tig. 412), which is often found growing on marine algae. The
difference between the markings of the upper and lower valves is
here distinctly seen ; for while both are traversed by striae, which
are resolvable under a sufficient power into rows of dots, as well as
by a longitudinal line which sometimes has a nodule at each end
(as in JVavicala), the lower valve (a) has also a transverse line, form-
ing a stauros, 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. 412 it not
only holds together the
two new frustules resul-
ting from the subdivi-
sion of the lowest cell, a,
which are not yet com-
pletely separated the one
from the other, but it may
be observed to invest the
two frustules b and c, which

have not merely separated, FiG iU._Gomphonema gemimUinh more u u
but are themselves begin- magnified : A, side view of frustule ; B, front view ;
ning to undergo binary sub- C, frustule in the act of division,
division ; and it may also

be perceived to invest the frustule d, from which the frustule e, being
the terminal one, has more completely freed itself.

In the family Cymbellece, on the other hand, both valves possess
the longitudinal line with a nodule in the middle of its length ; but
the valves have the general form of those of the Eunotiew, and the
line is so much nearer one margin than the other that the nodule
is sometimes rather marginal than central, as we see in Cocconema
(fig. 419,/).

The Gomphonemece, like the Meridiece and Licmophorecr, have
frustules which are cuneate or wedge-shaped in their front view (figs.
413, 414), but are distinguished from those forms by the presence of
the longitudinal line and central nodule. Although there are some
free forms in this family, the greater part of them, included in the
genus Gomphonema, have their frustules either affixed at their bases
or attached to a stipe. This stipe seems to be formed by an exu-
dation 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 frustules ceases ; but when

N H

546 MICROSCOPIC FORMS OF VEGETABLE LIFE

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 can take place. It is in this manner
that the dichotomous character is given to the entire stipe (fig. 413).
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 Naviculecz, the members
of which are distinguished by the symmetry of their frustules, as
well in the lateral as in the front view, and by the presence of a
median longitudinal line and central nodule in both valves, -In the
genus Navicula and its allies the frustules are free or simply
adherent to each other ; while in another large section they are in-
cluded within a gelatinous envelope, or are enclosed in a defi-
nite tubular or gelatinous frond. Of the genus Navicula 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 Pleurosignia, which is now generally adopted ;
but his separation of another set of species under the name Pinnu-
laria (which had been previously applied by Ehrenberg to designate
the striated species), on the ground that its strife (costse) 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 striae and coste?. Mr. Slack has
since given an account of the resolution of the so-called costse of
twelve species of Pinnularice into ' beaded ' structures. 1 The beauti-
ful genus Stauroneis, which belongs to the same group, differs from
all the preceding forms in having the central nodule of each valve
dilated laterally into a band free from stria?, which forms a cross
with the longitudinal band. The multitudinous species of the genus
Navicula are for the most part inhabitants of fresh water ; and they
constitute a large part of most of the so-called 'infusorial earths' which
were deposited at the bottoms of lakes. Among the most remarkable
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 Navicular and Pinnulariw.
The Polierschiefer, or 'polishing slate,' of Bilin in Bohemia, the
powder of which is largely used in Germany for the same purpose,
and which also furnishes the fine sand used for the most delicate
castings in iron, occurs in a series of beds averaging fourteen feet in
thickness, and these present appearances which indicate that they
nave 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 aggregation
of frustules of Navicuke &c. which have been consolidated by heat.
The species of Pleurosigma, on the other hand, are for the most part
either marine or are inhabitants of brackish water, and they compara-
tively 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.'
1 Monthly Microscopical Journal, vol. vi. 1871, p. 71.

DIATOMA CEM

SCHIZONEME^S

547

Of the members of the sub-family Schizonemea;, consisting of
those JSfaviculece in which the frustules are united by a gelatinous
envelope, some are remarkable for the great external resemblance
they bear to acknowledged algse. This is especially the case with
the genus 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
iile or without any definite arrangement (fig. 415), all these frustules
having arisen from the binary division of one individual. In the genus
Mastogloia, which is specially distinguished by having the annulus
furnished with internal costye projecting into the cavity of the frus-
tule, each frustule is separately supported on a gelatinous cushion
/(fig. 416, B), which may itself be either borne on a branching stipe

Fig. 415. — Schizonema Grevillii : A, natural size ; B, portion magnified five
diameters ; C, filament magnified 100 diameters ; D, single frustule.

>(A), or may be aggregated with others into an indefinite mass (fig.
417). The careful study of these composite forms is a matter of
great importance, since it enables us to bring into comparison with
•each other great numbers of frustules which have unquestionably
a common descent, and which must therefore be accounted as of the
same species, and thus to obtain an idea of the range of variation,
prevailing in this group, without a knowledge of which specific defini-
tion is altogether unsafe. Of the very strongly marked varieties which
may occur within the limits of a single species, we have an example
in the valves C, D, E, F (fig. 416), 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

n n 2

548 MICKOSCOPIC FORMS OF VEGETABLE LIFE

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 ex-
plored localities ; such differences as have already been pointed out
being, probably, in a large proportion of cases, the result of the mul-
tiplication 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 any
department of natural history, the more does it prove that the ranger

Fig. 416.

Fig. 41G. — 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. 417. — Mastogloia lanceolata.

of variation is far greater than had been previously imagined y
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
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 ;.

DISTRIBUTION OF DIATOMS

549

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
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 search and furnish specimens
of the tribe.' Such is their abundance in some rivers and estuaries,
that their multiplication is affirmed by Professor Ehrenberg 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. D. Hooker upon the Diatomacea- of the southern
seas ; for within the Antarctic Circle they are rendered peculiarly
conspicuous by becoming enclosed in the newly formed ice, and by
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 commu-
nication between the ocean waters and the bowels of a volcano, such
as there are other reasons for believing to be occasionally formed,
would account for the presence of Diatomacece 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 re-
spiration and decomposition would be continually imparting to them.
It is interesting to observe that some species of marine diatoms are
found through every degree of latitude between Spitzbergen and
Victoria Land, whilst others seem limited to particular regions. One
of the most singular instances of the preservation of diatomaceous
forms is their existence in guano, into which they must have passed
from the intestinal canals of the birds of whose accumulated excre-
ment that substance is composed, those birds having received them, it

55o

MICROSCOPIC POEMS OP VEGETABLE LIFE

is probable, from shell-fish, to which these minute organisms serve as-
ordinary food.

The indestructible nature of the silicified casings of Biatomacece
has also served to perpetuate their presence in numerous localities
from which their living forms have long since disappeared ; for the
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-

Fig. 418. — Fossil Diatomaceae &c. from Oran : a, a, a, Coscinocliscus ; b, b, b,.
Actinocyclus ; c, Dictyochya fibula; d, Lithasteriscus radiatus; e, Spongolithis
acicularis ; /, /, Grammatophora yparallela (side view) ; g, g, Grammatojphora
angulosa (front view).

nate, in the neighbourhood of the Mediterranean, with calcareous
strata chiefly formed of Foraminifera, the whole series being the re-
presentative of the chalk formation of Northern Europe, in which
the silex that was probably deposited at first in this form has under-
gone conversion into flint, by agencies hereafter to be considered*
Of the diatomaceous composition of these strata we have a character-
istic example in fig. 418, which represents the fossil Diatomacece of
Oran in Algeria. The so-called ' infusorial earth ' of Richmond in
Virginia, as well as 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

DISTRIBUTION OF DIATOMS

551

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. 419), 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 berg-
meld "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

Fig. 419. — Fossil Diatomacece Sec. from Mourne Mountains, Ireland : a, a, a,
GaiUonella [Melosira) procera and G. granuJata ; d, d, d, G. biseriata (side view) ;
b, b, Surirella plicata ; c, S. craticula ; k, S. caledcnica ; e, Gomphonema gracile ;
/, Cocconema fusidium; g, Tabellaria vulgaris ; h, Pinnularia dactijlus ; i, P .
nobilis; I, Synedra ulna.

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, 1 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 decaved vegetation floating on the
surface of the water. Their colour is usually a yellowish-brown of a

552 MICROSCOPIC FORMS OF VEGETABLE LIFE

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 are held in
suspension by it, only subsiding after some little time has elapsed.'
Notwithstanding every care, the collected specimens are liable to be
mixed with much foreign matter ; this may be partly got rid of by
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 suf-
ficient 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 sea-weeds, and in the fine mud or sand of soundings or
dredgings ; they are frequently found also, in considerable numbers,
in the stomachs of Holothurise, Asciclians, 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 animals
that may fall in his way, rare and beautiful forms having been
obtained from the interior of JVoctiluca. 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 follow-
ing 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 (muriatic) 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

COLLECTION AND MOUNTING OF DIATOMS

553

sand in the earth or guano, this sediment will consist almost entirely
of Diatomacece, 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 stirring 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
pour.ed off ; and this process may be repeated three or four times at
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 higher powers — a separation which
is attended with great convenience.1 It sometimes happens that
fossilised diatoms are so strongly united to each other by siliceous
cement as not to be separable by ordinary methods ; in this case,
small lumps of the deposit should be boiled for a short time in a weak
alkaline solution, which will act upon this cement more readily than
on the siliceous frustules ; and as soon as the lump is softened, so as to
crumble to mud, this must be immediately washed, in a large quantity
of water, and then treated in the 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 Diatomacece 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 back-ground 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

1 A somewhat more complicated method of applying the same principle is described
by fMr. Okeden in the Quart. Journal Microsc. Science, vol. iii. 1855, p. 158.
The Author believes, however, that the method above described will answer every
jmrpose.

2 For other methods of cleaning and preparing diatoms, see Quart. Journ. of
Microsc. Science, vol. vii. 1859, p. 167, and vol. i. n.s. 1861, p. 143; and Trans, of
Microsc. Soc. vol. xi. n.s. 1863, p. 4. A little book entitled Practical Directions
for Collecting, Preserving, Transporting, Preparing, and Mounting Diatoms (New
York, 1877), containing papers by Professors A. Mead Edwards, Christopher Johnson,
and Hamilton L. Smith, will be found to contain much useful information.

554

MICROSCOPIC FORMS OF VEGETABLE LIFE

residue) ; and the sediment, after being washed, should be boiled in
water with a small piece of soap, whereby the diatoms will be cleansed
from the flocculent matter which they often obstinately retain.1
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, objec-
tives of very large aperture are required, and all the improvements
which have recently been introduced in the construction and mode
of using the sub-stage condenser require to be put into practice.
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 as can be
made amenable to the powers possessed by our best recent optical
appliances ; and for the leisure of a professional or commercial man
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 may be readily distin-
guished under a simple microscope may be taken up by a camel's-
hair pencil which has been so trimmed as to leave two or three hairs
projecting beyond the rest. But the 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 parts separate
from each other to an amount proportionate to the pressure ; and, on
being brought up to the object, first pushed to the edge of the fluid
on the slide, may generally be made to seize it. A very experienced
American diatomist, Professor Hamilton Smith, strongly recom-
mends a thread of glass drawn out to capillary fineness and flexibility,
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 finger ' which have
been devised by American diatomists.2

Phseosporese. — The greater number of the sea-weeds exhibit a
higher type of organisation than any that has hitherto been described.
The old classification of sea-weeds into Melanosporece, Rhodosporece,
and Chlorosporece, according as their colouring matter is olive-brown,
red, or green, cannot altogether be retained. Under the head of
Phceosporece is now included a very large number of the brown ano!

1 See Prof. H. L. Smith in Amer. Journ. of Microscopy, vol. v. 1880, p. 257.
It is important that the soap should be free from kaolin, silex, or any other insoluble
matter.

2 For a description of those of Prof. Hamilton Smith and Dr. Eezner, see Journ.
of Boy. Microsc. Soc. vol. ii. 1879, p. 951 ; and that of Mr. Veeder, vol. iii. 1880,
p. 700, of the same Journal.

PHJEOSPOREJE

555

olive-brown sea-weeds. In ascending this series we shall have to
notice a gradual differentiation of organs, those set apart for
reproduction being in the first place separated from those appro-
priated 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 sea-weeds, such as the common Fucus and Laminaria, we
observe a certain foreshadowing of the distinction between root,
stem, and leaf, this distinction is very imperfectly carried out, the
root-like and stem-like portions serving for little else than the
mechanical attachment of the leaf -like part of the plant. There is
not yet any departure from the simple cellular type of 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 fronds. The cells of the Plueosporeai contain a substance
closely resembling starch, and an olive-brown pigment, which they
share with the Fucacece, known as phycophwin. The group of olive-
green sea-weeds presents us with the lowest type in the family
FctocarjXLcece, 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 sea-
weed, which is very commonly found growing upon larger alga?, 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
that seems to have suggested the name of the genus (from the Greek
o-^aKeAo?, gangrene). This apparent decay possibly consists in the
resolution of the endochrome of the terminal cells into motile bi-
ciliated antherozoids, which, when mature, escape by an opening with
a long tubular neck, which forms itself in the wall of the sphacele.
The same happens with the terminal cells of the peculiar lateral
branchlets, which are known as propagative buds. J anczewski, how-
ever, believes that these so-called antherozoids are really the zoospores
of parasitic fungi, belonging to the family Chytridiacece, with which
the sphaceles of the Sphacelariaceo3 are liable to be infested. It is
doubtful whether there is any true process of sexual reproduction in

556

MICROSCOPIC FORMS OF VEGETABLE LIFE

the Sphacela/riacece. The ordinary mode of propagation of the
Pkceosporece is by non-sexual zoospores ; and these are of two kinds,
produced respectively in unilocular and multilocular zoosporanges.
The former are comparatively large, nearly spherical, ovoid or pear-
shaped cells, the contents of which break up into a large number of
zoospores. The multilocular zoosporanges have the appearance of
jointed hairs, and are divided internally into a number of chambers,
each of which gives birth to a single zoospore. The zoospores from
the unilocular sporanges appear in all cases to germinate directly,
while those from the multilocular sporanges sometimes coalesce in
pairs before germinating. The different families of Phceosporece
present a most interesting gradual transition from the conjugation of
swarm-cells to the impregnation of a female 'oosphere' by male anthe-

rozoids. In Ectocarpus, Giraudia, and
Scytosijyhon, conjugation takes place be-
tween swarm-cells from the multilo-
cular sporanges which appear to be
exactly alike, but a slight differentia-
tion is exhibited in one of them coming
to rest and partially losing its cilia
before conjugation takes place (fig. 420,
II). In Cutleria and Zanardinia the
differentiation is more complete. The
male and female swarm-cells are pro-
duced 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 reproduc-
tive bodies are true ' oospheres,' being
from the first motionless masses of pro-
toplasm not provided with cilia. In the
family Laminariacece, belonging to the
Phceosporece, are included many of the
largest of the sea-weeds, chiefly natives
of southern seas, the frond often attain-
ing enormous dimensions, and exhibiting rudimentary differentiation
into rhizoids or organs of attachment, stem, and leaves. Such are
Lessonia, which stows to a great height and resembles a branching tree
with pendent leaves two or three feet long ; Macrocystis, where the
stalk-like base of each branch of the leaf is hollowed out into a large
pear-shaped air-bladder ; JVereocystis, Laminaria, and others.

In the Fucacese the generative apparatus is contained in the
globular ' conceptacles,' which are usually sunk in the tissue near the
extremities of the fronds. In some species, as Fucus platycarpus,

m

Fig. 420. — Process of conjugation
in Ectocarpits siliculosus. (From
Vines's 'Physiology.')

oogones ;

the same conceptacles contain both ' antherids ' and
others these two sexual elements are disposed in different conceptacles
on the same plant ; whilst in the commonest of all, F. vesiculosus (blad-
der-wrack), they are limited to different individuals. When a section

FUCACEjE

557

is made through one of the flattened conceptacles of F. platycarpus,
its interior is seen to be a nearly globular cavity (fig. 421), 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. 422, A), the antherids, whose granular contents
acquire an orange hue, and gradually shape themselves into oval
bodies (B), each with an orange- coloured spot and two long
vibratile cilia, which, when discharged by the rupture of the con-
taining cell, have for a time a rapid, undulatory motion whereby

Fig. 421. — Vertical section of conceptacle of Fucus plat y carpus lined with filaments;
among which lie the antheridial cells and the oogones containing oospheres.

these ' antherozoids ' are diffused through the surrounding liquid.
Lying amidst the mass of hairs, near the walls of the cavity, are-
seen (fig. 421) numerous dark pear-shaped bodies, which are the
oogones, 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
communicate a rotatory motion by the vibration of their own cilia.
In the hermaphrodite Fuci this takes place within the conceptacles,

558 MICROSCOPIC FORMS OF VEGETABLE LIFE

: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 oogones also discharge their oospheres, which,
meeting with antherozoids, are fecundated by them. The fertilised
oospores soon acquire a new and firm envelope ; and, under favour-
able circumstances, they speedily begin to develop themselves into
new plants. The first change is 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 par-
tition is speedily observable in its interior, its single cell being sub-
divided into two ; and by a continuation of a like process of biparti-

Fig. 422. — Antlierids and antherozoids of Fucus platycarjius : A, branching
articulated hairs, detached from the walls of the conceptacle, bearing antherids in
different stages of development ; B, antherozoids, some of them free, others still
included in their antheridial cells.

tion, first a filament and then a frondose expansion is produced, which
gradually evolves itself 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
antherozoids, set free from the orange-yellow (male) conceptacles,
be mingled with the fluid, they will speedily be observed, with the
aid of a magnifying power of 200 or 250 diameters, to go through
the actions just described ; and the subsequent processes of germi-
nation may be watched by means of the ' growing slide.' 1 The

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.

559

winter months, from December to March, are the most favourable
for the observation of these phenomena ; but where Fuci abound,
some individuals will usually be found in fructification at almost
any period of the 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 1 bladders ' of the 1 bladder-wrack ' and
other species of Fucus, imbedded in the frond, and the ' berries ? of
Sargassum bacciferum, the ' gulf-weed ' of the Atlantic, which are
•elevated on pedicels above the surface of the water.

Among the Floridese, or red sea-weeds, also, we find various
simple but most beautiful forms, which connect this group with the
lower alga?, especially with the family Chcetojjhoracece ; such delicate

Pig. 423. — Arrangement of tetraspores in Carpocaidon mcrfiterraneum: A, entire
plant ; B, longitudinal section of spore-bearing branch. ( X.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.)

feathery or leaf-like fronds belong for the most part to the family
Ceramiacece, some members of which are found upon every part of
our coasts, attached either to rocks or stones or to larger algae, and
often themselves affording an attachment to zoophytes and polyzoa.
They chiefly live in deeper water than the other sea- weeds, 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
them from an excess of light, since otherwise they become unhealthy.
Various species of the genera Ceramium, Griffithsia, Callithainnion,

$60 MICROSCOPIC FORMS OF VEGETABLE LIFE

and Ptilota, are extremely beautiful objects for low powers when
mounted in glycerin jelly. In many of them the phenomenon to
which we have previously referred under the name of ' continuity of
protoplasm ' is very beautifully exhibited. The colour of the red
sea-weeds is due to the presence of a pigment known as rhodosjjermin
or phyco-erythrin, soluble in water, 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 sea-weeds is the production and liberation
of ' tetraspores ' (tig. 423, 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.

Fig. 424. — Nemalion multifidum : I, a branch with a carpogone, c, and pollinoids,
sp ; II, III, commencement of the formation of the fructification ; IV, V, de-
velopment of the spore-cluster ; t, denotes the trichogyne, c the carpogone and
fructification. (From G-oebel's ' Outline of Classification.' The Clarendon Press.)

If the second binary division takes place in the same direction as
the first, the tetraspores are arranged in linear series ; but if its
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 conjunction,
the ' tetraspores ' of the Floridece must be regarded, like the
' zoospores ? of the Ulvacece, as gonids analogous rather to the buds
than to the seeds of higher plants. It is now known that a true
generative process takes place in this group ; but the sexual organs

FL0EIDEJ3

56i

are not usually found on the plants whicn produce tetraspores ; so
that there would appear to be an alternation between the two modes
of propagation. 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 ' antherozoids,' but minute rounded particles, known
as pollinoids, 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 trichophore, 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 passes
down its tube to the trichophore, and thence to the carpogone \ one
of the cells of the carpogone contains the oosphere, 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 escape,
and then germinate. In the true Corallines, which are Moridere
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 sea-weeds, as, for example, in
Dudresnaya, the process of fertilisation is more complex than this,
and consists of two distinct stages. First, the trichogyne is impreg-
nated by the pollinoids ; and secondly, the fertilising principle js
then conveyed from the trichophore-cells at the base of the tricho-
gyne to the cells which ultimately produce the carpospores, and which
may be at a considerable distance from the trichogyne, even on a
different branch. This transference is effected by means of long
simple or branched tubes which are known as ' fertilising tubes.'
The sexual mode of reproduction has, however, at present been
observed in comparatively few species of sea-weed ; and, considering
the number of species of Florideai found on our coasts, there is no
branch of microscopical observation which is more likely to reward
the young investigator with new discoveries.

o o

562

CHAPTER IX

FUNGI

Fungi, as already mentioned, differ essentially from alga? 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 parasite^ 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 hyphen,
slender filaments containing protoplasm and a nucleus (except possibly
in some of the most simple forms), but no chlorophyll and rarely any
pigment. The cell- wall is composed of a substance differing some-
what in its properties from ordinary cellulose, since it is not coloured
blue by iodine after treatment with sulphuric acid ; it is known as
fungus-cellulose or fungin. These hyphse may be quite distinct or
very loosely attached to one another ; those which penetrate the soil,
or the tissue of the ' host ' on which the fungus is parasitic, constitute
the mycele. In the larger fungi, such as the mushroom, the
portion above the soil is composed of a dense mass of these hypha?,
lying side by side, constituting a so-called pseudo-par erichy me, but
never a true tissue. In some families the hyphae have a tendency
to become agglomerated into balls of great hardness called selerotes,
which have the power of maintaining their vitality for very long
periods. The modes of reproduction of fungi, both sexual and non-
sexual, are very various. Among the latter the most common are by
non-motile spores or gonids, and by zoospores. The former are very
minute bodies, each composed of a single cell, or rarely of several
cells, which are either formed within a spore-case or sporange, or
are detached from the extremity of hyphse by a process of pinching
off or abstriction. From their extreme lightness they are wafted
through the air in enormous numbers, and thus bring about the
extraordinarily rapid spread of many fungi, such as moulds. The
zoospores are, like those of the lower algae, minute naked masses of
protoplasm provided with one or more vibratile cilia, by means of

MYXOMYCETES

563

which they move very rapidly through water, and finally force their
way into the tissue of the host, where the zoospore loses its cilia,
invests itself with a cell- wall, and proceeds to germinate, which is
effected, as is also the case with 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 re-
production 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 separate 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 : 1 —

The Myxomycetes, Myxogastres, or Mycetozoa, are a group of
very singular organisms, on the very confines of the animal and
vegetable kingdoms, doubtfully included among the fungi, and be-
lieved by many to have a closer affinity to the rhizopods. They appear
indeed at one period of their life-history to have an animal, at another
period a vegetable existence. Several species are not uncommon on
decayed wood, bark, heaps of decaying leaves, &c. The ' plasmode >
of uEthalium septicum, known as 'flowers of tan,' forms yellow
flocculent masses in tan-pits. The development of other species
is represented in fig. 425. Commencing with the germination 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 (D), and an amceba-like body (E) escapes from it, consisting
of a little mass of protoplasm, with a round central nucleus enclosing
a nucleolus and a contractile vesicle, and having amoeba like move-
ments connected with the protrusion and withdrawal of peculiar
processes or pseudopodes. This soon elongates (F), and becomes
pointed at one end, whence a long flag 'ellum 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 par-
ticles which it engulfs within its soft protoplasm. These swarm-cells

1 [The classification of fungi here adopted is essentially that of De Bary in his
Comparative Morphology and Biology of the Fungi, Mycetozoa, and Bacteria.
Owing to the very large recent additions to our knowledge of the structure of fungi,
it has been found necessary entirely to rearrange this portion of Dr. Carpenter's
work.— Ed.]

0 0 2

564

FUNGI

may multiply by bipartition to an indefinite extent ; but after a
time a < conjugation' takes place between two of these myxamoebce
(H) their substance undergoing a complete fusion into one body (I),
from which extensions are put forth (K); 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 ingestion and assimila-

Fig 425 — Development of Myxomycetes : A, plasmode of Didymmm serpula; B,
successive stages, a, a', b, of sporanges of Arcyria flava ; C, ripe spore ot
Physarum album ; D, its contents escaping ; E, F, G, the swarm-spore first be-
coming flagellated, and then amoeboid ; H, conjugation of two amceboids, wnicn, at
I, have fused together, and, at J, are beginning to put out extensions and ingest
nutriment, of which two pellets are seen in its interior.

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 undulatory move-
ments, 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,

MYXOMYCETES ; CHYTRIDIACEiE ; USTILAGINEJE 565

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,
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
speranges, 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 in most genera tubes or threads of
different forms occur among the spores, and constitute the capillitium.
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 Chytridiaceae 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 resting-spore. They are parasitic on fresh-
water organisms, both animal and vegetable ; and their chief interest
to the microscopist is that their zoospores have apparently frequently
been mistaken for antherids of the ' host.'

The TJstilaginese are fungi parasitic 011 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 ex-
tensively 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

566

FUNGI

generations, forms previously considered to belong to widely separated
groups being now known to be stages in the cycle of development of
the same species. A striking instance of this is furnished by the
well-known and very destructive disease of wheat and other grasses
known as ' mildew,' produced by the attacks on the leaves of the
parasitic fungus Puecinia graminis. It was long ago observed that
wheat was especially liable to this disease in the vicinity of barberry
bushes ; and it is now known that a fungus parasitic on barberry
leaves, formerly known as sEcidium berberidis, is the ' secidiospore '
generation of the same species of which Puecinia graminis is the

Fig. 426. — Puecinia graminis. From De Bary's 1 Comparative Morphology and
Biology of the Fungi.' (The Clarendon Press.)

' teleutospore ' generation. The complete cycle of development of
the best known Uredinew, such as the mildew, is this. The form
known as Puecinia graminis produces teleutospores, thick-walled
spores, borne usually in pairs, at the extremity of elongated cells known
as basids or sterigmata. Each of these teleutospores gives rise, on
germinating within the tissue of the grass, to a hypha or promycele,
the terminal cells of which develop, on slender basids, each a single
spore or sporid. These sporids will germinate only on the leaves of the

UKEDINEJE ; PERONOSPOREiE

567

barberry, where they produce, first of all, a mass of interwoven hyphae
within the tissue, and then the peculiar reproductive bodies known
as cecidia (fig. 427). The ' tecidium ' 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 cecidiospores, which are pro-
duced in rows or chains springing from basids at the base of the recep-
tacle. These are accompanied, often on the other surface of the leaf,
by spermogones, smaller spherical or flask-shaped receptacles, which
also eventually break through the epiderm, and are filled with barren
hypha? known as paraphyses. Among these are other shorter hyphro
or ' sterigmata,' from the extremities of which are abstricted narrow
ellipsoidal cells, the spermatid. 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 stems of grasses,
either producing the teleutospore-form directly, or giving rise to a
third * uredo-form.' This consists of filiform basids, each of which
bears a round oval spore, the uredosjwre, which germinates very rapidly,

A B

Fig. 427. — JEcidium tussilaginis : A, portion of the plant, magnified ; B, section

of one of the ' secidia ' with its spores.

•constantly reproducing the same form. The same mycele which
produces the uredo-form also gives rise subsequently to the teleuto-
spore-form. The fungus usually hibernates and remains in a state of
rest in the teleutospore-form.

Of the Peronosporeae (fig. 428) 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 distinct sep-
tated hyphae, are produced the sexual organs, ooyones and antherids.
Fertilisation is not effected by means of motile antherozoids, as in
■other classes of fungi and of algae, but the antherid puts out a
cylindrical or conical tube-like process, the fertilisation-tube. The
antherids and oogones 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 the oogone,
and discharges into the latter the contents of the antherid, thus
causing its protoplasmic contents or ' oosphere ' to develop into the

568

FUNGI

impregnated ' oospore.' The further history of the oospore is singu-
larly 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 addition to the
sexual organs of reproduction, many species of Peronosporese also
produce non-sexual spores or gonids, which are borne on special
branches springing erect from the mycele, the sporophores, or goni-

Fig. 428. — A-G, Cystopvs candidus ; H, Pliytophthcra infestans. A, branch of
rnycele growing at the apex, t, with haustoria, h, between the cells of the pith ©f
Lepidiwm sativum ; B, branch of mycele bearing gonids; C, D, E, formation of
swarm-spores from gonids ; F, swarm-spores geiminating ; G, swarm-spores
germinating on a stomate and piercing the epiderm of the stem of a potato at EL
After De Bary ; magnified about 400 times. Frcm ' Outlines of Classification and
Special Morphology of Plants,' by Dr. K. Goebel.

diophores. A similar difference is exhibited in the further develop-
ment 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
species which are parasitic on living plants, such as Phytophthora
infestans which produces the potato-disease, and Cystopus candidus,

SAPROLEGXIEJE ; MUCORIM

569

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, they there germinate ; the germinating tube passes
through a stomate, and the mycele is developed with great rapidity
within the tissue of the host. The 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 surface of the
plant, sending out processes which pene-
trate to its interior, though otherwise
germinating only on cut surfaces.

The Saprolegniese are saprophytic or Q{
parasitic fungi, nearly allied to the
Peronosporece, and differing from them
chiefly in two points : although organs
are known, in many species closely re-
sembling the antherids of the Perono-
sporece, the act of impregnation has not
actually been observed, the oospore being,
at least in many cases, apparently pro-
duced parthenogenetically, i.e. without
impregnation. In some species a single
oospore is produced within each oogone,
but more often the contents of the latter
break up into a number of oospores, each
of which gives rise to a mycele, or breaks
up into zoospores. In some genera, e.g.
Achlya (fig. 429), zoospores are also pro-
duced in very large numbers by the break-
ing 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
ferox on the living flesh of the animal.

The Mucorini are filamentous fungi,
resembling the two last orders in their
vegetative development, but differing in
their mode of reproduction. To this family
belong some of the most common moulds
which make their appearance on damp or
decaying organic substances. The ordi-
nary mode of non-sexual reproduction is by
endogenous spores or gonids, produced within a sporange (fig. 430, A).
These are borne at the ends of sporangiophores, long erect unseptated
hypha?, springing directly from the mycele or from the original
germinating filament. Several other kinds of non-sexual spores
occur in the family, including chlamydospores, reproductive cells
formed within the ordinary cells of the hypha?. Sexual reproduc-
tion takes place by means of zygospores (C), but is at present known
only in a few species. Either from ordinary hyphre or from sporan-
giophores spring a pair of short branches, the extremities of which

Fig. 429. — Two sporanges of
Achlya. From Goebel's
' Outlines of Classification
and Special Morphology.'

5;o

FUNGI

become firmly attached to one another. These 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 is 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

Fig. 430. — B, mycele (three days old) of Phycomyces nitens, grown in a drop of
mucilage with a decoction of plums ; the finest ramifications are omitted ; g, the
conidiophore of Mucor mucedo in optical longitudinal section ; C, a germinating
zygospore of Mucor mucedo ; the germ-tube, k, puts out a lateral conidiophore g.
In D are conjugating branches, b b, the extremities of which, a a, 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 Groebel's 4 Outlines of Classification
and Special Morphology.'

cut off coalesce to form the zygospore, which often swells to a consider-
able size, and its outer coat becomes frequently beautifully covered
with warts or other protuberances. After a period of rest the zygo-
spore germinates, its inner coat of cellulose bursting through the outer
warty and cuticularised epis]?ore, and developing into the first germi-
nating filament.

ENTOMOPHTHOREiE ; ASCOMYCETES

571

Very nearly allied to the Mucorini are the Entomophthoreae,
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 condi-
tion, the spore-bearing filaments of this plant stand out from the body
of the fly like the ' pile ' of velvet, and the spores thrown oft' from
these in all directions form a white circle round it, as it rests motion-
less on a window-pane. The filaments which show themselves ex-
ternally are the fructification of the fungus which occupies the inte-
rior of the fly's body, and this orginates 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 off 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 intro-
duced (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 fungous growth
spreads through the body and destroys the life of the insect ; it
then seems to growT 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 hypha?. 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 wTithin elongated sacs or tubes
known as asci. These are commonly collected together in masses ;
the collection of hyphse which give birth to the asci is known as the
hymene, the mass of tissue enclosing or bearing the hymenes as the
receptacle or fructification. Its form and structure vary greatly in
the different sections of the family. The ascospores are always pro-
duced within the ascus by free-cell formation, and their number is
always four or a 4 power ' of four, most commonly eight. The asci
are usually surrounded by enlarged club-shaped or sterile hyphse, the
paraphyses. In many Ascomycetes, in addition to the ascospores,
ordinary exogenous spores or conids are produced at the extremity of
sporophores or conidiophores (fig. 431, A). This is the case with a
large number of moulds or mildews, of which the common blue mould,
PenicilHum 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, the sexual mode of repro-

572 FUNGI

duction sets up (fig. 431, B-H). One of the branches of the mycele
elongates, and coils spirally upon itself into a corkscrew-like body,
the carpogone or ascogone, which 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

Fig. 431. — Development of Eurotium repens : A, small part of a mycele with the
conidiophore, c, and young ascogones, as; B, the spiral ascogone, a s, with the
antheridial branch,^) ; 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) ; a s, the ascogone ; G, an ascus ; H, an ascospore.
After De Bary. A, magnified 190, the rest 600 times. :

asci, each containing numerous spores, which germinate directly into
a new mycele. The enveloping tissue, together with the asci, is
known as the sjoorocarj).

In some Ascomycetes a tendency is exhibited for the formation of
sclerotes, dense hardened masses of interwoven hyphre. An example
of this is furnished by the structure known as ' ergot/ the sclerote
of a fungus of this kind, Claviceps purjmrea, which attacks the ovary
of rye and other grasses. Many species of Peziza have a peculiar
form known as the botrytis form, reproduced by conicls only, and

ascomycet.es

573

long believed to be altogether distinct from the Ascomycetes. Of this
nature is the so-called Botrytis bassia?ia(i\g. 432), a kind of mould, the
growth of which is the real source of the disease termed muscardine,
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

Fig. 432. — Botrytis bassiana : A, the fungus as it first appears at the orifices of the
stigmas ; B, tubular filaments bearing short branches, as seen two days after-
wards; E, magnified view of the same; C, D, appearance of filaments on the fourth
and sixth days ; F, masses of mature spores falling off the branches, with filaments
proceeding from them.

in necklace-like filaments, very nearly as in the ordinary ' bead-
moulds.' The spores of this fungus, floating in the air, enter the
breathing- pores which open into the tracheal system of the silk-
worm ; they first develop themselves within the air-tubes, which
are soon blocked up by their growth ; and they then extend them-
selves through the fatty mass beneath the skin, occasioning the de-
struction of this tissue, which is very important as a reservoir of

574

FUNGI

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
Sphceria takes place in the bodies of certain caterpillars, in New
Zealand, Australia, and China ; and being thus completely pervaded
by a dense substance, which, when dried, has almost the solidity of
wood, these caterpillars come to present the appearance of twigs,
with long slender stalks that are formed by the growth of the fungus
itself. The Chinese species is valued as a medicinal drug.

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) cerevisice,
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 contain a large number of globular or ovoid cells,
averaging about yJ^^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. 433. 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-

1 It appears from the researches of Pasteur that, although the presence of albu-
minous matter (such as is contained in a saccharine wort, or in the juices of fruits)
favours the growth and reproduction of yeast, yet that it can live and multiply in a
solution of pure sugar, containing ammonium tartrate with small quantities of mineral
salts, the decomposition of the ammonia salt affording it the nitrogen it requires for
the production of protoplasm, while the sugar and water supply the carbon, oxygen,
and hydrogen.

SACCHAROMYCETES ; BASIDIOMYCETES

575

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,
live, or six, which remain in connection with each other whilst the
plant is still growing, but which separate if the fermenting pro-
cess 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
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 forma-
tion of ascospores ; and hence Torula is regarded as a low or degraded
type of that order. Many other fungi of like simplicity have the
power to act as ' ferments ' ; thus in wine-making the fermentation
of the juices of the grapes or other fruit employed is set going by

a b e d

Fig. 433. — Saccliaromyces cerevisice, or yeast-plant, as developed during the process
of fermentation : a, b, c, d, successive stages of cell-multiplication.

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. 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, Mucor, or Aspergillus, iprovided the temperature
be kept up to blood-heat ; and this even though the solution has
been previously heated to 284°Fahr., a temperature which must kill
any germs it may itself contain.

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 familiar of our fungi,
such as the genera Agaricus, Boletus, Polyporus, Lycoperdon,
Phallus, &c. They are saprophytes, obtaining their nourishment
from the decaying vegetable matter in the soil, stumps of trees
<fcc, <fec, among which the mycele penetrates, consisting often of a
dense weft of septa ted hypl^e, the ' spawn ' of the mushroom. The
aerial portion, known as the receptacle or fructification, bears either

576

FUNGI

externally, as in the case of the mushroom (fig. 434), or internally,
as in the case of the Lycoperdon, or ' puff-ball,' the fertile portion
or hymene. On this hymene project the extremities of special
hyphfe, 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 quanti-
ties. In the Hymenomy-
cetes, of which the com-
mon mushroom (Agari-
cus campestris) may be
taken as a type, the re-
ceptacle has the form of
a capshaped pileus (fig.
435), raised on a stalk
or stipe, the whole com-
posed of a pseudo-paren-
chyme consisting of a
dense agglomeration of
parallel hyphae, the cor-
tical portion of which
is slightly differentiated
into an epiderm. In
the family to which the
mushroom belongs, the
hymene is borne at
the edge of narrow
gill-like projections or
lamellce radiating from
the apex of the stipe
on the under side of the
pileus. Among the
basids are seen other
cells of similar shape
and usually larger size,
also the extremities of
special hyphae, called
Fig. 434. — Agaricus campestris, formation of the cystids, the function
hymene: A and B slightly magnified; C, a part f M h j obscure,
of B, magnified 550 times. The portion marked

with fine dots is protoplasm. The basidiospores vary

greatly in colour in dif-
ferent genera. They are always unicellular, and the membrane
consists of two coats, the endospore and exospore, the former of
which consists of pure cellulose, while the latter is more or less
cuticularised. On germinating the endospore bursts through the
exospore, and grows into a germinating filament, from which is
developed the mycele, and on this ultimately the receptacles.

Lichens. — The microscopic study of this group has latterly
acquired a new interest for the botanist, from the remarkable

LICHENS

577

discovery announced in its complete form by Schwendener in 1869 1
(and now accepted by the highest authorities), that instead of
constituting a special type of Thallophytes, parallel to Ah/re (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 composite structures, having an algal base, on which
fungi have sown themselves and live parasitically. As, however,
they do not furnish objects of interest to the ordinary microscopist
(the peculiar density of their
structure rendering a minute
examination of it more than
ordinarily difficult), nothing
more than a general account
of their curious organisation
will here be attempted. The
algal portion of a lichen be-
longs to one or other of the
lower groups, and consists
of cells termed gonids —
usually green, but sometimes
red or bluish-green — inter-
spersed among long cellular
filaments. The proportion
between these two compo-
nents 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 in-
creases in thickness by the
formation of new layers upon
its free surface, has no very
defined limit, and, in conse-
quence of the slight adhesion
of its components, is said to
be ' pulverulent.' But in
the more complex forms of lichens the thallus is mainly composed
of long hyphse, which dip down into the superficial layers of the
bark of the trees on which they grow, and form by their interweav-
ing a hard crustaceous 'thallus' in which the gonids are imbedded,
sometimes irregularly, sometimes in definite layers, known as the
gonidial layer, covered by an envelope of interlacing filaments. It
is from this algal portion of the structure that the soredes of lichens

1 See his memorable work XJcber die Algentypen cler Flechtcngonidien (Baselj
ISO!)). •

Fig. 435. — Agaricus campcstris, natural size,
(From Goebel's ' Classification and Morpho-
logy of Plants.')

578

FUNGI

are formed, which are little projections of the surface, composed of
single or aggregate gonids, invested by hyphse, 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 entophytic 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
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

Fig. 436. — LejJtogiavi scotinum : Vertical section of the gelatinous tliallus, 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.')

fungus-hypha*, affording an example of the singular kind of mutual
dependence known as commensalism or symbiosis. The formation
of sexually produced ' spores ' usually takes place in asci arranged
vertically in the midst of straight elongated sterile cells termed
paraphyses, so as to form a layer that lies either on the surface
of cup-shaped receptacles termed apotheces, or is completely en-
closed within peritheces. Each of the asci contains a definite number
of ascospores (usually eight, but always a ' power ' of two), 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 pollinoids or ' sperm at ia '
and the female trichogyne. These pollinoids are produced within
.antherids which are often specially designated * spermogones,' formed

LICHENS 579

within these cavities, and, when mature, 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 j 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.'

Fig. 437. — Examples of various alga? which are employed as the gonids of lichens:
h indicates always the hypha of the fungus ; g' the gonid : A, germinating
spore, s, of Physcia parietina, the germ-tube of which adheres closely to Proto-
coccus viridis ; B, a filament of Scytonema with hyphaa of Stereocaulon ramulosus
twined round it; C,from the thallus of the lichen Physma chalaganum — a hyphal
branch is entering a cell of the Nostoc filament (gonid) ; D, from the thallus of
the lichen Synalissa symplwrea — the gonids are the alga Glaeocajjsa ; E, from
the thallus of the lichen Cladonia furcata ; the gonids, which are being sur-
rounded by the hypha?, are the cells of Protococcus. After Bornet. A, C, D, E
magnified 950 ; B, 650 times. (From Goebel's ' Classification and Special Mor-
phology of Plants.')

The fungus- constituent of lichens belongs, in the great majority
of cases, to the Ascomycetes, in a very few to the Basidiomycetes.
The gonids have been referred to a very large number of genera of
algae, among which may be mentioned Protococcus, Chroococcus,
Glozocapsa, Palmella, Scytonema, Nostoc, and Chroolejms.

The Bacteria or Schizomycetes. — At the close of this chapter we
place the Bacteria or Schizomycetes ; that it is their place in the
natural system may be doubted and easily traversed. Cohn does
not hesitate to say that to account them offshoots of the Fungi

58o

FUNGI

is to ' contradict all trustworthy observations ' ; 1 and it may be fairly
admitted that what we know of their development does not suggest
close kinship with the Fungi. But, great as is 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, as a rule, no chlorophyll.

There can be no doubt that some forms of the Bacteria manifest
affinity with the chlorophyllaceous Algse ; 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. That the
affinities of the Bacteria as a complete group are closer with the
Flagellata than is generally admitted, the present Editor is con-
vinced ; and whenever the saprophytic Flagellata — which are the in-
dispensable agents, not in the putrefactive fermentation by which
infusions and gelatine masses are broken up, but by which great
masses of organic tissue are reduced — and at the same time the-
Bacteria, as a whole, have been broadly and comprehensively worked
out, it will be found that both their morphological and physiological
affinities are of the closest order.

It is impossible to take what we know of such a form as B. lineolay.
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,2
where the process is identical. True the Cercomonas has a conju-
gating and subsequent resting stage, after which swarms emerge-
from spores thus formed. But a fuller knowledge of B. lineola is
certainly wanting before we can deny the further analogy. And
this incidence is the more suggestive, since it is characteristic of the
whole group of saprophytic monads, and their function is identical
with that of the saprophytic Bacteria.

No doubt if such affinity were established, it would lead to much
rearrangement at the base of the organic series.

Since there is an apparent and highly suggestive leaning of the
Bacteria to those forms of Algse which form the group of Nostocacece,
these also would be brought nearer the Flagellata ; while the Myce-
tozoa will have singular points of contact with these, one of which,
has reference to the mode of sporing of one at least of the flagellate
saprophytes,3 while it is suggestive that the same grouping, should
the affinity be established, would involve a connection with the Algse
and the Fungi.

1 Colin, Beitrage, ii. 188. & Manual of the Infusoria, i. 259.

5 J.B.M.S. vol, v. ser. ii. pp. 189-90, fig. 16, Plate V.

FORMS OF BACTERIA

5&I

It is only definite results leading to a comprehensive view of the
morphology of the Bacteria as a whole that can render generalisation
in this matter safe.

By the word Bacteria will be understood rod-shaped organic ■
forms. This is the characteristic of the group. They are often less
than Ifx in breadth, and may be sphere-like or cylindrical cells.
The motile forms, whether longer or shorter, are possessed, as a rule,
of fine flagella. The mode of multiplication commonly observed is by
, fission or bipartition. The products of successive fissions may remain
together in a single filiform row loosely attached, or attached by the
unbroken filament of the flagella, or they may at once separate
from the primal cell.

Of the nature of this simplest cell we have hitherto learnt
comparatively little ; the protoplasm is generally homogeneous, and
in some species it forms chlorophyll, although the absence of it and
other colouring matters is a distinct characteristic of the group.
The most remarkable of the coloured forms uniformly tinged red
was found and named by Professor E. Ray Lankester ; 1 other forms
have been found by Van Tieghem, Engelmann, and Zopf.

Within the protoplasm of the Bacteria, however, no nuclei have
hitherto been discovered, but a delicate investing envelope, probably
a mere thickening of the outmost area of the protoplasm, which is
often also gelatinous in its outer portions.

Many forms of Bacteria have the power of entirely free move-
ment. Frequently this movement is coincident with a revolution on
the longer axis of the rod, curved or straight, and in the vast
majority of cases this is directly correlated with a vortical action of
a front flagellum — an action which may be seen with the utmost
ease, if the proper means be employed, in the case of Spirillum
volutans, and less easily, but with almost equal certainty, in the
majority of other forms, not excluding B. termo.

The simplest forms in which Bacteria are found are as isolated cells
of round or ovate shape : these are known as Micrococci ; but many
which are placed under this head are in reality immature and de-
veloping monads of the saprophytic group.

The rod-like forms are found isolated and free, or in chains.
The short rods are known as Bacteria ; the long rods as Bacilli.
Some which are fusiform in appearance are known as Clostridia.

The coiled rods or spiral forms are either (1) closely coiled, when
they are known as Spirillum and Spirochcete (more threadlike) ; or
(2) those more openly coiled are known as Vibriones.

There are also very elongated filiform varieties known as Lepto-
thrix, sometimes though rarely branched ; and Beggiatoa, the filaments
•of which fixed at one extremity stretch the other free in the sur-
rounding fluid.

But all these may be united by some interfusing gelatinous
material in which all action ceases, or is of the most limited kind ;
and these living films, which appear on the surface or suspended in
i)he interior of putrescent fluids, are known as Zodglaece, They may

1 Quart. Journ. 31ic?-os. Sci. new series, xiii. 408.

•582

FUNGI

also be found on the surfaces of solid bodies, where the putrefactive
ferment is in action.

The only rational attempt which has been made to classify the
Bacteria is into I. Endosporous Bacteria ; and II. Arthrosporous
Bacteria. That this classification can be permanent is highly improb-
able ; it at best roughly covers the present state of our knowledge.

I. The endosporous forms are those whose multiplication is brought
about by the formation within a cell of a minute globular or oval
body, which, while the surrounding protoplasm of the mother-cell
disappears (is assimilated ?), gradually reaches its mature condition.
The formation of spores occurs most abundantly when the sub-
stratum is almost exhausted, or the
vital processes are in some way
interrupted.

II. Arthrosporous forms are re-
produced by the separation of single
members from their connection with
a group, which then give origin to
new generations. They are separate
individuals in which micrococci or
spores appear, from which arise forms,
ultimately similar to those from
which the forms bearing the cocci

#%4

originated.

A chosen illustration of the
endosporous Bacteria is Bacillus
megaterium. It was first observed
on boiled cabbage leaves, and is con-
sidered by De Bary as an ' exceed-
ingly instructive form.' It is 2'bfx
in short diameter and about four times as long as this. It is illus-
trated in fig. 438 : a represents a motile chain of the Bacilli in active

Fig. 438. — Bacillus
(From De Bary's
Morphology of Fungi.')

megaterium.
' Comparative

This is magnified 250 diameters, b is two active rods,
magnified 600 diameters.

vegetation.

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, dtof represent successive stages of a
pair of rods in the act of forming spores, e an hour later than d, and J
an hour later than e, The cells which did not contain spores disappeared
or perished, r is a quadri-cellular rod with ripe spores. gl is a five-
celled rod with three ripe spores placed in a nutrient solution after
several days' desiccation. g2 is the same an hour after ; g3 is the same
after another two hours and a half. hl is two spores with the walls of
the mother-cells dried and placed in a nutrient solution ; h2is the same
forty-five minutes later ;i,k,l three stages of germination of the spore.

Bacillus anthracis and B. subtilis are forms that range under the
same division. B. anthracis is the form which has been proved to
be the virus of splenic fever or charbon. It is found in great pro-
fusion in the blood and tissues of animals attacked by this disease.
The filaments or rods approach in thickness in strong specimens
grown in the blood of smitten animals, and they occasionally attain
three or four, sometimes five times this length. They are connected

ENDOSPOROUS AND ARTIIROSPOROUS BACTERIA

together in the blood in straignt rods varying m

straight rods varying in length. Fig. 439
shows two filaments grown on a microscopic slide (De Bary) in a
solution of meat extract, partly in an advanced state of spore-forma-
tion. At the upper part of the figure two ripe spores have escaped.
These spores on germination elongate and give rise to new groups of
rods and filaments.

B. subtilis, 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 com-
paratively 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. 439, represents
the development of B. subtilis : 1 shows frag-
ments 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 re-
leased ; 5, germ -tubes with the two extremities
remaining connected and already greatly in-
creased in size. The whole represents a magni-
fication of 600 diameters.

A sufficiently detailed illustration of the
development of the arthrosporous Bacteria may
be seen in Beggiatoa alba. In the refuse waters
discharged from factories, especially the sul-
phuretted effluents of sewage works, is found
this form — the ' sewage fungus ' of engineers. It
may have a thickness of 5/j, and it may be as
attenuated as to measure only l^t. It is attached
in an erect manner to objects in the impure
water it affects (fig. 440) ; and the filaments
consist of rows of cells, and in the protoplasm of
these granules of sulphur are enclosed. The
filaments readily break up into cells about as
long as they are broad, and become at length
active, but eventually attach themselves to some
object and come to rest, when they multiply by fission and accumu-
late in masses of zoogloea. 1 They may develop into rods, and these
again into the filaments after the rods have passed through the
swarming state.'

Spirally twisted forms also arise in this species. These break
into coiled parts, possessed of flagella, and exhibit extremely active
movement. The flagella in these are as strong and easily seen as
in the Spirillum volutans, and these forms were known at an earlier
period as Ophidomonas.

Fig. 440 shows at 1 a group of the attached filaments of Beggiatoa
alba : 2 to 5 show portions of filaments of differing diameters ; 5 shows a
filament in the act of multipartition. The small dark circles through-
out represent the granules of sulphur ; 6 to 8 show fragments rich

Fig. 439. — A, Bacillus
anthracis ; B, B. sub-
tilis. (From De Bary's
' Fungi.')

584

FUNGI

in sulphur with transverse septation developed by treatment with
methyl- violet solution. In 8 the formation of cocci and spores is
seen ; 9 shows the result of filaments having broken up into spores ;

Fig. 440. — Beggiatoa alba. (From De Bary's ' Comparative Morphology of Fungi.')

10 shows spores in movement. 1 is magnified 540 diameters, the
remainder 900 diameters.

Figure 441 shows the growth of the curved and spiral forms
of the same : A is a group of attached filaments ; B to H show por-

NUTRITION OF BACTEHIA

5 8 S

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.

The Bacteria behave very variously under the same conditions
of supply or exclusion of oxygen. Some require free oxygen in
quantity, as e.g. B. subtilis ; while in others the vital activities are
promoted by its exclu-
sion. But there can be
no doubt that gradual
modification of either
condition will bring about
adaptations. Nageli has
shown that there are
forms which usually de-
pend on oxygen which
continue to vegetate
when free oxygen ceases.

Their nutrition is car-
ried 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 patho-
genic forms to these is
one of the most interest-
ing problems in micro-
scopic biology. That
they are physiological
modifications of the sa-
prophytic forms appears
■per se a possibility ; but
in the light thrown upon
biological change and sur-
vival by the hypothesis
of the origin of species,
the suggestion incites to
practical inquiry and re-
search. If the parasitic
Bacteria are physiological
modifications of the saprophytic forms, to know the path by which
they biologically became such may be to put more into the hands of
medicine than could be accomplished by any other means.

Bacterium termo is the most universally present and abundant of
the saprophytic forms. It is l/x to 1'5/a long, and 0*5 to 07/a broad,
usually of dumb-bell form. These Bacteria are usually seen in f vacil-
lating ' movement in their free state ; each cell bears a flagellum at
each end, as B, D (fig. -±42), whilst the double cells bear a flagellum
at each extremity. The formation of the second flagellum takes place

Fig. 441. — Beggiatoa alba, curved and spiral
forms. (From De Bary's ' Comparative Morpho-
logy of Fungi.')

586

FUNGI

by the drawing out of a filament of protoplasm between two cells-
that are separating from each other (as in fig. 443, a, b)} the rupture
of which gives a new fiagellum to each. Their flagella are so minute
as to be among the most ' difficult ' of all microscopic objects, their
diameter being calculated from 200 measurements by Dr. Dallinger
at no more than ^ooVooth °^ 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 zoogloea-^lm. These

A B

C D

Fig. 442. — A, Bacterium termo, each cell furnished with a single fiagellum. Magni-
fied 4,000 diameters B, C, D, Bacterium lineola, each cell when separated
having a fiagellum at either end. Magnified 3,000 diameters. (Dallinger.)

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. rugula, seen in fig. 443.

Fig. 443. — Four individuals of Vibrio rugula, each showing fiagellum at one or
both ends ; two other individuals, a and b, separating from each other, and draw-
ing out a protoplasmic filament to form their second flagella. Magnified 2,000
diameters. (Dallinger.)

They are slightly curved rods and threads from 6/i to long, and
varying in thickness from 0'5p to 2/x. They have well-marked flagella,
one at each end. They appear in vegetable infusions, causing fer-
mentation of cellulose.

The Spirilla are the largest forms in the group, characterised by
their spirally formed cells and their graceful spiral motion. They
are fairly represented in fig. 444 by Spirillum undula (A), and
Spirillum volutans (B). The threads of the former are from 1'Xfx to
1'4/x in thickness, and from 9/x to 12/x in length. They are intensely
active, and possess a fiagellum at either end. They are found in
varying decomposing infusions.

Spirillum volutans was known to and named by Ehrenberg. It
is from l*5/x to 2*3//, in thickness, and varies from 25/x to 30/x or more
in length. They have distinctly granular contents. They have a

1 Journ. of Boy. Micros. Soc. vol. i. (1878), p. 175

2 Ewart, loc. cit.

GERMINATION OF BACTERIA

5«7

very easily demonstrable fiagellum at each end of the spiral ; a fia-
gellum was distinctly suggested by Ehrenberg on account of the vor-
tical action visible in the fluid before this spirillum as it advanced.

On the whole it is surprising to find at this time what difficulty
there appears on the part of some botanists to distinguish these mo-
tile filaments in the Bacteria. With the superb 6mm. power of Zeiss
(apochromatic dry N.A. 0-95), all but the most difficult of these can
be seen with relative ease on a dark ground with a 12 or 18 eye-
piece, provided they be examined alive with the fiagella in motion.
For the more difficult ones (B. termo and B. lineola) more careful
arrangements are required.

The germinating power of the spores of Bacteria may be brought
into operation at once on their reaching ripeness, or they may be
desiccated for an indefinite time, and again, on reaching suitable
surroundings, will germinate as before. This power is held in vari-
ous degrees by different forms, but the whole subject needs more

A.

Fig. 444. — A, Spirillum unchda, showing fiagellum at each end. Magnified 3,000
diameters. B, Spirillum volutans. Magnified 2,000 diameters. (Dalhnger.)

uniform and exhaustive inquiry. The spores of B. subtil is retain
their vitality for years if kept in a dry air, while those of
B. anthracis are stated by Pasteur to remain alive in absolute-
alcohol 1 ; 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 boiling
nutrient fluid until the end of the third hour, when all were de-
stroyed. So Buchner found that the same spores were wholly killed
only after three or four hours' boiling ; 2 while Pasteur states that
groups of uncertain spores can withstand a temperature of 130° C.
There is, however, uncertainty, because a want of uniformity, in the
results from various sources. 20° to 25° C. may be taken as the
average degree of temperature at which these organisms will freely
germinate ; but B. termo, for example, has been known to germinate
from 5-5° C. to 40° C.

1 ' Charbon et Septicemic,' Compt. Bend, lxxxv. p. 99.

2 Naegeli, Unters. iiber nietlerc Pilze, 1882, p. 220.

588

FUNGI

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.1 When these facts are allowed
their due weight, no difficulty can be felt in admitting the action of
Bacteria &c. in producing decomposition under conditions which
might at first view be fairly supposed to preclude the possibility of
their presence. This action is altogether analogous to that of the
yeast-plant in producing saccharine fermentation • and the careful
-.and exact experiments of Pasteur, repeated and verified in a great
variety of modes by Professors Lister, Tyndall, and others, seem
to leave no reasonable doubt on these two points — (1) that putre-
factive fermentation does not take place, ;even in liquids which
are peculiarly disposed to pass into it, except in the presence of
Bacteria-germs ; 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 'splenic fever' is producible by the inoculation of Bacillus
anthracis, and the typhoid fever of the pig by inoculation with another
species of Bacillus, the plants having been in both cases 1 cultivated,'
so as to be free from other contaminating matter. Again, it has
been ascertained by careful microscopical examination of the fluid of
the vaccine vesicle, that it is charged with a multitude of minute
^granules not above o^owoth °^ an inch in diameter ; and it has
been further determined that these, rather than the fluid in which
they are suspended, are the active agents in the production of a
-similar vesicle in the skin into which they are inserted. This vesicle
must contain hundreds or thousands for every one originally intro-
duced ; and it is obvious that their multiplication has so strong an
analogy to that of Bacteria, as to suggest the idea that it must take
place by a like process of cell-development. Similar observations
have been made upon glanders, sheep-pox, and cattle-plague ; so that
^an animal suffering under either of these terrible diseases is a focus
of infection to others, for precisely the same reason that a tub of fer-
menting beer is capable of propagating its fermentation to fresh
wort. A most notable instance of such propagation is afforded by the
spread of the disease termed ' pebrine ' among the silkworms of the
south of France, which, according to Pasteur, is caused by a minute
organism named Nosema Bombycis, the mortality caused by it being
estimated to produce a money loss of from three to four millions
sterling annually for several years following 1853, when it first
broke out with violence. It has been shown by microscopic investi-
gation that in silkworms strongly affected with this disease, every
tissue and organ in the body is swarming with these minute cylin-
drical corpuscles about 4'2/x, long, and that these even pass into

1 As it seems unquestionable that among the higher Fungi ' conjugation ' often
takes place at a very early stage of growth, it seems a not very improbable surmise
that the ' granular spheres ' observed by Mr. Ewart in Bacillus and Spirillum, which
seem to correspond with the ' microplasts ' observed by Prof. E. Ray Lankester in his
Bacterium ritbescens, may be a product of conjugation in the micrococcus stage of
these organisms.

MICROSCOPIC STUDY OF BACTERIA

5«9

the undeveloped eggs of the female moth, so that the disease is
hereditarily transmitted. And it has been further ascertained bj
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 be
completely exterminated.

Bacteriology is now so distinctly a branch of biological science
that it would be out of place here to even present a summary of its
voluminous details and methods of research. The microscope in its
most perfect form is an indispensable adjunct to the rapidly pro-
gressive 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 micro-
scopist can be a successful bacteriologist. But for the methods of
the bacteriological laboratory we must refer the reader to treatises
on this branch of science,1 it being enough here to remark that the
employment of nutrient gelatines, nutrient agar-agar, and other
similar media poured over glass plates, and into test tubes, so as by
inoculation to obtain plate and tube cultures of specific and isolated
forms with their characteristic appearances, is one of the essential
methods. Cleanliness and care, as well as practice in manipulation,
are the essentials in effecting these ; but as they solidify, the inoculated-
bacteria, instead of moving freely as they would in a liquid medium,
are fixed to one spot, where they develop in a characteristic manner
showing their own morphological features.

The pathological value of these researches is great beyond our
present ability to estimate, and must have an apparently increasing
value. But it is a branch of microscopy 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.

1 The English student will find an admirable aid in the Manual of Bacteriology,,
hy Professor E. Crookshank.

590

CHAPTER X

MICBOSCOPIC STBUCTUBE OF THE HIGHER CRYPTOGAMS

HepatlC98. — Quitting now the algal and fungoid types, and
entering the series of terrestrial cryptogams, we have first to notice
the little group of Hepaticce, or liverworts, which is intermediate
between lichens and ordinary mosses, agreeing rather with the
algal thallus of the former in its general mode of growth, whilst
closely resembling the latter in its fructification. This group pre-
sents numerous objects of great interest to the microscopist ; and no
species is richer in these than the very common Marchantia poly-
morplia, which may often be found growing between the paving-
stones of damp courtyards, but
which particularly luxuriates in
the neighbourhood of springs or
waterfalls, where its lobed fronds
are found covering extensive sur-
faces of moist rock or soil, adher-
ing by the radical filaments (rhi-
zoids) which arise from their lower
surface. At the period of fructi-
fication these fronds send up
stalks, which carry at their sum-
mits either round shield-like discs,
or radiating bodies that bear some
resemblance to a wheel without
its tire (fig. 445). The former
carry the male organs, or an-
therids ; while the latter in the first instance bear the female
organs, or archegones, which afterwards give place to the sporanges
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
(tig. 446, A, a, a,) bounded by raised bands (c, c) ; every one of these
spaces has in its centre a curious brownish- coloured body (b, b), with
an opening in its middle, which allows a few small green cells to be
seen through it. When a thin vertical section is made of the frond

1 In some species the same shields bear both sets of organs ; and in Marchantia
anclrogyna we find the upper surface of one-half of the shield developing antherids,
whilst the under surface of the other half bears archegones.

Fig. 445. — Frond of Marchantia poly-
morpha, with gemmiparous concep-
tacles, and lobed receptacles bearing
archegones.

STRUCTURE OF MARCHANT1A

591

•(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, a) of closely set cells, from whose under surface
the rhizoids arise ; at the sides by walls (c, c) of similar solid
parenchyme, the projection of whose summits forms the raised
bands on the surface ; and above by an epiderm (/;, 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
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
Greek ctto/jm, mouth) forms a sort
of shaft (g), composed of four or
Ave rings (like the ' courses ' of
bricks in a chimney) placed one
upon the other (h), every ring-
being made up of four or live
cells ; and the lowest of these
l ings (i) 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 pIG. 446.- Structure of frond of Marchan-

manner each of the air-chambers
of the frond is brought into com-
munication with the external
atmosphere, the degree of that
communication being regulated

tia polymorpha : A, portion seen from
above; a, a, lozenge-shaped divisions;

b, b, stomates in the centre of the lozenges ;

c, c, greenish bands separating the
lozenges. B, vertical section of the frond,
showing a, a, the dense layer of cellular
tissue forming the floor of the air-
chamber, d, d, the epidermal layer, b, b,
forming its roof ; c, c, its walls ; /,/, loose
cells in its interior ; g, stomate divided per-
pendicularly ; h, rings of cells forming its
wall ; i cells, forming the obturator-ring.

by the limitation of the aperture.
We shall hereafter find that the
leaves of the higher plants con-
tain intercellular spaces, which
also communicate with the ex-
terior by stomates, but that the structure of these organs is far less
complex in them than in this humble liverwort.

The frond of Marchantia usually bears upon its surface, as shown
in fig. 445, a number of little open basket-shaped gemmiparous con-
ceptacles (fig. 447), 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 gemmae,
each composed of two or more layers of cells ; and their wall is sur-
mounted by a glistening fringe of 1 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 B, at the

592 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMS

spontaneously detach

time when the ' conceptacle ' and its contents are first making their
way above the surface. The little gemmffi are at first evolved as
single globular cells, supported upon other cells which form their
footstalks ; these single cells, undergoing binary subdivision, evolve
themselves into the gemmae ; and these gemmae, when mature,

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 grow-
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 Marchantia puts forth seem
to have this origin. The very
curious observation was long ago
made by Mirbel, who carefully
watched the development of these
gemma3, that stomates are formed
on the side which happens to be
exposed to the light, and that
rhizoids are put forth from the lower
side, it being apparently a matter
of indifference which side of the
little gemma is at first turned up-
wards, since each has the power
of developing either stomates or
rhizoids according to the influence it
receives. After the tendencv to
the formation of these organs has
once been given, however, by the
sufficiently prolonged influence of
light upon one side and of darkness and moisture on the other, any
attempt to alter it is found to be vain ; for if the surfaces of the
young fronds be then inverted, a twisting growth soon restores them
to their original aspect.

When Marchantia vegetates in damp shady situations which
are favourable to the nutritive processes, it does not readily produce
the true fructification, which is to be looked for rather in plants
growing in more 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

Fig. 447. — Gemmiparous conceptacles
of Marchantia poly morplia: A, con-
ceptacle fully expanded, rising from
the surface of the frond, a, a, and
containing gonidial gemmae 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.

STRUCTURE OF MARCIIANTIA

593

* antherozoids ' like those of Chara, and surmounted by a long neck
that projects through the mouth of the flask-shaped cavity. The
wheel-like receptacles (fig. 445), on the other hand, bear on their
under surface, at an early stage, concealed between membranes that
connect the origins of the lobes with one another, a set of archegones,
shaped like flasks with elongated necks (fig. 448) : each of these has
in its interior an ' oosphere ' 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 asrgreeration of cells that constitute the im-
mediate progeny of the original germ -cell. These
cells, discharged by the bursting of the sporange,
^re 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 spores ex-
tend themselves (fig.
449), tearing apart the
cell - membrane ; and
they do this so suddenly
as to jerk forth the
spores which may be
adherent to their coils,
and thus assist in
their dispersion. The
spores, when subjected
to moisture, with a
moderate amount of
light and warmth, de-
velop themselves into
little collections of cells,

which gradually assume the form of flattened fronds
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.

Marchantia is the type of the section known as the thalloid
Hepaticse. Another section, the foliose Hepaticse, 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

Q Q

i

m

Fig. 448. — Archegones of Mar-
chantia polymorjiha, in succes-
sive stages of development.

Fig. 449.— Elater
and spores of
Marchantia.

and thus the

594 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMS

with small semi-transparent leaves. This distinct differentiation of
stem and leaves indicates a decided advance in organisation, and
marks the passage from the thallophytic to the cormophytic type of
structure.

Musci. — There is not one of the tribe of Mosses 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-

A

c

Fig. 450. — iStructure of mosses : A, plant of Funaria Jiygrometrica, showing, / the
leaves, it the sporanges supported upon the setae or footstalks s, closed by the opercule
o, and covered by the calypter c. B, sporanges of Encalyptra vulgaris, one of them
closed and covered with the calypter, the other open ; u, u, the sporanges ; o, o, the
opercules ; c, calypter ; p, peristome ; s, s, setae. C, longitudinal section of very
young sporange of Splachnum ; «, solid tissue forming the lower part of the capsule ;
c, columel ; Z, space around it for the development of the spores ; e, epidermal
layer of cells, thickened at the top to form the opercule o ; jp, two intermediate
layers, from which the peristome will be formed ; s, inner layer of cells forming
the wall of the cavity.

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 seem to prefigure the woody portion of the stem of
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 simple
structure, is commonly found on the seta or bristle-like footstalk
bearing the fructification, and sometimes on the midribs of the leaves.
The rhizoids of mosses, like those of Marchantia, consist of long

FRUCTIFICATION OF MOSSES 595

tubular cells of extreme transparency, within which the protoplasm
may frequently be seen to circulate, as in the elongated cells of
Char a.

What has commonly been regarded as the 'fructification' of.
mosses — namely, the 'urn' or 'capsule' filled with spores, which is

Fig. 451. — Antherids and antherozoids of Polytriclium commune : A, group of
antherids, mingled with hairs and sterile' filaments (paraphyses). Of the three
antherids, the central one is in the act of discharging its contents ; that on the
left is not yet mature ; while that on the right has already emptied itself, so that
the cellular structure of its walls becomes apparent. B, cellular contents of an
antherid, previously to the development of the antherozoids; C, the same,
showing the first appearance of the antherozoids; D, the same, mature and
discharging the antherozoids.

borne at the top of a long footstalk that springs from the centre of
a cluster of leaves (fig. 450, A) — is not the real fructification, but
its product; for mosses, like liverworts, possess both antherids and
archegones. These organs are sometimes found in the same envelope
(or perigone), sometimes on different parts of the same plant, some-
times only on different individuals ; but in either case they are.

Q Q 2

596 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMS

usually situated close to the axis, among the bases of the leaves.
The 'antherids' are globular, oval, or elongated bodies (fig. 451, A),
composed of aggregations of cells, of which the interior are ' sperm-
cells,' each of which, as it comes to maturity, develops within itself
an ' antherozoid ' (B, C, D) • and the antherozoids, set free by the
rupture of the cells within which they are formed, make their
escape by a passage that opens for them at the summit of the antherid.
The antherids are generally surrounded by a cluster of hair-like
filaments, composed of cells joined together (fig. 451, A), which are
called paraphyses ; these seem to be i sterile ; or undeveloped anthe-
rids. In the ' hair-moss,' Polytrichum commune, one of the largest
of our mosses, common on dry heaths, these antherids are collected
into conspicuous star-like clusters at the extremities of the branches
of the ' male ' plants. These are to be seen about April, and at the
same time the archegones may be detected concealed among the
leaves on the ' female ' plants ; while the capsules, or sporanges, in
this and most other mosses, make their appearance late in the
summer, and remain through the winter. The ' archegones ' bear a
general resemblance to those of Marchantia (fig. 448), and the
fertilisation of their contained 'obspheres,' or 'germ-cells,' is accom-
plished in the manner already described. The fertilised ' embryo-
cell' becomes gradually developed by cell-division into a conical
body elevated upon a stalk ; and this at length tears across the walls
of the flask-shaped archegone by a circular fissure, carrying the
higher part upwards on its summit as a calypter or ' hood ' (fig. 450,
B, c), while the lower part remains to form a kind of collar round
the base of the stalk, known as the vagine.

The urn, theca, or sporange, which is the immediate product of
the generative act, is closed at its summit by an opereule, or lid
{fig. 450, B, o, o), which falls off when the contents of the sporange

Fig. 452.— Mouth of sporange of Funaria, Fig. 453— Double peristome

showing the peristome in situ. of Fontinalis antipyretica.

are mature, so as to give them free exit ; and the mouth thus laid
open is surrounded, in many mosses, by a beautiful toothed fringe,
which is termed the peristome. This fringe, as seen in its original
undisturbed position (fig. 452), is a beautiful object for the binocular
microscope ; it is very ' hygrometric,' executing, when breathed on,

SPORANGE OF MOSSES

597

a curious movement which is probably concerned in the dispersion
of the spores. In figs. 453-455 are shown three different forms of
peristome, spread out and detached, illustrating the varieties which
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

Fig. 454. — Double peristome of Fig. 455. — Double peristome

Bryum intermedium. , of Cinclidium arcticum.

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. 450, C), the space (!) in which the
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 sporogone. Thus, the 'spore-capsule'
in liverworts and mosses, being the immediate product of the act of
fertilisation (which constitutes the point of departure of each ' new
generation'), is to be considered as the progeny of the plant that
bears it ; which, supplying the nutriment at whose expense it
develops itself, acts as its ' nurse.'

The development of the spore into a new plant commences with
the rupture of its firm yellowish-brown outer coat or exospore, and
the protrusion of its cell-wall proper or endospore, from the

598 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMS

projecting extremity of which new cells are put forth by a process
of outgrowth, forming a sort of confervoid filament (as in fig.
462, C). At certain points of this filament its component cells
multiply by subdivision, so as to form rounded clusters or buds, from
every one of which an independent plant may arise ; so that several
individuals may be evolved from a single spore. And as a numerous
aggregate of spores is developed, as we have seen, from a single
germ-cell, the rapid extension of mosses is thus secured, although no
separate individual ever attains more than a very limited size.

The tribe of Sj)hagnacece, or 1 bog-mosses/ is now separated by
muscoloaists from true mosses on account of the marked differences
by which they are distinguished, the three groups, HepafAcce,
Bryacece (or ordinary mosses), and Sphagnacece being ranked as to-
gether forming the group of Muscinece.
b b b The stem of Sphagnacece is more dis-

tinctly differentiated than that of
Bryacece 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 layers, 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
Sphagnacece exhibit a very curious de-
parture from the ordinary type ; for
instead of being small and polygonal,
they are large and elongated (fig. 456) ;
Fig. 456.— Portion of the leaf of they contain no chlorophyll, but have
%%%m «P-al fibres loosely coiled in tfieir in-

fibres and communicating aper- tenor ; and their membranous walls
tures; and the intervening have large rounded apertures, by which
SelLgid'cH^lloul cavities freely communicate with

cells. one another, as is sometimes curiously

evidenced by the passage of wheel-
animalcules that make their habitation in these chambers. Between
these coarsely spiral cells are some thick-walled narrow elongated
cells containing chlorophyll ; these, which give to the leaf its firm-
ness, do not, in the very young leaf, differ much in appearance from
the others, the peculiarities of both being evolved by a gradual pro-
cess of differentiation. The antherids, or male organs, of Sphagnacece
resemble those of liverworts, rather than those of mosses, in their
form and arrangement ; they are grouped in ' catkins ' at the tips
of lateral branches, each of the imbricated perigonal leaves enclosing
a single globose antherid on a 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

BOG-MOSSES

599

leaves at the end cf 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 on different branches, and in some instances
on different plants. The ' sporange ' which is formed as the product
of the impregnation of the germ-cell is very uniform in all the
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 Sj)hagnacece sometimes develop a smaller kind, the ' microspores,'
formed by a further division of the mother-cells ; the significance of
these is unknown. The ordinary spores when germinating do not
produce the branched confervoid 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
woody 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 Fio. 457.-Oblique section of foot-
the leaf -stalk of the common brake which stalk of fern leaf, showing
gives to the transverse section the mark- bundle of scalariform ducts,
ing commonly known as £ King Charles

in the oak.' A thin section, especially if somewhat oblique (fig. 457),
displays extremely well the peculiar character of the ducts of the
fern, which are termed ' scalariform ' from the resemblance of the
regular markings on their walls to the rungs of a ladder. These

600 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMS

bundles of scalariform ducts or { tracheides ' are usually surrounded by-
sheaths of sderenchyme, tissue composed of cells the walls of which
have become very hard and of a deep brown colour. These scleren-
chymatous sheaths are a very conspicuous feature in a transverse
section of the stem or rhizome of most ferns, and are the principal
agent in giving it strength and solidity.

What is usually considered 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..

Fig. 458. — Leaflet of Poly- Fig. 459. — Portion of frond of Hcemionitis,

podium, with sori. with sori.

as in the common Polypodium (fig. 458), and in Aspidium (fig.
460) ; but sometimes these ' sori ' are elongated into bands, as in
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 Hcemionitis (fig. 459) ; or they may form merely a
single band along its borders, as in the common Pteris (brake-fern).
The sori are sometimes ' naked ' on the under surface of the fronds ;
but they are frequently covered with a delicate membrane termed
the indusium, which may either form a sort of cap upon the summit
of each sorus, as in Aspidium (fig. 460), or a long fold, as in Scolo-
pendritmn and Pteris ; or a sort of cup, as in Deparia (fig. 461).
Each of these sori, when sufficiently magnified, is found to be made
up of a multitude of sporanges, or spore-capsules (figs. 460, 461)i,

FRUCTIFICATION OF FEENS

60 1

which are sometimes closely attached to the surface of the frond,
but more commonly spring from it by a pedicel or footstalk. The
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. 461). This ring has an
elasticity superior to that of all the rest of the capsular wall, caus-
ing 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 ') and Ophioglossum (adder's tongue) the sporanges
have no annulus. 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 ad-
vanced the sporanges may all be closed ; others, however, may often
be met with in which some of the sporanges are closed and others are

Fig. 4G0. — Sorus and indusium of Fig. 461. — Sorus and cup-shaped

Aspidium. indusium of Deparia prolifera.

open ; and if these be watched with sufficient attention the rupture
of some of the sporanges and the dispersion of the spores may be
observed to take place whilst the specimen is under observation in
the field of the microscope. In sori whose sporanges have all burst,
the annuli connecting their two halves are the most conspicuous
objects, looking, when a strong light is throw upon them, like
strongly banded worms of a bright brown hue. This is particularly
the case in Scolopendrium, whose elongated sori are remarkably beau-
tiful objects for the microscope in all their stages ; until quite
mature, however, they need to be brought into view by turning back
the two indusial folds that cover them. The commonest ferns,
indeed, which are found in almost every hedge, furnish objects of no
less beauty than those yielded by the rarest exotics ; and it is in
every respect a most valuable training to the young to teach them
how much may be found to interest, when looked for with intelli-
gent eyes, even in the most familiar, and therefore disregarded, speci-
mens of Nature's handiwork.

The ' spores ' (fig. 462, A) set free by the bursting of the spor-

602 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMS

anges usually have a somewhat angular form, and are invested by a
yellowish or brownish outer coat, the exosjwre, which is marked very
much in the manner of pollen-grains (fig. 507) 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. 462, B, a) of the internal cell- wall or endospore through an aper-
ture in the outer spore- coat ; and moisture being absorbed through
this, the cell becomes so distended as to burst the external unyielding
integument, and soon begins to elongate itself in a direction opposite
to that of the first rhizoid. A production of new cells by subdivi-

Fig. 462. — 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-
thallium taking the form of a leaf -like expansion ; a, first, and b, second rhizoid ;
c, d, the two lobes, and e, the indentation between them ; /, /, first-formed part of
the prothallium ; g, external coat of the original spore ; h, h, antherids.

sion then takes place from its growing extremity ; this at first pro-
ceeds in a single series, so as to form a kind of confervoid filament
(C); but the multiplication of cells by subdivision soon takes place
transversely as well as longitudinally, so that a flattened leaf -like
expansion (D) is produced, so closely resembling that of a young
Marchantia as to be readily mistaken for it. This expansion, which
is termed the prothallium, varies in its configuration in different
species, but its essential structure always remains the same. From
its under surface are developed not merely the rhizoids (a, b), which
serve at the same time to fix it in the soil and to supply it with
moisture, but also the antherids and archegones, which constitute
the true representatives of the essential parts of the flower of higher
plants. Some of the former may be distinguished at an early period
of the development of the prothallium (A, h) ; and at the time of its

SEXUAL GENERATION OF FEENS

603

A

B

C

Fig. 4(33. — Development of the antherids and anthe-
rozoids of Pteris serrulata : A, projection of one
of the cells of the prothallium, showing the anthe-
ridial cell b, with its sperm-cells e, within the cavity
of the original cell a. B, antherid completely
developed ; a, wall of antheridial cell ; e, sperm-
cells, each enclosing an antherozoid. C, anthero-
zoid more highly magnified, showing its large ex-
tremity a, its small extremity b, and its cilia d, d.

A B

complete evolution these bodies are seen in considerable numbers,
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. 463, A, 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 no
part in the formation of
the antherozoids ; but
the protoplasmic con-
tents of the large central
cell divide by free-cell
formation into a large
number of cells known as
the a ntherozoid-mother-
cells (c) • each of these
again breaks up into
four cells, not at first
provided 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 sperm -
cells themselves burst,
and give exit to their
antherozoids (C), which
execute rapid move-
ments of rotation on
their axes, partly de-
pendent on the long
cilia with which they
are furnished.

The archegones are
fewer in number, and
are found upon a dif-
ferent part of the pro-
thallium. Each of them
originates in a single cell of its superficial layer, which undergoes
subdivision by a horizontal partition. Of the two cells thus produced
the upper gives origin, by successive subdivisions, to the £ neck ' of
the archegone, which, when fully developed (fig. 46-i), is composed
of twelve or more cells, built up in layers of four cells each, one upon
another, so as to form a kind of chimney or shaft. The lower of the

Fig. 464. — Archegone of Pteris" serrulata: A, as
seen from above ; a, a, a, cells surrounding the
base of the cavity ; b, c, d, successive layers of
cells, the highest enclosing a quadrangular orifice.
B, side view, showing A, a, cavity containing the
germ-cell, a\ b, b, walls of the archegone, made
up of the four layers of cells, b, c, d, e, and having
an opening,/, on the summit; c, c, antherozoids
within the cavity ; g, large extremity ; h, vibra-
tile cilia ; i, small extremity in contact with the
germ-cell, and dilated.

604 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMS

two first-formed cells becomes the central cell of the archegone ;
and this again undergoing horizontal subdivision, the lower half be-
comes the oosjyhere 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. 464, B, a). The
oosphere, when fertilised by the penetration of the antherozoids,
becomes the £ embryo-cell ' of a new plant, the development of which
speedily commences.1 In the aberrant group of Ophioglossacece
(adder's-tongue ferns), the development of the prothallium takes
place underground, in the form of a small roundish tuber; composed
of parenchymatous tissue containing no chlorophyll, and producing
antherids and archegones on its upper surface.

The early development of the embryo- cell takes place according
to the usual method of repeated subdivision, producing a homo-
geneous globular mass of cells. Soon, however, rudiments of special
organs begin to make their appearance ; the embryo 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 flower-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 5 the embryos originated by the generative act ;
which embryos finally develop themselves, not, as in mosses, into

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 antherids, and bear abundance of
archegones, those which have remained behind in their growth are beginning to be
covered with antherids. If the crop be now kept with little moisture for several
weeks, and then suddenly watered, a large number of antherids and archegones
simultaneously open ; and in a few hours afterwards the surface of the larger pro-'
thallia will be found almost covered with moving antherozoids. Such prothallia as
exhibit freshly opened archegones are now to be held by one lobe between the forefinger
and thumb of the left hand, so that the upper surface of the prothallium lies upon the
thumb ; and the thinnest 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 Osmimda regalis will be found to
afford peculiar facilities for observation of the development of the antherids, which
are produced at its margin.

EQUISETACE.E

605

mere sporogones, but, as in Phanerogams, into entire plants, com-
plete in everything but the true generative organs, which evolve
themselves from the detached spores.

The singular discovery has recently been made by the researches
of De Bary, Farlow, and others, that the ordinary alternation of
'fenerations in ferns may be interrupted by the suppression either of
the spoi'opliyte, the asexual 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-fozmina and Folystichum angulare, and is shown by the pro-
duction of prothalloid structures bearing antherids and archegones on
the fronds in the place of ordinary sori. The latter occurs not un-
frequently in Fteris serrulata, the sporophytic generation springing
directly from the prothallium without the intervention of archegones
and antherids.

The little group of Equisetacese (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
silex, that even when its organic portion has been destroyed by pro-
longed maceration in dilute nitric acid, a consistent skeleton still
remains. This mineral, in fact, constitutes in some species not less
than 13 percent, 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 silicious particles are distributed in two lines, parallel to the
axis ; others, however, are grouped into oval forms, connected with
each other, like the jewels of a necklace, by a chain of particles
forming a sort of curvilinear quadrangle ; and these (which are, in
fact, the particles occupying the guard-cells of the stomates) are
arranged in pairs. Their form and arrangement are peculiarly well
seen under polarised light, for which the prepared epiderm is an
extremely beautiful object ; and it is asserted by Sir D. Brewster
(whose authority upon this point has been generally followed) that
each silicious particle has a regular axis of double refraction. What
is usually designated as the fructification of the Equisetacese forms a
cone or spike at the extremity of certain of the stem-like branches
(the real stem being a horizontal rhizome), and consists of a cluster
of shield-like discs, each of which carries a circle of sporanges or
spore-capsules, that open by longitudinal slits to set free the spores.
Each of these spores has attached to it two pairs of elastic filaments
(tig. 465) called elaters ; these are at first coiled up around the
spore, in the manner represented at A, though more closely applied
to the surface ; but, on the liberation of the spore, they extend them-
selves in the manner shown at B, the slightest application of mois-
ture, however, serving to make them close together (the assistance
which they afford in the dispersion of the spores being no longer
required) when the spores have alighted on a damp surface. If a
number of these spores be spread out on a slip of glass under the

6o6 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMS

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 condi-
tion when the effect of the moisture has passed off. If one of the
sporanges which has opened, but has 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 arche-
gones, 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 Rhizocarpese (or Pepper-worts), which either
float on the surface or creep along shallow bottoms. These all agree
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 anthe-
rozoid. In this, as we shall presently see, there is a distinct fore-

Fig. 465. — Spores of Equisetum, with their elaters.

shadowing of the mode in which the generative process is performed
in flowering plants, the ' microspore ' obviously corresponding to
the pollen-grain, while the 1 megaspore ' may be considered to repre-
sent the primitive cell of the ovule.

Another alliance of Ferns is to the Lycopodiacese (Club-mosses),
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
spores 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
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

SELAGINELLEiE AND EHIZOCAKPEJE

607

inflammability that causes their being used in theatres to produce
' artificial lightning.' But in the allied groups of Selaginellece and
Isoetece there are (as in the R1iizocarpeaj) two kinds of spore pro-
duced in separate sporanges j one set producing * megaspores,' from
which archegone-bearing prothallia are developed, and the other
producing ' microspores,' which, by repeated subdivision, give origin
to antherozoids without the formation of prothallia. It is a very
interesting indication of a tendency towards the phanerogamic type
of sexual generation, that the prothallium in this group is chiefly
developed within the sporange, forming a kind of ' endosperm,' only
the small part which projects from the ruptured apex of the spore
producing one or more archegones. The arborescent Lepidodendra
and SigiUarice 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 its basal portion, and ' microspores ' in
those of its upper part. Some of the best seams of coal appear to
have been chiefly formed by the accumulation of these ' megaspores.'

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 the ' 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 true seed, and supplies the material for its early development.
In the lower Cryptogams we have seen that the fertilised oospore
is thrown at once upon the world (so to speak) to get its own living ;
but in ferns and their allies the ' embryo- cell ' is nurtured for a
while by the prothallium of the parent plant. While the true
reproduction of the species is effected by the proper generative act,
the multiplication of the individual is accomplished by the production
and dispersion of ' gonidial ' spores ; and this production, as we have
seen, takes place at very different periods of existence in the several
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 3/archantia,
evolved from the spore and bearing the antherids and archegones,
is that which seems naturally to constitute the plant ; hut that which
represents this phase in the ferns is the minute Marchantia-like
prothallium. In ferns, on the other hand, the product into which

6o8 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMS

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 and
Special Morphology, and De Bary's Comparative Anatomy of the Phanerogams
and Ferns, translations of both of which have been published by the Clarendon
Press ; and especially to Bennett and Murray's Handbook of Cryptogamic TSotany,
published by Longmans (London, 1889).

6og

CHAPTER XI

OF THE MICBOSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

Between the two great divisions of the Vegetable Kingdom which
are known as Cryptogarma and Plianerogamia the separation is by
no means so abrupt as it formerly seemed to be. For, as has been
already shown, though the Cryptogamia were formerly regarded as
altogether non-sexual, a true generative process, requiring the
concurrence of male and female elements, is traceable almost through-
out the series. And in the higher types of that series we have seen
a foreshadowing of those provisions for the nurture of the fertilised
embryo which constitute the distinctive characters of the Phanero-
gamia. On the other hand, although we are accustomed to speak of
Plianerogamia as 'flowering plants/ yet not only are the conspicuous
parts of the flower often wanting, but in the important group of
Gymnosperms (including the Coniferce and Cycadece) the essential
parts of the generative apparatus are reduced to a condition closely
apjDroximating 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-
moving ' antherozoids ' that the contents of the sperm -cell find their
way to the germ-cell ; these are conveyed to it, throughout the
phanerogamic series, by an extension of the lining membrane of the
sperm-cell or pollen-grain into a tube, which penetrates to the germ-
cell, contained in the interior of the body called the ' ovule.' Again
(2), while the ' germ-cell ' or oosphere in the higher Cryptogams is
contained in a structure that originated in a spore detached from the
parent plant, it is not only formed and fertilised in all Phanerogams
whilst still borne on the parent fabric, but continues for some time
to draw from it the nutriment it requires for its development into the
' embryo.' And at the time of its detachment from the parent the
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 ovules, which when fertilised and matured become seeds,
are developed from specially modified leaves, which remain open in
Gymnosperms, but which in all other Phanerogams fold together so
as to enclose the ovules within an ' ovary.' Each ovule consists of a

R R

6lO MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

* nucellus ' surrounded by ' integuments ' which remain unclosed at
the apex, leaving open a short canal termed the ' micropyle ' or
' foramen.' One cell of the nucleus 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
1 oosphere ' is developed ; in Phanerogams by free cell-formation, but
in Gymnosperms indirectly after the formation of a ' corpuscle,' which
represents the archegone of Selaginella. By a further process of
free cell-formation the remainder of the embryo-sac comes to be
filled with cells, constituting what is termed the ' 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. — No marked change shows itself in general
organisation as we pass from the cryptogamic to the phanerogamic
series of plants. For 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 luoody 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, not only the pith
and the cortex, with the ' medullary rays,' which serve to connect
them, but the 'cambium layer' intervening 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 growing points of the root-fibres, in the leaf -buds
and leaves, and in the flower-buds and sexual parts of the flower ;
it is only when these organs attain an advanced stage of develop-
ment that ivoody structure is found in them, its function (as in
the stem) being merely to give 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 perse-
verance, may be made of leaves, flowers, and certain fruits. All the
softer and more pulpy tissue of these organs is composed of cells,
more or less compactly aggregated together, and having forms that
approximate more or less closely to the globular or ovoidal, which
may be considered as their original type.

As a general rule, the rounded shape is preserved only when the
cells are but loosely aggregated, as in the parenchymatous (or pulpy)

PARENCHYMATOUS TISSUES

6ll

substance of leaves, which often forms a distinct layer known as the
' spongy parenchyrne ' immediately beneath the epiderm of the upper
surface (fig. 466), and it is then only that the distinctness of their
walls become evident. When the tissue becomes more solid, the
sides of the vesicles are
pressed against each ^ j,

other, so as to flatten
them and to bring them
into ' close apposition,
and then the cavities
of adjacent cells are
separated by a single par-
tition wall. Frequently
it happens that the pres-
sure is exerted more in
one direction than in
another, so that the form
presented by the outline
of the cell varies accord-
ing to the direction in
which the section is
made. This is well
shown in the pith of the

young shoots of elder,

Fig. 466. — 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 ;
c, cells of parenchyrne ; d, their protoplasmic-
contents.

lilac, or other rapidly
growing trees, the cells of which, when cut transversely, generally
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. 466. A very good example of
such a cellular parenchyrne is to be found in the substance known as
rice-paper, which is made by cutting the herbaceous stem of a
Chinese plant termed xiralia papyrifera vertically round and round

6l2 MICBOSCOPIC STBUCTUKE OF PHANEKOGAMIC PLANTS

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. 467, B ; but if the stem be cut transversely, ,
their outlines are seen to be circular or nearly so (A). When, as
often happens, the cells have a very elongated form, this elongation
is in the direction of their growth, which is that, of course, wherein

there is least resistance. Hence
their greatest length is nearly
always in the direction of the
axis ; but there is one remark-
able exception, that, namely, -
which is afforded by the ' medul-
lary rays ' of exogenous stems
whose cells are greatly elongated
in the horizontal direction (fig.
489, a), their growth being from
the centre of the stem towards-
its circumference. It is obvious
that fluids will be more readily
greatest elongation, being that in

Fig. 468. — Section of stellate
parenchyme of rush,

transmitted in the direction of

vegetable
stellate

cells pre-
cell, repre^
forming the

which they will have to pass through the least number of partitions ;
and whilst their ordinary course is in the direction of the length of
the roots, stems, or branches, they will be enabled by means of the
medullary rays to find their way in the transverse direction. One

of the most curious varieties of
form which
sent is the
sented in fig. 468,
spongy parenchymatous 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 :
such is the case in the Nuvhar
lutea (yellow water-lily), the foot-
stalks of whose leaves contain
large air-chambers, the walls of
which are built up of very regular
cubical cells, whilst some curi-
ously formed large stellate cells
project into the cavity which they
bound (fig. 469). The dimensions
of the component vesicles of cel-
lular tissue are extremely variable ; for although their diameter
is very commonly between ^ Joth and 7^3-0 th of an inch, they occa-
sionally measure as much as ^th. of an inch across, whilst in-
other instances they are not more than -g-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

Fig. 409. — Cubical parenchyme, with
stellate cells, from petiole of Nu/phar
lutea.

STRUCTURE OF THE CELL

6l3

•cells previously in existence. The original cell- wall must therefore
-always be single. It is only in older thick -walled cells that a line of
demarcation becomes obvious in the form of an intermediate lamella,
at one time called 1 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 ob-
vious" that ' intercellular spaces ' must exist between them ; and as the
tissue develops, these spaces often increase greatly in size. They are
called ' schizogenous ' if formed simply by the parting of cells from
one another ; 4 lysigenous ' 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
then* condition ; this is the case with the pulp of ripe f raits, such as
the strawberry or currant (the snowberry is a particularly favour-
able subject for this kind of examination), and with the parenchyme
•of many fleshy leaves, such as those of the carnation (Dianthus
caryophyllus) or the London pride (Saxifraga umbrosa). Such cells
usually contain evident nuclei which are turned brownish-yellow by
iodine, whilst their membrane is only turned pale yellow, and in this
way the nucleus may be brought into view, when, as often happens,
it is not previously distinguishable. If a drop of the iodised solution
of chloride of zinc be subsequently added, the cell-membrane becomes
of a beautiful blue colour, whilst the nucleus and the granular proto-
plasm 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 pri / no /'dial or parietal
utricle, its contents being made to coagulate and shrink, so that it
detaches itself from the cellulose wall with which it is ordinarily in
contact, and shrivels up within its cavity, as shown in fig. 466. It
would be a mistake, however, to regard this as a distinct membrane ;
for it is nothing else than the peripheral layer of protoplasm, natu-
rally somewhat more dense than that which it includes, but passing
into it by insensible gradations.

It is probable that all cells, at some stage or other of their
growth, exhibit, in a greater or less degree of intensity, that curious
movement of cyclosis which has been already described as occurring
in the Characece, 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 examples 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, Vallisneria spiralis and Anacharis alsinastrum (or
Elodea canadensis), which are peculiarly fitted for the exhibition
•of this interesting phenomenon. Vallisneria is an aquatic plant

6l4 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

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 of mould,
which, after the roots have been inserted into it, should be closely
pressed down, the jar being then filled with water, of which a portion
should be occasionally changed.1 The jar should be freely exposed
to light, and should be kept in as warm but equable a temperature
as possible. The long grass-like leaves of this plant are too thick to
allow the transmission of sufficient light through them for the purpose
of this observation, and it is requisite to make a thin slice or shaving
with a sharp knife. If this be taken from the surface, so that the
section chiefly consists of the superficial layer of cells, these will be
found to be small, and the particles of chlorophyll, though in great
abundance, will rarely be seen in motion. This layer should there-
fore be sliced off (or, perhaps still better, scraped away) so as to bring
into view the deeper layer, which consists of larger cells, some of them
greatly elongated, with particles of chlorophyll in smaller number,,
but carried along in active rotation by the current of protoplasm ;
and it will often be noticed that the directions of the rotation in
contiguous cells are opposite. If the movement (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 phenome-
non, 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 objective of i-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 Leland enables the borders of the proto-
plasmic current, which carries along the particles of chlorophyll,,
to be distinctly defined ; and this beautiful 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

1 Mr. Quekett found it the most convenient method of changing the water in the
jars in which Chara, Vallisneria, &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.

CYCLOSIS OY PliOTOPLASM

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 Chara or in Vallisneria, the ^>
inch_ object glass being here preferable to the -}-in., 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 ^J^th to ^^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 ^yth of an inch
j>er minute, being sufficient to carry them several times round the
cell within that period. As in the case of Vallisneria, the motion
may frequently be observed to take place in opposite directions
in contiguous cells. The thickness of the layer of protoplasm in
which the granules are carried round is estimated by Mr. Wenham
at no more than 2o1y<roth °^ an mcn- When high powers and
careful illumination are employed, delicate ripples may be seen in the
protoplasmic currents. 1

Cyclosis, however, is by no means restricted to submerged plants ;
for it has been witnessed by numerous observers in so great a variety
of other species that it may fairly be presumed to be universal. It is
especially observable in the hairs of the epidermal surface. Such
hairs are furnished by various parts of plants ; and what is chiefly
necessary is, that the part from which the hair is gathered should be
in a state of vigorous growth. The hairs should be detached by
tearing off with a pair of fine pointed forceps the portion of the
epiderm from which they spring, care being taken not to grasp the
hair itself, whereby such an injury would be done to it as to check
the movement within it. The apochromatic hair should then be
placed with a drop of wTater under thin glass ; and it will generally
be found advantageous to use a Jj-inch with the 12 or the 18 eye-
piece objective with an achromatic condenser. The nature of
the movement in the hairs of different species is far from being
uniform. In some instances, the currents pass in single lines
along the entire length of the cells, as in the hairs from the filaments
of Tradescantia virginica, or Virginian spiderwort (fig. 470, A) ; in
others there are several such currents which retain their distinctness,
as in the jointed hairs of the calyx of the same plant (B) ; in others,
again, the streams coalesce into a network, the reticulations of which

1 Quart. Journ. of Microsc. Science, vol. iii. (1855), p. 277. . ■'•

6l6 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

change their position at short intervals, as in the hairs of Glaucium
luteum ; whilst there are cases in which the current flows in a slug-
gish uniformly moving sheet or layer. Where several distinct cur-
rents exist in one cell, they are all found to have one common point
of departure and return, namely, the nucleus (B, a), from which it
seems fairly to be inferred that this body is the centre of the vital
activity of the cell. In all cases in which the cyclosis is seen in the

hairs of a plant, the cells of the
epiderm also display it, provided
that their walls are not so opaque
or so strongly marked as to pre-
vent the movement 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
speedily as possible, since it loses
its vitality when thus detached
much sooner than do the hairs.
Even when no obvious movement
of particles is to be seen, the
existence of a cyclosis may be
concluded from the peculiar
arrangement of the molecules
of the protoplasm, which are re-
markable for their high refrac-
tive power, and which, when
arranged in a ' moving train,'
appear as bright lines across the
cell ; and these lines, on being
carefully watched, are seen to
alter their relative positions.
The leaf of the common Plantago
(plantain) furnishes an excellent
example of cyclosis, the move

ment being distinguishable at the

same time both in the cells and
in the hairs of the epiderm torn
from its stalk or midrib. It is
a curious circumstance that when

Fig. 470— Rotation of fluid in hairs of

Tradescantia virgmica : A, portion of

epiderm with hair attached; a, b, c,

successive cells of the hair ; d, cells of

the epiderm ; e, stomate. B, joints of a

beaded hair showing several currents ; a plant which exhibits the cyclo-

a, nucleus. sis js kept in a cold dark place

for one or two days, not only is
the movement suspended, but the moving particles collect together
in little heaps, which are broken up again by the separate motion
of their particles, when the stimulus of light and warmth occasions
a renewal of the activity. It is well to collect the specimens about
midday, that being the time when the rotation is most active, and
the movement is usually quickened by artificial warmth, which, in-
deed, is a necessary condition in some instances to its being seen at
all. The most convenient method of applying this warmth, while

STRUCTURE OF THE CELL-WALL

B

the object is on the stage of the microscope, is to blow a stream of
air upon the thin glass cover through a glass or metal tube pre-
viously 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 ifcs simplest condition such a
deposit forms a thin uniform
layer over the whole internal
•surface of the cellulose wall,
scarcely detracting at all from
its transparency, and chiefly
distinguishable by the ' clotted '*
appearance which the membrane
then presents (fig. 467, A).
These dots, however, are not
pores, as their aspect might
naturally suggest, but are merely
points at which the deposit is
wanting, so that the original cell- wall there remains unthickened.
A more complete consolidation of cellular tissue is effected by
deposits of sclerogen (a substance which, when separated from the
resinous and other matters that are commonly associated with it,
is found to be allied in chemical composition to cellulose) in succes-

Fig. 471. — Tissue of the testa or seed-coat
of star-anise : A, as seen in section ;
B, as seen on the surface.

Fig. 472. — Section of cherry-stone,
cutting the cells transversely.

Fig. 473. — Section of coquilla
nut, in the direction of the
long diameter of the cells.

sive layers, one within another (fig. 471, 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-

6l8 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

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 parti-
tion is the only obstacle to the communi-
cation between their cavities (figs. 471-
473). 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 endo-
sperm of the seed of Phytelephas (known
as ' vegetable ivory ') are made up ; and we

Fig. 474.-Spiral cells of leaf see th? use of . this very curious arrange-
of Onciclium. ment in permitting the cells, even after

they have attained a considerable degree
of consolidation, still to remain permeable to the fluid required for
the nutrition of the parts which such tissue encloses and protects.

The deposit sometimes assumes, however, the form of definite
fibres, which lie coiled up on the inner surface of the cells, so as to
form a single, a double, or even a triple or quadruple spire (fig.
474). Such spired cells are found abundantly in the leaves of certain

orchidaceous plants, immediately
beneath the epiderm, where they
are brought into view by vertical
sections ; and they may be obtained
in an isolated state by macerating
the leaf and peeling off the epiderm
so as to expose the layer beneath,
which is then easily separated into
its components. In an orchid-
aceous plant named Saceolabium
yuttatum the spiral cells are un-
usually long, and have spires wind-
ing in opposite directions, so that,
by their mutual intersection, a
series of diamond-shaped markings
is produced. Spiral cells are often
found upon the surface of the
testa or outer coat of seeds ; and
in Collomia grandifiora, 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. 475), springing out,
as it were, from the surface of the seed, to which they give a,
peculiar flocculent appearance. This very curious phenomenon may:

Fig. 475. — Spiral fibres of seed-coat of
Collomia,

STARCH-GRAINS

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 may
be exactly focussed to the object, the power employed being the
1-inch, |-inch, or Vinch. The cover of the aquatic box being
then removed, a small drop of water should be placed on that part
of its internal surface with which the slice of the seed had been in
contact ; and the cover being replaced, the object should be im-
mediately looked at. It is important that the slice of the seed
should be very thin, for two reasons ; first, that the view of the
spirals may not be confused by their aggregation in too great
numbers ; and second, that the drop of water should be held in its
place by capillary attraction, instead of running down and leaving
the object, as it will do if the glasses be too widely separated.

In some part or other of most plants we meet with cells contain-
ing granules of starch, which specially abound in the tubers of the
potato and in the seeds of cereals. Starch-grains are originally
formed in the interior of chlorophyll-corpuscles, and therefore within

Fig. 476. — Cells of peony filled Fig. 477. — Granules of starch as

with starch. seen under polarised light.

the protoplasm-layer of the cell ; but as they increase in size, the
protoplasm-layer thins itself out as a mere covering film, and at last
almost entirely disappears. So long as the starch-grains remain
imbedded in the protoplasm-layer they continue to grow ; but when
they accumulate so as to occupy the cell-cavity their growth stops.
They are sometimes minute and very numerous, and so closely
packed as to fill the cell-cavity (fig. 476) ; in other instances they
are of much larger dimensions, so that only a comparatively small
number of them can be included in any one cell ; while in other
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 hilum, 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 hilum (fig. 477) ; 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

620 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

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 the one now generally adopted,
though the alternative theory of formation by the ' apposition ' of
successive layers has many advocates. These layers differ in their
proportion of water, the outermost layer, which is the most solid,
having within it a watery layer, this, again, being succeeded by a
.firm layer, which is followed by a watery layer, and so on, the
proportion of water increasing towards the centre in both kinds of
.layer, and attaining its maximum in the innermost part of the
grain, where the 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
starch-grains of different plants that yield this product in abundance
may be microscopically distinguished from one another — a circum-
stance of considerable importance in commerce. The largest starch-
grains in common use are those of the plant (a species of Canna)
known as tous-les-mois. The average diameter of those of the potato
is about the same as the diameter of the smallest of the tous-les-mois,
and the size of the ordinary starch-grains of wheat and of sago is
.about the same as that of the smallest grains of potato-starch ; whilst
the granules of rice-starch are so very minute as to be at once dis-
tinguishable 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 contain-
ing 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 Papaverace?e, of which our
common field-poppy is an example, many Composite such as the
dandelion and lettuce, Convolvulacese, Euphorbiacese or spurges,
Apocynacese, Moraceae including the mulberry tfcc.

Deposits of mineral matter in a crystalline condition, known as
raphides, are not unfrequently found in vegetable cells, where they
are at once brought into view by the use of polarised light. Their
designation (derived from 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 are
scarcely less abundant ; thus, instead of bundles of minute needles,
single large crystals, octohedral or prismatic, are frequently met

RAPHIDES

621

with, and the prismatic crystals are often aggregated in beautiful
stellate groups. The most common material of these crystals is
oxalate of lime, which is generally found in the stellate form ; and no
plant yields these stellate raphides so abundantly as the common
rhubarb, the best specimens of the dry medicinal root containing as
much as 35 per cent, of them. In the epiderm of the bulb of the
onion the same material occurs in the octohedral 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 ^Vth of
an inch, while others measure no more than x ^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 Cactus tribe, when aged, have their
tissues so loaded with raphides as to become quite brittle, so that
when some large specimens of C. senilis, said to be a thousand years
old, were sent to Kew Gardens from South America, some years
since, it was found necessary for their preservation during transport
to pack them in cotton like jewellery. Raphides are probably to be
considered as non-essential results of the vegetative processes, being
for the most part produced by the union of organic acids generated
in the plant with mineral bases imbibed by it from the soil. The
late Mr. E. Quekett succeeded in artificially producing raphides.
within the cells of rice-paper, by first filling these with lime-water
by means of the air-pump, and then placing the paper in weak
solutions of phosphoric and oxalic acids. The artificial raphides of
phosphate of lime were rhombohedral • while those of oxalate of
lime were stellate, exactly resembling the natural raphides of the
rhubarb. Besides the structures already mentioned as affording good
illustrations of different kinds of raphides, may be mentioned the
parenchyme of the leaf of Agave, Aloe, Cycas, Uncephalartos, &c. ;
the epiderm of the bulb of the hyacinth, tulip, and garlic ; the bark of
the apple, Cascarilla, Cinchona, lime, locust, and many other trees ; the
pith of Elceagnus, and the testa of the seeds of Anagallis 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 woody fibre or
frosenchymatous tissue. This, however, can only be regarded as a
variety of cellular tissue ; for it is composed of peculiarly elongated
cells (fig. 493), usually pointed at their two extremities so as to
become spindle-shaped, whose wTalls 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 wToody 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

622 MICROSCOPIC STEUCTUEE OF 'PHANEROGAMIC PLANTS

■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 1 stringiness ' to various
esculent vegetable substances, are commonly known under the
name of fibro-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 fibro-vascular bundles, which are the chief
strengthening elements of such organs as the stem, branches, leaf-
stalks, flower-stalks &c, are, in the higher
plants, structures of considerable com-
plexity ; in Exogens they consist of three
distinct portions, the xylem-^OTtion. com-
posed chiefly of the different kinds of
vessels hereafter to be described, 2,j)hloem-
portion composed of prosenchymatous
tissue and ' sieve-tubes,' and a formative
cam&mm-portion.

A peculiar set of markings seen on
the woody fibres of the Coniferce, and of
some other tribes, is represented in fig.
478 ; 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

-[-I ArrQ c n indicates the boundary of a lenticular

Fig. 478. — Section ol coniferous . . J

wood in the direction of the cavity which intervenes between the ad-
trachei'des, showing their jaCent cells at this point. There are
JW«^£ varieties in this arrangement so eharac-
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
order, are known as 1 bordered pits,' and the elongated cells in which
they occur as ' tracheides.'

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 may be single, double, or even quadruple, this last character pre-

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 as having
any more claim to the designation of vessels, than have the elongated cells of the
woodv tissue.

STRUCTURE OF WOODY TISSUE

623

-senting itself in the very large elongated fibre-cells of Nepenthes
(Chinese pitcher- plant). Such vessels are especially found in the deli-
cate membrane (medullary sheath) surrounding the pith of Exogens,
and in the 1 xylem-portion ' of the woody bundles of Exogens and
Endogens ; thence they proceed to the leaf- stalks, through which
they are distributed to the leaves. By careful dissection under the
microscope these fibro -vascular bundles may be separated entire ;
but their structure may be more easily displayed by cutting round,
but not through, the leaf -stalk of the strawberry, geranium, &c. and
then drawing the parts asunder. The membrane composing the
tubes of the vessels will thus be broken across ; but the fibres within,
being elastic, will be drawn out and unrolled. Spiral vessels are
sometimes found to convey fluid, whilst in other cases they contain air
only ; the conditions of this difference are not yet certainly known.

Although fluid generally finds its way with tolerable facility
through the various forms of cellular tissue, especially in the direc-
tion of the greatest length of the cells, a more direct means of con-
nection between distant parts is required for its active transmission.
This is afforded by the peculiar kind of vessel 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 evident, both in the contraction of their diameter at regular
intervals, and in the persistence of remains of their partitions (fig. 493,
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. 479, 2) are indistinguishable from
the spired 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 to be annular (fig. 479, 1). Intermediate forms between the
spiral and annular ducts, which show the derivation of the latter
from the former, are very frequently to be met with. The spirals are
sometimes broken up still more completely, and the fragments of the
fibre extend in various directions, so as to meet and form an irregular
network lining the duct, which is then said to be reticulated. The
continuance of the deposit, however, gradually contracts the meshes,
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. 479, 3). The scalariform ducts of ferns may be re-
garded as a modification of the spiral ; but spiral ducts are fre-
quently to be met with also in the rapidly growing leaf-stalks of
flowering plants, such as the rhubarb. Not unfrequently, however,
we find all forms of ducts in the same bundle, as seen in fig. 479.
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 ;

624 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

and, generally speaking, they are larger in woods of dense texture,
such as oak and mahogany, than in those of which the fibres,
remaining unconsolidated, can serve for the conveyance of fluid.
They are entirely absent in the Conifer en.

The vegetable tissues whose principal forms have been now de-
scribed, but among which an immense variety of entail is found, may
be either studied as they present themselves in thin sections of the
various parts of the plant under examination, or in the isolated
conditions in which they are obtained by dissection. The former
process is the most easy, and yields a large amount of information ;
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 sub-
stances, such as ' vege-
table ivory,' the ' stones '
of fruit, the 'shell' of
the cocoa-nut, &c. can
scarcely be obtained ex-
cept by slicing and grind-
ing ; and these may be
mounted either in Canada
balsam or in glycerin-
jelly. In cases, however,
in which the tissues are
of only moderate firm-
ness, the section may be
most readily and effectu-
ally made with the ' mi-
crotome ' ; and there are
few parts of the vege-
table fabric which may
not be advantageously
examined by this means,
any very soft or thin por-
tions being placed in it
between two pieces of
cork, elder-pith, or carrot.
In certain cases, how-
ever, in which even this compression would be injurious, the sec-
tions must be made with a sharp knife, the substance being laid om
the nail or a slip of glass. In dissecting the vegetable tissues
scarcely any other instrument will be found really necessary than
a pair of needles (in handles), one of them ground to a cutting edge.
The adhesion between the component cells, fibres, &c. is often
sufficiently weakened by a few hours' maceration to allow of their
readily coming apart, when they are torn asunder by the needle-
points beneath the simple lens of a dissecting microscope. But if
this should not prove to be the case, it is desirable to employ some
other method for the sake of facilitating their isolation. None is so-
effectual as the boiling of a thin slice of the substance under exami-

Fig. 479. — Longitudinal section of stem of Italian
reed : a, cells of the pith ; b, fibro-vascular
bundle, containing 1, annular ducts ; 2, spiral
ducts ; 3, pitted ducts with woody fibre ; e, cells
of the epiderm.

STRUCTURE OF STEMS

625

nation either in dilute nitric acid or in a mixture of nitric acid and
chlorate of potassa. This last method (which was devised by
Schultz) is the most rapid and effectual, requiring only a few
minutes for its performance ; but as oxygen is liberated with such
freedom as to give an almost explosive character to the mixture, it
should be put in practice with extreme caution. After being thus
treated the tissue should be boiled in alcohol, and then in water ;
and it will then be found very easy to tear apart the individual cells,
ducts, ifcc. 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 is included the stem with its branches, and the root with its
ramifications) always has for the basis of its structure a dense cellular
parenchyme ; though, in an advanced stage of development, this
may constitute but a small portion of it. In the midst of the
parenchyme we generally find fibro-vascular bundles, consisting of
woody fibre, with ducts of various kinds, and (very commonly) spiral
vessels. It is in the mode of arrangement of these bundles that the
fundamental difference exists between the stems which are commonly
designated as endogenous (growing from within), and those which
are more correctly termed exogenous (growing on the outside) ; for
in the former the bundles are dispersed throughout the whole
diameter of the axis without any peculiar plan, the intervals between
them being filled up by cellular parenchyme ; whilst in the latter
they are arranged side by side in such a manner as to form a cylinder
of icood, 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 bark. These two plans of axis-formation
respectively characteristic of those two great groups into which
Phanerogams are subdivided — namely, the Monocotyledons and the
Dicotyledons — will now be more particularly described.

When a transverse section (fig. 480) of a monocotyledonous stem
is examined microscopically, it is found to exhibit a number of fibro-
vascular bundles, disposed without any 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
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 thev form is at once distinguished in transverse
section by the closeness of its texture (fig. 481). 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 absolutely 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 fibro-vascular

s s

626 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

bundles ; so that a sort of indication of the distinction between pith,,
wood, and bark is here presented. This distinction, however, is
very imperfect ; for we do not find either the central or the peripheral
portions ever separable, like pith and bark, from the intermediate

Fig. 480. — Transverse section of stem of young palm.

woody layer. In its young state the centre of the stem is always
filled up with cells ; but these not unfrequently disappear after a
time, except at the nodes, leaving the stem hollow, as we see in the

whole tribe of grasses. When a
vertical section is made of a 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 towards the surface
of the stein, and assist, by their in-
terlacement with the outer bundles,
in forming that extremely tough in-
vestment which the lower ends of
these stems present. New fibro-
vascular bundles are being continu-
ally formed in the upper part of the
stem, in continuity with the leaves
which are successively put forth at
Fig. 481.— Portion of transverse its summit ; but while these take part-
section of stem of Wanghie cane. {n the elongation of the stem, they

contribute but little to the increase of
its diameter. For those which are most recently formed only pass
into the centre of the stem during the higher part of their course,
and usually make their way again to its exterior at no great distance
below ; and, when once formed, they receive no further additions.

STRUCTURE OF STEMS

627

It was from the idea formerly entertained 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 continu-
ally receiving additions to its interior, that the term endogenous was
given to this type of stem-structure ; but, from the fact just stated
regarding the course of the fibro- vascular bundles, it is obvious that
such a doctrine cannot be any longer
admitted.

In- the stems of dicotyledonous phane-
rogams, 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 divi-
sion is always seen in a transverse sec-
tion (fig. 482) between three concentric
areas — the pith, the tcood, and the
bark — the first («) being central, the last
(b) peripheral, and- these having the
wood interposed between them, its circle
being made up of wedge-shaped bundles
(d, d), kept apart by the medullary rays
composed of unchanged cellular tissue
(c, c) that pass between the pith and the bark. The pith (fig. 483, a)
is almost invariably composed of cellular tissue only, which usually
presents (in transverse section) a hexagonal areolation. AVhen
newly formed it has a greenish hue, and its cells are filled with
fluid ; but it gradually dries up and loses its colour ; and not un-
frequently its component cells are torn apart by the rapid growth
of their envelope, so that irregular cavities are found in it ; or if

Fig. 483. — Transverse section of stem of Clematis: a, pith; b, b, b, woody bundles;

c, c, c, medullary raj s.

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 medullary sheath.

The woody portion of the stem (fisr. 483, by b) is made up of woody

ss2

Fig. it 2. — Diagram of the first
formation of an exogenous
stem : a, pith ; b b, bark ; c c,
plates of cellular tissue (me-
dullary rays) left between the
woody bundles d d.

628 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

fibres, usually with the addition of ducts of various kinds ; these,
however, are absent in one large group, the Conifer m or fir- tribe
with its allies (figs. 487-490), in which the prosenchymatous cells or
tracheides are of unusually large diameter, and are marked by the
bordered pits already described. In any stem or branch of more
than one year's growth the woody structure presents a more or less
distinct appearance of division into concentric rings, the number of
which varies with the age of the tree (fig. 484). 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

Fig. 484. — Transverse section of stem Fig. 485. — Portion of the

of Rhamnus (buckthorn), showing same more highly

concentric layers of wood. magnified.

figured appearance to the transverse section (figs. 484, 485). 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. 483. 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.

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 no doubt, however, that such is not the universal rule ;
and that we should be more correct in stating that each layer indi-
cates an epoch of vegetation, which, in temperate climates, is usually
(but not invariably) a year, but which is commonly much less in the

STEUCTUKE OF STEMS

629

case of trees flourishing in tropical regions. Thus among the latter
it is very common to Unci 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 processes
of vegetation produced by seasonal changes — as by heat and drought
in a tree that flourishes best in a cold damp atmosphere, or by a fall
of temperature in a tree that requires heat — would appear from the
frequency with which a double or even a multiple succession of rings
is found in transverse sections of wood to occupy the place of a
single one. Thus in a section of hazel stem (in the Author's posses-
sion), of which a portion is represented in tig. 486, between two
layers of the ordinary thickness there intervenes a band whose
breadth is altogether less than that of either of them, and which is

a b c

Fig. 486. — Portion of transverse section of stem of hazel, showing, in the portion

a, b, c, six narrow layers of wood.

yet 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 matters deposited within their
component cells and vessels, are spoken of collectively under the
designation duramen or ' heart-wood.' On the other hand, it is
through the cells and ducts of the outer and newer layers that the
sap rises from the roots towards the leaves ; and these are conse-
quently designated as alburnum or 4 sap-wood.' The line of demar-
cation between the two is sometimes very distinct, as in lignum vitse
and cocos-wood ; and as a new ring is added every year to the ex-
terior of the alburnum an additional ring of the innermost part of
the alburnum is every year consolidated by internal deposit, and is
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
plates of cellular tissue (fig. 483, c, c), not usually extending to any

630 MICEOSCOPIC STEUCTUEE OF PHANEROGAMIC PLANTS

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. 485, 487), and their real nature is best
understood by a comparison of longitudinal sections made in two

Fig. 487. — Portion of transverse section of the stem of cedar: a, pith;
b, b, b, woody layers ; c, bark.

different directions — namely radial and tangential — with the trans-
verse. Three such sections of a fossil coniferous wood in the
Author's possession are shown in figs. 488-490. The stem was of
such large size that, in so small a part of the area of its transverse
section as is represented in fig. 488, the medullary rays seem to run
parallel to each other, instead of radiating from a common centre.
They are very narrow ■ but are so closely set together that only two

or three rows of tracheides (no
ducts being here present) in-
tervene between any pair of
them. In the longitudinal
section taken in a radial direc-
tion (fig. 489) and consequently
passing in the same course with
the medullary rays, these are
seen as thin plates (a, a, a)
made up of superposed cells
very much elongated, and cross-
ing in a horizontal direction the
Fig. 488.— Portion of transverse section of tracheides which lie parallel to
i large stem of coniferous wood (fossil), one another vertically. And

showing part of two annual rings divided jn the tangential section (fig.
at a, a, and traversed by very thm but 1 • i ■ . -i • tV

numerous medullary rays. 490), which IS taken HI a direc-

tion at right angles to that of
the medullary rays, and therefore cuts them across, we see that each
of the plates thus formed has a very limited depth from above clown-
wards, and is composed of no more than one thickness of cells in the
horizontal direction. A section of the stem of mahogany taken in
the same direction as the last (fig. 491) gives a very good view of the
cut ends of the medullary rays as they pass between the prosenchy-

STRUCTURE OF STEMS

63I

matous 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.

Fig. 489. — Portion of vertical section of the
same wood, taken in a radial direction,
showing the trachei'des with 'bordered
pits,' without ducts, crossed by the medul-
lary rays, a, a.

I fit I

Fig. 490. — Portion of vertical
section of the same wood,
taken in a tangential direc-
tion, so as to cut across the
medullary rays.

In another fossil wood, whose transverse section is shown in
fig. 492, and its tangential section in fig. 493, 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
(a a, a a) intervening between the closely
set prosenchymatous cells, among which
some large ducts are scattered ; whilst in
the tangential section they are observed
to be not only deeper than the preceding
from above downwards, but also to have
a much greater thickness. This section
also gives an excellent view of the ducts,
bb, b b, which are here plainly seen to be
formed by the coalescence of large cylin-
drical cells lying end to end. In another
fossil wood in the Author's possession the
medullary rays constitute a still larger
proportion of the stem ; for in the trans-
verse section (fig. 492), they are seen as
very broad bands (b, b), alternating with
plates of woody structure (a a), whose
thickness is often less than their own ;
whilst in the tangential section (fig. 495)
the cut extremities of the medullary rays occupy a very large part
of the area, having apparently determined the sinuous ccurse of the

Fk

491. — Vertical section of
mahcgauv.

632 MICROSCOPIC STEUCTUEE OF PHANEROGAMIC PLANTS

prosenchymatous cells, instead of looking (as in fig. 490) as if they
had forced their way between these cells, which there hold a nearly

a a

Fig. 492. — Transverse section of a
fossil wood, showing the medullary
rays, a, a, a, a, a, a, running nearly
parallel to each other, and the
openings of large ducts in the midst
of the prosenchymatous tissue.

Fig. 498. — Vertical (tangential) sec-
tion of the same wood, showing the
prosenchymatous cells separated
by the medullary rays, and by the
large ducts, b b, b b.

straight and parallel course on either side of them. The medullary
rays maintain a connection between the external and the internal

Figs. 494 and 495. — Transverse and vertical sections of a fossil wood,
showing the separation of the woody plates, a a, a a, by the very
large medullary rays, b b, b b.

parts of the cellular tissue or fundamental par end ty me of the stem,,
which have been separated by the interposition of the wood.

STEUCTURE OF STEMS

633

The bark is usually found to consist of three principal layers :
the external or epiphlceum, which includes the suberous (or corky)
layer ; the middle, or mesophlceum, also termed the cellular envelope ;
and the internal, or endophlosum, which is more commonly known
as the liber. The two outer layers are entirely cellular, and are
chiefly distinguished by the form, size, and direction of their cells.
The epnphlceum is generally composed of one or more layers of colour-
less or brownish cells, which usually present a cubical or tabular
form, and are arranged with their long diameters in the horizontal
direction ; it is this which, when developed to an unusual thickness,
forms cork, a substance which is by no means the product of one
kind of tree exclusively, but exists in greater or less abundance in
the bark of every exogenous stem. The mesoplilceum consists of
cells, usually containing more or less chlorophyll, prismatic in their
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 close
proximity on its inner surface. The liber may generally be found to
be made up of a succession of thin layers, equalling in number those
of the wood, the innermost being the last formed ; but no such
succession can be distinctly traced either in the cellular envelope or
in the suberous layer, although it is certain that they, too, augment
in thickness by additions to their interior, whilst their external por-
tions are frequently thrown off in the form of thickish plates, or
detach themselves in smaller and thinner laminae. 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 tracheides,
ducts, spiral vessels, &c. These materials are so arranged as to
augment the fibro-vascular bundles of the wood on their external
surface, thus forming a new layer of c 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
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

634 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

the next by the interposition of a thick layer of cellular substance.
Sometimes wood is formed in the bark (as in Calycanthus), so that
several woody columns are produced, which are quite independent of
the principal woody axis, and cluster around it. Occasionally the
woody stem is divided into distinct segments by the peculiar thick-
ness of certain of the medullary rays, and in the stem, of which
fig. 496 represents a transverse section, these cellular plates form
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 actively performing their functions for a short time, we
find a circle of fibro-vascular bundles, as represented in fig. 482,
interposed between the central (or medullary) and the peripheral
(or cortical) portions of the fundamental tissue, these fibro-vascular

Fig. 496. — Transverse section of the Fig. 497. — Portion of transverse

stem of a climbing plant (Aristo- section of Arctium (burdock),

lochia ?) from New Zealand. showing one of the fibro-vascu-

lar bundles that lie beneath
the cellular epiderm.

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. 497) ; and sections of these, which are very
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

STRUCTURE OF STEMS AND ROOTS

^35

the woody stems of perennial exogens that those characters are
displayed which separate them most completely from the ferns and
their allies, whose steins 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 passes
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 1 definite ' or ' closed.'
The ' open ' fibro-vascular bundles of exogens and of gymnosperms
may be stated to consist of three distinct parts : the xylem portion,
which consists chiefly of ducts, of the nature of spiral, annular, or
pitted vessels, and which is the portion of the bundle nearest to the
centre of the organ ; the phloem or ' bast ' portion, which consists
largely of prosenchymatous cells, among which are almost always
1 sieve-tubes ' with their 'sieve-plates,' and which is the peripheral
portion of the bundle ; while between them is the formative cam-
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 fibro-vascular
bundle lie side by side, as is usually the case, the bundle is said to
be collateral ; when either portion encloses the other like a cylinder,
it is concentric.

The structure of the roots of endogens and exogens is essentially
the same in plan as that of their respective stems. Generally
speaking, however, the roots of exogens have no pith, although they
have 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 branches which proceed from the larger root-fibres
a central bundle of vessels will be seen enveloped in a sheath of
cellular substance ; and this investment also covers in the end of
the branch, which is usually somewhat dilated, and is furnished at
its extremity with one or more layers of cells, which are constantly
being thrown off, known as the pileorhiza ovroot-cap. The structure
of the branches of the root may be well studied in the common
duckweed, every floating leaf of which has a single root hanging down
from its lower surface. The central fibro-vascular cylinder which is
characteristic of the finer roots of exogens, as well as of endogens, is
surrounded by a single layer of cells very clearly differentiated from
the surrounding fundamental tissue, known as the bundle-sheath.
We have already seen the peculiar form assumed by the bundle-
sheath in the stem of ferns and other vascular cryptogams.

The structure of stems and roots cannot be thoroughly examined
in any other way than by making sections in different directions

636 MICROSCOPIC STEUCTUEE OF PHANEROGAMIC PLANTS

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 its
gum, before being submitted to the cutting process. If the wood
be dry, it should first be softened by soaking for a sufficient length
of time in water, and then treated with spirit, and afterwards -with
water, like green wood. Some woods are so little 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 as
that which is cut fresh. When a piece, of the 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 few thick slices should first be taken, to reduce its
surface to an exact level ; the surface should then be wetted with
spirit, the micrometer-screw 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 hoiv thin
he may venture to cut his sections without a breach of continuity,
and the micrometer-screw should be turned so as to give the required
elevation. If the surface of the wood has been sufficiently wetted,
the section will not curl up in cutting, but will adhere to the sur-
face of the razor, from which it is best detached by dipping the
razor in water so as to float away the slice of wood, a camel-hair
pencil being used to push it of! 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 weak
spirit or in glycerin-jelly. Where a mere general view only is needed,
dry mounting answers the purpose sufficiently well ; and there are
many stems, such as that of Clematis, of which transverse sections
rather 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
glass until they turn brown, may be mounted with advantage in
Canada 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

STRUCTURE OF LEAVES

r>37

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
after the manner of other hard substances which need to be reduced
by 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

Fig. 498. — Epiderm of leaf of Fig. 499. — Epiderm of leaf of Indian

Yucca, showing stomates. com (Zea Mais), showing stomates.

stratum (figs 502, 504, a, a). The shape of these cells is different in
almost every tribe of plants ; thus in the epiderm of the Yucca (fig.
498), Indian corn (fig. 499), Iris (fig. 503), and most other mono-
cotyledons, they are elongated, and present an approach to a
rectangular contour, their margins being straight in the Yucca
and Iris, but minutely sinuous or crenated in the Indian corn.
In most dicotyledons, on the other hand, the cells of the epiderm
depart less from the rounded form, but their margins usually
exhibit large irregular sinuosities, so that they seem to fit together like
the pieces of a dissected map, as is seen in the epiderm of the apple
(fig. 500, b, b). 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
that parallel arrangement of the veins which their leaves almost
constantly exhibit.

638 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

The cells of the epiderm are colourless, or nearly so, having no or
but little chlorophyll in their interior ; and their walls are generally
thickened by secondary deposit, especially on the side nearest the
atmosphere. This outermost hardened continuous wall of the
epidermal layer of cells is known as the cuticle. The deposit (cutin)

Fig. 500. — 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, b,
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.

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
European plants the epiderm contains but a single row of cells,
which, moreover, are usually thin-walled ; whilst in the generality

A B

Fig. 501. — Portion of epiderm of upper surface of leaf of
Bochea falcata as seen at A from its inner side, and at B
from its outer side: a, «, small cells forming inner layer;
b, b, large prominent cells of outer layer ; c, c, stomates dis-
posed between the latter

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

STRUCTURE OF LEAVES

639

epiderm of a plant indigenous to temperate climates would not afford
a sufficient protection to the interior structure against the rays of a
tropical sun ; whilst the less powerful heat of this country would
scarcely overcome the resistance presented by the dense and non-
conducting integument of a species formed to exist in tropical
climates.

A very curious modification of the epiderm is presented by
Rochea falcata, which has the surface of its ordinary epiderm (figs.
501,. 502, a, a) nearly covered with a layer of large prominent
isolated cells, b, b. A somewhat similar structure is found in
Mesemhryanthemum 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 dew-drops. 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 Elceagnus and other shrubs
and herbs are very beautiful objects under the microscope. In
numerous other cases, again,
we find the surface beset with
true hairs, which occasionally
consist of single elongated
cells, but are more commonly
made up of a linear series,
attached end to end. Some-
times these hairs bear little
glandular bodies at their ex-
tremities, by the secretion of
which a peculiar viscidity is
given to the surface of the leaf,
stem, or flower- stalk, as in
many kinds of rose, geranium,
<fcc. ; in other instances, the
hair has a glandular body at

its base, containing a peculiar secretion ; when this secretion is of
an irritating quality, as in the nettle, it constitutes a ' sting.' A
great variety of such organs may be found by a microscopic
examination of the surface of the leaves of plants having any
kind of superficial investment to the epiderm. Many connecting
links present themselves between hairs and scales, such as the
stellate hairs of Deutzia scab?*a, which a good deal resemble those
within the air-chambers of the yellow water-lily (fig. 469). The so-
called ' glands ' or 1 tentacles ' of the sundew (Drosera) are not
really hairs, but outgrowths of the internal tissue of the leaf, each
being penetrated by a fibro-vascular bundle.

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, kc. are still marked out in silex, and (unless the dissipa-
tion of the organic matter has been most perfectly accomplished)
are most beautifully displayed by polarised light. Such silicified

Fig. 502. — Portion of vertical section of leaf
of Rochea, showing the small cells, a, a,
of the inner layer of epiderm ; the large
cells, b, b, of the outer layer ; c , one of the
stomates ; d, d, cells of the paren chyme ;
L,. cavity between the parenchymatous
cells into which the stomate opens.

64O MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

epidermis 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 palece (chaff-scales)
of most grasses are furnished are strengthened by the like silicious
deposit ; and in Festuca pratensis, 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 are remarkably
beautiful objects for the polariscope,

In nearly all plants which possess a distinct epiderm, this is
perforated by the minute openings termed stomates (figs. 500, 502, c, c),
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. 503, h, b), 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 by two
pairs of cells, and is somewhat
quadrangular (fig. 498) ; 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 (fig.
499). In the stomates of no
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
there is no distinct epiderm, so there are no stomates. In the erect
leaves of grasses, the Iris tribe, &c. they are 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 has been estimated that no fewer than 160,000 are con-

Fig. 503. — Portion of epiderm of leaf of Iris
germanica torn from its surface, and
carrying away with it a portion of the
parenchymatous layer in immediate con-
tact with it : a, a, elongated cells of the
epiderm ; b, b, cells of the stomates ; c, c,
cells of the parenchyme ; d, d, impressions
on the epidermal cells formed by their
contact ; e, cavity in the parenchyme, cor-
responding to the stomate.

STRUCTURE OF LEAVES

641

. tained in every square inch of the under surface of the leaves of
Hydrangea and of several other plants, the greatest number seem-
ing always to be present where the upper surface of the leaves is
entirely destitute of these organs. In Iris germanica each surface
has nearly 12,000 stoma tes in every square inch ; and in Yucca each
surface has 40,000. In Oleander, Banksia, and some other plants
the stomates do not open directly upon the lower surface of the
epiderm, but lie in the deepest part of little pits or depressions,
which are excavated in it and lined with hairs ; the 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. 502, 504, and 505. In close apposition with the cells
of the upper epiderm (fig.
504, a, a), which may or may
not be perforated with the
stomates (c, c, d, d), we find a
layer of soft thin-walled cells,
with their longest diameter
at right angles to the surface
of the leaf, and containing
a large quantity of chloro-
phyll ; these generally press
so closely one against an-
other that their sides be-
come mutually flattened, and
no spaces are left, save where
there is a definite air-chamber
into which the stomate opens
(fig. 504, e) ; and the com-
pactness of this superficial layer is well seen when, as often happens, it
adheres so closely to the epiderm as to be carried away with this when
it is torn off (fig. 503, c, c). This layer, usually peculiar to the upper
surface of leaves, 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
epiderm, which the parenchyme only touches at certain points, its
lowest layer forming a sort of network, the so-called ' spongy paren-
chyme ' (fig. 500, d, d) with large interspaces, into which the stomates
open. It is to this arrangement that the darker shade of green
almost invariably presented by the upper surface of leaves is prin-
cipally due, the colour of the component cells of the parenchyme
not being deeper in one 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 paren-

T T

Fig. 504. — Vertical section of epiderm and of
portion of subjacent parenchyme of leaf of
Iris germanica taken in a transverse direc-
tion : a, a, cells of epiderm ; b, b, cells at the
sides of the stomates ; c, c, guard-cells ; d, d,
openings of the stomates ; e, e, cavities in the
parenchyme into which the stomates open ;
/, /, cells of the parenchyme.

642 MICKOSCOPIC STKUCTURE OF PHANEROGAMIC PLANTS

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. 505), 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 leaves ;
for if 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.
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

Fig. 505. — 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, b, stomata cut
through longitudinally ; c, c, green cells of parenchyme ;
d, d, colourless tissue, occupying interior of leaf.

becomes continuous with the epiderm, to the cells of which (fig. 505,
a) 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
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 ex-
ercise 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

STRUCTURE OF FLOWERS

643

their leaves ; in those, on the other hand, whose leaves are furnished
with reticulated veins to which the epiderm adheres (as is the case in
by far the larger proportion), this can only be detached by first
macerating the leaf for a few days in water; and if their texture
should be 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. Yery good sections of most leaves may be made by
a shar»p 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 low
magnifying power, are very striking microscopic objects j 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 FlG< 506.-Cells from petal of

he finds abundant sources of Pelargonium.
gratification, not merely to his

love of knowledge, but also to his taste for the beautiful. The general
structure of the sepals and petals, which constitute the ' perianth ' or
floral envelope, closely corresponds to that of leaves, the chief differ-
ence lying in the peculiar change of hue which the chlorophyll almost
invariably undergoes in the latter class of organs, and very frequently
in the former also. There are some petals, however, whose cells
exhibit very interesting peculiarities, either of form or marking, in
addition to their distinctive coloration , such are those of the Pelar-
gonium, of which a small portion is represented in fig. 506. The dif-
ferent 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.

t t 2

644 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

This method of preparation, however, does not give a true idea of
the structure of the cells ; for each of them has a peculiar mammil-
lary protuberance, the base of which is surrounded by hairs ; and
this it is which gives the velvety appearance to the surface of the
petal, and which, when altered by drying and compression, occa-
sions the peculiar spots represented in fig. 506. Their real character
may be brought into view by Dr. Inman's method, which consists
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 ari instant over
the spirit lamp, after which it is to be covered with a thin glass.
The boiling 'blisters ' it, but does not remove the colour ; and on
examination many of the cells will be found showing the mammilla
very distinctly, with a score of hairs surrounding its base, each of
these slightly curved, and pointing towards the apex of the mammilla.
The petal of the common scarlet pimpernel (Anagallis arvensis),
that of the common chickweed (Stellaria media), together with many
others of a small and delicate character, are also very beautiful
microscopic objects ; and the two just named are peculiarly favour-
able subjects for the examination of the spiral vessels in their natural
position. For the 'veins' which traverse these petals are entirely
made up of spiral vessels, none of which individually attain any
great length, but one follows or takes the place of another, the
conical commencement of each somewhat overlapping the like termi-
nation of its predecessor ; and where the ' veins ' seem to branch,
this does not happen by the bifurcation of a spiral vessel, but by
the ' splicing on ' (so to speak) of one to the side of another, or of
two new vessels diverging from each other to the end of that which
formed the principal vein.

The anthers and pollen-grains, also, present numerous objects of
great interest, both to the scientific botanist and to the amateur
microscopist. In the first place, they 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
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 loeuli or ' pollen-
chambers ' that afterwards present themselves ; and these cells give
origin to the pollen-grains, whilst the ordinary parenchyme remains
to form the walls of the pollen- chambers. The pollen-grains are
formed within ' mother- cells,' the endoplasm of each breaking up
into four segments. These become invested by a double envelope, a
firm extine, and a thin inline, and they are set free, when mature,
by the bursting of the pollen-chambers. It is not a little curious

POLLEN-GRAINS

645

that the layer of cells which lines the pollen-chambers should exhibit,
in a considerable proportion of plants, a strong resemblance in struc-
ture, though not in form, to the elaters of Marchantia (fig. 449).
For they have in their interior a fibrous deposit, which sometimes
forms a continuous spiral (like that in fig. 474), as in Narcissus and
Hyoscyamus ; but it is often broken up, as it were, into rings, as in
the Iris and hyacinth ; in many instances it forms an irregular
network, as in the violet and saxifrage ; in other cases, again, a
set of interrupted arches, the fibres being deficient on one side, as in
the yellow water-lily, bryony, 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 polJen-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. 507, 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
away. Sometimes the pores are covered by little disc-like pieces or
lids, which fall off when the pollen-tube 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 5 and the
pollen-tubes, at first mere protrusions of the inner coat of their cell,
insinuating themselves between the loosely packed cells of the stigma,
grow downwards through the style, sometimes even to the length of
several inches, until they reach the ovary. The first change, namely
the protrusion of the inner membrane through the pores of the exterior,
may be made to take place artificially by moistening the pollen
with water, thin syrup, or dilute acids (different kinds of pollen-

646 MICKOSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

grains requiring different modes of treatment) ; but the subsequent
extension by growth will only take place under the natural con-
ditions. By treating some pollen- grains, as those of Lilium
japonicum, L. rubrum, Or L. auratum, with the viscid liquid abun-
dantly secreted by the stigma, not only may the extrusion and
lengthening of the pollen-tubes be watched, but the grains with their
extruded tubes may be preserved almost unchanged by mounting in
this liquid.

The darker kinds of pollen may be generally rendered trans-
parent by mounting in Canada balsam * or, if it be desired to avoid
the use of heat, in the benzol solution of Canada balsam, setting
aside the slide for a time in a warm place. For the less opaque
pollens the dammar solution is preferable. The more delicate
pollens, however, 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 Amaran-
thacecE, Cichoriacew, Cucurbi-
tacece, Malvacece, and Passi-
ftoreai ; others are furnished
also by Convolvulus, Cam-
panida, (Enothera, Pelar-
gonium (geranium), Polygo-
num, Sedum, and many other
plants. It is frequently
preferable to lay down the
entire anther, with its ad-
herent pollen-grains (where
these are of a kind that hold

Fig. 507.-Pollen-grains of-A, Althcea rosea *?.**)» aS an °Pa(lue °bject '
(hollyhock) ; B, Cobcea scandens ; C, Passi- this may be done With great
flora ccerulea; D, Ipomcea purpurea, advantage in the case of the

common mallow {Malva syl-
vestris) or of the hollyhock [Althcea rosea), the anthers being picked
soon after they have opened, whilst a large proportion of their pollen
is yet undischarged, and being 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
upon a black surface. They are then, when properly illuminated,
most beautiful objects for objectives of §-, 1, 1^-, or 2-in. focus,
especially with the binocular microscope. 1

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)
general diffusion of sulphur (such as occurred in the neighbourhood of Windsor
in 1879) has frightened ignorant rustics into the belief that the ' end of the world '
was at hand. Its true nature is at once revealed by placing a few grains of it under
the microscope.

POLLEN-GRAINS AND OVl'LKS

647

There are, in fact, few more* interesting objects for the young
microscopist than pollen-grains, both from the ease with which they
can always be procured, and the almost infinite variety and beauty
in their forms. Some of the commonest weeds, such as the dandelion
and groundsel, are distinguished by the beauty of their pollen-grains.
The grains are sometimes nearly or quite spherical, as in the hazel,
birch, or 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,
Cineraria, and many other plants. Sometimes the grains are united
together by delicate threads, as in the Rhododendron and Fuchsia ;
and this union is much more complete in the Orchidece and Ascle-
piadece, where the whole of the pollen in each anther-lobe is glued
together by a viscid substance into a club-shaped poUiniam, or pollen-
mass. In what are called anemophilous flowers, in which the pollen
is carried through the air by the agency of the wind, the grains are
small, 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 Epilobium (willow-herb) and
CEnothera (evening primrose) are very favourable objects for ob-
serving the emission of pollen-tubes and their entrance into the
stigma.

The structure and development of the ovules that are produced
within the ovary at the base of the pistil, and the operation in which
their fertilisation essentially consists, are subjects of investigation
which have a peculiar interest for scientific botanists, but which, in
consequence of the special difficulties that attend the inquiry, are
not commonly regarded as within the province of ordinary micro-
scopists. Some general instructions, however, may prove useful to
such as would like to inform themselves as to the mode in which the
generative function is performed in phanerogams. In tracing the
origin and early history of the ovule, very thin sections should be made
through the flower-bud, both vertically and transversely ; but when
the ovule is large and distinct enough to be separately examined, it
should be placed on the thumb-nail of the left hand, and very thin
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 on by artificially applying the pollen to the stigma of several
flowers, and then examining one or more of the styles daily. ' If

648 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS

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 nigrum and rubrum (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 Euphrasia, it being only necessary to tear open with a needle
the bvary 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
A B c sections being made,

whereby the origin of
the embryo may be dis-
cerned ; and for this pur-
pose, CEnothera (evening
primrose) has been had
recourse to by Hof-
meister, whilst Schacht
recommends Lathrcea
squamaria, Pedicularis
palustris, and particu-
larly Pedicularis sylva-
tica.

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 verv
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. 508, A) presents a regular reticulation upon its surface,
pits, for the most part hexagonal, being left between projecting walls ;
that of the pink (D) is regularly covered with curiously jagged divisions,
every one of which has a small bright black hemispherical knob in its
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 Bignoniacew 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 the Eccremocarpus scaber, a half-hardy climbing plant
common in our gardens ; and when its membranous c wing 9 is
examined under a sufficient magnifying power, it is found to be

Fig. 508. — Seeds as seen under a low magnifying
power : A, poppy ; B, Amaranthus (prince's
feather) ; C, Antirrhinum majus (snapdragon) ;
D, Dianthus (clove-pink) ; E, Bignonia.

STRUCTURE OF SEEDS

649

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) are 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, being 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 ; but the most remarkable development of these
organs is said by Mr. Quekett to exist in a seed of Calosanthes
indica, an East Indian plant, in which the wing extends more than
an inch on either side of the seed. Some seeds are distinguished by
a peculiarity of form which, although readily discernible by the
naked eye, becomes much 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 Cyanthus 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 : — Anagallis,
Anethum graveolens, Begonia, Carum carui, Coreopsis tinctoria,
Datura, Delphinium, Digitalis, Elatine, Erica, Gentiana, Gesneria,
Hyoscyamus, Hypericum, Lepidium, Limnocharis, Linaria, Lychnis,
Mesembryanthemum, Nicotiana, Origanum onites, Orobanche, Petunia,
Reseda, Saxifraga, Scrophularia, Sedum, Sempervivum, Silene,
Stellaria, Symphytum asperrimum, and Verbena. The following
may be mounted as transparent objects in Canada balsam :
Drosera, Hydrangea, Jlonotropa, Orchis, Parnassia, Pyrola, Saxi-
fraga. 1 The seeds of umbelliferous plants generally are remarkable
for the peculiar vittw, or receptacles for essential oil, which are
found in the closely applied pericarp or seed-vessel which encloses
them. Various points of interest respecting the structure of the
testa or envelope of seeds, such as the fibre-cells of Cobaia and
Collomia, the stellate cells of the star-anise, and the densely con-
solidated tissue of the ' shells ' of the coquilla-nut, cocoa-nut, &c.
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 testa,
and closely adheres to it. The ' bran ' detached in grinding consists

1 A part of these lists have been derived from the Micrograph ic Dictionary.

650 MICBOSCOPIC STEUCTUEE OF PHANEEOGAMIC PLANTS

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 en-
velope in a single grain of a mixture consisting of chicory with only 5 per cent, of
roasted corn.

65i

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 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, beneath the great sub-kingdoms —
Vertebrata, Mollusca, Articulata, and Radiata — marked out by
Cuvier, is characterised by the extreme 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 protophytes) 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 & morula or ' mulberry -mass ' of
cells, corresponding to the 1 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 ' invagination ' of
that layer into the space left void by the dissolution of the central
cells of the 'morula.' This gastrula- stage? as we shall see hereafter,

1 The terms epiblast and hypoblast are generally used by English embryologists
in place of the ' ectoderm ' and ' endoderm ' used here.

'z 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
of 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 ' invaginate gastrulae.'

652 MICROSCOPIC FORMS OF ANIMAL LIFE

remains permanent in the great group of Ccelenterata, though the
endoderm and ectoderm are separated from each other in its higher
forms by the development of generative and other organs between
them. But in all classes above the ccelenterates 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 morula-
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 (plastid), which is
limited by a cell- wall, and contains a 'nucleus.' For they consist
of particles of protoplasm, termed 'cytodes,' of indefinite extent,
which have neither cell- wall nor nucleus, but which yet take in and
digest food, convert it into the material of their own bodies, cast out
the indigestible portions, and reproduce their kind, with the regu-
larity and completeness that we have been accustomed to regard
as characteristic of higher animals. With regard, however, to this
apparent absence of a nucleus we have to bear in mind that the
progress of 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 connected
gradation by which they pass into undoubted rhizopods, leave no
doubt of their claim to a place in the animal kingdom.

Monerozoa.

A characteristic example of this lowest protozoic type is presented
by the Protomyxa aurantiaca (fig. 509), a marine ' moner ' of an
orange -red colour, found by Professor Haeckel upon dead shells of
Spirula near 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 proto-

PKOTOZOA — PROTOMYXA

653

plasmic threads, along which stream in all directions orange-red
granules, obviously belonging to the body itself, together with foreign
organisms (6, 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
«ast out. Neither nucleus nor contractile vesicle is to be discerned^
but numerous floating and inconstant vacuoles (a) are dispersed

Fig. 509— Protomyxa aurantiaca: A, encysted sta to spore ; B, inci-
pient formation of swami- 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, a, and captured prey, b, c.

through the substance of the body. After a time the currents
become slower ; the ramified extensions are gradually drawn in-
wards ; 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 (B), which, at
first inactive, soon begin to move within the cyst, and change their
shape to that of a pear with the small end drawn out to a point.

654 MICROSCOPIC FORMS OF ANIMAL LIFE

The cyst then bursts, and the red pear-shaped bodies issue forth
into the water (C), moving freely about by the vibrations of fiagella
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 plastidules 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 Amoabce, putting forth
inconstant pseudopodial processes, and engulfing nutrient particles-
in their substance (D). Two or more of these amcebiform bodies

Fig. 510. — Vamjjyrella spirogyrce as seen at A sucking out contents
of Spirogyra-ceW- ; 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.

unite to form a ' plasmodium,' as in the Myxomycetes ; its pseudo-
podial extensions send out branches which inosculate to form a net-
work • and the body grows, by the ingestion of nutriment, to the
size of the original. In this cycle of change there seems no interven-
tion of a generative act, the coalescence of the amcebiform 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 Vampyrella,
of which one form (fig. 510) has long been known in its encysted
condition as a minute brick-red sphere attached to the filaments of

VAMPYEELLA; AKCHERINA

655

the conjugate Spirogyra ; whilst another (fig. 511, a, a) similarly
attaches itself to the branches of Gomphonema. The walls of the
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
' tetraspores ' (fig. 511, b, d) ; these escape by openings in the cyst
(fig. 510, C), and soon take the spherical form, emitting very slender
pseudopodial filaments (figs. 510, D, 511, B) like those of an Actino-
2)hrys, 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 V. spirogyrce is
watched in water containing some filaments of Spirogyra, it may be
seen to wander until it meets one of these filaments, to which, if it
be healthy and loaded with chlorophyll, it attaches itself. It soon
begins to perforate the wall of the filament ; and when the interior
of this has been reached, its endoplasm, carrying with it the chloro-
phyll-granules it includes, passes slowly into the body of the Vam-
pyrella. 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.
gomphonematis in like manner creeps over the stems and branches of
the Gomphonema (fig. 511, e), adapting itself to the form of its sup-
port ; and as soon as it has reached one of the terminal silicious 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 ( f) \
whilst another portion of the protoplasm finds its way between the
two silicious 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
Gomphonema-cell 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. spyrogyros, 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 Vampyrella — Archerina Boltoni
— which is remarkable for being chlorophyllogenous ; this species
presents another interesting peculiarity. ' Groups of ghost-like out-
lines corresponding to chlorophyll-corpuscles and their radiant fila-
mentous 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 Chi amy -
domyxa.

656 MICROSCOPIC FORMS OF ANIMAL LIFE

Intermediate between the foregoing and the 1 reticularian ' rhizo-
pods, to be presently described, is another simple protozoon dis-
covered in ponds in Germany by MM. Claparede and Lachmann,
and named by them Lieberkuehnia Wageneri.1 The whole sub-
stance of the body of this animal and its pseudopodial extensions
(fig. 512), is composed of a homogeneous, semi-fluid, granular proto-
plasm, the particles of which, when the animal is in a state of

Fig. 511. — Vampyrella gomphonematis : A, colony of
Gomphonema attacked by Vampyrellce ; 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 Gomphonema-ceMs, the emptied frustules of
which, g, h, are cast forth. B, isolated Vampyrella creeping
about by its extended pseudopodia.

activity, are continually performing a circulatory movement, which
may be likened to the rotation of the particles in the protoplasmic
network within the cell of a Tradescantia. It is a marked peculiarity

1 Etudes sur les Infusoires et les Bhizopodes, Geneva, 1858-1861. The beautiful
figure of Lieberkilhnia, given by M. Claparede, has been reproduced by the Author
in Plate L of his Introduction to the Study of the Foraminifera.

LEEBEBKUEHNIA

657

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
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
back with it a granule that
was seen advancing in the
opposite direction. Even in
the broader processes granules
are sometimes observed to come
to a stand, to oscillate for a
time, and then to take a retro-
grade course, as if they had
been entangled in the opposing
current, just as is often to be
seen in Char a. 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. Xot 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.

Fig. 512. — Lieberkuelmia Wageneri.

658

MICROSCOPIC FOKMS OF ANIMAL LIFE

But if watched sufficiently long its activity is resumed, so that it
may be presumed to have been previously satiated with food, which
is undergoing digestion during its stationary period. No encysting
process has been noticed in Lieberkuehnia ; but Cienkowsky 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 study.

Rhizopoda.

We now arrive at the group of rhizopods, or ' root-footed ' animals,
first established by Dujardin for the reception of the Amoeba 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
pseudopodia (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 Monerozoa, already described, would be
included in it ; but it seems on various grounds desirable to limit the
term Rhizopoda to those Protozoa in which the presence of a nucleus,
the differentiation of an ectosarc (or firmer superficial layer of proto-
plasm) from the semi-fluid endosarc, together with the more definite
form and restricted size, indicate a distinct approach to the condition
of 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 twenty years ago,1 based on the characters of
the pseudopodial extensions, have been accepted by more recent
systematists, he thinks it best still to adhere to it, as seeming to him
to be, on the whole, most natural.

I. In the first division, Reticularia, the pseudopodia freely
ramify and inosculate, so as to form a network, exactly as in Lieber-
kuehnia, 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
exclusively marine, constitute the group of Foraminifera, whose
special interest to the microscopist entitles it to separate considera-
tion ; and it is only for convenience that two Reticularia which
inhabit fresh water also, and the envelopes of whose bodies are

1 Natural History Review, 1861, p. 456 ; and Introduction to the Study of the
Foraminifera, 1862, chap. ii.

RHIZOPODA

659

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
exercise it to find a suitable situation for their attachment, the
capture of their food being effected by their pseudopodial network.

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
suffioiently soft and homogeneous (at least in the lower types), to coalesce
when they come into contact with each other. These have usually
(probably always) a contractile vesicle as well as a nucleus ; and the
higher forms of them are characterised by the enclosure of symbiotic
yellow corpuscles (zoochlorellce) in the substance of their endosarc.
By far the larger number of this group also have skeletons of
mineral matter, which are always silicious ; 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 twTo groups of Heliozoa (sens, strict.) and Radiolaria, the latter
being distinguished by the presence of a central capsule or mass of
protoplasm surrounded by a special envelope, the better development
of the skeleton, the greater tendency of the pseudopodia to coalesce
with one another, and the not infrequent presence of 4 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 lobate, 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 inhabit-
ants of fresh water, and some are even found in damp earth.

Reticularia. — This type is very characteristically represented by
the genus Gromia (fig. 513), some of whose species are marine, and
are found, like ordinary Foraminifera, among tufts of corallines,
alg?e, etc. ; whilst others inhabit fresh water, adhering to Conferva*
and other plants of running streams. It was in this type that the
presence of a nucleus, formerly supposed to be wanting in Reticularia

u u 2

66o

MICKOSCOPIC FORMS OF ANIMAL LIFE

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 x^th to xV^n °^ 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-
kiihnia. 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 are 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 silicious valves of
diatoms, shown in fig.
513) may be distinguished
in the midst of the sar-
codic substance. By the
same agency the Gromia
sometimes creeps up the sides of a glass vessel. In the intervals of
quiescence, on the other hand, the whole sarcodic body, except a
film that serves for the attachment of the test, is withdrawn into its
interior.

Another example of the reticularian group is afforded by the
curious little Microgromia socialis (fig. 514), first discovered by Mr.
Archer, and further investigated with great care by Hertwig, 1 which

Fig. 513. — Gromia oviformis, with its
pseudopodia extended.

' Ueber Microgromia in Arehiv fur Milcr. Anat. bd. x. Supplement.

3IICR0GE0MIA

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 orifice, from which the sarcode-body extends itself

Fig. 514. — Microgromia socialis: A, colony of individuals in extended state,
some of them undergoing transverse fission; B, colony of individuals
(some of them separated from the principal mass) in compact state ; C, D,
formation and escape of swarm-spore, seen free at E.

giving off very slender pseudcpodia which radiate in all directions.
A distinct nucleus can be seen in the deepest part of the cavity ;
while a contractile vesicle lies embedded in the sarcodic substance
nearer the mouth. Multiplication by duplicative subdivision has
been distinctly observed in this type ; but with a peculiar departure

662

MICROSCOPIC FORMS OF ANIMAL LIFE

from the usual method. A transverse constriction divides the body
into two halves — as shown in two individuals of colony A — each half
possessing its own nucleus and contractile vesicle ; the posterior seg-
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 monad, or assumes the form of an Actinophrys,
moving about by three or four pointed pseudopodia — probably in
each 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. — The Actinophrys sol, sometimes termed the 'sun-
animalcule ' (fig. 515), 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, enclosing a distinct nucleus ; but
the peripheral part has a 'vesicular' aspect, as in the type next
to be described (fig. 516). This appearance is due to the number
of ' vacuoles ' 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. 515;,
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 vacuoles 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
gradually as it refills. The body of this animal is nearly motionless,
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 with

HELIOZOA

663

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 lar^e and
vigorous enough to struggle to escape from its entanglement, It may
usually be observed that the neighbouring pseudopodia bend over and
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 not 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

Fig. 515. — Actinophrys sol: A, figure showing the wide vacuolated cortical
layer or ectosarc (22) and the fine granulated endosarc (at) ; n, central
nucleus, ax, axial filaments of pseudopodia ; cv, contractile vacuole ; x food-
mass inclosed in a large fcod- vacuole. B, a colony of four individuals, after
treatment with acetic acid ; r,m, and jv, as before ; r, r, 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.)

visible movement of the latter, much in the same manner as in Gromia.
When in either of these modes the food has been brought to the
surface of the body, this sends over it on either side a prolongation of
its own sarcode-substaiice ; and thus a marked prominence is formed
(fig. 515, A, n), which gradually subsides as the food is drawn more
completely into the interior. The struggles of the larger animals, and
the ciliary action of Infusoria and Eotifera, may sometimes be
observed to continue even after they have been thus received into

664

MICROSCOPIC FORMS OF ANIMAL LIFE

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
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 on,
a kind of slow circulation takes place in the entire mass of the endo-

Fig. 516. — Actinosphzrium Eiclicrnii : m, endosarc; r, ectosarc;
c, c, contractile vacuoles.

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
extemporises itself for the occasion ; but if small it sometimes glides
along a pseudopodium from its base to its point, and escapes from
its extremity.

The ordinary mode of reproduction in Actinojihrys seems to be
by binary subdivision, its spherical body showing an annular con-

HELIOZOA

665

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,
and there is not sufficient evidence that it has any true generative
character. Certain it is that such a junction or c zygosis ' may take
place, not between two only, but even several individuals at once,
their number being recognised by that of their contractile vesicles ;
and that, after remaining thus united for several hours as a colony,

Fig. 517. — Marginal portion of ActinosjphcBrium Eicliornii
as seen in optical section under a higher magnifying
power : m, endosarc ; r, ectosarc ; a, a, a, pseudopodia ;
n, n, nuclei with nucleoli ; /, ingested food-mass.

they may separate again without having undergone any discoverable
change.

Under the generic name Actinophrys was formerly ranked the
larger but less common Heliozobn, now distinguished as Actino-
sphcerium Eicliornii (fig. 516) ; 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 superficial or cortical zone of the body. Several
nuclei (n, n) are usually to be seen imbedded in the protoplasmic mass.
The general life-history of this type corresponds with that of the pre-

666

MICROSCOPIC FORMS OF ANIMAL LIFE

ceding, but its mode of reproduction presents some marked peculi-
arities. In many, if not in all cases it commences, as first observed by
Kolliker, with the conjugation of two separate individuals. The binary
segmentation is preceded 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 mass is
replaced by a sort of
morula, each spherule
of which shows the dis-
tinction between the
central and cortical
regions, the former in-
cluding a single nucleus,
whilst the latter is.
strengthened by silicious
deposit into a firm in-
vestment. After re-
maining in this state
during the winter the
young Actinosjyhceria
come forth in the spring
without this silicious
investment, and gradu-
ally grow into the like-
ness of their parent.1

A large number of
new and curious fresh-

Fig. 518.- Clathrulma elegans: A, complete water forms of this type
organism ; B, swarm-spore showing nucleus, n, are being frequently
and two contractile vesicles near its opposite end. brought under notice,

of which the Clathrulina
elegans (fig. 518) may be specially mentioned as presenting an
obvious transition to the Polycystine type. This has been found
in various parts of the Continent, and also (by Mr. Archer 2) in
Wales and Ireland, occurring chiefly in dark ponds shaded by
trees and containing decaying leaves. Its soft sarcode-body, which
is not differentiated into ectosarc and endosarc, is encased by a
silicious capsule of spherical form, regularly perforated with oval

1 On the results of the artificial division of Actinosphcerium see K. Brandt, Ueber
Actinosphcerium Eichornii, Halle a/S., 1877 ; Gruber, Berichte d. Naturf. Ges. zu
Freiburg i/B., 1886 ; Nussbaum, Arch. f. Mikr. Anat. xxvi.

2 See his memoir on Fresh-water Kadiolaria in Quart. Journ. of Microsc. Sci.
n.s. vol. ix. 1869, p. 250.

HELIOZOA; LOBOSA

667

apertures, and supported on a long silicified peduncle. The body
itself and the pseud opodia which it puts forth through the aper-
tures of the capsule seem closely to correspond with those of
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 silicious 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, and
ultimately produces the capsule and stem characteristic of its type.
In the second mode numerous small rounded sarcode-masses, each

produced within the capsule, in what
made out ; and every one of these is

possessm&

a nucleus, are
manner cannot be clearly
enveloped in a firm en-
velope, set round with
short spines, probably
silicious. These cysts
remain for months with-
in the common capsule ;
and when the time arrives
for their further develop-
ment the sarcode-cor-
puscles slip out of their
cysts, and escape through
the orifices of the capsule
as flagellated monads of
oval form (fig. 518, B),
each having a nucleus,
71, near the base of the
flao-ella, and two

VI L

con-

Fig. 519. — Diagrammatic representation of Amoeba
proteus : E C, ectosarc ; E N, endosarc ; C V, con-
tractile vesicle ; N, nucleus ; P, pseudopodia ;
V I L, villous tuft.

tractile vesicles near its

opposite end. After swarming for some hours in this condition,
they change to the free Actinophrys form, and finally acquire the
silicious capsule and stem of the Clathrulina.

Lobosa. — No example of the rhizopod type is more common in
streams and ponds, vegetable infusions, &c. than the Amoeba
(fig. 519) ; 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 here clearly marked, so that the body approaches
much more closely in its characters to an ordinary * cell ' composed
of cell- wall and cell- contents. It is through the ' endosarc ' alone,
E N", 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

668

MICROSCOPIC FORMS OF ANIMAL LIFE

the introduction of alimentary particles, or for the extrusion of effete
matter ; 1 and thus there is no evidence, in Amoeba and its immediate
allies, of the existence of any more definite orifice, either oral or anal,
than exists in other rhizopods. The more advanced differentia-
tion of the ectosarc from the endosarc of Amoeba is made evident
by the effects of reagents. If an Amoeba radiosa be treated with
a dilute alkaline solution, the granular and molecular endosarc
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 Cohn 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 Amoeba, 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
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 an Amoeba moves from place to
place, a protrusion like the finger of a glove being first formed, into

1 This remarkable character has been stated by Professor Huxley ill 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 thej bubble without bursting it, the walls closing together, and re-
covering their continuity as soon as the rod is drawn away.'

LOBOSA

669

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 Amoeba is described by most observers as a ' rolling 7
movement, this being certainly the aspect which it commonly
seems to present ; but it is maintained by MM. Claparede and
Lachmann 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 Amoeba en-
counters particles 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.

If may often be seen that portions of the sarcode-body of an
Amoeba, 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
has 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
fixed point, or retracting the lobe into the body, causes the connect-
ing band to thin away until it separates ; and the detached portion
speedily shoots out pseudopodial processes of its own, and comports
itself in all respects as an independent Amoeba. Multiplication
also takes place l)y regular binary subdivision. Various observers
have seen phenomena which they have supposed to be evidence of the
formation of ' swarm-spores ' 1 or of the development of cysts, but it
must be borne in mind that a large number of protozoa pass during
the course of their life through amcebiform stages, some of which
may have been taken as true species of Amoeba. No sexual act has
been certainly recognised as part of the life-history of Amoeba, the
union of two or more individuals, which may be occasionally wit-
nessed, having more the character of the ' zygosis ' of Actinophrys.

A sarcodic organism discovered by Greef, and named by him
Pelomyxa pcdustris (fig. 520), which spreads over the bottom of
stagnant ponds in the condition of slimy masses of indefinite form,
exhibits a further advance upon the Amoeban type. The substance
of its body, which may be of the size of two millimetres, exhibits a
very clear differentiation between the homogeneous hyaline ectosarc
(B, a, d), and the contained endosarc, which contains such a multi-
tude of spherical vacuoles, b, as to have a ' vesicular ' or frothy
aspect. When it feeds upon the decomposing vegetable matter at
the bottom of the pool it inhabits, its body 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,

1 Prof. A. M. Edwards (U.S.A.) in Monthly Microsc. Joarn. vol. viii. 1872, p. 29.

670

MICEOSCOPIC FORMS OF ANIMAL LIFE

e, e, and also many hyaline globular brilliant bodies, f,f, which are
regarded by Greef as germs or swarm- spores developed from nucleoli
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 amoeba, either by general undulations of its surface, 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 amoebiform bodies break forth, each
having its nucleus and contractile vesicle. These at first live as

Fig. 520. — Pelomyxa jpalustris : A, as it appears when in amoeboid
motion; B, portion more highly magnified, showing a, a, the hyaline
ectosarc ; b, one of the vacuoles of the endosarc ; c, rod-like bodies (pro-
bably Bacteria) scattered through the endosarc ; d, protruded exten-
sion of ectosarc with endosarc passing into it ; e, e, nuclei ; /, /, globular
hyaline bodies.

Amoebrp, but afterwards pass into a resting state, assuming a spherical
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 enclosed in a ' test ' composed of a horny
membrane, apparently resembling in constitution the chilin which
gives solidity to the integuments of insects ; it is usually discoidal

RHIZOPODA ; LOBOSA

671

(fig. 521, 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. 513). In each case a de-
tached portion of the sarcodic body will put forth pseudopodia of
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 enclosed. In Arcella it has been
observed that the pseudopodia of two or more individuals unite by
bridges of protoplasm, and afterwards separate ; and it seems to be
almost certain that this is a true 1 conjugation,' and not a mere
' zygosis.' A remarkable method of reproduction has been observed
by Gruber in Eaglypha alveolata ; in an active form highly refractive

Fig. 521. — Testaceous forms of Amoeban rhizopods : A, Difflugia
proteiformis; B, Difflugia oblonga ; C, Arcella acuminata ; D,
Arcella clentata.

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 proto-
plasmic body preceding that of its nucleus.

Many testaceous amozbans 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 Quadrula symmetrica repre-
sented in fig. 522 ; the sarcode-body is here encased in a pear-shaped
test, of glassy transparence, made up of a great number of square

672

MICROSCOPIC FOEMS OF ANIMAL LIFE

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 nucleolus • and in front of this are contractile
vesicles, usually two in number.1

Coccoliths and Coccospheres. — This would seem the most appro-
priate place for the description of certain peculiar little bodies found
very extensively diffused over the deep-sea bottom, especially abound-
ing in the Globigerina-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 Professor Huxley first
found the Coccoliths (fig. 5237
1, 2) which Dr. Wallich in
1860 found aggregated in
the spherical masses which he
designated as ' coccospheres '
(a). Regarding the gelati-
nous matrix in which they
were imbedded as a new type
of the Monerozoa described by
Haeckel, having the condition
of an indefinitely extend-
ed plasmodiiom, Professor
Huxley proposed to designate
it by the name Batbybius,
indicative of its habitat in
the depths of the sea ; and

tj, KOO n 7 7 , • this idea was accepted by

.bio. 522. — Quadrula symmetrica^ with * J

extended pseudopodia. Haeckel, whose representa-

tion of a living specimen of
Bathybius, with imbedded coccoliths, is given in fig. 523, .3. The
observations made in the ( Challenger ' Expedition, however, have
not confirmed this view ; the supposed Bathybins being a gelatinous
precipitate, consisting of sulphate of lime, slowly deposited in water
to which strong spirit has been added. Whatever be their nature,
coccoliths and coccospheres are bodies of great interest ; since their
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
discolilhs and cyatholiths. The former are round or oval discs, having

1 See especially the recent admirable work of Professor Leidy on the fresh-water
rhizopods of the United States, 1HS0. 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.

COCCOLITHS AND COCCOSPHERES

673

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. 523, 5). In general, the
' discoliths ' are slightly convex on one side, slightly concave on the
other, and the rim is raised into a prominent ridge on the more
convex side ; so that when viewed edgewise they present the appear-
ances shown in figs. 8, 9. Their length is ordinarily between ^oVoth
ancl Wo otn °* an incn ; but ^ ranges from -^j^th. to yyi^th. The
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. They are convex on one face and flat or concave on the

Fig. 523. — Coccoliths and Coccospheres : 1, 2, 7, cyatholiths seen
obliquely ; 3, coccosphere with imbedded cyatholiths ; 4, coccoliths im-
bedded in supposed protoplasmic expansion ; 5, discolith seen in front
view ; 6, cyatholith seen in front view, showing (1) central corpuscle, (2)
granular zone, (3) transparent outer zone ; 8, 9, discoliths seen edgewise.

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-
times divided into two. The zone (2) immediately surrounding
the central corpuscle is usually more or less distinctly granular,
and sometimes has an almost bead-like margin. The narrower-
outer zone (3) is generally clear, transparent, and structureless,,
but sometimes shows radiating striae. When viewed sidewise or
obliquely, however, the ' cyatholiths ' are found to have a form
somewhat resembling that of a shirt-stud (figs. 1, 2, 7). Each con-
sists of a lower plate, shaped like a deep saucer or watch-glass ; of
a smaller upper plate, which is sometimes flat, sometimes more or
less concavo-convex ; of the oval, thick-walled, flattened corpuscle,
which connects these two plates together at their centres ; . and of

MICKOSCOPIC FOEMS OF ANIMAL LIFE

•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 y^L^-th to south' of an inch, those of
^-gL-Qth of an inch and under being always circular. It appears
from the action of dilute acids upon the coccoliths that they must
mainly consist of calcareous matter, as they readily dissolve, leaving
scarcely a trace behind. When the cyatholiths are treated with
very weak acetic acid, the central corpuscle rapidly loses its strongly
refracting character; and there remains an extremely delicate,
•finely granular membranous framework. When treated with iodine
they are stained, but not very strongly, the intermediate sub1
stance being the most affected. Both discoliths and cyatholiths are
completely destroyed by strong hot solutions of caustic potass or
soda. The coccosphcres (fig. 3) are made up by the aggregation " of
bodies resembling c cyatholiths ' of the largest size in all but the
absence of the granular zone ; they sometimes attain a diameter of
y^th of an inch. What is their relation to the coccoliths, and
under what conditions these bodies are formed, are questions whereon
no positive judgment can be at present given.

Sporozoa.

The term Sporozoa has been applied by Leuckart to a group of
protozoic animals of which the well-known Gregarinida are certainly
members, and to which the Myxosporidia and Sarcosporidia 1 may
also belong. They are especially characterised by the peculiarities
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 Gregarinida lead a parasitic life, and may often be met with
in the intestinal canal or other cavities of earthworms, insects, (fee.
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 two-
thirds of an inch.2 A sort of beak or proboscis frequently projects
from one extremity ; and in some instances this is furnished with a
circular row of hooklets, closely resembling that which is seen on
the head of Taenia. There is here a much more complete differentia-
tion between the cell-membrane and its contents than exists either
in Actinophrys or in Amoeba; and in this respect we must look upon
Gregarina as representing a decided advance in organisation. Being
nourished upon the juices already prepared for it by the digestive
operations of the animal which it infests, it has no need of any such
apparatus 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,

1 Consult the memoir by Dr. E. Blanchard in Bull. Soc. Zool. France, x. p. 244.

2 See Prof. Ed. Van Beneden on Gregarina gigantea (found in the intestinal
canal of the lobster) in Quart. Journ. Microsc. Sci. n.s. vol. x. 1870, p. 51, and vol.
xi. p. 242.

GREGARINIDA

■whose contents are usually milk-white and minutely granular, there
is generally seen a pellucid nucleus ; and when, as often happens,
the cell undergoes duplicative subdivision, the process commences in
a constriction and cleavage of this nucleus. The membrane and its
contents, except the nucleus, are soluble in acetic acid. The move-
ments of the body are of very various kinds ; there is a forward
movement which may be due, as suggested by Lankester, to the
undulations of the body. The cell itself may undergo contraction, and
consequent change in form, which may or may not be accompanied
by locomotion ; circular constrictions may extend along the body ;
or the cell may bend on itself and again straighten out. By Van
JBeneden the contractility of the cell is localised in a layer of the

Fig. 524. — Cyst of Monocystis agilis, the Gregarinid of the earthworm
(750 diams.), showing ripe chlamydospores and complete absence of
any residual protoplasm in the cyst. (After Professor Ray Lankester.)

ectoplasm, which he has found to consist of a layer of contractile
fibrils. When the process of ency station commences 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 undergoes a singular change.
The nucleus disappears, and the sarcodic raa&s breaks up into a
series of globular particles, which gradually resolve themselves (as
shown at b, c, d, e, fig. 525) into forms very like those of Naviculces
and a cyst more advanced, and greatly magnified, is shown in fig. 524.
These ' pseudo-navicellsg ' or ' spores,' as it is better to call them, are
set free in time by the bursting of the capsule that incloses them ; and

676

MICROSCOPIC FORMS OF ANIMAL LIFE

they develop themselves into a new generation of Gregarime, first
passing through an amoeba-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
become more globular in shape, and are then encysted by the forma-

Fig. 525. — Gregarina Scenuridis, from testis of Tubifex rivulcrum
two adults uniting : a, 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
Kolliker.)

Fig. 526. — Coccidium oviforme (Leuckart) from the liver of the rabbit:
a, cyst just formed ; b, condensed contents, the outer envelope has
disappeared ; c, contents divided into four sporoblasts ; d, the sporo-
blasts have become rounded and clearer internally ; e and /, formation
of the falciform germ ; (j and Ji, spores more highly magnified — g from
the side, li from in front.

SPOROZOA

677

tion 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 ' is the same as that of the ordinary encysting process,
there seeems no sufficient reason for regarding it, like the ' conju-
gation ' of protophytes, as a true generative act.

The Coccodia (fig. 526) 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, such as Drepani-
dium ranarum, Lankester, may be migrating young.

Of the imperfectly known Myxosporiclia it may be said that
their spores are the bodies which are known as ' psorosperms ' ;
while the bodies observed by Rainey and others, and wrongly
regarded as the cause of the cattle plague, are sarcocystids which
live in the muscular fibre of mammals.

Professor Haeckel's memoirs on Monera and the Gastrcsa Theory will be found in
successive numbers of the Jenaische Zeitschrift beginning with 1868, and in a collected
form in the two parts of his Biologisclie Studien. The first of his memoirs on
Monera is translated in Quart. Journ. Microsc. Sci. n.s. vol. ix. 1869; and the first
of his papers on the Gastrcea Theory in vol. xiv. 1874, of the same journal. See also
the valuable series of papers on the Freshwater Rhizopods, by Mr. Wm. Archer, in the
current series of the Quart. Journ. Microsc. Sci. ; the important memoirs of Hertwig
and Lesser in the Archiv fur mikr. Anat. (especially the suppl. Heft to Bd. X. 1874),
and the Presidential, Addresses of Professor Alhnanto theLinnean Society for 1876 and
1877 (in Nos. 69 and 71 of its Journal) on 1 Recent Researches on some of the more
simple Sarcode-Organisms,' of which the Author has freely availed himself ; as also the
chapters dealing with the Rhizopoda and Sporozoa in Professor Biitschli's edition of
Bronn' s'K lassen and Ordnungen des Thierreichs, 1880, and Professor Ray Lankester's
article, ' Protozoa,' in the ninth edition of the Encyclopedia Britannica, 1885.

678

CHAPTER XIII

ANIMALCULES— INFUSORIA AND BOTIFEEA

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, larvse of worms,
molluscs, (fee. 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 loiv, and the other of comparatively high
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 the
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 Rotifer a or Rotatoria is, on 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

CI LI ATE INFUSORIA

679

is difficult to decide what is their relationship to other groups of
'Vermes.' 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.

Section I. — Infusoria.

This term, as now limited by the separation of the Rhizopoda
on the one hand, and of the Rotifera on the other, is applied to a>
far smaller range of forms than was included by Professor Ehren-
berg under the name of ' polygastric ' animalcules. For a large
section of these, including the Desmidiacea^ Diatomacece, Volvocinece,
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 ciliate 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 currents ; 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 oesophageal 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 digestive vesicle
(fig. 531) ; 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 ' ; while their rhythmical
action resembles that of the circulatory and respiratory 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 active free-swimming
Infusoria are directed, so as to avoid obstacles and find out passages,

68o

MICKOSCOPIC FOKMS OF ANIMAL LIFE

seems to indicate that another portion of their protoplasmic sub-
stance must have to a certain degree the special endowments which
characterise the nervous systems of higher animals. Altogether, it
may be said that in the ciliate Infusoria the life of the single cell
finds its highest expression.^

Before proceeding to the description of the ciliate Infusoria,
however, it will be 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-history
of several among them. The monads, properly so called,2 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 Monas lens, long familiar
to microscopists as occurring in stagnant waters and infusions of
decomposing organic matter, is a spheroidal particle of protoplasm,
from o oo 0th to s^Vo^h 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 flagellum, 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 aperture that forms itself in some part of the oral
region near the base of the flagellum. In some true Monadinw
neither nucleus nor contractile vesicle is distinguishable, but in
the majority a nucleus can be clearly seen. The life-history of
several simple Monadinm 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 among zoologists from the time when it was first definitely put
forward by Von Siebold (Lehrbuch der vergleich. Anat. Berlin, 1845) 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 Dujardm (Hist. Nat. des
Infusoires, Paris, 1841). Of late, however, there has been a decided convergence of
opinion in the direction above indicated ; which has been brought about in great
degree by the contrast between the pro to zoic 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 Infusorien,'
Jenaische Zeitschr. Bd. vii. 1873). An excellent summary of the whole discussion
was given by Professor Allman in his Presidential Address to the Linnean Society in
1875.

2 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.

THE SAPROPHYTIC ORGANISMS

and thoroughness by Messrs. Dallinger and Drysdale, of whose im-
portant observations a general summary will now be given.1

The present Editor, while adopting the lead of Dr. Carpenter,
greatly doubts the suitability or correctness of the saprophytic monad
forms appearing at this place in the organic series. They possess
features that ally them, as has been already suggested, much more
nearly to the vegetable series, and indicate affinities with certain
Nostocacepe and the Bacteria ; while a leaning to the Mycetozoa and
the* chlorophyllaceous Algae, and even some forms of Fungi, is quite
apparent to the careful student.

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 on the
destructive ferment. But where 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 Bacteria, 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 these organisms is represented in fig. 1, Plate
XIII, A. It has been named by Saville Kent 2/onas Dallingeri,
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. l, A, represents the normal form of the
organism. It has a long diameter of about the g^^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 sud-
denly appears a constriction across its body, as in fig. 2. This is at
once accompanied by an apparent effort of the opposite nagella to
pull against each other ; the consequence is a very rapid stretching
of a neck of sarcode between two halves of the body, as at fig. 3.
This becomes longer, as at 4, and attains the length of two nagella
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 6 and 7.

This, in the course of from two to three minutes, is once more
begun and carried on in each half successivelv, 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 appears in the unaltered
and unchanged field of observation normal forms, with a remarkable

1 See their successive papers in the Monthly Microsc. Journ. vol. x. 1873,
pp. 53, 245 ; vol. xi. 1874, pp. 7, 69, 97 ; vol. xii. 1874, p. 261 ; and vol. xiii. 1875,
p. 185 ; and Proceed. Boy. Soc. vol. xxvii. 1878, p. 332. But especially for the latest
results with recent objectives, Journ. Boy. Micro. Soc. vol. v. 1885, p. 177; vol. vi.
p. 193 ; vol. vii. p. 185 ; vol. viii. p. 177.

682

MICROSCOPIC FORMS OF ANIMAL LIFE

diffluent or amoeba-like envelope, as seen in figs. 8 and 9, A. These
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 fig. 10, A. This leads to the rapid
approach of the oval bodies of the two organisms, as in fig. U, B,
resulting in their fusion, as in figs. 12, 13, 14, and a still condition
of the sac (fig. h) 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 sjjores, as shown in fig. 15. These are opaque or semi-opaque,
but by observation upon them at a temperature of 65° to 70° Fahr.,
they in the course of thirty minutes become transparent, elongate,
as in figs. 16 and 17, and, continuing to grow, assume the conditions
and sizes represented in figs. 18 and 19 ; and we were able to trace
them through all their changes of growth from the spore into the
adult condition, as at fig. 20, until they entered upon and passed
through the self-division into two described and figured in A.

The next form, though even more simple in appearance, has a
much more complex morphological history. It is seen in its normal
form in fig. l, C. It has but one flagellum, and, as we believe, on
that account has a much more restricted power of movement. It*
is from the --^-jjth to the -^Lyth of an inch in long diameter. In
its motion at one stage of its life its oval body becomes uncertain in
form, as seen in 2, 3, 4, C ; but when this has continued for not
more than a minute, the flagellum falls in upon the body, as in 4,
and the organism becomes perfectly still. In this condition, after a
space ranging from ten to twenty minutes, two white bars .at right
angles suddenly appear, as in fig. 5 ; this is almost immediately
followed by another and a similar one at right angles to the first, as
in fig. 6. Then the circumference of the flattened sphere twists,
leaving the centre unaffected, so that the body assumes a turbined
appearance, as seen in fig. 7. After this the interior substance breaks
up, and becomes a knot of slightly moving but compact forms, as in
fig. 8 ; which remains in this state for from fifteen to twenty
minutes, and then becomes dissociated, as in fig. 9 ; so that we have'
here a complex form of multiple partition, giving rise to enormous
numbers, because, 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. 1 1 , 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. 12, resulted.

This remained from thirty to thirty- six hours absolutely inert p
but at the expiration of that time it burst, as seen in fig. 13, D, and
poured out an enormously diffusive fluid, which as it flowed into the
surrounding water appeared like a denser fluid, diffusing itself through
one of less density ; but no spores were at this stage at all apparent. It-

Plate XIII.

SAPEOPHYTIC LIFE-HISTORIES

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
became larger and larger, growing as seen in 14, 15, 16, 17, until
the adult size was reached, as at 18, 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 XIII.

The monad has been named by S. Kent Dallingeria Drysclali.
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. l, is a long oval, slightly constricted
in the middle, and having a kind of pointed neck (a), from which
proceeds a flagellum about half as long again as the body. From
the shoulder-like projections behind this (b, c,) arise two other long
and fine flagella, which are directed backwards. The sarcode-body
is clear, and apparently structureless, with minute vacuoles dis-
tributed through it ; and in its hinder part a nucleus (d) is dis-
tinguishable. The extreme length of the body is seldom more than
the ^o^th of an inch, and is often the ^-J^th. This monad swims
with great rapidity, its movements, which are graceful and varied,
being produced by the action of the flagella, which can not only
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
flagella anchor themselves and coil into a spiral, and the body then
darts forwards and upwards, until the anchored flagella straighten
out again, when the body falls forward to its horizontal position, to
be again drawn back by the spiral coiling of the anchored flagella.
This monad multiplies by longitudinal fission, the first stage of
which is the splitting of the anterior flagellum into two (fig. 2, a, b),
and a movement of the nucleus (c) towards the centre. In the course
of from thirty to sixty seconds the fission extends down the neck (fig.
3, a ) ; a line of division is also seen at the posterior end (c), and the
nucleus (b) shows an incipient cleavage. In a few seconds the
cleavage -line runs through the whole length of the body, the separa-
tion being widest posteriorly (fig. 4, a) ; and in from one to four
minutes the cleavage becomes almost complete (fig. 5), the posterior
part of the body, with the two halves (a and b) of the original nucleus^
being now quite disconnected, though the anterior parts are still
held together by a transverse band of sarcode, as seen in fig. 6, which
continues to rapidly elongate, as in fig. 7, and becomes the length of
two side flagella, as in fig. 8. The forms then approach and rapidly
recede from each other, snapping the cord, as in figs. 9 and 10. In
this way two forms exist instead of one ; and each of these almost

684

MICROSCOPIC FORMS OF ANIMAL LIFE

immediately enters upon and passes through the same process of fission,
which from first to last is completed in from four to seven minutes ;
and being repeated at intervals of a few minutes, this mode of multipli-
cation produces a rapid increase in 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 flagella and the great
development of the nucleus, and afterwards in the formation of a
transverse granular band across the middle of the body (fig. 11, E).
One of these altered forms, swimming into a group in the ' springing '
state, within a few seconds firmly attaches itself to one of them, which
at once unanchors itself, and the two swim freely and vigorously about,
shown in fig. 12, 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 flagella of the ' springing '
form being drawn in (fig. 13, 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. 1 4, a. This is a cyst filled with repro-
ductive particles of such extraordinary minuteness that, when
•emitted from the ends of the cyst (fig. 15, a) after the lapse of four
or five hours, they can only be distinguished under an amplification
of 5000 diameters, with perfect central illumination, i.e. the full
cone of a large-angled condenser. Yet these particles, when con-
tinuously watched, are soon observed to enlarge and to undergo
elongation (figs. 16, 17, 18, 19, 20), and within two hours after their
■emission from the sac the anterior nageHum, and afterwards the two
lateral flagella (fig. 19), can be distinguished. Slight movements then
commence, the neck-like protrusion shows itself and in about
half an hour more the regular swimming action begins. About
four hours after the escape of its germ from the sac, the monad
acquires its characteristic form (fig. 2l), 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 the entire life-cycle of this monad
has thus been elucidated ; and it will now be sufficient to notice the
principal diversities observed by Messrs. Dallinger and Drysdale in
the life-cycles of the other monadine forms which they have studied.

The hi- flagellate or ' acorn ' monad of the same observers (identi-
fied by Kent with the Polytoma uvella of Ehrenberg) presents some
remarkable peculiarities in its mode of reproduction. Its binary
fission extends only to the protoplasmic substance of its body, leaving

SAPROPHYTIC LIFE-HISTORIES

685

its envelope entire ; and by a repetition of the process, as many as six-
teen segments, each attaining the likeness of the parent, are seen thus
enclosed, their flagella protruding through the general investment.
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 protoplasm. It is, like the previous pro-
cess, non-sexual or gonidial, the true generative process consisting
here, as in the preceding cases, in the 4 conjugation ' of two indi-
viduals, with the usual results.

The hooked monad {Heteromita uncinata, Kent) is another bi-
nagellate form, usually ovate with one end pointed, and from y^^th
to 4-^-^th of an inch in length, being distinguished from the pre-
ceding by the peculiar character of its flagella, of which the one that
projects forward 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
flagellum. Multiplication takes place by transverse fission, and con-
tinues uninterruptedly for several days. A difference then becomes
perceptible between larger and smaller individuals, the former
being further distinguished by the presence of what seems to be a
contractile vesicle in the anterior part of the body. Conjugation
occurs between one of the larger and one of the smaller forms, the
latter being, as it were, absorbed into the body of the larger ; and
the resulting product is a spherical cyst, which soon begins to
exhibit a cleavage-process in its interior. This continues until the
whole of its sarcodic substance is subdivided into minute oval
particles, which are set free by the rupture of the cyst, and of
which each is usually furnished with a single flagellum, by whose
lashing movement 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 calycine monad of the same observers (Tetramitus rostratus,
Perty) has a length of from -?nVoth to y^^th of an inch, and a
compressed body tapering backwards to a point. Its four flagella
(which constitute its generic distinction) arise nearly together from
the flattened front of the body, and its swimming movement is a
graceful gliding. Near the base of the flagella are a pair of contractile
vesicles, and further behind is a large nucleus. Multiplication takes
place by longitudinal fission, which is preceded by a change to a semi-

686

MICROSCOPIC FORMS' OF ANIMAL LIFE

amoeboid state. This gives place to a more regular pear-like form,
the four flagella issuing from the large end ; and the fission commences
at 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 amoeboid condition, in which the
creature not only moves, but also feeds, like an Amoeba (devouring all
the living and dead Bacteria in its neighbourhood), occurs previously
to ' conjugation ' ; and this takes place between two of the amoeboid
forms, which begin to blend into each other almost immediately
upon coming into contact. The conjugated bodies, however, swim
freely about for a time, the two sets of flagella apparently acting in
concert. But by the end of about eighteen hours the fusion of
the bodies and nuclei is complete, the flagella are lost, and a
spherical distended sac is then formed, which, in a few hours more
without any violent splitting or breaking up, sets free innumerable
masses of reproductive particles. These under a magnifying power
of 2500 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. Dallinger and Drysdale has reference to the temperatures
respectively endurable by the adult or developed forms of these
monads, and by their reproductive germs. A large number of experi-
ments upon the several forms now described indubitably led to the
conclusion that all the adult 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 150° Fahr. But, on the other hand, the reproductive granules
emitted from the cysts that originate in ' conjugation ' were found
capable of sustaining a fluid heat of 220°, and a dry heat of about
30° more, those of the Cercomonad surviving exposure to a dry heat of
300° Fahr. This is a fact of the highest interest in its bearing on the
question of ' spontaneous generation,' or abiogenesis ; since it shows
that germs capable of surviving desiccation maybe everywhere diffused
through the air, and may, on account of their extreme minuteness
(as they certainly do not exceed ^ooVtro^n °^ an ^ncn ^n 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 Monthly Micros. Journ. vol. xi. 1874, p. 97 ; ibid.
-vol. xv. 1876, p. 165 i and Proceed. Boy. Soc. vol. xxvii. 1878, p. 343.

THE MONAD NUCLEUS WITH RECENT LENSES '687

saprophytic organism 1 of special character, have been discovered
during 'a recent period. But it will he of more moment here to note
to what an extent in this series of observations the new homogeneous
objectives, especially in their apochromatic form, have been success- •
fully employed in enlarging the area of knowledge.

The present Editor has gone carefully over the great 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 L50 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
Dr. Dallinger upon the nucleus of the nucleated forms of these monads.

- Briefly to present the facts, we may recall the part taken in the
act of fission in the form last described (Dallinyeria Drysdali). It
will be seen by reference that it appeared to us that the nucleus fol-
lowed the processes inauyuratedby the somatic 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 y^th inch and -^th inch homo-
geneous of N.A. L50 by Powell and Lealand, and an apochromatic
of TVth inch N.A. 1*40 by the same Firm ; and also by the use of
the beautiful 3 mm. and 2 mm. N.A. 1*40 of Zeiss (apochromatic),
it can be seen with comparative ease that it is in the nucleus that
all the activities of the body are originated.

This may be followed from a study of Plate XIV. Fig. 1 , A,
represents the nucleus of the form drawn . at fig. 1, E, Plate
XIII. In long diameter it is of an average length of ^joirooth °f an
inch ; but 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 substances, as seen in fig. 6, A,
Plate XIV. But directly the process of fission is to be inaugurated,
•we need not wait to see its first action in the splitting of the
flagellum, as in fig. 2, E, Plate XIII, ; 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 either side of
the nucleus, as in fig. 1, b, A, Plate XIV. A clear space is left at c,
and no change has taken place in the body-sarcode, a, a, a. But
shortly an incision takes place in the nucleus, as at d, fig. 2, and
this is immediately followed by the incision f in the body-s;a code,
and the process goes on simultaneously in nucleus and body, as in
fig. 3, until the division of the nucleus is completely effected, and the
total severance of the body follows.

But as soon as the nucleus is divided, the plexus, which has been
during division, as in fig. 3, condensed over part of each dividing
half, at once distributes itself evenly again, as in fig. 6, A, and re-
mains so until another change is inaugurated in the form to which
the nucleus belongs.

1 Joam. of Jtoyal Micros. Soc. vol. v.

688 MICKOSCOPIC FORMS OF ANIMAL LIFE

Not less remarkable is this in the conjugation of the same form.
With the old lenses we 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 XIII.
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 highly
refractive plexus -like condition into a large milky structureless state,
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
fig. 4, A, Plate XIV, 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 nucleolar body through the substance of the whole, as in
fig. 5, A.

In B, Plate XIY, the nucleus only, separate from the body of
the organism known as Tetramitus rostratus, is shown as we can
reveal it with recent German and English apochromatic objectives.
This entire organism is relatively large, and its nucleus will average
in long diameter the To^00th of an inch.

Hence it affords a still better means of study. Now this
organism divides by fission for a very considerable time, but at length
many forms become amoeboid — acting precisely as an amoeba, 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.

We could with the old objectives determine nothing more than
the fact that the amoeboid form had supervened ; but now it is easy to
show that the nucleus in the body of a form not yet amoeboid is under-
going change upon which the amoeboid state 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 observed 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. 1, B, Plate XIV, 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 be inaugurated, when the change seen in fig. 2r
followed by the changes and deeper division seen in figs. 4, 5, 6,
7, and 8, which, after the state of the nucleus seen in fig. 4 has
been reached, is shared in by initiation of division in the substance
of the entire body.

It thus appears that a form of haryo kinesis 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

STUDIES OF THE NUCLEUS IN SAPROPHYTIC ORGANISMS.

FLAGELLATA

689

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 to be the protoplasmic ectosarc proceeds from the anterior
extremity of the body (fig. 527, cl), forming a kind of funnel, from the
bottom of which the fiagellum 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 where the ectosarc seems
softer than that which envelops the rest of the body. Towards the
base of the collar a nucleus (n) is seen ; while near the posterior
termination of the body is a single or double contractile vesicle (cv).
The body is attached by a pedicle proceeding from its posterior
extremity, which also seems to be a prolongation of the ectosarc.
These animalcules multiply by longitudinal fission ; and this, in
some cases (as in the genus Jlonosiga), proceeds to the extent of a
complete separation of the two bodies, which henceforth, as in the
ordinary Monadina,
live quite independ-
ently of each other.
But in other forms, as
Codosiga, the fission
does not extend through
the pedicel, and the
twin bodies being thus
held together at their
bases, and themselves
undergoing duplicative
fission, clusters are pro-
duced which spring
from common pedicels
(fig. 528) ; 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 miniature the polype-cell of a Campanu-
laria forms itself round the body of the monad, which can retract
itself into the bottom of it ; and in the genus Salpingoeca both
calyx and collar are present. In some forms of this group multi-
plication seems to take place, not by fission, but by gemmation :
and, as among hydroid polypes, the gemma! -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, whilst in others they

Fig. 527. — Single zoijid of Codosiga umbellata : cl,
collar; n, nucleus; cv, double contractile vesicle.

1 See his memoirs in Ann. Xat. Hist. ser. 3, vol. xviii. 18(JG ; ibid. ser. 4, vol. i.
186S ; vol. vii. 1871 ; and vol. ix. 1872.

2 See 'his Manual of the Infusoria, 1880-82, 2 vols, and 1 vol. of platas.

690

MICROSCOPIC FORMS OF ANIMAL LIFE

take on a ramifying arrangement. While some of these composite
organisms are sedentary, others, as Dinobryon, are free- swimming.

Two solitary flagellate forms, Antliophysa and Anisonema, may
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 stitF
bristle, of uniform diameter throughout, which moves by occasional
jerks, and the other a very delicate tapering flagellum, which is
in constant vibratory motion. If, as appears from the recent observa-
tions of Biitschli, the well-known Astasia — of which one species has
a blood-red colour, and sometimes multiplies to such an extent as
to tinge the water of the ponds it inhabits — has a true mouth for the

Fig. 528. — Codosiga umhellata : Colony-stock, springing from single
pedicel tripartitely branched.

reception of its food, it must be regarded as an animal, and sepa-
rated from the Euglena (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 no longer any doubt that the well-known Noctiluca
miliar is — 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 ' Fhigdlata. This animal, which is of spher-
oidal form, and has an average diameter of about ^-th 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 Zeitschrift f. Wissensch. Zool. Bd. xxx,
of which an abridgment (with plate) is given in Quart. Joam. Micros. Sci» vol. xix.
1879, p. (j'd.

NOCTILUCA

69 r

the light ; and its tail-like appendage, whose length about equals
its own diameter, and which serves as an instrument of locomotion,
may be discerned with a hand -magnifier. The form of Noctiluca is
nearly that of a sphere, so compressed that while on one aspect (fig.
529, 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,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 there
extends a slightly elevated ridge, c, which commences with the
appearance of a bifurcation at the end of the atrium farthest from

Fig. 529. — Noctiluca miliar is as seen at A on the aboral side, and at
B on a plane at right angles to it : a, entrance to atrium; b, atrium ;
c, 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 ninety 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 oesophagus, 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 floor there arises a long

1 The organ here termed ' tentacle ' is commonly designated flagellum ; while
what is here termed the flagellum is spoken of by most of those who have recognised
it as a cilium. The Author agrees with M. Robin in considering the former organ,
which has a remarkable resemblance to a single fibrilla of striated muscle as
one peculiar to Noctiluca, and the latter as the true homologue of the flagellum of
the ordinary Flagellata. It is curious that several observers have been unable to dis-
cover the so-called cilium, which was first noticed by Krohn. Professor Huxley sought
for it in at least fifty individuals without success ; and out of the great number which
he afterwards examined he did not get a clear view of it in more than half a dozen.

Y Y 2

692

MICROSCOPIC POEMS OF ANIMAL LIFE

flagellum, which vibrates freely in its interior. The central proto-
plasmic mass sends off 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. 530). Besides these branching-
prolongations, there is sent off from the central protoplasmic mass a
broad, thin, irregularly quadrangular extension (fig. 529, B,/), which
extends to the superficial rod-like ridge, and seems to coalesce with
it • its lower free edge has a thickened border ; whilst its upper-
edge becomes continuous with a plate-like striated structure, g, which
seems to be formed by a peculiar duplicature of the body- wall. At
one side of the protoplasmic mass is seen a spherical vesicle, h, of.'

PlG. 580. — Portion of superficial protoplasmic reticulation formed
by ramification of an extension a of central mass. (Magnified
1000 diameters.)

about s o30 oths 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 nucleus.

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 oesophagus by extensions of its sub-
stance, which enclose 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 then-
way into the radiating extensions of the central mass (as shown in
fig. 529, B), and are ensheathed by the protoplasmic substance which
goes on to form the peripheral network (fig. 531). 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

NOCTILUCA

693

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 has 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
cyelosis (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. 529. There is no contractile vesicle.

The peculiar ' tentacle' of Noctiluca 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

Fig. 531. — Pair of digestive vesicles of Xoctiluca lying in course of exten-
sion of central protoplasmic mass, o, to form peripheral reticulation,
b, and containing remains of Algae. (Magnified 4S0 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. But 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 Xoctiluca 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 JToctilucce, they may be obtained by the tow-net in un-
limited quantities ; and when transferred into a jar of sea-water,
they 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

694

MICROSCOPIC FORMS OF ANIMAL LIFE

their light, which is of a beautiful greenish tint, and is vivid enougli
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 Noctiluca, on the stage of the microscope,
produce a luminosity strong enough to be visible under a half -inch
objective, lasting with full intensity for several seconds, and then
gradually disappearing. He 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 Cienkowsky, even a small portion of the proto-
plasm of a mutilated Noctiluca will (as among rhizopods) reproduce
the entire animal. Multiplication by fission or binary subdivision,
beginning in the enlargment, 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 process.
The tentacle and flagellum disappear, and the mouth gradually
narrows, and at last closes up ; the meridional groove also disappears,
so that the animal becomes a closed hollow sphere. The nucleus
elongates, and becomes transversely constricted, and its two halves
separate, each remaining connected with a portion of the protoplasmic
network. This duplicative subdivision is repeated over and over
again, until as many as 512 'gemmules' are formed, each consisting
of a nuclear particle enveloped by a protoplasmic layer, and each
having its flagellum. The entire aggregate forms a disc-like mass
projecting from the surface of the sphere ; and this mass sometimes
detaches itself as a whole, subsequently breaking up into individuals ;
whilst, more commonly, the gemmules detach themselves one by one,
the separation beginning at the margin of the disc, and proceeding
towards its centre. The gemmules are at first closed monadiform
spheres, each having a nucleus, contractile vesicle, and flagellum ;
the mouth is subsequently formed, and the tentacle and permanent
flagellum afterwards make their appearance. A process of 4 conjuga-
tion ' has also been observed, alike in ordinary Noctilucce 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 thrown 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 Noctiluca has been the subject of numerous memoirs, of which the following
are the most recent : Cienkowski, Arch. f. Micros. Anat. Bd. vii. 1871, p. 131, and
Bd. ix, 1873, p. 47 ; Allman, Quart. Journ. Micros. Sci. n.s. vol. xii. 1872, p. 327 ;
Bobin, Journ. cle VAnat. et de Physiol, torn. xiv. 1878, p. 586; Vignal, Arch, de
Physiol. se*r. ii. torn. v. 1878, p. 415; Stein, Der Organismus der Infusionsthiere,
hi. 2, 1883; and Biitschli, Morphol. Jahrbuch, x. 1885, p. 529. For' the group of
which it and Leptodiscus (Hertwig) are the representatives, Ray Lankester has sug-
gested the name Eh ynchofiagellata.

DINOFLAGELLATA

695

The name Cilio-flagellata and the definition of the group must
both be altered, now that Klebs and Biitschli have shown that what
were regarded as cilia in the transverse grooves of their bodies is
really a fiagellum ; the name to be used is DinoflageUata? 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 Peridinium observed
by Professor Allman in 1854 was present in such quantities that
it imparted a brown colour to the water of some of the 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. 532, A, B) has a form approaching the spherical,
with a diameter of from ^^th to 3 J0 0th of an inch, and is
partially divided into two hemispheres by a deep equatorial furrow,
a, whilst the flagellum-bearing hemisphere, A, has a deep meridional
groove on one side, b, extending from the equatorial groove to the
pole, the fiagellum taking its origin from the bottom of this vertical

Fig. 532. — Peridinium uberrimum: A, B, front and back views ;
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
reseaches 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. 532, 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.
There is reason to believe that conjugation obtains in certain cases :
(llenodinium cinctum 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- water, but the most remarkable marine
forms of the cilio-flagellate group belong to the genus Geranium (fig.
533), in which the cuirass extends itself into long horny appendages.

1 Or, more correctly, Dinomastif/ophora,

696

MICROSCOPIC FORMS OF ANIMAL LIFE

Inrthe Ceratium 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,
one of them being always longer than the other ; whilst the posterior
is prolonged straight backwards. The anterior and posterior halves
of the cuirass are separated by a ciliated furrow, from one point of
which the flagellum arises ; and at the origin of this is a deep
depression into which the flagellum may be completely and suddenly
withdrawn. The Author has found the Ceratium tripos extremely

Fig. 533. — 1, Ceratium tripos; 2, Ceratium furca.

abundant in Lamlash Bay, Arran, where it constitutes a principal
article of the food of the Antedons that inhabit its bottom.1

Suctoria. — The suctorial Infusoria constitute a well-marked
group, all belonging to one family, Acinetina, the nature of which
has been until recently much misunderstood, chiefly on account of
the parasitism of their habit. 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 them-
selves by flexible peduncles, sometimes to the stems of Vorticellina-;,
but also to filamentous Algre, stems of zoophytes, or to the bodies
of larger animals. Their nutriment is obtained through delicate

1 See Allman in Quart. Micros. Journ. vol. iii. 1855, p. 24; H. James-Claik in
Ann. Nat. Hist. ser. iii. vol. xviii. 18GG, p. 429 ; and Bergli, Morphol. Jahrbuch, vii.
1881, p. 177.

A CINETINA

697

tubular extensions of the ectosarc, which act as suctorial tentacles
(fig. 534), 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 Actinophrys, 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-

Fig. 534. — Suctorial Infusoria : 1, Conjugation of Podophrya
quadripartita ; 2, formation of embryos by enlargement and sub-
division of the nucleus ; 8, ordinary form of the same ; 4, Podo-
phrya elongata.

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 Acinetina cannot be fed with
indigo or carmine ; but, so far as can be ascertained by observation
of what goes on within their bodies, there is a general protoplasmic
cyclosis without the formation of any special 'digestive vesicles.'
The ordinary forms of this group are ranked under the two genera
Acineta and Podophrya, which are chiefly distinguished by the
presence of a firm envelope or lorica in the former, while the body
of the latter is naked. In one curious form, the Ophryodendroii) the
suckers are borne in a brush-like expansion on a long retractile

698

MICROSCOPIC FORMS OF ANIMAL LIFE

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
longitudinal fission, but this is rare in the adult state. Some-
times external gemmce are developed by a sort of pinching off of a
part of the free end of the body, which includes a portion of the
nucleus ; the tentacula of this bud disappear, but its surface be-
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 ordinary method of
propagation in this type. These originate in the breaking up of
the nucleus into several segments, each of which incloses itself in
a protoplasmic envelope ; and this becomes clothed with cilia, by
the vibrations of which the embryos are put in motion within the

Fig. 585. — Immature forms of Podophrya.quadripartita : 1, Amoe-
boid state (Trichophrya of Claparede and Lachmann) ; 2, the
same more advanced ; 8, incipient division into lobes.

body of the parent (fig. 534, 2), from which they afterwards escape
by its rupture. In this condition (a) they swim about freely, and
seem identical with what has been described by Ehrenberg as a
distinct generic form, Megatricha. And, according to the observa-
tions of Mr. Badcock,1 these Megatricha-iorms, multiply freely by
self-division. After a short time, however, they settle down upon
filamentous Algae 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.
535, ], 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

1 Journ. of Boy. Micros. 80c. vol. iii. 1880, p. 563.

CILIATA

699

footstalk is formed, and they gradually assume the characteristic
form of Podophrya quadrijjartita. 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. 534, ]) ; but whether this always
precedes the production of internal embryos, or is any way prepara-
tory to propagation, has not yet been ascertained.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 little flattened,
tapering gradually from the base to the point. Their size is ex-
tremely variable, the largest that have been observed being about
T^oth of an inch in length, and the smallest about -rsiyo^th. "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 save 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
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 zoospores 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 will, 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.

In the ciliated Infusoria, the differentiation of the sarcodic sub-
stance into ' ectosarc ' or cell-wall, and ' endosarc ' or cell-contents,

1 The Acinetina were described both by Ehrenberg and Dujardin ; but the first
full account of their peculiar organisation was given by Stein in his Organismus iter
InfusionsthiercJien.' Misled, however, by their parasitic habits, Stein originally sup-
posed them not to be independent types, but to be merely transitional stages in the
development of VcrticellincB and other ciliate Infusoria ; this doctrine he loug
since abandoned. Much information as to this group will also be found in the
beautiful Etudes sar les Infusoireset les RJiizojwcles of MM. Claparede and Lach-
mann, Geneva, 1858-61.

yoo

MICROSCOPIC FORMS OF ANIMAL LIFE

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 Diatomacese, seems to be the representative of the
carapace of Arcella &c. as of the cellulose coat of protophytes.
It is sometimes hardened, so as to form a ' shield ' that protects

Fig. 536. — A, Kerona silurus : a, contractile vesicle ; b,
mouth ; c, c, animalcules swallowed by the Kerona, after
having themselves ingested particles of indigo. B,
Paramecium caudatum : a, a, contractile vesicles ;
The dotted lines indicate currents.

b, mouth.

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 4 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 Costonella,
-described by Haeckel, the body is enclosed in a silicious lattice-work
shell, usually bell-shaped or helmet-shaped, which bears so strong
a resemblance to the shells of many Radiolaria as to be easily mis-
taken for them. The form of the body is usually much more
definite than that of the naked rhizopods, each species having its
characteristic shape, which is only departed from, for the most part,
when the animalcule is subjected to pressure from without, or when
its cavity has been distended by the ingestion of any substance
above the ordinary size. The cilia and other mobile appendages of

VORTICELLA

701

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 striae may be
distinguished, and as this striation is also distinguishable in the
eminently contractile foot-stalk of Vorticella1 (fig. 537, B) there seems
good reason to regard it as indicating a special modification of pro-
toj>lasinic substance, which resembles muscle in its endowments.
Hence this is termed the 'myo-
phan-'layer.' Beneath this, in cer-
tain species of Infusoria, there is
found a thin stratum of condensed
protoplasm, including minute 'tri-
chocysts,'" which resemble in
miniature the ' thread-cells ' of
zoophytes ; and this, where it
exists, is known as the ' tricho-
cyst-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 animal-
cule is living.

The vibration of ciliary fila-
ments, which are either disposed
along the entire margin of the
body, as well as around the oral
aperture (fig. 537, A, B), or are
limited to some one part of it,
which is always in the immedi-
ate vicinity of the mouth, 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 undu-
lations, 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

1 On the morphology of the YoriicellhiEe see Biitschli. Morphol. Jahrb. xi.

Fig. 537. — Group of YcrticeUa nebulifera
showing, A, the ordinary form ; B, the
same with the stalk contracted ; C. the
same with the bell closed ; D. E. F, suc-
cessive stages of fissiparous multiplica-
tion.

p. 553.

702

MICKOSCOPIC FOBMS OF ANIMAL LIFE

(like the spines of Echini) by the contraction of the integument from
which they arise, in such a manner that the animalcule crawls by
their means over a solid surface, as we see especially in Trichoda
lynceus (fig. 541, P, Q). In Chilodon and Nassula, again, the mouth
is provided with a circlet of plications or folds, looking like bristles,
which, when imperfectly seen, received the designation of ' teeth ; ;
their function, however, is rather that of laying hold of aliment-
ary particles by their expansion and subsequent drawing together
(somewhat after the fashion of the tentacula of zoophytes) than of
reducing them by any kind of masticatory process. Some, like
Opcdina, are entoparasitic, and have no mouth • a form allied to
Opcdina (Anoploplirya circulans) lives in the blood of Asellus
aquaticus ; other entoparasites, such as Triehonympha in the ' white
ant,' still possess their mouth. The curious contraction of the foot-
stalk of the Vorticella (fig. 537), again, is a movement of a different
nature, being due to the contractility of the tissue that occupies
the interior of the tubular pedicle. This stalk serves to attach the
bell-shaped body of the animalcule to some fixed object, such as a
leaf or stem of duck-weed ; and when the animal is in search of
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 Kuhne) by electrical excitation. The only special
i impressionable ' organs 1 for the direction of their actions with the
possession of which Infusoria can be credited are the delicate
bristle-like bodies which project in some of them from the neighbour-
hood of the mouth, and in Stentor from various parts of the surface.
The red spots seen in many Infusoria, which have been designated
as eyes by Professor Ehrenberg, from their supposed correspondence
with the eye-spots of JRotifera, 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 — Erytliropsis agilis — 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 has observed that if light be
allowed to fall on a part only of a colony of Ophridimn versatile all
the members soon congregate to the illuminated portion.

The interior of the body does not always seem to consist of a

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.

INFUSORIA

703

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 Diatomacese seem to be the ordinary food of many ; and the in-
solubility 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. 541, 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 Entomostraca, for example, are so voraciously
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 oesophagus being-
occupied by other particles subsequently ingested. (This 'moulding,'
however, is by no 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 after
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. 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.

704

MICROSCOPIC FORMS OF ANIMAL LIFE

Contractile vesicles (fig. 536, 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 contents, as to be quite unclistinguishable, 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 size and place, in different individuals of the
same species ; hence they are obviously quite different in character
from the ' vacuoles.' In Paramecium there are always to be observed
two globular vesicles (fig. 536, B, a, a), each of them surrounded by
several elongated cavities, arranged in a radiating manner, so as to
give to the whole somewhat of a star-like aspect, as seen in fig. 538,
l, v, v ; and the liquid contents are seen to be propelled from the
former into the latter, and vice versa. Further, in Stentor, a com-
plicated network of canals, apparently in connection with the con-
tractile vesicles, has been detected in the substance of the ' ectosarc,'
and traces of this may be observed in other Infusoria. In some of
the larger animalcules it may be distinctly seen that the contractile
vesicles have permanent valvular orifices opening outwards, and that
an expulsion of fluid from the body into the water around it is
effected by their contraction ; in some vorticellids the 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 is
still very imperfect ; for, although various modes of multiplication
have been observed among them, it still remains doubtful whether
any process takes place that can be regarded, like the conjugation
of the Monadina, as analogous to the sexual generation of higher
organisms. 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 else-
where) 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. 537, D, E, F), in other species transversely
(fig. 541, C, D) ; whilst in some, as in Chilodon cucullulus (fig. 539),
it has been supposed to occur in either direction indifferently. But
it may fairly be questioned whether, in this last case, one set of thr
apparent ' fissions ' is not really ' conjugation ' of two individuals.
This duplication is performed with such rapidity, under favourable
circumstances, that, according to the calculation of Professor Ehren-
berg, no fewer than 268 millions might be produced in a month by
the repeated subdivisions of a single Paramecium. When this fission
occurs in Vorticella (fig. 537), it extends down the stalk, which thus
becomes double for a greater or less part of its length ; and thus a

SUBDIVISION OF INFUSORIA 705

whole bunch of these animalcules may spring (by a repetition of the
same process) from one base. In some members of the same family
arborescent structures are produced resembling that of Codosiga
(fig. 528) by the like process of continuous subdivision. Another
curious result of this mode of multiplication presents itself in the
family Ophrydina, masses of individuals which separately resemble

1 2 3 4

g 7 8 c 10 11 12

13 14 15 16 17 18

Fig. 538. — Sexual (?) reproduction of Infusoria.

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 of JSTostoc, 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

z z

7o6

MICROSCOPIC FORMS OF ANIMAL LIFE

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

A B C D E F

Fig. 539. — Fissiparous multiplication of Cliilodon cucullulus : A,
B, C, successive stages of longitudinal fission(?) ; D, E, F, succes-
sive stages of transverse fission.

E £

which is about y^^th of an inch in length, with those of the composite
masses, some estimate may be formed of the number included in the
latter ; for a cubic inch would contain nearly eight millions of them
if closely packed ; and many times that number must exist in the
larger masses, even making allowance for the fact that the bodies

of the animalcules are
separated from each
other by their gelatinous
cushion, and that the
masses have their cen-
tral portions occupied
by water only. Hence
we have, in such clus-
ters, a distinct proof of
the extraordinary ex-
tent to which multipli-
cation by duplicative
subdivision may pro-
ceed without the inter-
position of any other
operation. These ani-
malcules, however, free
themselves at times
from their gelatinous
bed, and have been ob-
served to undergo an
4 encysting process ' cor-
responding with that
of the Vorticellina.

mm

Fig. 540. — Encysting process in Vorticella micro-
stomia : A, full-grown individual in its encysted
state; «, retracted oval circlet of cilia; b, nucleus;
c, contractile vesicle ; B, a cyst separated from its
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,
(1, 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 gemnmles is discharged.

The chemical composi-
tion of this jelly or zoocytium has been investigated by Halliburton,
who finds that it resembles vegetable cellulose in its general pro-
perties, but differs from it and agrees with the form of cellulose manu-
factured by the Tunicata in being less easily converted into sugar.
Many, perhaps all, ciliated Infusoria at certain times undergo an

LIFE-HISTORY OF INFUSORIA

707

•encysting process, 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
cyst, the movements 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-
tions are either lost or retracted, as is well seen in Vorticella
(fig. 540, A). A new wreath of cilia, however, is developed near
the base, and in this condition the animal detaches itself from its
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 altogether quiescent through the whole period of its
torpidity ; so that, however long may be the duration of its imprison-
ment, it emerges without any essential change in its form or con-
dition. But in other cases this process seems to be subservient
either to multiplication or to metamorphosis. For in Vorticella the
substance of the encysted body (B) appears to break up (C, D) into
eight or nine segments, which, when set free by the bursting of the
cyst, come forth as spontaneously moving spherules. Each of these
soon increases in size, develops a ciliary wreath within which a mouth
makes it appearance, and gradually assumes the form of the Tricho-
dina grandinella of Ehrenberg. It then develops a posterior wreath
of cilia and multiplies by tranverse fission ; each half fixes itself by
the end on which the mouth is situated, a short stem becomes de-
veloped, and the cilia-wreath disappears. A new mouth and 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
cyst differing in many respects from that of the animalcule which
became encysted. According to M. Jules Haime, by whom this
history was very carefully studied,2 the form to be considered as the
larval one is that shown in fig. 541, A, E, which has been described
by Professor Ehrenberg under the name of Oxytricha. This possesses
a long, narrow, flattened body, furnished with cilia along the greater
part of both margins, and having also at its two extremities a set of
larger and stronger hair-like filaments ; and its mouth, which is an
oblique slit on the right-hand side of its fore-part, has a fringe of
minute cilia on each lip. Through this mouth large particles are not
^infrequently swallowed, which are seen lying in the midst of the

1 Everts, TJntersuchungen an Vorticella nebulif 'era, quoted by Professor Allman,
loc. eit.

2 Annalcs cles Sci. Nat. ser. iii. tome xix. 1853, p. 109.

7o8

MICROSCOPIC FORMS OF ANIMAL LIFE

endosarc without any surrounding vesicle ; and sometimes even an
animalcule of the same species, but in a different stage of its life, is
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 1 vacuoles ' make their appearance (G) ; and in the

Fig. 541. — Metamorphoses of Trichoda lynceus: A, larva (Oxytricha) ; B, a
similar larva after swallowing the animalcule represented at M ; C, a very-
large individual on the point of undergoing fission ; D, another in which
the process has advanced further ; E, one of the products of such fission ;
F, the same body become spherical and motionless; 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 Aspidiscce, one as seen side-
ways, moving on its bristles, the other as seen from below (magnified twice
as much as the preceding figures).

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

REPRODUCTION OF INFUSORIA

709

within its cavity (M). The body thus discharged (N) does not differ
■much in appearance from that of the Oxytricha before its encyst-
ment (F), though of only about two-thirds its diameter ; but it soon
develops itself (0, P, Q) into an animalcule very different from
that in which it originated. First it becomes still smaller by the
discharge of a portion of its substance ; numerous very stiff bristle -
like organs are developed, on which the animalcule creeps, as by
legs, 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 sort of an inverted
•cone whereby a current is brought towards the mouth. This latter
form had been described by Professor Ehrenberg under the name of
Aspidisca. It is very much smaller than the larva, the difference
being, in fact, twice as great as that which exists between A and
P, Q (fig. 541), since the last two figures are drawn under a magni-
fying power double that employed for the preceding. How the
Aspidisca-iorm in its turn gives origin to the Oocytricha-ioim
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 resem-
blance 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 1 encysted ! condition that their dispersion chiefly takes place,
since they have been found to endure desiccation in this state,
although in their ordinary condition of activity they cannot be dried
up without loss of life. When this circumstance is taken into
account, in conjunction with the extraordinary rapidity of multipli-
cation of these animalcules, there seems no difficulty in accounting
for the universality of their diffusion. It may be stated as a general
fact that wherever decaying organic matter exists in a liquid state,
and is exposed to air and warmth, it speedily becomes peopled with
some or other of these minute inhabitants ; and it may be fairly
presumed that, as in the case of the Fungi, the dried cysts or germs
of Infusoria are everywhere floating about in the air, ready to de-
velop themselves wherever the appropriate conditions are presented ;
while all our knowledge of their history seems further to justify the
belief that (in some instances, at least) the same germs may develop
themselves into a succession of forms so different as to have been
regarded as distinct specific or even generic types.

A very important advance was supposed to have been made in
this direction by the asserted discovery of M. Balbiani 1 that a true
process of sexual generation occurs among Infusoria, his observa-
tions having led him to the conclusion that male and female organs
are combined in each individual of the numerous genera he has
examined, but that the congress of two individuals is necessary for

1 See his ' Recherches BUI les Phenomenes sexuels des Infusoires ' in Dr. Brown-
Sequard's Journal de la Plujsiologie for 1861. An abstract of these researches is.
contained in the Quart. Journ. of Micros. Sci. for July and October 1862.

7io

MICEOSCOPIC POEMS OF ANIMAL LIFE

the impregnation of the ova, those of each being fertilised by the
spermatozoa of the other. He regards the ' nucleus ' as an ovarium
or aggregation of germs, whilst the ' nucleolus ' is really a testis or
aggregation of spermatozoids. The particular form and position
which these organs present, and the nature of the changes which
they undergo, vary in the several types of Infusoria ; but as we have
in the common Paramecium aurelia an example which, although
exceptional in some particulars, affords peculiar facilities for the
observation of the process, and has been most completely studied
by M. Balbiani, it is here selected for illustration. This animalcule,
as is well known, multiplies itself with great rapidity (under favour-
able circumstances) by duplicative subdivision, which always takes
place in the transverse direction, and the condition represented in
fig. 538, l, 2, is not, as has been usually supposed, another form of
the same process, but is really the sexual congress of two individuals
previously distinct. When the period arrives at which the Para-
mecia are to propagate in this manner, they are seen assembling
upon certain parts of the vessel, either towards the bottom or on the
walls ; and they are soon found coupled in pairs, closely adherent
to each other, with their similar extremities turned in the same
direction, and their two mouths closely applied to each other, but
still continuing to move freely in the liquid, turning constantly
round upon their axes. This conjugation lasts for five or six days,
during which period very important changes take place in the
condition of the reproductive organs. In order to distinguish
these the animalcules should be slightly flattened by compression.,
and treated with acetic acid, which brings the reproductive appa-
ratus into more distinct view, as shown in fig. 538, 1-5. In l each
individual contains an ovarium, a, which is shown to present in the
first instance a smooth surface ; and from this there proceeds an
excretory canal or oviduct, c, that opens externally at about the
middle of the length of the body into the buccal fissure, e. Each
individual also contains a seminal capsule, b, in which is seen lying
a bundle of spermatozoids curved upon itself, and which communi-
cates by an elongated neck with the orifice of the excretory canaL
The successive stages by which the seminal capsule arrives at this
condition from that of a simple cell, whose granular contents resolve
themselves (as it were) into a bundle of filaments, are shown in
G-10. In (i the surface of the ovary, a, is seen to present a lobulated
appearance, which is ©ccasioned by the commencement of its reso-
lution into separate ova ; while the seminal capsule is found to have
undergone division into two or four secondary capsules, b, b, each of
which contains a bundle of spermatozoa now straightened out. This
division takes place by the elongation of the capsule into the form
represented in 11, and by the narrowing of the central portion
whilst the extremities enlarge, the further multiplication being
effected by the repetition of the same process of elongation and
fission. In 3, which represents one of the individuals still in con-
jugation, the four seminal capsules, b, b, are represented as thus
elongated in preparation for another subdivision ; whilst the ovary
a, a, has begun, as it were, to unroll itself, and to break up into*

REPRODUCTION OF INFUSORIA

7H

fragments which are connected by the tube m. It is in this con-
dition that the object of the conjugation appears to be effected, by
the passage of the seminal capsules of each individual, previously to
their complete maturation, into the body of the other. In 4- is
shown the condition of a Paramedian ten hours after the conclusion
of the conjugation ; the ovary has here completely broken up into
separate granular masses, of which some, a, a, remain unchanged,
whilst others, 0, 0, 0, 0, either two, four, or eight in number, are
converted into ovules that appear to be fertilised by the escape of
the spermatozoa from the seminal capsules, these being now seen in
process of withering. Finally, in 5, which represents a Paramecium
three days after the completion of the conjugation, are seen four
complete ova, 0, 0, 0, 0, within the connecting tube, m, m ; whilst
the seminal capsules have now altogether disappeared. In fig.
538, 13-18, are seen the successive stages of the development of
the ovule, which seems at first (1.3) to consist of a germ-cell
having within it a secondary cell containing minute granules, which
is to become the ' vitelline vesicle.' This secondary cell augments
in size, and becomes more and more opaque from the increase of its
granular contents (14, 15, 1G), forming the ' vitellus ' or yolk, in
the midst of which is seen the clear 'germinal vesicle,' which shows
on its wall, as the ovule approaches maturity, the ' germinal spot '
(17). The germinal vesicle is subsequently concealed (18) by the
increase in the quantity and opacity of the vitelline granules. The
fertilised ova seem to be expelled by the gradual shortening of the
tube that contains them ; and this shortening also brings together
the scattered fragments of the granular substance of the original
ovarium, so as to form a mass resembling that shown in 1, a, by
the evolution of which, after the same fashion, another brood of ova
may be produced. 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 interpreting the phenomenon, it is clearly comparable to
the sexual reproduction 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. 538, 21). In
Vorticella, according to several recent observers, what has been
regarded as gemmijoarous multiplication — the putting forth of a bud
from the base of the body — is really the conjugation of a small
individual in the free- swimming stage with a fully developed fixed
individual, with whose body its own becomes fused. But it is
doubtful whether such conjugation has any reference to the encyst-
ing process. According to Butschli and Engelmann, the conjugating
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. The same
excellent observers adduce strong grounds for distrusting Balbiani's

712

MICROSCOPIC FORMS OF ANIMAL LIFE

assignment of sexual characters to the nucleus and nucleolus. For
although a striation may be observed on the surface of the latter,
no one has witnessed its subdivision into spermatozoidal filaments.
And if embryos are really produced at the expense of the nucleus,
what Balbiani described as sexual ova are really non-sexual gemmules,
each consisting (like the zoospore of protophytes) of a segment of
the nucleus surrounded by an envelope of protoplasm. 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. 538, 19, 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 way ; 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
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. Two observers, independently of each other,
examining the same monad, if their inquiries were not sufficiently
prolonged, might, with the utmost truthfulness of interpretation,
assert opposite modes of development. Competent optical means,
careful interpretation, close observation, and time are alone capable
of solving the problem.

t V .

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

1 There can be no doubt that Stein was wrong in his original doctrine that the
fully developed Acinetina are only transition stages in the development of Vorti-
cellina and other ciliated Infusoria. But the balance of evidence seems to the writer
to be in favour of his later statement, that the bodies figured in fig. 538, 19, are
really infusorian embryos, and not parasitic Acinetse.

WHEEL-LIKE ORGANS OF ROTIFERS

713

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 sea-shore ; but the great majority known to us belong to
fresh water, and are to be found in ditches, ponds, reservoirs, lakes,,
and slowly running streams — sometimes attached to the leaves and
stems of water-plants, sometimes creeping on the Algae, sometimes
swimming freely through the water. They are met with also in gut-
ters on the house-top, in water-butts, on wet moss, grass, and liver-
worts, in the interior of Volvox globator and Vaucheria, in vegetable
infusions, on the backs of Entomostraca, in the viscera of slugs, earth-

A B

Fig. 542. — Rotifer vulgaris, as seen at B, with the wheels drawn in, and
at A with the wheels expanded : b, eye-spots ; c, wheels ; d , antenna ;
e, jaws and teeth ; /, alimentary canal ; g, cellular mass inclosing it ;
7i, longitudinal muscles ; i, i, tubes of water-vascular system ; k,
young animal ; /, cloaca. |

worms and Naiades, and in the body-cavities of Synaptce — in fact,
in almost every place where there are moisture and food. The
wheel-like organs from which the class derives its designation are
most characteristically seen in the common Rotifer (fig. 542), where
they consist of two disc-like lobes or projections of the body whose
margins are fringed with long cilia ; and it is the uninterrupted suc-
cession of strokes given by these cilia, each row of which nearly re-
turns (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 arranged

7*4

MICKOSCOPIC FOKMS OF ANIMAL LIFE

so as to fulfil three different purposes, viz. to bring food to the
mouth, to conduct it through the alimentary canal, and to enable the
animal to swim.

The great transparence of the Rotifera permits their general
structure to be easily recognised. They have usually an elongated

Fig. 543. — Brachionus rubens'. sp, styligerous prominences; cw, coronal
wreath.; ts, tactile styles; <7, dorsal antenna; ft', ft', lateral antennae; lm,
longitudinal muscles ; ce, cesophaguw ; oy, ovary ; om, ovum ; g, germ ;
vt, vibratile tags ; intestine ; /, foot ; t, toes ; gn, brain ; e, eye ; mx,
mastax; ti, tropin; gg, gastric glands; 8, stomach; 7c, longitudinal
canals ; cv, contractile vesicle ; cl, cloaca ;fg, foot-gland. (After Dr.; Hudson.)

form, similar on the two sides ; but this rarely exhibits any traces of
segmental division. The body is covered with an envelope of two
layers. The inner of these is a soft lining to the outer, which may
be soft and flexible, or membranous, and of very varying degrees of
stiffness, or even of an inflexible substance capable of resisting the

INTERNAL ANATOMY OF ROTIFERS

715

Fig. 544.-

Malleate.

action of caustic potash. In this latter condition it is called a lorica.
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 any
solid object ; and this is their ordinary position when keeping their
' wheels ' in action for a supply of food or of water ; they have no-
difficulty, however, in letting go their hold and moving through the
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 by the posterior extremity to the
spot on which they have at first fixed themselves, and their cilia are
consequently employed for no other purpose than that of creating
currents in the surrounding water.

In considering the internal struc-
ture of Rotifera we shall take as its
type the arrangement which it pre-
sents in Brachionus rubens (fig. 543),
a common large and handsome animal,
and one that bears the temporary
captivity of a compressorium remark-
ably well.

Its vase- shaped lorica is hard and
transparent ; open in front to allow
the protrusion of the head, and closed
behind, except where a small aper-
ture permits the passage of the foot.
The anterior dorsal edge bears six
sharp spines, and the ventral edge has
a wavy outline. The head is shaped like a truncated cone, with the
larger end forward, is rounded at each side, and carries on its front
surface three protuberances (sp), 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 buccal
funnel. At the bottom of the buccal funnel is the mastax (mx), a
muscular bulb containing the jaws or trophi (ti). These latter are
hard, glassy bodies consisting of two hammer- like pieces called
mallei (fig. 544) and a third anvil-piece called an incus. Each
malleus (rtis) is in two parts — the manubrium (mm), or handle,
and the uncus (us), of five finger-like processes, which unite to
form the hammer's head. The incus (is), or anvil, is formed of two
prism-shaped bodies, or rami (rs), pointed at their free ends, and
attached at their broad ends to a thin plate called the fulcrum (fm),
which, seen ventrally or dorsally, looks like a rod. These various
parts are connected by muscular fibres, and so acted on by muscles
attached to themselves, and to the interior of the mastax, that the
unci rise and fall at the same time that the rami open and shut.
The food is torn by the unci, crushed by the rami, and then passes
between the latter down a short oesophagus (02) 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

•n us, uncus.
ms, malleus \ ' , .

' I mm, manubrium.

is, mens

1 7's, ramus.
\fm, fulcrum.

7 1 6 MICEOSCOPIC FOEMS OF ANIMAL LIFE

the name of an intestine (i). The lower end of the intestine gene-
rally expands into a cloaca (cl), into which open the ducts of the
ovary (oy), and contractile vesicle (cv). Just above the mastax,
and sometimes just below it, on the oesophagus, are what are sup-
posed to be salivary glands ; while attached to the ivpper end of
the stomach are two gastric glands (gg), often possessing visible
duets. There are two further glands (fg) in the foot, which is
itself a prolongation of the ventral portion of the trunk below the
aperture of the cloaca. These foot-glands secrete a viscid substance
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 withdrawing the head and foot
within the lorica can be readily seen, and these parts are driven out
again by the pressure of transverse muscular fibres acting on the
fluids of the body.

Fig. 5-15. Fig. 546.— Male : e, eye ; 1c, longi-

tudinal canals ; vt, vibratile tag ;
cv, contractile vesicle ; ss, sperm-
sac ; p, penis ; /, foot ; fg, foot-
gland.

On either side of the body is a tortuous tube commencing in a
plexus in the head and running down to open on the contractile
vesicle (cv). These tubes bear little tags (vt), each of which appears
to contain a vibrating cilium. The real structure of these bodies is
uncertain, and the use of the whole apparatus is much disputed ;
but the tags are possibly very minutely ciliated funnels, 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

The ovary is large and its germs are conspicuous. The animal is
oviparous and the huge egg is easily discharged through the oviduct
and cloaca owing to the very fluid condition of its contents. It is
retained by a thread till hatched at the bottom of the lorica. There

1 But see Dr. Hudson's Presidential Address, Journ. of the Boyal Micros. 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.

CLASSIFICATION OF EOTIFEES

717

are three kinds of eggs : the common soft-shelled eggs, which are
large, oval, and produce females ; similar soft eggs, which are
smaller, more spherical, and produce males ; and ephippial eggs (fig.
545), with thick cellular coverings, often ornamented with spines.
These latter can be dried completely without losing their vitality,
and so, lying buried in the mud of dried-up ponds, preserve the
species for next year. There is a bilobed nervous ganglion (gn)
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 pass nerve-threads to a dorsal antenna (a) and to two
lateral antennw {a') 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 antenna has a similar bundle and lies
sheathed in a tube (fig. 547) which has its base just above
the nervous ganglion, and passes thence between the two Jjtl/
central anterior spines of the lorica. It is furnished with (l
a muscle, by means of which the bunch of setee at the free
extremity can, by invagination, be drawn within the tube. // P

The male (fig. 546) is but a third of the length of the Fig. 547.
female, and is unlike it in shape. It has a cylindrical
trunk, small foot, and flat, round head, surrounded by a simple ring
of long cilia. It has no lorica nor any alimentary tract of any kind,
but it has a nervous system similar to that of the female, a red eye,
and antenna?. Its excretory and muscular systems are also of the
female pattern. The only other internal organ is a large sperm-sac
(ss) ending at its lower extremity in a protrusile, ciliated, hollow
penis (p), whose outlet holds the position of the anus in the female ;
that is, on the dorsal surface, at the base of the foot.

The Rotifera have been divided by Dr. Hudson and Mr. P . H. Gosse1
into four orders, according to their powers of locomotion. These are :

1. Rhizota (the rooted). Fixed when adult.

2. Bdelloida (the leech-like). That swim with their ciliary
wreath, and creep like a leech.

3. Ploima (the sea-ivorthy). That only swim with their ciliary
wreath.

4. Scirtopoda (the skippers). That swim with their ciliary wreath
and skip with arthropodous limbs.

The order Bhizota contains two families, chiefly differing from
each other in the position of the mouth, which in the Flosculariada^
(figs. 1 and 2, Plate XV) is central, lying in the body's longer axis,
but in the Melicertaclce (fig. 3, Plate XV) 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 gelatinous tube ;
or by large fpecal pellets also regularly deposited.

1 The Botifcra, or Wheel-animalcules. Longmans, 1889.

71 8 MICROSCOPIC FORMS OF ANIMAL LIFE

The second order, Bdelloida (fig. 7, Plate XY), while having many
points in common with the Melicertadce, have a foot peculiarly their
own. It has several false joints that can be drawn one within the
other like those of a telescope. The corona consists of two nearly
circular discs, 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 dustiness.
This is due to their secreting round their bodies (after having drawn in
both head and foot) a gelatinous covering which retains the body-fluids
safe from evaporation.1 This process 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 Potifera 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, Plo'ima, is divided into a loricate and an illoricate
group, which are not, however, very sharply separated ; as in some
cases the outer layer of the skin is, though horny, yet thin and
flexible. Brachionus rubens (fig. 543), which has already been fully
described, is a good type of the loricata, and Gopeus cerberus (fig. 6,
Plate XY) of the illoricata. Most of the species of this order have
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, Scirtopoda, contains but one family, Pedalionidce,
and has only two genera, Pedalion and Hexarthra, and each of these
has but one known species. Pedalion (figs. 4, 5, 8, Plate XY) 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 Arthropoda, and worked by pairs of opposing
muscles which traverse then: entire length. These limbs are ar-
ranged round the body, some on the dorsal, some on the ventral
surface, and all running parallel to the body's longer axis. In
Hexarthra, on the contrary, all the limbs are on the ventral surface,
and are arranged radiatingly. There is no foot in either Potif eron ;
but in Pedalion there are two ciliated club-like processes at the pos-
terior extremity, rising above the dorsal surface and secreting
a similar viscous fluid to that secreted in the toes of other
Rotifera.

1 See Davis in Monthly Micros. Journ. vol. ix. 18G3, p. 207 ; Slack, at p. 241 of
•same volume ; and the report of a discussion on the subject at the Boyal Microsco
laical Society, Journ. of Boyal Micros. Soc. 1887, p. 179.

f L, A T hi AV

W»st Ns-.vmaTi oViromo

Tvicicai Rptifers.

BIBLIOGRAPHY OF ROTIFERS

719

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 Nile in 1853 ; their arthropodous limbs
give them a strong resemblance to a Nauplius larva, and make it
probable that the Rotifera will have to be placed between the Vermes
and Arthropoda.1

1 The following treatises and memoirs) in addition to those already referred to)
contain valuable information in regard to the life-history of animalcules and their
principal forms: — Ehrenberg. Die Infusionsthierchen, Berlin, 1838; Dujardin,
Histoire naturelle des Zoophijtes infusoires, Paris, 1841 ; Pritchard, History of
Infusoria, 4th ed. London, 18G1 fa comprehensive repertory of information) ;
Stein, Der Organismus der Infusionstliiere, Leipzig: Erste Abtheilung, 1859; Zweite
Abtheilung, 1867 ; Dritte Abtheilung, Halfte I. 1878. Saville Kent's Manual of the
Infusoria, 1880-1 ; and Professor Biitschli's Protozoa (1880-1) in the new edition of
Bronn's Thier-reichs. For the Bhizopoda and Infusoria specially see Claparede
and Lachmann, Etudes sur les Infusoires et les Bhizopodes, Geneva, 1858-61;
Cohn, in Siebold und Kolliker's Zeitschrift, 1851-4 and 1857 ; Lieberkiihn, in
Midler's Archiv, 1856, and Ann. of Nat. Hist. 2nd ser. vol. xviii. 1856 ; Engehnann.
Zur Katurgeschichte der Infusionstliiere, 1862; and Professor Biitschli's Studien
vber die Conjugation der Infusorien &c., 1876. For the Botifera specially see
Leydig, in Siebold und EolU'ker's Zeitschrift, Bd. vi. 1854 ; Gosse on Melicerta
ringens, in Quart. Journ. of Micros. Sci. vol. i. 1853, p. 1 ; on the Manducatory
Organs of Botifera, Phil. Trans. 1856; Huxley on Lacinularia socialis in Trans.
Micros. Soc. ser. ii. vol. i. 1853, p. 1 ; Colin, in Siebold unci Kolliher's Zeitschrift,
Bde. vii. ix. 1856, 1858; Dr. Moxon, Trans. Linn. Soc. 1864; Karl Eckstein, Siebold
und Kolliber's Zeitschrift, 1883; Bourne, Botifera, in the 9th edition of the Hncy-
clopcedia Britannica ; Joliet, ' Monographie des Melicertes,' Archiv. zool. exper. ser.
ii. torn. i. p. 131 ; and Plate, Jenaische Zeitschr. xix. p. 1. The Botifera, or Wlieel-
animalcules, by Hudson and Gosse : Longmans, 1889. Mr. Slack's Marvels of Pond
Life, 2nd edit. (London, 1871), contains many interesting observations on the habits of
Infusoria and Botifera.

720

MICROSCOPIC FORMS OF ANIMAL LIFE

CHAPTER XIY

FOB A MINIFEBA AND BADIOLABIA

Returning now to the lowest or rhizopod type of animal life
(Chapter XII), we have to direct our attention to two very remarkable
Series of forms, almost exclusively marine, under which that type
manifests itself, all of them distinguished by skeletons so consolidated
by mineral deposit as to 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 no other protection
than a membranous envelope. In the second group, the Radiolaria,
the skeleton is always silicious and may be either 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 Foraminifera
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
Radiolaria do the same, though in far less measure, for the silex.
And both extract from sea- water the organic matter universally dif-
fused through it, converting it into a form that serves for the nutri-
tion of higher marine animals.

Section I. — Foraminifera.

The animals of this group belong to that reticularian form of
the rhizopod type in which — with a differentiation between the
containing and the contained sarcodic substance which is involved
in the formation of a definite investment — a distinct nucleus
(sometimes single, in other cases multiple) is probably always
present.1 The shells of Foraminifera are, for the most part, poly-

1 The absence of a nucleus was long supposed to be a characteristic of ihe animal
of the Foraminifera', and its presence in Gromia (first detected by Dr Wallich)

FORA MIXIFERA

72 I

thalamous, or many-chambered (Plates XVI and XVII), 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 cephalopoda 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 each
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 ' poly
thalamous' forms, therefore, and the monothal anions or single-
chambered, of which we have already had an example in Gromia,
and of which others will be presently described, is simply that,
whereas any buds produced by the latter detach themselves to form
separate individuals, those put forth by the former remain in con-
tinuity with the parent stock and with each other, so as to form a
1 composite ' animal and a ' poly thalamous ' shell.

According to the plan on which the gemmation takes place will
be the configuration of the shelly structure produced by the seg-
mented body. Thus, if the bud should be put forth from the
aperture of a Lagena (Plate XVII, fig. 12) in the direction of the
axis of its body, and a second shell should be formed around this
bud in continuity with the first ; and this process should be succes-
sionally repeated, a straight rod-like shell would be produced, whose
multiple chambers communicate with each other by the openings that
originally constituted their mouths, the mouth of the last-formed
chamber being the only aperture through which the protoplasmic
body, thus composed of a number of segments connected by a
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 size, or nearly so, in which case the entire rod will approach
the cylindrical form, or will resemble a line of beads ; but it often

was regarded as differentiating that type from the Foramiuifera proper. But the
researches of Hertwig and Lesser, F. E. Schulze, Biitselili. and others, having esta-
blished its presence in several true Foraminifera. and the Author's own observations
having confirmed these, its general presence may be fairly assumed.

722

MICROSCOPIC FORMS OF ANIMAL LIFE

happens that each segment is somewhat larger than the preceding
(fig. J6), so that the composite shell has a conical form, the apex of
the cone being the original segment, and its base the one last formed.
The method of growth now described is common to a large number
of Foraminifera, chiefly belonging to the sub-family JSfodo.sarinm ;
but even in that group we have every gradation between the recti-
lineal (fig. ig) and the spiral mode of growth (fig. 2-2) ; 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. 548 ; but more commonly there is-
so little difference between the successive segments, after the spire
has made two or three turns, that the breadth of each whorl scarcely
exceeds that of its predecessor, as is well seen in the section of the.

Fig. 548. — Foraminifera: — Penerojplis and Orhiculina.

Botalia represented in tig. 566. An intermediate condition is pre-
sented by Botalia, 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 form 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 form 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 XVII, fig. 23) and Nonionina ;
whilst of the latter we find a typical representation in Botalia
Beccarii (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

Plate XVI

A TYPICAL GROUP OF FOBAMINIFERA 1 to 11

FORAMINIFERA

frequently happens that the last-formed whorl encloses the preced-
ing to such an extent that they are scarcely, or not at all, visible
externally, as is the case in Cristell 'aria (tig. 17), Polystomella (fig. -23),
and Nonionina. The turbinoid spire may coil so rapidly round an
elongated axis that the number of chambers in each turn is very
small; thus in Globigerina (figs. 20, 2i, Plate XVII) there are
usually only four : and in Valvulina the regular number is only
three. Thus we are led to the biserial arrangement of the chambers,
which is characteristic of the textularian group (fig. 8, a, b, and 9,
Plate XVI), 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

Fig. 549. — Discorbina globularis (Bosalina varzans, Schultze),
with its pseudopodia extended.

chamber of its own side, as will be understood by reference to fig.
564, A, which shows a ' cast ' of the sarcode- body of the animal. On
the other hand, we tind in the nautiloid spire a tendency to pass
(by a curious transitional form to be presently described) into the
cyclical mode of growth ; in which the original segment, instead of
budding forth on one side only, develops gemma- all round, so that
a ring of small chambers (or chaniberlets) is formed around the
primordial chamber, and this in its turn surrounds itself after the
like fashion with another ring ; and by successive repetitions of the
same process the shell comes to have the form of a disc made up of
a great number of concentric rings, as we see in Orbitolites (fig. 551)
and in Cycloclypeus (fig. 569).

3 a 2

724

MICKOSCOPIC FORMS OF ANIMAL LIFE

These and other differences in the plan of growth were made by
M. D'Orbigny the foundation of his classification of this group,
which, though at one time generally accepted, has now been aban-
doned by most of those who have occupied themselves in the study
of the 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 be noticed.1

Two very distinct types of shell structure prevail among ordinary
Foraminifera — namely, the porcellanous and the hyaline or vitreous.
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 extremely
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 Miliola, but which is an
almost constant characteristic of Peneroplis (fig. 548). But no
difference of texture accompanies either this or any other kind of
inequality of surface, the raised and depressed portions being alike
homogeneous. In the shells of the vitreous or hyaline type, on the
oilier 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 Globigerinctfrubrdj Truncatulina rosea, and Polytrema
miniaceuni, 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 perforations, 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
' pimctations ' on the surface of the shell, as is shown in fig. 549 ;
whilst in other cases they are so minute as only to be discernible in

1 This subject will In; found amply discussed in the Author's Introduction to the
Study of-the Fdra/wiinifera, published by the Ray Society, to which work he would
refer such of his readers as may desire more detailed information in regard to it.

Plate. XVII

A TYPICAL GEOUP OF FORAMINIFERA 12 to 26

¥ 0 R A MINI FE R A

725

thin sections seen by transmitted light under a higher magnifying
power, as is shown in figs. 574, 575. When they are very numerous
and closely set, the shell derives from their presence that kind of
opacity which 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 substance,
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 Xummulites (fig. 573). 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 pseuclopodial extensions
of the sarcode-body through every part of the external wall of the
chambers occupied by it (fig. 549) ; 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 nummulines 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 tubulation or non-
tubulation of foraminiferal shells is the key to a very important
physiological difference between the animal inhabitants of the two
kinds respectively ; for whilst every segment of the sarcode-body in
the former case gives off 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 be 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

726

MICROSCOPIC FORMS OF ANIMAL LIFE

very important difference in the conformation of the shell — viz.
that whilst the aperture of communication between the chambers
and between the last chamber and the exterior is usually very small
in the 1 vitreous ' shells, serving merely to give passage to a slender
stolon or thread of sarcode from which the succeeding segment may
be budded off, it is much wider in the ' porcellanous ' shells, so as to
give passage to a ' stolon ' that may not only bud off new segments,
but may serve as the medium for transmitting nutrient material
from the outer to the inner chambers.

Between the highest types of the porcellanous and the vitreous
series respectively, which frequently bear a close resemblance to
each other in form, there are certain other well-marked differences
in structure, which clearly indicate their essential dissimilarity.
Thus, for example, if we compare Orbitolites (tig. 551) with Cyclo-
clypeus we recognise the same plan of growth in each, the chamber-
lets being arranged in concentric rings around the primordial
chamber ; and to a superficial observer there would appear little
difference between them. But a minuter examination shows that
not only is the 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, between 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 lamina? 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, however, is seen in
Polystomella and in Operculina ; and an account of it will be given
in the description of those types.

Porcellanea. — Commencing, now, with the porcellanous series,
we shall briefly notice some of its most important forms, which are
.so related to each other as to constitute but the one family Miliolida.
Its simplest type is presented by the Cornuspira of our own coasts,
found attached to seaweeds and zoophytes ; this is a minute spiral
shell, of which the interior forms a continuous tube not divided into
chambers ; the latter portion of the spire is often very much flattened
out, as in Peneroplis, so that the form of the mouth is changed from
a circle to a long narrow slit. Among the commonest of the Fora-
minifera, and abounding near the shores of almost every sea, are
some forms of the milioline 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

PORCELLANOUS FORAMENIEERA

727

are characterised will be best understood by examining, in the first
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. Now it is a very common habit in the milioline
type for the chambers of the later convolutions to extend themselves
over those of the earlier, so as to conceal them more or less com-
pletely; and this they very commonly do somewhat unequally, so that
more of the earlier chambers are visible on one side than on the
other. Mil iolce thus modified (fig. 1, PI. XVI) have received the names
of QuinquelocuJina 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 Bilocirfina, there is often such an
increase in the breadth of the chambers as altogether changes the
usual proportions of the shell, which has almost the shape of an egg
when so placed that either the last or the penultimate chamber faces
the observer. It is very common in milioline shells for the external
surface to present a 'pitting,' more or less deep, a ridge-and-furrow
arrangement (fig. 3), or a honeycomb division ; and these diversities
have been used for the characterisation of species. Xot 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 1 in the structure of the shells of this
group. They find that while two forms, which they distinguish as
form A and form B, are similar externally they differ in internal
structure, form B having its initial chamber much smaller than
that of form A, and this 4 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. The ob-
servations of the French naturalists referred to open out a new field
of inquiry, and one which is enjoying the attention of several works
in this department of research.

Reverting again to the primitive type presented in the simple
1 Bulletin Soc. Geol. ser. iii. vol. xiii. p. 273.

728

MICROSCOPIC FORMS OF ANIMAL LIFE

spiral of Cornuspira, we find the most complete development of:
it in Peneroplis (tig. 548), a very beautiful form, which, although,
not to be found on our own coasts, is one of the commonest of all
Forarninifera in the shore-sands and shallow- water dredgings of
warmer regions. This is normally a nautiloid shell, of which the
spire flattens itself out as it advances in growth. It is marked
externally by a series of transverse bands, which indicate the posi-
tion of the internal septa that divide the cavity into chambers ; and
these chambers communicate with each other by numerous minute
pores traversing each of the septa, and giving passage to threads
of sarcode that connect the segments of the body. At a is shown
the ' septal plane ' closing in the last-formed chamber, with its single
row of pores 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 closely
set strice that cross the spaces between the successive septal bands ;
these markings, however, do not indicate internal divisions, and are
due to a 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
Nautilus, 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 arrow-shaped septal plane of
Dendritina, with all its pores fused together (so to speak) into one-
dendritic aperture, and a like gradation being presented between
the ordinary forms and the ' spiroline ' varieties, with oval or even
circular septal plane, into which both Peneroplis and Dendritina
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 Orbiculina (fig. 548).
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 Peneroplis in its general form,
but that its principal chambers are divided by ' secondary septa '
passing at right angles to the primary into ' chamberlets ' occupied
by sub-segments of the sarcode-body. Each of these secondary
septa is perforated by an aperture, so that a continuous gallery is
formed, through which (as in fig. 551) 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
any row normally communicates with two chamberlets in each of the

PORCELLANOUS FORAMINIFEKA

adjacent 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 rise to the peculiar shape represented
in fig. 548, in the illustration on the extreme right, which is the
common aduncal type of this organism. But sometimes even at an
early age the growing margin extends so far round on each side
that 'its two extremities meet on the opposite side of the original
spire, which is thus completely enclosed by it ; and its subsequent
growth is no longer spiral but cyclical, a succession of concentric
rings being added, one around the other, as shown in the middle
illustration in the same figure. This change is extremely curious, as
demonstrating the intimate relationship between the spiral and the
cyclical plans of growth, which at first sight appear essentially distinct.
In all but the youngest examples of Orbiculina the septal plane pre-
sents more than a single row of pores, the number of rows increasing
in the thickest specimens to six or eight. This increase is associated
with a change in the form of the sub-segments of sarcode from little
blocks to columns, and with a greater complexity in the general
arrangement, such as will be more fully described hereafter in
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. 550) 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 with
each other, as in the preceding type, by a continuous gallery ; and
they communicate with those of the next row by a series of multiple
pores in the principal septa, such as constitute the external orifices of
the last-formed series, seen on its septal plane at «, a.

The highest development of that 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 basin, has lately proved to be
scarcely less abundant in certain parts of the existing ocean. The
largest recent specimens of it, sometimes attaining the size of a
shilling, have hitherto been obtained only from the coast of New
Holland, the Fijian reefs, and various other parts of the Polynesian
Archipelago ; but discs of comparatively minute size and simpler
organisation are to be found in almost all foraminiferal sands and
dredgings from the shores of the warmer regions of the globe, being
especially abundant in those of some of the Philippine Islands, of the

73Q

MICROSCOPIC FORMS OF ANIMAL ( LIFE

Red 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 the structure represented in
hg. 551, where we see on the surface (by incident light) a number
-of rounded elevations, arranged in concentric zones around a sort of

nucleus (which has been laid open in the figure to show its internal
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

OKBITOLITES

73t

light through them ; but in those which are too opaque to be thus
seen through, it is sufficient to rub clown 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
ring ; so that each circular zone of chamberlets might be described
as a continuous annular passage dilated into cavities at intervals.
On the other hand, each zone communicates with the zones that are
internal and external to it by means of passages in a radiating
direction ; these passages run, however, not from the chamberlets of
the inner zone to those of the outer, but from the connecting pass-
ages 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. 551. — Orbitolites. Ideal representation of a disc of complex type.

internal and external to it. The radial passages from the outermost
annulus make their way at once to the margin, where they termi-
nate, forming the ' pores ' which (as already mentioned) are to be seen
on its exterior. The central nucleus, when rendered sufficiently
transparent by the means just adverted to, is found to consist of a
' primordial chamber ' («), usually somewhat pear-shaped, that com-
municates by a narrow passage with a much larger ' circumambient
chamber' (6), which nearly surrounds it, and which sends off a vari-
able number of radiating passages towards the chamberlets of the first
zone, which forms a complete ring round the circumambient chamber. 1

1 Although the above may be considered the typical form of the Orbitolite, yet,
in a very large proportion of specimens, the first few zones are not complete circles,
the early growth having taken place from one side only ; and there is a very beautiful
variety in which this one-sidedness of increase imparts a distinctly spiral chai*acter
to the early growth, which soon, however, gives place to the cyclical. In the Orbito-
lites italica (fig. 553), brought up from depths of 1500 fathoms or more, the 'nucleus '

732

MICROSCOPIC FORMS OF ANIMAL LIFE

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. 552) 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 4 primordial ' pear- shaped segment, «, 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, has budded off three stolons, which

swell into new sub-
segments from which
the first ring is formed.
Scarcely any two speci-
mens are precisely alike
as to the mode in
which the first ring-
originates from the
' circumambient sesr-
ment ' ; for sometimes
a score or more of
radial passages extend
themselves from every
pari: of the margin of

the latter (and this, as
corresponding with the
plan of growth after-
wards followed, is
probably the typical
arrangement) • whilst
in other cases (as in
the example before us)
the number of these
primary offsets is ex-
tremely small. Each zone is seen to consist of an assemblage of
ovate sub-segments, whose height (which could not be shown in the
figure) corresponds with the thickness of the disc ; these sub-seg-
ments, 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 be-
tween the sub-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 Cornusjnra.
with an interruption at every half-turn, as in Spiroloculina, the growth after-
wards becoming purely concentric.

Fig. 552. — Composite animal of simple type of Orbito-
lites complanata : — a, central mass of sarcode ;
b, circumambient segment, giving off peduncles, in
which originate the concentric zones of sub-segments
connected bv annular bands.

ORBITOLITES

733

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
accident hac repeatedly furnished proof — that, a small portion of a
disc, entirely separated from the remainder, will not only continue

Fig. 553. — Disc of OrbitoJites italica, Costa, sp. (O.tenuissima, Carp.),
formed round fragment of previous disc.

to live, but will so increase as to form a new disc (fig. 553), the want
of the 1 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. In all the larger specimens of OrbitoJites we observe that
the marginal pores, instead of constituting but a single row, form
many rows one above another ; and, besides this, the chamberlets
of 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 OrbiruJina. When
a vertical section is made through such a disc, it is found that these
oblong chambers constitute two superficial layers, between which

734

MICROSCOPIC FORMS OF ANIMAL LIFE

are interposed columnar chambers of a rounded form ; and these-
last are connected together by a complex series of passages, the
arrangement of which will be best understood from the examination
of a part of the sarcode-body that occupies them (fig. 554). 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 annulus, but
each of them disconnected from its neighbours, and communicating
only by a double footstalk with the two annular ' stolons,' a a', 6 6',
which obviously correspond with the single stolon of 'simple' types
(fig. 552). 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 sarcocle 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
pores, would

ranges

of

doubtless

marginal

act as-

-Portion of animal of complex

Fig. 554.

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
d d, the lower layer, connected with the
annular bands of both zones ; e e and
e e', vertical sub-segments of the two
zones.

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 hirye 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, send-

ARENA CEO US F OK A MINI FE RA

ing 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 jjresented by the sarcode-body, it seems reasonable to
infer that gemmnles, 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.

Areiiacea. — In certain forms of the preceding family, and espe-
cially in the genus MUiola, wTe not unfrequently find the shells en-
crusted with particles of sand, wmich are imbedded in the proper
shell-substance. This incrustation, however, must be looked on as
(so to speak) accidental, since we find shells that are in every
other respect of the same type altogether free from it. A similar
accidental incrustation presents itself among certain ' vitreous ' and
perforate shells ; but there, too, it is usually on a basis of true shell,
and the sandy incrustation is often entirely absent. There is, how-
ever, a group of Foraminifera in which the true shell is constantly
and entirely 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 1 vitreous r
series ; whilst yet they graduate into one another in such a manner
as to indicate that all the members of this ' arenaceous ' group are
closelv related to each other, so as to form a series of their own.
And it is further remarkable that while the deep-sea dredgings
recently carried down to depths of from 1,000 to 2,500 fathoms
have brought up few forms of either ' porcellanous ' or ' vitreous *
Foraminifera that were not previously known, they have added
greatly to our knowledge of the ' arenaceous' types, the number and
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
of the Foraminifera into the arenaceous and calcareous groups does

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 *
Report (p. 224;) describes a remarkable allied type from the Southern Ocean —
Keramosphcera 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.9. Challenger
£1864), illustrated by 116 plates.

736

MICROSCOPIC FORMS OF ANIMAL LIFE

not correspond to any natural arrangement ; for, although the rule
is tolerably constant in many groups, there are others, notably
certain sub-families of Textulariidce, 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 that of a large pea, formed
of an aggregation of sand-grains, minute foraminifers, <fcc, held
together by a tenacious protoplasmic substance. On tearing these
open the whole interior was found to have the same composition,
-and no trace of any structural arrangement could be discovered in
their mass. Hence they might be supposed to be mere accidental
agglomerations were it not for their conformity to the 'monerozoic'
type previously described ; for, just as a simple 'moner,' by a differen-
tiation of its homogeneous sarcode, becomes an Amoeba, so would
one of these uniform blendings of sand and sarcode by a separation
of its two components — the sand forming the investing ' test ' and
the sarcode occupying its interior — become the arenaceous Astro-
rhiza. This type, which abounds on the sea-bed in certain localities,
presents remarkable variations of form, being sometimes globular,
sometimes stellate, sometimes cervicorn. But the same general
arrangement prevails throughout, the cavity being occupied by a
dark-green sarcode, while the 'test' is composed of loosely aggregated
sand-grains not held together by any recognisable cement, and has
no definite orifice, so that the pseudopodia must issue from inter-
stices between the sand-grains, which spaces are probably occupied
during life with living protoplasm that continues to hold together
the sand-grains after death. These are by no means microscopic
forms, the ' stellate ' varieties ranging to 0*3 or even 04 inch in
diameter, and the 'cervicorn' to nearly 05 inch in length.1 A much
larger form was found by Mr. Brady among the dredgings lately
made it the Faroe Channel (see his ' " Challenger " Report,' p. 242) ;
Syrinrjammina 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.

More recently another large and interesting type belonging to
the same group has been obtained by Mr. Wood-Mason, of the
Indian Museum, from the Bay of Bengal.- This has received the
generic name Masonella. The test consists of a thin sandy disc,
nearly half an inch in diameter, either flat or saucer-shape, with
a central chamber and simple or branched radiating tubuli open
at the periphery.

The purely arenaceous Foraminifera are ranged by Mr. H. B.
Brady3 (by whom they have been specially studied) under two

1 See the description and figures of this type given by the Author in Quart.
Journ. Micros. Sci. vol. xvi. 1870, p. 221.

2 Ann. and Mag. Nat. Hist. 1889, ser. vi. vol. iii. p. 293, woodcuts.

5 See his ' Notes' in Quart. Journ. of Micros. Soc. n.s. vol. xix. 1879, p. 20, and
vol. xxi. 1881, p. 31.

saccamaiixa and pilulixa

737

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. 555, a, b,
c). This type, which occurs in extraordinary abundance in certain
iocalities (as the entrance of the Christiania fjord, and still further

Fig. 555. — Arenaceous Foraminifera : a, Saccammina spherica; b, the same
laid open ; c, portion of the test, enlarged to show its component sand-
grains ; d, Pilulina Jeffreysii; e, portion of the test enlarged, showing
the arrangement of the sponge-spicules.

north on the shores of Franz Josef Land), is of peculiar interest
from the fact that a closely allied species (Saccammina Carteri) is,
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 Pih/Iina, from
its resemblance to a homoeopathic 'globule' (fig. 555, 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-
-stance of which very fine sand-grains are dispersed. This ' felt ' is

3 B

738

MICROSCOPIC FORMS OF ANIMAL LIFE

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 tenacity, therefore, seems entirely due to the
wonderful manner in which the separate silicious fibres are 'laid.5
It is not a little curious that these two forms should present them-
selves in the same dredging, and that there should be no perceptible
difference in the character of their sarcode bodies, which, as in the
preceding case, have a dark -green hue. The Marsipella elongata
(fig. 556, d), on the other hand, is somewhat fusiform in shape, and
has its two extremities elongated into tubes, with a circular orifice
at the end of each. The materials of the ' tests ' differ remarkably
according to the nature of the bottom whereon they live. When

Pig. 556. — Arenaceous Foraminif era : a, b, upper and lower aspects of Hap lo-
•phragm/ium globigeriniforrne\ c, Hormosina globulifera; d, Marsipella
elongata', e, terminal portion, and /', middle portion of the same, enlarged ;
<;, Thurammtna 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 sand-grains ; but when
they are brought up from a bottom on which sand predominates,
the larger part of the 4 test ' is made up of sand-grains and minute
Foraminifera, with here and there a sponge-spicule (fig. 556, 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

LIT U OLID A

739

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. 5-37, 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 right 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 dredgings in which the triradiate forms are rare,
seems to preclude the idea that these long single rods are broken
rays of the latter. It is undoubtedly in this group that we are to
place the genus Halvphysema^ 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

Lituclida. — 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 Lituoline 'tests?
often simulate in a very curious way those of the simpler types of
the vitreous series. Thus, the long spirally coiled undivided sandy-
tube of Ammodiseits is the isomorph of SpiriUina. In the genu£
Haplopliracjinivm (tig. 556, a, b, and Plate XV, tig. fi) we have singular
imitations of the Globigerine, Kotaline, and Nonionine types ; and in

1 See Saville Kent in Anil, of Nat. Hist. ser. v. vol. ii. 1S78 ; Professor Ray Ban-
kester in Quart. Juurn. Micros. Sci. vol. xix. 1878, p. 476; and Professor Mubius'
Foraminifcra von Mauritius, 1S80. . .

3 r> 2

740

MICROSCOPIC FOEMS OF ANIMAL LIFE

Thurammina papillata (fig. 556, 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. 556, h),
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 globulifera (fig. 556, c), which is composed
of a succession of globular chambers rapidly increasing in size, each
havino; a narrow tubular neck with a rounded orifice, which is
received into the next segment. In other species of the same genus
there is a nearer approach to the ordinary Nodosarine type, their
tests being sometimes constructed with the regularity characteristic
of the shells of the true Nodosaria, Plate XVII, 16, whilst in other

Fig. 557. — Arenaceous Foraminfera : a, b, exterior and sectional views of
Mheophax sabulosa ; c, Mhabd&mmina abyssorum ; d, cross- section of one
of its arms ; e, llheophax scorpiurus ; /, Hormosina Carpenteri.

cases the chambers are less regularly disposed (fig. 557,/), 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 Nodosarine forms
of the genus JRheophax, in which not only are the sand-grains of the
test very coarse, but small Foraminifera are often worked up with
them (fig. 557, e). A straight, many-chambered form of the same
genus (fig. 557, a, b) is remarkable for the peculiar finish of the neck
of each segment ; for whilst the test generally is composed of sand-
grains, as loosely aggregated as those of which the test of Astrorhiza
is made up, the 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 nautiloid

CYCLAMMINA

741

shells, both of the 1 porcellanous ' and the £ vitreous ' series ; and the
most remarkable of these is the Cyclammina cancellata (fig. 558),
which has been brought up in considerable abundance from depths
ranging downwards to 1900 fathoms, the largest examples being
found within 700 fathoms. The test (fig. 558, 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
between these chambers being left by a fissure at the inner margin
of the spire, as in Operculina (fig. 570). One of the most curious
features in the structure of this type is the extension of the cavity
of each chamber into passages excavated in its thick external wall

Fig. 558. — Cyclammina cancellata, 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 unfrequently happens that the
outer layer of the test is worn away, and the ends of the passages
then show themselves as pores upon its surface ; this appearance,
however, is abnormal, the passages simply running from the chamber-
cavity into the thickness of its wall, and having (so long as this is
complete) no external opening. This ' labyrinthic ' structure is of
great interest, from its relation not only to the similar structure
of the large fossil examples of the same type, but also to that which
is presented in other gigantic fossil arenaceous forms to be presently
described.

Although some of the nautiloid Lituolcv are among the largest
of existing Foraminifera, having a diameter of 0*3 inch, they are
mere dwarfs in comparison with two gigantic fossil forms, whose

742

MICBOSCOPIC FORMS OF ANIMAL LIFE

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-

V V

Fig. 559. — General view of the internal structure of Parkeria : In the hori-
zontal section, l\ l'2, P, ?4 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\ r'!, c3, 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. W. K. Parker. A section of the sphere taken through its
centre (fig. 559) 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 Parkeria and Loftusia ' in Philosophical Transac-
tions, 1809, 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 Hijdroida (see the memoir by Professor Alleyne Nicholson, published
in 1886 by the Palajontographical Society).

PARKER I A

743

Fig. 560. — Portion of one of the lamellae
of Parkeria, showing the sand-grains of
which it is built up, and the passages
extending into its substance.

•chamberlets, 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 chamberlet is itself penetrated by extensions
•of the cavity into its substance, as in the Cyclammina last described ;
and these passages are separated by partitions very regularly built
up of sand-grains, which also close in their extremities, as is shown
in fig. 560. The concentric spheres are occasionally separated by
walls of more than ordinary thickness, and such a wall is seen in
fig. 559 to close in the last-formed series of chamberlets. But these
walls 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-
nals. The ' nucleus ' is always
composed of a single series of
chambers arranged end to end,
sometimes in a straight line, as
in hg. 559, c1, c2, c:i, c4, sometimes
forming a spiral, and in one in-
stance returning upon itself.
But the outermost chamber en-
larges, and extends itself over the
whole ' nucleus,' very much as the
' circumambient ' chamber of the
Orbitolite 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 concen trie-
spheres are successivel}7 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
Alveolina (fig. 550), and attaining a length of three inches, has been
described by Mr. H. B. Brady {Joe. cit.) under the name Loftusia, after
its discoverer, the late Mr. W. K. Loftus, who brought it from the
Turko-Persian frontier, where specimens were found in considerable
numbers imbedded in ' a blue marly limestone,' probably of early
Tertiary age.

There is nothing, it seems to the Author, more wonderful in
Nature than the building up of these elaborate and symmetrical
structures by mere ' jelly-specks,' presenting no trace whatever of
that definite c organisation ' which we are accustomed to regard as
necessary to the manifestations of conscious life. Suppose a human
mason to be put down by the side of a pile of stones of various shapes
and sizes, and to be told to build a dome of these, smooth on both
■surfaces, without using more than the least possible quantity of a
very tenacious but very costly cement in holding the stones together.
If he accomplished this well, he would receive credit for great in-
telligence and skill. Yet this is exactly what the se little ' jelly-specks '

744

MICROSCOPIC FORMS OF ANIMAL LIFE

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 sandy bottom one species picks up the coarser 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 finer
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 minutest sand-grains and the terminal portions of sponge- -
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 the
next, while forming the walls of its chambers of ordinary sand-grains
rather loosely held together, shapes the conical mouths of the suc-
cessive chambers by firmly cementing to each other the quartz-grains
which border it. To give these actions the vague designation ' in-
stinctive ' does not in the least help us to account for them ; since
what we want is to discover the mechanism by which they are worked
out ; and it is most difficult to conceive how so artificial a selection,
can be made by creatures so simple.

Vitrea. — Returning now to the Foraminifera which form true-
shells by the calcification of the superficial layer of their sarcode-
bodies, we shall take a similar general survey of the vitreous series,
whose shells are perforated by multitudes of minute foramina (fig.
549). Thus, Spirillina has 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 Lagenida, which also contains a series of transition forms,
leading up gradationally to the ' polythalamous ' nautiloid type. In
Lagena (Plate XVII, figs. 12, 13, 14, 15) the mouth is narrowed andi
prolonged into a tubular neck, giving to the shell the form of a micro-
scopic flask : this neck terminates in an everted lip, which is marked!
with radiating furrows. A mouth of this kind is a distinctive
character of a large group of many-chambered shells, of which each
single chamber bears a more or less close resemblance to the simple
Lagena, and of which, like it, the external surface generally presents ;
some kind of ornamentation, which may have the form either of
longitudinal ribs or of pointed tubercles. Thus the shell of JVodo--
saria (Plate XVII, fig. 16) is obviously made up of a succession,
of lageniform chambers, the neck of each being received into the
cavity of that which succeeds it ; whilst in Cristellaria (fig. 17)
we have a similar succession of chambers, presenting the characteristic
radiate aperture, and often longitudinally ribbed, disposed in a
nautiloid spiral. Between Nodosaria and Cristellaria, moreover,
there is such a graclational 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,,.

GLOBIGEEIXIDA

745

( >£ which many are to be met with on our own shores, but which are
more abundant on those of the Mediterranean and especially of the
Adriatic, can be traced backwards in geological time at least as far
as the 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 t]ie mode in which they are set one upon another, vary in such
a manner as to give rise to very marked differences in the general
configuration of the shell, which are indicated by the name it bears.

Globigerinida. — Returning once again to the simple ' monothala-
mous ' condition, we have in 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 Orbulina is really
a detached generative segment of Globigerina, with which it i&
generally found associated. The shell of Globigerina consists of an
assemblage of nearly spherical chambers (fig. 561), having coarsely

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, may not communicate directly with each
other, but open separately into a common ' vestibule ' which occupies,
the centre of the under side of the spire. This type has recently
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 1,260 to
2,000 fathoms in the middle of the northern parts of the Atlantic
Ocean. The younger shells, consisting of from eight to twelve
chambers, are thin and smooth, but the older shells are thicker,
their surface is raised into ridges that form a hexagonal areolation
round the pores (fig. 562) ; and this thickening is shown by examina-
tion of thin sections of the shell to be produced by an exogenous
deposit around the original chamber- 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 Dr. Wallich. But the sweeping of the upper waters

Fig. 561. — Globigerina bulloides as seen in three positions.

746

MICROSCOPIC FORMS OF ANIMAL LIFE

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 Glohigerince, 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 live
times the diameter of the shell (fig. 563). 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

d

562. — Globigerina conglobaia (Brady) : a,b, c, bottom specimens;

(I, section of shell.

itself on each of the spines, creeping up one side to its extremity,
and passing down the other with the peculiar flowing movement
already described. The whole of this sarcodic extension is at once
retracted if the cell which holds the (xlobi^erina receives a sudden
shock, or a drop of any irritating fluid is added to the water* it con-
tains. It is maintained by Sir Wyville Thomson that the bottom -
deposit is formed by the continual ' raining down ' of the Globigerina1
of the upper waters, which (he affirms) only live at or near the sur-
face, and which, when they die, lose their spines and subside. The

GL0BIGEE1NIDA

747

Author, however, from his own examination of the Globigerina ooze,
is of opinion that the shell forming its surface-layer must live on the
hottom, being incapable of floating in consequence of their weight ;
und that if they have passed the earlier part of their lives in the
upper waters they drop clown 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 form in 2f 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 the
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 Carpen-
teria has been given. This
may be regarded as a highly
developed form of Globi-
gerina, its first-formed por-
tion having all the essential
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 ch ambers — is here re-
tained with a curious modification ; for the central vestibule 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
Balanus (acorn-shell), for which this type was at first mistaken. The
principal chambers are partly divided into chamberlets by incomplete
partitions, as we shall find them to be in Eozoon. The presence of
sponge-spicules in large quantity in the chambers of many of the
best preserved examples of this type was for some time a source of
perplexity ; but this was explained by the late Professor Max
Schultze,1 who showed how the pseudopodia of this rhizopod have
the habit like those of Haliphysema of taking into themselves sponge-
spicules, which they draw into the chambers, so that they become
incorporated with the sarcode-body. It should be added that Pro-
1 Archiv f. Nahirgesch. xxix. 18G8, p. 81.

Fio. 563. — Globigerina, as captured by tow-net,
. floating at or near surface.

748

MICROSCOPIC FORMS OF ANIMAL LIFE

fessor Schultze, with whom Mr. H. J. Carter,1 Mr. H. B. Brady,2 and
Dr. Goes 3 are in agreement, regard Carpenteria as allied to Polytrema..
Some interesting observations have been made by Professor Mobius 4
on a large branching and spreading form of Carpenteria which he.
recently met with on a reef near Mauritius, and to which he has given
the name of C. rhapliidodendron.

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 TextuJarian and the Rotalian. For,
notwithstanding the marked difference in their respective plans of
growth, the characters of the individual chambers are the same,
their walls being coarsely porous, and their apertures being oval,,
semi-oval, or crescent-shaped, sometimes merely fissured. In Textu-
laria (Plate XYI, 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. 564. — Internal silicious casts representing the forms of the segments of
the animals of, A, Text id aria; B, Botalia.

t internal casts ' (fig. 564, A) as exhibit the forms and connections of
the segments of sarcode by which the chambers were occupied during
life. In the genus Bulimina 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 Textularinue is entirely replaced by a sandy test, that
some systematists prefer to range this group among the Arenacea.

In the Rotalian series the chambers are disposed in a turbinoid
spire, opening one into another by an aperture situated on the lower

1 Annals and Mag. Nat. Hist. ser. iv. vols. xvii. xix. xx.

2 ' Challenger ' Iieport.

5 K. Svenska Vet. Handlingar, xix. No. 4, p. 94.

4 See his Foraminifera von Mauritius, 1880, plates v. vi.

ROTA LI A

749

and inner side of the spire, as shown in Plate XVII, fig. 22, the forms
and connections of the segments of their sarcode-bodies being shown
in such ' internal casts ' as are represented in fig. 564, 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 Texlularia) by large
apertures, whilst they communicate
with those directly above and below
by the ordinary pores of the shell.
The simple or smooth varieties of this
genus forming the sub-genus Gypsina
present great diversities of shape, with
great constancy in their internal struc- Fig. 565 .— Tinoporus baculatus.
ture, being sometimes spherical, some-
times resembling a minute sugar-loaf, and sometimes being irregu-
larly flattened out. The typical form (fig. 565), 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 coast and among the
Polynesian islands. To the simple form of this genus we are
probably to refer many of the fossils of the Cretaceous and
early Tertiary period that have been described under the name
Orbitolina, some of which attain a very large size. Globular Orbi-
tolince, which appear to have been artificially perforated and strung
as beads, are not unfrequently found associated with the ' flint-imple-
ments ' of gravel-beds. Another very curious modification of the
Hotaline type is presented by Polytrema, which so much resembles
a zoophyte as to have been taken for a minute millepore, but which
is made up of an aggregation of ' Globigerine ' chambers communi-
cating with each other like those of Tinoporus, and differs from that
genus primarily in its erect and usually branching manner of growth
and the freer communication between its chambers. This, again, is
of special interest in relation to Eozoon, showing that an indefinite
zoophytic mode of growth is perfectly compatible with truly fora-
miniferal structure.

In Rotalia, properly so called, we find a marked advance towards
the highest type of forarainiferal structure, the partitions that

MICROSCOPIC FORMS OF ANIMAL LIFE

divide the chambers being in the best developed examples composed

of two laminae, and spaces being left between them which give-

solid

ages with

passage to a system of canals whose general distribution is shown
in fig. 566. 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 Calcarina, which

has been so designated
from its resemblance to a
spur- rowel (fig. 571). The
club-shaped append-
which this shell
provided entirely be-

1S

long to the 'intermediate
skeleton ' b, which is quite
independent of the cham-
bered structure a ; and this
is nourished by a set of
canals containing prolonga-
tions of the sarcode-body
which not only furrow 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 foraminifer does the
' canal system ' attain a like
development ; and its dis-
tribution in this minute

Fig. 566. — Section of Hot alia SchroeteHana^eax
its base and parallel to it, showing, a, a, the
radiating intersteptal canals ; b, their internal
"bifurcations : c, a transverse branch; d, tubulated
wall of the chambers.

shell, which has been made out by careful microscopic study, affords
a valuable clue to its meaning in the gigantic fossil organism
Eozoon canadense. The resemblance which Calcarina bears to the
radiate forms of Tinoporus (fig. 565), which are often found with
them in the same dredgings, is frequently extremely striking ; and
in their early growth the two can scarcely be distinguished, since
both commence in a 'Rotaline' spire with radiating appendages;
but whilst the successive chambers of Calcarina continue to be
added on the same plane, those of Tinoporus are heaped up in less
regular 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 Foraminfera, to which
the name Fiisnliiia (indicative of its fusiform or spindle-like shape)
has been given (tig. 567). In general aspect and plan of growth it
so much resembles Afveolina 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

FUSUUNA

751

elongated extensions, which correspond to the ' alar prolongations r
of other spirally growing Foraminifera, but which, instead of wrapping
round the preceding whorls, are prolonged in the direction of the
axis of the spire, those of each whorl projecting beyond those of the
preceding, so that the shell is elongated with every increase in its
diameter. Thus it appears that in its general plan of growth
Fusulina bears much the same relation to a symmetrical Rotaline or
Nummuline shell that Alveolina bears to OrbicuJina ; 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 tubulation of its component shells as to
prevent him from confidently affirming it, yet the apj)earances he
could distinguish were decidedly in its favour. And having since
received from Dr. C. A. White specimens from the Upper Coal
Measures of Iowa, U.S.A., which are in a much more perfect state of

Fig. 567. — Section of Fusulina limestone.

preservation, he is able to state with certainty, not only that Fusulina
is tubular, but that its tubulation is of the large coarse nature that
marks its affinity rather to the Rotaline than to the Nummuline
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.

Nummulinidae, — 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 the completeness of the
wall that surrounds each segment of the body (the septa being
generally double instead of single), the density and fine porosity of
the shell- substance, and the presence of an intermediate skeleton/

752 MICROSCOPIC FORMS OF ANIMAL LIFE

with a ' canal system ' for its nutrition. It is true that these cha-
racters are also exhibited in the highest of the Rotaline series, whilst
they are deficient in the genus Amphistegina, which connects the
Nummuline series with the Rotaline ; but the occurrence of such
modifications in their border forms is common to other truly natural
groups. With the exception of Amphistegina, all the genera of this
family are symmetrical in form, the spire being nautiloid in such
•^ls 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 Nuinmuline
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 Opercnlina and Nummulites in having its
chambers extended by ■ alar prolongations ' over each surface of
the previous whorl ; but on the under side these prolongations are
almost entirely cut off from the principal chambers, and are so dis-
placed as apparently to alternate with them in position, so that M.
D'Orbigny, supposing them to constitute a distinct series of chambers,
described its plan of growth as a biserial spire, and made this the
character of a separate order.1

The existing Niimmulinida? 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 i 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.
No 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. 508) of the sarcode-body and
canal system of the large P. craticnlata 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 Nuinmuline plan of growth,
the ical nature of which whk first elucidated by Messrs. Parker and Rupert Jones,
see the Author's Introduction to the Study of the Foraminifera (published by the
Ray Society).

a It was by Professor Ehrenberg that the existenoe 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

POLYSTOMELLA

753

we see that the segments of the sarcode-body are smooth along their
anterior edge 6, ft1, 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, c1,
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

Fig. 568. — Internal cast of Polystomella craticulata : a, retral processes
proceeding from the posterior margin of one of the segments ; b, bl, smooth
anterior margin of the same segment ; c, cl, stolons connecting successive
segments, and uniting themselves with the diverging branches of the meri-
dional canals; d, d\ d-, three turns of one of the spiral canals; e, el, e:,
three of the meridional canals; f,fl,f 2, their diverging branches.

divide the segments ; whilst from each of these there passes off
towards the surface a set of pairs of diverging branches,/*,/*1,,/"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 enclosed by another convolution,

organischen Lebens,' in Abhandlungen der Jeonigl. ATcad. der Wissenschaften,
Berlin, 1855. It was soon afterwards shown by the late Professor Bailey (Quart. Journ.
Micros. Sci. vol. v. 1857, p. 83; that the like infiltration occasionally takes place in
recent Foraminifera, enabling similar ' casts ' to be obtained from 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 ' Expedition, the most notable from
the coast of Australia.

3 c

754 MICEOSCOPIC POEMS OF ANIMAL LIFE

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 layer that forms the proper walls of the chambers, and

Fig. 569. — Cycloclypeus — external surface and vertical and horizontal sections.

which fills up with a solid ' boss ' what would otherwise be the de-
pression at the umbilicus of the spire. The substance of this ' boss J
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 XVII, 23, the umbilical boss
in P. cris])a, however, being much smaller in proportion than it

Fig. 570. — OpercuUna laid open to show its internal structure : a, marginal
cord seen in cross-section at a' ; b, b, external walls of the chambers ;
c, c, cavities of the chambers ; c c, their alar prolongations ; d, d, septa
divided at d' d' and at d" so as to lay open the interseptal canals, the
general distribution of which is seen in the septa e, e ; the lines radiating
from e, e point to the secondary pores ; g, g, non-tubular columns.

is in P. craticulata. There is a group of Foraminifera to which the
term Nonionina is properly applicable, that is probably to be con-
sidered as a sub-genus of Polystomella, 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 septal plane, and in the absence of the ' retral pro-

XUMMULIXE FORAMIXIFERA

755

cesses ' of the segments of the sarcode-body, the external walls of
the chambers being smooth. This form constitutes a transition to
the ordinary Xummuline type, of which Polystomella is a more aber-
rant modification.

The Nummuline type is most characteristically represented at the
present time by the genus Opercul hu>, which is so intimately united
to the true Nummulite by intermediate forms that it is not easy to
separate the two, notwithstanding that their typical examples are
widely-dissimilar. The former genus (fig. 570) 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 the
septa, are seen at b, b • and these are bounded at the outer edge of

Fig. 571. — Calcaritia laid open to show its internal structure : a, chambered
portion ; b, 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. 574), 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 at c, c ;
while the ' alar prolongations ' of those cavities over the surface of
the preceding whorl are shown at c', c' . The chambers are separated
by the septa, d, d, d, formed of two lamina? of shell, one belonging
to each chamber, and having spaces between them in which lie the
4 interseptal canals,' whose general distribution is seen in the septa
marked e, e, and whose smaller branches are seen irregularly divided
in the septa <7 , d', whilst in the septum d" one of the principal
trunks is laid open through its whole length. At the approach of
each septum to the marginal cord of the preceding is seen the
narrow fissure which constitutes the principal aperture of communi-

3 o 2

756

MICROSCOPIC FORMS OF ANIMAL LIFE

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. 576.
The external walls of the chambers are composed of the same finely
tubular shell-substance that forms them in the Nummulite ; but, as
in that genus, not only are the septa themselves composed of vitreous
non-tubular substance, but that which lies over them, continuing
them to the surface of the shell, has the same character, showing
itself externally in the form sometimes of continuous ridges, some-
times of rows of tubercles, which mark the position of the septa
beneath. These non- tubular plates or columns are often traversed
by branches of the canal system, as seen at g, g. Similar columns
of non-tubular substance, of which the 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 speci?nens 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 Nummidites, 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 arid frequently
of enormous thickness, that may be traced from the Atlantic shores
of Europe and Africa, through Western Asia to Northern India and
China, and likewise over vast areas of North America (fig. 572).
The diameter of a large proportion of fossil Nummulites ranges
between half an inch and an inch ; but there are some whose
diameter does not exceed TVth of an inch, whilst others attain the
gigantic diameter of 4^ inches. Their typical form is that 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 are
some Nummulites which closely approximate Operculinw 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 encloses by the ' alar prolongations ' of its
own chambers (the peculiar arrangement of which will be presently
described), that the spire is scarcely if at all visible on the external*
surface. It is brought into view, however, by splitting the Num-
mulite through the median plane, which may often be accom-
plished simply by striking it on one edge, with a hammer, the opposite

Nl'MMULIT'ES

757

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. Nummulites 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. 573,
which is taken from one of the commonest and most characteristic

A

Fig. 572. — A, piece of Nummulitic 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. 573.— Vertical section of portion of Nummulites laevigata : a, margin
of external whorl ; b, one of the outer row of chambers ; c, c, whorl invested
by a ; d, one of the chambers of the fourth whorl from the margin ; e, e',
marginal portions of the inclosed whorls ; /, investing portions of outer
whorl ; g, g, spaces left between the investing portion of successive whorls ;
h, h, sections of the partitions dividing these.

examples of the genus : thus in some, as Jf. distans, they keep their
own separate course, all tending radially towards the centre ; in
others, as N. Icevigata, their partitions inosculate with each other, so
as to divide the space intervening between each layer and the next
into an irregular network, presenting in vertical section the appear-
ance shown in fig. 573 ; whilst in X. garansensis they are broken

758 MICROSCOPIC FORMS OF ANIMAL LIFE

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 whorls, yet there is evidence that

Fig. 574. — Portion of a thin section of Nummulites Icevigata taken in the
direction of the preceding, highly magnified to show the minute structure
of the shell : a, a, portions of the ordinary shell-substance traversed by
parallel tubuli ; b, 5, portions forming the marginal cord, traversed by
diverging and larger tubuli ; c, one of the chambers laid open ; d, d, d,
pillars of solid substance not perforated by tubuli.

the segments of sarcode which they contained were not cut off from
communication with the exterior, but that they may have retained
their vitality to the last. The shell itself is almost every-
where minutely porous, being penetrated by parallel tubuli, which
pass directly from one surface to the other. These tubes are shown,
as divided lengthwise by a vertical section, in hg. 574, a, a ; whilst
the appearance they present when cut across in a horizontal section

is shown in fig. 575, the
transparent shell -substance
a, a, a being closely dotted
with ' minute punctations
which mark their orifices-
In that portion of the shell,
however, which forms the
margin of each whorl (fig.
574, b, b), the tubes are larger,
and diverge from each other
at greater intervals ; and it
is shown by horizontal sections
that they communicate freely
with each other laterally, so
as to form a network such as
is seen at b, b, fig. 576. At
certain other points, d, d, d,
tig. 574, 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 surfaces
have been much exposed to attrition, it commonly happens that the
pillars of the superficial layer, being harder than the ordinary shell-
substance, and being consequently less worn down, are left as

IB ill

writ-

c

Fici. 575. — Portion 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 d.

NOIMULITES

759

b 1

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-
whorl : this passage is

ceding

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 tig. 573, b. There
is also, as in OpercuUna, a
variable number of isolated
pores in most of the septa,
forming a secondary means of
communication between the
chambers. The canal system
of Xummidites seems to be ar-
ranged upon essentially the
same plan as that of Oper-
cuUna ; 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. 575, 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

Fig. 576. — Internal cast of two of the cham-
bers of Nummulites striata, with the
network of canals, b, in the marginal
cord communicating with canals passing
between the chambers.

well seen in the ' internal cast
represented in fig. 576.

A very interesting modifi-
cation of the Nummuline type
is presented in the genus
Heter'ostegina (fig. 577), which
bears a very strong resemblance
to Orbiculina in its plan of
growth, whilst in every other
respect it is essentially dif-
ferent. If the principal cham-
bers of an Opemdina were
divided into chamberlets by
secondary partitions in a direc-
tion transverse to that of the
principal septa, it would be
converted into a Heterostegina,
just as a Peneroplis would be converted by the like subdivision into
an 0rbiculi7ia. Moreover, we see in Heteroxteyina, as in Orbiculina,
a great tendency to the opening out of the spire with the advance of

Fig. 577. — Hctcrosteghia.

760 MICKOSCOPIC FORMS OF ANIMAL LIFE

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, Cycloclypens, 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 Foraminifera, 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 Nummuline type in every cha-
racter which essentially distinguishes
the genus that there cannot be a
doubt of the intimacy of their rela-
tionship. It will be seen from the
examination of that portion of the
figure which shows Cycloclypens 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 lamella?,
in the central portion of the disc
than they do nearer its edge that
new lamellae 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
traversed by columns of non-tubular
substance, which spring from the
septal bands, and gradually increase
in diameter with their approach to
the surface, from which they project in the central portion of the
disc as glistening tubercles.

The Nummulitic limestone of certain localities (as the south-west
of France, Southern Germany, North-eastern India, etc.) contains a
vast abundance of discoidal bodies termed OrbiUndcs (fig. 572, B),
which are so similar to Nummulites as to have been taken for them,
but which bear a much closer resemblance to Cycloclypens. These
are only known in the fossil state; and their structure can only be
ascertained by the examination of sections thin enough to be trans-
lucent. When one of these discs (which vary in size, in different
species, from that of a fourpenny-piece to that of half a crown or
even larger) is rubbed down so as to display its internal organisation,
two different kinds of structure are usually seen in it, one being
composed of chamberlets of very definite form, quadrangular in some

a

Fig. 578. — Section of Orbitoidcs
Furtisii, parallel to the surface,
traversing, at a, a, the superficial
layer, and at b, b, the median
layer.

ORBITOIDES

761

species, circular in others, arranged with a general but not constant
regularity in concentric circles (figs. 578, 579, b, b); the other, less
transparent, being formed of minuter chamberlets which have no
such constancy of form, but which might almost be taken for the

a b

Fig. 579. — Portions of the section of Orliitoides Fortisii, shown in fig. 578,
more highly magnified : a, superficial layer ; b, median layer.

pieces of a dissected map (a, a). In the upper and lower walls of
these last, minute punctations may be observed, which seem to be
the orifices of connecting tubes whereby they are perforated. The
relations of these two kinds of structure to each other are made

Fig. 580. — Vertical section of Orbitoides Fortisii, showing the large
central chamber at a, and the median layer surrounding it,
covered above and below by the superficial layers.

evident by the examination of a vertical section (fig. 580), which
shows that the portion b, figs. 578, 579, 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 been already described
as prevailing in Cycloclypeus, the most
satisfactory indications to this efiect
being furnished by the silicious 'internal
casts ' to be met with in certain Green-
sands, which afford a model of the sar-
code-body of the animal. In such a
fragment (fig. 581) we recognise the
chamberlets of three successive zones
seems normally to communicate by one or two passages with the
chamberlets of the zone internal and external to its own ; whilst
between the chamberlets of the same zone there seems to be no direct

Fig. 581. — Internal cast of por-
tion of median plane of Orbi-
toides Fortisii, showing, at
a a, a' a', a" a", six chambers
of each of three zones, with
their mutual communications;
and at b b, b' b', b" b", portions
of three annular canals.

a. a

a", each of which

762 MICROSCOPIC FORMS OF ANIMAL LIFE

connection. They are brought into relation, however, by means
of annular canals, which seem to represent the spiral canals of the
Nummulite, and of which the ' internal casts' are seen at b b, b' b't
b" b".

A most remarkable fossil, referable to the foraminiferal type,
has been recently 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. 582), is found
in beds of Serpentine limestone that occur near the base of the

Fig. 582. — Vertical section of Eozcdn canadense, showing alternation of
calcareous (light) and serpentinous (dark) lamellae.

Laurent ian 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 Dr. (now Sir W.) Dawson, of Mon-

EOZOON

treal, who at once recognised its foraminiferal nature,1 the calca-
reous layers presenting the characteristic appearances of true shell, so-
disposed 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 periods, and also at the present time, abundant evidence has
already been adduced. Having himself taken up the investigation
(at the instance of Sir William Logan), the Author was not only able
to confirm Dr. Dawson's conclusions, but to adduce new and im-
portant evidence in support of them.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 mineral 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 may
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 separa-
tion of its chambers ; it has its parallel in Carpenteria ; whilst in the
high development of its ' intermediate skeleton ' and of the ' canal
system ' by which this is formed and nourished, it finds its nearest
representative in Calcarina. Its calcareous layers were so super-
posed one upon another as to include between them a succession

1 This recognition was due, as Dr. Dawson has explicitly stated in his original
memoir (Quart. Journ. of Geol. Soc. vol. xxi. p. 54), to his acquaintance, not merely
with the Author's previous researches on the minute structure of the Foraminifera,
but with the special characters presented by thin sections of Calcarina which had
been transmitted to him by the Author. Dr. Dawson has given an account of the
geological and mineralogical relations of Eozcon, as well as of its organic structure, in
a small book entitled The Dawn of Life.

2 For a fuller account of the results of the Author's own study of Eozoon, and of the
basis on which the above reconstruction is founded, see his papers in Quart. Journ.
of Geol. Soc. vol. xxi. p. 59, and vol. xxii. p. 219, and in the Intellectual Observer,
vol. vii. 1865, p. 278 ; and his ' Further Researches' in Ann, of Nat. Hist. June 1874.

3 See the memoirs of Professors King and Rowney in Quart. Journ. of Geol. Soc.
vol. xxii. p. 185, and Ann. of Nat. Hist. May 1874.

1 Among these the Author is permitted to mention Professor Geikie, of Edinburgh,
who has thus studied the older rocks of Scotland, and Professor Bonney, of London, who
has made a like study of the Cornish and other Serpentines. By both these eminent
authorities he is assured that they have met with no purely mineral structure in the
least resembling Eozoon, either in its regular alternation of calcareous and serpen-
tinous lamellae, or in the dendritic extensions of the latter into the. former ; and while
they accept as entirely satisfactory the doctrine of its organic origin maintained by
the Author, they find themselves unable to conceive of any inorganic agency by which
such a structure could have been produced.

764 MICROSCOPIC FORMS OF ANIMAL LIFE

of 'storeys' of chambers (fig. 583, A1, A1, A2, 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 ,6, 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 ' lamella?, B, B, which form the
boundaries of the chambers beneath and above, serving (so to speak)
as the ceiling of the former, and as ike 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 ' Num-
muline ' layer (fig. 585)
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.
585), bearing a singu-
lar resemblance in its
occasional Avaviness to
that of the crab's claw.
The thickness of this
interposed layer varies
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 lacunar or
interspaces between the outside of the proper chamber-walls and
the 'intermediate skeleton,' exactly as in Calcarina, the exten-
sions of the sarcode-body which occupied them having apparently
been formed by the coalescence of the pseudopodial filaments that
passed through the tubulated lamella).

F13. 583. — Portion of the calcareous shell of Eozoim
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, <z, but occasionally separated by a septum,
b, b ; A-, A-, chambers of upper storey ; B, B,
proper walls of the chambers, formed of a finely
tubular or Nummuline substance ; C, C, inter-
mediate skeleton, occasionally traversed by large
stolon-passages, D, connecting the chambers of
different storeys, and penetrated by the arbores-
cent systems of canals, E, E, E.

EOZOON

7^5

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 sarcode-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. 584), which, though altogether dissimilar in arrangement,
is essentially analogous in character to the ' internal casts ' repre-
sented in figs. 564, 568. This cast presents us, therefore, with a.
model in hard serpentine of the soft sarcode-body which originally
occupied the chambers, and extended itself into the 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.

Fig. 584. — Decalcified portion of Eozoon canadense shell, showing the ser-
pentinous internal cast of the chambers, canals, and tubuli of the original,
presenting an exact model of the animal substance which originally filled
them.

We see that each of the layers of serpentine, forming the lower part
of such a specimen, is made up of a number of coherent segments,
which have only undergone a partial separation ; these appear to
have extended themselves horizontally without any definite limit ;
but have here and there developed new segments in a vertical 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-body
that originally passed into them. But this is not all. In specimens
in which the Nummuline layer constituting the ' proper wall ' of the
chambers was originally well preserved, and in which the decalcifying
process has been carefully managed (so as not, by too rapid an evolu-
tion of carbonic acid gas, to disturb the arrangement of the serpen-
tinous residuum), that layer is represented by a thin white film
covering the exposed surfaces of the segments ; the superficial aspect

766 MICROSCOPIC FORMS OF ANIMAL LIFE

of which, as well as its sectional view, are shown in fig. 584. 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 ' asbestiform layer '), but which are fre-
quently grouped in converging brush-like bundles, so as to be very
close to each other in certain spots at the surface of the film, whilst
widely separated in others. Now these fibres, which are less than
To~ijiroth °^ an incn in diameter, are the £ internal casts • of the tubuli
of the Nummuline 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 Opercidina. Thus

Fig. 585. — Vertical section of a portion of one of the calcareous lamellae of
Eozodn canadensc : a a, Nummuline layer perforated by parallel tubuli,
which show a flexure along the line a' a' ; beneath this is seen the inter-
mediate skeleton, c, C, traversed by the large canals, and by oblique
cleavage i>lanes, which extend also into the Nummuline 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. 584
it is to be observed that the segments are confusedly heaped together
instead of being regularly arranged in layers, the lamella ted 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

EOZOON

767

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 Globigerincv, 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 thatr 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 Bau des Eozoon Canadense ' (1878)
has been recently published by Professor Mobius, of Kiel, in which
the structure of Eozoon is compared with that of various types of
Foraminifera, and, as it differs from that of every one of them, is
affirmed not to be organic at all, but purely mineral. Upon this the
Author would remark, that if the validity of this mode of reasoning
be admitted, any fossil whose structure does not correspond with that
of some existing type is to be similarly rejected. Thus the Stroma-
topora of Silurian and Devonian rocks, which some palaeontologists
regard as a coral, others as polyzoary, others as a calcareous sponge,
and others as f oraminifer, would not be a fossil at all, because it differs

768 MICROSCOPIC FORMS OF ANIMAL LIFE

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 Mobius that as the supposed canal
system of Eozoon has not the constancy and regularity of distribu-
tion which it presents in existing Foraminifera, it must be accounted
a mineral infiltration. To this the Author would 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 Mobius could command, has satisfied
him that in well-preserved specimens the canal system, so far from
being vague and indefinite, has a very regular plan of distribution ;
(2) that this plan does not differ more from the arrangements
characteristic of the several types of existing Foraminifera than
these differ from each other, its general conformity to them being
such as to satisfy Professor Max Schultze (one of the ablest students
of the group) of its foraminiferal character ; and (3) that not only
does the distribution of the canal system of Eozoon differ in certain
essential features from every form of mineral infiltration hitherto
brought to light, but that canal systems in no respect differing from
each other in distribution 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 Mobius altogether ignores.

The argument for the foraminiferal nature of Eozoon is essentially
a cumulative one, resting on a number of independent probabilities,
no one of which, taken separately, has the cogency of a proof ; yet
the accordance of them all with that hypothesis has an almost
demonstrative value, no other hypothesis accounting at once for the
whole assemblage of facts. As it is the Author's intention to set
forth this in the best and completest form he can devise, at the
earliest possible period, he would beg fov & suspension of judgment on
the part of those who have credited Professor Mobius with having
completely settled the question, the small amount of evidence con-
tained in his memoir bearing no comparison to that of an opposite
bearing of which the Author is in possession. 1

1 The work, which the death of the Author unfortunately prevented his publish-
ing himself, will, it is hoped, be before long made accessible to the student under the
editorship of Professor Kupert Jones. It is, however, to be noted that Mr. J. W. Gregory,,
who has had an opportunity of examining the so-called Tudor specimen of Eozoon,,
communicated to the Geological Society, on March 11, 1891, a paper, of which the'
following is an abstract : —

After careful examination of all tbe slides and figures, and after consideration of
Sir W. Dawson's interpretation, the author is absolutely unable to recognise in the-
specimen any trace of the ' proper wall,' ' canals,' or ' stolon passages ' which are
claimed to occur in Eozoiin, or any reasons for regarding the calcite bands as the
' intermediate skeleton ' of a foraminifer. There are points in Sir W. Dawson'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 on 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. Selwyn and Vennor in the Huronian.

COLLECTING FORAMINIFERA

769

Collection and Selection of Foraminifera. — Many of the Fora-
minifera attach themselves in the living state to sea-weecls, zoophytes,
&c. ; and they should therefore be carefully looked for on such
bodies, especially when it is desired to observe their internal organ-
isation and their habits of life. They are often to be collected in
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 1
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 living 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
liner 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
lead shells from a mass of sand brought up by the dredge, a very
different method should be adopted. The whole mass should be
exposed for some hours to the heat of an oven, and be turned 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 ttc. subside. Another
method, devised by Mr. Legg, consists in taking advantage of the
relative sizes of different kinds of Foraminifera and of the substances
that accompany them. This, which is especially applicable to the
sand and rubbish obtainable from sponges (which may be got in
1 Trans, of Micros. Soc. ser. ii. vol. ii. 185 i, p. 19.

3 D

J JO MICROSCOPIC FORMS OF ANIMAL LIFE

large quantity from the sponge-merchants), consists in sifting the
whole aggregate through successive sieves of wire-gauze, commencing'
with one of ten wires to the inch, which will separate large extraneous-
particles, and proceeding to those of twenty, forty, seventy, and 100"
wires to the inch, each (especially that of seventy) retaining a much
larger proportion of foraminiferal shells than of the accompanying
particles ; so that a large portion of the extraneous matter being thus-
got rid of, the final selection becomes comparatively easy. Certain
forms of Foraminifera are found attached to shells, especially bivalves
(such as the Chamidct) with foliated surfaces ; and a careful exami-
nation of those of tropical seas, when brought home ' in the rough,r
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 between the lips, being the instrument which may be 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 they are to be viewed
by transmitted or by reflected light. In the former case they should
be mounted in Canada balsam, the various precautions to prevent
the retention of air-bubbles, which have been already described, being
carefully observed. In the latter no plan is so simple, easy, and;
effectual as attaching them with a little gum to wooden slides.
They should be fixed in various positions, so as to present all'
the different aspects of the shell, particular care being taken
that its mouth is clearly displayed ; and this may often be most
readily managed by attaching the specimen 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 then 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
separate slides and finished oft1 each one by itself.

Section II. — Radiolaria.

It has been shown that one series of forms belonging to the
rhizopod type is characterised by the radiating arrangement of their
rod-like pseudopodia, suggesting the designation Heliozoa or 'sun-
animalcules ' ; and that even among those fresh-water forms that do
not depart widely from the common Actinophrys there are some
whose bodies are enclosed in a complete silicious skeleton. Now
just as the reticularian type of rhizopod life culminates in the marine

RADIOLABIA

771

calcareous-shelled Foraminifera, so does the heliozoic type seem to
culminate in the marine Itadiolaria ; which, living for the most
part near the surface of the ocean, form silicious skeletons (often of
marvellous symmetry and beauty) that fall to the bottom on the
death of the animals that produced them, and may remain unchanged,
like those of the diatoms, through unlimited periods of time. Some
of these skeletons, mingled with those of diatoms, had been detected
by Professor Ehrenberg in the midst of various deposits of foramini-
feral origin, such as the calcareous Tertiaries of Sicily and Greece,
and of Oran in Africa ; and he established for them the group of

a

\

Fig. 58G. — Fossil Badiolaria from Barbadoes : a, Podocyrtis mitra; b,
BhabdolitJms sceptrum ; c, Lyclinocanium falciferum ; d,Eucyrtidiun/
tubulus; e, Flustrella concentrica; /, Lyclinocanium I 'ucerna ; g,Eucyr-
tidium elegans ; li, Dictyospyris clathriLS ; i, Eucyrtidium Mongolfieri ;
A-, Stephanolithis spinescens ; I, S. nodosa ; m, Lithocyclia ocellus ; n,
Cephalolithis sylvina ; 0, Podocyrtis cothuruata ; p, Rhabdolithus pipa.

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. 586).
Nothing, however, was known of the nature of the animals that
formed them until they were discovered and studied in the living state
by Professor J. Miiller,1 who established the group of Itadiolaria,

1 ' Ueber die Tlialassicollen, Polycystinen, und Acanthometren des Mittel-
meeres,' in Abhandlungen der Tcdnigl. Akad. der Wissensch. zu Berlin, 1858, and
separately published ; also ' Ueber die Lm Hafen von Messina beobachteten Poly-
cystinen,' in the MonatsbericJite of the Berlin Academy for 1835, pp. 071-67(5.

3 d 2

772

MICKOSCOPIC FORMS OF ANIMAL LIFE

including therein, with the Polycystina of Ehrenberg, the Acantho-
metrina first recognised by himself, and the Thalassicolla which had
been discovered by Professor Huxley. Not long afterwards appeared
the magnificent and 'epoch-making' work of Professor Haeckel; 1
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

Fig. 587. — Polycystina: A, HaMowima hystrix; B, Pterocanium, with animal.

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. The pseudopodia radiate in all
directions (tig. 587) from the deeper portion of the extracapsular
sarcode ; they have generally much persistency of direction and very
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 along
them ; and the mode in which they obtain food-particles (consisting

1 Die Had iolarien (Rhizopoda Radiaria), Berlin, 1802. This great work has
lately been followed by a gigantic monograph published in the ' Challenger' lieports,
which extends over 1800 pages, and is illustrated by 140 plates. In it are described
■4318 species, of which 3508 are new to science.

RADIOLARIA

773

B

mm

pgooo2gQ§a

of diatoms and other minute alga?, marine infusoria, <kc), 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 Zooxanthelhe, as K. Brandt has proposed to
call them, so often seen in these cells, are not confined to Radiolaria,
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 1 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 Radiolaria skeletal structures are developed in the
sarcode-body, either inside or outside the capsule, or in both positions ;
sometimes in the form of
investing networks having
more or less of a spher-
oidal form (fig. 589, l, 2),
or of radiating spines,
3, or of combinations of
these, 4, 5. But in many
cases the skeleton con-
sists only of a few scat-
tered spicules ; and this
is especially the case in
certain large composite
forms or 1 colonies ; (fig.
59-4) which may consist
of as many as a thousand
zooids, aggregated toge-
ther in various forms, disc -
oidal, cylindrical, spher-
oidal, chain-like, or even
necklace-like. The ' colo-
nies' seem to be produced,
like the multiple segments of the bodies of Foraniinifera, by the
non-sexual multiplication of a primordial zooid ; but whether this
multiplication takes place by fission, or by the budding off of por-
tions of the sarcode-body, has not yet been clearly made out. The
emission of flagellated zoospores, provided with a very large nucleus,
and in some cases with a rod-like crystal, has been observed in many
radiolarians ; but of the mode in which they are produced, and of
their subsequent history, very little is at present known. Until 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 be framed ; and nothing more will be here attempted
than to indicate some of the principal forms under which the radio-
larian type presents itself.2

1 See especially K. Brandt, Verhandl. Physiol. Gesellsch. Berlin, 1881-82, p. 22 ;
Mitth. ZooJ. Stat. Keapel, iv. p. 191 ; P. Geddes, Nature, xxv. p. 303.

- Considerable attention has been given to the question of the classification of
the Radiolaria by Haeckel and by R. Hertwig, Jenaische Dcnkschr. ii. 1S79, p. 129.

Fig. 588. — Polycystina: A, Podocyrtis Schom*
burgkii ; B, Rhopalocanium ornatum.

774 MICROSCOPIC FORMS OF ANIMAL LIFE

Disci da. — Among the beautiful silicious structures which are
met with in the racliolarian sandstone of Barbadoes (fig. 586) there
is none more interesting than the skeleton of Astromma (fig. 590),

Fig. 589. — Various forms of Radiolaria (after Haeckel) : 1, Mthw.08Jphcsra
sij'houophcra; 2, Actinomma inerme ; 3, Acanthometra xipliicdiitha;
4, Aracnnosphcera obligaoantha', 5, Chulococcus viminalis.

in which we have a remarkable example of the range of variation
that is compatible with conformity to a general plan of structure.
As in other forms of Haeckel's group of Discida, there is in this
skeleton a combination of radial and of circumferential parts, the

RADIOLARIA

775

former consisting of solid spoke-like rods, whilst the latter is com-
posed of a silicious network more or less completely filling up the
spaces between the rays. The radial part of the skeleton predomi-
nates in the beautiful four-rayed example represented at D, having

Fig. 590. — Varietal modifications of Astromma.

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. Tn the five-rayed
•specimens A and B, on the other hand, the radial portion is much

Fig. 591. — Periclilamydium prcetextum. Fig. 592. — Stijlodyctyci gracilis.

less developed, whilst the circumferential becomes more discoidal.
And in C and E, while the circumferential network forms a penta-
gonal disc, the radial portion is represented only by solid projections
at its angles. The transition between the extreme forms is found

776

MICKOSCOPIC FORMS OF ANIMAL LIFE

to be so gradual when a number of specimens are compared that no*
lines of specific distinction can be drawn between them ; and the
difference in the number of rays is probably of no more account in
these low forms of animal life than it is in the discoidal diatoms..
Other discoidal forms, showing a like combination of radial and
circumferential parts, are represented in figs. 591 and 592, and also-
in fig. 586, e, m.

Entosphserida. — In this group the silicious shell is spheroidal,
and is formed within the capsule ; and it is not traversed by radii,
although prolongations of the shell often extend themselves radially
outwards, as in Cladococcus (fig. 589, 5). Sometimes the central
sphere is inclosed in two, three, or even more concentric spheres-
connected by radii, as in the beautiful Actinomma (fig. 589, 2), re-
minding us of the wonderful concentric spheres carved in ivory by
the Chinese. One of the most common examples of this group is-
the Haliomma Humboldtii (fig. 593) 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

the capsule. This shell may, as in the-
preceding, be a simple sphere com-
posed of an open silicious network, as
in Ethmosphcera (fig. 589, l) ; or it may
consist of two or three concentric
spheres connected by radii ; or, again,
it may put forth radial outgrowths,
which sometimes extend themselves
to several times the diameter of the-
shell, and ramify more or less
minutely, as in Araclmosphcera (fig.
589, 4). But more frequently the shell
Fig. 593.— Haliomma Humboldtii. opens out at one pole into a form

more or less bell-like, as in Podocyrtis-
(fig. 588, A, and fig. 586, «, 0), Rhopcdocanium (fig. 588, B), and.
Pterocanium (fig. 587, B); or it may be elongated into a somewhat
cylindrical form, one pole remaining closed, while the other is more
or less contracted, as in Eucyrtidium (fig. 586, d, y, i). The transi-
tion 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. 587, 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 less tenacious protoplasm, resembling that of which the

1 The general plan of structure of the Polycystina and- the signification of their
immense variety of forms, were ably discussed by Dr. Wallich in the Trans, of the
Micros. Soc. n.s. voL xiii. 18(55, p. 75.

COLLOZOA

777

pseudopodia are composed. One species, the Acantliometra echin-
oides, which presents itself to the naked eye as a crimson-red
point, the diameter of the central part of its body being about
I 0(i0 0ths of an inch, is very common on some parts of the coast of
Norway, especially during the prevalence of westerly winds ; and
the Author has himself met with it abundantly near Shetland, in the
floating brown masses termed madre by the fishermen (who believe
them to furnish food to the herring), which consists mainly of this
Acanthometra mingled with Entomostraca.

Phseodaria. — Among the most important of the Eadiolaria
collected by the ( Challenger ' are the comparatively large (as much as
1 mm. in diameter) single-celled forms which are remarkable for the
constant presence of large, dark brown granules, which are 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 extracap-
sular silicious skeleton,
according to the struc-
ture of which the group
is subdivided.

Collozoa. — To this
group belong those re-
markable composite
forms, which, exhibit-
ing the characteristic
radiolarian type in their
individual zooids, are
aggregated into masses in which the skeleton is represented only
by scattered spicules, as in Splubrozoum (fig. 594) and Thalassicolla.
These 1 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 inoscu-
late with each other so as to form a network. Towards the inner
surface of this coat are scattered a great number of oval bodies
resembling cells having a tolerablv distinct membraniform 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 are disposed. Each of these groups is protected by a

773 MICEOSCOPIC FOEMS OF ANIMAL LIFE

silicious skeleton, which sometimes consists of separate spicules (as
in fig. 594), but which may be a thin perforated sphere, like that of
certain Polycystina, sometimes extending itself into radial pro-
longations. The internal portion of each mass is composed of an
aggregation of large vesicle-like bodies imbedded in a softer sarcodic
substance. 1

From the researches made during the 1 Challenger ' Expedition,
it 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, but also at considerable depths. Their silicious skeletons
accumulate in some localities (in which the calcareous remains of
Foraminifera are wanting) to such an extent as to form a 1 radio-
larian ooze ' ; and it is obvious that the elevation of such a deposit
into dry land would form a bed of silicious sandstone resembling the
well-known Barbadoes rock, which is said to attain a thickness of
1,100 feet, or a similar ronk of yet greater thickness in the Nicobar
Islands. Few microscopic objects are more beautiful than an
assemblage of the most remarkable forms of the Barbadian Poly-
cystina (fig. 586), especially when seen brightly illuminated upon
a black ground ; since (for the reason formerly explained) their
solid forms then become much more apparent than they are when
these objects are examined by light transmitted through them. And
when they are mounted in Canada balsam the black-ground illu-
mination is much to be preferred for the purpose of display,
although minute details of structure can be better made out when
they are viewed as transparent objects with higher powers. Many
of the more solid forms when 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. No 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.2

1 See Professor Huxley (to whom we owe our first knowledge of these forms) in Ann.
Nat. Hist. ser. ii. vol. viii. 1851, p. 433; also Professor Miiller, of Berlin, in Quart. Journ.
Micros. Sci. vol. iv. 1856, p. 72, and in his treatise Ucbcr die Thalassicollen, Poly-
cystinen, unci Acantdiometren cles Mitt elm eeres, the magnificent work of Professor
Haeckel, Die Radiolarien, and the monograph by K. Brandt, published in the Fauna
unci Flora cles Golfes von Neapel, 1885, 'Die kuloniebildenden Radiolarien
(Sphferozoeen) des Golfes von Neapel.'

2 For a fuller description of the fossil forms of this group see Professor Ehrenberg's
memoirs in the Monatsbericlite of the Berlin Academy for 1846, 1847, and 1850 ; also
his Microgeologie, 1854 ; and Ann. of Nat. Hist. vol. xx. 1847. The best method of
separating the Polycystina from the Barbadoes sandstone is described by Mr. Fur-
long in the Quart. Journ. of Micros. Sci. n.s. vol. i. 18(31, p. 64.

779

CHAPTER XV

SPOXGES AND ZOOPHYTES

I. Sponges.

We now leave the Protozoa and commence the study of the Metazoa,
or those forms in which the egg-cell undergoes subdivision, the result-
ing elements of which do not separate or lead an independent
existence, but combine to form an organic whole, various parts
undertaking various functions. Of these Metazoa the simplest ex-
amples are to be found among Spoxges. 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, 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 composed of cells ; some of these cells have a striking
resemblance to the collared Flagellata (fig. 527), whilst others re-
semble Amcebce (fig. 519). These units are held together by a con-
tinuous connective-tissue-like substance which clothes the skeletal
framework that represents our usual idea of a sponge, and is itself
made up of distinct cellular elements. In the simpler forms of
sponges, however, this framework is altogether absent ; in others it
is represented only by calcareous or silicious ' spicules,' which are
dispersed through the sarcodic substance (fig. 596, B) ; in others,
again, the skeleton is a keratose (horny) network, which may be
entirely destitute (as in our ordinary sponge) of any mineral support,
but which is often strengthened by calcareous or silicious spicules
(fig. 596) ; whilst in what may be regarded as the highest types of
the group, the silicious component of the skeleton increases, and the
keratose diminishes until the skeleton consists of a beautiful silicious
network resembling spun glass. But whatever may be the condi-
tion of the skeleton, that of the body that clothes it remains
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 outicards by
repeated ramification, sending their zooid-bearing branches to

;8o

SPONGES AND ZOOPHYTES

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. 595, b, b) with which the surface a, a of the living sponge
is beset lead to incurrent passages that open into chambers lying
beneath it (c, c), and open into the ' ampullaceous sacs,' or, as they
are now more often called, ' flagellate chambers,' from the pressure
round their walls of the flagellate or collared cells. The water
drawn in by their agency is driven outwards through a system of
excurrent canals, which, uniting into larger trunks, proceed to the
oscula or projecting vents, cl, from each of which, during the
active life of the sponge, a stream of water, carrying out excremen-
titious matter, is continually issuing. The in-current brings into
the chambers both food-material and oxygen ; and from the manner
in which coloured particles experimentally diffused through the
water wherein a sponge is living are received into its sarcodic sub-
stance, 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. 1

The continuous sar-
code-substance or 'me-
soderm ' 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 has been shown by
Professor C. Stewart and Dr. von Lendenfeld, sensory organs, formed
of groups of cells with long projecting filaments, are to be seen on
the surface of many sponges. Any one of these amceboids, again,
detached from the mass, may lay the foundation of a new 'colony/
In the aggregate mass produced by its continuous segmentation

1 The view that sponges are essentially protozoic colonies has found its chid
supporters in Prof. H. James Clark, Mr. H. J. Carter, and Mr. Saville Kent ; but
the now known facts as to the history of their development are decisive as to the
metazoic nature of the group. For an excellent summary of this aspect of the ques-
tion see Prof. F. E. Schulze, Sitzuugsber. lerlin Akad. 1885, p. 179.

Fig. 595. — Diagrammatic section of a sponge : a, a,
superficial layer ; b, inhalant apertures or pores ; c, c,
flagellated chambers ; d, exhalant oscule ; e, deeper
substance of the sponge.

SPONGE STRVCTURE

78l

certain globular clusters are distinguishable, each having a cavity in
its interior ; and the amoeboids that form the wall of this cavity
become 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 incurrent passage, and on the
other with the excurrent canal-system leading to the oscula. Be-
sides this reproduction by 1 microspores,' there is another form
of non- sexual reproduction by macrospores, which are clusters of
amoeboids encysted in firm capsules, frequently strengthened on
their exterior by a layer of spicules of very peculiar form. These
' seed-like bodies,' which answer to the encysted states of many
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 amceboids, each of which may found a new colony.
A true process of sexual generation, moreover, is known to take place
in sponges, certain of the amceboids, like certain cells of Volvox, '
becoming ' sperm-cells,' and developing spermatozoa by the meta-
morphosis of 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
wralls of the canals, swim forth from the vents, and for a time move
actively through the water. According to Professor Haeckel, the
fertilised germ-cells are to be regarded as true ova, and the products
of their segmentation as morula?, which, by invagination, become
gastrulce ; and he argues that the whole system of canals and
ampullaceous sacs is really, like the system of canals in the .sponge-
like AJcyonium, an extension of the primitive gastric cavity, the
oscula of sponges being the undeveloped representatives of the
polypes of the zoophyte. 1

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 bases ;
but smaller and simpler sponges, such as Grantia, have no horny
skeleton, and their calcareous spicules are imbedded in the general
substance of the body. Sponge- spicules are much more fre-
quently silicious than calcareous ; and the variety of forms pre-
sented by the silicious spicules is much greater than that which

1 See Chapter V. of Mr. Saville Kent's Manual of the Infusoria, and Chapter V. of
vol. i. of Mr. Balfour's Co mparative Embryology, as well as Professor Haeckel's im-
portant work on the Calcareous Sponges.

7&2 SPONGES AND ZOOPHYTES

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 :

A

Fig. 59(3. — A, section through PhakelUa ventilabrurn', var. connexiila, taken at right
angles to the surface, to' show the arrangement of the parts of a sponge : p, pores
on the surface leading to ic, the inhalent canals, then to the flagellated chambers,
fc, and thence to the exhalent canals, ec, to o, the oscula in the dermal membrane,
dm. B, more highly magnified view of the internal portion (choanosome) of
Axinella pa radoxa x 290: me, mesodermal cells. Other letters as in A. (After
Kidley and Dendy.)

SPONGE-SPICULES

783

■ at

tliey are sometimes pointed at both ends, sometimes at one only ;
one or both ends may be furnished with a head like that of a
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-
vals, giving them a jointed appearance. 1 The more recent authorities-
on Sponges, such as Professor Sollas 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
spongologists the polyaxial
spicules are the most primi-
tive, there is no practical
objection to our noticing them
in the reverse order ; a method
which will be found to conduce
to simplicity of description.
In the examination of spicules,
it is necessary, first of all, to
distinguish between axes and
rays ; thus in the Monaxonida
the megaloscleres have but a
single axis, but the growth
from the point of origin may
be 011 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
three axes and three rays ; but

-at

2

Fig. 597.— Structure of the chelae of Mo-
naxonid Sponges : 1, tridentate anisochela
from in front ; la, from the side ; 2, 2ar
front and side views of a palmate isochela ;
t, t', tubercle; at, at', anterior tooth or
palm ; It, It' , lateral tooth or palm ; s, shaft -
/, fimbria. (After Ridley and Dendy.)

111

In the Calcispongiae there are
some sponges, such as Venus'

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. 27U-332 ; and in his Monograph of the
British Spongiadce, published by the Kay Society. The Calcareous Sponges have
been made by Professor Haeckel the subject of an elaborate monograph, Die Kalk-
schwamme, Berlin, 1872. For compendious enumerations and classifications of the
various kinds of spicules, see Professor Sollas, art. ' Sponges,' in Encycl. Britannica,
and Messrs. Ridley and Dendy, Bepcrt on the 'Challenger ' Monaxonida, pp. xv-xxi!

734

SPONGES AND ZOOPHYTES

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 hexactinellid. In others, such as Geodia and the Lithistid
Sponges there are four axes, whence such forms are called tetraxonid.
Lastly there are the least regular rnegaloscleres 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
Dencly 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 be simple sigmiform microscleres, or the
shape may be complicated by the inner margin of the shaft or hook
thinning out to a fine knife-edge. The most complex forms of this
group are the microscleres which the just-quoted authors denominate
chelce. They describe these scleres (fig. 597) 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 \W~b 71 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

^ „nn . . . , T^,7 7 fimbriae ( /). If the two ends of the
Fig. 598. — Anisochelae of Clado- x ' i i -77
rhiza inversa showing n, the spicule are equal, we have isoclielce ; it un-
nucleus of the mother-cell of equal, anisochelce. The stellate micro-
tia spicule from in front, o, scleres may De spiral, have a shaft with
and from the side, 0. x 600. . J 1 ' . . . „ . .

(After Eidley and Dendy.) spinose whorls, or a cylindrical shatt with

a toothed whorl at either end. The
spicules of sponges cannot be considered, like the raphides 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. 598),
and the special cells which produce them are distinguished as sili-
coblasts ; in this there is first developed a central organic thread
around which concentric layers of silica or chalk are laid down.

There is an extremely interesting group of Sponges in which the
horny skeleton is entirely replaced by a silicious framework of great
firmness and of singular beauty of construction. This framework
may be regarded as fundamentally consisting of an arrangement of
six-rayed spicules, the extensions of which come to be, as it were,
soldered to one another \ and hence the group is distinguished as
sexradiate. Of this type the beautiful Euphctella of the Manila
seas — which was for a long time one of the greatest of zoological
rarities, but which now, under the name of ' Venus' flower-basket,'
is a common ornament of our drawing-rooms — is one of the most

SPONGES

785

characteristic examples.1 Another example is presented by the-
Holtenia Carpenteri, of which four specimens, dredged up from a
depth of 530 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
on 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 Euplectella, 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.2
One of the greatest features of interest in this Holtenia is its.
singular resemblance to the Ventriculites of the Cretaceous formation.
Subsequent investigations have showed 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 pumiceus 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 Spongilla 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, &c. between the tide-
marks. The various species of Grantia in which, of all the marine
sponges, the flagellate cells can most readily be observed, belong to-
this last category. They have a peculiarly simple structure, each
being a sort of bag whose wall is so thin that no system of canals is.
required, the 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 |-inch objective on some
of the cells of the gelatinous substance scraped from the interior of
the bag ; or they may be seen in situ by making very thin trans-
verse sections of the substance of the sponge. It is by such sections
alone that the internal structure of sponges, and the relation of
their spicular and horny skeletons to their fleshy substance, can be
demonstrated. They are best made by the imbedding process. In

1 The structure and arrangement of the soft parts of Euplectella aspergiUum have
been investigated by Prof. F. E. Schulze, Trans. Boyal Soc. of Edinburgh, xxix.
p. 661.

2 See his elaborate memoir in Phil. ' Trans. 1870, and his Depths of the Sea>
1872, p. 71.

3 E

786

SPONGES AND ZOOPHYTES

order to obtain the spicules in an isolated condition, the animal
matter must be got rid of either by incineration or by chemical
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 nitro- muriatic acid, until its animal
substance is dissolved away ; if, on the other hand, they be 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
on several accounts to dispense with it. The spicules, when obtained
in a separate state, should be mounted in Canada balsam. Sponge
tissue may often be distinctly recognised in sections of agate,
chalcedony, and other silicious concretions, as will be more fully
stated hereafter.1

II. Zoophytes (Coslenterata).

Under the general designation Zoophytes it will be still con-
venient to group those animals which form composite skeletons or

' polyparies ' of a more or less
plant-like character, associating
with them the Acalephs, which
are now known to be the ' sexual
zobids'of polypes, but excluding
the Polyzoa on account of their
very different structure, not-
withstanding 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 cavity
(though sometimes extending
itself almost indefinitely) being-
lined by the original endoderm,
and their surface being covered
by the original ect&derm, and
these two lamellae not beino-
separated by the interposition
of any body-cavity or cmlom.
It is a fact of great interest
that although the product of
the development of a 'morula is
here a distinctly individualised
polype, in which several mutu-
ally dependent parts make up
■a single organic whole, yet these parts still retain much of their
independent protozoic life ; which is manifested in two very re-

1 A complete and valuable handbook to the Sponges has been lately published by
Dr. (1. C Vosmaer as vol. if, of Bron-h's tClassen it ml Ordnungen (lets ThierreichSj
Leipzig, 1S87. Compare also the article by Professor Hollas in the ninth edition of the

-Fn;. 501). — Longitudinal section of the body
of a hydra killed in full digestion : ec,
ectoderm; en, endoderm; nip, muscular
processes; d, a diatom; /', toed. (After
T, J. Parker.)

C'CELENTEEATA

787

markable modes. In the first place, the digestive sac is observed
to be lined by a layer of amoeboid cells, which send out pseudopodial
prolongations into its cavity (fig. 599) 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
Myriotliela ; 1 the like has been since shown by Mr. Jeffery Parker
to be true of the ordinary Hydra ; 2 and Professor E. Pay Lankester
has made the same observation upon the curious little Medusa {Limno-
cpdium), lately found in a fresh-water tank in this country, whither
it has undoubtedly been introduced ; while the observations of
Krukenberg have shown that a similar process obtains among the sea-
anemones.3 (It may be mentioned in this connection, that Metschni-
koff has seen the cells which line the alimentary canal of the lower
planarian worms gorging themselves with coloured food-particles,
exactly in the manner of Amoeba and the liver-fiuke, and that a
number of larvae are known to obtain their nourishment in the same
way.4) The second ' survival ' of protozoic independence is shown
in the extraordinary power possessed by Hydra, Actinia, &c. of
reproducing the entire organism from a mere fragment. This great
division includes the two principal groups the Hydrozoa and the
Actinozoa, the former comprehending the Polypes, and the latter
the Anemones. In the Hydrozoa the mouth is placed on a projecting
oral cone, while in the Anthozoa it is sunk below the level of the oral
circlet of tentacles, and the cavity developed from and connected
with the digestive cavity separates its wall from the body- wall and
is traversed by a series of vertical partitions or septa. As most of
the hydroid polypes are essentially microscopic animals, they need
to be described with some minuteness ; 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 Hydra 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, &c. Two species are common in this
country, the H. viridis or green polype, and the H. vulgaris, Avhich
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

fusca, is distinguished from both the preceding by the length of
its tentacles, which in the former are scarcely as long as the body,
whilst in the latter they are, when fully extended, many times longer

Encyclopaedia Britannica; the' Challenger' Reports by Professor Scliulze, Messrs.
Ridley and Dendy, Polejaeff, and Sollas ; and the numerous memoirs of Professors
O. Schmidt and Scliulze, and of Dr. von Lendenfeld.

1 Phil. Trans. 1875, p. 552. It should be noted that Professor Claus called
attention to the ingestion of foreign bodies by amoeboid cells of Monupliytes in 1874.
See his Schriften Zool. Inhalts. (Wien, 1874), p. 30

2 Prop, of Boy. Soc. vol. xxx. 1880, p. 61.

3 Quart. J ourn. Micros. Sci. n.s. vol. xx. 1880, p. 371.

4 Consult an interesting article on ' Intercellular Digestion,' by Metschnikoff, in
Revue Scientifique, ser. iii. vol. xi. p. 083.

3 e 2

788

SPONGES AND ZOOPHYTES

(fig. 600 ]). The body of the Hydra consists of a simple bag or sac,,
which may be 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-
globe, whilst, if distended with food, it may present the form of an
inverted flask or bottle, or even of a button. At the upper end of
this sac is a central opening, the mouth ; and this is surrounded by
a circle of tentacles or ' arms/ usually from six to ten in number,
which are 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 it&
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.'2
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 1 urticating 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
doubtless intended to give to the
Fig. COO.— Hydra fusca, with a young bud organ a great prehensile power,
at b, and a more advanced bud at c. 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 by a vesicle at its base.

1 On the specific characters of Hydra consult Haacke, Jcnaische Zeitschr. xiv..
X>. 133 ; and Jickeli, Zool. Anzciq. v. p. 401.

2 To this intermediate layer, Mr. Cx. C. Bourne applies the term mesoglcea. For an
account of its variations and structure among the Ccelenterata, and a discussion of its
homology with the mesoderm of higher Metazoa, see his essay on Fangia in vol..
xxvii. of the Quart. Journ. Micros. Sci. n.s.

HYDROZOA

789

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 fusca,
whose arms, when ex-
tended in search of
prey, are not less than
seven or eight inches in
length ; whilst they are
.sometimes so contract-
ed, when the stomach
is filled with food, as
to appear only like little
tubercles around its en-
trance. By means of
these instruments the
Hydra is enabled to
draw its support from
animals whose activity,
as compared with its
•own slight powers of
locomotion, might have
been supposed to re-
move them altogether

from its reach ; for
when, in its movements
through the water, a
minute worm or a water-
flea happens to touch
one of the tentacles of
the polype, spread out as these are in readiness for prey, it is
immediately seized by this ; other arms are soon coiled around it,
and the unfortunate victim is speedily conveyed to the stomach,
within which it may frequently be seen to continue moving for
some little time. Soon, however, its struggles cease, and its outline
is obscured by a turbid film, which gradually thickens, so that at
last its form is wholly lost. The soft parts are soon completely dis-
solved, and the harder indigestible portions are rejected through the
mouth. A second orifice has been observed at the lower extremity

Fig. 601. — Gampanularia gchttinosa.

790

SPONGES AND ZOOPHYTES

of the stomach ; but this would not seem to be 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 Hydrozoa,
brings into mutual connection the lower extremities of the stomachs
of all the individual polypes.

The ordinary mode of reproduction in this animal is by a 1 gemma-
tion ' resembling that of plants. Little bud-like processes (fig. 600,.
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 veneration of buds is
sometimes observed on the young
polype before quitting its parent ;
and as many as nineteen young
Hydrce in different stages of
development have been seen thus-
connected with a single original
stock (tig. 602). 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 Hydra may
be divided, each may retain its
vitality, ami 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 appear
that sometimes one individual Hydra develops only the male cysts
or sperm-cells, while another develops only the female cysts or ovi-

mm

■ft

mm
1

Fig. 602. — Hydra fusca in gemmation
mouth ; />, base : c
buds.

origin of one of the

HYDROZOA

791

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-
tweeen April and July. According to Ecker, the eggs of II. viridis
produced early in the season run their course in the summer of
the same year ; while these 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 light, towards which the Hydra seeks to move itself, than
with reference to the search after food.1

The compotni'l Hydroids may be likened to a Hydra whose
gemma?, instead of becoming detached, remain permanently connected
with the parent ; and as these in their turn may develop gemma?
from their own bodies, a structure of more or less arborescent
character, termed a yolypary, 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
continous 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 polyparyof 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 cosnosarc, and through it the nutrient matter circulates. The
'zooicls,' or individual members of the co]ony, are of two kinds : one
the ])oly]nte, or alimentary zooid, resembling the Hydra in essential

1 A very full account of the structure and development of Hydra has been
published by Kleinenberg, of whose admirable monograph a summary is given by
Professor Airman, with valuable remarks of his own, in Quart. Journ. Micros. Sci. n.s.
vol. xiv. 1874, p. 1. See also the important paper by Mr. Jeft'ery Parker already
cited. On the chlorophyll corpuscles of H. viridis consult Brandt, Mitth. Zoo/,
Stat. Neapel, iv. p. V.)l ; Hamaim, Z00J. Anzeig. vi. p. 307 ; and Lankester Quart.
Journ. Micros. Sci. n.s. xxii. p. 22U.

SPONGES AND ZOOPHYTES

structure, and more or less in aspect ; the other, the gonozooid, or
sexual zooid, developed at certain seasons only, in buds of particular
shape.1

The simplest division of the Hydroida is that adopted by Mr.
Hincks,2 who groups them under the sub-order Athecata said Thecata,
the latter being again divided into the Thecaphora and the Gymno-
chroa. In the first, neither the ' polypites ; nor the sexual zooids
bear true protective cases ; in the second the polypites are lodged in
•cells, or, as Mr. Hincks prefers to call them, calycles, many of which
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 Medusae, (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 planulce, 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, after which the ' polypary ' characteristic of the genus
is gradually evolved by the successive development of polype-buds
from the first-formed polype and its subsequent offsets. The Medusae
of these polypes (jig. 605) belong to the division called ' naked-eyed,'
on account of the eye-spots usually seen surrounding the margin of
the bell at the base of the tentacles.

A characteristic example of this production of medusa-like
' gonozooids ' is presented by the form termed Syncoryne Sarsii (fig.

1 A useful list of the principal terms used in describing hydroids, with definitions,
will be found on pp. 1(5 and 17 of Professor Allman's Report oil the Hydroida (Plu-
viulariidce) of the Challenger.

- History of British Hydroid Zoophytes, 1808.

DEVELOPMENT OF HYDROZOA

793

•603) belonging to the sub-order Athecata. At A is shown the ali-
mentary zodid, 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 Medusa? of the genus Syncoryne (as now restricted) have the
form named Sarsia in honour of the Swedish naturalist Sars. Their
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 gonozooids
are produced in the common
Hydra, as already described, and
that of Syncoryne. In Tubu-
laria the gonozooids, though
jDeruianently attached, are fur-
nished with swimming bells,
having four tubercles repre-
senting marginal tentacles. A
common and interesting species,
Tubularia indivisa. receives its
specific name from the infre-
quency with which branches are
given off from the stems, these for
the most part standing erect and
parallel, like the stalks of corn,
upon the base to which they are
attached. This beautiful zoo-
phyte, which sometimes grows
between the tide-marks, but is
more abundantly obtained by
dredging in deep water, often
attains a size which renders it
scarcely a microscopic object, its
stems being sometimes no less

than a foot in height and a line in diameter. Several curious
phenomena, however, are brought into view by microscopic examina-
tion. The polype-stomach is connected with the cavity of the
stem by a circular opening, which is surrounded by a sphincter ;
and an alternate movement of dilatation and contraction takes
place in it, fluid being apparently forced up from below, and then
•expelled again, after which the sphincter closes in preparation for

1 Hincks. op. ext. p. 49.

Fig. 603. — Development of Medusa buds
in Sylicoryne Sarsii: A, an ordinary
polype, with its club-shaped body covered
with tentacles ; B, a polype putting forth
medusoid gemmae ; a, a very young bud ;
b, a bud more advanced, the quad-
rangular form of which, with the four
nuclei whence the cirrhi afterwards
spring, is shown at d ; c, a bud still
more advanced.

794

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 again
by a new growth from the stem beneath ; and this exuviation and
regeneration may take place many times in the same individual.1

It is in the families Campanulariida and tiertidariida (whose
polyparies are commonly known as 1 corallines '), that the horny
branching fabric attains its completest development, not only afford-
ing an investment to the stem, but forming cups or cells for the
protection of the polypites, as well as capsules for the reproductive
gonozooids. Both these families thus belong to the sub-order Theeaia*
In the Campanulariida the polype-cells are campanulate or bell-
shaped, and are borne at the extremities of ringed stalks (fig. 601, 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. 601). .In both the general structure of the individual polypes
(fig. 601, B, d) closely corresponds with that of the Hydra j 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 tabular 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 ' coenosarc 7
(fig. 601,/) contained in the stem and branches that new polype-
buds (b) are evolved ; these carry before them (so to speak) a portion
of the horny integument, which at first completely invests the bud ;
but as the latter acquires the organisation of a polype, the case
thins away at its most prominent part, and an opening is formed
through which the young polype protrudes itself.

The origin of the reproductive capsules or ' gonothecre ' (e) is
exactly similar, but their destination is very different. Within
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, but (like the flower-buds of a plant) expand
one after another at the mouth of the capsule, withering and drop-
ping off softer they have matured their generative products. In the
Sertulariida, on the other hand, the medusan conformation is wanting,
as the gonozooids are always fixed ; the reproductive cells (fig. 604, a)
which were shown by Professor Edward Forbes to be really meta-

1 The British Tubv.lariida form the subject of a most complete and beautiful
monognvph by Professor Allman, published by the Ray Society.

COLLECTING ZOOPHYTES

795

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 gonothecse, sometimes perhaps on
distinct polyparies — spermatozoa and ova ; and the latter are ferti-
lised'by the entrance of the former whilst still contained within
their capsules. The fertilised ova, whether produced in free or in
attached medusoids, develop themselves in the first instance into
ciliated 'gemmules," or planula?, which soon evolve themselves into
true polypes, from every one of which a new composite polypary
may spring.

There are few parts of our coast which will 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
after a storm. Many kinds,
however, can only be ob-
tained by means of the
dredge. Of the remarkable
forms dredged bv the 1 Chal-
lenger ' mention can onlv
be made here of the gigantic
Tubularian — Jlonocaulus —
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 Streptocaidus pulcherrimus, 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 Goadby's solution, Deanes gelatine,
glycerin -jelly, weak spirit, diluted glycerin, a mixture of spirit and
glycerin with sea-water, or any other menstruum, by means of

Fig. 604. — Sertulari-a cuprcssina : A, natural
size ; B, portion magnified.

.796

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 2 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 eartheirware 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
throughly 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 inedusoid Acalpphce (or ' jelly-fish '). We
ziow know that the small free-swimming medusoids belonging to

1 See Mr. J. W. Morris in Quart. Joum. of Micros. Sci. n.s. vol. ii. 18G2, p. 116.

2 Archives de Biologic, vi. p. 115.

JELLY-FISHES

797"

the 'naked-eye' group, of which Thaumcmtias (fig. 605) 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. 605. — A, Thaumaniias piloseUa, one of the ' naked-eyed ' Medusae : a a,
oral tentacles ; b, stomach ; c, gastro-vascular canals, ha-vin* the ovariesr
d d, on either side, and terminating in the marginal canal, e e. B, Thau-
mantias Eschscholtzii, Haeckel.

budded off, endowed with independent organs cf nutrition and
locomotion, whereby they become capable of maintaining their own
existence and of developing their sexual products. The general con-
formation of these organs will be understood from the accompanying

793

SPONGES AND ZOOPHYTES

figure. Many of this group are very beautiful objects for micro-
scopic 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 Medusce 1 or Aca-
lepele which are commonly known as k 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 zoophytic ; their development into Medusae
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 Rliizostoma found commonly
swimming round our coasts, and the beautiful Cltrysaora remarkable
for its long ' furbelows ' which act as organs of prehension, are oceanic
acalephs developed from very small polypites, which fix themselves
by a basal cup or disc. The embryo emerges from the cavity of its
parent, within which the first stages of its development have taken
place, in the condition of a ciliated ' planula,5 of rather oblong form,
very closely resembling an infusory 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 titba, which is the polype stage of
the ( 'hrysaora and other jelly-fish, leads in every important particular
the life of a Hydra ; propagates like it by repeated gemmation, so
that whole colonies are formed as offsets from a single stock ; and
can be multiplied like it by artificial division, each segment develop-
ing itself into a perfect Hydra. There seems to be no definite limit
to its continuance in this state, or to its power of giving origin to
new polype-buds ; but when the time comes for the development of
its sexual gonozodids, the polype quits its original condition of a
minute bell with slender tentacles (fig. (50(5), 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, a (fig. GOO, II), somewhat of the form of the
■original polype, being still left at the attached extremity. ' The
number of circles is indefinite, and all are not formed at once, new
constrictions appearing below, after the upper portions have been de-
tached ; as many as thirty or even forty have thus been produced in
bne specimen. The constrictions then gradually deepen, so as tb divide
the cylinder into a pile of saucer-like bodies, the division bein£

1 See Professor Claus, tfutersuchungen itber die Organisation ijna Eritwickelutig
der Medusen, Prague and Leipzig, 1S88.

REPRODUCTION OF ACALEPHS

799

most complete above, and 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? (tig. 607, 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
bulb which remains at the base of the pile. At last the topmost
and largest disc begins to exhibit a sort of convulsive struggle ; it

Fig. 606. — I, two Hydrce tubce (Scyj)]iisto)na-stAge) of Cyanea
rapillata, with two (a, b) undergoing fission (Strobila-st&ge).
II, a and b of fig. I three days later. In a the tentacles are
developed beneath the lowest of the Epliyrce, from the stalk
of the Strobila, which will persist as a Hydra tube. (After
Van Beneden.)

becomes detached, and swims freely away ; and the same series of
changes takes place from above downwards, until the whole pile of
discs is detached and converted into free-swimming Medusa?. But
the original polypoid body still remains, and may return to its
original polype-like mode of gemmation, becoming the progenitor of
a new colony, every member of which may in its turn bud off a pile
of Medusa discs. - t .

The bodies thus detached have all the essential characters of the
adult Medusae, Each consists of an umbrella-like disc divided at
its edge into a variable number of lobes, usually eight ; and of a

8oo

SPONGES AND ZOOPHYTES

stomach, which occupies a considerable proportion of the disc, and
projects downwards in the form of a proboscis, in the centre of which,
is the quadrangular mouth (fig. 607, 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 Medusa? are very voracious, and grow rapidly, so as to attain

Flft. 607. — Development cf Chrijsaora from Hydra tuba: A,
detached individual viewed sideways, and enlarged, showing
the proboscis a, and b the bifid lobes ; B, individual s ^en from
above, showing the bifid lobes of the margin, and lb : quadri-
lateral mouth; C, one of the bifid lobes still more enlarged,
showing the rudimentary eye (?) at the bottom of the eleft;
D, group of young MedtfsBB, as seen swimming in the water,
of the natural size.

a very large size. The Cyanece and Chrysaorce, which are common all'
round our coasts, often have a diameter of from six to fifteen inches
while Rhizostoma sometimes reaches a diameter of from two to three
feet. The quantity of solid matter, however, which their fabrics con-
tain is extremely small. It is not until adult age has been attained
that the generative organs make their appearance, in four chambers
disposed around the stomach, which are occupied by plaited membran-
ous ribands containing sperm-cells in the male and ova in the female ;
and the embryoes 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 themselves
in the first instance into hydi old polypes, from which medusoids are-
subsequently budded off".

ACTINOZOA

80 1

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 Author 2 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 generative or sexual act, whilst in the other it is
by a process of gemmation or budding. Thus the Medusae 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 Thecata as Ca mpanulariida and Sertulariida, whose Medusa -
buds are of small size and simple conformation, and not unfrequently
do not detach themselves as independent organisms ; whilst the
Medusan phase of life is the most conspicuous in the ordinary Acalephs,
their zoophytic stage being passed in such obscurity as only to be
detected by careful research. The Author's views on this subject,
which were at first strongly contested by Professor E. Forbes, and
other eminent zoologists, have now come to be generally adopted.3

Actinozoa. — Of this group the common sea-anemones may be
taken as types, constituting, with their allies, the order Zoanthariay
or helianthoid polypes, which have numerous tentacles disposed in
several rows. Next to them come the Alcyonaria, consisting of
those whose polypes, having always eight broad short tentacles,
present a star-like aspect when expanded ; as is the case with various
composite sponge- like bodies, unpossessed of any hard skeleton, which
inhabit our own shores, and also with the red coral and the 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 Gorgonice or sea-fans. A third order, Rugosa, consists of fossil
corals, whose stony polyparies 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 Zoantharia the common Actinia or ' sea anemone ' may
be taken as the type, the individual polypites of all the composite
fabrics included in the group being constructed upon the same model.0

1 See his treatise on The Alternation of Generations, a translation of which has
been published by the Ray Society.

2 Brit, and For. Med. Chir. Review, vol. i. 1848, p. 192 et seq.

3 Compare Huxley, Anatomy of Invertebrated Animals, p. 133; and Balfour,
Comparative Embryology, i. p. 151.

4 Professor Haeckel, led by the study of Ctenaria ctenophora, associates the
Ctenophora with the Hydrozoa (Sitzungsber. Jenaische Gesellschaft, May 16, 1879).

5 On the anatomy of Actinia and its allies, see O. and R. Hertwig's monograph
in vols. xiii. and xiv. of the Jenaische Zeitschrift.

3 F

802 SPONGES AND ZOOPHYTES

In by far the larger proportion of these zoophytes, the bases of the
polypites, as well as the soft flesh that connects together the members
of aggregate masses, are consolidated by calcareous deposit into stony
corals ; and the surfaces of these are beset with 'cells,' usually of a
nearly circular form, each having numerous vertical plates or lamellce,
radiating from its centre towards its circumference, which are
formed by the consolidation of the lower portions of the radiating
partitions that divide the space intervening between the stomach and
the general integument of the animal into separate chambers. This
arrangement is seen on a large scale in the Fungia or ' 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 Caryo-
phyllia, 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-hydrogen microscope. An exceedingly use-
ful method of preparing sections of corals has been devised by Dr. G.
von Koch ; the corals with all their soft parts in plate are hardened
in absolute alcohol, and then placed in a solution Of copal in chloro-
form. After thorough 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 on 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 Medusa?, as well as on other parts of their bodies. If a
tentacle of any of the sea-anemones so abundant on our coasts (the
smaller and more transparent kinds being selected in preference) be
cut off, and be subjected to gentle pressure between the two glasses
of the aquatic box or the compressorium, multitudes of little dart-
like organs will be seen to project themselves from its surface near
its tip ; and if the pressure be gradually augmented, many additional
darts will every moment come into view. Not only do these organs
present different forms in different species, but even in one and the
same individual very strongly marked diversities are shown, of
which a few examples are given in fig. 608. At A, B, C, D is
1 See ZoohgUeher Anzeiger, i. p. 86; and Proc. Zool. Soc. London, 1880, p. 24.

A LC YON ARIA

803

shown the appearance of the ' filiferous capsules,' whilst as yet the
thread lies coiled up in their interior ; and at E, F, G, H are seen
•a few of the most striking forms which they exhibit when the thread
or dart has started forth. These thread-cells are found not merely in
the tentacles and other parts of

the external

integument

of Ac-

tinozoa, but also in the long fila-
ments which lie in coils within
the chambers that surround the
stomach, in contact with the
sexual organs which are attached
to the lamellae dividing the cham-
bers. The latter sometimes con-
tain ' sperm-cells ' and sometimes
ova, the two sexes being here
-divided, not united in the same
individual. What can be the
office of the filiferous filaments
thus contained in the interior of
the body it is difficult to guess
at. They are often found to pro-
trude from rents in the external
tegument, when any violence has
been used in detaching the animal
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
^ Jpijth of an inch in length ; while
the thread or dart, in Coryyiactis
Allmanni, when fully extended,
is not less than ith of an inch
or thirty
of its capsule

Of the Alcyonaria a character-
istic example is found in the Alcy-
onium digitatum of our coasts ;
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 fingers.' When a specimen of
this is first torn from the rock to which it has attached itself, it
-contracts into an unshapely mass, whose surface presents nothing
1 See Mr. Gosse's Naturalist's Rambles on the Devonshire Coast, and Professor
Mobius, ' Ueber den Bau u.s.w. der Xesselkapseln einiger Polvpen und Quallen,' in
Abhandl. Naturw. Vereins zu Hamburg. Band v. 18C6. On the relations of stinging
cells to the nervous system, see Dr. v. Lendenfekl, Quart. Journ. Micros. Sci°
xxvii. p. 893.

3 r 2

-seven
1

^th of
times the

length

Fig. COS.— Filiferous capsules of Acti-
nozoa : A, B, Cori/nactis Allmanni;
C, E, F, Caryophi/Ui a Smitliii ; D .G,
Actinia crassicornis ; H, Actinia Can-
dida.

n.s.

804

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, 1 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 pinnae or filaments, fringing-
each margin, and arching onwards ; and with a higher power these
pinn?e 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
edges.' (Gosse.) When a mass of this sort is cut into, it is found
to be channelled out somewhat like a sponge by ramifying canals ; the
vents of which open into the stomachal cavities of the polypes, which
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

FtG. 609. — spicules of Alcyonium Fig. 010. — A, Spiculesof Gorgonia guttata ;
ana" Gorgonia. B, spicules of Muricea plongata:

their transparent bodies) by means of cilia lining the internal surfaces
of the polypes ; but no cilia can be discerned on their external sur-
faces. The tissue of this spongy polypidom is strengthened through-
out, like that of sponges, with mineral spicules (always, however, cal-
careous), which are remarkable for the elegance of their forms ; these
are disposed with great regularity around the bases of the polypes,,
and even extend part of their length upwards on their bodies. In
the Gorgonia or sea-fan, whilst the central part of the polypidom is
consolidated into a horny axis, the soft flesh which clothes this axis
is so full of tuberculated spicules, especially in its outer layer, that,
when this dries up, they form a thick yellowish or reddish incrusta-
tion upon the horny stem. This crust is, however, so friable that it
may be easily rubbed down between the fingers, and when examined
with the microscope it is found to consist of spicules of different
shapes and sizes, more or less resembling those shown in figs. 609, 610,
sometimes colourless, but sometimes of a beautiful crimson, yellow,

CTENOPHORA

805

or purple. These spicules are best seen by black -ground illumination,
especially when viewed by ihe binocular microscope. They are, of
course, to be separated from the animal substance in the same
manner as the calcareous spicules of sponges ; and they should be
mounted, like them, in Canada balsam. The spicules always
possess' an organic basis, as is proved by the fact that when their
lime is dissolved by dilute acid a gelatinous-looking residuum is left,
which preserves the form of the spicule.

The Ctenophora, or ' comb-bearers,' are so named from the comb-
like arrangement of the rows of tiny ' paddles ' by the movement of
which the bodies of these animals are propelled. A very beautiful
.and not uncommon representative of this order is furnished by the
Cydippe pileus (rig. 611), very commonly known as the Bcro'e, which
designation, however, properly appertains to another animal (fig. 612)
■of the same grade of organisation. The body of Cydippe is a nearly

Fig. CAl.—Cijdippe pileus, with Fig. 612.— Beror Forslcalu,

its tentacles extended. showing the tubular pro-

longations of the stomach.

globular mass of soft jelly, usually about ;sHhs 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 rows
of flattened filaments, far larger than ordinary cilia, but lashing the
water in the same manner ; they sometimes act quite independently
of one another, so as to give to the body every variety of motion, but
sometimes work altogether. If the sun-light should fall upon them
when they are in activity, they display very beautiful iridescent
•colours. In addition to these ' paddles ' the Cydipp* is furnished
with a pair of long tendril-like filaments, arising from the bottom of
a pair of cavities in the posterior part of the body, and furnished with
lateral branches (A) ; 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

3o6

SPONGES AND ZOOPHYTES

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 quadritid cavity bounded by four folds which seem to repre-
sent the oral proboscis of the ordinary Medusa? (fig. 606) ; 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.1 From the cavity of the stomach tubular prolongations-
pass off beneath the ciliated bands, very much as in the true Bero'e
(B). These may easily be injected with coloured liquids by the
introduction 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 Bero'e?
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
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 even by fragments of their bodies, being augmented
by agitation of the water containing them. All the C tenophora are
reproduced from egi^s. and are already quite advanced in their deve-
lopment by the time they are hatched. Long before they escape,
indeed, they swim about with great activity within the walls of their
diminutive prison, their rows of locomotive paddles early attaining
a large size, although the long flexile tentacles of Cydippe are then
only short stumpy protuberances. By (,<i/<>/>/<(>i(t and Ctenoplana
the Ctenophora appear to be allied to the Planarian Worms. 3

Those wlio iimv desire to acquire a more systematic and detailed acquaintance
with the zoophyte group may he especially referred to the following treatises and
memoirs, in addition to those alread] cited, and to the various recent systematic
treatises on zoology : -Dr. Johnston's History of British Zoophytes ; Professor Milne-
Edwards' 'Rechercb.es Bur Les Polypes,' and his 'Histoire des Corallaires' (in the
Suites ,) Buffon 1 'aris, 1857 : Pr< ifessi >r Van Beheden, ' Bur les Tubulaires ' and ' Surles.
Campanulaires,' in M&m. de VAcad. Boy.de Bruxelles, torn, xvji., and his 4 Recherehes.

sur l'Hi-t. Nat. des Polypes qui freipientent les Cotes de Belgique,' op. cit. torn.,
xxxvi. ; Sir J. G. Daly ell's Hare and llona rlcable Annuals of Scotland, vol. i. ;
Tremhlev's Mem. pour servvr & Vhistoiri d'uri gewre de "Polype d'eau donee; M.

1 It, is commonlj stated that the two branches of the alimentary canal open on
tin- Burfaee hv two pores situated in the hollow of the fringe, one on either side of the
nervous ganglion. The Author, however, has not been ahh: to satisfy himself of the
existence of such excreton pores in the ordinary Cydippe or BeroS, although he has
repeatedly in jected their whole alimentary canal and its extensions, and has atten-
tively watched the currents produced by ciliary action in the interior of the hi furcat-
ing prolongations, which currents always appear to him to return as from eaical
extremities. He is himself inclined to believe that this arrangement has reference
solely to the nutrition of the nervous ganglion and tentacular apparatus, which lies
imbedded (so to speak) in the .bifurcation of the alimentary cana l, so as to he ahh:
to draw its supply of nutriment direct from that cavity.

? On the anatomy of Beroe", see Eimer, Zoologische Stiidien ait f Capri. I.Ueber
Jieroe OVatUS, Leipzig, 1H7:>.

3 See Korotneff, Zeitsehr.f. Wis 8. Zool. xliii. p. '24'2.

CCELENTERATA

807

Hollard's ' Monographic du Genre Actinia ' in Ann. des Sci. Nat. ser. iii. torn. xv. ;
Professor Max Schultze, 'On the Male Reproductive Organs of Campanularia gehtcu-
lata ' in Quart. Journ. Micr. Sci. vol. ifl. 1855, p. 59 ; Professor F. E. Schulze's memoirs
on Gordylaphora lacustris, Leipzig, 1871, and on Syncoryne, 1878 ; Professor Agassiz's
beautiful monograph on American Medusae, forming the third volume of his Contri-
butions to the Natural History of the United States of America; Mr. Hincks'
British Hydroid Zoophytes ; Professor Allman's admirable memoirs on Cordylophora
and Myriothela in the Phil. Trans, for 1853 and 1875; Professor Lacaze-Duthiers'
Hist. Nat. du Corail, Paris, 1864, and his essays on the Development of Corals, in
vols. i. and ii. of the Archives de Zool. experimental ; Professor J. R. Greene's
Manual of the Sub-Kingdom Coslenterata, 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-
paedia. The Ctenophora are specially treated of in vol. iii. of Professor Agassiz's
Contributions to the Natural History of the United States. See also Professor
Alex. Agassiz's Sea-side 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. 848 ; Dr. D. Macdonald in
Trans. Boy. Soc. Edinb. vol. xxiii. p. 515 ; Mr. H. N. Moseley, ' On the Structure of
a Species of Millepora,' in Phil. Trans. 1877, p. 117, and 'On the Structure of the
Siylasteridce,' ibid. 1878, p. 425; and on the Acalephce, Professor Haeckel's Beitrdge
zur Natu rgeschichte der Hydromedusen ; the masterly work of the brothers Hertwig,
Das Nervensystem und die Sinnesorgane der Med usen, 1878 ; and the memoir of
Professor Schafer, ' On the Nervous System of Aurelia aurita,' in Phil. Trans. 1878,
p. 563. Of later treatises Professor Ray Lankester's article on Hydrozoa, in the 9th
edition of the Encyclopaedia Britannica; the 'Challenger' Beports of Professor
Allman on the Hydroida (Plumulariidse only), Professor Haeckel on the Medusae,
Professor Moseley on Deep-sea Corals, Dr. R. Hertwig on the Actiniaria, Professors.
E. P. Wright and Studer on the Alcyonaria, and Mr. George Brook on the Antipatharia ;
the monographs by Dr. A. Andres on Actiniae and by Dr. C. Chun on Ctenophora;,
published in the Fauna und Flora des Golfes von Neapel, should be consulted.

8o8

CHAPTER XVI

E CHINODEB MA T A

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 one's self 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-
dermata, even a general account of whose complex organisation
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 sea- side observer, brings into view an order of facts
of the highest scientific interest.

It is in the structure of that calcareous skeleton which probably
exists under some form in 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 4 spines,'
which may have the form of prickles of no great length, or may be
stout club-shaped bodies, or, again, may be very long and slender
rods. The intimate structure of the shell is everywhere the same ;
for it is composed of a net/work, 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 areola or interspaces freely communicating with each
other (figs. 613, 614). These 'areola?,' and the solid structure which
surrounds them, may bear an extremely variable proportion one to
the other ; so that in two masses of equal size the one or the other
may greatly 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. 613, or may possess a considerable degree of com-
pactness, if the solid portion be 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-

STRUCTURE OF ECHINOIDS

809

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 1 rosette ' which is contained
in the tip of every one of the tubular suckers put forth by the living
Echinus from the ' ambulacral pores 5 that are seen in the rows of

Pig. 613. — Section of shell of Echinus
showing the calcareous network of
which it is composed : a a, portions
of a deeper layer.

Fig. (514. — Trans- verse section of cen-
tral portion of spine of Heterocen-
irotus, showing its more open net-

smaller plates interposed between the larger spine-bearing plates of
its box-like shell. If the entire disc be cut of}', 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. 616.

The most beautiful display of this reticulated structure, however,
is shown in the conformation of the ' spines ' of Echinus, Cidaris, &c.
in which it is combined with solid ribs or pillars, disposed in such a
manner as to increase the strength of these organs, a regular and
•elaborate pattern being formed by their intermixture, which shows
■considerable variety in different species. When we make a thin
transverse section of almost any spine belonging to the genus
Echinus (the small spines of our British species, however, being
exceptional in this respect) or its immediate allies, we see it to be

8io

ECHIN0DER3IATA

made up of a number of concentric layers, arranged in a manner that
strongly reminds us of the concentric rings of an exogenous tree

Fig. 615. — Transverse section of spine oiJEchinometra

(fig. 615). 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.
614) ; and this is bounded
by a r ow of transparent
spaces (like those at a ofi
b b% c c', etc. fig. 617),
which on a cursory in-
spection might be sup-
posed to be void, but are
Fro. oio.— One of the segments of the calcareous found on closer examina-
skeleton oi an ambulacra! disc of Echinus. tion to be the sections of

solid ribs or pillars, which
run in the direction of the length of the spine, and form the exterior
of every layer. Their solidity becomes very obvious when we

SPINES OF ECHINOIDS

either examine a 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 tig. 617, 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 wrorks, 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 a transverse section

Fig. (il7. — Portion of transverse section of spine of Heterocentrotus

mammillatus.

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 basal ; and in any intermediate
part of the spine, so many layers will be traversed as have been
formed since the spine first attained that length. The basal portion
of the spine is enveloped in a reticulation of a very close texture,
without concentric layers, forming the cup 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 Heterocentrotus
furnish sections most remarkable for size and elaborateness, as well
as for depth of colour (in which last point, however, the deep purple
spines of Echinus lividus are pre-eminent) ; but for exquisite

$12

ECHINODERMATA

neatness of pattern there are no spines that can approach those
of Echinometra (fig. 615). The spines of Stomopneustes variolaris
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. Dalyell) these
spines are cast off and renewed every year, each new formation
thus going to make an entire spine, instead of making an addition
to that previously existing. Most curious indications are some-
times afforded by sections of Echinus-spines of an extraordinary
power of reparation inherent in these bodies. For 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

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 Cidaris 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
Echini. This is usually found to close in the spine at its tip also ;

1 See the Author's description of such reparations in the Monthly Microscopical
Journal, vol. iii. 1870, p. 225.

SPIKES ; PEDICELLAEIiE

813

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. 618,
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 1 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. 619. — Spine of Spatangus.

of Spatangus (fig. 619) 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 PediceHarw (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 Journ. Boy. Micros. Soc. 1884, p. 845.

2 A number of rare spines are described and figured by Prof. H. W. Mackintosh
in vols.xxvi. (p. 475) andxxviii. (pp. 241 and 259) of the Trans. Boy. Irish Academy.

5 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. Sc. Nat. (5), vols. xii. and xiii. ; Mr. Sladen's-

8 14

ECHINODERMATA

Another example of the same structure is found in the peculiar
iramework 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 1 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-

a

Pig. 020. — 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; a, 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.

what 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-
verse section of the tooth presents the form of a J_. This keel is
composed of cylindrical rods of carbonate of lime, having club-shaped
■extremities lying obliquely to the axis of the tooth (fig. 620, A, d) ;
these rods do not adhere very firmly together, so that it is difficult
to keep them in their places in making sections of the part. The

essay in Ann. and Mag. Nat. Hist. (5), vi.p. 101 ; and M. Foettinger's paper in vol. ii.
p. 455, of the Archives de Biologie.

1 See his memoir, ' On the Structure and Growth of the Tooth of Echinus,' in
Phil. Trans, for 1861, p. 887. See also Giesbrecht, ' Der feinere Bau der Seeigel-
zahne,' Murph. Jahrbuch, vi. p. 79.

CALCAREOUS TISSUE

8l5

•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 the tooth below, as 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. Salter ' 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 Ophiuroidea
('sand-stars ' and 'brittle stars') have the same texture as those of
the shell of Echinus. And this
presents itself, too, in the spines or
prickles of their surface when
these (as in the great Goniaster
equestris or 'knotty cushion-star')
are large enough to be furnished
with a calcareous framework. An
example of this kind, furnished by
the Astrophijton, is represented in
fig. 621. The spines with which
the arms of the species of Ophiothrix
(' brittle star ') are beset are often
remarkable for their beauty of conformation ; those of 0. penta-
phyUum, one of the most common kinds, might serve (as Professor
E. Forbes justly remarked), in point of lightness and beauty, as

Fig. 621. — Calcareous plate and claw
of Astrophijton.

8i6

ECHINODERMATA

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 Pentacrinns asterius, 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, Arc. 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
l'> ntacrini 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 Echinodermata can only be displayed by thin
sections made upon the general plan already described in Chapter VI.
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 prevent the interspaces of the network from being-
clogged by the particles abraded in the reducing process, an illus-
tration of a section cut from a spine of Echinometra is given in
fig. 615. A section of the shell, spine, or other portion of the
skeleton should first be cut with a fine saw, and be rubbed on a flat
file until it is about as thin as ordinary card, after which it should
be smoothed on one side by friction with water on a Water-of-Ayr

1 The < ( -iii-ecus skeleton even of living Echinoderms has a crystalline aggregation,
as is very obvious in the more solid spines of Echinovictra- &C. ; for it is difficult, in
sawing these across, to avoid their tendency to cleavage in the oblique plane of
calcite. And the Author is informed by Mr. Sorby that the calcareous deposit which
fills up the areola- of the fossilised skeleton has always the same crystalline system
with the skeleton itself, as is shown not merely by the uniformity of their cleavage,
but by their similar action on polarised light.

8 See figs. 74-7C> of the Author's memoir on ' Shell Structure ' in the Heport of
the British Association, 1847.

PREPARING SPIXES

8l7

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^ Xext, 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 be observable, or some minute air-bubbles should
show themselves between the glass and the under surface, it is desir-
able 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 be in the balsam
covering the part of the glass on which it is laid. The surface now
brought uppermost is then to be very carefully ground 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), and 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 ^ver the specimen, he must have con-
tinual recourse to the microscope during the latter stages of his
work ; and he should bear constantly in mind that, as the specimen
will become much more translucent when mounted in balsam and
covered with glass than it is when the ground surface is exposed, he
need not carry his reducing process so far as to produce at once the
entire translucence he aims at, the attempt to accomplish which
would involve the risk of the destruction of the specimen. In
' mounting ' the specimen liquid balsam should be employed, and
only a very gentle heat (not 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 has 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

3 G

8i8

ECHINODERMATA

glass 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 1 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 press 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
underlaid 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, Mr. 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 beauti-
ful structures, which represent, in the class Holotliurioidea the solid
calcareous skeleton of the classes already noticed. The greater
number of the animals belonging to this order are distinguished by
the flexibility and absence of firmness of their envelopes ; and ex-
cepting in the case of the various species which have a set of calcar-
eous plates, disposed around the wall of the pharynx, we do not find
among them any representation, that is apparent to the unassisted
eye, of that skeleton which constitutes so distinctive a feature of the

1 See his memoir in the Linnean Transactions, xxv. p. 365 ; see also Bell,
Journ. Boy. Micros. Soc. 1882, p. 227.

HOLOTHURIA.N SPICULES

819

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

Fig. 622. — Holothurioidea : I, Stiphopus Kefersteinii ; a, calcareous
plate of same ; b, c, calcareous plates of Holothnria vagabunda ;
d, the same of H. inhabilis ; e, the same of H. botellus ; /, of El.
partialis ; g, of H. edulis.

the substance of the skin. Various forms of the plates which thus
present themselves in Holothuria are shown in fig. 622. In the
Synapta, one of the long-bodied forms of this order, which abounds
in the Mediterranean Sea, and of which two species (the S. digitata
and S. inhcerens) occasionally occur upon our own coasts,'2 the cal-
careous plates of the integument have the regular form shown at A,

A B C

Fig. 623. — Calcareous skeleton of Si/napta : A, plate imbedded in
skin ; B, the same, with its anchor-like spine attached ; C, anchor-
like spine separated.

fig. 623 ; and each of these carries the curious anchor-like appendage
C, which is articulated to it by the notched piece at the foot, in the
manner shown (in side view) at B. The anchor-like appendages

1 For an account of a very remarkable form see Moseley ' On the Pharynx of an
unknown Holothurian, of the family Dendrochirota?, in which the calcareous skeleton
is remarkably developed,' Quart. Journ. Micros. Sri. n.s. xxiv. p. 255.

- ' On the spicules of Synapta, together with some general remarks on the archi-
tecture of Echinoderm spicules,' consult R. Semon. Mitth. Zool. Stat. Neapel, vii.
p. 272.

3 g 2

820

E C HINODE KM AT A

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 ' an-
chors,' but is furnished with very remarkable wheel-like plates.;
those represented in fig. 624 are found in the skin of Chiridota
violacea, & species inhabiting the western parts 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 figures 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 class
has some kind of calcareous skeleton disposed in a manner conform-
able to the examples now cited ; and it is now generally acknow-
ledged that the marked peculiarities by 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

Fig. 624.-Wheel-like plates from skin of these sections, when dry, are
Chiridota viola ecu. most advantageously mounted

in the same medium, by which
their transparence is greatly increased. All the objects of this class
are most beautifully displayed by the black-ground illumination, and
their solid forms are seen with increased effect under the binocular.
The black-ground illumination applied to very thin sections of Echinus
spines brings out some effects of marvellous beauty ; and even in these
the solid form of the network connecting the pillars is better seen
with the 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 knowledge of which we are chiefly in-
debted to the painstaking and widely extended investigations of
Professor J. Midler.3 All that our limits permit is a notice of two of
the most curious forms of these larv a' by way of sample of the won-

1 No systematic account of a species of Holothurian can be regarded as complete
which does not contain an account of the form of its spicules, when these are present.
Figures of various forms will he found in Professor Kemper's Reise.n im Archipel tier
Philippine u : Holot Itii rieu, Dr. Theel's ' Challenger ' Reports, and the memoirs of
Professors Bell, Ludvvig, and Selenka.

2 It may be here pointed out that the reticulated appearance is sometimes de-
ceptive, what seems to be solid network being in many instances a hollow network
of passages channelled out in a solid calcareous substance. Between these two con-
ditions, in which the relation between the solid framework and the intervening space
is completely reversed, there is every intermediate gradation.

5 Of later works consult especially the ' Selections from Embryological Mono-
graphs, ii. Echinodermata,' edited by Mr. A. Agassi/,, in vol. ix. of the Memoirs of the
Museum of Comparative Zoology

LARVAL ECHINODERMS

821

derful phenomena which his 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 zooids have, by
secondary adaptations to their mode of life, acquired a type quite
different from that which characterises the adults ; for instead of a
radial symmetry they exhibit a bilateral, the two sides being pre-
cisely alike, and each having a ciliated fringe along the greater part
or the whole of its leno-th. The

two fringes are

united

by

a

superior and an inferior trans-
verse ciliated band, and be-
tween these two the mouth of
the zooid is always situated.
The external forms of these
larva3, however, vary in a most
remarkable degree, owing to the
unequal evolution of their dif-
ferent parts : and there is also
a considerable diversity in the
several orders as to the propor-
tion of the fabric of the larva
which enters into the compo-
sition of the adult form. When
the young begins to acquire the
characters of the fully developed
star-fish and sea-urchin, the
parts which are not retained
shrivel up, and their substance
goes to feed the young form.

One of the most remarkable
forms of Echinoderm larva? is
that which has received the
name of Bvpinnaria (tig. 625),
from the symmetrical arrange-
ment of its natatory organs. The mouth (a), which opens in the
middle of a transverse furrow, leads through an oesophagus, a', to a
large stomach, around which the body of a star-tish is developing
itself ; and on one side of this mouth are observed the intestinal
tube and anus (6). On either side of the anterior portion of the
body are six or more narrow tin-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-tish, 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-tish ; for the ceso-

Fig. 625. — Bipinnaria asterigera, or larva
of star-fish: a, month; a', oesophagus ; b,
intestinal tube and anal orifice ; c, furrow
in which the mouth is situated; dd\ bi-
lobed peduncle ; 1, 2, 3, 4, 5, 6, 7, ciliated
arms.

822

ECHINODERMATA

phagus of the latter 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 Bipinnarian larva by the forcible contractions
of the connecting stalk, as soon as the calcareous consolidation of its
integument has taken place and its true mouth has been formed, but
long before it has attained the adult condition ; and as its ulterior
development has not hitherto been observed in any instance, it is not
yet known what are the 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 Asieroidea,
there is no internal calcareous framework ; such a framework, how-
ever, is found in the larvae of the Eehinoidea and Ophiuroidea, of
which the form delineated in fig. 626 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 hours after
its emersion the embryo changes from the spherical into a sub-
pyramidal form with a flattened base ; and in the centre of this
base is a depression, which gradually deepens, so as to form a mouth
that communicates with a cavity in the interior of the body which
is surrounded by a portion of the yolk-mass that has returned to the
liquid granular state. Subsequently a short intestinal tube is found,
with an anal orifice opening on one side of the body. The pyramid
is at first triangular, but it afterwards becomes quadrangular ; and
the angles are greatly prolonged round the mouth (or base), whilst
the apex of the pyramid is somH hues much extended in the opposite
direction, but is sometimes rounded off into a kind of dome (fig.
626, A). All parts of this curious body, and especially its most
projecting portions, are strengthened by a framework of thread-like
calcareous rods (e). In this condition the embryo swims freely
through the water, being propelled by the action of the cilia, which
clothe the four angles of the pyramid and its projecting arms, and
which are sometimes thickly set upon two or four projecting lobes
(/) ; and it has received the designation of PI ulcus. The mouth is.
usually surrounded by a sort of proboscis, the angles of which are
prolonged into four slender processes (g, </, g, g), shorter than the four
outer legs, but furnished with a similar calcareous framework.

The first indication of the production of the young Echinus from
its ' pluteus ' is given by the formation of a circular disc (fig. 626.
A, c) on one side of the central stomach (b) j and this disc soon
presents five prominent tubercles (B), which subsequently become
elongated into tubular processes, which will form the 'sucking-

LARVAL ECHINI

823

A

feet ' of the adult. The disc gradually extends itself over the stomach,
and between its tubules the rudiments of spines are seen to protrude
(D) : these, with the tubules, increase in length, so as to project against
the envelope of the pluteus, and to push themselves through it; whilst,
at the same time, the original angular appendages of the pluteus
diminish in size, the
ciliary movement be-
comes less active, being
superseded by the action iJ^^fAi
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-
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
augments in size, multiplies the number of its plates, cirrhi, and

Fig. 626. — Embryonic development of Echinus: 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 ;
(I, d, d, d, four arms of the pluteus-bcdy ; e, cal-
careous framework ; /, ciliated lobes ; g, g, g, g,
ciliated processes of the proboscis ; B, disc with
the first indication of the sucking-feet; C, disc,
with the origin of the spines between the tubular
sucking-feet ; D, more advanced disc, with the feet,
g, and spines, x, projecting considerably from the
surface. (X.B. — In B, C, and D, the Pluteus is not
represented, its parts having undergone no change,
save in becoming relatively smaller.)

824

ECHINODEEMATA

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 Echinoclerm types, with references to the principal memoirs
which treat of it, will be found in Chapter XX. of Mr. Balfour's
' Comparative Embryology.' In collecting the free-swimming larvae
of Echinoclerm a ta the stick-net should be carefully employed in the
manner already described, and the search for them is of course
most likely to be successful in those localities in which the adult
forms of the respective species abound, and on warm calm days, in
which they seem to come to the surface in the greatest numbers.
The following mode of preparing and mounting them has been kindly
communicated to the Author by Mr. Percy Sladen : — ' For killing
and preserving echinoderm zooids, I have come to prefer either osmic
acid or the picro-sulphuric mixture of Kleinenberg of one-third
strength. The latter, of course, destroys all calcareous structures ;
but the soft parts are preserved in a wonderful manner. If the
diluted Kleinenberg's mixture is used, let the zooids remain in it for
one or two hours ; then wash them thoroughly in 70 per cent, spirit,
until all trace of acid is removed ; 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
zooids in a watch-glass of sea-water, and add a drop of the 1-per-cent.
solution. They should not remain even in this weak solution for
more than a minute, and should then be thoroughly washed in a
superabundance of 35 per cent, spirit, to prevent the deposit of
crystals of salt consequent on the action of the osmic acid. Then
transfer the specimens to 70 per cent, spirit, and proceed as in the
other case.'

One of the most interesting to the microscopist of all Echino-
dermata is the Antedon2 (more generally known as Cornatula), or
'feather-star' (tig. 627), which is the commonest existing representa-
tive of the great fossil scries of Crinoidea, 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. 628 ; 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-
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 ahoral (under) side of the central disc (fig. 627) ;

1 An abbreviated development, in which there is no free-swimming larva, is now
known to be more common than was once supposed: among Holothurians Cucu-
maria crocea, among Ophiuroids Ophiacantha vivypara, and among Ecliinoids
Hcmiaster cavernosas may be cited as examples.

2 See the Author's ' [Researches on the Structure, Physiology, and Develoj >ni

of Antedon rosaccus,' Part I. in Phil. Trans. 18(50, p. 071."

ANTEDON

825

so that, DOtwithstandingits locomotive power, it is nearly as station-
ary in its free adult condition as it is in its earlier pentacrinoid
stage. The pentacrinoid larva1 — 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
viewed by a strong incident light (fig. 628, 3); and a series of specimens
in different stages of development shows most curious modifications
in the form and arrangement of the various component pieces
of its calcareous skeleton. In its earliest stage (fig. 628, 1) the
body is inclosed in a
oalyx composed of
two circles of plates,
namely, five basals,
forming a sort of
pyramid whose apex
points downwards, and
is attached to the
highest joint of the
stem ; and five orals
superposed on these,
forming when closed
a like pyramid whose
apex points upwards,
but usually separating
to give passage to the
tentacles, of which a
circlet surrounds the
mouth. In this con-
dition there is no
rudiment of arms. In
the more advanced
stage shown at 2,

the arms have begun to make their appearance, and the skeleton
when carefully examined is found to consist of the following pieces,
as shown in fig. 628, 1, b, b, the circlet of basals supported on the
top of the stem ; r\ the circle of first radiate, now interposed between
the basals and the orals, and alternating with both ; between two
of these is interposed the single anal plate a ; whilst they support
the second and the third radial s (r2, r3), from the latter of which
the bifurcating arms spring ; finally, between the second radials we
see the five orals lifted from the basals on which they originally
rested by the interposition of the first radials. In the more advanced
stage shown in fig. 628, 3, we find the highest joint of the stem
beginning to enlarge, to form the centro-dors<d plate (2, c d), from
which are beginning to spring the dorsal cirrhi (cir) that serve to
anchor the animal when it drops from the stem ; this supports the

i The pentacrinoid larva? of Antedon have been found abundantly 1 attached to
sea-weeds and zoophytes) at Millport, on the Clyde, and in Lamlash Bay, Arran ; m
Kirkwall Bay, Orkne'y ; in Lough Strangford, near Belfast, and m the Bay of Cork ;
and at Ilfracombe and in Salconibe Bay, Devon.

Fig. G27. — Antedon (Comatula), or feather-star,
seen from its aboral side.

ECHDsODERMATA

Fig. 628. — Penta< rinoid larvaol Antedon. 1. Skeleton of early pentacrinoid,
under black-ground ill iim i nation , showing its componeni plates: b, b,
basals, articulated below to tlit: highest point of the stem; r1, r1, first
radials, between two of which i* seen the hingle anal plate, a ; T2, second
radials ; r5, third radials, giving off the hifurcating arms at their summit;
o, o, orals. 2, 8. Back and front views of a more advanced pcntacrinoid,
as seen by incident light, one of the pair of arms being cut away in fig. i\
in order to bring the mouth and its surrounding parts into view : I), h,
basals ; r', r-, r*, first, second, and third radials; a, anal, now carried
upwards by the projection of the vent, V, <>, 0, orals; cir, dorsal cirrhi>
developed from the highest joint of the stem.

ANTEDON

827

basah', on which rest the first radials (rl) ; whilst the anal plate is
now lifted nearly to the level of the second radials (r-) by the
development of the anal funnel or vent to which it is attached. The
oral plates 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, 0), 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 centro-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 zooid ' or pseudembryo, which was.
first observed by Busch, and has been since carefully studied by
Professors Wyville Thomson 1 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 sea-weed (very
commonly Laminaria), 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 rosaceus' in Phil. Trans, for 1865, p. 518.

2 Archiv f. Mikrosk. Anat. Bd. xii. p. 583.

5 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 Boy at Society for
1876, p. 211, and in a subsequent note, p. 451. Of the further contributions recently
made to our knowledge of it the memoir of Dr. H Ludwig ' Zur Anatomie der
Crinoideen ' (Leipzig, 1877), forming part of his Morphologische Studien an Echino-
dermen, 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.

828

CHAPTER XVII

POLYZOA AND TUNICATA

As in previous editions of this work the Author followed the pre-
valent 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, tinally, 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
groups are found on almost every coast, and are most interesting
objects for anatomical examination, as well as for observation in the
living state.

Polyzoa. — The group which is known under this name to British
naturalists (corresponding with that which by Continental zoologists
is designated Bryozoa) was formerly ranked as an order of zoophytes,
and it has been entirely by microscopic study that its comparatively
high organisation has been ascertained. The animals of the Polyzoa,
in consequence of their universal tendency to multiplication 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 4 polypidom ' of a zoophyte, which has
been appropriately designated the polyzoary. The individual cells
of the polyzoary are sometimes only connected with each other by
their common relation to a creeping stem or stolon^ as in Lagun*
cula (fig. 029) ; but more frequently they bud forth directly, one from
another, and extend themselves in different directions over plane
surfaces, as is the case with Fhi8trcei Lepralice, &c. (fig. 630) ; whilst
not unfrequently the polyzoary develops itself into an arborescent
structure (fig. 031), which may even present somewhat of the density
and massiveness of the stony corals. Each individual is composed
externally of a sort of sac, of which the outer or tegumentary layer
is either simply membranous, or is horny, or in some instances
calcified, so as to form the cell ; this investing sac is lined by a more
delicate membrane, which closes its orifice, and which then becomes
continuous with the wall of the alimentary canal ; this lies freely in
the visceral sac, floating (as it were) in the liquid which it contains.

The principal features in tin? structure of this group will be hcst
understood from the examination of a characteristic example, such as
the Layimcula repem, which is shown in the state of expansion at A,
fig. 629, and in the state of contraction at B and C. The mouth is

POLYZOA

829

surrounded by a circle of tubular tentacles, which are clothed by
vibratile cilia ; these tentacles, in the species we are considering,
vary from ten to twelve in number, but in some other instances they
are more numerous. By
the ciliary investment
of the tentacles the
Polyzoa are at once dis-
tinguishable from those
hydroid polypes to
which they bear a
superficial resemblance,
and with which they
were at one time con-
founded ; and accord-
ingly, whilst still ranked
among zoophytes, they
were characterised as
ciliobrachiate. The ten-
tacula are seated upon
an annular disc, which
is termed the lopho-
pltore, 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
D, fig. 630, representing
a portion of the ten-
tacular circle on a
larger scale, a a being
the tentacula, b h their
internal canals, c the
muscles of the tentacula,
d the lophophore, and e
its retractile muscles.
The mouth situated in
the centre of the lopho-
phore, as shown at A,
leads to a funnel-shaped
cavity or pharynx, b,
which is separated from
the oesophagus, d, by a
valve at c ; and this
oesophagus opens into
the stomach, wrhich
occupies a considerable part of the visceral cavity. (In the Bawer-

1 This communication between the tentacular and visceral cavities is denied by
Dr. Vigelius, who has recently made a careful search for it.

Fig. 629.— Structure of Laguncula repenH{X&\\ Bene-
den). A, polypide expanded ; B, polypide retracted ;
C, another view of the same, with the visceral
apparatus in outline, that the manner in which it
is doubled on itself, with the tentacular crown and
muscular svstem, may be more distinctly seen :
a a, tentacula; b, pharynx; c, pharyngeal valve;
d, oesophagus; e, stomach; /, its pyloric orifice;
<7, cilia on its inner surface ; //, biliary follicles lodged
in its wall; f, intestine; k, particles of excremen-
titious matter ; 1, anal orifice; TO, testis ; n, ovary;
0, ova lying loose in the perivisceral cavity ; p, out-
let for their discharge ; q, spermatozoa in the peri-
visceral cavity ; r, s, t, u, v, iv, x, muscles. D, por-
tion of the lophophore more enlarged: a a, tenta-
cula ; b b, their internal canals ; c, their muscles ;
(J, lophophore ; e, its retractor muscles.

POLYZOA AND TUNICATA

bankia and some other Polyzoa a muscular stomach or gizzard for the
trituration of the food intervenes between the oesophagus and the
true digestive stomach.) The walls of the stomach, It, 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, g ;
.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 (i)
opens, by a pyloric orifice,/', which is furnished with a regular valve ;
within the intestine are seen at k particles of excrementitious matter
which are discharged by the anal orifice at I. No special 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-
ally flowing over them.

The production of
gemmae 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 stmi or ' stolon,' where the cells are distinct
one from the other, as in L<u/iincul(t. Tn the latter case 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 zooid, 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 shapes itself into the
alimentary canal with its tentacular appendages. Of the produc-
tion of gemmre from the polypides themselves the best examples are
furnished by the Flustr.ce and their allies. From a single cell of the
Flustra five such buds may be sent off', which develop themselves

Pig. (>;i0. — Cells of Polyzoa : A, Masttgopliora Hyiirf-
mawni\ B, Cribrilina figularis] C, Umbouula
verrucosa.

POLYZOA

83 1

into new polypides around it ; and these in their turn produce buds
iroin 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,
liaving escaped from this into the visceral cavity, as at 0, are fer-
tilised by the spermatozoa which they there meet with, and are
finally discharged by an outlet at p, 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, it, v, 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 Seria-
laria having a branching polyzoary that spreads itself on sea-weeds
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,
but also by plexuses of nerve-fibres, which may be distinctly made

1 For further details consult Haddon ' On Budding in Polyzoa,' Quart. Journ.
Micros. Sci. xxiii. p. 516.

- See his memoir in Wiegmami's Archiv, 1860, p. 311, translated in Quart. Journ.
of Micros. Sci. n.s. vol. i. 1861, p. 300 ; Rev. T. Hincks' ' Note on the Movements of
the Vibracula in Caberea bori/i, and on the supposed common Nervous System in
the Polyzoa,' Quart. Journ. Micros. Sci. xviii. p. 7.

832

POLYZOA AND TUNICATA

out with the aid of chromic acid in the cylindrical joints of the poly-
zoary. His views, however, have not been universally accepted,
observers of great histological experience maintaining that what he
regards as nerve-fibres are only connective tissue.

Of all the Polvzoa of our own coasts the Membra n iporidce, or
'sea-mats' (Flustra, 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, on both sides, and it was calculated by Dr. Grant that as
a single square inch of an ordinary Flustra contains 1,800 such cells,
and as an average specimen presents about ten square inches of
surface, it will consist of no fewer than 18,000 polypides. The want
of transparence in the cell- wall, 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 Bowerbankia, a polyzoon
with a trailing stem and separated cells like those of Laguncula, which
is very commonly found clustering around the base of masses of
Flustrse. 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 18-">7, and subjected it to a far more
minute examination than any polyzoon had previously received ;'
and it is one of the best adapted of all the marine forms yet known
for the display of the beauties and wonders of this type of organisa-
tion. The Alcyonidium, however, is one of the most remarkable of
all the marine forms for the comparatively large size of the tentacular
crowns, these, when expanded, being very distinctly visible to the
naked eye, and presenting a spectacle of the greatest beauty when
viewed under a sufficient magnifying power. The polyzoary of this
genus has a spongy aspecl and texture, very much resembling that of
certain Alcvonian 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 film, are in striking contrast with the larger
solid-looking polypes of the Alcyonium. The opacity of the polyzoa ry
of the Alcyonidium renders it quite unsuitable for the examination of
anything more than the tentacular crown and the (esophagus which
it surmount s, the stomach and the remainder of the visceral appa-
ratus being always retained within the cell. It furnishes, however,
a most beautiful object for the binocular microscope, when mounted
with all its polypides expanded.'- Several of the fresh -water Polyzoa
are peculiarly interesting subjects for microscopic examination, alike

1 See his memoir ' On the Minute Structure of Home of the Higher Forma of
Polypi,' in the Phil. Trana. for ls;57, p. 887.

2 Mr. J. Lamas has recently detected calcareous spicules in Alegonidiii m geloti-
nosnm, and finds that they are more abundant in older than in younger colonies. See
Proceedings of the Liverpool Geological Society, v. p. '241.

GEOUPS OF POLYZOA

*33

on 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 horse-shoe -shaped lophophore. By this peculiarity the
fresh-water Polyzoa are distinguished from the marine ; and they,
with -the marine Eltabdopleura, may be further distinguished by the
possession of an epistome, or movable process above the mouth,
whence Professor Allman calls them the Pkylactolcemata, as com-
pared with the others which are Gymnolcemata, or have no epistome.
The cells of the Pkylactolcemata are for the most part lodged in a
sort of gelatinous sub-stratum which spreads over the leaves of
aquatic plants, sometimes forming masses of considerable size ; but
in the very curious and beautiful Cristate!! a the polyzoary 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 either opens outside (Ectoprocta) or
within (Entoprocta) the circlet of tentacles ; the former comprise
three groups : — I. CJieilostomata, in which the mouth of the cell is
sub -ter mined, or not quite at its extremity (fig. 630), is somewhat
crescentic in form, and is furnished with a movable (generally mem-
branous) lip, which closes it when the animal retreats. This includes
a large part of the species that most abound on our own coast, not-
withstanding their wide differences in form and habit. Thus the
polyzoaries of some (as Fiustra) are horny and flexible, whilst those
of others (as Eschara and Retepora) are so j:>enetrated with calcareous
matter as to be quite rigid ; some grow as independent plant-like
structures (as Bugida and Gemellaria), whilst others, having a like
arborescent form, creep over the surfaces of rocks or stones (as
Hippothoa) ; and others, again, have their cells in close apposition,
and form crusts which possess no definite figure (as is the case with
LepraUa and Membranipora). 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.
631 ). This includes a comparatively small number of genera, of which
Crisia and Tubulipora contain the largest proportion of the species
that occur on our own coasts. III. The distinguishing character of
the third order, Ctenostomata, 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 itr
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 Fa rr ell a and Boiuerbankia),
sometimes fleshy and coalescent (as in Alcyonidium). IV. In the
Entoprocta, which are represented by Loxosoma and Pedicel Una,
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's beautiful Monograph of the British Fresh-water Polyzoa,
published by the Ray Society, 1857 ; and J. Jullien, ' Monograph ie des Bryozoaires
d'eau douce,'' Bull. Soc. Zool. de France, x. p. 91.

3 ii

834

POLYZOA AND TUNICATA

that of VorticeUa (fig. 537). As the Polyzoa 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, avieidaria and
vibracula. The avicularia or 'bird's head processes/ so named from
the striking resemblance they present to the head and jaws of a bird

(fig. 631, 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 Bugula and
BiceUaria, where they are
present at all, they are
' pedunculate/ or mounted
on foot-stalks (B). Under
one form or the other, they
are wanting in but few of
the genera belonging to
this order ; and their pre-
sence or absence furnishes
valuable characters for the
discrimination of species.
Each avicularium has two
' mandibles/ of which one
is fixed, like the upper jaw
of a bird, the other mov-
able, like its lower jaw ; the
latter is opened and closed
by two sets of muscles
which are seen in the interior of the 'head,' .and between them is a
peculiar body, furnished with a pencil of bristles, which is probably a

Fig. 681. — A, portion of BiceUaria ciliata, en-
larged ; B, one of the ' bird's-head ' processes of
Bugula avicularia, more highly magnified, and
seen in the act of grasping another.

tactile

organ, being

brought

forwards when the mouth is open, so

1 For a more detailed account of the structure and classification of the marine
Polyzoa see Professor Van Beneden's ' Recherches sur les Bryozoaires de la cote
d'Ostende' in M6m. de UAcud. Boy, de Bruxelles, torn. xvii. ; Mr. G. Busk's
Catalogue of the Marine Polgzoa in the Collection of the British Museum ; Mr.
Hincks' British Marine Polgzoa, 18H0 ; and Nitsche, ' Beitriige zur Kenntniss dor
Bryozoen,' in Zeitschrift f. Wiss. Zool. Bde. xx. xxi. xxiv. Of the more important
recent publications we may note Mr. Busk's Reports on the Polyzoa of the Challenger
voyage; Mr. Harmer, ' On the Structure and Development of Loxoeoma' and 'On
the Life-history of Pedicellina' in vols. xxv. and xxvi. of the Quart. Journ. of
Micros. Sci.; J. Barrois, 'Recherches sur l'Emhryologie des Bryozoaires,' Lille, 1H77,
and other memoirs ; AV. J Vigelius, ' Morphologische [Jntersuchungen iiber Flustra
Membranaceo-truncata,' Biolog. Cent ralblatt, iii. p. 705, and Bijdragen tot de
Dierkunde, xi. For a general account see Professor Ray Lankester's article ' Polyzoa,'
in the 9th edition of the Encgclo^cedia Britannica.

AVICULARIA AND VIBKACI LA

»35

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 portion of the polyzoary of
Bugula 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 Pedicellaria? 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 vibracula, which are long bristle-shaped organs (fig. 630,
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
slowly 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. Out of 191 species of Cheilostomatous
Polyzoa described by Mr. Busk, no fewer than 126 are furnished
either with avicularia, or with vibracula, or with both these organs.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 Chordata. 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 inclosureof their bodies in a ' tunic,'
which is sometimes leathery or even cartilaginous in its texture, and
which sometimes includes calcareous spicules, whose forms are often
very beautiful. They are often found to resemble the Polyzoa in
their tendency to produce composite structures by gemmation ; but in
their habits they are for the most part very inactive, exhibiting
scarcely anything comparable to those rapid movements of expansion
and retraction which it is so interesting to watch among the Pol vz< >,t :
whilst, with the exception of the Salpidaz and other floating species
which are chiefly found in seas warmer than those that surround our
coast, and the curious Appendicularia to be presently noticed, they
are rooted to one spot during all but the earliest period of their lives.
The larger forms of the Ascidian group, which constitutes the bulk
of the class, are always solitary ; not propagating by gemmation,

1 See Mr. G. Busk's ' Remarks on the Structure and Function of the Avicularian
and Vibracular Organs of Polyzoa' in Tntiis. Micros. Soc. ser. ii. vol. ii. 1854,
p. 26 ; and Mr. A. W. Waters, ' On the use of the Avicularian Mandible in the deter-
mination of the Cheilostomatous Bryozoa,' Jouru. Hoy. Micros. Soc. (2), v. p. 774.

3 11 2

836

POLYZOA AND TUNICATA

except in the case of the Clavelinida?. Although of special importance
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. 632 B and 634 ; 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 Polyzoa
affords a more beautiful spectacle. It is peculiarly remarkable in one
species that occurs on our own coasts, the Corella parallel oar amma,1
in which the wall of the branchial sac is divided into a number of
areola?, 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 sys-
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 twor
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 pause
occurs, and the first course is resumed. The length of time inter-
vening between the changes does not seem by any means constant.
It is usually stated at from half a minute to two minutes in the com-
posite forms ; but in the solitary Corella parallelogra/mmar(&, species,
very common in Lamlasb Bay, Arran), the Author has repeatedly
observed an interval of from five to fifteen minutes, and in some
instances he has seen the circulation go on for half an hour, or even
longer, without change — always, however, reversing at last.

The compound Ascidians are very commonly found adherent to-
sea- weeds, 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 enables their structure
to be clearly discerned without dissection, but allows many of their
living actions to be watched. Of these we have a characteristic
example in Amaroucium proliferum, of which the form of the com-

T See Alder in Ann. of Nat. Hist. ser. iii. vol. xi. 1803, p. 157; and Hancock in

Jouru. Linn. Soc. ix. p. 333.

TUNICATA

«37

posite mass and the anatomy of a single
ng. 632. Its clusters appear almost com
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 to enter bv one set and to be
ejected by the other, indicating that
all the machinery of active life is
going on within these apathetic
bodies. In the family Polyclvaida
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
of the thorax is seen the oral oritice,
c, which leads to the branchial sac e ;
this is perforated by an immense
number of slits, which allow part of
the water to pass into the space
between the branchial sac and the
muscular mantle. At k is seen the

individual are displayed in
pletely inanimate, exhibiting

c

•'. .

if Jx^1

v....

\

Fig. 632. — Compound mass of Amaroucium proliferum with the anatomy of a
single zooid : A, thorax; B, abdomen; C, post-abdomen; c, oral orifice;
e, branchial sac ; /, thoracic blood-vessel ; /, atriopore ; i', projection over-
hanging it; j, nervous ganglion; k, oesophagus; /, stomach surrounded by
digestive tubuli ; m, intestine ; n, anus opening into the cloaca formed by
the mantle ; o, heart ; o', pericardium ; j?, ovarium ; p', egg ready to escape ;
q, testis ; r, spermatic canal ; r', termination of this canal in the cloaca.

oesophagus, which is continuous with the lower part of the pha-
ryngeal cavity ; this leads to the stomach, I, which is surrounded
by glandular follicles; and from this passes off the intestine, m, which
terminates at n in the vent. A current of water is continually

833

POLYZOA AND TUNICATA

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 pharyngeal sac,
passes through the alimentary canal, giving up any nutritive
materials it may contain, and carrying away with it any excre-
mentitious matter to be discharged ; and this having met the
respiratory current in the cloaca, the two mingled currents pass forth
together by the atrial orifice, i. The long post-abdomen is principally
occupied by the large ovarium,^, which contain 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 influence from the testis, q, which
discharges its products by the long spermatic canal, r, that opens into
the cloaca, /•'. At the very bottom of the post-abdomen we find the
heart, o, inclosed in its pericardium, o' . In the group we are now
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 bythe side of the intestine in the
abdominal portion of the body. Tin1 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 Botryllians, whose
beautiful stellate gelal [nous inerusl ations are extremely common upon
sea-weeds and submerged rocks (fig. \\'.Y.\). The anatomy of these
animals is very similar to that of the A mnj-oKciiuti already described ;
with this exception, that the body exhibits no distinction of cavities,
all the organs being brought together in one, which must be con-
sidered as thoracic. In this respect there is an evident approximation
towards the solitary species.1

This approximation is still closer, however, in the ' social ' Asci-
dians, or Cldvettinidce, 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 i)ionj special information respecting the com pound Ascidians see' espe-
cially the admirable monograph of Professor Milne- I'M wards on that group; M r. Lister's
memoir, ' On the Structure and Functions of Tubular and Cellular Polypi, and of
Ascidiae,' in the Phil. Trans. 1884 ; and thearticde ' Tunicata,' by Professor T. Rupert
Jones, in the Cijclopccdia of Anatomy and Fhi/niolof/i/

TUNIC AT A

839

pides of Laguncula, the chief difference being that a regular cir-
culation takes place through the stolon in the one case, such as has
no existence in the other. A better opportunity of studying the
living actions of the Ascidians can scarcely be found, than that which
is afforded by the genus Perojihora, first discovered by Mr. Lister,
which occurs not unfrequently on the south coast of England and in
the Irish Sea, living attached to sea-weeds, 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

A

Fig. 633. — Botryllus violaceus : A, cluster on the surface of a Fucus;
B, portion of the same enlarged.

into the space between the sac and the mantle, and is thus dis-
charged immediately by the atrial funnel. Whatever little particles,
animate or inanimate, the current of water brings flow into the
sac unless stopped at its entrance by the tentacles, which do not
appear fastidious. The particles which are admitted usually lodge
somewhere on the sides of the sac, and then travel horizontally until
they arrive at that part of it down which the 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. Iii 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

840

POLYZOA AND TUXICATA

any group contains two distinct canals, which send off branches
into each peduncle. One of these branches terminates in the heart,
which is nothing more than a contractile dilatation of the principal
trunk ; this trunk subdivides into vessels (or rather sinuses, which are
mere channels not having proper walls of their own), of which some
ramify over the respiratory sac, branching off at each of the passages
between the oval slits, whilst others are first distributed to the
stomach and intestine, and to the soft surface of the mantle. All

these 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 is
interrupted by a ligature ;
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
segmentation of the yolk,
whereby a ' mulberry mass '
is produced, a sort of ring
is seen encircling its central
portion ; but this soon
shows itself as a tapering •
tail-like prolongation from
one side of the yolk, which
gradually becomes more
and more detached from
it, save at the part from
which it springs. Either

Fig. 634. — Diagrammatic longitudinal section of
Ascidiu show ing the heart, the blood- vessels,
the branchial sac, the alimentary canal, &c.
from the left side : br.si., branchial siphon;
at.si., atrial siphon ; t., test ; m,t mantle ;
br.s., branchial sac; p.br., peribronchial
cavity ; cl., cloaca ; n.g., nerve ganglion ;
fit., tentacle; gl., neural gland; tie. a., oeso-
phageal aperture ; at., stomach ; /., intestine ;
r., rectum; a., anus; ov, genital organs;
g.d., genital ducts; h., heart; cap., cardio-
splanchnic vessel; v.t., vessel to the test;
t.k., terminal knob on vessel in test ; v.t'.,
vessel from the test; v.st., vessel to the
stomach &c. ; v.w., vessel to the mantle;
v.m'., vessel from the mantle ; d.v., dorsal
vessel ; tr., transverse vessel of branchial
sac ; l.v., fine longitudinal vessel of branchial
sac ; sg., stigmata of branchial sac ; v.v.,
ventral vessel ; br.c, branchio-cardiac
vessel ; sp.br., splanchno-branchial vessel.
(After Herdman.)

whilst the egg is still within

the cloaca, or soon after it has escaped from the vent, its envelope
bursts, and the larva escapes, and in this condition it presents very

TUNICATA

84 r

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 amass 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
heart 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 exception of some species of
Molgula.1

This larval condition is represented in a very curious adult free
swimming form, termed Appendiculairiaz which is frequently to be
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 the body
differs greatly 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 Ascidians derived a new interest and im-
portance from the discovery, made by Kowalevsky in 1867, that their free-swimming
larva? 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 Ampliioxus, the lowest vertebrate at present known)
affording strong reason for belief in the derivation of the vertebrate and tunicate
tvpes from a common original. See his memoir ' Entwickelungsgeschichte der
e'infachen Ascidien ' in Mem. St. Petersb. Acad. Set. torn. x. 1807, and the abstract
of it in Quart. Journ. Micros. Sci. x. n.s. 1870, p. 59 ; also Professor Haeckel's History
of Creation, ii. pp. 152, 200. Further information will be found in chap. 11. of vol.
ii. of the late Professor Balfour's Comparative Embryology, and an application of the
facts of development to the philosophy of the subject in Professor Ray Lankester's
Degeneration (London, 1880J.

842 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 Professor Huxley, however, it appears that
the direction of this current is by no means constant • since, although
it usually enters by the mouth and passes out by the ciliated canals,
it sometimes enters by the latter and passes out by the former. The
caudal appendage has a central axis (notochord), above and below
which is a riband-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 Midler, Huxley,
Leuckart, and Gegenbaur, neither of these excellent observers has
met with this appendage ; but it has been since seen by Professor
Allman, who describes it as an egg-shaped gelatinous mass, in which
the body is imbedded, the tail alone being free ; whilst from either
side of the central plane there radiates a kind of double fan, which
seems to be formed by a semicircular membranous lamina folded upon
itself. It is surmised by Professor Allman, with much probability,
that this curious appendage is ' nid amenta!,' having reference to the
development and protection of the young ; but on this point further
observations are much needed ; and any mieroscopist 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. Joum. of Micros. Sci. vol. iv. 185(5, p. 181 ; also-
Allman in the same journal, \ <>1. vii. is.")'.), p. !S(> ; Gegenbaur in SieboUl and Kolliker's
Zeitsch/rift, Bd. vi. 185."), p. km;; Leuckart's Zoologische Untersuchungen, Heft. ii.
1851; and Fol's ' Etudes sur les Appendiculaires ' in Archiv. zool. exptr. torn, i.
1872, p. 57. For the Tunicata generally, see Professor T. Rupert Jones in vol. iv. of the
Cyclop, of Anatomy and Physiology ; Professor Herdman's recently published article,
'Tunicata,' in the l»tli edition of the Encyclopedia Britannica ; Mr. Alder's ' Observa-
tions on the British Tunicata' in Ana. of Nat. Hist. ser. iv. vol. xi. 18(58, p. 158; and
Mr. Hancock's memoir' On the Anatomy and Physiology of the Tunicata ' in the Jonrua I
of the Linnean Society, vol. i\. p. 809. Great additions to our knowledge have been
recently made In Professor Herdman, whose reports on the forms collected by H.M.S.
Chalic nger should be consulted, and by Professors Van Beneden and eTulin (see espe-
cially their memoirs in the Archives tic Biologic). See also Roule, ' Recherches sur les
Ascidies simples des cotes de Provence,' Aim. Museum Marseilles, ii. ; Seeliger,
'Die Entwickelungsgeschichte der Socialen Ascidien,' JenaiscJie Zeitsehr. xviii.
p. 528; Salensky, ' Neue Untersuchungen iiber die embryonale Entwickelung der
Salpen,' Mitt//. Zoo/. Stat. Neapel, iv. pp. !)<), 827; and Ulianin, ' Die Arten des
Gattung Dolioluni im Golfe von Neapel,' in the Fauna and Flora des Golfes von
Neapel, x.

843

CHAPTER XVIII

MOLL USC A AND BRA CHIOPODA

The various forms of ' shell-fish/ with their ' naked ' or shell-less
allies, furnish a great abundance of objects of interest to the micro-
scopist. of which, however, the greater part may be grouped under
three heads — namely (1) the structure of the shell, which is most
interesting in the Conchifera (or Lamellibranciiiata) and Braciiio-
PODA,inbothof 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 is 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 MargCbritacece,
which includes the Jleleagrina or 'pearl oyster ' and its allies, the
common Pinna rankino- amongst the latter. In all these shells we
readily distinguish the existence of two distinct layers ; an external,
of a brownish yellow colour ; and an internal, which has a pearly
or ' nacreous ' aspect, and is commonly of a lighter hue.

The structure of the outer layer may be conveniently studied in
the shell of Pinna, in which it commonly projects beyond the inner,
and there often forms laminae 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
of an assemblage of segments of basaltic columns (fig. 638). This
outer layer is thus seen to be composed of a vast number of prisms.

844

MOLLUSCA AND BRACHIOPODA

having a tolerably uniform size, and usually presenting an approach
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.
635) to be themselves com-
posed of a very homogeneous
substance, but to be 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. 636), its
"hexagonal divisions bearing a strong resemblance to the walls of
"the cells of the pith or bark of a plant. By making a section Of the
.shell perpendicularly to its surface, we obtain a view of the prisms
cut in the direction of their length (fig. 637) ; these are frequently
seen to be marked by delicate transverse striie (fig. 638) 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 prism-
atic cells viewed longitu-
dinally, and will be seen to
be more or less regularly
marked by the transverse
striae just alluded to. It
sometimes happens in re-
cent but still more com-
monly in fossil shells, that
Fig. 636.— Membranous basis of the same. the decay of the animal

membrane leaves the con-
tained prisms without any connecting medium ; as they are then
quite isolated, they can be readily detached one from another ; and
each one may be observed to be marked by the like striations,
which, when a sufficiently high magnifying power is used, are seen
■to be minute grooves, apparently resulting from a thickening of the
intermediate wall in those situations. These appearances seem best

Fig. 635. — Section of shell of Pinna, taken
transversely to the direction of its prism.

STRUCTURE OF SHELLS

accounted for by supposing that each is lengthened by successive
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 laminae 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 thev are
progressively lengthened and
consolidated at their lower ex-
tremities, would appear also
from the fact that where the
shell presents a deep colour (as
in Pinna nigrind) this colour
is usually disposed in distinct
strata, the outer portion of each layer being the part most deeply
tinged, whilst the inner extremities of the prisms are almost colour-
less.

This c prismatic ' arrangement of the carbonate of lime in the
shells of Pinna and its allies has been long familiar to concholo-
gists, and regarded by them as the result of crystallisation. When

Fig. 637. — Section of the shell of Pinna
in the direction of its prisms.

Fig. 638.— Oblique section of prismatic shell-substance.

it was first more minutely investigated by Mr. Bowerbank 1 and the
Author,2 and was shown to be connected with a similar arrangement
in the membranous residuum left after the decalcification of the shell -
substance by acid, microscopists generally 3 agreed to regard it as a

1 'On the Structure of the Shells of Molluscous and Conchiferous Animals,' in
Trans. Micros. Soc. ser. i. vol. i. 1844, p. 123.

« ' On the Microscopic Structure of Shells ! in Reports of British Association for

1844 and 1847.

5 See Mr. Quekett's Histological Catalogue of the College of Surgeons Museum..
and his Lectures on Histology, vol. ii.

846

MOLLUSCA AND BBACHIOPODA

£ calcified epidermis,' the long prismatic cells being supposed to be
formed by the coalescence of the epidermic cells in piles, and giving
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 Professor Huxley 1 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 Margaritacem and some
other families has a ' nacreous ' or iridescent lustre, which depends
(as Sir D. Brewster has shown3) upon the striation of its surface
with a series of grooved lines, which usually run nearly parallel to
each other (fig. 639). As these lines are not obliterated by any
amount of polishing, it is obvious that their presence depends upon
something peculiar in the texture of this substance, and not upon
any mere superficial arrangement. When a piece of the nacre (com-
monly known as 'mother-of-pearl ') of the Meleagrina or ' pearl-oyster'
is carefully examined, it becomes evident that the lines are produced
by the cropping out of laminae of shell situated more or less obliquely
to the plane of the surface. The greater the dip of these lamime, 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 themseh es 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 (/or. 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 t he
hard calcareous lamime 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,

1 See his article, ' Tegumentary Organs,' in Cyclop cedia of Anatomy and
Physiology, supplementary volume, pp. 4s;>-4l)2.

- The j/rr/ostrorji m is the yellowish-brown membrane covering the surface of
many shells, which is often (but erroneously) termed their epidermis.

3 Phil. Trans. 1814, p. :j'.)7.— The late Mr. Barton (of the Mint) succeeded in
producing an artificial iridescence on metallic buttons by drawing closel y approxi-
mating lines with a diamond point upon the surface of the steel die by which they
were struck.

STRUCTURE OF SHELLS

847

these being frequently no more than y^j^th of an inch apart. But
when the nacre is treated with dilute acid, so as to dissolve its cal-
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 surface of
the shell of Meleagrina, and possessing a nacreous structure corre-
sponding to that of ' mother-of-pearl.' Such concretions are found in
many other shells, especially the fresh-water mussels, Unio and Ano-
don ; but these are usually less remarkable for their pearly lustre ;
and, when formed at the edge of the valves, they may be partly or

Fig. 039. — Section of nacreous lining of shell of Meleagfina
m a rga ritifera (Pearl-oyster) .

even entirely made up of the prismatic substance of the external
layer, and may be consequently altogether destitute of the pearly
character.

In all the genera of the Margaritacece we find the external layer
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 Unionichr (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 Ostreacea (or oyster tribe), also, the greater
part of the thickness of the shell is composed of a ' sub-nacreous '
substance, representing the inner layer of the shells of Margaritacea?,
its successively formed laminae, however, having very little adhesion

§48

MOLLUSCA AND BKACHIOPODA

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
brownish-yellow colour. In these and some other cases a distinct
membranous residuum is left after the decalcification of the prismatic
layer by dilute acid ; and this is most tenacious and substantial,
where (as in the Margaritacece) there is no proper periostracum.
Generally speaking, a thin prismatic layer may be detected upon the
external surface of bivalve shells, where this has been protected
by a periostracum, or has been prevented in any other manner
from undergoing abrasion ; thus it is found pretty generally in
Chama, Trigonia, and Solen, and occasionally in Anomia and Pecten.

In many other instances, however, nothing like a cellular struc
ture can be distinctly seen in the delicate membrane left after decal-
cification ; and in such cases the animal basis bears but a very small
proportion to the calcareous substance, and the shell is usually ex-
tremely hard. This hardness ap-
pears to depend upon the mineral
arrangement of the carbonate of
lime ; for whilst in the prismatic
and ordinary nacreous layer this
has the crystalline condition of
cd/rite, it can be shown in the hard
shell of Pholas to have the arrange-
ment of arragonite, the difterence
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. 640), in
which the carbonate of lime seems
to be deposited in nodules that
possess a crystalline structure re-
sembling that of the mineral
termed iracdlite. 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 is the case, for example, with most of the
PectinidtH (or scallop tribe), also with some of the Mytilacem (or
mussel tribe), and with the common Oyster. In the internal layer
of by far the greater number of bivalve shells, however, there is not
the least approach to the nacreous aspect ; nor is there anything
that can be described as definite structure ; and the residuum
left after its decalcification is usually a structureless ' 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 which is in contact with the internal
surface of the preceding, and extends beyond it — does not express

Fig. 640.— Section of hinge-tooth of
Mya arena rid.

SHELLS OF LAMELLIBRANCHS

849

the whole truth ; for it takes no account of the fact that most shells
are composed of two layers of very different texture, and does not
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 fig. 641 will
clearly show the mode in which the operation is effected. This figure
represents a section of one of the valves of Unio occidens, taken per-
pendicularly to its surface, and passing from the margin or lip (at
the left hand of the figure) towards the hinge (which would be at
some distance beyond the right). This section brings into view the
two substances of which the shell is composed, 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 a a', b b\ c c',
circ. These lines evidently indicate the successive formations of this

Fig. 641. — Vertical section of the lip of one of the valves of the
shell of Unio : a, b, c, successive formations of the outer
prismatic layer ; a', b', c', the same of the inner nacreous layer.

layer, and it may be easily shown by tracing them, towards the
hinge on the one side and towards the margin on the other, that at
every enlargement of the shell its whole interior is lined by a new
nacreous lamina in immediate contact with that which preceded it.
The number of such laminae, 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, however, new laminae of both
layers still continue to be added, and thus the lip becomes thickened
by successive formations of prismatic structure, each being applied
to the inner surface of the preceding, instead of to its free margin.
A like arrangement may be well seen in the Oyster, with this differ-
ence, that the successive layers have but a comparatively slighj
adhesion to each other.

The shells of Terebratulw and of most other Bnicltiopodx 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 Ion- flat-
tened prisms (fig. 642, A, b), which are arranged with such obliquity

850 MOLLUSCA AND BEACHIOPODA

that their rounded extremities crop out upon the inner surface of the
shell in an imbricated (tile-like) manner (a). All true Terebratulidd*,
both recent and fossil, exhibit another very remarkable peculiarity ;
namely, the perforation of the shell by a large number of canals,

A B

.Fig. 642. — A, internal surface, a, and oblique section, b, of shell of Waldheimia
australis ; B, external surface of the same,

which generally pass nearly perpendicularly from one surface to the
other (as is shown in vertical sections, fig. 643), and terminate inter-
nally by open orifices (tig. 642, A), whilst externally they are covered

A

w

by the periostracum (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 sometines happens,
a new internal layer is formed
as a lining to the preceding
(fig. 643, A, d d). Hence the
diameter of these canals, as
shown in different transverse
sections of one and the same
shell, will vary according to
the part of its thickness which
the section happens to tra-
verse. The shells of different
species of perforated Brachio-
jjod;<, 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. 644-646. 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

Fig. 643.— Vertical sections of shell of Wald-
heimia australis, showing at A the canals
opening by large trumpet- shaped orifices
on the outer surface, and contracting at
d, d into narrow tubes ; and showing at B
a bifurcation of the canals.

SHELLS OF BRACHIOPODA

85I

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, Fschara, Leprcdia, &c. into passages excavated in
the walls of the cells of the polyzoary. Professor Sollas 1 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 Rliynchondlidw, which is represented by only

Fig. 644. Fig. 645. Fig. 646.

Fig. 644. — Horizontal section of shell of Terebratula bullata (fossil, Oolite).
Fig. 645. „ „ Megerlia lima (fossil, Chalk).

Fig. 646. „ „ Sjjiriferina rostrata (Triassic).

■six recent species, but which contains a very large proportion of
fossil Brachiopods, these canals are almost entirely absent ; so
that the uniformity of their jDresence in the Terebratulid(f, and their
general absence in the RltynchonelUdie, supplies a character of
great value in the discrimination of the fossil shells belonging
to these two groups respectively. Great caution is necessary,
however, in applying this test ; mere surface markings cannot 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 Spirjferidm and Strophomenidce, on the
other hand, some species possess the perforations, whilst others are
destitute of them ; so that their presence or absence the re 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. Boy. Dublin Soc. v. 318.

2 For a particular account of the Author's researches on this group see his memoir
on the subject, forming part of the introduction of Mr. Davidson's Afc nograph of tin-

3 i 2

852

MOLLUSCA AND BKACHIOPODA

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 ' porcellanous ' 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 laminae placed
side by side, which separate one from another, apparently in the
planes of rhomboid al cleavage, when the shell is fractured ; and, as
was first pointed out by Mr. Bowerbank, each of these lamina? 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 Cyprcsa 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 Mr. (now Sir) John Tomes to exist in the enamel
of the teeth of Rodentia, and by Professor Rolleston in that of the
elephant.

The principal departures from this plan of structure are seen in
Patella, Chiton, Haliotis, 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 rufus,
a thin oval plate of calcareous texture is found imbedded in the
shield-like fold of the mantle covering the fore part of its back ; and
if this be examined in an early stage of its growth it is found to
consist of an aggregation of minute calcareous nodules, generally
somewhat hexagonal in form, and sometimes quite transparent,
whilst in other instances it presents an appearance closely resem 1)1 ing
that delineated in tig. 640. In the epidermis of the mantle of some
species of Doris, on the other hand, we find long calcareous spicules,,
generally lying in parallel directions, but not in contact with each
other, giving firmness to the whole of its dorsal portion ; and these
are sometimes covered with small tubercles, like the spicules of

British Fossil Brachiopoda. published by the Pahrontographieal Society. A very
remarkable example of the importance of the presence or absence of the perforations
in distinguishing shells whose internal structure shows them to be generically dif-
ferent, whilst from their external conformation they would be supposed to be not
only r/cneri call // but sprrifical 'I ;/ identical, will be 'found in the Ann. N«t. Hist.
ser. hi. vol. xx. 1867, p. 08. '

SHELLS OF MOLLUSCA

Qorcfonia. 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
Limax.

The structure of shells generally is best examined by making
sections in different planes as nearly parallel as may be possible to
the surfaces of the shell, and other sections at right angles to these ;
the former may be designated as horizontal, the latter as vertical.
Nothing need here be added to the full directions for making such
sections which have already been given. Many of them are beautiful
and 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 unpossessed of shells ; and the
structure of the few that we meet with in the genera Xautilus, Argo-
nauta ('paper nautilus'), and Spirula does not present any peculi-
arities that need here detain us. The rudimentary shell or sepiostairp
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. 640. 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 lamina',
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

$54

MOLLUSCA AND BRACHIOPODA

it is connected above and below, and the sinuosity of the thin
intervening lamina?, 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 lamina?. 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, and 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
polariscope.

Palate of Cephalophorous Molluscs. — The organ which is some-
times 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. 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. 647.— Portion of the left half of the palate s]it up and spread out SO as
of Helix hortcns, iW rows of teeth near t f nearly flat surface.

the edge separated from each other to snow . . J

their form. On the interior of the tube,

as well as on the flat expan-
sion of it, we find numerous transverse rows of minute teeth, which
are set upon flattened plates, each principal tooth sometimes
having a basal plate of its ow n, whilst in other instances one plate
carries several teeth. Of the former arrangement we have an
example in the palate of many terrestrial Gastropods, such as the
snail (Helix) and slug (Limax), in which the number of plates in
each row is very considerable (figs. 647, 048), amounting to 180
in the large garden slug (Limax maximus) ; whilst the latter prevails
in many marine Gastropods, such as the common whelk (Buccinum,
undatum), the palate of which has only three plates in each row, one
bearing the small central teeth, and the two others the large lateral
teeth (tig. 651). The length of the palatal tube and the number of
rows of teeth vary greatly in different species. Generally speaking,
the tube of the terrestrial Gastropods is short, and is contained
entirely within the nearly globular head ; but the rows of teeth
being closely set together are usually very numerous, there being
frequently more than 100, and in scfnie species as many as 160 or
170 ; so that the total number of teeth may mount up, as in Helix
pomatia, to 21,000, and in Limax maximus to 26,800. The trans-
verse rows are usually more or less curved, as shown in fig. 648y

PALATES OF GASTROPOJ > A

855

Fig. 648. — Palate of Hyalinia cellaria.

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 manv instances it extends
far beyond the head, which may, indeed, contain but a small
part of it. Thus in a common limpet (Patella) we find the principal
part of the tube to lie folded up, but perfectly free, in the abdominal
cavity, between the greatly elongated intestine and the muscular
foot • and in some species its length is twice or even three times as
great as that of the entire animal. In a large proportion of cases
these palates exhibit a very marked separation between the central
and the lateral portions (tigs.
649, 650), the teeth of the cen-
tral band being frequently small
and smooth at their edges,
whilst those of the lateral are
large and serrated. The palate
of Trochus zizyphinus, repre-
sented in fig. 649, 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 vet
more complex type, however, is
found in the palate of HaHotis.
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 Literal
band in fig. 649 ; whilst in Doris tubercidata the central band is
the part most developed, and contains a number of rows of conical

Fig. 649.— Palate of Trochus zizyphinus.

856

MOLLUSC A AND BRACHIOPODA

teeth, standing almost perpendicularly, like those of a harrow
(fig. 650).

Many other varieties might be described did space permit ; but
Ave 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 animals
we find the roof of the mouth
furnished with a large strong
horny plate, against which the
flat end of the tongue can work.
On the other hand, the flattened
portion of the palate of Bucci-
ii hid (whelk) and its allies is
used by these animals as a file,
with which they bore holes
through the shells of the molluscs
that serve as their prey ; this
they are enabled to effect by everting that part of the proboscis-
shaped mouth whose floor is formed by the flattened part of the
tube, which is thus hrought to the exterior, and by ing 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 hack 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 ( Jastropods is that
of a 'cavity of reserve,' from which a new toothed surface may be
continually supplied as the old one is worn away — somewhat as the
front teeth of the rodents are constantly being regenerated from the
surface of the pulps which occupy their hollow conical bases — as fast
as they are rubbed down at their edges, or as a nail is constantly
being worn away at its free end, and fashioned anew in its
'bed.'

The preparation of these palates for the microscope can, of course,
be only accomplished by carefully dissecting them from their attach-
ments within the head ; and it will be also necessary to remove the

Fig. G50.— Palate o: Lcris tuberculata.

1 Ann. Nat. Hist. ser. ii. vol. x. 1852, p. 413.

DEVELOPMENT OF MOLLUSCA

857

membrane that forms the sheath of the tube, when this is thick
enough to interfere with its transparence. The tube itself should be
slit up with a pair of hne 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 Goad by '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. 651), and
a gorgeous play of colours being exhibited
when a selenite plate is placed behind the
object, and the analysing prism is made to
rotate. 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
which a general account will be here given. Attached to the gills of
fresh-water mussels (Unio 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 Glochidia, but are
now known to be their own progeny in an early phase of develop-
ment. When they are expelled from between the valves of their
parent, they attach themselves in a peculiar manner to the fins and
gills of fresh-water fish. In this stage of the existence of the young
Anodon, its valves are provided with curious barbed or serrated
hooks (fig. 652, 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 (aad) which draws the valves
together, and the long, slender byssus-filament (hy) which makes
its appearance while the embryo is still within the egg-mem-
brane, lying coiled up between the lateral lobes. The hollow of
each valve is filled with a soft granular-looking mass, in which
are to be distinguished what are perhaps the rudiments of the

1 For additional details on the organisation of the palate and teeth of the
Gastropod molluscs, see Mr. "VV. Thomson in Cyclop. Anat. and Physiol, vol. iv.
pp. 1142, 1143, and in Ann. Nat. Hist. ser. ii. vol. vii. p. 86; Professor Troschel, Das
Gebiss der Schnecken, Berlin, 1856-79 ; A. Riicker, 'Ueber die Bildung der Badula
bei Helix pomatia,' Bericht oberhess. Gesellsch. Giessen, xxii. p. 209 ; P. Geddes, ' On
the Mechanism of the Odontophore in certain Molluscs,' Trans. Zool. Soc. x. p. 485.

2 See Balfour's Comparative Embri/oloc/y, vol. i. chap. ix.

858

MOLLUSCA AND BRACHIOPODA

branchiae 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 cf nsh, 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
nidamentum. The nature of this envelope, however, varies greatly ;
thus, in the common Limnceus stagnates, 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, and
which is attached to the leaves or stems of aquatic plants ; in the
Buccinum undatum, or common whelk, it is a membranous case,

Fig. 652. — A, Glochidium immediately after it is hatched : ad, ad-
ductor; sh, shell; by, hyssus-cord ; ,v, sense-organs. B, the same
alter it lias heen on the fish for some weeks: or, liranchiiB ; auv,
auditory sack;/, food; a.ad and />.ad, anterior and posterior
adductors; al, mesenteron ; mt, mantle.

connected with a considerable number of similar cases by short stalks,
so as to form large globular masses which may often be picked up on
our shores, especially between April and June; in the Purpura
lapillus, or 1 rock-whelk,3 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 the tide marks, great numbers being often found
standing erect side by side ; whilst in the Nudibranchiate order
generally (consisting of the Doris, Eol 18} and other 'sea-slugs') it
forms a long tube with a membranous wall, in which immense
numbers of eggs (even half a million or more) are packed closely
together in the midst of a jelly-like substance, this tube being disposed
in coils of various forms, which are usually attached to sea-weeds or
zoophytes. The course of development, in the first and last of these
instances, may be readily observed from the very earliest period down

1 See the Rev. W. Houghton, ' On the Parasitic! Nature of the Fry of the Ano-
donta cygneaj in Quart. Joum. Micron. Sci. n.s. vol. ii. lH(il, p. 162, and especially
Balfour, op. pit. pp. '220-223. On the embryonal bys;!us-gland of Anudonta, see
J. Carriere, Zoolog. Anzeig. vii. p. 41. ' ui -fs

DEVELOPMENT OF DORIS

to that of the emersion of the embryo, owing to the extreme trans-
parence of the nidamentum and of the egg-membranes themselves.
The first change which will be noticed by the ordinary observer- is
the ' segmentation ' of the yolk-mass, which divides itself (after the
manner of a cell undergoing binary subdivision) into two parts, each
of the'se two into two others, and so on until a morula, or mulberry-
like mass of minute yolk-segments, is produced (fig. 653, A-F),
which is converted by 1 invagination ' into a ' gastrula,' whose form

Fig. 653— Embryonic development of Doris Ulamellata : A, ovum, consist-
ing of enveloping membrane, a, and yolk, b; 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 ; d, foot' ;' g, hard plate or operculum attached to it ;
h, stomach ; i, intestine ; m, n, masses (glandular ?) at the sides of the
oesophagus; o, heart (?) ; s, retractor muscle (?) ; t, situation of funnel;.
v, membrane enveloping the body; x, auditory vesicles; ;/y mouth.

86o

MOLLUSCA AND BRACHIOPODA

is shown at G. This 1 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 fringing a sort of fold of the ecto-
derm termed the velum, which afterwards usuallv gives origin to a
pair of large ciliated lobes (H-L, c) resembling those of Rotifers.
The velum is so little developed in Limncens, however, that its
existence was commonlv 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 lirst changes which is seen in
it consists in its extension into a sort of fin-like membrane on either
side, the edges <>f which are fringed with long cilia (fig. 653, 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 vehicles ' (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 i he 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 liotifera, serving also
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 its swimming movements are
less active, in consequence 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 Bee his valuable ' Observations on the Development of Lint turn a stuyiuit/s and
on the. early stages of other Mollusca ' in Quart. Styu/rn, Micros. Set. October 1874 ;
and ' On the Developmental History of the Mollusca,' Phil. Tram. 1875. Bee a lso
Lereboullet, ' Recherches sur le Developpement du Limnee,' in Ann. des Sci.Nat.
Zool. 4e serie, torn, xviii. p. 47.

DEVELOPMENT OF PURPURA

86 r

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 Purpura lapillus1 and it is probable that something
of the same kind exists also in Buccinum, as well as in other Gas-
tropods of the same extensive order (Pectinibranchiata). Each of
the cajmiles already described contains from 500 to 600 egg-like
bodies (fig. 654, 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. 655, B), that each of
them must include an amount of
substance eaual 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 Buccinum) 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
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. 654. 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 detiniteness 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 1 cephalic ; and the
' visceral ' portions of the
former into distinct organs

instance a narrow transparent border is seen around the whole
embryonic mass, which is broader at the cephalic portion (I) ; next,

Fig. 654. — Early stages of embryonic
development of Purpura lapilhis : 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.

mass (H). and the evolution of tho
very speedily commences. In the first

1 See Trans. Micros. Sec. ser. ii. vol. ii. 1854, p. 83.

862

MOLLUSCA AND BRACHIOPODA

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. 655 ; 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. ('>.">.">. — Later stages of embryonic development of Purpura lapilhis'.
A, conglomerate mass of vitelline segments, to which were attached the
embryos a, b, c, d, e. B, full-size embryo in mere advanced stage of
development.

stages of (development represented in fig. 654, H-K. After a short
time, however, it becomes apparent that the most advanced embryos
-are beginning to swallow the yolk-segments of the conglomerate mass,
and capsules will not unfrequently bo met with in which embryos
of various sizes, as a, b, c, d, e (fig. 655, 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 down.
For during the time in which they are engaged in appropriating this
additional supply of nutriment, although they increase in size, yet
they scarcely exhibit any other change ; so that the large embryo,
fig. 655, e, is not apparently more advanced, as regards the formation
of its organs, than the small embryo, fig. 654, K. So soon as this
operation has been completed, however, and the embryo has attained
its full bulk, the evolution of its organs takes place very rapidly ; the

DEVELOPMENT OF PURPURA

863

ciliated lobes are much more highly developed, being extended in a
long sinuous margin, so as almost to remind the observer of the
' wheels ' of Rotifera, and being furnished with very long cilia (tig.
655, 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, lire, during the evolution of which (and while they areas yet far
from complete) the capsule thins away at its summit and the embryos
make their escape from it.1

It happens not unfrequently that one of the embryos which a
-capsule contains does not acquire its ' supplemental ' yolk in the
manner now described, and can only proceed in its development as far
as its original yolk will afford it material ; and thus, at the time when
the other embryos have attained their full size and maturity, a strange-
looking creature, consisting of two large 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 thev
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, no additional supply whatever having
been acquired by them, so that their development has been arrested
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 may happen to be spending the months of August
and September in a locality in which the Purpura 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 Purpura, as he
believes that it may be advantageously adopted in many other cases. This consists
in cutting off the two ends of the capsule (taking care not to cut far into its cavity K
and in then forcing a jet of water through it by inserting the end of a fine-pointed
syringe into one of the orifices thus made, so as to drive the contents of the capsule
before it through the other. These should be received into a shallow cell and first
examined under the simple microscope. For some further observations on the de-
velopment of Purpura, see Professor Haddon, ' Notes 011 the Development of the
Mollusca,' Quart. Journ. Micros. Sci. xxii. p. 367.

2 Fuller details on this subject will be found in the Author's account of his re-
searches in Trans. Micros. Soc. ser. ii. vol. iii. 1855, p. 17. His account of the
process was called in question by MM. Koren and Danielssen, who had previously
given an entirely different version of it, but was fully confirmed by the observations
of Dr. Dyster. See Ann. Xat. Hist. ser. ii. vol. xx. 1857, p. 10. The independent

864

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-historv will ere lon°" 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 JIussel {Mytilus),1 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 riband-
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 (if 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 t<> 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 "t" each cilium is made. Few spectacles are more
striking to the unprepared mind than the exhibition of such won-
derful activity ;i> w ill then hecome 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 tin- gills themselves, so as to eli'ect the aeration of the
blood, hut also directs a portion of this current to the mouth, so
as t<> 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 Neritina flv/viatiUa (MUller's
Archiv, 1857, p. 101), and abstract in Ann. of Nat. Hist. ser. ii. vol. xx. 1857, p. 190)
showed the mode of development in that species to be the same in all essential par-
ticulars as that (if l'n i /in r<i . The subject has again been recently studied with great
minuteness by Selenka, Niederlandisches Archiv fur Zoolor/ic, Bd. i. July 18(!'2.

1 This shellfish may be obtained, not merely at the sea-side, but likewise at the1
shops of the fishmongers who supply the humbler classes, even in midland towns.

SENSE-ORGANS OF MOLLUSCA

865

-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 has made the
interesting discovery that many of the Chitonidce are provided with
a large number of minute eyes on the exposed areas of the outer
surfaces of their shells ; as the fibres of the optic nerve are directed
to the rods from behind these eyes are of the ordinary invertebrate
type, and differ therein from the just mentioned eyes of Pecten, or
those which are found on the back of Onchidium, which resemble
the vertebrate retina in having the optic fibres inserted into the front
aspect of the layer of rods.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 Slug, at the points of these organs. In the latter case the ten-
tacles are furnished with a very peculiar provision for the protection
of the eyes ; for when the extremity of either of them is 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
Avithin 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 most
Gastropods (fig. 653, 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
Tight through it. The very early appearance of the auditory vesicles
in the embryo Gastropod has been already alluded to. Those who
have the opportunity of examining young specimens of the common
Pecten will find it extremely interesting to watch the action of the
very delicate tentacles which they have the power of putting forth

1 See Mr. S. J. Hickson on ' The Eye of Pecten' in Quart. Jottm. Micros. Sci.
vol. xx. n.s. 1880, p. 4-13.

2 See Professor Moseley ' On the Presence of Eyes in the Shells of certain Chitonida\
and on the Structure of these Organs,' in Quart. Joum. Micros. Sci. xxv. p. 37.

3 K

866

MOLLUSC A AND BKACHIOPODA

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 Cephalopods. — Almost any species of cuttle-
fish (Sejria) or squid (Loligo) 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 which it may be
in proximity. The alternate contractions and extensions of these
pigment-cells, or chromatopJiores, maybe 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, A: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 sea-weeds,
zoophytes, etc. ; 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.' 2

1 Much valuable information concerning the sensory organs of molluscs will be
found in Dr. H. Simroth's memoir, ' Ueber die Sinneswerkzeuge unserer einheim-
ischen Weichthiere,' Zcitschr. f iir Wiss. Zool. xxvi. p. 227.

2 For further information regarding the chromatophores see an essay by Dr.
Klemensiewicz in the Sitzung&lxri elite of the Vienna Academy, vol. lxxviii. p. 7.

The following works and memoirs on the Mollusca generally may be consulted by
the student : S. P. Woodward, A Manual of the Mollusca, 3rd ed. London, 1875*;
Keferstein, in Bronn's Klassen und Orel uungen des TJiierreichs; the article ' Mollusca,'
by Professsor Ray Lankester, in the iitb edition of the Encyclopedia Britanwica ;
and M. P. Fischer's Manuel de Conrh yl/olog/c. Paris, 1881-87; as well as the
numerous reports on the Mollusca collected by H.M.S. Challenger.

867

CHAPTER XIX

WOEMS

Under the general designation of Worms naturalists at present
group a number of Metazoa, which differ considerably among them-
selves, and exhibit on the one hand very simple, and on the other
somewhat complex plans of organisation ; the assemblage is, indeed,
hardly anything else than a zoological lumber-room, from which,
with the progress of research, group after group may be expected to
be removed. Among others there are included in it the Entozoa or
intestinal worms, the Rotifera or wheel animalcules, TurheUaria, and
Annelida, 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 physiologist 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 Taenia (' tape- worm'), which may be taken as the typo
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 1 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
eo-o-s ; 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 recent results of micro-

1 The most important work on human entozjic parasites is that by Prof esse r
Leuckart, Die mensehlichen Parasiten, of which a second elition is now in course
of publication; of this — the first portion has already been translated into En^li^h
by Mr. W. E. Hovle.

3 k

868

WOEMS

scopic inquiry have been more curious than the elucidation of the
real nature of the bodies formerly denominated cystic Entozoa, which
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
hooklets 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 Ccenunis, 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 Tcenim
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.

The higher forms of Entozoa, belonging to the Xematoid or
thread-like order — of w hich the common Ascaris may be taken as
a type, one species of it (the A. lumbricoides or ' round worm') being a
common parasite in the small intestine of man, while another (the
Oxyuris vermicularis or ' thread-worm ' ) is found rather in the lower
bowel — are much less profoundly degraded in their organisation ; they
have a distinct alimentary canal, which commences with a mouth at
the anterior extremity of the body, and which terminates by an anal
orifice near the other extremity ; and they also possess a regular
arrangement of circular and longitudinal muscular fibres by which
the body can be shortened, elongated, or bent in any direction. The
•smaller Nematode worms, by some or other of which almost every
vertebrated animal is infested, are so transparent that every part of
their internal organisation may be made out, especially with the
assistance of the compressor, without any dissection ; and the study
of the structure and actions of their generative apparatus has yielded
many very interesting results, especially in regard to the first forma-
tion of the ova, the mode of their fertilisation, and the history of
their subsequent development. Some of the worms belonging to
this group are not parasitic in the bodies of other animals, but live
in the midst of dead or decomposing vegetable matter. Others, such
as Gordius or the ' hair-worm,' are parasitic for the greater part of

NEMATODES AND TREMATODES

869

their existence, but leave their host for the purpose of maturing
their generative products ; in these later stages the G or dins 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 Anguilluhc are little eel-like won qs.
of wliich one species, A.Jtttviatilis, is very often found in fresh water
amongst Desrnidiece, Con few ce, &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. glutinis (A. aceti) is found in sour paste,
and was often found in stale vinegar, until the more complete
removal of mucilage and the addition of sulphuric acid, in the
course of the manufacture, rendered this liquid a less favourable
' habitat ' for these little creatures. A writhing mass of any of these
species of 'eels: is one of the most curious spectacles which the
microscopist can exhibit to the unscientific observer ; and the
capability which they all possess (in common with Rotifers and
Tardigrades) of revival after desiccation, at a very remote interval,
enables him to command the spectacle at any time. A grain of
wheat within which these worms (often erroneously called 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 j)ortion. and putting it under the microscope with a little
water under a thin glass cover, it will be found after a short time (if
not immediately) to be a wriggling mass of life, the apparent fibres
being really Anguillulce or ' eels ' of the microscopist. If the seeds
be soaked in water for a couple of hours before they are laid open,
the eels will be found in a state of activity from the first ; their
movements, however, are by no means so energetic as those of the
A. glutinis or ' paste eel.' This last frequently makes its appearance
spontaneously in the midst of paste that is turning sour ; but the
best means of securing a supply for any occasion consists in allowing
a portion of any mass of paste in which they may present themselves
to dry up, and then, laying this by so long as it may not be wanted,
to introduce it into a mass of fresh paste, which if it be kept warm
and moist will be found after a few days to swarm with these curious
little creatures.

Besides the foregoing orders of Entozoa, the Trematode group>
which is more closely allied to the Cestoda than to the Nematodes
must be named ; of this the Distoma hepaticum, 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 Planarice (fig. 656) ; and also for the curious

TV OEMS

phenomena" of its development, several distinct forms being passed
through between one sexual generation and another. These have
been especially studied in the Distoma, which infests Paludina,
the o\~a 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 zooids, 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 larva1 are provided with
pigment-spots or rudimentary optic organs, although these organs are
wanting in the fully developed Distoma, the peculiar ' habitat ' of
which would render them useless. 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 Planarian
tribe present themselves among collections both of marine and of
fresh-water animals (particular species inhabiting either locality)
and on account of the curious organisation which many of these
possess. .Most of the members of this tribe have elongated, Flattened
bodies, and move by a sort of gliding or crawling action over the
surfaces of aquatic plants and animals. Some of the smaller kinds
are sufficiently transparent to allow of their internal structure being
seen by transmitted light, especially when they are slightly com-
pressed ; and the opposite figure (tig. Gf)6) displays the general
conformation of their principal organs as thus shown. The body
lias the flattened sole-like shape of the Trematode Entozoa ; its
mouth, w hich is situated at a considerable dist ance 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 (h) 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 oritice ; 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 A. P. Thomas,
Quart. Joum. Micros. Sri. xxiii. p. 1 ; and Professor K. Leuckart, Archiv filr Natur-
gesch. xlviii. \>. 80. On its anatomy, see Dr. F. Sommer, ZeiUchr. J'iir Wiss. Zooh
xxxiv.

PLAXARIA

87»

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 Entozoa, extend through a
large part of the body, their ramifications proceeding from the two
oviducts (kj A), which have a dilatation (I) at their point of junction.
There is still much obscurity
about the history of the em-
bryonic development of these
animals, as the accounts given
of it by different observers by
no means harmonise with each
other.1 The Planaria>, how-
ever, do not multiply by eggs
alone ; for they occasionally un-
dergo spontaneous fission in a
transverse direction, each seg-
ment becoming a perfect animal ;
and an artificial division into
two or even more parts may be
practised with a like result. In
fact, the power of the Planarice
to reproduce portions which
have been removed seems but
little inferior to that of the
Hydra : a circumstance which
is peculiarly remarkable when
the much higher character of
their organisation is borne in
mind. They possess a distinct
pair of nervous ganglia (f, f),
from which branches proceed to
various parts of the body ; and
in the neighbourhood of these
are usually to be observed a
number (varying from two to
forty) of ocelli or rudimentary
eyes, each having its refracting
body or crystalline lens, its pig-
ment-layer, its nerve-bulb, and
its cornea-like bulging of the
skin. The integument of many
■of these animals is furnished

with cells containing rods or spindles which are very possibly
comparable to the 4 thread-cells ' of zoophytes.2

1 See Balfour's Comparative Embryology, vol. i. pp. 159-1(52.

2 For further information regarding the Turbcllaria consult Dr. L. Graff's article
on Planarians in the 9th edition of the Encyclopcedia Britannica, and his magnifi-
cent Monographic tier Turbellariden, Leipzig, 1882; A. Lang, Die Polycladni,
Leipzig, 1884; P. Hallez, Contributions a Vhistoire naturcUe des Tnrbellaru-s,
Lille, 1879. On transverse fission, see Bell, Journ. Boy. Micros. Soc. (2), vi. p. 1107.

Fig. 65G. — Structure of Boiycelis levi-
gatus (a Planarian worm) : a, mouth,
surrounded by its circular sucker ; b,
buccal cavity ; c, oesophageal orifice ;
d, stomach ; e, ramifications of gastric
canals ; /, cephalic ganglia and their
nervous filaments; g, g, testes; //,
vesicula seminalis ; ft, male genital
canal ; 1% k, oviducts ; /, dilatation at
their point of junction; m, female
genital orifice.

872

WORMS

Annelids, — 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 fresh water or
live on land. The body in this class is usually elongated and nearly

always presents a well-marked seg-
mental 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 seg-
mental division is very indistinctly
seen, on account of the general
softness of the integument. A
large portion of the marine An-
nelids have special respiratory ap-
pendages, into which the fluids of
the body are sent for aeration,
and these are situated upon the
land (tig. 657) in those species
which (like the Serpula, Terebellay
Sabellaria, etc.) have their bodies
inclosed by tubes, either formed of
a shelly substance produced from
their own surface, or built up by
the agglutination of grains of sand,
fragments of shell &c.] ; whilst they
are distributed along the two sides
of the body in such as swim freely
through the water, or crawl over
the surfaces of rocks, as is the case
with the Nereida; or simply bury
themselves in the sand, as the
Ari'iiicuhi or 'lob-worm.' In these
respiratory appendages the circu-
lation 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

n, venous sinus surrounding cesopha- species to OCCUpy the space that

gus; n. inferior intestinal v<-ss,-l ; i,ltervenes between the outer sur-

O, o, ventral trunk ; p. lateral vascular
branches. lace °1 the alimentary canal and

the inner wall of the body, and to

pass from this into canals which often ramify extensively in the

respiratory organs, but are never furnished with a returning series

of passages ; and second, a fluid winch is usually red, contains few

floating particles, and is inclosed in a system of proper vessels that

1 For an interesting account of the formation of these tubes see Mr. A. T. Watson' s-
paper in Jonrn. Boy. Micr. Soc. lb'JO, p. 085.

Flo. 657. — Circulating apparatus of
Tcrcbrllii c<>nr]tih"</a : a, labial ring;
b, b, tentacles; c, first segment of
the trunk; <1, skin of the back; e,
pharynx; /'.intestine; (j, longitudinal
muscles of the inferior surface of the
body; //, glandular organ ; ^organs
of generation ; j, feet ; k, k, branchiae ;
Z, dorsal vessel acting as a respiratory
heart ; in, dorso-intestinal vessel ;

DEVELOPMENT OF WORMS

873

communicates with a central propelling organ, and not only carries
away the fluid away from this, but also brings it back again. In
Terebella we find a distinct provision for the aeration of both fluids ;
for the first is transmitted to the tendril-like tentacles which sur-
round the mouth (fig. 657, b, b), whilst the second circulates through
the beautiful arborescent gill-tufts (k, k) situated just behind the
head. The former are covered with cilia, the action of which con-
tinually 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 Annelids 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 Annelids 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, &c. with which the mouth is com-
monly armed in the free or non-tubicolar sj)ecies, which are
eminently carnivorous.

The early history of the development of Annelids, too, is ex-
tremely curious ; for they come forth from the egg in a condition
very little more advanced than the ciliated gemmules of polypes,
consisting 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 off
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,

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 not 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
pre vented 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.

•8/4

WOEMS

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
moving condition, before they settle down to the formation of a
tube.1

To carry out any systematic observations on the embryonic
development of Annelids the eggs should be searched for in the
situations which these animals haunt ; but in places where Anne-
lids 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 oft' 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 (fig. 658) bears a

strong resemblance in many particulars
to the ' bipinnarian ' larva of a star-
fish, having an elongated body, with
a series of ciliated tentacles {(I) sym-
metrically arranged ; these tentacles,
however, proceed from a sort of disc
which somewhat resembles the ' lopho-
phore ' of certain Polyzoa. The mouth
(e) is concealed by a broad but pointed
hood or ' epistome ' (a), which some-
times closes down upon the tentacular
disc, but is sometimes raised and ex-
tended forwards. The nearly cylin-
drical body terminates abruptly at the
other extremity, where the anal orifice
of the intestine (b) is surrounded by a
circlet of very large cilia. This animal
swims with great activity, sometimes
by the tentacular cilia, sometimes by
the anal circlet, sometimes by both
combined ; and besides its movement
of progression it frequently doubles
itself together, so as to bring the anal
extremity and the epistome almost into
contact. It is so transparent that the
anus; r, stomach; d, ciliated whole of its alimentary canal may be
tentacles ; e, mout h. ;LS distinctly seen as that of Laguncula ;

and, as in that polyzoon, the aliment-
ary masses often to be seen within the stomach (c) are kept in a
continual whirling movement by the agency of cilia with which its
walls are clothed. This very interesting creature was for a long
time a puzzle to zoologists ; since, although there could be little
doubt of its being a larval form, there was no clue to the nature of
the adult produced from it until this was discovered by Krohn in

m

Fig. 058. — Actinotrocha branchi-
ata : a, epistome or hood ; b,

1 For further information on this subject see Balfour's Comparative Em bryology,
vol. i. chap. xii. and the memoirs there cited.

LARVJE OF WOEMS

«75

1858 to be a Gephyrean worm (Phoronis).1 An even more extra-
ordinary departure from the ordinary type is presented by the larva
which has received the name Pilidium (fig. 659), its shape being
that of a helmet, the plume of which is replaced by a single Ion"
bris.tle-like appendage that is in continual motion, its point moving
round and round in a circle. This curious organism, first noticed
by J ohannes Midler, has been since ascertained to be the larva of

Fig. GZ9. — Pilidium 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.

some species of the Xemertine worms, which belong to the division
Anopla, a group in which there are no stylets to the proboscis.2

Among the animals captured by the tow-net the marine
zoologist will not be unlikely to meet with an Annelid which,
although by no means microscopic in its dimensions, is an admirable
subject for microscopic observation, owing to the extreme trans-

1 'Ueber Pilidium and Actinotrocha ' in Miiiler's ArcJriv, 1858, p. 293. For
more recent observations upon this interesting creature, see Balfour's Comparative
Embryology, vol. i. pp. 299-302 ; and a paper on ' The Origin and Significance of the
Metamorphosis of Actinotrocha,' by Mr. E. B. Wilson (of Baltimore), in Quart.
Joum. Micros. Sci. April 1881.

- See especially Leuckart and Pagenstecher's ' Untersuchungen iiber niedere
Seethiere ' in Miiiler's Archiv, 1853, p. 569 ; and Balfour, oj). cit. p. 165. The Author
has frequently met with Pilidium in Lamlash Bay.

Sy6 worms

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

B

Fig. GOO. — Structure and development of Tomoptcris onisciformis : A, portion
of caudal prolongations, coi i t:ii 11 in</ tlx; 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, a, b\ E, one of the pinnulated segments, showing the position of the
ciliated canal, c, and its rosette-like discs, a, b ; showing also the incipient
developnienl of I lie o\ a,, d, at I lie exl remity of i lie segment ; l<\ ceplialic gan
glion, with its pair of auditory (?) vesicles, a a, and its two ocelli, b b ; G, very
young Tomopteris, showing at a the larval antennae; b b, the incipient
long antennae of the adult ; c, d, e,ff four pairs of succeeding pinnulated
segments, followed hy bifid tail.

TOMOPTEEIS

877

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. 660, B), each of them carrying at its extremity^,
pair of pinnules, by the movements of which it is rapidly propelled
through the water. The full-grown animal, which measures nearly
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-
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 be 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. JSo 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, and on a larger scale
at fig. D ; each of these orifices (D, a, b) is surrounded by a sort of
rosette, and the rosette of the larger one (a) is furnished with
radiating ciliated ridges. The two branches incline towards each
•other, and unite into a single canal that runs along for some dis-

878

tance in the wall of the body, and then terminates in the perivisceral
cavity, and the direction of the motion of the cilia which line it is
from without inwards.

The reproduction and developmental history of this Annelid
present many points of great interest. The sexes appear to be
distinct, ova being found in some individuals and spermatozoa in
others. The development of the ova commences in certain *' germ-
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. Jn 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 tills 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, lias not yet
been ascertained. < >f 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 tin; number of the segments of the body (which
successively augment by additions at the posterior extremity), but
also in that of the antenna1. 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', <(, a, behind
which there are five pairs of bifid appendages, b, c, <7, e, f, in the
first of which, 6, one of the pinnules is furnished with a seta. In
more advanced larva* 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 antennae
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,

9

NAIS

879

and the microscopist can scarcely have a more pleasing object for
study.1 Its elegant form, its crystal clearness, and its sprightly,
graceful movements render it attractive even to the unscientific
observer ; whilst it is of special interest to the physiologist 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.2

Among the fresh -water Annelids those most interesting to the
microscopist are the worms of the JVais 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 off 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

1 See the memoirs of the Author and M. Clapaivde in vol. xxii. of the Linnean
Transactions and the authorities there referred to ; also a recent memoir by Dr. F.
Vejdovsky in Zcitsclirift f. Wiss. Zool. Bd. xxxi. 1878.

2 See his memoirs on the Annelida of La Manche in Arm. <lcs Sci. Nat. ser. ii.
Zool. torn. xix. and ser. iii. Zool. torn. xiv. ; and Professor Mcintosh in Naturcy
xxxii. p. 478.

88o

WOEMS

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
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 hand-
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 Oligochasta,' Report Brit.
Assoc. 1885, p. 1096.

2 Among the various sources of information as to the anatomy and physiology of
the Annelids the following may he specially mentioned: the ' Histoire Naturelle des
Anneles Marins et d'Eau douce ' of M. de Quatrefages, forming part of the Suites <l
Buffon ; the successive admirable monographs of the late Professor Ed. Claparede,
Recherclies Anatomiques sur les Annelides, Turbellaries, Opalines et Gregarin.es,
observes dans les Hebrides, Geneva, 18(51 ; Recherckes Anatomiques sur Ips Oligo-
clietes, Geneva, 1862; Beobach t u ngen iiber Anatomie und JEntwickelungsgescMchtfi

Wirbellnsfit Thiere an der Kiiste von Normandie, Leipzig, 18(53; andJLes Annelides
ChStopodes <lu Golfe de Naples, Geneva, 1868-70 ; the monograph of Dr. Ehlers, Die
Borstenieitnner (Annelida Ch (etoj)oda), 1864-68. With the exception of Professor
Mcintosh's article in the il neijelojned ia lirita nnica and Professor Vcjdovsky's System
und Morphologic der Oligochaten, Prague, 1884, most of the recent papers on
Annelids have dealt with small groups only, but of these a very large number has
appeared. hor the descript ions of new forms the memoirs of Grube and Mcintosh
are especially to be consulted ; Kleinenberg, Hatschek, and Salensky have written
the most important contributions to our knowledge of development; Bergh, Bourne,
Eisig, Perrier, and Whitman have, among others, added to our knowledge of their
.anatomy and morphology.

88i

CHAPTER XX

C BUST ACE A

Passing to the division of Arthropods, in which the body is
furnished with distinctly articulated or jointed limbs, some of which
are always modified to serve as mouth-organs, we come first to the
class of Crustacea, which ordinarily includes (when used in its
most comprehensive sense) all those animals belonging to this group
which are fitted for aquatic respiration, though the king-crab
(Limulus) has closer relations to the scorpions, and the Pycnogonids
to the spiders. It thus comprehends a very extensive range of
forms ; for although we are accustomed to think of the crab, lobster,
cray-fish, and other well-known species of the order Decapoda (ten-
footed), as its typical examples, yet all these belong to the highest
of its many orders ; and among the lower are many of a far simpler
structure, and 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 sea-weeds 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.
661) 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.

3 L

882

CEUS.TACEA

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
antennas 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 (6) situated in the

Pig. 661. — Ammothea pycnogonoides : a, narrow oesophagus ;
h, stomach; c, intestine ; d, digestive cteca of the foot-jaws ;
c, e, digestive caeca 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 crecal 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
out 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 csecal 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
between the widely extended stomach and the walls of the body and

PYCNOGOKEPA ; ENTOMOSTRACA

883

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 havo
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 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 live pairs
have their function limited to locomotion, the respiratory organs
being attached to the parts in the neighbourhood of the mouth ;
whilst in the Branchiopoda, or ' gill-footed ' tribe, the members
(known as 'fin-feet') serve both for locomotion and for respiration,
and the number of these is commonly large, being in Apus 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 common
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 Arachnida 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 Pvcnogonids are Dr. A. Dohrn's Die Pantopoden des Golfes von Xeapel
&c, Leipzig, 1881 ; Dr. P. P. C. Hoek's 1 Keport on the Pycnogonida of the Challenger,"
1881, and his ' Xouvelle Etude sur les Pycnogonides,' in Archives de Zool. Exptr. ix.
p. 445; and Professor G. O. Sars' report in the Zoology of the Norwegian North Sea
Expedition

3 l 9.

884

CKVSTACEA

number of 'fin-feet,' swim with an easy gliding movement, sometimes
on their back alone (as is the case with Branchipus) and sometimes
with equal facility on the back, belly, or sides (as is done by Artemia
salina, the ' brine-shrimp '). Some of the most common forms of
both tribes will now be briefly noticed.

The first group contains two orders, of which the first, Ostracoda,
is distinguished by the complete inclosure of the body in a bivalve
shell, by the small number of legs, and by the absence of an external
egg-sac. One of the best known examples is the little 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 antennas 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 antenna- 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, <»t* 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 fche 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 sub-
stance.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, most ly adapted for swim-
ming, the fifth pair, however, being rudimentary in the genus Cyclops,
the commonest example of the group. This genus receives its name
from possessing only a single eye, or rather a single cluster of ocelli ;
which character, however, it has in common with the two genera
already named, as well as with Daphnia, and with many other

1 On the recent British Ostrac >da see the monograph by (l. S. Brady in vol. xxvi.
of the T ransactiouH of the Liunratt Society of London; compare also Zenker,
* Monographic der Ostrucoden,' Arcliiv firr Natttrf/. xx. 1854 Claus has an essay on
the development of QiJliris, Marburg, 1808; see also Dr. Brady's ' Clt allaiycr ' Report.

KNTOMOSTKACA

885

Entomostraca. It contains numerous species, some of which belong
to the fresh 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 ; the ordinary water with which London is sup-
plied frequently contains large numbers of them. 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
collected by the tow-net. The body of the Cyclops is soft and gela-
tinous, and it is composed of two distinct parts, a thorax (fig. 662, a)
and an abdomen (b), 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 antennae (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 ' in axillae,' 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- 662.-A, female of Cyclops quadricornis :
n i "i • i i ,i fl.body; b, tail ; c, antenna; d, antennule ; er

rag fertilised, undergo the feet . ^ piumoSe seta? of tail. B, tail, with

earlier Stages of their de- external egg-sacs. C, D, E, F, G, successive

velopment. The Cyclops is st*ges 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 Claclocera 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

1 See for British forms Professor G. S. Brady's Monograph of the free and
semi-parasitic Copepoda of the British Islands, published by the Ray Society,
1878-80.

CRUSTACEA

adapted for swimming, and a single eye. The commonest form of
this is the Daphnia pulex, which is Sometimes called the ' arborescent
water-flea,' from the branching form of its antemiEe. 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, Phyllopoda, includes those Branchiopoda whose
body is divided into a great number of segments, nearly all of which
are furnished with leaf -like appendages, or ' fln-feet.' The two
families which this group includes, however, differ considerably in
their conformation ; for in that of which the genera Apus and Xebalia 1
are representatives, the body is inclosed in a shell, either shield-like
or bivalve, and the feet are generally very numerous ; Avhilst in that
which contains Branchipus and A Hernia, 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 '2h 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, ami
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 Brancliipus
stagrtalis lias 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
antennas), whence it has received the name of CheirocephdluB ; but

1 Professor Claus 1ms pointed out the relations of Nebalia to the Malacostraca, or
higher division of the Crustacea, and has suggested for the group which they re-
present the name of LeptosU ■(tea. See the Zeit8cTw. fuT Wiss. Zool. 1872, p. ,3'28 ;
and Claus, Untcrsuchimgen zwr Erfurscliang der geucalogischcn Griiudlage des
Crustaceen-Systems, Wien, 1870; but a different view is taken by Professor G. O.
Sars in his Eeport on the Challenger Phyllocarida.

ENTOMOSTEACA

88;

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 Branchipus 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 set?e springing from it.' Unfortunately, however, it is a
comparatively rare animal in this country. The Artemia 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 coni-
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 on being moistened, after having remained for a long
time in the condition of fine dust. Most Entomostraca, too, are
killed by severe cold, and thus the whole race of adults perishes
every winter ; but their eggs seem 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
dn 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

888

CRUSTACEA

single fertilised female of the common Cyclops quadricornis 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
oft" with it. In a very short time afterwards another brood of eggs
is seen in the cavity, and the same process is repeated, the shell
being again exuviated after the young have been brought to maturity.
At certain times, however, the Daphnia may be seen with a dark
opaque substance within the back of the shell, which has been called
the ephippium^ from its resemblance to a saddle. This, when care-
fully examined, is found to be of dense texture, and to be composed
of a mass of hexagonal cells ; and it contains two oval bodies, each
consisting of an ovum covered with a horny casing, enveloped in a
capsule which opens like a bivalve shell. From the observations of
Sir J. Lubbock,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 curious 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, that
the ' ephippial ' eggs are true sexual products, since males are to-
be found at the time when the ephippia are developed ; whilst it
is certain that the ordinary eggs can be produced non-sexually,
and that the young which spring from them can multiply the race
in like manner. The young which are produced from the ephippial

1 For an interesting account of the parthenogenetic development of Aj)us and its
allies see the sixth of Von Siebold's Beitriige zur Parthenogenesis cler Arthrojwden
(Leipzig, 1871). A new explanation of the facts has recently been made by Mr. H..
Bernard, who finds that many of the AjJOtlidcc are hermaphrodite {Jenaische Zeitschr..
xxv. p. 337).

2 ' An Account of the two Methods of Reproduction in Daphnia, and of the
Structure of the Ephippium,' in Phil. Trans. 1857, p. 79.

ENTOMOSTRACA

889

eggs seem to have the same power of continuing the race by non-
sexual reproduction as the young developed under ordinary cir-
cumstances.

In most Entomostraca the young at the time of their emersion
from the egg differ considerably from the parent, especially in having
only the thoracic portion of the body as yet developed, and in pos-
sessing but a small number of locomotor appendages (see fig. 662,
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 moultings 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
seta? 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

Closely connected with 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 certain Entomostraca ; but more commonly it is between the
earlier forms of the two groups that the resemblance is the closest,

1 For a systematic and detailed account of this group see Dr. Baird's Natural
History of the British Entomostraca, published by the Kay Society. The numerous
essays by Professor Claus should also be consulted.

2 It is now generally recognised that these should be placed with the Copepoda,
which may be divided into the Eucopepbda and the Branchiura ; the former are
divisible into the Gnathostomata, most of which are non-parasitic, and have been
already described under Copepoda, and the Siphonostomata, of which Lernaa is an
example.

Crustacea •-•

most of the Suctoria undergoing such extraordinary changes in their
progress towards the adult condition that, if their complete forms
were alone attended to, they might be excluded from the class
altogether/ as has (in fact) been clone by many zoologists. Of the
suctorial Crustacea which form the group Branchiura 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 branchiopods, are chiefly adapted for
swimming ; and the tail, also, is a kind of swimmeret. This little
animal can leave the fish upon which it feeds, and then swims freely in
the water, usually in a straight line, but frequently and suddenly
changing its direction, and sometimes turning over and over several
times in succession. The stomach is remarkable for the large cajcal
prolongations which it sends out on either side, immediately beneath
the shell ; for these subdivide and ramify in such a manner that they
are distributed almost as minutely as the caecal prolongations of the
stomach of the Planaria (fig. 65G). The proper alimentary canal, how-
ever, is continued backwards from the central cavity of the stomach, as
an intestinal tube, which terminates in an anal orifice at the extremity
of the abdomen. A far more remarkable departure from the typical
form of the class is shown in the Lerncea, which is found attached
to the gills of fishes. This creature has a long suctorial proboscis ;
a short thorax, to which is attached a single pair of legs, which meet
at their extremities, where they bear a sucker which helps to give
attachment to the parasite ; a large abdomen ; and a pair of pendent
egg-sacs. In its adult condition it buries its anterior portion in the
soft tissue of the animal it infests, and 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 Cyclop* (fig. 662, C, D),
which they much resemble ; and only attain the adult form after a
series of metamorphoses, in which they cast oil' their locomotive
members and eyes. It is curious that the original form is retained
with comparatively slight change by the males, which increase but
little in size, and are so unlike the females that no one would suppose
the two to belong to the same family, much less to the same species,
but for the study of their development.1

From the parasitic suctorial Crustacea the transition is Dot

1 As the group of suctorial Crustacea is rather interesting to the professed
•naturalist than to the amateur microseopist, even an outline view of it would be un-
suitable to the present work; and the Author would refer such of his readers as may
desire to study it to the excellent treatise by Dr. Baird already referred to. Of the
numerous recent essays and memoirs those of Professor Claus should by all means
be consulted. •

CIMIPEDIA

89I

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 o-eneral 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
<nven 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

Fig. 663. — Development of Balanus balanoides : A, earliest
form; B, larva after second moult; C, side view of the same;
D, stage immediately preceding the loss of activity; a,
stomach (?) ; b, nucleus of future attachment (?).

study of which has contributed most essentially to the elucidation
of their real nature. The observations of Mr. J. V. Thompson 1 with
the extensions and rectifications which they have subsequently
received from others (especially Mr. Spence Bate 2 and Mr. Dar-
win 3) show that there is no essential difference between the early
forms of the sessile Cirripeds (Balanidce or 'acorn-shells ') and of the
pedunadated (Lepadidce or 'barnacles'); for both are active little
animals (fig. 663, A), possessing three pairs of legs and a pair of
compound eyes, and having the body covered with an expanded

1 Zoological Researches, No. IV. 1830, and Phil. Trans. 1835, p. 355.

2 ' On the Development of the Cirripedia ' in Ann. Nat. Hist. ser. 11. vol. vin.
1851 p 321

Monograph of the Sub-Class Cirripediar published by the Ray Society.

CRUSTACEA

carapace, like that of many entomostracous crustaceans, so as in no
essential particular to differ from the larva of Cyclops (fig. 662, C).
After going through a series of metamorphoses, one stage of which
is represented in fig. 663, 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 with an adhesive sucker and hooks, and
of six pairs of posterior legs adapted for swimming. This bivalve
shell, with the members of both kinds, is subsequently thrown off' ;
the animal then attaches itself by its head, a portion of which, in
the barnacle, becomes excessively elongated into the ' peduncle ' of
attachment, whilst in Balanus it expands into a broad disc of
adhesion ; the first thoracic segment sends backwards a prolongation
which arches over the rest of the body, so as completely to inclose
it, and of which the exterior layer is consolidated into the ' multi-
valve ' shell ; whilst from the other thoracic segments are evolved
the six pairs of cirrhi, from whose peculiar character the name of
the group is derived. These are long, slender, many -join ted, ten-
dril-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 Phyllosornata or ' glass-crabs,'
the envelope is formed by the transparent horny layer alone ; and
in many of the small crabs belonging to the genus Portuna the
whole substance of the carapace beneath the horny investment
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
parts of the shell is chiefly contained, may be easily brought into
view by grinding away from the inner side as flat a piece as can be
selected, having first cemented the outer surface to the glass slide^
and by examining this with a magnifying power of 250 diameters,
driving a strong light through it with the achromatic condenser -y

1 Valuable details as to the structure of this group will be found in Dr. P. P. C.
Hoek's report on the Cirripeds collected by H.M.S. CliaUenger.

MALACOSTEACA

893

whilst the tubular structure of the thick inner layer may be readily
demonstrated by means of sections parallel and perpendicular to
its surface. This structure, which resembles that of dentine, save
that the tubuli do not branch, but remain of the same size through
their whole course, may be particularly well seen in the black extremity
of the claw, which (apparently from some peculiarity in the mole-
cular arrangement of its mineral particles) is much denser than the
rest of the shell, the former having almost the semitransparence
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 1 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 areolae 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 fro^s, the colours of which are often in remarkable
conformity with those of the bottom of the rock-pools fre-
quented by these creatures ; whilst in the shrimps there is seldom
any distinct trace of the areolated layer, and the calcareous portion
of the skeleton is disposed in the form of concentric rings, which
seem to be the result of the concretionary aggregation of the calci-
fying 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 macrourous (long-tailed) or to

1 The Author is now quite satisfied of the correctness of the interpretation put by
Professor Huxley (see his article, 'Tegumentary Organs,' in the Cyclop. Auat. and
Phijs. vol. v. p. 487), and by Professor W. C. Williamson (' On some Histological
Features in the Shells of Crustacea ' in Quart. Journ. Micros. Sci. vol. viii. 18(H),
p. 38) upon the appearances which he formerly described {Report of British Asso-
ciation for 18-47, p. 128) as indicating a cellular structure in this layer.

2 Consult Braun, ' Ueber die histologischen Vorgange bei der Hautung von
Astacus fluviatilis,' Arbeit. Zool. Inst. Wiirzbitrg, ii. p. 121.

; CRUSTACEA

the brachyourous (short-tailed) division, of the group ; and the forms
of these larvae are. so peculiar, and so entirely different from any of'
those into which they are ultimately to be developed, that they were
considered as belonging to a distinct genus, Zoeay until their real
nature was first ascertained by Mr. J. \. Thompson. Thus, in the
earliest state of Carcinus mamas (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. 664, A) furnished with a long
j^rojecting 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 ' macrourous ' 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

Fi<;. — Metamorphosis of CurcinuH dkcikis : A, first, or Zotia
stage; B, second or Megalopa stage; C, third stage, in which
it begins to assume the adult form; D, perfect form.

special chamber formed by an extension of the carapace beneath the
body) for the latter : while the abdominal segments increase in size
and become furnished with appendages (false feet) of their own. In
the crabs, or ' brachyourous ' 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 Megalopa, when it was sup-
posed to be a distinct type. In the next stage, C, we find the
abdominal portion reduced to an almost rudimentary condition, and
bent under the body ; the thoracic limbs are more completely adapted
for walking, save the first pair, which are developed into chelce or
pincers ; and the little creature entirely loses the active swimming
habits which it originally possessed, and takes on the mode of life
peculiar to the adult. 1

1 On the metamorphoses of Crustacea and Cirripedia, see especially'the TJnter-
suchung&n iiber Crustaceen of Professor Clans, Vienna, 1H7(5. A number of

COLLECTING CRUSTACEA

895

In collecting minute Crustacea the ring-net should be used for
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 larvse,
and the smaller Medusce. The water containing these should be put
into a large glass jar, freely exposed to the light ; and, after a little
practice, the eye will become so far habituated to the 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 larva-,
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
glycerin -jelly as the best medium.

interesting facts and speculations on the Crustacea will be found in F. Miiller's Facts
and Arguments for Darwin (London 1869). The recent work of Reichenbach on the
Development of the Crayfish is contained in vol. xxix. of the Zeitschr. f. Wiss. Zo< /.
p. 123, 1877, and vol. xiv. of the Abhandl. Senclcenberg. Naturf. Gesells. 188(5. See
also the essay, by W. K. Brooks, On the Development ol Lucifer, in Phil. Trans,
1882, p. 57.

896

CHAPTER XXI

INSECTS AND ABACHNIDA

There is no class in the whole animal kingdom which affords to the
microscopist such a wonderful variety of interesting objects, and
such facilities for obtaining an almost endless succession of novelties,
as that of insects. For, in the first place, the number of 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 afford to him who knows how to employ his
materials a considerable number of microscopic objects of very
different kinds ; and thirdly, although some of these objects require
much care and dexterity in their preparation, a large proportion
may be got out, examined, and mounted with 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 antennce, although not such beautiful objects as those of
many other Diptera, are still well worth examination ; its tongue or
' proboscis ' is a peculiarly interesting object, though requiring some
care in its preparation ; its spiracles, which may be easily cut out
from the sides of its body, have a very curious structure ; its
alimentary canal affords a very good example of the minute distri-
bution of the trachece • its wing, 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 areolation of surface,
when examined by light reflected from its surface at a particular
angle ; its foot has a very peculiar conformation, which is doubtless
connected with its singular power of walking over smooth surfaces
in direct opposition to the force of gravity, and on the action of
which additional light has lately been thrown ; while the structure
and physiology of its sexual apparatus, with the history of its develop-
ment and metamorphoses, would of itself suffice to occupy the whole
time of an observer who should desire thoroughly to work it out, not
only for months, but for years. 1 Hence, in treating of this department
in such a work as the present, the Author labours under the embarras

1 See Mr. Lowne's valuable treatise on The Anatomy and Physiology of the
Bloiv-fly, 1870, of which a new edition is now (1891) being published.

MOUNTING INSECTS

897

desrichesses ; for, to enter into such a description of the parts of tin-
structure of insects most interesting to the microscopist as should
be at all comparable in fulness with the accounts wliich it has heen
thought desirable to give of other classes would swell out the
volume to an inconvenient bulk j and no course seems open but to
limit, the treatment of the subject to a notice of the kinds of
objects which are likely to prove most generally interesting, with a
few illustrations that may serve to make the descriptions more clear,
and with an enumeration of some of the sources whence a variety
of specimens of each class may be most readily obtained. And this
limitation is the less to be regretted, since there already exists 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), Keuroptera (dragon-fly,
May-fly, &c.),LTymenoptera (bee, wasp, &c), and Diptera (two-winged
flies) — may be mounted entire as opaque objects for low magnifying
powers, care being taken to spread out their legs, wings, <fcc. 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. Full directions on this point, applicable to small and
large insects alike, will be found in the various text-books of ento-
mology. There are some, however, whose translucence allows them
to be viewed as transparent objects, and these are either to
be mounted in Canada balsam or in Deane's medium, glycerin
jelly, or Warrant'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 bug 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 larvae of the Diptera and Neuro-
ptera, which are so transparent that their whole internal organisa-
tion can be made out without dissection, are very beautiful and
interesting objects when examined in the living state, especially
because they allow the circulation of the blood and the action of the
dorsal vessel to be discerned. Among these there is none prefer-
able to the larva of the Ephemera marginata (day-fly), which is dis-
tinguished by the possession of a number of beautiful appendages
on its body and tail, and is, moreover, an extremely common
inhabitant of our ponds and streams. This insect passes two or
even three years in its larval state, and during this time it
repeatedly throws off its skin ; the cast skin, when perfect, is an
object of extreme beauty, since, as it formed a complete sheath to
the various appendages of the body and tail, it continues to exhibit

1 An excellent introduction to the study of injects will be found in 'The Structure
and Life-history of the Cockroach, by L. C. Miall and A. Denny (London, 1886).

3 M

SgS

INSECTS AND ARACHNID A

their outlines with the utmost delicacy ; and by keeping these larvse
in an aquarium, and by mounting the entire series of their cast
skins, a record is preserved of the successive changes they undergo.
Much care is necessary, however, to extend them upon slides in con-
sequence of, their extreme fragility ; and the best plan is to place
the slip of glass under the skin whilst it is floating on water, and to
lift the object out upon the slide. Thin sections of insects, cater-
pillars, (fee. which bring the internal parts into view in their normal
relations, may be cut with the microtome by first soaking the body
(as suggested by Dr. Halifax) in thick gum-mucilage, which passes
into its substance, and gives support to its tissues, and then inclos-
ing it in a casing of melted paraffin made to fit the cavity of the
section-instrument.

Structure of the Integument. — In treating of those 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 clermo-
skeleton is continuous, the cells which secrete it lying beneath the
parallel laminae of which it is made up ; on the surface the chitinous
cuticle may be seen to be marked out into a number of polygonal
(usually hexagonal) areas w hich correspond to the subjacent secret-
ing cells. Of this we have a very good example in the snperjicia
layers (fig. 677. 15) of the thin horny lamellae or blades which consti
tute the terminal portion of the antenna of the cockchafer (fig. 67G)
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 ant
exhibits the hexagonal cellular arrangement very disti nel ly through-
out ; and the broad flat expansion of the leg of the Crabro (' sand-
wasp') affords another beautiful example of a distinctly cellular
arrangement of the outer layer 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 any distinct organi-
sation, though it may be usually separated into several lamella1,
which are sometimes traversed by tubes that pass into them from
the inner surface, and extend towards the outer without reaching it.

Tegumentary Appendages. — The surface of the insects is often
beset, and is sometimes completely covered, with appendages having

INTEGUMENT

899

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 lamella3, often with an intervening
deposit of pigment, on which, especially in Lepidoptera, their colour
depends. Certain scales, however, especially in the beetle tribe,
have a metallic lustre, and exhibit brilliant colours that vary with
the mode in which the light glances from them ; and this ; irides-
cence,' which is specially noteworthy in the scales of the 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. 665, 666), 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 hair-like on the latter. A
peculiar type of scale, which has been distinguished by the designa-
tion phi mule, is met with among the Pieridce, one of the principal
families of the diurnal Lepidoptera. The ' plumules ' are not flat,
but cylindrical or bellows-shaped, and are hollow ; they are attached
to the wing by a bulb at the end of a thin elastic peduncle that
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 Lyoi nido' 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, 314.

3 M 2

goo

INSECTS AND ARACHNID A

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 Morpho Mene-
laus (fig. 665), the ordinary and ' battledore ' scales of the Polyom-
matus Argus (figs. 666, 667), and the scales of the Lepisma saccharina:
(fig. 668), which are now only used for testing objects of low or
medium power, were the recognised tests for objects of high power ;
while the exhibition of alternating light and dark bands on a
Podura scale was regarded as a first-rate performance. It is easy
for anyone possessed of a good apochromatic objective of 6mm,.
(\ 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 Podurcu
scale, and yet on no one object has-
microscopy lavished so much labour for
so many years.

The easier test scales are furnished-
by the Lepidoptera (butterflies and
moths), and among the most beautiful
of these, both for colour and for regu-
larity of marking, are those of the
Morpho Menelaus (fig. 665). These are-
of a rich blue tint, and exhibit strong*
longitudinal stria1, 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. 665), probably on the in-
ternal surface of one of the membranes. The large scales of the Poly-
ommatua Argus ('azure blue' butterfly) resemble those of the Menelaus*
in form and structure, but are more delicately marked (fig. 666).
Their ribs are more nearly parallel than those of the Menelaus scale,
and do not show the same transverse striation. When one of these
scales lies partly over another, the effect of the optical intersection,
of the two sets of ribs at an oblique angle is to produce a set of
interrupted striations (b), very much resembling those of the Podura
scale. The same butterfly furnishes smaller scales, which are com-

T

FlG. CG5. — Scale of Morpho
Menelaus,

SCALES

901

monly termed the 'battledore' scales, from their resemblance in
form to that object (fig. 666, a). 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. 667) ; 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 Thysanura, but

Fig. 666. — Scales of Polyommatus Argus
(azure blue) : a, battledore scale ; b,
interference striae.

Fig. 667. — Battledore scale of
Polyommatus Argus (azure
blue).

now separated by Sir John Lubbock,4 on account of important
differences in internal structure, into the two groups Collembola and
true Thysanura. Of the former of these the Lepismidce constitute
the typical family ; and the scale of the common Lepisma saccha-
rina, or ' sugar-louse,' 5 very early attracted the attention of

1 See Watson, loc. cit.

2 ' The Markings on the Battledore Scales of some of the Lepidoptera ' in Monthly
Microscopical Journal, vol. vii. 1872, pp. 1, 250.

3 See 'Proceedings of the Microscopical Society,' op. cit. p. 278.

4 See his Monograph of the Collembola and Thysanura, published by the Ray
Society, 1872.

5 This insect may be found in most old houses, frequenting damp, warm cupboards,
and especially such as contain sweets; it may be readily caught in a small pill-box,
which should have a few pin-holes 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.

902

INSECTS AND ABACHXLDA

rnicroscopists 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 striae, representing the different structural arrange-
ments of its two superficial membranes. One of its surfaces (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. 668). 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
Pod ara ; but where the crossing is
tr.msverse 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. Morehouse/* The
minute beaded structure observed by Dr. Koyston-Pigotl 1 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 L< "jns/na , the Machilis pol i/noda, the very distinct
ribbing (tig. 6(j'J) 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 to structure between the superposed

1 The Achromatic Microscope, p. 50.

* See liis appendix to Sir John Lubbock'H Monograph.

•"' Monthly Microscopal Journal, vol. xi. 1874, p. 13, and vol. xviii. 1H77, p. 81.
4 Ibid. vol. ix. 1873, p. 03.

Fig. 668. — Scale of Lepisma
savcharina.

SCALES

903

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 ap-
pendage by which they can leap like fleas. This is particularly
well developed in the species now designated Lepidocyrtus curvi-
collis, 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.5 Its scales are of different sizes and
of different degrees of strength of marking
(fig. 670, A, B), and 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 moderate aperture now suffices to re-
solve every dark band into a row of dis-
tinct 'exclamation marks.' A certain
longitudinal continuity may be traced be-
tween the ' exclamation marks ' in the
ordinary test scale ; but this is much more
apparent in other scales from the same
species (fig. 671), as well as in the
scales of various allied types, which were
carefully studied by the late Mr, R. Beck.2
In certain other types, indeed, the scales
have very distinct longitudinal parallel
ribs, sometimes with regularly disposed Fig. 669— Scale of Machilis
cross-bars ; these ribs, being confined to one polypoda.
surface only (that which is in contact with

the body), are not subject to any such interference with their optical
continuity as has been shown to occur in Lepisma ; but more 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 been recently stated by Mr. J oseph

1 See Mr. Joseph Beck in Sir J. Lubbock's Monograph, p. 255.

2 Trans. Micros. Soc. n.s. vol. x. 1862, p. 83. See also Mr. Joseph Beck, 111 the
appendix to Sir John Lubbock's Monograph, and in Monthly Microscopical Journal.
iv. p. 253.

904

INSECTS AXD ARACHNID A

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 i in. obj. show the ' exclama-
tion markings,' whilst under a ^ 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 larvae, 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 sure to exhibit

Fig. 670. — Test scales of Lepictocyrtus curvi- Fir.. 671. — Ordinary scale
coUis : A, lar<,'<- Btrongly marked scale; B, of Jjcpidocyrtus carvi-

small scale, more faintly marked. 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 lias a cylindrical shaft (fig. 672, B) with closely set
whorls of spiny protuberances, four or five in each whorl ; the
highest of these whorls is composed of mere knobby spines ; and the
hair is surmounted by a curious circle of six or seven large filaments,
attached by their pointed ends to its shaft, whilst at their free ex-
tremities they dilate into knobs. An approach to this structure is
seen in the hairs of certain Myriopods (centipedes, galley- worms, &c),
of which an example is shown in fig. 672, A ; but a beautiful photo-

J Jov/rn. Boy. Micros. Soc. vol. ii. 1879, p. 810.

HAIRS

905

1

micrograph of the hair of Polyxenus lagurus, of the family Poly-
desmidce (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 de- 3
scribed in Chapter VII. pp. 388-390. The tribe
of Curculionicke, 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
Curculio im perialis, 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
Curculio niche, 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 ascertain 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 Lepicloptera are best mounted as
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. 672. — A, hauf of
Myriopod\ B, hair of
Dermestes.

go6

INSECTS AND AR ACHNID A

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 wdiat 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 wrell as
of scientific interest, are sure to reward such as may prosecute it
with any assiduity. Further information may often be gained by
softening such parts in potash and viewing them in fluid. The

scales of the wings of Lepido-
ptera etc. are best transferred
t o the slide by simply pressing a
p< irtion of the wing either upon
the slip of glass or upon the
cover ; if none should adhere
the glass may first be gently
breathed on. Some of them
are best seen when examined
' dry,' whilst others are more
clear when mounted in fiuid ;
and for the determination of
their exact structure it is well
to have recourse to both these
Fig. G73.— Head and compound eyes of the methods. Hairs, Oil the other
bee, showing the oceUites in ^ on one , } b t mounted in

side, A, and displaced on the other, B ; , , '
a, a, a, stemmata ; b, b, antennae. balsam.

Parts of the Head. -The
eyes of insects, situated upon the upper and outer part of the head,
are usually very conspicuous organs, and are frequently so large as
to touch each other in front (iig. 673). We find in their structure
a remarkable example of that multiplication of similar parts which
seems to be the predominating 'idea' in the conformation of arti-
culated animals ; for each of the large protuberant bodies which we
designate as an eye is really a ' compound 5 eye, made up of many
hundred or even many thousand minute conical ocelli (I>). Ap-
proaches to this structure are seen in Entomostraca ; but the
number of 'ocellites' thus grouped together is usually small.
In the higher Crustacea, however, the 'ocelli' are very numerous ;
and their compound eyes are constructed upon the same general
plan as those of insects, though their shape and position are often
very peculiar. The individual ocelli are at once recognised when
the 'compound eyes' are examined under even a low magnifying
.power by the 'faceted' appearance of the surface (fig. 673, A),
which is marked out by very regular divisions either into hexagons

EYES

90/

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
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 bettle 25,000.
The structure of the arthropod eye
is best explained by a comparative
account of the various stages of
complication which it presents.
In various larvae the cuticular
layer is modified to form a single
lens, behind which are simple, sepa-
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 these last combine to form
groups, 'retinuke': the sensitive
cells may become divided into two
regions, an outer one, which is
' vitreous' and refractive in function,
while the inner part remains sensi-
tive : the corneal surface may be-
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.
675) 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

Fig. 674. — Diagram of a section of the
composite eye of Melolontha vul-
garis (cockchafer) : a, facets of the
cornea ; b, transparent pyramids
surrounded with pigment ; c, fibres
of the optic nerve; d, trunk of the
optic nerve.

from

mixing

no rays, save

with each other ; and
those which pass

m

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. 673) 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 com-
bined action of the entire aggregate will probably only afford but

Fig. 07-3. — Part of the compound eye
of Phryyanca ; the retinal cells are
seen to be united into a retinula (r)
which is differentiated into a rhab-
dom [m) posteriory; cc, crystalline
cone : /. facet of compound eye ;
pg} pigment. (After Grenadier.)

908

INSECTS AND ABACHN1DA

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 taken with
the retina removed at the back of the eye of one of the higher
vertebrates.

The demonstration has just reached us as these sheets are pass-
ing through the press, and the present Editor is indebted for a
knowledge of the following details to the courtesy of a private com-
munication from Professor Exner.

The general result of the researches on the subject is presented
in fig. 675a, which is the image at the back of the compound eye

of Lampyris splendid n J a
(fire-fly) in the position
in which it would be por-
trayed upon the retina, but
mainlined 120 diameters.
On to the window pane a
letter R cut out in black
was fixed ; the distance of
the window from the eye
w as 225 cm. while the dis-
tance of the church from the
w indow through which it is
seen in the magnified image
was 135 paces.

The result is unmistak-
able ; there may appear to be
some matters of interest still
needing interpretation, but
those are explained in the
monograph by Exner, giving
the complete details of the
method he adopted and the
mathematical explanation of
the results he obtained. The
rectitude of the image and
the reversion of the R are
certainly noteworthy; and
as a contribution to our
knowledge of the physiology
of sight in insects and other animals with compound eyes, the im-
portance of the result obtained by the ingenuity and skill of Professor
Exner is great, giving us a new start on solid ground. The mathe-

Fig. 075 A. — Image of a window with the
letter R on one of its panes, and a church
beyond, taken through the compound eye
of Lampyris splendid 'u la, and magnified
120 diams.

EYES

909

uiatics of the question are fully discussed by Exner 1 in a memoir, to
which the student must be referred for complete information. 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. G 75a
was this : The eye of the Lampyris 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 Lampyris. The
whole was placed upon an ordinary cover-glass, and this being fixed
by its edges to a slide or object-carrier with a circular aperture cut in
it, as in fig. 675b, C ; a is the slide with an aperture less in diameter
than the cover-glass h cut through it ; c is the fluid-medium of

Fig. 675b. — Diagrammatic illustration of the method by which
the image in fig. 675a was photo-micrographed.

?i=l*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-
be now examined with a microscope (the C of Zeiss was employed)
the 'lenses' will be distinctly seen, but if the focus be readjusted

1 Sitzungsber.AJcad. Wissench. Wien, Bd. xcviii. (1881)), pp. 13, 1-43 ; also Die Phy-
siologic der Facettirten Ait gen von Krebscn und Insccten (Leipzig und Wien, 1891).

a A critical history of the discussion will be found in Chapter VII. of Sir J.
Lubbock's Senses of Animals (London, 1888). 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, TJntcrsuchungen uber
das Sehorgan der Arthropoden &e. (Gottingen, 1879) ; Carriere, Die Schorgane der
Tliiere &c. (Munich and Leipzig, 1885) ; Hickson, ' The Eye and Optic Tract of In-
sects,' Quart. Joum. Micros. Sci. xxv. p. 215; Lankester and Bourne, "The Minute
Structure of the Lateral and Central Eyes of Scorpio and Limulus,' Quart. Jonrn.
Micros. Sci. xxiii. p. 177 ; Lowne ' On the Compound Vision and the Morphology
of the Eye in Insects,' Trans. Linn. Soc. (2), ii. p. 3S9 ; Patten, ' Eyes of Molluscs
and Arthropods,' Mitth. Zool. Stat. Neapel, vi.

9IO INSECTS AND AKACHNIDA

to the focal plane of the image in the eye this image will be seen
and magnified. This will be understood from D (fig. 675b), where
e, f represent the image, h the cornea with its ' lenses ' g, 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 diagrammatically
at E (fig. 675b), where % indicates the cornea of the eye exposed
to air, k. the image thrown through the ' lenses ' as a unified
picture at the focal point of the microscope, and / is the sensitised
plate on which the image was photographed.

This is doubtless but the beginning of much similar work to be
done by others in Germany and elsewhere ; but this piece of admir-
able 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 fluid or ' aqueous
humour.' In other instances, again, this space is occupied by a
double-convex body, which seems to represent the ' crystalline
lens,' and this bo ly 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 Arc. Besides
their 'compound' eyes, insects usually possess a small number of
' simple* eyes (termed ocelli or stemmata) seated upon the top of the
head (tig. 673, a, a, a). Each of these consists of a single very con-
vex corneule, to the back of which proceeds a bundle of rods that
are in connection with fibrils of the optic nerve. Such ocelli are
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 insects
may be adopted, according to the manner wherein they are to be
viewed. For the observation of their external faceted surface by
reflected light it is better to lay down the entire head, so as to
present a front face or a side face, according to the position of the
eyes, the former giving a view of both eyes when they approach
each other so as nearly or quite to meet (as in fig. 673), whilst the
latter will best display one when the eyes are situated more at the
sides of the head. For the minuter examination of the ' corneules,'
however, these must be separated from the hemispheroidal mass
whose exterior they form by prolonged maceration, and the pig-
ment must be carefully washed away by means of a fine camel-hair
brush from the inner or posterior surface. In flattening them out
upon the glass slide one of two things must necessarily happen,
either the margin must tear when the central portion is pressed
down to a level, or, the margin remaining entire, the central por-

tion must be thrown into plaits, so that its corneules overlap one
another. As the latter condition interferes with the examination
of the 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 be cut very thin,
and should be mounted in Canada balsam. The following are some
of the insects whose eyes are best adapted for microscopic pre-
parations : Coleoptera, Cicindela, Dytiscus, Melolontha (cockchafer),
Lucanus (stag-beetle) ; Orthoptera, Acheta (house and field crickets),
Locusta : Hemiptera, Notonecta (boat-fly) ; Xeuroptera, Libellula
(dragon-fly), Agrion ; Hymenoptera, Yespida? (wasps) and Apida*
(bees) of all kinds ; Lepidoptera, Vanessa (various species of), Sphinx
ligustri (privet hawk-moth), Bombyx (silkworm moth and its allies);
Diptera, Tabanus (gad-fly), Asilus, Eristalis (drone-fly), Tipula (crane-
fly), Alusca (house-fly), and
many others.

The antennce, which are
the two jointed appendages
arising from the upper part
of the head of insects (fig.
673, b 6), present a most
wonderful variety of confor-
mation in the several tribes
of insects, often differing
considerably in the several
species of one genus, and
even in the two sexes of the
same species. Hence the
characters which they 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, etc. distinguished by the toothed or serrated form of
the antenna?, and hence called Serricornes ; 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 Clavicornes ;
in another, again, of which the HydrophUus, or large water-beetle,
is an example, the antenna? are never longer, and are commonly
shorter, than one of the pairs of palpi, whence the name of Palpi-

Fig. G76. — Antenna of jlcloJonUia
(cockchafer).

912 INSECTS AND AKACHNIDA

comes is given to this group ; in the very large family that in-
cludes the Lucani, or stag-beetles, with the Scarabcei, of which the
cockchafer is the commonest example, the antenna? terminate in a
set of leaf -like appendages, which are sometimes arranged like a fan
or the leaves of an open book (fig. 676), are sometimes parallel to
each other like the teeth of a comb, and sometimes fold one over
the other, thence giving the name Lamellicornes ; whilst another
large family is distinguished by the appellation Longicornes, from
the great length of the antennae, 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 dis-
tinguish the group to which any specimen belongs. As every treatise
on entomology contains 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, Hydro-
philus, Aphodius, Melolontha, Cetonia, Curculio; Orihoptera, Forficula
(earwig), Blatt;i (cockroach) ; Lepidoptera^ Sphingida? (hawk-moths),
and Xoctuina (moths) of various kinds, the large c plumed ' antenna?
of the latter being peculiarly beautiful objects under a low magni-
fying power ; />/'/'/' r<>. Culicidse (gnats of various kinds), Tipulidse
(crane-flies and midges), Tabanus, Eristalis, and Muscida* (Mies of

various kinds). All the
larger antenna1, 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 antennae of gnats
and midges arc so liable
to distortion when thus
mounted that it is better
to set them up in fluid, the
head with its pair of an-
tennae being thus preserved
together when not too
large. A curiou < i of organs has been recently discovered in the
antenna* of many Insects, which have been supposed to constitute
collectively an apparatus for hearing. Kach consists of a cavity
hollowed out in the horny integument, sometimes nearly spherical,
sometimes flask -shaped, and sometimes prolonged into numerous
extensions formed by the folding of its lining membrane ; the mouth
of the cavity seems to be normally closed in by a continuation of this
membrane, though its presence cannot always be satisfactorily deter-
mined ; whilst to its deepest part a nerve-fibre may be traced. The
expanded lamella' of the antenna1 of A/rlolontha present a great dis-
play of these cavities, which are indicated in fig. 077, A, by the
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

A B

Fig. 077. —Minute structure of leaf-like expan-
sions of antenna of Melolontha: A, their in-
ternal layer; B, fcheuf superficial layer.

THE MOUTH OF INSECTS

913

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 Coleoptera, or beetles, that
we find the several parts of which the mouth is composed in their
most distinct form ; for, although some of these parts are much more
highly developed in other insects, other parts may be so much altered
or so little developed as to be scarcely recognisable. The Coleoptera
present the typical conformation of the 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 maxillce, 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 1 ah rum ; -1, a lower lip or labium ; 5, one or two pairs
of small jointed appendages, termed palpi, attached to the maxill;e,
and hence called maxillary palpi ; 6, a pair of labial palpi. The
labium 2 is often composed of several distinct parts, its basal portion
being distinguished as the mentum or chin, and its anterior portion
being sometimes considerably prolonged forwards, so as to form an
organ which is properly designated the ligula, 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 fly kind, in which it forms the
chief part of what is commonly called the 'proboscis' (fig. 678) ;3
and it also forms the ' tongue ' of the bee and its allies (fig. 679).

1 See the memoir of Dr. Hicks, ' On a new Structure in the Antennae of Insects,'
in Trans. Linn. Soc. xxii. p. 147; and his 'Further Remarks' at p. 383 of the
same volume. See also the memoir of M. Lespes, ' Sur l'Appareil auditif des Insectes,'
in Ann. des Sci. Xat. ser. iv. zool. torn. ix. p. 258; and that of M. Claparede, ' Sur les
pretendus Organes auditifs des Coleoptcres lamellicornes et autres Insectes,' in Ann.
des Sci. Nat. ser. iv. zool. torn. x. p. 236. Dr. Hicks lays great stress on the ' bleach-
ing process ' 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 a
drachm and a half ; mix these in a small wide bottle containing about an ounce ; wait
five minutes and then add about a drachm and a half of strong hydrochloric acid.
Chlorine is thus slowly developed, and the mixture will retain its bleaching power
for some time.

2 The labium and the labial palps are, morphologically, a second pair of maxilla?
which have undergone more or less fusion of the basal parts along the median line.

3 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
reader is referred to Mr. Suffolk's memoir, ' On the Proboscis of the Blow-fly,' in
Monthly Microsc. Journ. i. p. 831, and to Mr. Lowne's treatise on The Anatomy
unci Physiology of tlie Blow-fly, p. 41 (a new and elaborate edition of this work ik
in process of publication).

INSECTS AND ARACHNID A

Fig. 078. — A, tongue of common fly: a, lobes of ligula; b, portion inclosing
the lancets, formed by the metamorphosis of the maxillae; c, maxillary
palpi. B, a portion of some of the pseudo-tracheae more highly magnified.

MOUTH-PARTS OF INSECTS

915

The ligula of the common fly presents a curious modification of the
ordinary tracheal 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 contiDuous 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. 678, 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 setcr, which, when closed together, are received into
a hollow on the upper side of the labium (fig. 678), 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, however, two or more of these organs may
"be wanting, so that their number is reduced from six to four, three,
or two. In the HyniPMoptera (bee and wasp tribe) the labrum and
the mandibles (fig. 679, b) much
resemble those of mandibulate
insects, and are used for corre-
sponding purposes ; the maxilla?
(c) are greatly elongated, and
form, when closed, a tubular
sheath for the ligula, or 'tongue,'
through which the honey is drawn
up ; the labial palpi (d) also are
greatly developed, and fold to-
gether, like the maxilla?, 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 divi-
sions, 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 ex-
tended to a great distance beyond
the other parts of the mouth ;
but when at rest it is closely
packed up and concealed between
the maxilla?. ' 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

1 According to Dr. Anthony (Monthly Microscopical Journ. vol. xi. p. 242), these
' pseudo-trachea? ' are suctorial organs, which can take in liquid alike at their ex-
tremities and through the whole length of the fissure caused by the interruption of
the rings, the edges of this fissure being formed by the alternating series of ' ear-like
appendages ' connected with the terminal ' arches,' the closing together of which
converts0 the pseudo-trachea? into a complete tube. Dr. Anthony considers each of
these ear-like appendages to be a minute sucker, ' either for the adhesion of the fleshy
tongue, or for the imbibition of fluids, or perhaps for both purposes,' The point i«
well worthy of further investigation.

o x 2

Fig. 079. — Parts of the mouth of Apis
mellifica (honey-bee) : a, mentum; b,
mandibles ; c, maxilla? ; d, labial palpi ;
e, ligula, or prolonged labium, com-
monly termed the 1 tongue.'

INSECTS AND ARACHNID A

constant succession of short and quick extensions and contractions
of the organ, which occasion the fluid to accumulate upon it and
to ascend along its upper surface, until it reaches the orifice of the
tube formed by the approximation of the maxillae above, and of the
labial palpi and this part of the ligula below.'

By the plan of conformation just described we are led to that
which prevails among the Lepidoptem, 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 maxillae are immensely
elongated, and are united together along the median line to form the
kaustelUum, or true ' proboscis,' which contains a tube formed by the
junction of the two grooves that are channelled out along their
mutually applied surfaces, 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 haustellium varies
greatly : thus in such Lepidoplera 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 -

I'n.. OHO. — Hiinstcllium (proboscis) of \'<t iicr.sn .

inches in length, and in most butterflies and moths it is about as
Long as the body itself; in Ain]>hon;/x, one of the Sphingidce, it is
more than nine inches Long, or about three times the length of the
body. This haustellium, which, when not in use, is coiled up in a
spiral beneath tin- mouth, is an extremely beautiful microscopic
object, owing t<> the peculiar banded arrangement it exhibits (tig.
680), which is probably due to the disposition of its muscles. In
many instances the two halves maybe 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, may
be 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
great rapidity) is effected by the agency of the respiratory apparatus.
The proboscis of many butterflies is furnished, for some distance from

PARTS OF THE BODY

917

its extremity, with a double row of small projecting barrel-shaped
bodies (shown in fig. 680), 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 ' musgular
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 larvre, 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 larva3, however, these
partitions are very indistinct ; and the walls of the ' dorsal vessel '
are so thin and transparent that it can with difficulty be made out,
a limitation of the light by the diaphragm being often necessary.
The blood which moves through this trunk, and which is distributed
by it to the body, is a transparent and nearly colourless fluid, carry-
ing with it a number of £ oat-shaped ' corpuscles, by the motion of
which its flow can be followed. The current enters the £ dorsal
vessel ' at its posterior extremity, and is propelled forwards by the
contractions 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
of the Ephemera marginata (day-fly), the extreme transparence of

1 The student who desires to carry further the study of the digestive apparatus
should consult Professor Plateau's memoir, ' Eecherches sur les Phenomenes de
la Digestion chez les Insectes,' Mem. Acad. Boy. de Belgique, xli.

918

INSECTS AXD ARACHNID A

which renders 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 walls : but it is not improbable
that in the perfect insect the case may be different. In many aquatic
larva1, 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 Annelida.1

The circulation may be easily seen in the wings of many insects
in their pupa state, especially in those of the Neuroptera (such as
dragon-flies and day-flies) which pass this part of their lives under
water in a condition of activity, the pupa of Agrion jmella, 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 of
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*
Tlx- 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 sufficient to keep the body at rest
without doing it any injury.

The respiratory apparatus of insects affords a very interest-
ing series of microscopic objects j for, with great, uniformity in its
general plan, there is almost infinite variety in its details. The
aeration of the blood in this class is provided for, not by the trans-
mission of the fluid to any special organ representing the lung of a
vertebrated animal or the gill of a mollusc, but by the introduction
of air into every part of the body, through a system of minutely
distributed trorln n , or air-tubes, which penetrate even the smallest
and most delicate organs. Thus, as we have seen, they pass into
the haustellium, or 'proboscis,' of the butterfly, and they are minutely

1 Seethememoirsf.n Corethra phimicornis,hy Professor Rymer Jones, in Trans.
Micros. Soc. o.B. vol. xv. Ihct. p, 90; by Professor E.Kay Lankester, in the Popular
Science "Review for October iMi;;, ; ar.d by Dr. A. Weismann, in Zeitschr.f. Wiss. Zool.
Bd. xvi. p. 45. On the circulatory system of insects consult Graber, ' Ueber den pro-
pulsatorisfhen Appaxat der Insecten,' Arch, fiir Mikr. Anat. ix. p. 139,

RESPIRATORY APPARATUS

919

distributed in the elongated labium or ' tongue,' of the fly (fig. 678).
Their general distribution is shown in fig. 681, where we see two
long trunks (/) passing from one end of the body to the other, and
connected with each other by a transverse canal in every segment ;
these trunks communicate, on the one hand, by short wide passages
with the 'stigmata,' 'spiracles,' or 'breathing pores ' (g), through
which the air enters and is discharged ; whilst they give off branches
to the different segments,
which divide again and
again into ramifications of
extreme minuteness. They
usually communicate also
with a pair of air-sacs (h)
which is situated in the
thorax ■ but the size of
these (which are only found
in the perfect insect, no
trace of them existing in
the larvse) varies greatly
in different tribes, being
usually greatest in those
insects which (like the bee)
can sustain the longest and
most powerful flight, and
least in such as habitually
live upon the ground or
upon the surface of the
water. The structure of
the air-tubes reminds us
of that of the ' spiral
vessels' of plants, which
seemed destined (in part
at least) to perform a
similar office ; for within
the membrane that forms
their outer wall an elastic
fibre winds round and
round, so as to form a

spiral closely resembling Fig. 681.— Tracheal system of Nepa (water-
in its position and func- scorpion) : a, head ; 6, first pair of legs ; ; c, first
, • xi • 1 • • segment of thorax ; d, second pair of wings ; e,

tions the spiral Wire spring se*ond pair of legs; /, tracheal trunk; g, one

of flexible gas pipes ; with- of the stigmata; h, air-sac.

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
tracheal system has been dissected out, and so pressed in mounting
that the sides of the tubes are flattened against each other (as has
happened in the specimen represented in fig. 682), the spire forms
two layers which are brought into close apposition, and a very
beautiful appearance, resembling that of watered silk, is produced

920

INSECTS AND AEACKSIDA

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 1 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 each
margin of the under sur-
face. In most larva-,
nearly every segment is
provided with a pair, but
in the perfect insect
several of them remain
closed, especially in the
thoracic region, so that
their number is often con-
siderably reduced. The
structure of the spiracles
varies greatly in regard to
complexity in 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
which particles of dust, soot, ifcc. which would otherwise enter the
air-passages are filtered out ; and this sieve may be formed by

th<' i nt erlacemont of the

Fig. 682. — Portion of a large trachea of Dyti, C
with some of its principal branches.

branches of minute arbo-
rescent growths from the
binder of the spiracle, as
^ in the common fly (fig.
i4\f,/ 6b 3), or in the DytisGus ;
) ) or it may ho a membrane
: perforated wit h minute
holes, and supported upon
wii a framework of bars that
w is prolonged in like manner
from the thickened margin
of the aperture (fig. 684),
as in the larva of the
Mrlohmtlm (cockchafer). Not unfrequently the centre of the aper-
ture is occupied by an impervious disc, from which radii proceed
to its margin, as is well seen in the spiracle of Tipula (crane-
fly),1 In those aquatic larvae which breathe air we often find one

1 Consult Landois unci Thiele, ' Der Tracheenverschluss hei den Insecten,' Zeit-
schr/ftf. Wm. Zoul. xvii. p. 187.

Fic. <;h:{. — Spiracle of common fly.

RESPIRATORY APPARATUS

921

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 larvae of the ynat tribe may frequently be observed
in this position.

There are many aquatic larvae, 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 Ephemera (day-
fly), the body of which is furnished with a set of branchial appendages
resembling the ' tin-feet ' of branchiopocls, whilst the three-pronged
tail also is fringed with clusters of delicate hairs which appear to
minister to the same function. In the larva of the Libellula
(dragon-fly) the extension of the surface for aquatic respiration
takes plate within the termination of the intestine, the lining
membrane of which is folded into an immense number of plaits,
each containing a minutely ramified system of tracheae ; the water
slowly drawn in through the anus
for bathing this surface is ejected
with such violence that the body
is impelled in the opposite direc-
tion ; and the air taken up by its
tracheae is carried through the
system of air-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 extraordinary
copiousness of the distribution of
the tracheae in the intestinal folds.

The main trunks of the tracheal
system, with their principal ramifi-
cations, may generally be got out with little difficulty by laying
open the body of an insect or larva under water in a dissecting
trough, and removing the whole visceral mass, taking care to leave
as many as possible of the branches, which will be seen pro-
ceeding to this from the two great longitudinal trachea3, to whose
position these branches will serve as a guide. Mr. Quekett recom-
mends the following as the most simple method of obtaining a
perfect system of tracheal tubes from a larva. A small opening
having been made in its body, this is to be placed in strong acetic
acid, which will soften or decompose all the viscera ; and the 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

Fig. GS4. — Spiracle of larva of
cockchafer.

922

INSECTS AND AKACHNIDA

by mounting them in fluid (weak spirit or Goadby's solution), using
a shallow cell to prevent pressure. The finer ramifications of the
tracheal 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
tracheae 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.

Wing's. — 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 horny layer, within
which prolongations tracheae are nearly always to be found, whilst
they also include channels through which blood circulates during
the growth of the wing and for a short time after its completion.
This is the simple structure presented to us in the wings of JVeuro-
ptera (dragon-Hies kc.), Hymenoptera (bees and wasps), Diptera
(two-winged Hies), and also of many 11 omoptera (Cicada1 and Ajiliidcs) ;
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 Xeuroptera, since it is the distinguishing feature of
this order that the veins, after subdividing, reunite again, so as to
form a close network ; whilst in the Hymenoptera and Diptera such
reunions are rare, especially towards the margins of the wings, and
the areola; are much larger. Although the membrane of which
these wings are composed appeal's 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 areola) lias a hair in
its centre. In order to make this observation, as well as to bring
out the very beautiful iridescent hues which the wings of many
minute insects (as the Aphides) exhibit when thus viewed, it is con-
venient to hold the wing in the stage-forceps for the sake of giving
it every variety of inclination ; and when that position has been
found which best displays its most interesting features, it should be
set up as nearly as possible in the same. For this purpose it should
be mounted on an opaque slide, but instead of being laid down
upon its surface the wing should be raised a little above it, its
' stalk ' being held in the proper position by a little cone of soft wax,
in the apex of which it may be imbedded. The wings of most
Hymenoptera are remarkable for the peculiar apparatus by which

WINGS ; SOUND-ORGANS

923

those of the same side are connected together, so as to constitute in
flight but one large wing ; this consists of a row of curved hooklets
on the anterior margin of the posterior wing, which lay hold of the
thickened and doubled down posterior edge of the anterior wing.
These hooklets are sufficiently apparent in the wings of the common
bee, When examined with even a low magnifying power ; but they
are 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
Tineidce or ' clothes-moths,' form very beautiful opaque objects for
low powers, the most beautiful of all being the divided wings of
the Fissijwnnes or -plumed moths,' especially those of the genus
Pterophorus.

There are many insects, however, in which the wings are more or
less consolidated by the interposition of a layer of horny substance
between the two layers of membrane. This plan of structure is
most fully carried out in the Coleoptera (beetles), whose anterior
wings are metamorphosed into elytra or ' wing-cases ' ; and it is
upon these that the brilliant hues by which the integument of many
of these insects is distinguished are most strikingly displayed. In
the anterior wings of the Forjiculidaj1 or earwig tribe, the cellular
structure may often be readily distinguished when they are viewed
by transmitted light, especially after having being mounted in Canada
balsam. The anterior wings of the Orthoptera (grasshoppers,
crickets, &c), although not by any means so solidified as those of
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
as to be very showy objects (as are also the posterior fan-like wings)
for the electric or gas microscope, although their large size and the
absence of any minute structure prevent them from affording much
interest to the ordinary microscopist. We must not omit to men-
tion, however, the curious sound-producing apparatus which is
possessed by most insects of this order, and especially by the common
house-cricket. This consists of the ' tympanum,' or drum, which is
a space on 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 tympanum, furnished with numerous
teeth : and it is believed that the sound is produced by the rubbing
of the two bows across each other, while its intensity is increased
by the sound-board action of the tympanum. The wings of the
Fxdgorido- (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
Uieadce and Aphides, which are associated with them in the order
Homoptera. In the order Ilemiptera, to which belong various kinds
of land and water insects that have a suctorial mouth resembling
that of the common bug, the wings of the anterior pair are usually
of parchmenty consistence, though membranous near their tips, and

924

INSECTS AND ARACHNID A

are often so richly coloured as to become very beautiful 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 Xotonecta
(water-boatman) and the Xepa (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 five,"2 but that
number is subject to reduction ; and the vast order Coleoptera is
subdivided into primary groups, according as the tarsus consists of
five, four, or three segments. The last joint of the tarsus is usually
furnished with a pair of si rong hooks or claws (figs. 685, 686) j 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 4 sole ' ; this is especially the
case in the family Curculionichv ; a pair of the feet of the ' diamond
beetle ' mounted so that one shows the upper surface made resplendent
by its jewel-like scales, and the other the hairy cushion beneath, is
a very interesting object. In many insects, especially of the fly
kind, the foot is furnished with a pair of membranous expansions
termed pulvilli (fig. 685) ; 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

1 See his memoir, ' On a new Organ in Insects,' in Journ. Linn. Soc. vol. i. 185(5,
p. 18G ; his ' Further Remarks on the Organs found on the Bases of the Halteres
and Wings of Insects,' in Trans. Linn. Soc. xxii. p. 141 ; and his memoir, 1 On
certain Sensory Organs in Insects hitherto undescrihed,' in Trans. Linn. Soc.
xxiii. p. 189.

i See, however, Professor Huxley (Anat. of Invertebral <■ . I nimaU, p. 348), who,
regarding the 'pulvillus ' of the cockroach as a joint, finds the number to be six.

FEET

925

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 : whilst,
on the other hand, that the
feet pour forth a secreted
fluid is evidenced by the
marks left by their attach-
ment on a clean surface of
glass. Although, when all
the hairs have the strain
put upon them equally, the
adhesion of their discs suf-
fices to support the insect,
yet each row may be de-
tached separately by the
gradual raising of the tarsus
and pul villi, as when we
remove a piece of adhesive
plaster by lifting it from
the edge or corner. Flies are
often found adherent to window-panes in the autumn, their reduced
strength not being sufficient to enable them to detach their tarsi.1
A similar apparatus on a far larger scale presents itself on the foot
of the Dytiscus (fig. 686, 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 (a) is extremely large, and is furnished with strong
radiating fibres ; a second (6) is a smaller one formed 011 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
structure, the large suckers being furnished, like the hairs of the
fly's foot, with secreting sacculi, which pour forth fluid through the

1 See Mr. Hepworth's communications to the Quart. Juurn. Microsc. Sci. vol. ii.
1854, p. 158. and vol. iii. 1855, p. 312. See also Mr. Tuffen West's memoir, ' On the
Foot of the Fly,' in Trans. Linn. Soc. xxii. p. 393; Mr. Lowne's Anatomy of
the Blow-fly, p. 19; H. Dewitz in Zoologisclu r Angt i;;er, vi. p. 278; and G. Sim-
mermacher in Zeitschr.f. Wiss. Zool. xl. p. 481.

926

INSECTS AND ARACHNID A

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 Curcu-
lionidce.1 The leg and foot of the Dytiscus, if mounted without
compression, furnish a peculiarly beautiful object for the binocular
microscope. The feet of caterpillars differ considerably from those
of perfect insects. Those of the first three segments, which are
afterwards to be replaced by true legs, are furnished with strong-
horny claws ; but each of those of the other segments, which are
termed ' pro-legs,' is composed of a circular series of comparatively
•slender curved hooklets, by which the caterpillar is enabled to cling
to the minute roughnesses of the surface of the leaves &c. 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

Fig. 080. — A, foot of Dytiscus, showing its apparatus of suckers : a, b, large
in kers ; 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
division of the order is a 'sting,' and in the other is an 'ovipositor'
or instrument for the deposition of the eggs, which is usually also
provided with the means of boring a hole for their reception. The
former group consists of the bees, wasps, ants, &c. ; the latter of the
saw-flies, gall-flies, ichneumon-flies, &c. These two sets of instru-
ments are not so unlike in structure as they are in function.2 The

1 See Mr. Lowne, ' On the so-called Suckers of Dytiscus and the Pulvilli of Insects,'
in Monthly Microsc. Jourti. v. p. '207.

2 See Kraepelin, ' Untersuchungen iiber den Bau, Mechanismus und Entwicke-
lungsgeschichte der bienenartigen Thiere,' in Zeitschr, f. Wiss. Zool. xxiii. p. 28!) ;
Dewitz, ' Ueber Bau und Entwickelung des Stachels und der Legeschoide,' op. cit.
xxv. p. 174; and 'Ueber Bau und Entwickelung des Stachels der Ameisen/ op cit.
xxviii. p. 527,

STINGS AND OVIPOSITORS

927

* sting ' is usually formed of a 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 Ichneumonicto), 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 has a peculiarly
irritating effect upon the vegetable tissues, occasioning the production
of the ' galls,' which are new growths that serve not only to protect the
larvae, but also to afford them 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 Siricido-, 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 consists
of a pair of ' saws ' which are not unlike the ' stings ' of bees &c.
but are broader and toothed for a greater length, and are made to
slide along a firm piece that supports each blade, like the 1 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 Assure . beneath a sort of arch formed
by the terminal segment of the body. When a slit has been made
by the working of the saws they are withdrawn into this sheath ;
the ovipositor is then protruded from the end of the abdomen (the
body of the insect being curved downwards), and, being guided into
the slit by a pair of small hairy feelers, there deposits an egg.1
Many other insects, especially of the order Diptem, 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
of this is furnished by the gad-fly [Tahanus), whose ovipositor is

1 The above is the account of the process given by Mr. J. W. Gooch, who has
informed the Author that he has repeatedly verified the statement formerly made by
him {Science Gossip, Feb. 1, 1873), that the eggs are deposited, not, as originally
stated by 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.

928

INSECTS AND ARACHNID A

composed of several joints, capable of being drawn together or
extended like those of a telescope, and is terminated by boring
instruments ; and the egg being conveyed by its means, not only
into but through the integument of the ox, so as to be imbedded in
the tissue beneath, a peculiar 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. G87. — Various e^'gs, chiefly of the Acarina, etc.

going parts are besl seen when mounted in balsam, save in the case
of the muscles and poison-apparatus of the sting, which are better pre-
served in fluid or in glycerin jelly.

The sexual organs of insects furnish numerous objects of extreme
interest to the anatomist and physiologist ; but as an account of
them would be unsuitable to the present work, a reference to a
copious source of information respecting one of their most curious
features, and to a list of the species that afford good illustrations,
must here suffice.1 The eggs of not only the class Insecta, but of

1 Bee the memoirs of M. Lacaze-Duthiers/' Sxir l'Armuro (u'nitale rtes Insectes,' iti
Ann. cles Sci. Nat. ser. iii. zool. tomes xii. xiv. xvik xviii .\ix.; and M. Ch. Robin's

EGGS

929

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 held. In tig. 687 we
give a group of eggs, all but the central form being eggs of 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. 688). The
most interesting belong for the most part to the order Lepidoptera ;
and there are few among these that are not worth examination,
some of the commonest (such as those of the cabbage butterfly,

Fig. 688. — 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 (Ceryra
vinula), the privet hawk-moth (Sphinx Ugustri^ the small tortoise-
shell butterfly ( Vanessa urticce), the meadow-brown butterfly (Hip-
parchia janira), the brimstone-moth (Rumia cratcegata), and the
silkworm (Bombyx mori) may be particularly specified ; and, from
other orders, those of the cockroach (Blatta orientalis), field-cricket
(Acheta campestris), water-scorpion (Nepa ranatra), bug (Cimex

Memoire but les Objets qui peuvent etre conserves en Preparations mieroscopiques
(Paris, 1856), which is peculiarly full in the enumeration of the objects of interest
afforded by the class of Insects.

3 o

930

INSECTS AND ARACHNID A

lectularius), cow-dimg fly (Scatopliaga stereo raria), and blow -fly
(Musca vomitoria).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 some
of the best binocular effects.

The remarkable mode of reproduction that exists among the
Aphides must not pass unnoticed here, from its curious connection
with the non-sexual reproduction of Entomostraca and Rotifer a. 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 S9ason : 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 Professor Huxley 2 that the broods of viviparous
Aphides originate in ova which are not to be distinguished from those
deposited by the perfect winged female. Nevertheless, this non-
sexual or agamic reproduction must be considered analogous rather
to the 'gemmation' 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 be caused to continue (as it would
seem) indefinitely, without any recurrence to the sexual process.
Further, it seems now certain that this mode of reproduction is not
at all peculiar to the Aphides, but that many other insects ordinarily
multiply by 'agamic ' propagation, the production of males and the
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
perfect 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
drones; and others in the ordinary cells, which become workers or
neuters. It has long been known that these last are really un-
developed females, which, under certain conditions, might become
queens ; and it has been observed by bee-keepers that worker-bees,

1 Compare R. Lenekart in Arcliiv f. Anat. 1858, p. ;><>, ' debet die Mfcropyle and
den feinern Bau der Schalenhaut bei den Insecteneietn,' and A. Brandt, Ueber das
Ei unri Heine Jiilrf n nynhitte,, Leipzig, 1878.

2 'On the Agamic Reproduction and Morphology of Aphis1 in Trans. Linn.
Soc. xxii. p. 193.

DEVELOPMENT OE INSECTS

931

in common with virgin or unimpregnated queens, occasionally Jay
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 alone ■ 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 is a study of peculiar
interest, from the fact that it may be considered as divided (at
■least in such as undergo a 1 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 by
which the imago or perfect insect is produced within the body of
the pupa. Various circumstances combine, however, to render the
study a very difficult one ; so that it is not one to be taken up by
the inexperienced microscopist. The following summary of the
history of the process in the common blow-fly, however, will prob-
ably be acceptable. A gastrula with two membranous lamelhe
having been evolved in the first instance, the outer lamella very
rapidly shapes itself into the form of the larva, and shows a well-
marked segmental division. The alimentary canal, in like manner,
shapes itself from the inner lamella, at first being straight and
very capacious, including the whole yolk, but gradually becoming
narrow and tortuous as additional layers of cells are developed
between the two primitive lamella?, from which the other internal
organs are evolved. When the larva eomes forth from the egg it
• still contains the remains of the yolk ; it soon begins, however, to
feed voraciously ; and in no long period it grows to many thousand
times its original weight, without making any essential progress in
development, but simply accumulating material for future use. An
adequate store of nutriment (analogous to the ' supplemental yolk '
of Purpura) having thus been laid up within the body of the
larva, it resumes (so to speak) its embryonic development, its passage
into the pupa state, from which the imago is to come forth, involving
a degeneration of all the larval tissues ; whilst the tissues and
organs of the imago 'are redeveloped from cells which originate
from the disintegrated parts of the larva, under conditions similar
to those appertaining to the formation of the embryonic tissues from
the yolk.' The development of the segments of the head and body
in insects generally proceeds from the corresponding larval segments ;
but, according to Dr. Weismann, there is a marked exception in the
case of the Dipfera and other insects whose larvae are unfurnished
with legs, their head and thorax being newly formed from 'imaginal
discs,' which adhere to the nerves and tracheae of the anterior
extremity of the larva ; 2 and, strange as this assertion may seem,

1 See Professor Siebold's memoir, On true Parthenogenesis in Moths and Bees,
translated by W. S. Dallas (London, 1857) ; and his Beitrage zur Parthenogenesis
der Arthropoden (Leipzig, 1871).

2 See his ' Entwickelung der Dipteren ' in Zeitschrift f. Wiss. Z00J. xiii. and xiv.;
]Mr. Lowne's A natomu of the Blow-fly (1st ed.), pp. 6-9, 113-121 ; and A. Ivo\valevskvr

3 o 2

932

INSECTS AND ARACHNID A

it has been confirmed by the subsequent investigations of Mr,
Lowne.

The Arachnida, or scorpions and pseudo-scorpions, and the Ara-
neida or spiders, present much that is of interest even to the unscien-
tific who use the microscope only for pleasure. The general remarks
which have been made in regard to insects are equally 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
having eight legs instead of six. The Acarina are the true ' mites ' j
they generally have the legs adapted for walking, and some of them
are of active habits. The common cheese-mite, as seen by the naked
eye, is familiar to everyone ; 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 jonce. is one of the most amusing objects that can be
shown to the young. There are many other species, which closely
resemble the cheese -mite in structure and habits, but which feed
upon different substances : and some of these are extremely destruc-
tive.

The Acarina arc the smallest of the Arthropoda, and are specially
w ell fitted for microscopical examination; indeed, with the exception
of the Ixodida (including the Argosince), which attain a substantial
size, 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. Many
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 Lei080ma palmacittcf n m, Teyeocranus ce/)heiformis, T.
dentatus, and the adults of (U-yrij magus plumiger and G. jud/mifer
are admirable. They arc all British, and are found respectively on
lichen ;it the Land's End, on the fallen bark and needles of fir-trees,,
on fallen oak-wood, in the fodder in stables, and on cellar- walls.
Many of the Trombidiida and Hydrachnidce also are very beautiful ;
and the Dermaleichi, especially the males, and such creatures as
Myobia, Listrophorus, &c. are extremely curious. With the excep-
tion of the Phytoptida?, all Acarina in the adult stage have eight
legs and the constriction between cephalo-thorax and abdomen is.
far less marked than in insects and spiders — in many genera it is
wholly lost. The sexes are distinct and often very different from;
each other ; the reproduction is oviparous or ovo-viviparous — pos-
sibly in rare and exceptional instances viviparous. The ova are
usually elliptical or ova] ; in those which have a hard shell a
curious stage known as the 'deutovium ' exists : as the egg increases,
in size the shell splits into two sym metrical halves, which remain
attached to the lining membrane, but are widely separated, the

Beitrage zur Kemitnis der Nachenibrvonalcn-Entwickelung dev Musciden,' ZcilscJir..
f. Wish. Zool. xliv. p. 542.

PLATE XVIII.

West.NewTna.-n clrrorno

Ac ar ina.

MITES

933

membrane becoming the external covering in the space left. The
eggs of the so-called stone-mite (Petrobia lapichun) 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 "pyrifbrme ; they are good microscopical objects. The larva?
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 Orthoptera ; it
usually undergoes several ecdyses. In many species of the Oribatidcn
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 XIX, fig. 2). In the Trombidiidw, Tyroglyplii, &c.
the nymphs usually greatly resemble the adults ; in the Oribatidc-
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 tropin are extremely different in the respective families,
or even genera. In the more highly organised of the Gamastdce
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 anchylosed

T P 1 '

maxilla? and probably upper lip and lingua. Up the centre or 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 wholly within it, the muscles which withdraw them
often arising from quite the posterior end of the body. These man-
dibles are different in the two sexes, and those of the male often
have most remarkable appendages. One of the best examples is
that of Gamasus terribUis, a species found in moles* nests by Mr.
Michael. Professor Canestrini, of Padua, also has figured some very
singular forms. In the Oribatidw, Tetranyelais, the Sareoptidas,
&c. the mandibles are also chelate, but of two joints only, shorter,
more powerful, and not capable of such great protrusion. In the
Hydracltnidce, Trombidiince, kc. 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 maxillae
are large toothed crushing organs in the Oribatida ; they are very

1 Journ. cle VAnat. et de la Physiol., Robin, May 1870.

934

INSECTS AND ARACHNID A

strongly developed in Hoplophora, which is a wood-boring creature.
In other families they are more commonly joined, forming a maxil-
lary lip with a flexible edge for sucking purposes. The maxillary
palpi vary greatly ; in the Sarcoptidce, Myobia, &c. they are anchy-
losecl to the lip : in the Phytopti ISTalepa is of opinion that they are
needle-like piercing organs, but these may well be the maxilla?. In
some predatory forms, as Cheyletns, 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 Trorobidiidce and Gamasidce) ; five is the
most usual number. They are terminated by a sucker as in the Sar-
coptidce, where it is often very large ; or by a claw or claws, or both
together. In some parasitic species the claws are developed in a
special manner for holding the hairs of the host ; thus Myobia 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 lias a similar arrangement on the third leg.
Both these genera contain species which are parasites of the mouse,
and easily obtained. In the Oribatidcr, Tyroylyphi, kc. the legs
are all strictly walking organs ; but in Chey/etus, most GamasidcBt
Arc. the first pair are tactile, and not used in locomotion. The legs
generally correspond on the two sides of the body, but in Freyana
hi U rojmSy an extraordinary parasite of the cormorant discovered by
Mr. Michael (Plate N X. fig. .'>), 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 OribatidcB and
most Gamasida ; partly so in the Ixodidce ; 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 Leiosomapaltn<<-
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 Glycvphagu8 plumiger they are
elegant plumes ; in some Sarropfidfc, e.g. Sytubtotrs fripdis, some of
the simple setiform hairs are three times the length of the body ;
in the Trombidiida the body-hairs are often extremely fanciful. The
setiform hairs are the principal organs of touch, those on the front
legs being specially important. So sensitive are they that Clicyb'ttis
and some Gamasids, which are predatory and capture such active
creatures as Thysanaridce, are entirely eyeless, and trust to the
tactile sense only. Haller was of opinion that certain specialised

PLATE XIX.

Ac anna.

MITES

935

hairs had an auditory function. In the Ixodiche 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-
mata: The Gamasidce, Oribatidce, Tyroglyphidce, Sarcoptidce, etc.
are entirely without special organs of vision. The Hydrachnidce
have two pairs of simple eyes, each pair being so close together as
to look like a single eye. The 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 oesophagus, provided with distensor muscles on each side,
so as to make it a sucking organ ; it usually passes right through or
close under the great ganglion known as the brain ; in some species,
as Damceus genicidatus, 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 caecal ap-
pendages, not numerous, but often very large, occasionally larger
than the organ itself. A valve in many cases separates the ven-
triculus from the hind-gut, Avhich is commonly divided into what
may be called colon and rectum. In the Gamasidce a single very
large Malpighian vessel on each side of the body enters between
the two last-named divisions of the alimentary canal. These vessels
run right along the side of the body, and strong pulsation may be
seen in them. In the Oribatidce they are absent, their function
being apparently performed by supercoxal glands. The Tyro-
glyphidce, Sarcoptidce, Phytoptidce, etc. are without special respir-
atory organs ; the Oribatidce and some Uropoda have simple un-
branched tracheae, much in the same condition as those of Peripahis.
The other Gamasidce, the Trombidiidce, Gheyletidce, Ixodidce, Arc.
usually have branched trachea*, like insects ; air-sacs are occasion-
ally found, but not anything like the tracheal lungs or gills (so
called) of spiders and scorpions. The principal nerve-centre is
much concentrated, and consists usually of either a large supra -
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 Ixodes, 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 Araclinida ; thus in female Oribatidce they consist of a central
ovary, with an oviduct springing from near each end, in which the

93^

INSECTS AND ARACHNID A

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 may be found in most Gamasidce,
Hydrachniclce, &c. but without the ovipositor. Spermatheca1 are
often found in the Gamasidce, Tyroglyphidre, etc., and accessory
glands frequently accompany the vagina in almost all families. The
male system varies greatly, but is frequently constructed on similar
lines, preserving somewhat of the ' ring ' form.

The 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 Dermanyssus, however, are more leathery in texture,
and are parasitic during their whole lives, the former on bats, the
latter on birds. This family have the true stigmata, one on each
side of the ventral surface, usually between the second and third
pairs of legs ; these do not communicate directly 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 Argasides
must be included iii this group ; their tenacity of life and power of
existing without food are marvellous ; their bite is severe, but the
terrible stories told of the results of the bite of the Persian Argas
have not been supported on investigation.

The Oribatidce are mostly wholly chitinised, the chitin being
very hard and brittle. The stigmata are in tin? acetabula of the
legs. The pseudo-stigmata (hearing organs) of tins family have
been before referred to. Oribatidce are vegetable feeders, living in
moss, lichen, fungus, dead woorj, under bark of trees, Arc. and some
few species on aquatic plants. They are widely distributed from
the arctic regions to the equatorial, Hoplophora has the power of
withdrawing the Legs wholly within the carapace, and then shutting
down the cephalo-thorax against the abdomen, so as to close the
opening, when it appears like a chitinous ball ; from this power it
has been called the 'box-mite.' The sexes have not any external
difference.

The Trombidiidai are a large and varied group, mostly predatory

PLATE XX.

Ac anna.

MITES

937

and with soft, often velvety skins, frequently of scarlet and other
brilliant colours. The large Trombidium holosericum is a well-known
microscopical object. The Tetranycki are usually included in this
family ; they are, however, rather doubtful members ; they are the 1 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
tire, in gardens and is a beautiful object. The hexapod larvae 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
autumnalis, and are known in England as the ' harvest-bug,' and
in France as the rouget. The BdeJUdcR 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 Hydrachnidce, or water-mites, as well as the Trombidiidce,
have the two stigmata in the rostrum ; the legs are swimming
-organs, the sexes often very different ; they live in fresh water and
are often parasitic in their immature, but not in the adult stages.
They are mostly soft-bodied and often of brilliant colours.

The Limnocaridce are sometimes treated as a sub-family of the
Hydrachnidw, but are crawling, not swimming creatures, and are
found in fresh water ; but the Halicaridm, 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 Jfyobiidce 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, &c. ; the
genus Glyciphagus contains many singular and interesting forms, as
G. platygaster and G. Krameri, found in moles' nests. It is in this
family that the curious hypopial stage exists ; some of the indi-
viduals of some species, instead of following the ordinary life-history,
are thanged 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 Tarsonemidcv are minute creatures, some leaf-miners, some
parasitic on bees &c.

The Sarcoptidcp are divided into two great sub-families, the Sar-
coptince, or itch-mites, of which the well-known Sarcoptes scabiei of
man (Plate XX, fig. 4) is the type, and the Analgesince, or bird-parasite
mites ; all have soft bodies with finely striated cuticles. Sarcoptes

938

INSECTS AND ARACHN1DA

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 Analgesnce (Dermcdeichi) 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 i?i general form, but very different in other respects, is
Demodex folliculoriun, 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 Acanis will often be found within.
Similar parasites exist on the dog and pig.

There are numerous other curious and interesting forms which
cannot be included in any of the families mentioned above.

The number of objects furnished to the microscopist by the
spider 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
i organs of the latter.

The structure of the mouth is always mandibulate, and is less
complicated than that of the mandibulate insects. The respiratory
apparatus is not tracheal, as in insects and some Acarhta, but is
• 'instructed upon a very different plan, for the 'stigmata,' winch
are usually four in number on each side, open upon a like number
of respiratory sacculi, each of which contains a series of leaf-like
folds of its lining membrane upon which the blood is distributed so-
as to afford a large surface to the air.

In the structure of the limbs, the principal point worthy of
notice is the peculiar appendage with which they usually terminate,
for the strong claws, with a pair of which the last joint of the
foot is furnished, have their edges cut into comb-like teeth, which
appear to be used by the animal as cleansing instruments, and in
many cases for the manipulation of the silk of their snares. But a
feature deserving study by the microscopist is the physical cause of
the exquisite sensitiveness of these ' feet.' By resting these upon a

SPIDERS

939

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 taughtened
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 parts 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

Fig. 689. — Foot, with comb-like claws, of the
common spider (Epe'ira).

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. 690, A), those which
lie across the polygons of
the scaffolding are stud-
ded at regular intervals
with viscid globules, as
seen in fig. 690, 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 spicier, and can be erected or depressed, and one, many,
or all of the tubes crowning a spinneret may be caused to exude,
and have drawn from it or them the silk as the spider determines.
There can be no doubt that there is a 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 :

A

B

Fig. 690. — Ordinary thread (A) and viscid
thread (B) of the common spider.

94Q

INSECTS AND ARACHNIDA

(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, and the
habits of spiders offer a good scope for industrious study in the field.

94i

CHAPTER XXII

VEB TEBBA TED ANIMALS

We are now arrived at the highest division of the animal kingdom,
in which the bodily fabric attains its greatest development, not only
as to completeness, but also as to size ; and it is in most striking
contrast with the class we have been last considering. 1 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 ban 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 consti-
tute the loAvest types of the animal series, and the complex fabric of
man or other vertebrates, yet it appears from recent researches 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 ioraminiferal
shell does to the sarcodic substance which fills its cavities and
extends itself over its surface. For, as was first pointed out by

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 as the most recent
and elaborate treatises that have appeared in this country : The translation of
Strieker's Manual of Histology, published by the New Sydenham Society; the
Handbook for the Physiological Laboratory, by Drs. Burdon- Sanderson, Michael
Foster, Brunt on. and Klein ; the translation of the 4th edition of Professor Frey's
Histology and Histo- Chemistry of Man ; the ' General Anatomy ' of the 9th edition
of Quain's Anatomy, 1882; and the Atlas of Histology, by Professor Klein and Mr.
Noble Smith, 1880-1 (a new edition is now in course of publication).

942

VEKTEBEATED ANIMALS

Dr. Beale 1 the smallest living ' elementary parr ' of every organised
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 maybe termed formed material,
may present every gradation of character from a mere inc-rganfc
deposit to a highly organised structure, but is in every ease altogether
incapable of self-increase. A very detinite 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 1 endosarc 1 and the * eetosare ' 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 tin4 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 indepond
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 aotil it v. It is of such
cells, retaining more or less of their characteristic spheroidal shape,
that every mass of faJt% whether large or small, is chietly made up.
In a large number of cast's the cell shows itself in a somewhat
different form, the ( elementary part' being a c<>r|>use|e of pro to
plasm of which the exterior has undergone b Blight consolidation,
like that which constitutes the 1 primordial utricle' of the vegetable
cell or the ' eetosare ' of the A //'" but in which there is no proper-
distinction between 'cell-wall' and 'cell-contents.' This condition,
which is characteristically exhibited by the nearly globular colourless
corpuscles of the blood, appears to be common to all cells in t he in
cipient stage of their formation, and the progress of their develop
ment consists in the gradual differentiation of their parts, the 'cell-

1 Professor Beale's views are most systematically 6.Ypoiiinle<l in his lectures On the
Structure of the Simple Tissue* of the Human tioily, lwjl ; in his How to Work
with the Microscope, 5th edition, 18H0 ; and in the introductory portion of his DOW
edition of Todd and Bowman 'a Physiological Anatomy, 1S07. The principal reiultl
of the inquire* of German histologists on thin point, are well stated in a paper l»y
Dr. Duffm on ' Protoplasm, and the part it play* in the Actions ..f Living Being! ' in
Quart. Journ. Micros. Sci. n.s. vol. iii. 1868, p. 251. The &nthor feeli 11 naoBMftry,
however, to express his dissent from Professor Boale's view,-, in on. important pari loolflr,
viz. his denial of 'vital ' endowments to the ' formed material ' ol any of Hie titfKMI ;
since it seems to him illogical to designate contractile muscular fibre f for example)
as 'dead,' merely because it has not the power of lelf-reparation.

CELLS

943

wall' becoming distinctly separated from the 4 cell- contents,' and
these from the ' nucleus,' and the original protoplasm being very
commonly replaced more or less completely by some special product
(such as fat in the cells of adipose tissue, or haemoglobin 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
jihrous 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 1 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-iauclei, and that the fibres hold the
same relation to them that the ' walls ' and ' contents ' of cells do to
their germinal corpuscles. The transition from the one type to the
other is well seen in fibro-cartilage, in which the so-called 4 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 ' lacunar ' and
' canaliculi ' which are excavated in it (fig, 692) give lodgment to a
set of radiating corpuscles closely resembling those just described ;
and these are centres of ' germinal matter,' which appear to have an
active share in the formation and subsequent nutrition of the osseous

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 the recently published 3rd edition of Professor Strassburger's
Zellbildung unci ZelltlicUung, 1880. See also Dr. Klein's ' Observations on the
Structure of Cells and Nuclei ' in Quart. Journ. Micros. Soi. n.s. vol. xviii. 1878, p. 815,
and vol. xix. 1879, pp. 125, 404 ; and chap. xliv. of his Atlas of Histology. The
numerous essays of Flenmhng, in recent volumes of the Archiv f. mikr. Anat; Gruber,
on the Nucleus of Protozoa, in vol. xl. of the Zeitschr. f. Wiss. Zool.; and Carnoy, in
La- Cellule, may be studied by those who desire to carry further the history of the
cell.

944

VEKTEBRATED ANIMALS

texture. In dentine (or tooth-substance) we seem to have another
form of the same thing, the walls of its 'tubuli ' and the 'intertubular
substance' being the ' formed material ' that is produced from thread-
like prolongations of 1 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 stated,
indeed, with considerable confidence that the bodies of even the
highest animals are everywhere penetrated by that protoplasmic
substance of which those of the lowest and simplest arc entirely com-
posed ; and that this substance, which forms a continuous network
through almost every portion of the fabric, i> the main instrument
of the formation, nutrition, and reparation ot 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 essentia] 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 hone, Mich as in that
forming the scapula (shoulder-blade) of a mouse ; but they are dis
played more perfect lv by artificial sections, the details of 1 lie 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 lind 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 ' hit t ice w ork ' 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 lishes,
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 Vertebral*.
In the most developed kinds of 'Hat' 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 Vertebral a, the whole
thickness is usually more or Less 'cancellated/ that is, divided up
into minute medullary cavities. When we examine, under a low

1 This term is used in its most general sense, M including not only Mm proper
internal skeleton, but also the hard parts protecting the exterior of 1 1 ■ « * body, vrhlofc

form the dermal skeleton.

STRUCTURE OF BONE

945

magnifying power, a longitudinal section of a long bone, or a section
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, and are filled like it with marrow.
In the shafts of ' long ' bones these canals usually run in the direction
of their length, but are connected here and there by cross-branches ;
whilst in the flat bones they form an irregular network. On 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. 691) is the centre of a rod of bony
tissue (1), usually more or less circular in its form, which is arranged
around it in concentric rings, resembling those of an exogenous
stem. These rings are marked out and divided by circles of little
dark spots, which, when closely examined (2), are seen to be minute
flattened cavities excavated in the solid substance of the bone, from
the two flat sides of which
pass forth a number of
extremely minute tubules,
one set extending inwards,
or in the direction of the
centre of the system of
rings, and the other out-
wards, or in the direction
of its circumference ; and
by the inosculation of the
tubules (or canaliculi) of
the different rings with
each other a continuous
communication is esta-
blished between the ceil- FlG 691.— Minute structure of bone as seen in
tral Haversian canal and transverse section : 1, a rod surrounding an
the outermost part of the Haversian canal 3 showing the concentric
, , , , 1 , arrangement of the lamella? ; 2, the same, with

bony rod that surrounds the iacun£e and canaliculi; 4, portion of the
it, which doubtless minis- lamellae parallel with the external surface,
ters to the nutrition of

the texture. Blood-vessels are traceable into the Haversian canals,
but the ' canaliculi ' are far too minute to carry blood-corpuscles ; they
are occupied, however, in the living bone by threads of protoplasmic
substance, which bring the segments of ' germinal matter ' contained
in the lacunae into communication with the walls of the blood-
vessels.

The minute cavities or lacunct (sometimes but erroneously termed
'bone-corpuscles,' as if they were solid bodies), from which the cana-
liculi proceed (fig. .692), are highly characteristic of the true osseous
structure, being never deficient in the minutest parts of the bones
of the higher Vertebrata, although those of fishes are occasionally
destitute of them. The dark appearance which they present in
sections of a dried bone is not due to opacity, but is simply an optical
effect, dependent (like the blackness 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

3 p

946

VERTEBEATED ANIMALS

in the several classes of Vertebrate, and even in some instances in
the orders, so that it is often possible to determine the tribe to which
a bone belonged by the microscopic examination of even a minute
fragment of it. The following are the average dimensions of the
lacuna?, in characteristic examples drawn from the four principal
classes, expressed in fractions of an inch: —

Man .
Ostrich
Turtle .
Conger-eel

Lonn Diameter
1-1440 to 1-2400
1-1333 1-2260
1-375 „ 1-1150
1-550 1-1135

Short Diameter
1-4000 to 1-S000
1-5425 „ 1-9650
1-4500 .. 1-5S40
1-4500 .. 1-8000

The lacuna? of birds are thus distinguished from those of mam-
mals by their somewhat greater length and smaller breadth, but

they differ still more in the
remarkable tortuosity of their
canaliculi, which wind back-
wards ami forwards in a very
irregular manner. There is an
extra* ordinary increase in lengt h
in the lacuna* of reptiles, with-
out a corresponding increase in
breadth ; and this is also seen
in SOUK1 jis/it s, though in gen-
eral the lacuna' of the latter
are remarkable for their angularity of form and the fewness of their
radiations, as shown in tig. 693| which represents the lacuna' and
•canaliculi in the bony scale of the Lepidosteu* ('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

lacuna* in any bone do not bear any relation to the size of the animal

Fig. 692. — Lacuna' of osseous substance
«, central cavity ; h, its ramifications.

Fig. (51)3. — Section (if tin- bony scale of LtpidoiitUi i a, show-
ing the regular distribution ci the laonnae and <•( the connecting

canaliculi; b, small portion more highly magnified,

to which it belonged ; thus there is little or no perceptible diflerence
between their size in the enormous extinct [guanodon and in the
smallest lizard now inhabiting the earth. J>ut they bear a close rela-
tion to the size of the blood-corpuscles in the several classes ; and
this relation is particularly obvious in the ' perennibranchiate '-
Batrachia, the extraordinarily large size of whose blood-corpuscles
will be presently noticed.

TEETH

947

Proteus .
Siren

Menopoma

Lepidosiren

Pterodactvle

Long Diameter
1-570 to 1-980
1-290 „ 1-450
1-450 1-700
1-375 1-494
1-445 ,. 1-1185

Short Diameter
1-885 to 1-1200
1-540 „ 1-975
1-1300 „ 1-2100
1-980 „ 1-2200
1-4000 „ 1-5225 1

In preparing sections of bone it is important to avoid the pene-
tration of the Canada balsam into the interior of the lacunae and
canaliculi, since when these are filled by it they become almost
invisible. Hence it is preferable not to employ this cement at all,
except it may be in the first instance, but to rub down the section
beneath the finger, guarding its surface with a slice of cork or a slip
of gutta-percha, and to give it such a polish that it may be seen to
advantage even when mounted dry. As the polishing, however,
occupies much time, the benefit which is derived from covering the
surfaces of the specimen with Canada balsam may be obtained
without the injury resulting from the penetration of the balsam into
its interior, by adopting the following method. A quantity of
balsam proportioned to the size of the specimen is to be spread upon
a glass slip, and to be rendered stiffer by boiling, until it becomes
nearly solid when cold ; the same is to be done to the thin glass
cover ; next, the specimen being placed on the balsamed surface of
the slide, and being overlaid 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 quickly
pressed down, and the slide to be rapidly cooled, so as to give as
little time as possible for the penetration of the liquefied balsam into
the lacunar system. The same method may be employed in making
sections of teeth.2 The study of the ossein or organic basis of bone
should be pursued by macerating a fresh bone in dilute nitro -hydro-
chloric acid, then steeping it for some time in pure water, and
tearing thin shreds from the residual substance, which will be
found to consist of an imperfectly fibrillated material, allied in its
essential constitution to the ' white fibrous ' tissue.

Teeth. — The intimate structure of the teeth in the several classes
and orders of Yertebrata 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 the shark tribe, as in many other
fishes, the general structure of this dentine is extremely analogous 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. 694), and each of these canals

1 See Professor J. Quekett's memoir on this subject in the Trans. Micros. 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 Boyal College of Surgeons,
vol. ii. ' •

2 Some useful hints on the mode of making these preparations will be found m
the Quart. Journ. Micros. Sci. vol. vii. 1859, p. 258.

948

VERTEBRATE!) ANIMALS

being surrounded by a system of tubuli (tig. 695), which radiate into
the surrounding solid substance. These tubuli, however, do not enter
lacuna?, 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. 696). In the teeth of the higher Vertebrata, however,
we usually find the centre excavated into a single cavity (tig. 097),
and the remainder destitute of vascular canals ; but there are inter-
mediate cases (as in the teeth of the great fossil sloths) in which the
inner portion of the dentine is traversed by prolongations of this
cavity, conveying blood-vessels, which do not pass into the exterior

Fig. 09.4. — Perpendicohvi Motion <>f

tooth of Lammi, moderately en-

larged, showing network of me-
dullary i-aiials.

FlQ. 695i — Trans\ ri se Motion of DOT

tion of tooth of Pristis, more highly

magnified, showing orifices of me-
dullary cuiuiIh, with NVHteuiN of

radiating and inotoolating tuhn I i .

layers. The tubuli of the ' non-vascular ' dentine, which exist* 1>\

itself in the tooth < »f nearly .ill mammalia, and w hich in t he elephant
18 known as ' ivory,' all radiate from tho central oa\ ity, and pass

towards the surface of the tooth in a nearly parallel course. Their

diameter at their largest part averages th of an inch ; their

smallest branches are immeasurably tine. rl Jhe tubuli in their course
present greater and lesser undulations : the former are few in number,
but the latter are numerous ; and ai they occur at tin- game pari of
the course of so vend contiguous tubes they 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, puc
cessive stages of calcification. The tubuli are occupied, during the
life of the tooth, by delicate threads of protoplasmic substance, ex-
tending into them from the central pulp.

Two other substances, one of them harder and the other sofler

TEETH

949

Fig. G96. — Transverse
Myliobates (eagle
opaque object.

section of tooth of
ray), viewed as an

than dentine, are frequently found associated with it ; the former is
termed enamel, and the latter cementum or crusta petrosa. The enamel
is composed of long prisms, closely resembling those of the 1 prismatic '
shell-substance formerly described, but on a far more minute scale, the
diameter of the prisms not being more in man than ^Wth 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 stria?, 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. 697, 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-
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 Avith it
at the grinding surface of the
tooth ; and there is in such
teeth no continuous layer of
enamel over the crown. This

arrangement provides by the unequal icear of these three sub-
stances (of which the enamel is the hardest, and the cementum the
softest) for the constant maintenance of a rough surface, adapted to
triturate the tough vegetable substances on which these animate 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

Fig. 697. — Vertical section of human molar
tooth : a, enamel ; 6, cementum or crusta
petrosa; c, dentine or ivory; osseous
excrescence arising from hypertrophy of
cementum ; e, pulp-cavity ; /, osseous lacuna-
at outer part of dentine.

95°

VEETEBEATED AM3IAL3

in the embryonic tooth : and he has further shown that it is much.'
more frequently present than used to be supposed. The ce))ie)itum,
or crusta petrosa, 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 eementum has no
such vascularity, but forms a thin layer (tig. l>i)7, M, which envelopes
the root of the tooth commencing near the termination of the eap
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 Kdentata, as well as of mam
reptiles and ti>hes. it forms a thick continuous envelope over t ho
whole surface, until worn away at the crown.1

Dermal Skeleton. — The -kin of tishes, of ;i few amphibians, of
most reptiles, and of few mammals, is strengthened by plates of a
bdrny, Cartilaginous, bony, or even enamel-like texture, which are
sometimes fitted together at their edges, BO 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 CSSe that they are usually known as SCal*f, Although
we are accustomed t«» associate in our minds the 'scales' of lishes
with those of reptiles, yet essentially different structures have been

included under this name.
^KkAaBilHl^ those of the former and of

4/V',' ••• '"IN 'ft*" many of the latter being

iV* r&.'i&fi'fir ' de\ eloped in the stihsfanci

'•'ltV''dF5wSpV » ' .^f4wkj-> • of the true skin (with a

SnmwBwfi7f if i\ i layer of which, in addition

'" epidermis, they are

v\ '^M'-'i'*?? " H^S!*' ;uU;l> co\rit i|), and bear

"^ttffl^^^o^1^ ^llmw t '' j\> resemblance to car-

y'w .'. . :\im9^9WWBmiu^ '■ " V" tila^c and hone in their

Fio. 698.— Portion of ikin of tola, riewed M an ,, x ,u,v composition ;

opaque object whilst others, such as the

scales of snakes or t he tor-
toise-shell, are formed upon the surface of the true skin, and are
to be considered as analogous to nails, hoofs, Ac. and other- 'epi-
dermic appendages/ In nearly all the existing fishes the scales are
flexible, being but little consolidated by calcareous deposit ; and in
some species they are so thin and tr.-mspa rcn t 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

1 The student is 1 ecoinn:end»'d to a >i i - i 1 1 1 M r. ( . S . T. , j i n< , ' , M < i n u 1 1 1 < 1 1 1 >t n I a I
Anatomy, Human and Cotnjxinit i rr (tfnl edition, London, lHHih. If lu; ilcHirvH to
go further, the first part of Baunie's Odontologuche Fortchungen (Leipzig, 1889)

should be consulted.

SCALES OF FISHES

951

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 havre scales of the ctenoid kind (that is, furnished at their
posterior extremities with comb-like teeth, fig. 699), when dried
with its scales in situ, is a very beautiful opaque object for the low
powers of the microscope (fig. 698), especially with the binocular
arrangement. Care must be taken, however, that the light is made
to glance upon it in the most advan-
tageous manner, since the brilliance with
which it is reflected from the comb-like
projections entirely depends upon the
angle at which it falls upon them. The
only appearance of structure exhibited by
the thin flat scale of the eel, when ex-
amined microscopically, is the presence of
a layer of isolated spheroidal transparent
bodies, imbedded in a plate of like trans-
parence ; these, from the researches of
Professor W. C. Williamson 1 upon other
scales, appear not to be cells (as they
might readily be supposed to be), but con-
cretions of carbonate of lime. When the
scale of the eel is examined by polarised
light its surface exhibits a beautiful St.
Andrew's cross ; and if a plate of selenite FlG 699 _Scale of soie? viewed
be 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 lamime of a structureless trans-
parent substance like that of cartilage ; the outermost of these
laminse is the smallest, and the size of the plates increases pro-
gressive^ from without inwards, so that their margins appear on the
surface as a series of concentric lines ; and their surfaces are thrown
into ridges and furrows, which commonly have a radiating direction.
The inner layer is composed of numerous lamina? of a fibrous

i See his elaborate memoirs, ' On the Microscopic Structure of the Scales and
Dermal Teeth of some Ganoid and Placoid Fish,' in Phil. Tra ns 1849 ; '^ le -
gations into the Structure and Development of the Scales and Bones ot Uishes, m
Phil. Trans. 1851.

g$2 VEETEBKATED 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, reach, ike The structure of the ctenoid
scales (fig. 699), which we find in the sole, perch, pike, fax 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 Agassi/,
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 con-
solidated to a considerable extent by the calcification of their soft
substance ; but still 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 gemoid scales, on the other hand, t he w hole substance of
the scale is composed of a materia] 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 t he bones of the
vertebrate skeleton, and being very frequently identical with that
of the bones of the same tish, as is the cast' with the Lrpidosteiis (tig.
693), one of the few existing representatives of this order, which, ill
former ages of the earth's history, comprehended a large number of
important families. Their name (from yiu-us, splendour) is bestowed
on account of the smoothness, hardness, and high polish of the outer
surface of the BCales, which is 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 plaooid type, which charac-
terises 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 BOale usually consists
of a flattened plate of a rounded shape, with a hard spine projecting
from its centre ; in the sharks (to which tribe belongs the 'dog tish '
of our own coast) the scales have more of the shape of teeth. This
resemblance is not confined to external form ; for their intimate
structure strongly resembles that of dentine, their dense substance
being traversed by tubuli, which extend from their centre to their
circumference in minute ramifications, without any trace of osseous
lacuna*. These tooth-like scales are often so small as to he invisible
to the naked eye ; but they are well seen by drying a piece of the skin
to which they are attached, and mounting it in Canada balsam ; and
they are most brilliantly shown by the assistance of polarised light.

HAIR

953

A like structure is found to exist in the ' spiny rays ' of the dorsal fin,
which, also, are parts of the dermal skeleton ; and these rays usually
have* a central cavity filled with medulla, from which the tubuli
radiate towards the circumference. This structure is very well seen
in thin sections of the fossil ' spiny rays,' which, with the teeth and
scales, are often the sole relics of the vast multitudes of 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,
nails, elates, and horns (when not bony) of mammals are all epi-
dermic appendages ; that is, they are produced upon the surface, not
within the substance of the true skin, and are allied in structure to
the epidermis, being essentially composed of aggregations of cells
tilled 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 1 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.

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
sometimes be distinctly made out to consist of flattened scales
arranged in an imbricated manner, as in some of the hairs of the

iFor further information regarding the scales of fishes, see the papers by O.
Hertwig in vol. viii. of the Jenaische Zeitschrift; and vols. u. and v. ot the
Morpholog. Jahrbuch.

954

YERTEBKATED ANIMALS

sable (fig. 700) ; 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
almost the entire hair seems made up of thin M ailed polygonal cells
(fig. 701). The hair of the reindeer, though much larger, has a very

Fig. 700. — Hair of sable, showing large Fig. 701. — Hair of nuisk d»vr, consis'

rounded cells in it- interior, covered ing almost entirely of polygonal cells,

by imbricated scales or flattened c ells.

similar structure : and its cells, except Qear the root, are occupied
with hair alone, so as to Beem black by transmitted light, except
when penetrated by tin- fluid in which they are mounted. In the
hair of the mouse, squirrel, and Other small rodents (tig. 702, A, !>),
the cortical substance forms a tube, which we see crossed at intervals-
by partitions thai arc sometimes complete, sometimes only partial ;
^ _ (, these are the walls of the single

or double line of cells, of which
t he medullary subst a nee is made

op, The hairs i >f i he bal t ribe
are commonly distinguished by

t he project ions on I heir surface,
winch are formed by extensions
of the component scales of the
COH LOa] substance : t liese are

particularly well seen in the

hairs of one of the Indian

species, which has a set. oi
whorls of b n narrow leaflets
(so to speak) arranged at

regular intervals on its stem
(C). In the hair of the peccary
(fig. 7<M) the cortical envelope

lends Lnwardi a set o£ radial

prolongations, the interspaces
of which are occupied by the polygonal cells of the medullary tub
stance; and this, on a larger BCale, is fcfafc structure of the 'quills
of the porcupine, the radiating partitions of which, when seen

through the more transparent parts of the cortical iheath, give to

the surface of the latter a lluted appearance. The, hair of tin; ornitho-

Fig. 702. — A, small haiz of iqnirre] ; B, laigt
hair of nqnirrcl; C, Iniir of Indian l)iit.

HAIR

95S

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. When its
outer surface is examined, it is seen to
be traversed by irregular lines (fig. 704,

A), which are most strongly marked in Fig. 703.— Transverse section
foetal hairs ; and these are the indications of hair of peccary,

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 stria? 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. 704.— Structure of human hair: A, external surface of the shaft, show-
ing the transverse strias 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
pigmentarv matter; C, transverse section, showing the distinction be-
tween the "cuticular envelope, the cylinder of cortical substance, and the
medullary centre ; D, another transverse section, showing deficiency of
the central cellular substance.

cells ; but this ' 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

956

VEETEBEATED 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. Tranverse sections
of hairs are best made by gluing 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 lias already passed more
lightly, ami by picking out from the hither, and carefully washing,
the sections thus taken oil'.1

The stems of h <itli> r* exhibit the same kind of structure as hairs,
their cortical portion being the horny she it h that envelopes tho
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 he 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 \« rv distinctly seen in the
lateral barbs, which are sometimes found to be composed of single
files of pear-shaped cells, laid end t«. end ; hut 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

balsam, or boiling then 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 w ith slender flattened
filaments or 'barbules' ; the barbules of the distal side of each barb
are furnished on their attached half with cinwed hooks, whilst those
of the proximal side have thick turned up edges in their median
portion ; as the two sets of barbules that spring from two ad jacent
barbs cross each other at an angle, and M each hooked barhule of
one locks into the thickened edge of several barbules of the other,
the barbs are connected very firmly, in a mode very similar to that

1 On the minute structure of hair, consult Orimm's A tlat <!<■>■ nmiMehVUihtn mid
tierischoi Uaare (Lahr, 1884, 4to, with a preface by W. Woldeyer).

HORNS, HOOFS, CLAWS

957

in which the anterior and posterior wings of certain hymenopterous
insects are locked together. Feathers or portions of feathers of
birds 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 an observer at one point may seem very
dull to the eye of another in a different position. The small feathers
of humming-birds, portions of the feathers of the peacock, and
others of a like kind, are well worthy of examination ; and the
scientific microscopist, who is but little attracted by mere gorgeous-
oess, may well apply himself to the discovery of the peculiar
structure which imparts to these objects their most remarkable
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 papillae (fig. 705). When
transverse sections of these
cylinders are viewed by polar-
ised light, each of theni is

seen to be marked by a crOSS, FlG 705— 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 whalebone, which is formed from the surface of the
membrane that lines the mouth of the whale, and has no relation
to its true bonv 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. Vv ray, ' On the Structure of the Barbs, Barbules, and Barbicels of a
typical Pennaceous Feather,' in the Ibis for 1887, p. 420.

958

VERTEBRATE!) 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
hest 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
k white ' or * colourless." The
r>d present, in every instance,
the form of a flattened disc,
which is circular in man and
moM mammalia (tig. 707), but
i^ <»\ al in birds, rept ilcs (tig.
70»>), and tishes, as also in a
i. w mammals (all belonging to
the camel tribe). I n I he one
form, a-> 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 t hey spi >nt aneously
undergo when kept by means
of ;i 4 warm stage 1 at a tern
perature of aboul 100°, and
from the e fleets uf pressure in
breaking them up. The red
corpuscles in the blood of
0\ iparous Vertebrata are dis
t ingoilhed by t he presence of a
-central spot or nucleus} thil is mO*i distinctly brought into view
by treating the blood-discs with acetic acid, which causes the nucleus

•Fig. 706.

-Red corpuscle of frog's blood:
aa, their flattened face ; l>, particle turned
nearly edgeways; r, colourless corpuscle ;
d, red corpuscles altered b] dduted acetic
acid.

i

®

e

Fig. 707. — lied corpuscles of human blood :
represented at <i, as the\ an- k -en when
rather within tin- focus ..( the microscope;
and at b, as they appear when precisely in
the focus.

1 A very simple mode of applying continued wiirmth to tin r »l » j • -i • t nmler observa-
tion is to lay the slide on a thin plate of hrass or tin, about three inches longer tlian
t-he breadth oi tin Ir.v. aiM about two inches broad; which must be perforated
with u hole uhout one-fourth inch in diameter, at the distance of hulf the hreadth of
the stage from one end of it. When this plate is laid on the stage, and its hole is
brought into the optic axis, so as to allow the light reflected upwards from the mirror
to pass to the slide laid upon it, the plate will project uhout three inches on one side
of the stage — preferably the right. JJy placing a small lamp beneath thil projection
and keeping the linger of the left hand on the part of the plate close to the object (so
as to feel the degree of warmth imparted to itj, the bent given by the lamp may M
regulated by varying its position. For more exact i ad continuous regulation of the
■temperature the methods and apparatus described in this \olume at pp. 989-995
should be studied.

BLOOD-CORPUSCLES

959

to shrink and become more opaque, whilst rendering the remaining
portion extremely transparent (fig. 706, d). By examining un-
altered red corpuscles of the frog or newt under a sufficiently high
magnifying power the nucleus is seen to be traversed by a network
•of filaments, which extends from it throughout the ground sub-
stance of the corpuscle, constituting an intracellular reticulation.
The red corpuscles of the blood of mammals, however, possess no
distinguishable nucleus, the dark spot which is seen in their centre
{fig. 707, b) being merely an effect of refraction, consequent upon
the double concave form of the disc. "When these corpuscles are
treated with water, so that their form becomes first flat and then
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 the aspect of a pile
of coins ; or, if the stratum be too thin to admit of this, partially
overlapping, or simply adhering by their edges, which then become
polygonal instead of circular. The size of the red corpuscles is not
altogether uniform in the same blood ; thus it varies in that of man
from about the 4 fa 0th to the % fa th of an inch. But we generally find
that there is an average size, which is pretty constantly maintained
among the different individuals of the same species ; that of man may
foe stated at about ^ooth of an inch. The following table 1 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 I Camel

1-3542
1-3099
1-2745
1-3811

. 1-3254, 1-5921
Llama . . 1-3361, 1-6294
Javan chevrotain . 1-12325
Caucasian goat . 1-7045
Two-toed sloth . 1-2865

BIED3

1-1812, 1-3832 I Ostrich .
1-1830, 1-3400 Cassowary
1-1961. 1-4000 Heron .
1-2313, 1-4128 Fowl
1-1898, 1-4000 ! Gull

REPTILES AXD BATRACHIA

1-1231, 1-1882
1-1231, 1-2286
1-1555, 1-2743
1-1178, 1-2666
1-1274, 1-1800

Frog

Water-newt
Siren
Proteus .
Amphiuma

FISHES

1-2099, 1-2824 Pike
1-2142, 1-3429 Eel .
1-1777, 1-2824 Gymnotus

1-1649, 1-3000
1-1455, 1-2800
1-1913, 1-3491
1-2102, 1-3466
1-2097. 1-4000

. 1-1108. 1-1821
. 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

1 These measurements are chiefly selected from those given by Mr. Gulliver in
his edition of Hewson's Works, p. 236 et scq.

960

VERTEBRATE D ANIMALS

.0

0

O *>

.0

'1

/

the average dimensions of some of the most interesting examples of
the red corpuscles in the four classes of vertebrated animals, expressed
in fractions of an inch. Where two measurements are given they
are the long and the short diameters of the same corpuscles. (See also
fio-. 708.) Thus it appears that the smallest red corpuscles known
are those of the Ja van chevrotain (Trayulits javatticus), whilst the
largest are those of that curious group of Batrachia (frog tribe) which
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
DO fewer than 5 1 0 of them.
Those of the Ainphhtma
are --till larger.1 Accord-
ing to the estimate of
Vierordt, a cubic inch of
human blood contains up-
wards of e'ujhty millions
of red corpuscles and
nearly a tjnartt'r of a mil
lion t»f the colourless.

The nhit, or 'colour
less c< n'juiscles are more
readily distinguished in
(In- DJOOd of batrachians
than in that of man.
being in the former case
of much smaller size, as
w 1 II as ha\ i ng a oin ular
out iinej tig. 701'), r) ; whilst
in t he latter t heir size and
contour are ^-o nearly the
sumo that, as the red cor-
puscles themselves, when

ieeo in a single layer, Km i 0

but a tery pale hue, the

deficiency of colour does
not sensibly mark their

different* of nature. The

I h i i| iorf ion 1 if white to rstf
eorpu teles being scarcely
ever greater (in a healthy man) than 1 to 2.r>0, and often M Ion M
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, especially it' the glass
cover be moved a little on the slide, ho as to cause the red 0OT
puscles to become aggregated into rows and irregular 'masses. It is
remarkable that, not wif listamling the great variations in the sizes

1 A very interesting account of the ' Ktructuri' of tin- lied Corpntcltf <>f thi
AmpliiuuKi trtdddtyl/untf 1 lias been given by Dr. U. I>. Schmidt, <>f New Orleans, 111

the Jouru. Hoy. Microxc. Sue. vol. i. W.t, pp. 57, W7.

1%

Fig. 708. — Coiui>ar.iti\ 6 lissi ol red Idood-cor-
puscles: 1, man; % idephant; JJ, mubkdeer;
4, dromedary ; ostrich ; 15, pigeon ; 7, humming-
bird; B, crocodile ; '.». python; H>, proteus; 11,
perch; L2,pika; 18, shark.

BLOOD-CORPUSCLES

961

of the red corpuscles in different species of vertebrated animals, the
size of the white is extremely constant throughout, their diameter
being seldom much greater or less than 3oVotn °f an inch in the
warnr-blooded classes and 2 sVo^h 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.
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 ob-
served to undergo changes of form, and even to move from place to
place, after the manner of Amcebce. When thus moving they engulf
particles which lie in their course— such as granules of vermilion that
have been injected into the blood-vessels of the living animal— and
afterwards eject these in the like fashion.1 Such movements will

1 Metschnikoff lias 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. 710), and then disappeared inside the
cells. . . . From all these data we must assume with Metschnikoft 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 Bacteria, English edition, p. 136). ^

Fig. 709. — Altered white corpuscle of blood
an hour after having been drawn from the
finger.

962

VERTEBRATED ANIMALS

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, 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
tin. 700. Similar changes have been
Observe 1 aUo in the corpuscles lloat-
iwj; in the circulating fluid of the
higher invertcbrata, as the crab,
which resemble the ' white ' cor-
puscles of vertebrated blood, rather

than its 'red' corpuscles — these
last, in fact, being altogether peculiar t<. the circulating fluid of
vertebrated animals.

In examining the blood microscopically it is, of course, import-
ant 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 <>f thin glass of period flatness, and then, having
received a small drop of blood upon a ^lass slide, to lav the thin
glass cover /'"' "/><>" this, hut with its edge just touching the edge
of the dn»p : for tin- blood will then be draw n in by capillary at true
tiou. so as to spread in a uniformly thin layer between the two
glasses. Such thin til ins may be preserved in the liquid state by
applying a co\ er glass and cement ing it with gold-size before evapora
tion has taken place j DU1 it is preferable lirst to expose the drop to
the vapour of OSmioaoid, and then to apply a drop of B weak solution
of acetate of pota-s : after which a cover glass may be put on, and
secured with ^old-size in the usual way. Ii i> far simpler, however,
to allow such filmfl to dry without any COVer, and then merely to
COVer them for protection j and in this condition the general
chat of the corpuscles can l.e very well made out, Qotwith

standing that they have in B0m6 degree been shrivelled by the
desiccation they have undergone. And this method is particularly
serviceable as affording B 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 be pronounced upon with 1
high degree of probability.

Simple Fibrous Tissues.— A very beautiful example of a tissue of
this kind is furnished by the membrane of the common fowl's egg ;
which (as may be seen by examining an egg whose shell remains
soft for want of consolidation by calcareous particles) consist of two
principal layers, one serving as a basis of the shell itself, and the
other forming that lining to it which is known as the menibrana

Fig. 710.-- a. blood-cell of a frog in
the act of engulfing a rod of
Bacillus authraris, observed in the
living state in a drop of aqueous
humour: the same a few minutes
later. (After Metschnikoff ; highly
magnified.)

FIBROUS TISSUE

963

putaminis. The latter may be separated by careful tearing with
needles and forceps, after prolonged maceration in water, into several
matted lamellae resembling that represented in fig. 711 ; and similar
lamellae may be readily obtained from the shell itself by dissolving

Fig. 711. — Fibrous membrane Fig. 712. — White fibrous tissue

from egg-shell. from ligament.

away its lime 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 pecu-
liar tendency to fall into undulations, when it is
attempted to tear them apart from each other
(fig. 712). 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 brino-mo- into view certain
oval nuclear particles of 'o-erminal matter,

which are known as 'connective tissue cor-

Fig. 71o.— Portion of
young tendon, show-
ing the corpuscles
of 'germinal matter.',
with their stellate
prolongations, inter-
posed among its fibres.

puscles.' These are relatively much larger, and
their connections more distinct, in the earlier
stages of the formation of this tissue (fig. 713).
It is perfectly inelastic ; and we find it in such
parts as tendons, ordinary ligaments, fibrous

capsules, lie. whose function it is to resist tension without yielding
to it. It constitutes, also, the organic basis or matrix of bone ; for
although the substance which is left when a bone has been macerated
sufficiently long in dilute acid for all its mineral components to be

3 Q 2

964

VERTEBRATEP AXDIALS

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 lamelhv, 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 (tig. 714),
and frequently anastomose, so as to form a network. They are for
the most part between -^..th and , ^.....th of an inch in diameter ;
but they are often met with both larger and smaller. This tissue
does not undergo any change when treated with acetic acid. It
exists alone (that is, without any mixture of the white) in parts
which require a peculiar elasticity, Buch as t he middle coat of the
arteries, the 'vocal cords." the 'ligameiitum nucha* ' of quadrupeds,

the elastic ligament which
holds together the valves of

a bivalve si ud 1, and that by
w hich the claws of the feline
tribe are retracted when
n<>t in use ; and it enters
largely into the coin posit ior.
of i>rrt>t,tr or connective
t issue.

The tissue formerly
known to anatomists as
' cellular," but now more

properly designated conm e

live or areolar tissue, con-
sist a <»t a net w ork of minute
fibres and bands which are
interwoven in every direction, so as to leave innumerable areolm or
little spaces tli.it communicate freely with one another. Of these
fibres some are of the 'yellow ' or elastic kind, but the majority are
composed of the 1 white ' librous tissue ; and, as in that form of ele
mentary structure, they frequently present the condition of broad
flattened bands or membranous shredsin which do distinct fibrous
arrangement is visible. The proportion of the two forms varies,
according to the amount of elasticity, or of simple resisting pow er,
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,
&c. binds together the fat-cells into minute masses (fig 720), these
into large ones, and so on ; and in this way penetrates and forms
part of all the softer organs of the body. Bui whilst tlie librous
structures of which the 'formed tissue ' is composed have* a purely
mechanical function, there is good reason to regard the ' connective

Fig. 714. — Yellow fil.n.us ti^su.' from li^'u
nieiitum iiuclni- of culf.

SKIN

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 shreds 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. 713).

Skin; Mucous and Serous Mem-
branes.— The skin, which forms the ex-
ternal envelope of the body, is divisible
into two principal layers : the cutis vera
or ' true skin,' which usually makes up
by far the larger part of its thickness,
and the ' cuticle,' ' scarf skin,' or epi-
dermis, which covers it. At the mouth,
nostrils, and the other orifices of the
open cavities and canals of the body;
the skin passes into the membrane that
lines these, which is distinguished as
the mucous membrane, from the pecu-
liar glairy secretion of mucus by which

Fig. 715. — Vertical section of skin
of finger : A, epidermis, the
surface of which shows depres-
sions a a, between the emi-
nences b b, on which open the
perspiratory ducts s ; at m is
seen the deeper layer of the
epidermis, or stratum Malpighii.
B, cutis vera, in which are im-
bedded the sweat-glands d,
with their ducts e, and aggre-
gations of fat-cells/; g, arterial
twig supplying the vascular
papillae p ; t, one of the tactile
papillae with its nerve.

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 thin cuticular
layer, which, as it differs in many points from the epidermis, is dis-
tinguished as the epithelium. The substance of the 1 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-
dule of various kinds ; and in the skin we also find abundance of
Jierves and lymphatic vessels, as well as, in some parts, of hair-

966

VEETEBEATED ANIMALS

follicles. The general appearance ordinarily presented by a thin
vertical section of the skin of a part furnished with numerous sensory
papilla? is shown in rig. 715 : 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 papilhv, supplied with loops of blood-
vessels from the trunk. and a tactile papilla, f, with its nerve
twig. The spaces between the papilhv are rilled up by the soft
' Malpighian layer." ui. of the epidermis. A, in which it- 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 duct-, opening at >■ upon the surface, which presents
alternating depressions, a, and elevations, b. The distribution of
the blood-vessels in the skin and mucous membranes, which 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 membrane-, cm the other hand,
whose function is simply protective, the supply of blood-vessels is
more scanty.

Epidermic and Epithelial Cell-layers. The epidermis or 1 cuticle '
covers the whole exterior of the body as a thin semitranspare ut
pellicle, which i- 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 newc-t and deepest layer- gradually become the oldest
and most superficial, and are at last throw n ofl" by slow desquamation.
In their progress from tin- internal to t he external surface of the

epidermis t he cells undergo a serie- of well

^agr,/^ marked changes. When we examine the

(W~\ S -''^Sfcji innermost layer, we find it soft and granu

lar, Consisting Of nucleated cells w hich are
llatter in the upper than the lower strata,
which make up the layer. This was for
merlv considered as a distinct tissue, and
■'.•< ' was -uppo-ed to be (he peculiar scat of the

"^H^/iMflM colour of the skiii ; it recei\ed the desig

' ,,,/"' nation of Malpighian layer or nte mMcoturn,

i i< .. tic.— Ceils from the j>ig- The change in form is accompanied by a

""'"! ;"/'"""•'"'• change in the chemical composition of the

a, mgmentar\ grannies oon- ft . . 1

ccaiiii- tin- l'luck-us; bf the tissue, which seems to be due to the metamor
mmJoub. phosis of tin; contents of the cells into a

homy iubstance identical with that of which
hair, horn, nails, hoofs, ifcc are composed. Mingled with the epi-
dermic cells we find others which secrete colouring matter instead
of horn ; these, which are termed 1 pigment-cells,' are especially to
lie noticed in t he epidermis of the negro and other dark races, and
are most distinguishable in the Malpighian layer, their colour ap-
pearing to fade as they pass towards the surface. The most rema rk-
able development of pigment-cells in the higher animals, however,
is on the inner surface of the choroid coat of the eye. where they
have a very regular arrangement, and form several layers, known as

■EPIDERMIS

967

the pigmentum nigrum. When examined separately these cells are
found to have a polygonal form (fig. 716, «), 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 enclosed 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. 730, c c). The gradual formation of these prolongations
may be traced in the pigment-cells of the tadpole during its meta-
morphosis (tig. 717). Similar varieties of
form are to be met with in the pigmentary
cells of fishes and smal] 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
elevations of the true skin ; an arrangement
which is shown on a large scale in the thick
cuticular covering of the dog's foot, the sub-
jacent papillae being large enough to be dis-
tinctly seen (when injected) with the naked

eye. The cellular nature of the newly forming layers is best seen
by examining a little of the soft film that is found upon the surface
of the true skin, after the more consistent layers of the cuticle have
been raised by a blister. The alteration which the cells of the
external layers have undergone tends to obscure their character;
but if any fragment of epidermis be macerated for a little time in a
weak solution of soda or potass, its dry scales become softened, and
are filled out by imbibition into rounded or polygonal cells. The
same mode of treatment enables us to make out the cellular struc-
ture in warts and corns, which are epidermic growths from the
surface of papillae enlarged by hypertrophy.

The epithelium may be designated as a delicate cuticle, covering
all the free internal surfaces of the body, and thus lining all its
cavities, canals, &c. Save in the mouth and other parts m which
it approximates to the ordinary cuticle, both in locality and 111

Fig. 717. — Pigment - cells
from tail of tadpole : a a,
simple forms of recent
origin ; b b, more complex
forms subsequently as-
sumed.

968

VERTEBRATE!) ANIMALS

nature, its cells (fig. 7 IS) 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, fiat, 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 arc
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 0*861
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 n
covering. If the cylinders be closely pressed together, their form is
changed into prisms ; and such epithelium is often known as
•prismatic.' On the other hand, if the BUrfa >• on which it rests he
convex, th»- bases nr lower ends of the cylinders become smaller than

a.

Fig. 71s. — Detached enithelium-celltt :
a, with nuclei b, and nucleoli r,
from mucous membrane of the
mouth.

Fio. 710. — Ciliated epithelium !

'/, nucleated cell* resting on
tlieir smaller extremitiea ; /»,
cilia.

their tree 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 01 the intestine is a peculiarly good ex-
ample) is termed 'conical.' Hut between these primary tonus 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 rilin ; but these appendages are
more commonly found attached to the elongated tlian to the
flattened forms of epithelial cells (lig. 7 1 9). Ciliated epithelium is

found upon the lining membrane of the lir pamgtti in all air
breathing Yertebrata ; and it also presents itself in many other
situations, in w hich a propulsive power is needed to prevent an ac-
cumulation of mucous or oi her secret ions. < >\ving to the very slight
attachment that usually exists between the epithelium and the
membranous inrface whereon it li«-s there is usually no difficulty
vhatever in < <amininj« 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

969

swim freely cabout in water. If the thin fluid that is copiously dis-
charged from the nose in the first stage of an ordinary 1 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 proper adipose substance. The individual fat-cells
always present a nearly spherical or spheroidal form ; sometimes,
however, when they are closely pressed together they become some-
what polyhedral, from the flattening of their
walls against each other (fig. 720). Their
intervals are traversed by a minute network
of blood-vessels (fig. 734), 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,
md very dark towards their margin, in
onsequence of their high refractive power ;
ut if, as often happens in preparations that
ve been long mounted, the oily contents

ould have escaped, they then look like any other cells of the same
mi. Although the fatty matter which fills these cells (consisting

Fig. 720. — Areolar and adi-
pose tissue : a a, fat-cells ;
b b, fibres of areolar
tissue.

solution of stearine or margarine in oleine) is liquid at the
oilmary temperature of the body of a warm-blooded animal, yet its
"hrder portion sometimes crystallises on cooling, the crystals shoot-
in; from a centre, so as to form a star-shaped cluster. In examining
thi structure of adipose tissue it is desirable, where practicable, to
liae recourse to some specimen in which the fat-cells lie in single
la^s, and in which they can be observed without disturbing or
layag them open ; such a condition is found, for example, in the
meintery of the mouse ; and it is also occasionally met with m the
f at4posits which present themselves at intervals in the connective
tissL of the muscles, joints, &c. Small collections of fat-cells exist
in tl deeper layers of the true skin, and are brought into view by
vertjd sections of it (fig. 715, /). And the structure of large
mass^ of fat may be examined by thin sections, these being placed
undewater in thin cells, so as to take off the pressure of the glass-
-Goverroni their surface, which would cause the escape of the oil-

VERTEBRATE!) ANIMALS

Fig. 721. — Cellular cartilage of
mouse's ear.

particles. ' JSTo 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 1 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
external ear of a hat or mouse (tig.
731 )t the cells are packed as closely
together as are those of an ordinary
vegetable parenchyma; and this stems to be the early condition
of most cartilages that arc 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, <»r four (fig. 722), which are evidently
formed by a process of ' binary subdivision.' The substance of these

0$Uular cartilages is ent irely
destitute of blood-vessels,
being nourished solely by

imbibition from the blood

brought i<> the meinhram

<■( »\ ering theirsurface, I [enc

they may be compared, i
regard to t heir grade of o
gain sat ion, with the largr
algfe, which consist, lie
them, of aggregations of ces
held toget her by intereellui r
substance, wit hout vessels*!
any kind, and arc notiris'd
by imbibition through tKir
w hole surface. There ire
many eases, however, in
which the structureless her-
cellular substance is rep*ced
by bundles of fibres, sometimes elastic, but more commonlyion-
elastic ; such combinations, which are termed /////-"-cartilage are
interposed in certain joints, wherein tension as well as preilUI has
to be resisted ; as, for example, between the vertebrte of the pinal
column and the bones of the pelvis. In examining the sticture
of cartilage nothing me>re is necessary than to make vei thin
sections, preferably with the microtome. These sections ay be

Fig. 722.— Section of the branchial curtilage of
tadpole: a, group of four cells, wpavttilig
from each other; b, pair of COllfi m i i j . | -
tion; c c, nuclei of curtilage-cells; Iff, r.r.it;.
containing three cells (the fourth prnliubly
behind j.

GLANDS

07 I

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 tit to display the cha-
racteristic features of their structure.

Structure of the Glands. — The various secretions of the body (as
saliva, bile, urine, Arc.) are formed by the instrumentality of organ-
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 tilled 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 proeluct 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 Annelida 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 (tig. 723), fig. 728.— Ultimate follicles
nothing more being necessary than to of mammary gland, with
make sections of these organs thin enough containfn^nudeiT b.
to be viewed as transparent objects. The
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 ot the
kidney, whilst they are convoluted and tilled with a spheioida,
epithelium in the outer or « cortical.' Certain iiask-shaped dilata-
tions of these tubes include curious little knots of blood-vessels,
which are known as the < Malpighian bodies ' of the kidney . these
are well displayed in injected preparations. For such a full and

972

YERTEBKATED ANIMALS

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 ran 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
Aery little altered) will be readily separated.

Muscular Tissue. Although we are accustomed to speak of this
tissue as counting of • fibres,' yet the ultimate structure of the
' muscular fibre ' is very different from that of the 1 simple fibrous
tissues' already described. When we examine an ordinary
anuscle (or piece of 'flesh') with the naked eye, we observe that it

is made up <>t' ;i number of f<tsri, nli or bundles
< I | |Di| of fibres (tig. 11 I), 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

m. t h • microscope, they are found to I"' themselves

fasciculi, composed of minuter fibres bound
together by delicate filaments of connective
tissue. Hy carefully separating these we may
obtain the ultimate muscular fibre. This fibre
exists iind<T- two forms, the striutnl and the

Fig. 724. — Fascicular non ttriated, The former is chiefly distinguished

of striated muscular by the transversely stria ted appearance which
fibre, showiii'' ut a tin- /<» . , , e \ .

transverse striae, and 11 l"v^'",s 0^'- "'"1 which is duo to an

at 6 its junction with alternation oi light and (lark sp&oes along its

the tendon. whole extent; the breadth and distance of

these stri;e vary, however, in different fibres,
and even in different parts of the same fibre, according to then-
state of contraction or relaxation. Longitudinal strifl are also

frequently visible, which are due to a partial separation between
the component nbrillfe into which the fibre may be broken up.
When a fibre of this kind is more closely examined, it is seen to be
enclosed within a delicate tubular sheath, which is quite distinct on
the one hand from the connective tissue that binds tin; fibres into

MUSCLE

973

Fig. 725. — Striated muscular fibre, separating
into fibrillar.

fasciculi, and equally distinct from the internal substance of the
fibre. This membranous tube, which is termed the sarcolemma, 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-
tremes also are wider ; thus
its dimensions vary in the
frog from ^th to j^th.

of an inch, and in the skate from g^th to ^-J-ffth ; whilst in the
human subject the average is about ^-J-^th of an inch, and the
extremes about o-hyth and ?,^th.

The substance of the fibre, when broken up by ' teasing ' with
needles, is found to consist of very minute fibrillse, which, when
examined under a magnifying power of from 250 to 400 diameters,
are seen to present a slightly beaded form, and to show the same
alternation of light and dark spaces as when
the fibrillar are united into fibres or into
small bundles (fig. 725). The dark and light
spaces are usually of nearly equal length ;
each light space is divided by a transverse
line, called 1 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 composition of the fibre ; but
Mr. J. B. Haycraft has recently 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
truefocus Dobie'sline, D(fig. 725a), 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 the
other hand, the surface of the fibre be brought into focus, the convex

Fig. 725a. — Diagram of
striated fibrilla.

974

VEETEERATEP ANIMALS

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 tibre ; with the
exception that the alternation of light and dark stria1 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 ribrilhv. 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 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 fibrillar by dissection with a pair of needles under the simple
microscope. The general characters of the striated fibre are admir-
ably 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 als i 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 tins process for their more ready
reparation, especially in the ca-^e of the tongue. I >r. Iloale recom-
mends glycerin for the preparation, and glycerin media for the
preservation of objects of* thi^ class ; and states that the alternation
of light and dirk HMOefl in the librilla- is rendered more distinct by
such treatment. The f i I ni 1 In* 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 mad'- by the freezing microtome. Striated fibres,
separable with gnat facility into their component fibrillae, are
readily obtainable from the limbs of crust acea and of insects ; and
their presence is also readily distinguishable in the bodies of worms,
even of very lo\v organisation ; so that it may he regarded as charac-
teristic of the articulated series generally. On the other hand, the
molluscous classes are, for the most part, distinguished by the mm
striation of their fibre ; there are, however, some exceptions, such as
the muscles of the odontophore in the -nail and the powerful adductor
muscle of Peoten. Its presence seems related to energy and rapidity
of movement the non-striated presenting itself where the move-
ments are slower and feebler in their character.

The 'smooth' or rum ttirutied form of muscular fibre, which is
especially found in the walls of the itomaoh, Intestines, bladder, and
■other similar parts, is composed of flattened bands whose diameter is

1 Quart. Jonni. Micros. Sri. n.s. xxi. p. :;i)7. 'Flo- more recent views will be
found in Mr. C. F. Marshall's paper in vol. xxviii. of the same journal, and in the

memoirs cited by him.

MUSCLE ; NERVE

975

usually between ^^th and y<faft*fr of 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. 726, 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
cells having the same general characters, but shorter and wider in
form ; and although some of these approach very closely in their

Fig. 726.— Structure of non-striated
muscular fibre: A, portion of
tissue showing fusiform cells a «,
with elongated nuclei b b ; B. a
single cell isolated and more
highly magnified ; C, a similar
cell treated with acetic acid.

Fig. 727. — Ganglion-cells and nerve-
.tibres from a ganglion of lamprey.

general appearance to epithelium-cells, yet they seem to have quite
a different nature, being distinguished by their elongated nuclei, as
well as by their contractile endowments.

Nerve-substance.— Wherever a distinct nervous 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
-an^lionic centres, and the 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. , 2J ) :
whilst in other cases they seem to inosculate with those ot other
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

976

YEETE BKATED AXBLA LS

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 Yertebrata that peculiar hue which causes it 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 1 white substance of
Schwann,' whose outer and inner boundaries are marked out by two
distinct lines, giving to each margin of the nerve-tube what is de-
scribed as a 1 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 arc
foreign to their nature that their characters can only he 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 i> a protoplasmic substance, is the
i vs. ntial component of the nerve tibre,
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
uerve-tubes di tiers in dillerent nerves, being
sometimes as great as , t h of an inch,
and as small in other instances as , 0/lOI>th.
In many of the lower invertebrata, such as
Fig. T'JH.-di-latinniis ,,,-rvi- M"1"'" and C omat a I <i y wo seem fully
fibres, from olfactory norvr. justified by physiological evidence in re-
garding as nerves certain protoplasmic
fibres which do not possess the characteristic structure of ' nerve-
tubes,' and libres 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 1 gelatinous,' are considerably smaller
than the preceding, and do not exhibit any differentiation of parts
(fig. 728). They an? 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 libres, and also
with the radiating prolongations of the ganglion cells ; so that their
nervous character, which has been questioned by some anatomists,
seems established beyond doubt.

The ultimate distribution of the nerve-fibres is a subject on
which there has been great divergence of opinion, and one which can-
only be successfully investigated by observers of great experience.
The Author believes that it may be stated as a general fact, that in
both the motor and the sensory nerve-tubes, as they approach their

NERVE-FIBRES

977

terminations in the muscles and in the skin respectively, the
protoplasmic ax is -cylinder is continued beyond its envelopes,
often then breaking up into very minute fibrillar which inosculate
with each other, so as to form a network closely resembling that
formed by the pseudopodial threads of Rhizopods. Recent observers
have described the fibrillar of motor nerves as terminating in
'motorial end-plates ' seated upon or in the muscular fibres; and
these seem analogous to the little ' islets ' of sarcodic substance into
which those threads often dilate. Where the skin is specially
endowed with tactile sensibility we find a special papillary
apparatus, which in the skin may be readily made out in thin
vertical sections treated with solution of soda (fig. 729). 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. 715, p, 737), being destitute of nerve-fibres,
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
(tig. 729, 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 - fibres are more
readily seen, however, in
the ' fungiform ' papillae of

Fig. 729.— 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 papilla?, c c.

the tongue, to each of which several of them proceed ; these bodies,
which are very transparent, may be well seen by snipping off minute
portions of the tongue of the frog ; or by snipping off the papillae
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 papillae also is increased by treating them with a weak
solution of soda. Nerve-fibres have also been found to terminate
on sensory surfaces in minute ' end-bulbs ' of spheroidal shape and
about ^th of an inch in diameter, each of them being composed
of a simple outer capsule of connective tissue, filled with clear
soft matter, in the midst of which the nerve-fibre, after losing its
dark border, ends in a knob. The ' Pacinian corpuscles/ which are
best seen in the mesentery of the cat, and are from ^th to ^th of
an inch long, seem to be more developed forms of these ' end-bulbs.

For the sake of

obtaining

a general acquaintance

with

3 R

the

978

VEETEBEATED ANIMALS

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-hbres, 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-tibres are rendered tinner and more distinct. Again, the axis-
cylinders arc 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 (fflatinous
fibres are found in the greatest abundance in the sympathetic nerves ;
and their characters may be best studied in the smaller hranches of
that system. So tor the examination of the ganglionic cells, and of
their relation to the nerve-tubes, it is better to take some minute
ganglion as a whole (such as one of the sympathetic ganglia of the
frog, mou>e, or other small animal), then to dissect the larger
ganglionic masses, whose structure ean only be successfully studied
by such a> air proticmnt in this kind of investigation. The nerves
of the orbit i'f tin' i;Vfs (if lislie-, with the ophthalmic ganglion and
its branches, which may be very readily got at in the skate, and of
which the components maybe 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-fihres and ^anglion-eells. I«\>r 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 1 1 1 i ^ inquiry should acquaint themselves with the methods which
have been found most sueeessful in the hands of the able histologists
whose works have been already referred to.1

Circulation of the Blood. — One of the most interesting spectacles
that the microsoopist can enjoy is that which is furnished by the
circulation of the blood in the nijiillary 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

1 For farther information Hoarding the nervous system tin* memoir of F. Nannen
on ' The Structure and Combination of the Histological Elements of tin- Central
Nervous System' in Bergen's Mu&eumt Aartberetning for 1886 1 L887), P. 99, should

he consulted.

CIRCULATION OF BLOOD

979

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 fths 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 will serve to
keep it in place. The web being moistened with water (a precaution
which should be repeated as often as the membrane exhibits the
least appearance of dryness) and an adequate light being reflected
through the web from the mirror, this wonderful spectacle 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 un-
frequently be found, however, that the current of blood is nearly or
altogether stagnant for a time ; this seems occasionally due to the
animal's alarm at its new position, which weakens or suspends the
action of its heart, the movement recommencing again after the
lapse of a few minutes, although no change has been made in
any of the external conditions. But if the movement should not
renew itself, the tape which passes over the body should be slackened ;
and if this does not produce the desired effect, the calico envelope
also must be loosened. 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 observa-
tion. The movement of the blood will be distinctly seen by that of
its corpuscles (fig. 730), 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 alwavs in the same

3 it 2

980

YERTEBKATEP ANIMALS

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 b) towards their trunks (a) : the arteries, whose ultimate sub-
divisions discharge themselves into the capillary network, are for
the most part restricted to the immediate border- 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 eon
tents are much more plainly seen : and it may then be observed
that whilst the 1 red' corpuscles flow at a very rapid rate along the
centre of each tube, the 4 white* corpuscles, which are occasionally
discernible, move slowly in the clear stream near its margin.

The circulation may also be displayed in the tongue of the frog

b b

Fig. 730. — Capillary <inulution in u partlon <>f the web of a fn g*| foot:
</, trunk of vein ; 0, l>, itd LiuikIh-h ; c, c, pitfnu-nt-cellH.

by laying the animal (previously chloroformed) on its hack, with its
head close to the hole in the OOrk -plate, and, after securing the hodv

in this position, drawing oul the tongue with tin- forceps and fixing

it on the Other Bide of the hole with pins. So, again, the circ ula-
tion may be examined in the lungs — where it aH'ords a spectacle of
singular beauty — or in tin; no sr,,f,ri/ of the living frog by laying
open its body and drawing forth cither organ, the animal having
previously been made insensible by chloroform. Tim tadpole of the
frog, when sufficiently young, furnishes a good display of < becapilla i y
circulation in its tail ; and the difficulty of keeping it quiet during

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 boa temperature of
between 100° and 1 10° Fahr. j and, notwithstanding thai the musclai
of the body are thrown into a state of spasmodic rigidity by this
treatment, the heart continues to pulsate, and the "circulation il

CIRCULATION OF BLOOD

maintained.1 The larva of the water-newt, when it can be obtained,
furnishes a most beautiful display of the circulation, both in its
external gills and in its delicate feet. It may be inclosed in a large
aquatic box or in a shallow cell, gentle pressure being made upon
its body, so as to confine its movements without stopping the heart's
action. The circulation may also be seen in the tails of small fish,
such as the minnow or the stickleback, by confining these animals in
tubes, or in shallow cells, or in a large aquatic box ; 2 but although
the extreme transparence of these parts adapts them well for this
purpose in one respect, yet the comparative scantiness of their
blood-vessels prevents them from being as suitable as the frog's web
in another not less important particular. One of the most beautiful
of all displays of the circulation, however, is that which may be seen
upon the yolk-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 (as
it is in the bird) previously to the hatching of the egg, this bag
hangs down from the belly of the little creature on its emersion,
and continues to do so until its contents have been absorbed into
the body, which does not take place for some little time after-
wards. And the blood is 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.3 The tadpole, as every naturalist is aware, is
essentially a fish in the early period of its existence, breathing by
gills alone, and having its circulating apparatus arranged accord-
ingly : but as its limbs are developed, and its tail becomes relatively
shortened, its lungs are gradually evolved in preparation for its
terrestrial life, anoTthe 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. 731, l)
and at the bases of these, concealed by opercula or gill-flaps resem-
bling those of fishes, are seen the rudiments of the internal gills,
which soon begin to be developed in the stead of the preceding.

1 A special form of live-box for the observation of living tadpoles etc. contrived
bv Prof. F. E. Schultze, is described and figured in the Quart. Journ. Micros. Sci.
n.s. vol. vii. 1867, p. 261.

- A convenient trough for this purpose is described in the Quart. Journ. Micros.
Sci. vol. vii. 1859, p. 113.

5 See Mr. Whitney's account of 'The Circulation in the Tadpole in Trans.
Micros. Soc. n.s. vol. x. 1862, p. 1. and his subsequent paper ' On the Changes which
accompanv the Metamorphosis of the Tadpole ' in the same transactions, vol. xv.
p. 43. In" the first of these memoirs Mr. Whitney described the internal gills as
lungs, an error which he corrected in the second.

982

YEKTEBBATED AXDIALS

The external gills reach their highest development on the fourth or
fifth day after emersion : and they then wither so rapidly (whilst

Fig. 731. — Circulation in tin- tadpole.

1. Anterior portion of young tadpole, showing the external gills, wit li the incipient,
tufts of the internal gills, and the. pair of minute tuhes between the heart and the
spirally coiled intestine, which are the rudiments of the fatUTfl lungs.

2. More advanced tadpole, in which tin- external gills have almost disappeared :

a, renin uit of external grills on the left side ; />, operculum ; r, remnant of external gill
on the right side, turned in.

3. Advanced tadpole, showing the course of the general circulation: a, heart;

b, branchial arteries; c, pericardium; <L internal gill; r, lirst or cephalic trunk;
/, branch to lip ; 7, branches to head ; //, fecund or luanchial trunk; /.third trunk,
uniting with its fellow to form the abdominal aorta, which ir. continued as the caudal
artery, A:, to the extremity of the tail; I, caudal vein; m, kidney ; n, vena cava ;
<>, liver; jj, vena porta-; >/, inn, vimuni>, receiving the jugular vein, r, and the ab-
dominal veins, t, u, as also the branchial vein, v.

4. The branchial circulation on a larger scale: A, ]',, (', three primary branches of
the branchial artery ; a, cartilaginous archc ; 6, additional framework J c, c, twigs of
branchial artery; d,f, rootlets of branchial vein.

5. Origin of the vessels of the internal gills, g, from the roots of those of the
external.

6. The heart, systemic arteries, pulmonary arteries and veins, and lungs, in the
adult frog, the heart being turned up in the right-hand figure, to show the junction
of the pulmonary veins and their entrance into the left auricle.

CIKCULATION IN TADPOLE

983

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 us is the beating heart, a bulbous-
looking cavity, formed of the most delicate transparent tissues,
through which are seen the globules of the blood, perpetually, but
alternately, entering by one orifice and leaving it by another. The
heart (fig. 731, 3, a) appears to be slung, as it were, between two
arms or branches, extending right and left. From these trunks (b)
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, /, to
the thick fringed lip which surrounds the mouth ; after which it
suddenly curves upwards and backwards, so as to reach the upper
surface of the head, where it dips between the eye and the brain.
The second main trunk, h, seems to be chiefly distributed to the gill,
although it freely communicates by a network of vessels both with
the first or cephalic and with the third or abdominal 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
the spine, where it meets its fellow to form the abdominal aorta, i,
which, after giving off branches to the abdominal viscera, is con-

984

VERTEBRATE!) ANIMALS

tinued as the caudal artery, k, to the extremity of the tail. The
blood is returned from the tail bv the caudal vein, /, which is
gradually increased in size by its successive tributaries as it passes
towards the abdominal cavity : here it approaches the kidney, m,
and sends off a branch which incloses that organ on one side, while
the main trunk continues its course on the other, receiving tributaries
from the kidney as it passes. The venous blood returned from the
abdominal viscera, on the other hand, is collected into a trunk, />,
known as the portal rein, which distributes it through the substance
of the liver, o, as in man : and after travelling that organ it is dis-
charged by numerous tine channels, which converge towards the
great abdominal trunk, or W //" cava, >i, as it passes in close proximity
to the liver, onwards to the sinus r> nost(Sy 7. or rudimentary auricle
of the heart. This also receives the jugular petit, /\ from the head,
which first, however, passes downwards in front of the gill close to
its inner edge, and meets a vein, coming up from the abdomen,
after which it turns abruptly ill the direction of the heart. Two
other abdominal veins, meet and pour their blood direct into the

sinus vcno>us : and into this cavity i^ also poured the aerated blood
returned from tin- gill by the branchial rein, i\ of which only the
one on the right side can be distinguished. The lungs may be de-
tected in a rudimentary Btate, even in the very young tadpole,
being in that stage a pairof minute tubular sacs, united at the Upper
extremities, and lying behind the intestine and close to the spine.
1 hey may be be>t brought into view bv immersing the tadpole for a
few days in a weak solution <»t' ohromic acid, which renders the
tissue friable, so that the parts that OOQCea] them may be more
readily peeled away. Their gradual enlargement may be traced
during the period of the tadpole's transparence ; but they can only
be brought into view by dissection when the metamorphosis has
been completed. The following are Air. Whitney's directions for
displaying the cireulation in these organs : ' Put the young frog into
a wineglass ami drop on him a single drop of chloroform. This
suffices to extinguish sensibility. Then lay him mi the back on a
piece of cork and li\ him with small DUM passed through the web
of each foot. Remove the skin of the abdomen with a line pair of
sharp scissors and forceps. Turn aside the intestines from the l>j't
side, and thus expose the left lung, which mav HOW be seen as a
glistening transparent sac containing air bubbhs. 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 irantmitt I light, Unpin
the frog and place him on a slip of glass, and then transmit the
light through the everted portion of lung. Remember that the lung
is very elastic, and is emptied and collapsed by very slight pressure.
Therefore, to succeed with this experiment, the lung should be
touched as little as possible, and in the lightest manner, with the
brush. If the heart is acting feebly you will see simply a trans-
parent sac, shaped according to the quantity of air-bubbles it may
happen to contain, but void of red vascularity and circulation. But
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

INJECTED PREPAEATIOXS

985

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. 731, 6.

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,
from those who have made it
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. 733 — Section of the toe of a mouse : a, a, a, tarsal bones; b, digital artery;
c, vascular loops in the papillae forming the thick epidermic cushion on the under
surface ; d, distribution of vessels in the matrix of the claw.

results, being readily accessible elsewhere to such as desire to put it
in practice.1

Many anatomical parts, when well injected and mounted, become

1 See especiallv the article 'Injection' in the Micrograph ic Dictionary; M.
Robin's work, Du Microscope et des Injections ; Prof. H. Frey's treatise, Das Mikro-
skop und die mikroskopische Technik; Dr. Beale's How to Work with the Micro-

FlG. 732. — Transverse section of small intes-
tine of rat, showing the villi in situ.

g 86 VEETEBRATEP ANIMALS

objects of both interest and instruction. This is the case with the
villi of the intestine, seen in fig. 732. which presents a transverse
section, in which they are seen in situ. A thin section of the toe
of a mouse (fig. 733) 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-
ment of the vessels not in any way determining the function, but

Fig. 734.— Capillary network Via. 7:1.". -Capillary network of

around fat-cells. nuisrle.

merely administering to it, like the arrangement of water- or gas
pipes in a manufactory. Tims, in tiu'. 7 •">!. we see that the capil-
laries of adipose ub>t a nee are disposed in a network with rounded
meshes, so as to distribute the blood aiming the fat cells ; whilst in
fig. 735 we see the meshes enormously elongated, so as to permit
the muscular fibres to lie in them. Again, in tig. 7.*»t>, we observe
the disposition of the eapillaries around the orifices of the follicles
of a mucous membrane; whilst in lig. 737 we see the looped

Via. T.U',. — Distribution of eujii!- Fin. 7U7. — Distribution of capil-

laries in mucous membrane. laru-s in skin of finder.

arrangement which exists in the papillary surface of the skin, and
which is Subservient to the nutrition of the epidermis and to the
activity of the sensory nerves.

In no part of the circulating apparatus, however, does the
disposition of the capillaries present more points of interest than it
does in the respiratory organs. In bony fishes the respiratory surface
is formed by an outward extension into fringes of g%U$t each of w hich
consists of an arch with straight lamime hanging down from it, and

scope; the Handbook to the PJn/siolor/ical Lahore lor;/ ; and Rutherford's and

Schiifer's treatises on Practical Sutology,

RESPIRATORY ORGANS

98/

every one of these laminae (fig. 738) 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 Yertebrata,
the great extension of surface
which is effected in the latter
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 are
thin and membranous at the
lower part, there present a
simple smooth expanse ; and it
is only at the upper part, where
the extensions of the tracheal
cartilage form a network over
the interior, that its surface is
depressed into sacculi whose
lining is crowded with blood-
vessels (fig. 739). 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
on one side against its wall, so
as only to be exposed to the air

on its free surface. In the elongated lung of the snake the same
general arrangement prevails ; but the cartilaginous reticulation
of its upper part projects much further into the cavity, and incloses

Fig. 738. — 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 lamellae.

Fig. 739.

-Interior of upper part of
lung of frog.

■gS8 VERTEBRATES ANIMALS

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 ' (tig. 740, P>), each of
which has its own bronchia] tube (or subdivision of the windpipe)
and its own system of blood-vessels, which have very little com-

A B

Fig. 740. — Interior strm tur<- of lnn^ of fowl, us displayed 1>\ a section, A,
passing in tin- ilin-ition of u l»roncliiul tulif, uiul l»y another section B

Dotting it ticross.

Fig. 741. — Arrangement of the nipilhirh'H on tin- walls of the air-i dl < (A

tin- human lung.

munication with tliose of other lobules. Baoh lobule has a central

cavity, which closely resembles that of ;i frog's lung in tD \ n iature,
having its walls strengthened by a network of cartilage derived from
tin; bronchial tube, A, in the interspaces of which are opening! lead
ing to sacculi in their substance. But each of these cavities is sur-
rounded by a solid plexus of blood-vessels, which does not seem to be
covered by any limiting membrane, but which admits air from the
central cavity freely between its meshes ; and thus its capillaries are
in immediate relation with air on all sides — a provision that is ob-

LUNGS

989.

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. 741) is so disposed in the partitions
between these cavities that the blood is exposed to the air on both
sides. It has been calculated that the number of these air-cells
grouped around the termination of each air- tube in man is not less
than eighteen thousand, and that the total number in the entire
lung is six hundred millions.

1 On the respiratory organs of birds, see Campana, La Besjriration cles Oiseauxt.
Paris, 1875.

990

CHAPTER XXIII

APPLICATION OF THE MICROSCOPE TO GEOLOGICAL

IXVESTIGA 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 moans 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 l^oS, when I)r. H. C Sorby, F. K.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 Nicol. A description of his method is given by II. Witham
{1831). 2 Previous to 18f>S only those minerals could be examined
microscopically which possessed the necessary degree of transparency,
whilst rocks were largely closed secret s. Nc\ ert heless ( Wordier (in 1 S I f))
was able to determine the constituent minerals of many rocks by the
study of the powder under the microscope j a procedure which Fleurian
•de Bellevue had previously recommended in I SOU, and which is still
•employed by some mineralogists. Seven years before; I )r. Sorby \s paper
appeared, the German scholar Oschatz exhibited a series of thin
sections of minerals and locks and drew attention to their important
bearing upon structural studies, but the collection was regarded
more as a curiosity than as a scientific achievement, so great was
the stagnation that characterised those years.

The discovery of the method of preparation gave an enormous
impetus to geological research, and the germs falling upon tin; fruitful

1 Quart. Journ. Geol. Soc. vol. xiv. 1858, pp. £58-500.

^ Observations on Fossil Vegetables, Edinburgh and London, 1831.

5 The history of the application of the microscope to geology has been sketched
by F. Zirkel in his paper Die Einfiihrung des Mikroskops in das mineraloyisch-
c/eologische Studium, Leipzig, 1881.

MICROSCOPIC SECTIONS OF ROCKS

991

soil of scientific Germany led to the growth of a 'micro-petrology.'
Its development we owe to such Continental workers as Zirkel,
Vogelsang, Rosenbusch, Renard, and others.

In- order to examine minerals and rocks, sections must be pre-
pared thin enough to permit of the use of transmitted light ; for
this purpose they should be from about ^th to ^yh of an 'inch
thick.

A chip about an inch square is struck or cut off 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 by
hand or by means of a machine specially constructed for this purpose
(pp. 424, 425). 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 wdiich large air-bubbles
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-bubbles
have been included between the glass and the stone. Should they be
present in any quantity the wdiole 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 an
object-glass more suited to optical work. This transference is
effected by the application of a gentle heat to the slab until the
b>alsam becomes liquefied, when the section can be pushed with a
piece of wire on to a slide of fine material. Obviously a drop of balsam
should be poured upon the latter before the section is transferred.

1 Mr. F. G. Cuttell (52 New Compton Street, Soho) prepares good sections ;
Messrs. Voigt and Hochgesang (Gottingen, Rothe Str. 13) andR.Fuess (Berlin, S.W.
108 Alte Jacob Str.) do also most excellent work.

992 THE MICROSCOPE IX GEOLOGICAL INVESTIGATION

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 clown 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 heated with Canada balsam, in
order to give them the consistency necessary for the preparation of
thin sections. Isolated mineral grains and sands can be mounted
by means of Canada balsam dissolved in chloroform. The slide
must not be heated, but evaporation allowed to take place. Another
method is described by Thoulet ; 1 whilst May soft or decomposed
rocks should be mounted according to AYichmann's proposal.2

In the application of the microscope to penological and minera-
lngical 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 having been recently brought out by Messrs. J.
Swift and Son. which combines all that experience has led penologists
to consider desirable for niineralogical and petrological investigation,
a brief account of it is here subjoined It is specially adapted to the
study of tin- optical properties of minerals generally, and particularly
to that of the thin plates of minerals seen in ordinary sections of
rocks pre] tared for microscopical examination. The microscope is
shown in fig. 7 ll'.'

The eyepiece tube is slotted at F to receis e the micrometer scale
(shown detached at K), 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. ! 1< b »\\ tie- stage, which has neither sliding nor rotatory
movement a, is mounted tin- polariser, B, capable of independent, rota-
tion like the analyser, and upon the tul>e of the polariser is mounted
a toothed wlnrl of the same ize as that upon the analyser; this
wheel gears into a wheel carried by a tube which forms a telescopic
extension of the pinion wire, the object being to allow of the raising
or lowering of ( he body of the micrOSCOpC for focussing. The ana-
lyser and the polariser may thus he rotated synchronously without
disconnecting their toothed wheels. Now, in the mictoseopes

usually constructed tot petrological work the rotation of a small

1 Annales de Chvmde et de Physique (5), xx. pp. 862-482.

2 Tschermak's Mvneralogwche und Petrogr. Milt. Bd. v. 1889, p, '.v.\.

3 Mr. Watson, (it ELolborn. London, and .Messrs. I lenry Crouch, Limited, makt suit-
able instruments. Those constructed hv Zeiss, of .Jena ; Nuclei, of Paxil) Voi^t and
Hoch^esan^, of ( iiittin^en ; Fucss, of Berlin ; and Hartnack, of Potsdam, can also bt
recommended.

4 The instrument is protected by letters patent.

PETROLOGICAL MICROSCOPE 993

crystal on the stage between the polarising and the analysing prisms
is liable to put it out of position in regard to the cross-threads in

Pjq, 742— Swift's new petrological microscope.

the eye-niece, as the centring of the objective is scarcely ever so
perfect as not to produce some displacement : and, if the centring

994 THE MICROSCOPE IN GEOLOGICAL DTVESTIGATION

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
__i_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 looks strange when first looked at on a fixed stage with mov-
able 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 eye-piece, 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 ai d 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 eye-piece and the analysing prism IV (fig. 74*2)
is for the purpose of placing such plates as the j-undulation plate K
in position.

The great value of the instrument is in the facility with which
studies in convergent light can be performed. <* is a slide fitted
with a double convex lens which may be used for showing the
optical figures of crystals, and II is a similar slide carrying B lens
and diaphragm of small aperture used for showing optical pictures
in minute crystals. The polariser is fitted with two convergent lenses,
which work in conjunction with the lens Aon the slide ot* the stage,
when great convergence is required. This slide may he pushed in
without disturbing the object upon the stage. The achromatic con

denser, A, shown at the foot ot* the figure, also works in conjunction
with the sliding lens, A, w hen the highest angular aperture is required.
When convergent light is required tin- slide on the stage and
either (I or 1J are pushed in, and the eye-piece covered with the
analyser JV. The optical figures <»f the crystal then appear with
almost ideal clearness. It" this simple met hod is Compared with that
previously in use the superiority of the instrument will be im
mediately recognised. It is in fact the must perfect petrological
microscope yet issued, and is one which will suit equally the minera
logical and [o troingical student. The instrument was designed
by Mr. Allan Dick and marks a great advance in this branch of
microscopical science.

The microscopical in vest igat ion 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 test s

the mineral components of rocks of coarse texture, the ease is

different with those of extremely fine grain ; st ill more with such as
present an apparently homogeneous, compact, and glassy character.
The study reveals facts of the most striking significance, and wel-

COERODED CRYSTALS

995

come light has been thrown upon the question of the order and
method of formation of rock-components. 1

The material which issues from a volcano during an eruption
is rarely in a state of true igneous fusion. In most cases it contains
crystals and parts of crystals which have formed under high
pressure 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. Now, pressure lowers the melting
point of all substances. Accordingly, as the pressure is relieved
upon the lava getting at or near the surface, a rise in the temper-
ature of the fused mass must occur, and the crystals which are float-
ing in it at the time are liable to become corroded or redissolved.
Instances of this corrosion are numerous. The quartzes of the
quartz-porphyries have this corroded appearance j whilst the por-
phyritic constituents of the basic rocks (hornblende, olivine, &c.)
not infrequently show the same alteration (vide fig. 743 ; the dotted
line marks the original outline). In the case of the hornblende the
dissolved portions usually give rise to the forma-
tion of small grains of augite and magnetite, which
are then found incircling the 'mother-crystal.'

The rapid movement of the igneous mass may
cause the crystals to come into violent contact
with each other, fracture being the natural result.
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 lava
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

-i . , - j (• A ■ Fig. 713. — Corroded

microscope being frequently required tor their olivine in basalt
detection and determination. of Kilima Ndjaro,

This last stage of consolidation often induces the East Africa,
formation of glass and gives rise to the appearance
-of very remarkable products, which are known to be the result of
definite chemical compounds, endeavouring to crystallise under un-
favourable circumstances. Generally 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, £c. — and represent matter in a state

1 The reader is referred to the following works treating of the microscopic charac-
ters of minerals and rocks:— F. Fouqne'et Michelligvy, MmSralogie micrographique,
Paris, 1873 ; E. Hnssak, Anleitung zitm Bestimmen der gesteinsbildenden Mineralien,
Leipzig, 1885; E. Kalkowsky, Elemente der Lithologie, Heidelberg, 1886; A. V.
Lasaulx. Elemente der Petrographie, Bonn, 1875, and Einfuhrung in die Gesteins-
ieure, Breslau, 1886 (also edition in French) ; Levy et Lacroix, Les Mineraux des
Roches, Paris, 1888 ; F. H. Rcsenbusch, Mikroskopische Physiographic, 2nd edition,
vol. i. ;Die Mineralien' (translated into English by Iddings), vol. ii. ' Die massigen
Oesteine- ' and Hulfstaoellen zur mihrosTcopischen Mineralbestimmung in Gesteinru
(translated into English bv F. H. Hatch) ; F. Rutley, The Study of Pocks, 3rd edition,
1884, and Rock-fcrming Minerals, 1888; J. J. H. Teall, British Pe^Oflrop*y,_1888 \
Ch. Velain, Conferences de Petrographie, ler fascicule Paris, 1889; F. Zirkel,
Lehrhuch der Petrographie, 2 vols. Bonn, 1866; Basaltgestcine, Bonn, 1870; Die
mikroskopische Beschafenheit der Mineralien und Gesteine, Leipzig, 1873; Micro-
scopical Petrography (U.S. Geol. Exploration of 10th parallel), W ashington, 1S76.

996 THE MICROSCOPE IN GEOLOGICAL INVESTIGATION

intermediate between that of a glass and a crystalline body, hence-
are optically inactive. The conditions of their formation have been
experimentally determined by Vogelsang.

The bodies belonging to the highest stage of development are
called microlites (tig. 744). They differ from the crystallites in possess-
ing the internal structure of true 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 equi-
valent 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 nuxion->tructure.

Masses of molten material mav, however, consolidate at a con-
siderable depth beneath the surface ol the earth ; in such cases the
distinction between the first and second periods of crystallisation is
not no 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

I k. Til. — Mu-rolites. (After Zirkel.) Vu\. 746.—- Angite showing zonal

Htrui tuiv. 1 After Zirkul. 1

turn modified by them ; then is action and reaction between it and*
its cn\ ii-oiiiiH-nt . This remarkable property of all crystalline bodies
is well shown by the microscope. Crystals are constantly found
built up of different layers or Eones of material, unlike in their
optical characters, and hence dissimilar in chemical constitution.
This is the v" called zonal structure, and lb common to the felspars
and augites — in short, to nearly all minerals represent ing isomorphous
mixtures (fig. 745), In the case of the augites a difference in colour
often indicates its presence. This structure unquestionably signifies
changes in the environment <>f the crystals during their growth, the

precipitation of each successive /one affecting the ehemical con-
stitution of the succeeding <»ne. This structure may be experi
mentally produced by placing an artificial crystal in a solution of a
substance isomorphic with that of thfl crystal.

Another great service has been rendered by the microscope,
inasmuch as it has enabled the petrologilt to draw conclusions ai to
the physical condition of the fused mass or magma ;it the time
crystallisation commenced. All chemists are aware that when

FLUID INCLUSIONS— 'NEGATIVE' CRYSTALS

997

•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 maybe called glass- or stone-cavities.' 1
When formed under great pressure by the combined influence of
liquid water and fused mineral matter the crystals will contain
glass-cavities and also fluid inclusions.

Glass inclusions are very abundant in the porphyritic crystals
•of volcanic rocks and represent to some extent the composition of
the fused mass at the period of inclosure. The glass composing the
inclusions is often darker in colour than the glass forming the base
of the rock. This is probably due to the presence in the glass of
the inclusions of a greater amount of iron and the bases usually
associated with it. The glass often contains crystallites and micro-
lites, sometimes due to inclosure at the same time, sometimes to a
subsequent crystallising action set up by the glass. Gas bubbles
are also inclosed along with the glass.

The existence of fluid inclusions in crystals has long been known ;
but not until Dr. Sorby directed his attention to the subject was
their universal distribution in rock-constituents imagined, or their
bearing upon geological problems recognised. They are often very
minute, being frequently less than ^ 0 ft 0 0th of an inch in diameter.
They are rare or absent in rocks of the volcanic group, but are
especially characteristic of the plutonic rocks, such as granite,
gabbro, diorite, £e. Where glass inclusions are common, fluid
inclusions are rare or wanting.

Sometimes the fluid inclusions are so numerous in the quartzes
of the granites as to be, according to Dr. Sorby,'2 ' not above the
roVoth °f an incn apart. This agrees with the proportion of a
•thousand millions to a cubic inch, and in some cases they must bo
more than ten times as many.'

The forms of such inclusions vary, but they may be bounded by
planes corresponding to the external faces of the crystals, and are
then termed ' negative ' crystals.

There is usually an intimate relation between the number of
cavities in a crystal and the rate at which it was formed.
Generally speaking, it may be said that the more rapid the growth,
the more numerous the inclusions.

Not infrequently the cavities contain bubbles varying from
i . i o-th to g^^th of an inch in size. These bubbles sometimes
possess an apparently spontaneous movement, at other times heat
must be applied to produce a change of position.

According to Dr. Sorby 's experiments, the bubbles arise in con-
sequence of the contraction of the liquid on cooling from the high
temperature at which the cavities were filled.

The nature of the inclosed fluid has been determined with some
■accuracy. Generally the liquid is a solution of water charged with

1 Sorby, Quart. Journ. Geol. Soc. 1858 p. 242.

- Op. oft. p. 48f>.

998 THE MICKOSCOPE EN GEOLOGICAL INVESTIGATION

salts ; but it is seldom so concentrated as to cause the deposition in,
the cavities of little squares of salt. The presence has also been
established of liquid carbonic dioxide, the bubble of which dis-
appeared at about 32° C, the critical point for this gas.1

The discovery in the mineral components of plutonic rocks
of these fluid inclusions is manifestly of the highest importance.
Daubree's experiments have shown the enormous mineral-forming
powers possessed by overheated water, whilst the presence of liquid
carbonic dioxide testifies to the enormous pressure under which
plutonic rocks, such as granite and diorite, have consolidated.

Inclusions of gaseous matter are also common ; and it is self-
evident that the occurrence of one mineral in another is no rarity ; the
included mineral is cither contemporaneous or older, usually the
latter. To such microscopic inclusions of crystalline bodies is due t ho
remarkable colour of some minerals. Thus, red flakes of hematite
cause the deep red colour of the carnallite of Stassfurt and the
stibbite from the Fassathal in Tyrol. In fact, -so numerous and so
minute are tin* inclusions in some minerals that even with high

powers the minerals appear to lie charged with
the finest dust. The leucitc is a good instance
of this (fig. 7 16).

The foregoing allows us to conclude, that an
absolutely pure mineral is exceptional. Alf
BUCh mineral bodies contain inclosures of foreign

Ifa*. 746. Lencite from HUlttfer which have become entangled during

Kilima N<ljum. Bad their fori i ia t ion ; when they contain glass inclu-
Afnoa. sions they have been precipitated out of a mass

in the condition of igneous fusion. [1 follows,
therefore, that a characteristic of igneOUS locks is the presence of
amorphous glass in their composition, either as a glossy residue OT as
glaSS-inclusions. Still, the absence of such material does not always
demonstrate the mm igneous origin of the rock, for, on the Other
hand, plutonic rocks, such as granite, do not possess this feature.
Their geological occurrence shows them to be eruptive. (J lass
inclusions are certainly reported by Signiund 2 to be present in the
quart ZOS of the granites of the Monte M ulatto, near PrcdaUEO, in Sou 1 1 »

Tyrol, but V. Chrustechoff considers them products of contact meta

morphism.

\\ e have dealt hitherto more especially with igneous masses, but
the sedimentary rocks demand smuu attention.

The microscope enables us to recognise to some extent the sources
whence the materials composing clast ic:t rocks were derived. For
instance, t he presence Of quart /«•> containing QUmorOUS fluid inclusions
((■specially those of carbonic dioxide) and hair like crystals of rutih-
lead us to conclude they are derived from granites or similar rocks.
The cemented material can also be studied and its nature determined.

1 The application of the burning end of a ci^ar to tin action ih usually BOftoimt

to cause the bubble to diaaapMT,

■ ' Petrograpbiaohe Studi.n ani (lianit von I 'r< rlu/./(.,' Jnhrh. k.k.qcol. Bticht

unsttilt, Bd. xxix. 1H71). ]>]>. 805-810.
3 Greek nXturrbt > broken.

THE ACTION OF THERMAL WATERS

999

In some loose sands and sandstones there has sometimes occurred a
curious process which the microscope first brought under notice
This is the precipitation on the outer surface of rounded quartz-
grams of a greater or less amount or silica, which has been deposited
in crystalline continuity with that of the original nuclei (n>. 747)
The phenomenon is like that which happens
when an irregular fragment of a crystal is
placed in a concentrated solution of the same
salt slowly evaporating. Restoration of the
broken angles first takes place ; then deposi-
tion goes on over the whole exposed surface,
in perfect optical and crystalline continuity,
so as to change a broken fragment into a
definite crystal.

By the microscopical examination of
volcanic dust or ashes it is possible to deter-
mine the constitution of the igneous mass fig. 747.— Sand-grain (A)

1 , ■ . °. . , with much quartz de-

wnose eruption gave rise to such material. posited on the surface.
Thus the ashes and dust which fell at various (After Dr. Sorby.)
places after the great Krakatoa eruption

in 1883 were found to belong to an acid lava, a pyroxene andesite.1
Further, the identification of glacial boulders with rocks in situ
can only be satisfactorily carried out by a microscopical examination
of their thin sections. The occurrence has accordingly been demon-
strated of Norwegian rocks as boulders in the Eastern Counties,
whilst Swedish and Finnish rocks are common in the drift of North
Germany and Saxony. We now come to the discussion of the meta-
morphism to which all rock-masses are liable.

The metamorphism caused by atmospheric 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 con-
stituent is felspar, which decomposes under the influence of water
charged with carbonic acid into kaolin ; while the products of the.
decomposition of non-aluminous minerals are carbonates, ferric
oxide, and quartz. The minute accessory constituents, such as the
titanium -oxides, are not affected by these agencies, and hence are to
be found in all clays and sands.

Thermal waters charged with various substances are common in
all volcanic districts and play their part in the metamorphosis of
rocks. In this way a volcanic rock may become silicified through
the percolation of such solutions ; and microscopical examination has
shown that portions of the Roche Castle rock, in Pembrokeshire,
have had their porphyritic felspars changed into quartz by this
agency. Whilst treating of the metamorphosis caused by atmo-
pheric agencies, mention should be made of the fact that a movement
is now in progress to assist the selection of building stones suitable
for public edifices, by a microscopical examination of thin sections of

1 See the interesting paper by J. Murray and A. Renard on ' Volcanic Ashes and
Cosmic Dust' in Nature, 18S4, vol. xxix. p. 585.

IOOO THE MICROSCOPE Ds GEOLOGICAL INVESTIGATION

the rocks proposed to be employed. 1 Obviously an increase in volume
goes hand in hand with the decomposition of a constituent, and this
may seriously atfect the stability of the rock as a mass.

The intrusion of an igneous rock has generally an important
influence on the structure and minora logical composition of the
surrounding mass, portions of which it can include and partially
dissolve (contact-metamorphism). Sections from the junction of an
igneous rock with <>ne of sedimentarv origin are highly interesting.
The metamorphism is found to largely consist in the development of
new minerals, such as garnets, andalusite, mica, fluorspar, or a new-
crystalline structure out of non-crystalline sedimentary materials.
The formation of the new constituents points to the action of over-
heated water and gases of various nature, which accompany the
eruption. Of very common occurrence is the metamorphism of
fossiliferous chalk — which is amorphous Carbonate of lime into a
marble consisting of crystalline calcite in which no trace of organic
life can be discerned. The heat is often so intense as to fuse sand
stones into a brownish -_:las>..

It would be difficult t<» overestimate the utility of the micro
scope in questions relating to dynamic metamorphiBmi or that due

to ' earth st iv--. -s. '

The deformation by movement h a-, sometimes been so enormous,

that the rockfl bave undergone a complete metamorphism, the original

structure Itfinu' partially or even wholly effaced. .Mechanical energy
is of course largely transformed into heat ; hence under high pressure
plasticity may !>.• produced in I mm lies which are solid under ordinary

circumstances. -

Chemical reactions must occur ami must entail the formation of
new minerals; for Spring :i has demonstrated the truth that chemical
action can take place under excessive pressure without the applica-
tion of external heat.

These question! are now engaging wide rpread attention, and vre

ma v hope for u elcome light being cast upon the vexed Subjed of the
crystalline origin of t lie schists. These hit ter never show any t race of
glassy matter, but their struct u re often reminds one of thai <>f pi u tonic
rocks. The opinion i^ daily gaining ground t hat |omc of t hem (e.g. t he

amphibolites) are oothing but altered igneous masses, an opinion
which is strengthened by the know ledge that igneous rocks do pass
into schists under the influence of dynamic metamorphism. This

transition through molecular rearrangement first received its con-
firmation as a truth by the aid of the microscope. Let us take, for

instance, the case of a felspar-pyroxene rock. Under dynamic

metamorphism the twin lamellae 01 the lath shaped felspars become
bent, actual fracture of the crystals may occur, and possibly re -
crystallisation of the component substance in ritu ; the new felspar,
being granular, will arrange itself approximately along the planes

of schistosity.

The pyroxene, which in the case of a dolerite w as light chocolate,

1 Cf. KIooh, ZeiU. der deuUohen geol. Gfawllichqft, Bd> si. ihmh, p, ma.
* Tresca, ' Flow of Solids,' Proc. hint. Mrrh. Enq. 1676, p. B01,
s Bull. Acad. Belgique, torn. xlix. IhmO (|), p, :J2H.

METAMORPHISM IOOI

brown, becomes altered into a pale green constituent, whose optical
characters prove it to be hornblende. This latter also arranges
itself so that the longest diameter of the grain lies itself in the
planes of schistosity. Finally we obtain out of a rock, whose lighter
and darker constituents were distributed uniformly through the
mass, one in which the mineral components occur in wavy or
parallel layers. In conjunction with this transformation a migra-
tion or concentration of certain substances is rendered possible.
This generally shows itself in the white segregation veins common to
4 moved ' masses. Molecular tension can always be recognised by the
presence of optical anomalies. The extinction shadows sweep over
the sections as the stage is rotated, a phenomenon known as undulose
extinction. Strain eventually overcomes the limit of elasticity, and
there occurs granulation. The latter can be defined as the resolution
of a grain, or the edges of a grain, into smaller individuals no longer
in optical continuity with each other. As a rule, this alteration is
confined to quartz and felspar, and becomes only visible under crossed
nicols. It often leads to the formation of a quartz-felspar mosaic,
in which secondary aggregate any relics of the original mineral will
lie. Where large grains of the original constituents remain, the fine
secondary aggregate sweeps round them in the manner characteristic
of fluxion structure.

The quartz-granules of metamorphosed strata are sometimes
observed to have lost the fluid inclusions so generally found in the
quartz-grains of old sedimentary rocks. Hence it would appear that
expulsion of such liquids is also a result of metamorphic action.

Secondary minerals become developed through the same causes.
Pyroxene and olivine pass into hornblende, lime-soda felspars are
altered into albite (soda-felspar) and epidote &c. Mica, both white
and black, is generally developed along planes of movement, being
formed at the expense of the felspar or the ferro-magnesian con-
stituent. 1

The degree of metamorphism is greatly influenced by chemical
composition and varies accordingly. But it must be well understood
that metamorphism does not produce a radical change in the
elementary chemical composition. There has ensued rather a re-
crystallisation and a new association of the pre-existing elements
(Delesse). The chemical constitution of a hornblende schist formed
by the metamorphism of a dolerite is practically identical with that
of the dolerite ; the change has been here more mineralogical than
chemical.

In conclusion, dynamic metamorphism causes sandstones to pass
into quartzite or quartz schists ; when they are felspathic, into mica
schists. Argillaceous rocks are altered into phyllites &c. ; basic
igneous rocks into hornblendic, actinolitic or chloritic schists.

The optical methods now in use enable the petrologist to determine
the constituents of rock-masses with astonishing success. The colour
of the mineral in transmitted light, the crystallographic outlines,

1 Cf. ' Kecent Researches in the Metamorphism of Rocks,' by Dr. A. Geikie, in
Nature, vol. xxvii. 1882, p. 121.

1002 THE MICROSCOPE IX GEOLOGICAL INVESTIGATION

the direction of the cleavage planes, the polarisation tints, the posi-
tion of the axes of elasticity, as also of the true optical axes, all these,
with other minor properties, render his determinations of real value,
A valuable test is further that of pleochroism, which is well deve-
loped in such minerals as hornblende, cordierite, tourmaline Are.
This property has been artificially produced in colourless crystals by
Senarmont.

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 ol the crystal system to
which they belong. The most satisfactory explanation for the
anomaly of double refraction in crystals of the regular (cubic) system
is that which considers it as the result of tension (Germ. $}kHMun(/).
The admixture of an isomorphic substance can produce this dis-
turbance in the molecular equilibrium. Experiment lias proved that
compression, strain, or other mechani< al distortion, may cause amor-
phous bodies and crystals belon^in^ to the regular system to become
double -r-f'i \t< t in A uniaxial crystal becomes biaxial by the appli-
cation of pressure at right angles to its optical axis, and ordinary
glass may be Lri\en optical properties in the same manner.

Mention may well be made here of the anomalies presented by
the mineral l<-u- it.-, which is a most important constituent of the
lavas of Vesuvius and the nei«'hl)ourh«KHl of Koine. It crystallises
apparently in beautiful ieositet rahedra ( fig, 7 48 ). and to belong to

the regular system should remain dark
under crossed nicols, that is, isotropic.
The .small crystals certainly behave in
this manner, but the large! ones display
tin >r< or lc double i efracl ion \\ 1 1 1 1 de
eided traces of t win lamella' (tig. 7 1s).
This anomaly \\a> tor a long time inc\
plicable, till Khan showed1 that such
Crystals revert when healed to .r>0() " (J. to
a condition of perfect isotropy, which
property they again lose upon becoming

oooL The conclusion to be drawn from his
nlaasioal investigation is, thai the leucito
originally Otystallised in the regular sys-
tem and that il present opt ieal condition
U Owinfl to molecular change due to the
reduction of temperature consequent upon solidification. It is
worthy of notice that .M M. Pouque A&d Miehel Levy have synthe-
tically produced a Leucite rock, i In* leucites of which possessed the
optical anomalies described above.

I lie relation between optical characters and chemical constitu-
tion has received some degree of attention, and in the case of the
felspar group has been accurately determined. Only the 'quantitative''

Pxo. 748, — Leucite showing twin

striution under crossed nicols

(After Zirkel.)

i 1 For adeacsriptton of tlie so-rullcd ' Falnt/un^ MikroHkop,' Ht-e Groth'n PInjMt-

kahsche Krystallugraphic, Leipzig, 1885, p. 681.

PURIFYING CRYSTALS OF INCLUSIONS — ANALYSIS 1003

portion of the subject can be dealt with here, and we must abstain
from the discussion of those minerals whose microscopical appearance
leads the trained petrologist to draw qualitative conclusions. It is
interesting, however, to note that the dark reddish or violet-brown
colour of some monoclinic pyroxenes is due, according to Knop, to
the presence of a not unimportant quantity of titanium oxide in their
constitution. This was found in one instance to be as much as 4 57
per cent., the angle between the axis of least elasticity yand the
vertical crystallographic axis (commonly called the extinction angle)
being also very high. As a rule, the extinction angles of all mono-
clinic pyroxenes are found to vary between 36° and 54°. Hence
Professor Tschermak concluded that this was due to differences in
their chemical constitution, and suggested the varying amount of iron
as the cause. The subject has recently been investigated, though
with poor success, by Wiik,1 and later by Doelter. Asa general
rule, the extinction angle may be said to be somewhat less in those
augites which are poor in iron and alumina than in those rich in
these substances. In the case of the hornblendes the positive angle
of the optical axis increases, according to Tschermak, with the quantity
of iron, whilst Wiik considers the extinction angle to be proportionate
to the quantity of alumina. The diversity of opinion demands a
further inquiry.

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)Si6016) and
anorthite (Ca(Al2)Si208), the optical and chemical characters stand
in the closest possible relations to each other. Hence, given the
extinction angle on a known surface, the chemical constitution is
known and, roughly speaking, the specific gravity.

Strangely enough, the micro-spectroscope has not been ex-
tensively used by penologists. It has been recently employed by
Professor Orville Derby in the determination of the presence of
monazite in Brazilian sands.2 This mineral contains a large per-
centage of didymium, and accordingly, gives the bands characteristic
for that element.

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 formula? 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.3 Most satis-
factory results have been obtained by such means. In cases where
the greatest accuracy is necessary, the apparatus designed by Dr.

1 ' Om fbrhallandet mellan de optiska egenskaperna och den kemiska snraiiinn-
sattningen hos pyroxen- och amphibol-arterna '—Fiuska Vetensk. Soc F&rhdl, vol.
xxiv. 1882, and vol. xxv. 1883.

2 American Journal of Science, vol. xxxvii. 1889, p. 109.

3 For their mode of preparation see Rosenbusch, Mikroskopische Physiographic.
p. 206, et seq. (English edition by Iddings.)

1004 THE 3IICE0SC0PE IN GEOLOGICAL INVESTIGATION

P. Mann had better be employed.1 It is well to microscopically
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 micro-
chemistry.2 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. Tin4 cover-glass is accordingly
removed and the balsam dissolved in alcohol. A weak solution of
hydrochloric acid is then poured over the surface, w -hen, it soluble
silicates are present. Lrclatinisat ion will take place. Upon allow ing
the gelatinous mass to evaporate little squares of salt will form if
such a silicate i- present. Sometimes colouring substances maybe
used for the same purpose. My the treatment of a slide with nitric
acid a silicate like neplrdine becomes porous and permeable to
anilin blue, tm hsin, ifcc. In the case of nepheline the colouring
matter cannot he washed out, and hence 'staining' proves a delicate
test.

Where such a can -.' is possible, minute pieces <»t' the question*
able minerals should be isolated and treated singly. There are two

methods in use t'..r testing such particles micro-chemicall y. The
first is that proposed hy Uovricky, who employed pure hydro lluo
silicic acid ( II SiF, ), 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 undo- the microscope reveals the presence of delicate crystals
of the silieo-tli.or nil- of the metals present in the mineral. The
nature of the crystals may then be determined microscopically.

The second method is that proposed by Uehrens, and mostly

follow s the usual method of chemical analysis. The isolated partiole

is heated in a small platinum crucible with a onium fluoride, the

mass then evaporated with sulphun add 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, ean he identified by
optical methods. It is po ihle hy Hehrens' tests to detect the
presence of 0*0005 mgr. < 'a< I in ■ grain.

* Neuea Juhrbuch fur Mineralogie, &c. Bd. ii. 1884, p. ] 72.

2 The following works can be consulted on this inbjeol B. Borioky, EUnunti

enter neurit r/i r in / srh - ,„ i I; rtnk, mi se/i en Mini nil n„, I C, %U in n mil i,:r , PrftgOS, 1877 ;

T. H. Behrens, Mikrochemische Methoden zur Mineralanal i ■■<-. A msterdsm, ImhI ;
HauBhofer, Mikroskopische Beactionen, Braunschweig, 1886; K Lenient < t Etenard,
Reactions microchimia uea <> cristaux, &e. Bruxelles, 1886; Rosenbusch, Mikro
Jkopische Physiographie, vol. i. 1885, pp. 195-288 (English •••litiun by iddlngs) j

v. liutley, Bock-forming Minerals, London, ihmh.

FOSSILISED WOOL — COAL

1005

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 fossilised 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 organisa-
tion ; but such sections, though made with great facility when lime
is the fossilising material, require much labour and skill when silica
has to be dealt with. Occasionally, however, it has happened that
the infiltration has filled the cavities of the cells and vessels, with-
out 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 balsam.
Generally speaking, the lignites of the Tertiary strata present a
tolerably close resemblance to the woods of the existing period :
thus the ordinary structure of dicotyledonous and monocotyledonous
stems may be discovered in such lignites in the utmost perfection ;
and the peculiar modification presented by coniferous wood is also
most distinctly exhibited. As we go back, however, through the
strata to the Secondary period, we more and more rarely meet with
the ordinary dicotyledonous structure ; and the lignites of the earliest
deposits of these series are, almost universally, either gymnosperms 1
or palms.

Descending into the palaeozoic series, we are presented in the
vast coal formations of our own and other countries with an extra-
ordinary proof of the prevalence of a most luxuriant vegetation in a
comparatively early period of the world's history. The determina-
tion of the characters of the Ferns, Sigillariw, 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 recently brought to light

1 Under this head are included the Cycadece, along with the ordinary Coniferar.
or pine and fir tribe.

I006 THE 3IICK0SC0PE -IN GEOLOGICAL INVESTIGATION

numerous examples of coal-plants whose internal structure is suf-
ficiently well preserved to allow of its being studied microscopically ;
and the careful researches of Professor W. C. Williamson have shown
that they formed a series of connecting links between Cryptogamia
and flowering plants, being obviously allied to Equisetacea', Lyeo-
jjodiacece, &c, in the character of their fructification, whilst their
stem-structure foreshadowed both the 'endogenous' and 'exogenous'
types of the latter.1 Notwithstanding the general absence of any
definite form in the masses of decomposed vegetable matter of which
coal itself consists, the traces of struct ur' 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 ditfi'sed through this
a multitude of minute resinoid yellowish brow n granules, which are
sometimes aggregated in clusters and inclosed in sacculi ; and these
may now be pretty certainly affirmed to represent t ho spore*, while
the sacculi represent the sporuwjia, of gigantic Lycopodiae&t
of the Carboniferous flora. The larger t ho proportion of these
granules, the brighter and st longer is the tlame with which the coal
burns; thus in some blazing <nnn>1 coals they abound to such a
degree as to make up the greater proportion of their suh.;tance ;
whilst in init/iru'ltr or 'stone eoal ' the want of them is shown by
its dull and slow combustion. It is curious that the dispersion of
these resinoid granules through the black carlumaceoiis matter is
sometimes so regular as to give to transparent sections very much
the aspect of a section of vegetable cellular tissue, for which they

have been mistaken even l>y experienced microscopists ; but this
resemblance disappears under a more extended scrutiny, which
shows it to be altogether accidental. '*

Passing on now to the Animal Kingdom, we first cite some
parallel Cases in which the essential nature of deposits thai form a
very important part of the earth's crust has been determined by the
assistance of the microscope, and then select a few examples of
the most important contributions h hich it bfl i afforded to ouraoquaint'

ance with types of animal life long since extinct. 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 DiatOtnOCM in what is now dry
land, we can entertain DO doubt that this silicious deposit origin-
ally accumulated either at the hot torn of a fresh water lake or beneath
the waters of the ocean ; just as such deposits are formed at the
present time by the production and death of successive generations
of these bodies, whose indestructible casing! accumulate in the lapse

1 See his succession of memoirs on the cool-plant* In the recent rolnmsi of the

Phil Trans.

2 For notes upon methods to he employed in makin 1 prepm-ati* ns of coal, kuo

Eutley, Study of liocko, 1884, p, 71.

BOCKS IX FORMATION BY MICROSCOPIC ANIMALS IOO;

of ages, so as to form layers whose thickness is only limited by
the time during which this process has been in action. In like
manner, when we meet with a limestone rock entirely composed of
the calcareous shells of Foraminifera, some of them entire, others
broken up into minute particles (as in the case of the Fusulina
limestone of the Carboniferous period, and the nummulitic lime-
stone 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 Orhitolites, is at present
in process of formation on certain parts of the shores of Australia,
as the Author was informed by Mr. J. Beete Jukes, thus affording
the exact parallel to the stratum of Orhitolites (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 re-
searches of Professor W. C. Williamson 1 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. 749), consisting of calcareous and
silicious spicules of sponges and GorgonicE, and fragments of the
calcareous skeletons of echinoderms and molluscs. A collection of
forms strongly resembling that of the Levant mud, with the exception
of the silicious Diatomacea?, 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 the recent exploration 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 '2 of a sample of H cwt. obtained by the dredge from a depth
of nearly three miles, 1 the surface-layer was found to consist chiefly
of entire shells of Glohigerinabidloides, large and small, and of frag-
ments of such shells mixed with a quantity of amorphous calcareous
matter in fine particles, a little fine sand, and many spicules, portions

1 Memoirs of the Manchester Literary and Philosophical Society, vol. vii.
a The Depths of the Sea, p. 410.

1008 THE MICROSCOPE IN GEOLOGICAL INVESTIGATION

of spicules, and shells of Badiolaria, a few spicules of sponges, and a
few frustules of diatoms. Below the surface-layer the sediment be-
comes gradually more compact, and a slight grey colour, clue probably
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 tine state of

Fl«. 740. — BfiozOMOpkl OVffUUfllll in LtTMlt mud; A, (.', 1), silicioiiH
spiculen of Tethya', B, II, spicule* of ( if>(/i<i ; K, ciiUiu ^kiih spicule of
Grantia; F, G, M, O, portions of calcareous Hkeletonof Ee\inoa6Tin <it<i ;
1, calcareous spicule of ( lurijoni'i ; K, li, N, silicious spicules of sponges ;
P, portion of prismatic layer of biicll «»f Pinnn.

division, is in greatly preponderating proportion. ( me can have do
doubt, on examining this sediment, that it. is formed in the main by
the accumulation and disintegration of t Ik; shells of (,'lohit/rri »a ; the
shells fresh, whole, and living, in the surface-layer of the deposit;
and in the lower layers dead, and gradually crumbling down hy the
decomposition of their organic cement, and hy the pressure of the
layers above.' This white calcareous mud also contains in large

FOKAMINIFEBAL ORIGIN OF CHALK

1009

amount the ' coccoliths ' and < coccospheres 9 formerly described.
Now the resemblance which this Globigerina-mud, when dried, bears
to chalk is so close as at once to suggest the similar origin of the
latter ; and this is fully confirmed by microscopic examination. For
many samples of it consist in great part of the minuter kinds of
Foraminifera, especially Globig evince., 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 Inoceramus), form the great bulk of the
recognisable components ; whilst in other cases, again, the chief part

Fig. 750. — Microscopic organisms in chalk from Gravesend : a, b, c,'d,
Textularia globulosa; e, e, e, Botalia aspera; f, Textularia aculeata;
g, Planularia hexas; h, Navicula.

is made up of the shells of Cytherina, a marine form of entomo-
stracous crustacean. Different specimens of chalk vary greatly in
the proportion which the distinctly organic remains bear to the
amorphous residuum, and which the different kinds of the former
bear to each other ; and this is quite what might be anticipated when
we bear in mind the predominance of one or another tribe of animals
in the several parts of a large area ; but it may be fairly concluded,
from what has been already stated of the amorphous component of
the Globigerina-mud, that the amorphous constituent of chalk like-
wise is the disintegrated residuum of foraminiferal shells. But,
further, the Globigerina-mud now in process of formation is in some
places literally crowded with sponges having a complete silicions

IOIO THE MICROSCOPE IN GEOLOGICAL INVESTIGATION

skeleton ; and some of them bear such an extraordinarily close re-
semblance, alike in structure and in external form, to the Ventriculites
which are well known as chalk fossils, as to leave no reasonable doubt
that these also live as silicious sponges on the bottom of the creta-
ceous sea. Finally (as was first pointed out by Mr. Sorby) the cocco-
liths and coccospheres at present found on the sea -bottom are often
to be discovered by 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 Globigerina-mM<\ is being formed over the Atlantic sea -bed at the
present time.

In examining chalk or other similar mixed aggregation, whose
component particles are easily separable from each other, it is de-

Fki. 7 "» 1 . — Microscopic organisms in chalk from Mciulon, seen partly ub
opaque, and partly as transparent ohjccts.

•si i -able to separate, with as little trouble as possible, the larger and
more definitely organised bodies from the minute amorphous particles ;
and the mode of doing this will depend upon whether we are oper.it
ing upon the large or upon the small scale. If the former, a quantity
of soft chalk should be rubbed to powder with water by means of a
soft brush ; and this water should then be proceeded with according
to the method of levigation already directed for separating the*
Jjiatomacece. It will usually be found that the first deposits contain
the larger Foraminifera, fragments of shell, &c, and that the smaller

1 ' On the Organic Origin of the so-called " Crystalloids" of Chalk ' in Ami. Nat.
Hint. ser. iii, vol. viii. lbOl, pp. 190*300.

ORGANIC ORIGIN OF LIMESTONES

IOI I

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 organisms
thus separated should be dried and mounted in Canada balsam. If
the smaller scale of preparation be preferred, as much chalk scraped
line as will lie on the point of a knife is to be laid on a drop of water
on the glass slide, and allowed to remain there for a few seconds ■
the water, with any particles still floating on it, should then be re-
moved : and the sediment left on the glass should be dried and
mounted in balsam. For examining the 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-
creases 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.

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 ; and the
presence of animals of this group 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,
encrinite-stems, or the shells of molluscs. In the formation of the
coralline crag (Tertiary) of the eastern coast of England, polyzoaries
had the greatest share ; but the Tertiary limestone of which Paris is
chiefly built consists almost exclusively of the shells of Miliolida,
and is thus known as miliolite (millet-seed) limestone. In the vast
stratum of nummulitic limestone which was formed at the com-
mencement of the Tertiary period the microscope enables us t<»
see that the matrix in which the large entire nuiimiulites are im-
bedded is itself composed of comminuted fragments and young shells
of the same, together with minuter Foraminifera. In the Oolitic
(Secondary) formation, again, there are many beds which are shown
by the microscope to have been chiefly composed of foraminiferal
shells ; and in those portions which exhibit the 'roe-stone' arrange-
ment from which the rock derives its name (such as is beautifully
displayed in many specimens of Bath stone and Portland stone), it
is found by microscopic examination of transparent sections that
each rounded concretion is composed of a series of concentric spheres
formed by successive calcareous deposits upon a central nucleus,
which is sometimes a foraminiferal shell. In these and similar calca-
reous formations, the entire materials of which were obviously fur-
nished by the accumulation of animal remains, it not unfrequently
happens that all traces of their origin are obliterated by ' metamorphic '
action ; and thus a crystalline marble, whose particles present not
the least evidence of organic arrangement, may have been formed by
the metamorphosis of chalky, Oolitic, or nummulitic limestone. Now
there is very strong evidence that the vast mass of sub-crystalline
' carboniferous ' limestone which forms our coal-basins has had a
similar origin in foraminiferal and zobphytic life, the traces of

IOI2 THE MICROSCOPE IN GEOLOGICAL INVESTIGATION

which have been for the most part removed by the metamorphie
action involved in its upheaval. For where it has sustained but
little disturbance, the evidences of its organic (chiefly foraminiferal)
origin are unmistakable. Thus in the great plains of Russia there
are certain bands of limestone of this epoch, varying in thickness
from fifteen inches to five feet, and frequently repeated through a
vertical depth of two hundred feet over very wide areas, which are
almost entirely composed of the extinct genus Fustdina, Again,
those parts of the carboniferous limestone of Ireland which have
undergone least disturbance can be plainly shown, by the examina-
tion of microscopic sections, to consist of the remains of Foraminifera,
Polyzoa, fragments of corals, &C, And where, as not unfrequently
happens, beds of this limestone are separated by clay scams, these
are found to be loaded with 'mierozoa of various kinds, particularly
Foraminifera (of which the SocGQmina has come down to the present
time), and the beautiful polyzoaries known as Mace-corals.'

Mention has been already made of Professor Khren berg's very
remarkable disc< very that a large proportion (to say the least) of the
green smuls which present themselves in various stratified deposits,
from the Silurian epoch to the Tertiary period, and w hich in certain
localities constitute what is known as the Greensand formation (be-
neath the chalk), is composed of the rusts ol the Interior of minute
shells of Foraminifera and Mollusca, the shells themselves having
entirely disappeared. The mineral material of these casts has not
merely tilled the chambers and t heir coinmunicat ing passages, hut has
also penetrated, even to its minutest ramifications, the canal system
of the intermediate skeleton. The precise parallel to these deposits
presents itself in certain spots of the existing sea-bottom, such as
the Agulhas hank, near the ('ape of (J nod Hope, where the dredge
comes up laden with a green sand, which on microscopic examina
tion proves to consist almost entirely of ' internal casts ; of existing
Foraminifera.

It i», however, in the case <-t* the teeth, the bones, and the dermal
skeleton of \ ertehrated animals that the value of microscopic inquiry
beOOmeB most apparent ; since their structure presents so many
characteristics which are subject to well-marked variations in (heir
several classes, orders, and families that a knowledge of these cha
racter> tVerjuently enables the inicroscopist to determine the nature
of even the most fragmentary specimens with a posit iveness which
must appear altogether misplaced to such as have not studied the
evidence. It was in regard to teeth that the possibility of such
determinations was first made ch ar by the laborious researches of
Professor Owen ; 1 and the following may be given as examples pi
their value : A pock -for -mat ion extends over many parts of Russia
whose mineral characters might justify its being likened either to
the Qhl or to the A» //• Kc< | Sa m Kt . »ne of t his count ry, ami whose
position relatively to other strata is such thai there is great difficulty
in obtaining evidence from the usual sources as to its place in the
series. Hence the only hope of settling this question (which w§4

1 Bee his Vdontogtaphy,

DETERMINATION OF FOSSIL TEETH AND BONES 1013

one of great practical importance, since, if the formation were New
Red, coal might be expected to underlie it, whilst if Old Red no
reasonable hope of coal could be entertained) lay in the determina-
tion ot the organic remains which this stratum might yield • but
unfortunately these were few and fragmentary, consisting chiefly of
teeth, which are seldom perfectly preserved. From the gigantic size
of these teeth together with their form, it was at first inferred that
they belonged to saurian reptiles, in which case the sandstone would
have been considered as New Red ; but microscopic examination of
their intimate structure unmistakably proved them to beW to a
genus of fishes {Dendrodus) which is exclusively paleozoic, and thus
decided that the formation must be Old Red. So, again, the micro-
scopic examination of certain fragments of teeth found in a sandstone
of Warwickshire disclosed a most remarkable type of tooth- structure
(shown in fig. 752), which
was also ascertained to
exist in certain teeth that
had been discovered in
the 1 Keupersandstein ' of
Wiirtemberg ; and the
identity or close resem-
blance of the animals to
which these teeth belonged
having been thus esta-
blished, it became almost
certain that the Warwick-
shire and Wiirtemberg
sandstones were equiva-
lent formations, a point
of much geological import-
ance. The next question
arising out of this dis-
covery was the nature of
the animal (provisionally

termed Labyrinthodon, a name expressive of the most peculiar
feature in its dental structure) to which these teeth belonged.
They had been referred, from external characters merely, to the
order of saurian reptiles ; but it is now clear that they were
gigantic salamandroid Amphibia, having many points of relationship
to Ceratodus (the Australian ' mud-fish which shows a similar,
though simpler, dental organisation.

The researches of Professor Quekett on the minute structure of
bone1 have shown that from the average size and form of the lacuna1,
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

1 See his memoir on the ' Comparative Structure of Bone ' in the Trans. Micros.
Soc. ser. i. vol. ii. ; and the Catalogue of the Histological Museum of the Boy. Coll.
•of Surgeons, vol. ii.

Fig. 752. — Section of tooth of Labyrinthodon.

rQI4 THE MICROSCOPE IN GEOLOGICAL INVESTIGATION

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 on
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 bones
belonged to a large species of the extinct genus Pfrroifncti/hiSj a li ving
lizard whose wing was extended upon a single immensely prolonged
digit. No species of pterodactyle, howe\ er, at all comparable to
this in dimensions, was 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 bono, and di tiered
essentially from that of every known bird, that no one who placed
much reliance upon that evidence could entertain the slightest doubt
on the matter. By Professor Owen, however, the validity of thai
determination was questioned, and the hone was still maintained to
be that of a bird, until the question was finally set at rest, and the
value of tin* microscopic test triumphantly continued, by the discovery
of undoubted pterodactyle bones of corresponding and e\ en of greater
dimensions in the same and other chalk quarries.

The microscopic examination of the sediments now in course of
deposition on various parts of the great oceanic area, and especially
of the large number of samples brought up in the ( !hallenger ' sound
ings, lias led to this very remarkable conclusion — that the debris
resulting from the degradation of continental land masses are not
carried far from their shores, being f/itirr/ y ahsrnt from the bottom
of the ocean-basins. The sediments //e/v found were not of
organic origin, mainly consist of volcanic sands and nshrs, which are
found in volcanic areas, and of rhty that seems to have been pro
ducted by the disintegration of masses of pumice (vesicular lava),
which, after long floating and dispersion by surface-drift or ocean
currents, have become water- logged and have sunk to the hot torn.
As no ordinary silicious sand is found anywhere save in the neigh
bourhood of continents and cont mental islands, and as all oceanic
isl ands are the products of local volcanic outbursts, this absence of
all trace of submerged continental land over the great oceanic area
afl'ords strong confirmation to the belief which geological evidence
has been gradually tending to establish, that the, sedimentary rocks
which form the existing land were deposited in the immediate

ORIGIN OF OCEANIC AREAS

neighbourhood of pre-existing land, whose degradation furnished their
materials ; and consequently 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 inter-
planetary spaces. Thus the application of the microscope to the
study of these deposits brings us in contact with the greatest
questions not only of terrestrial, but also of cosmical physics, and
furnishes evidence of the highest value for their solution.

1 See Professor Geikie's lecture on ' Geographical Evolution ' in the Proc. Boy.
Geog. Soc. July 1879 ; also 1 A Search for Atlantis with the Microscope,' by the same
author, Nature, 1882, p. 25.

ioi6

CHAPTER XXIV

CR YS TALLISA TIOX. POLARISATION. MOLECULAR

COALESCENCE

Although by far the most numerous and most important applica-
tions of the microscope are those by which the structure and actions
of organised beings are made known to us, yet there are many
mineral substances which constitute both interesting and beautiful
objects, being remarkable either for t lie elegance of their forms, or
for the beauty of their colours, or for both combined. The natural
forms of inorganic substances, when in any way symmetrical, are so
in virtue of that peculiar arrangement of their molecules which is
termed crystal! isu.tioii ; and each subject which crystallises at all does
so after a certain type or plan, the identity or ditVerence of these
types furnishing characters of primary value to the mineralogist.
It does not follow, however, that the form of the crystal shall be
constantly the same for each substance ; <»n the contrary, the same
plan of crystallisation may exhibit itself under a great variety of
forms j and the study of these in such minute crystals as are
appropriate subjects for observation by the microscope is not only
a very interesting application of its powers, but is capable of afford-
ing some valuable hints to the designer. This is particularly the
case with crystals of snotr, which belong to the ' hexagonal system,'
the basis of every ligure being a hexagon of six rays ; for these rays
4 become incrusted with an endless variety of secondary formations
of the same kind, some consisting of thill lamina' alone, others of
solid but translucent prisms heaped .>ne upon another, and others
gorgeously combining lamina- and prisms in the richest profusion,' 1
the angles by which these figures are bounded being invariably G0°
or 120°. Beautiful arborescent forms are not unfrequently produced
by the peculiar mode of aggregation of individual crystals ; of this
we have of ten an example on a large scale on a frosted window ; but
microscopic crystallisations sometimes present the same curious
phenomenon (tig. 7o3). The Mineral Kingdom presents many
interesting microscopic objects : avanturine, lapis lazuli, crystallised
silver, &c. make very good specimens ; w hilst thin sections of granite,
gabbro, and other rocks of the more or less regularly crystalline

1 Glaisher on 1 Snow-crystals in 1855 ' Quart. Joum. Micros. Sci. vol. iii. 1855,
p. 179.

FORMATION OF CRYSTALS

1017

structure, also of agate, arragonite, piedmontite, zeolite, and other
minerals, are very beautiful objects for the polariscope.

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 salt, 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
will gradually extend downwards. If it should go on too slowly,
or should cease altogether, whilst yet a large proportion of
the liquid remains, the slide may be again warmed, and the part
already solidified may be re-dissolved, after which the process will
recommence with increased rapidity. This interesting spectacle
may be watched under any microscope, but the instrument specially
designed by 0. Lehmann1 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-ground illumi-
nation ; still more beautiful is the
spectacle when the polarising appa-
ratus is employed, so as to invest
the crystals with the most gorgeous
variety of hues.

By chemically precipitating crys-
talline products under the micro-
scope we can obtain a still deeper
insight into the crystallisation pro-
cess. One of the earliest workers
at this subject was Link in 1839.
He observed that the precipitates FlG- 7 53.-Crystallised silver.

first appeared in the form of very

minute liquid globules, which ran together and eventually by almost
insensible gradations passed into the solid and crystalline condition.
In fact, the deposition of crystalline precipitates out of solutions
seems mostly to occur in this way. Carbonate of lime is a good
instance of this, the liquid globules finally arranging themselves
into the little rhombohedra peculiar to the substance. On the
temperature of the glass slide during the solidification depend
the size and arrangement of the crystals. Thus santonine, 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. 754) ; but when the slip of glass is
cool the crystals are exceedingly minute. In the case of sulphate of
copper, Mr. R. Thomas 2 succeeded, by keeping the slide at a

1 ' Ueber Krystallanalyse,' Fogg. Ann. Bd. xiii. 1881, pp. 506-522.

2 See his paper ' On the Crystallisation at various temperatures of the Double

IOlS

CRYSTALLISATION, POLARISATION. ETC.

temperature of from 80° to 90°, in obtaining most singular and
beautiful forms of spiral crystallisation, such as that represented in
fig. 755. Mr. Slack has shown that a great variety of spiral and
curved forms can be obtained by dissolving metallic salts, or salieine,
santonine, &c. in water containing 3 or -i per cent, of colloid
silica. The nature of the action that takes place may be under-
stood by allowing a drop of the silica solution to dry upon a slide ;
the result of which will be the production of a complicated series of
cracks, many of them 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-

Fiii. 7." I. — Kadiatin^ c rystallisation of santonins.

tion above tin- slid*- to exhibit fragments of Newton's rings, when it
is illuminated with Powell ;ind Lealand'fl modification of Professor
Smith's dark ground illuminator for high powers, and \ iewed with
a ^th objective. With erystalline bodies these actions add to
the variety of colours to be obt lined with the polariscope, the

best slides exhibiting a Beries 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 '/w*- forms of peculiar beauty. O. Leh-
iiiinin has don.- excellent work in this department ; hut reference
must be had to his papers in the ' Zeitschrift fur K ryst allographie '
for a description of the phenomena such mixtures exhibit. The fol"

Salt, Sulphate of Magnesia and Snlphatc of Zinc,' in <t>utirt. ./num. Micros. Sri. n.s.
yi. pp. 137, 177. S. , ,,| II. \. Draper on 'Crystals for the Miero-polariscope,',
in Intellectual Ob.-irrrrr, vol. vi. 1966 p. 187.

' On the Employment of Colloid Silica in the preparation ' of CryHtaln tot tin-

Polariscope/ h) Monthly Mtrro*. J our it. v. p, 50.

DICHROISM

IOIQ

lowing 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
the development of colour. The substances marked d are distin-
guished by the curious property termed dichroism, which was first
noticed by Dr. Wollaston, arid specially investigated by Sir D.
Brewster. This property consists in the exhibition of different
colours by these crystals, according to the direction in which the
light is transmitted through them, a crystal of chloride of plati-
num, for example, appearing of a deep red when the light passes
along its axis, and of a vivid green when the light is transmitted in
the opposite direction, with various intermediate shades. It is only
possessed by doubly refracting substances ; and it depends on the

Fig. 755. — Spiral crystallisation of sulphate of copper.

absorption of some of the coloured rays of the light which is polar-
ised during its passage through the crystal, so that the two pencils
formed by double refraction become differently coloured, the degree
of difference being regulated by the inclination of the incident ray
to the axis of double refraction.

Acetate of Copper, d
„ Manganese
„ Soda
Zinc

Alum

Arseniate of Potass
Asparagine
Aspartic Acid
Bicarbonate of Potass
Bichromate of Potass
Bichloride of Mercury
Binoxalate of Chromium and
Potass

Bitartrate of Ammonia

„ Lime
: . „ Potass
Boracic Acid
Borate of Ammonia

„ Soda (boras)
Carbonate of Lime (from urine of
horse)

Potass
Soda ,

Chlorate of Potass , ,
Chloride of Barium
Cobalt

1020

CRYSTALLISATION. POLARISATION, ETC.

Chloride of Copper and Ammonia

„ Palladium, d

„ Sodium
Cholesterine
Chromate of Potass
Cinchonoidine
•Citric Acid
Cyanide of Mercury
Hippuric Acid
Hypermanganate of Potass
Iodide of Potassium

„ Quinine
Mannite
ATargarine
Murexide

Muriate of Ammonia
Nitrate of Ammonia
Barnes
Bismuth
Copper
Potass
Soda
Strontian
M Uranium
Oxalic Acid
Oxalate of Ammonia
,, Chromium

„ Chromium and Ammonia, d
„ Chromium and Potass, d
„ Lime
„ Potass
„ Soda
Oxalurate of Ammonia

M

H
II
»»

Phosphate of Ammonia

,. Ammonia co- Ma gnesian
(triple, of urine)
Lead. d

Soda

Platino-chloride of Thallium
Platino-cyanide of Ammonia, d
Prussiate of Potass (red)

„ (yellow)

Quinidine

Salicine

Saliginine

Santoniue

stearine

Sugar

Sulphate of Ammonia
Cadmium
Copper

Copper and Ammonia
Copper and Magnesia
( 'opper and Potass
Iron

Iron and Cobalt
Magnesia
Nickel
Potassa
„ Soda
Zinc
Tartarie Acid
Tartrate i if Soda
Urie Aeid
Urate of Ammonia
Soda

»»
•»
»

»»
'»
»»
»'

It not unfrequently happens tliat a remarkably beautiful speci
men of crystallisation develops itself, which the observer desires to
keep for display. In order to do this sueeessfully, it is necessary to
exclude the air ; and Mr. Warrington recommends castor nil as the
best preservative. A small quantity of this should he poured on
the crystallised surface, a gentle warmth applied, and a thin glaM
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-size or other varnish is to be applied. Although most of the
objects furnished by vegetable and animal structures, which are
advantageously shown by polarised light, have been already noticed
in their appropriate places, it will be useful here to recapitulate the
principal, with some additions.

Yi'ijitublr

Cuticles, Hair-, and Scales, from Leaves
Fibres of Cotton and Flax
Eaphides

Spiral cells and vessels
Starch-grains

Wood, longitudinal sectionsof, mounted
in balsam

Animal

Fibres and Spicules of Sponges
Polypidoms of Hydrozoa
Spicules of Gorgoniae

l'« »1 vzoaries

Tongues (Palates) of (iastcropods

mounted in balsam
Cuttle-lidi hone
Scale- of Fishes
Sections of F^-dn lls

M

M
>i
»»
>»

H

1 lairs
< Mulls
J lorn*
Of Shells
Skin
Teeth

Tendon, longitudinal

CONCRETIONARY SPHEROIDS

1021

Molecular Coalescence. — Remarkable modifications are shown
in the ordinary forms of crystallisable substances, when the aggre-
gation of the inorganic particles takes place in the presence of certain
kinds of organic matter ; and a class of facts of great interest in
their bearing upon the mode of formation of various calcified struc-
tures in the bodies of animals was brought to light by the ingenious
researches of Mr. Rainey,1 whose method of experimenting essentially
consisted in bringing about a slow decomposition of the salts of lime
contained in gum-arabic by the agency of subcarbonate of potash.
The result is the formation of spheroidal concretions of carbonate of
lime, which progressively increase in diameter at the expense of an
amorphous deposit which at first intervenes between 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. 756, b. The particles of such composite spherules
appear 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 a, a are gradually produced. The struc-
ture of these, especially when examined by polarised light, is found
to correspond very closely with that of the small calculous concre-
tions 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 auditory sacs of fishes, present an arrangement of their par-
ticles essentially the same.
Similar concretionary spher-
oids have already been men-
tioned 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 Mol-
lusca ; and we have a very
good example of them in
the outer layer of the en-
velope of what is commonly
known as a ' soft egg,' or an
1 egg without shell,' the cal-
careous deposit in the fibrous
matting already described
being here insufficient to
solidify it. In the external layer of an ordinary egg-shell, on the
other hand, the concretions have enlarged themselves by the pro-
gressive accretion of calcareous particles, so as to form a continuous
layer, which consists of a series of polygonal plates resembling those

i See his treatise ' On the Mode of Formation of the Shells of Animals, of Bone,
and of several other structures, by a process of Molecular Coalescence, demonstrable
in certain artificially formed products,' 1858 ; and his ' Further Experiments and
Observations ' in Quart. Journ, Micros. Set. n.s. vol. i. 18G1, p. 28,

1022

CEYSTALLISATION, POLARISATION. ETC.

of a tessellated pavement. In the solid ' shells 7 of the eggs of the
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 W. C. Williamson
on the scales of fishes, there can be no doubt that much of the
calcareous deposit which they contain is formed upon the same plan.

This line of inquiry has been contemporaneously pursued by
Professor Harting, of Utrecht, who, working on a plan funda-
mentally the same as that of Mr. Rainey (viz. the slow precipitation
of insoluble salts of lime in the presence of an organic ' colloid '),
has 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 O \ 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 muriate, nitrate,
or acetate of lime, and carbonate of potash or soda ; so that, being
very gradually dissolved, the two substances may conic slowly to act
upon each other, and may throw dow n their precipitate in the midst
of the 'colloid.' The whole is then covered with a plate of glass,
and left for some days in a state of perfect tranquillity ; when there
begins to appear at various spots on the surface minute points re-
flecting light, which gradually increase and coalesce, so as to form a
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 J so that
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
one employed. W hen 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 cedco-globuline. Besides the globular concretions
with the peculiar concentric and radiating arrangement obtained
by TTr. Rainej (fig. 7oG), Professor Harting obtained a great
variety of forms bearing some resemblance to the following : 1. The
' discoliths ' and ' cyatholiths ' of Professor Huxley. 2. The tuber
culated 'spicules 'of Alcyonaria, and the very similar spicules in the
mantle of some species of Doris. :». Lamella" of ' prismatic shell-
substance/ which are very closely imitated by crusts formed of
flattened polyhedra, found on the surface of the ' colloid.' 4. The

DETECTION OF MINUTE QUANTITIES OF POISON 1023

spheroidal concretions which form a sort of rudimentary shell within
the body of Limax. 5. The sinuous lamellae 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 chloride of
calcium, and transferring it thence to a concentrated solution of
carbonate of potass, with which, in some cases, a little phosphate of
soda 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 inquiry has been further pro-
secuted by Dr. W. M. Ord, with express reference to the formation
of urinary and other calculi."2

Micro-chemistry of Poisons.— By a judicious combination of
microscopical with chemical research, the application of reagents
may be made effectual for the detection of poisonous or other sub-
stances in quantities far more minute than have been previously
supposed to be recognisable. Thus it is stated by Dr. "VVormley 3
that micro-chemical analysis enables us by a very few minutes' labour
to recognise with unerring certainty the reaction of the xowoo^h Par^
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 apjDlication of ordinary chemical tests under
the microscope, but also by the use of other means of recognition
which the use of the microscope affords. Thus it has been shown that
by the careful sublimation of 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 may be applied to the volatile metals,
mercury, cadmium, selenium, tellurium, and some of their salts, and
to some other volatile bodies, as sal-ammoniac, camphor, an 1 sulphur.
The method of sublimation was afterwards extended to the vegetable

1 See Prof. Harting's Bcclierches de Morphologie synthttiquc sur la production
artificielle de quelques Formations Calcaires Inorganiques,publices par VAcadt'mic
Boyale Keerlandaise des Sciences, Amsterdam. 1872; and Quart. Journ. Micros.
Sci. xii. p. 118.

2 See his treatise On the Influence of Colloids upon Crystalline Form and
Cohesion, London, 1879.

5 Micro-chemistry of Poisons.

1024

CRYSTALLISATION,

POLARISATION, ETC.

alkaloids, such as morphine, strychnine, veratrine, etc. And subse-
quently it was shown that the same method could be further ex-
tended 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 admissible), the
detection of poisons and other substances in very minute quantity
can be accomplished with a facility and certainty such as were
formerly scarcely conceivable.

APPENDICES AND TABLES

USEFUL TO THE MICROSCOPIST

8 u

1027

APPENDIX A

TABLE OF NATURAL SINES

o

0'

V

1 Kf lO

OA/ 1 O

A C / '-i O

45 — j°

°

0'

30'=J°

45'=$°

0

•oooo

•0044

•0087

•0131

46

-

•7193

- .

•7224

•79fi 1
i — u i

• 7')Q (

1

•0174

•0218

•0262

•0305

47

•7313

•7343

■737*1

.7 1 A<>
4 -IVa

2

•0349

•0393

•0436

•0480

48

•7431

•7460

• 74.00

. 7 - 1 Q
/Oio

3

•0523

•0567

•0610

•0654

49

•7547

•7576

•7fi04

l Out

4

•0697

•0741

•0784

•0828

50

•7660

•7688

•771 fi

• 77 A A

5

•0871

•0915

•0958

•1002

51

•7771

•7799

.70K'J

I ooo

6

•1045

•1089

•1132

•1175

52

•7880

•7907

•7033

t you

7

•12] 9

•1262

•1305

•1348

53

•7986

•8012

•8038

•80A1

8

•1392

1435

•1478

•1521

54

•8090

■8116

■8141

•81

9

1564

•1607

•1650

•1693

55

•8191

•8216

•8241

10

•1736

•1779

•1822

•1865

56

•8290

•8315

•8339

11

•1908

•1951

•1994

•2036

57

•8387

•8410

•8434

o to ±

•84^7

12

•2079

•2122

•2164

•2207

58

•8480

•8503

•85°6

1 -8^10

OD-tJ

13

•2249

•2292

•2334

•2377

59

•8572

•8594

•861(5

' •8<;^8

14

•2419

•2461

•2504

•2546

60

•8660

•8682

•8703

15

•2588

•2630

•2672

•2714

61

•8746

•8767

•8788

•88H0

16

•2756

•2798

•2840

•2882

62

•8829

> *8850

•8870

•8800

17

•2924

•2965

•3007

•3049

63

•8910

•8930

•8949

18

•3090

•3132

■3173

•3214

64

•8988

•9007

•9026

•0044

19

•3256

•3297

•3338

•3379

65

•9063

•9081

•91 00

V L\J\J

20

•3420

•3461

•3502

•3543

1 66

•9135

•9153

•91 71

%J JL I JL

•01 88

21

•3584

•3624

•3665

•3705

67

•9205

•9222

•9239

22

•3746

■3786

•3827

•3867

68

•9272

•9288

•9304

23

•3907

•3947

•3987

•4027

69

•9336

•9351

•9367

24

•4067

•4107

•4147

•4187

70

•9397

•9412

•9426

•044 1

25

•4226

•4266

•4305

•4344

71

•9455

•9469

•9483

•0407

26

•4384

•4423

•4462

•4501

72

•9510

•9524

•9537

%J 'J *J 1

27

•4540

"4579

•4617

•4656

73

•9563

•9576

•9588

•oi;oo

28

•4695

•4733

•4771

■4810

74

•9613

•9624

•9636

29

•4848

•4886

•4924

•4962

75

•9659

•9670

•9681

30

•5000

•5038

•5075

•5113

76

•9703

•9713

•9724

•0734

31

%J JL 1

•51 50

•51 88

5225

•5262

77

•9744

•9753

9763

32

•5299

5336

•5373

•5410

78.

•9781

•9790

•9799

•0808

i'OV'O

33 1

•5446

•5483

•5519

"5556

79

•9816

•9824

•9832

•QR4.fl

34

•5592

•5628

•5664

•5700

80

•9848

•9855

•9863

•9870

35

•5736

•5771

•5807

•5842

81

•9877

•9884

•9890

■9896

36

•5878

•5913

•5948

•5983

82

•9903

•9909

•9914

•9920

37

•6018

•6053

•6088

•6122

83

•9925

•9931

•9936

•9940

38

•6157

•6191

•6225

•6259

84

•9945

•9950

•9954

•9958

39

•6293

•6327

•6361

•6394

85

•9962

•9966

•9969

•9972

40

•6428

•6161

•6494

•6528

86

•9976

•9978

•9981

•9984

41

•6560

•6593

•6626

•6659

87

•9986

•9988

•9990

0092

42

•6691

•6724

•6756

•6788

88

•9994

•9995

•9996

•9998

43

•6820

•6852

•6883

•6915

89

•9998

•9999

•9999

I 0000

44 1

•6946

•6978

•7009

•7040

90

1-0000

45

•7071

•7102

•7132

•7163

iVote.— The sine of any given angle is the length of the perpen licular 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 80o= perpendicular = J = .g
hypotenuse 1

3 U 2

1028

APPENDICES AND TABLES

APPENDIX B

TABLE OF BEFBACTIVE INDICES

(Suitable for experiment and study for obtaining media for mounting, and supplying contact

substances for homogeneous lens systems.)

Substance IvVt'nu'tive Tiulox

>> cii er .....

• r* D —

1 -TV?

fcain a .

• A* E

I .1.1.'

Sea-water ....

• • • A4 E

1 Oil)

Human blood

• A4 E

1 OOt

Alum (sat. sol.)

DIXViAM / ~< \0 L'„|,r \

ii.tuer^.j'1 r a nr. j

• A* E

l OOli

. A* n

AlHunH 11 ....

• A* E

1 till

AU>01Ul€ AIC0D01 •

• • Mn

l .Mill

( >il of Ambergris

• A* E

1 .>l>>

btilt (>at. s< >1.)

• A* K

Fluor Spar ....

. A* F.

Spermaceti (molted)

• • • • M F.

till
1 ill

Bees-wax (melted)

• A* E

1 l.l.i

hi I'mrs (sp. gl . Ir'.'l.i)

• A* |

1 1 1 ( >

Borax .....

• A4 F.

Naphtha ....

• A* r.

| I/O

4 kit ..1 I11 4 « # . • r\ / 1 ■ 1 W W > . \

IM1 *>I i U t | >* *T l T 1 n « * (^p. • 'VJUj

• A* E

1.1—0
1 | / O

Oil ot Linseed (sp. gT. •lJo2)

• A* D

1 1 B -

I astur Oil .

. A* E

L'4B7

Bees- wax (cold) .

' * - • A* E

T . 1 t|i|

Chloride 01 I in .

• A* p

1 . A • I

1 t>(M

Spermaceti (cold)

* »' • r* F.

1 .>().{

( >il ci < 'innamon .

• A* K

1 ,)().S

1 u 1 < >i < e< i.ir

A* D

1 .1 1 1)

fliiiii \ r'i 1 >in
tllllll. Mill )1(_,

• A* F.

1 . ) 1 _

Dammar ....

• A* D

L*680

Oil of Cloves

• A* d

1*680

Sugar .....

A4 i»

1-535

Felspar ....

• A4 F.

[•686 to 1-701

Cedrene ....

A4 p

1*689

Canada balsam

• A4 p

1*840

Oil of Fennel

• A4 p

1*644

Bock Crystal

A4 D

1*647

Bock Salt (sp. gr. 2-143)

• A4 i»

1 555

Nitrobenzene

• A4 P

1-558

Sty rax

• A4 p

1 :»H2

Benzylaniline

• A4 p

1*61 1

Methyldiphenylamine .

• A4 p

L-616

Balsam of Tolu

A4 E

1*618

Oil of Cassia

. fi |

1*626 to 10 17

Quinoline ....

• A4 n

1*688

Tourmalin (ordinary ray) .

• A4 D

1 030

Iceland Spar (ordinary ray)

• A4 K

1 or, 1

Monobromide of Naphthalin

. A4 n

1057

Bromide of Antimony

(approximately) ft ,,

1-680

Pipeline ....

• A4 D

1*684

USEFUL TO THE MICKOSCOPIST

IO29

Bisulphide of Carbon
Quinidine .
Zircon1.

Carbonate of Lead
Borate of Lead .
Sulphur (melted)
Phosphorus .
Kealgar (artificial)
Diamond (sp. gr. 3*4)
Chromate of Lead

(approximately) /x D
/* E
M- D
/* A

H- E
¥■ D

M E
^ E

1-687
1-700

1-782 to 2-015
1-81 to 2-08

1- 866

2- 148
2-224
2549
247

ft* 2-50 to 2-97

Glass

Crown ......

D

1-51 to

1-53

Plate

. fi

D

1-516

Flint

. fx

D

1-54 to

1 62

Dense Flint ....

. fj.

D

1-650

Extra Dense Flint

. fx

D

1-710

1 Boro-silicate Crown

. fx

D

1-51

Phosphate Crown

. fx

D

1-51 to

1-56

Barium Silicate Crown

. fx

D

P54 „

1-60

Boro-silicate Flint

. fx

D

155 „

1-57

Borate Flint ....

. /X

D

155 „

1-68

| Barium Phosphate Crown .

. fx

D

1'57 „

1-59

' Very heavy Silicate Flint .

. fx

D

]-96

Glass of Antimony

: H-

D

2-216

1030

APPENDICES AND TABLES

APPENDIX C

TABLE OF EXGLISH MEASURES AXD WEIGHTS, WITH THE IE

METRICAL EQUIVALENTS

The following are calculated from the values of the metre, determined
by Col. Clarke, R.E. (1867). as equal to ;>i>S>704;>>2 inches; and the weight
of a cubic foot of water at 62° F. found by Mr. II. J. Chancy (1890) to be
equal to 62-27*o01 lb. avoirdupois.

Length

Inch 2,">.'?*)<.)8 Centimetres.

Foot = 12 inches i>-04Ti>7 Poeimetres.

Yard = 3 feet - -1)1431) Metre.

Fathom 2 yards 1*82878

Pole = 54 yards .V029 1 Met res.

Chain - 4 poles 2*01 100 iVcametres.

Furlong 10 chain> 2*01160 Hectometres.

Mile s furlongs 1M»0*.)33 Kilometre.

Sl'l'KKKICIKS

Square Inch 6*45148 Square ( Cut iniet its.

•00045 Mdliare.
Font 144 Square Inches- '92901
Yard = 9 Feet H-.tC.l 12 Miliums.

•H80 11 Centiare.
Perch = 80| Yards - 2*62024 Declares.

Hood -40 Pel -dies . . = 10*11690 Ares.
Acre = 4 Roodl . , . -40*46784 „

You MM

Cubic inch 01688 Millistero.

„ Foot . . ■1728 Cubic Indu s 2K.1101 ( 'entistercs.

„ STard. • 27 „ let 7<>4.")34 Docisterei,

Minim. })\ . . .

I teaohm, *5 ■ 60 m

Ounce, f 5 H f "

Pint, 0 . 20
Gallon, 0 -80

f S

Gill . .
Pint . .
Quart .
Gallon .
Peeks .
Bushel
Quarter

Cai\k in
Apothecaries'

Ml

28* U671
568*88416
4*64667

) ( "ubic ( entimetrc or M illilitre.

( i ntimetres or \l illilit r< s.

- 284107 ( '« ntilitros.
5*08884 Decilitres.
I »< ciiin 1 1 < s, Milli ti re or lain |«

= 4 pills -
= 2 pints ■
= 4 quarts
■ 2 gallons =
= 4 pecks =
= 8 bushels ■

/ /;/ />' rial

I2-OSU54 Culiic Centimetres - 1*42088 Decilitre.

B416 .. „ .V0H384 Decilitres.

1*18667 Decimetre, MiUiitere, or Litre.

}•."» HW',7 .. I )i-c imetres, M illisteres, or Litres.

MYtflili Decalitres.
2*90086 Hectolitrei.

USEFUL TO THE MICKOSCOPIST

IO31

Weight

Apothecaries'1

= 6-48799 Centigrammes.

= 20 gr. = 1-29760 Gramme.
= 3 3= 60 gr. = 3-89280 Grammes.
= 85= 480 gr. = 3*11424 Decagrammes.
= 12 5 = 5760 gr. = 3*73708 Hectogi-ammes.

Avoirdupois

Drachm, dr = 27-34375 gr. = 1-77406 Gramme.

Ounce, oz. . . . =16 dr.= 437*5 gr.= 2-83850 Decagrammes.

Pound, lb. ... = 1G oz. = 7000 gr. = 4-54160 Hectogrammes.

Quarter, qr. . . . =28 lb = 12-71647 Kilogrammes.

Hundredweight, cwt. = 4 qr = 1-01732 Centner.

Ton =20 cwt = 1-01732 Tonneau.

1 lb. Avoirdupois = -822857 lb. Troy or Apothecaries'.
1 lb. Troy or Apothecaries' = 1-21527 lb. Avoirdupois.

- Grain, gr. .
Scruple, 3 .
Drachm, 5
Ounce, ^ .
Pound, lb.

TABLE OF METRIC MEASURES AND WEIGHTS, WITH THEIR

ENGLISH EQUIVALENTS

The metre was originally intended to be the T(j(ji»V?j(Jotn Part °f the dis-
tance 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. /x
Millimetre .
Centimetre .
Decimetre
Metre . .

Decametre .
Hectometre .
Kilometre

J— Millimetre
Centimetre
y^j Decimetre
T\ Metre . .
Unit ....

10 Metres . .
10 Decametres
10 Hectometres

= -00003937 Inch.

= -03937

= -39370

= 3-93704 Inches.

= 3-28087 Feet.

= 1-093623 Yard.

= 1-98840 Pole.

= 4-97101 Chains.

= 4-97101 Furlongs.

= -6213767 Statute Mile.

Superficies

Milliare . .

. =10

Centiare . .

. =1

Deciare . .

. =10

Are = Unit .

. =1

Hectare . .

. =1

Sq. Decimetres = 1-07641
„ Metre = 1-19601
„ Metres = 11-96011
„ Decametre =119-60115
„ Hectometre = 2-47110

Sq. Ft. = 155-00 Sq. In.
Square Yard.
,, Yards.

»> p
Acre Ft

Volume

Millistere .
Centistere .
Decistere
Stere = Unit
Decastere .
Hectostere .

= 1 Cubic Decimetre = 61*02537 Cubic Indus.
= 10 „ Decimetres = 610-25368
= 100 ., „ = 3-53156

= 1 „ Metre = 1-30798
= 10 „ Metres = 13-07985

= 10 Decasteres

= 130-79854

>>

Feet.
Yard.
Yards,

1032

APPENDICES AND TABLES

Capacity

Millilitre = Cubic Centimetre = 16*89148 M\. Miniins.
Centilitre =10 Centimetres = 2-81 ~rl o f 5. Drachms.
Decilitres =100 „ „ = 8*51900 i 5. Ounces = -70381 Imp. Gill.

Litre . = Millistere . . = 1*75958 0, Tint = 1*75958 „ Pint-
Decalitre = 10 Litres . . =2*19941 C, Gals. = 2*10941 Gals.

Hectolitre = 10 Decalitre- - 2*74926 .. Bush.

Kilolitre = 10 Hectolitres = 1 Store = 1 Cubic Metre - 3*48658 Qrs.

Weight

Milligramme = j\ Centigramme = '01.341 Grain.
Centigramme = ^ Decigramme = *15418 „
Decigramme = Gramme = 1*54181 „
Gramme . Unit = 15*41308 1 1030 Grains.

Decagramme
Hectogramme
Kilogramme
Myriagramuie
Quintal . .
Tonneau . .

<thrc>iricx Avoirdupois

= 10 Grammes 2*56886 5 5*68678 dr.

= 10 Decagrammes 8*21106 J - 3-52299 oz.

10 Heoiogrrunmes - 2*67588 lb. 2*2018687291 lb.

= 1U Kilourranmirs

= 10 Myria^'rammes
= 10 Quintals . . .

22*01869
L'96595 owt
•98298 ton.

Note. — The Centner of 5 myria^raumies, or 110-1 11)., is sometimes
called a Quintal. A ('t inner of KM) II.. and a Ton of 2000 lb. are used
in the United States. The Tonneau is sometimes oalled a Millier.

In the United States the measures for length, surface, volume, and
weight arc the same as the Knglish. Tli^,,. t . » r dry measure are derivod
from the 1 Winchester' gallon, which ■ 268*8 cubic inches; and tlioso for
iluid measure from the 'Winchester' tluiil gallon, which 2.51 cubic
inches.

WlNt H1STKK

Dry

Pint . . 5*60590 I >< cilitn s.

Gallon . V U)472 lito •

Peck . . 8*80944 „

Bushel , ■ 8*62877 Decalitres.

Quarter , 2*81902 Beotolita ,

INTo Mjbtbxc

Fluid

(.ill . . 1-lH-j'il Decilitre.

Pint . . =4*78108 Decilitres.
Quart . ■ 9*46826 „
I lallou . 8*78580 Litres.

M Kline into Winchbstes
Dry Fluid

Litre. . -1*81624 Pint Decilitre. - -s |;,:57 Gill.

Decalitre 1*18615 Peck. Litre . . ■1*05072 Quart.

Hectolitre 2*88787 Bushels. Decalitre - 2*64179 Gallons.

Kilolitre . ■ 8*54734 Quarters.

United States lluid gallon "83254 1 English gaJlojL
English gallon . . . m 1*201188 United States fluid gallon.
, United States dry gallon = '968778 English gallon.
English gallon . . . = 1*032228 United States dry gallon.

USEFUL TO THE MICKOSCOPIST

1033

CONVERSION OF BEITISH AND METRIC MEASURES

LIXEAL

Microvilli metres Jfre. into Inches %c.

v-
l
2
3
4
5
6
7
8
0
10

11

12
13
14
15
l(i
17
18
19
20

21
22
23
24
25
20
27
28
99
30

31

32
33
34
35
30
37
38
39
40

41
4 2
43
44
45
40
47
48
49
50

00

70

80

90
100
200
300
400
500
OOO

;oo

800
900

1000 ( =

ins.
•000039
•000079
•000118
•000157
•000197
•000236
•000276
•000315
•000351
•000394

•000433
•000472
•000512
•000551
•000591
•000630
•000669
•0007U9
•000748
•000787

•000827
•000866
•0009U6
•000945
•000984
•001024
•001063
•001102
•001142
•001181

•001220
•001260
•001299
•001339
•001378
•001417
•001457
•001496
•001535
001575

•001614
•001654
•001693
•001732
•001772
•001811
•001850
•001890
•001929
•001969

•002362
•002756
•003150
•003543
•003937
•007874
•011811
•015748
•019685
•023622
•027559
•031496
•035433
1 mm.)

111 111.

ins.

mm.

ins.

-1

JL

— 4

51

2-007892

Q

Ui 0/ 41

52

2-047262

. >

-11011 1
1 lolll

53

2-086633

4

•157482

54

2-126003

5

•196852

55

2-165374

0

56

2-204744

iy
4

• O t ^ \ ft 0

57

2-244115

ollaoo

58

2-283485

if

.or 1 Q 0 1

or>4ao4

59

2-322855

JLU (1 Clll.j

dVOI 04

GO (6 cm.)

2-362226

11

'43307a

01

2-401596

1 '

'472445

02

2*440967

-f 0

13

0ll5lO

03

2-480337

14

•551186

04

2-519708

15

•590556

05

2-559078

10

•G29927

00

2-598449

1 7

•669297

07

2-637819

18

•708668

08

2-677189

19

•748038

09

2-716560

20 (2 cm.)

•787409

70 (7 cm.)

2-755930

21

•826779

71

2-795301

2 J

•866150

72

2-834671

23

•905520

73

2-874042

J 4

•944890

74

2-913412

25

•984261

75

2-952782

20

1-023631

70

2-992153

2 7

1*063002

77

3-031523

28

1-102372

78

3-070894

29

1-141743

79

3-110264

30 (3 cm.)

1-181113

80 (8 cm.)

3149635

31

1-220483

81

3-189005

32

1-259854

82

3-228375

33

1-299224

83

3-267746

Q 1

± 0OO00D

O OK) t 1 ID

35

1-377965

85

3346487

30

1-417336

so

3385857

37

1-456706

87

3-425228

38

1-496076

ss

3-464598

39

1-535447

89

3-503968

40 (4 cm.)

1-574817

90 (9 cm.)

3-543339

41

1-614188

91

3-582709

4'i

1-653558

92

3-622080

43

1-692929

93

3-661450

44

1-732299

94

3700820

45

1-771669

95

3-740191

40

1-811040

90

3-779561

47

1-850410

97

3-818932

48

1-889781

9S

3858302

49

1 929151

99

3-897673

50 ( 5 cm.)

1-968522

100 (10 cm. =

1 ilecim.)

deckn,

ins.

1

3-937043

2

7'874Ut>6

3

11-811130

4

15-748173

5

19-685216

0

23-622259

7

27-559302

s

31-496346

9

35*433389

10 < =

1 inetre^ 39 370432

= 3-280869 ft

= 1-093623 yd.

1034

APPENDICES AND TABLES

ms.

25000

1

20i :
i

loOOo
1

10000

_ 1

900O

1__

..

__1_
700c

1

6O0O
1

r< "
1

4,, ,,,
L_

3" "

_1

2000
1

IOOO

1

90<>

1
8<Hi

1

7ou

I
GOO

1

BOO
1,

4.1 0
1

41111
1

1 015991

1- 269989
1(393318

2- 539977
2-822197
3171972
3628539
1-233295

5- 079951

6- 319913
8-466591

12-699886
25-399772

mm.
(•2S222
031750
086386
•042333

050800
•0664 1 1
•003499
072571

Inches St. into Micromilli 'metres Sr

mm. I

1US.

_1_
30O
1

•J s 0

1

200
_1_

150
1

100

I

50
1

5

20
J_
IK
t

X

IS

£
1

8
1

I

a

T«
X

TiB
:>

M

•081666
•101599
•126999
169332
•253998
•507995
1-015991
1*269989
1-587486
1-693318
2*116648
2*539977
3*174972
1-283296
4*762457
5*079954
6*349948
7*937429
9*524916

cm.

1*111240
1*269989

ms.

8

11

It?
£
4

15
l-;

8
15
10.

1

.1

4

5
6
I

S
9

to

1 1

ft

1 yd, =

0111.

1*428737

1- 587186
1*746234
1*904983
2 063732
2*222480
2*381229
2*539977
5079951
7*619932

deoim.

1*015991
1*269989
1*623986
L -777984
2*031982

2- 2851.79
2-539977
2*793976
8*047978

nn ttv

91 1392

Lines

Linos

per inch

in mill.

5,000

197

K»,( IOO

39 i

15,1 11 1"

590

l'i 1,000

7-7

L'.",,«i(iii

9M

:*o,i m mi

1,181

35,<M 1(1

1.37 s

40,om

1,575

45,000

1,772

50,(M 10

1 ,968

55, ( •( M )

2,105

60,000

2,362

65,oo(i

2,559

70,000

2,756

75,000

2,95::

80,000

3,160

s:,,oi to

3,34(5

90,000

3,513

95,000

8,740

100,000

3,937

110,000

1,331

120,000

4,724

130,000

5,1 18

140,000

5,612

150,000

5,905

160,000

6,299

170,000

»;,093

180,000

7,087

1 90,000

7,480

f/ir/irs tint/ .1/ V// i met res

Lined

Line*

per inch

in nun.

200,COO

7,87 l

210,01 0

8,268

220,000

8,661

281 1,000

9,o55

LMO.ooo

9.1 19

25o,0OO

9,848

260,000

10,286

270,000

L0.680

2so,ooo

1 1 ,( »2 1

29<>,o< 10

11,417

300. ouo

1 1,M 1

35(l.(M mi

lit, 780

400,000

15,748

150.0(H)

17,717

500,000

19,685

LlnM in ft

25,399-77

1

:.o,799

76,199

3

101,599

1

126,999

5

152,399

6

177,798

7

203,198

8

228,598

9

258,998

10

I'rtuM ions

of llll ilH'll

fi

1 5,0000)

5*08

10,01 >(»

2*5 1

20,01 "»

1 27

80,000

•847

10,01 10

•635

.Ml,) 11 Hi

•606

t;o,o< 10

123

70,01 10

•363

80,000

•317

911,000

•282

100,01 >o

2.VI

1 10,01 >o

•231

1 24 >,< N N »

•212

130,01 )()

•195

I 10,000

18!

150,000

166

160,000

1 59

170,000

149

|S<l,0< 10

111

L»0,0( 10

131

200,000

127

250,1 M30

•1016

800,000

0847

850,01 »o

•0726

100,000

•0635

150,000

•0564

500,000

()508

USEFUL TO THE 3IICKOSCOPLST

1035

The legal equivalent of the metre is 39*37079 inches, and of the kilo-
gramme 15432-34874 grains. In the above tables Colonel Clarke's
measures of 39-370432 inches in 1867 have been adopted as being the
more -accurate. In 1882 the metre was again measured by Rogers, who
found it equal to 39*37027 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. Its theoretical value, viz. the weight of a cubic deci-
metre of water, according to C olonel Clarke and Mr. Chaney,is 15413*08110
grains. If Piogers' value of the' metre were used it would be a trine less.
The kilogramme is some 19^ grains heavier than theoretically it ought to
be, and the litre, a vessel holding a kilogramme of water, has therefore a
greater capacity than a cubic decimetre.

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 diams.
12 dots can be counted in *3 of an inch. At what rate per inch is the
structure in the diatom '?

m magnifying power x number counted .

space counted
735 x 12

"3 inch

= 29,400 per 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*39977 mm.

735 x 12 = 1157-5 per mm.

•3 inch x 25-39977 1

(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 mm. = -03937 inch,

i mm

675'2 x 12 = 29400 per inch.
. x -03937

«

1036

APPENDICES AND TABLES

APPENDIX D

COMPABISON OF THE SCALES OF FAHRENHEIT1 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 bv 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 § for the Centigrade, or f for
Reaumur's.

To convert degrees of the Centigrade or Reaumur's into Fahrenheit's,
multiply the Centigrade by §, 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

F =

9° + 823
5

F

= ?-? + 32;
4

C =

5 (F-32)
9 '

C =

5 R
4 '

R =

4 (F-32)
9 '

R

4 C

IT'

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 MICROSUOPIST

1037

APPENDIX E

OPTICAL FORMULAE

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 /x the refractive index. Then

AC= - ; BC= st (i)

r — s r — s

Example explaining the method of treating the signs : First, it should
be particularly noticed that all curves which are convex to the left hand
have positive radii, and those turned the other way negative radii.

In a biconvex let r = 2, s = — 3, and t = 1 ; then by (i)

\C= _2x 1-= 2 = 2- BC= -3*l = -3 _3
2- (-3) 2 + 3 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 = 00 , and t = 1 ; then

AC= ~2x-1 =0; BC=— 1 =^ -= -1 . . (i)
-2- 00 ' -2-0- -00 v 7

c is therefore coincident with A.

The nodal points D and E may be found thus :

AD = 1 . li. ; B E = 1 . — (ii)

H r — s /x ?• — s

1 3

Example : In a meniscus r = — 3, s = -2, t = and n = ;

-3 1 _3 -3

1 *! 2 4 2 "4 2 3 1 rv

Ai) - 3 ' -3-(-2) 3 ' -T72 3 ' ~ 3 ' 4 2 W

2

D is measured £ inch to the right from A.

1 1

2 1 1

-3-(-2) ~ 3 -3 + 2 3*2 = 3
2

E is measured ^ inch to the right from B.

If the meniscus is turned round so that its convexities face the left
1 3

hand, r = 2, s = 3, £ = -, /* = -;

(ii)

o
2

1

4

~ B 2 3

14 2 1 1 1
a r» f . = . . - 1 •■= - .... (11)

Similarly BE= - Both are therefore measured to the left. The

IO38 APPENDICES AND TABLES

formulae (ii) are approximations, sufficiently accurate for genera] practical
purposes, but in cases of importance the following, longer but more
accurate, formulae should be used :

AD= — ; BE= — (iii)

Plano-convex Lens. — Let /=the principal focal point and //-the
semi-aperture ; then if parallel rays are incident on A, the plane side of
the lens, r= x . and by in) BE = 0. The nodal point is therefore at the
vertex B, and the focal length

B/» "5 : E/=B/ (iv)

h - 1

The spherical aberration

v -it^jy g?

8

Thus when n

«/= -4-5 ^ (v)

If the parallel ravs are incident on the convex aide A, s - » ,

B E = - - (ii), and the focal length

(vi). E/= r, (vii)

ft— 1 /a p 1

The spherical 1 aberration

0/ . (viii)

When /x= r.">Hi (plate glass)

bf -\\y\ (viii)

J

When n \ i>2 (Hint glass)

J/- ~ '8042 2; (viii)

y

To find the radius of a plano-convex lens, the re I", index and focus
E / being given !

r yV-1) (vii)

To find the radius of a plano-convex l<n-. the ref. index, the thickness,
and the focus B / being given :

r = - — ^ (vi)

ii

A plano-concii \ ■ - us follows a plano-convex ; /' will be negative,
which shows that the foonfl is virtual. (\.nea\es heing thin, / is usually

neglected.

Equi-convex and ei|ui-concave generally:

B /«» r 4X (ix)

Ivpii-convex more accurately:

1 1 1 « ,1 1 1T < '•< ,,,,,, Irani <>/,n, , ] -^7.

USEFUL TO THE MICROSCOPIST 1039

r _ t
2(^-1) 2 0* + l) '

-g ±= r t

Spherical 1 aberration

In an equi-convex lens when /x= ,

2

S/= - 1-6 t (xi)

To find the radius of either an equi-convex or equi-concave lens,
generally, the ref. index and the focus Bf being given :

r = 2fc-I)/ (ix)

To find the radius of an equi-convex lens, the ref. index, the thickness,
and the focus B/ being given :

r q (x)

fx + 1 w

Bi-convex and bi-concave, generally :

E/= — i=- • r— (xii)

/m — 1 s-r

Bi-convex, generally :

B/= — T • (xni

The same when about equi-convex, generally :

. B/=^i-s_V2o.Vi) (xiv)

Bi-concave t may be neglected B/= E/ practically.

Bi-convex more accurately, and converging and diverging menisci :

B/= —r (XV)

fc-l)|r.-8-(/x-l)-j>

When the light is travelling from right to left

A/-- L £ L_ (xv)

0*-l) J r-*-(u-l)H

Spherical aberration :

Example : Let r = 2, s = - 3, f = 1, and /x = ; then by (xv)
1 Heath's Geometrical Optics, 1887.

1040 APPENDICES AND TABLES

B/"e-iM»-<-«»-fi-i);ri(a*»-i-8

L S

= J n 1

L 14 7 "7'

-i s 5

Similarly A f - - 2%

7

By (xivi B/= = 2*

.> 5 .»

1 I H -

Therefore ixivl ami ixv> are resp< etivelv ' and ' .
The following is an exampb worth\ of note. Suppose

r- 8< t and > (/* — 1)-.

M

Thus let r = 6 , * = 5, c - 1, u - ".

/l 11\ 160

Thenbx (XV) B/ -il—'1 -J -810.

( - )

It will be obuerved that, although this meniscus is thickest in the
middle, it has, howevi r, u l.u ^c negative locus.
The nodal points <>f a sphere iuv at its centre.
The locus of a sphere, measured from the centre :

B/- (xvii)

• 2(p-l)

The focus of a sphere measured from its surface:

i;/=r/i->;) (xvii)

The focus of a heinisphen measured from the vertex, the light being
incident on the com ex surface:

B/- / n (vi)

But when the li^ht is incident on the plane surface

B/--^ (iv)

ft - 1

Wheil 1*5 the focus of a sphere measured from the surface = ^ 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 nodal points cross over.

To find the radii r and s of a crossed lens of minimum aberration for
parallel rays :

USEFUL TO THE MICEOSCOPIST IO41
r = 2(/z-l)(^) 2(^-1)^ + 2) .

m(2/x + i) y' ^(aT^ipr^ ' ■ ,(xvm)

For plate glass ^ = 1-516 ; r = -5935/; and s = - 3-944 /; (xviii)

S/= -1-025 £ (xvi)

For flint glass = 1*62 ; r = -653 /; and s= - 12-06/; (xviii)

S/=-798| (xvi)

Critical angle. — Let 6 be the critical angle for a ray passing out of a
denser medium into a rarer one.

Then = l (xix)

When 1:888, 0 = 48° 36^'; fi = 1 , 6 = 41° 48^ ; p-1-52 0 = 41° 8J';

p = l*62, 0 = 38° 7'.

Let /be the principal focus, and p = 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 r-£Li ,-Jt£; f.JOL, (xx)

p-f p -f P+P K ;

Let v be the distance from the object to /, and w be the distance from
/ on the other side of the lens to the conjugate image.
Then

v = p-f; w=p'-f; p = v+f; p' = w+f\ and viv=f~; v = L ;

w

w = J— (xxi)

If o be the size of the object and i the size of its conjugate image

i = ow = of=op/ = _of -°(v'-f) .
f v p p-f f

0-iv -jf_ip - if = * (P -/) .
/ w p' p'-f f

jj . : , JJ ,

• * % 0 0

of if r ip op' oiv /..x

V = -4-\ 10 = -J - ; / = -v-r- = . 1 — r- .... (xxn)

% O t + 0 % + O I

Examples : With an objective of J-mch 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 + -03)
p -03 25 5.

Therefore pf = 25- inches, the distance required (xxii)

2

Conversely, if the image of a diatom projected by a £-inch objective
measures 2 inches on the screen at 40£ inches from the optic centre
what is the size of the diatom ?

4 4

the size of the diatom required.

3 x

IO42 APPENDICES AND TABLES

The last formula of (xxii) 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 10 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.

- o IP -01 x 60 -o 1 ...
= = = ixxn)

t 2-4 2-4 4 v

To find F, the equivalent focus of two lenses in contact :

F= /^l ixxiiiV

where /is the focus of one lens and/' that of the second.

Example: It is required to make t combination of two plano-convex
lenses, the focus of one lens,/, being twice ft that of the other, and whose

8

combined focus F = *6, /x j find their radii (see rigs. 4, 6, 8, and 9).
Then /=2/'.

F= 2ff -V*-*/".

y

fmBVmV8 ana/=8F-l-8 .... (xxiii>

r in [)/ (' - 1 ) 1\S- 9; similarly r' !". . . (vii)

The focus for three lenses follows that for two, thus :

*'7f$rfi-fr (xxiii)-

which limy be vrritfc b J, i *..

To find P, the equivalent focus of two lcn e . nol in contact, generally,
F to be measured from the last h.mIuI point ( I ! ) .»f the second lens : I ,< I

d -the distance between the lenses:

Y f+r-d (xxlv)

More accurately let DB be the nodal points of the first lens and
D'K' those of the second, \ ) ; ,imJ A' IV being the respective vertices,
d = the distance from K to D'j then (j ( i ', the nodal points of the combi-
nation, are :

"-*-/+'/-* (xxvi>

and V . ft (xxvii)>

F is measured from one of the nodal points of the combination An

USEFUL TO THE 31ICJROSCOPIST

I043

example will be of interest. Let parallel rays fall on the convex face of
the field lens of a Huyghenian eyepiece ; find their focus.

Let /, the focus of the field lens = 3, and that of the eye lens /' = 1 ;

= and the distance between the surfaces, that is B A', = 1*8; t the

thickness of the field lens = f- ; and if that of the eye lens = — : A D = 0

10 20

(ii) ;BE= --= -*2 (ii). Similarly A'D' = 0 : B'E'= fitt •

A* a 10 "

<7 = EB + B A' = '2 + l-8-2. Now

G, = E,-3!^2 = E,-1 ' • ' • •
v_ 3x1 3 . ^

F~3TI^2 = 2 (XXV11>

"We see, therefore, that the equivalent focus is 1£ inch, but the
principal point, G\ from which the focus is measured is 1 inch to the left
from E' ; therefore the focal point is § inch to the right from E'. Now as
E' is inch to the left of B', the plane surface of the eye lens, it follows
that F, the focal point, is T% inch to the right of the plane surface of the
eye lens.

This explains 'the microscope objective of 10-ft. focus.'

The equivalent focus of the objective was 10 ft., but the nodal point,
G', from which that focus was measured was 9 ft. 11 J inches from the
objective, which would give \ inch as the working distance of the lens.
The objective in question has a double convex back lens and a plano-
concave front ; a small decrease in the distance between the lenses, such
as 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 optical tube length, the position of the principal
points of a combination plays an important part. Suppose the Huyghenian
eyepiece, in the preceding example, was mounted as an objective ; the
tube length would have to be measured from the right-hand principal
point of the eyepiece, wherever that might be, to the left-hand principal
point of the objective, which in the example before us is

G= 2x^ _3 (xxy>

0 + I — 4

G is therefore measured 3 inches to the left 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 3 or 4 inches in error.

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 = the optical tube
length measured in the manner indicated above ; then

M = F^ (xxvm>

If 0 = the focal length of the entire microscope, X.A. = the numerical
aperture, and e = the diameter of the eye-spot, then

1 Journal E.M.S.

3x2

<0 =

I044 APPENDICES AND TABLES

t-2(N.A.)0; N.A. = -i- (xxx)1

2 9

If X = the number of waves per inch of light of a given colour, L the
limit of resolving power of any objective is

L = 2X(N.A.)

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 (xxxii) or
(xxxiii) :

a = 1 (xxxii) ; a = 1 - ^ (xxxiii)

Next find x by (xxxiv) or (xxxv) :

T_2fr -!)/:_■, b t >(xxxiv); s = I*2 . . . (xxxv)

r 5
Then find co by (xxxvi) :

L_/ft±| + 4 (^ + 1) a 3 + (3 p + 2) 0* - 1) + ) (xxxvi)

L-jL' * + 4y2 + a)^ • • • • .'(xxxvii)

The aberration 8 P' ■ -coP'-?/'- . . . (xxxviii)

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.

for the first lens, = * - = + w'y8 . . . . (xxxvii)

for the second lens, A = — + 1 + »'fp . . . (xxxvii)

for both lenses, ~ = - f — - - + (co 4 co') ya . . (xxxix)

Therefore, for n lenses, — = 2 \.— + 2 co if , . (xxxix)

*x J *

The aberration 8 Q' = - 2to Q'Y and 8 F = -2 co FV .... (ll)

Example : Two" plano-convex lenses of equal foci have their convex
surfaces in contact ; find the aberration for parallel rays (fig. 7). Then

m= | ;/=/'• f={ (xzitf)

For the first lens r = co ; therefore x = - 1 (xxxiv) ; P = oo ; therefore

9

« = -1 (xxxii) ; and co = -' - (xxxvi).

2/J

For the second lens s = vj ; therefore # = 1 (xxxv) ; !l« -lj there-

1 Journal li.M.S.

USEFUL TO THE MICROSCOPIST

IO45

fore a ■= — 3 (xxxii) ; co' = (xxxvi) ; o + a/ = 2 w = ^ ;

6/3 3/3

/= 2 F (xxiii) ; therefore 2 a = ..2Q ;

*F= _2°ZV. (xl)

24 F3 6 F [ ;

This is half the aberration of an equi-convex lens (fig. 1) of the same
focal length as the combination where

V=-5-$ • . •••(»>

If the front lens of the combination be turned round so that its convex
surface faces the incident light the aberration is

SF= - A . t (xl)

12 F v 7

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

V-'-I-^ (™>

The following figures pictoriallv illustrate spherical aberration in
single lenses and in various combinations of two plano-convex lenses, all
having the same focus, F, the same aperture, and the same refractive
index, |. The dot nearer the lens is the focal point for the marginal, and
that farther away the focal point for the central rays ; the distance
between the dots is the spherical aberration h F

1 1

Fig. 1. Fig. 2.

Fig. 1. An equi-convex, r = F;

Fig. 8.

Fig. 2. A plano-convex, r = — ;

7 7

Fig. 3. A crossed convex, r = - F ; s = - - F (xviii) ;

12 2i

SF= -1*07 %
F

-•111

1 1

SF = - 1-6 ^ = --173 (xi)

F

SF= -1-1(3 2/2 = -*121 fviii)

F

(xvi)

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 =•£ (xxiii)

SF= --833^-
F

-•087 (xl>

Fig. 4. The same, only 2/=/';

8 F= -1-611 %, = --168 (xl)

F

1046

APPENDICES AND TABLES

Fig. 6. The same, only/ =2/' ;

SF= --5 5T= -'052
F

Fig. 5. The first lens inverted, f—f \

Fig. 8. The same, only 2/= f ;

Fig. 9. The same, only/=2/';

8F= - -376 I = - -039
F

(xl)

o~F = !"= --043 (xl)

SF= --623| = -'065 (xl)

(xl)

1 1

it

1 1

Fig. 4.

Fie.

Fig. ft.

1 1

Fig. 7.

Fio. H.

PlO. !>.

We see, therefore, that with the same focal length F the aberration of
fig. 1 i:> the greatest, and that of tig. 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 L,rlass, fx 1">1(), for parallel rays similar in arrange-

5 f

ment to fig. 9 is when f= f .

o

The Aplanat i<- Meniscus. A spherical refracting surface lias 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 /• = /i r, then thoso
rays will be refracted aplanatically to some other point, say I', which will
lie on the same side of the surface as 1''. This fact is of great service,
because it enables an aplanatic meniscus to bo constructed ; thus, if we
make r the radius of the curve A, we can make «, 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 F> 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
from P\ P will be negative and P' positive.

The formula; for finding /• and P' when P is given are :

11 P • P'= -,x P (xli)

r= —

+ 1 '

and those for finding r and P when P' is given are :

P'

r = -.. ;
M + J

1''

P= - -

(xlii)

USEFUL TO THE MICROSCOPIST

IO47

An excellent combination, suitable for a bull's-eye, can be made of an
aplanatic meniscus and a plano-convex of flint, or a crossed plate lens.
The following are the radii of some examples. A doublet of plate glass
/x = 1'516.

1st lens, a meniscus, diam. 1*7"; r= + •964// ; s= + 1'375".

2nd lens, bi-convex crossed, diam. 2-1" ; r' '= + T816"; s' = -12-07''.

The flatter side of the crossed lens to face the meniscus, the distance
between the lenses •05//, P=l*6", P' = 2-425", d F = - •168,/, angle 70°.

A better combination can be made by substituting a flint plano-
convex for the second lens, diam. 2-1"; fx = 1-62 ; r' = + 1-83"; S F = - •132//.
The aberration is therefore ,036// less than before.

The aberration may be further reduced by adding another meniscus
and by making all the lenses of flint ll = 1'62.

1st lens, a meniscus, diam. T65" ; r = + •958" ; s = + 1-35".

2nd lens, a meniscus having its concave side facing the convex side
of the first lens ; diam. 2-0" ; r' = + i'67" ; s' = + 2-55".

The third lens is a plano-convex, with its plane side facing the
convex side of the second meniscus ; diam. 2*1// ; r" = + 2-914" ; P = 1*55 ;
d F = - -0226 ; angle 70°.

The aberration is therefore '145 v less than that of the first example.
The distance between the lenses is '05" as before.

To find the radii r and s of a lens which will refract light from a point
jj to pointy/ with minimum aberration.

r= 2^ + 2)^ (xliv)

fi (2/x + l)K-4 +

^ (xlv)

Let 0 be the coefficient of lJj in formulae v, viii, xi, and xvi, then for

parallel rays in each particular case the lateral aberration = 3 . (xlvi)

1 V5 ••
Diameter of least circle of aberration = (xlvn)

Distance of least circle of aberration from focus = - ^ \J . (xlviii)

"When the rays are not parallel

1 3

(xbn)=a>p'y3 (xlvii) = 2-fi>i>V (xlviii) = -^p'-if

It is interesting to note that L = 2 (/*- 1)< {\\\\)

3 y2

Therefore, when \x = ^ = t.

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

m = 1 + ^ 0)

It may be of interest to note that formula (xx) on this page may be used
to determine the focus of spectacles required to bring the abnormal focus

IO48 APPENDICES AND TABLES

of either a presbyopic or myopic person to a normal focus. Make p the-
abnormal, and p' 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 ; it is usual to make p' 10 or 12 inches.

Achromatism

Let fx be the refractive index of a mean ray |E line nearly) fo a
certain material, fxc that for a blue ray, and pr that for a red ray ; the is-

persive power of the material is — — ^-r; this is usuallv written - £ , or xr..

The formula for achromatism is

S^ . 1 + lii' . 1 = 0;
/ M'-l /'

that is, ^ + Z , = 0 (U>

f f

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 :

imPJt±£3 (Bl)

If p is large,/ in the denominator maybe neglected; this will make
d half the sum of the foci, which is the formula for both the lluy^rhenian
and Ramsden eyepieces ; but when pm f, d is the sum of the t'oci.

Formula' relating t<> Spherical Mirrors

Let p = one focus, p' - its conjugate, / = principal focus, and r
= radius of curvature ; then in concave mirrors

P* . Pf .f= P p' ;/=r-

V = * \ p = 1 J .

-/'-'• p -f

p ■ I

r^PP. r 2/; V Zzl (xx).

P+P P J

To find p interchange p and />'.

If o is the size of an object . and i the size, of its image, and v the dis-
tance of the image from the principal focal point, then

0 p' .of / "v

t— — x- ; i=_i- (xxn).

p V

In convex mirrors prefix a negative sign, thus: r= —2/, and so on
with the other formula .

The formula- for mirrors may be derived from those of lenses by sub-
stituting-1 for p; thus r= -2/(vii).

Let y = the semi-aperture; then th<- spherical aberration

8f= -1 . y1 (v) or (viii)

J 8 /

A mirror to be aplanatic for parallel raya must have a parabolic curve.

A mirror to reflect ray3 diverging from a point j>, so that they may
converge aplanatically to another point p\ must be elliptical, having //•
and p' for its foci.

USEFUL TO THE MJCROSCOPIST

IO49

Formula relating to Prisms

Let 1 = the refracting angle of the prism, the angle of incidence on
the first surface, the angle of refraction at the first surface, yj/ the angle
of incidence in the prism at the second surface, and the angle of re-
fraction on emergence ; then the total deviation

D = <£ + ifr'-i; (j>' + f = t (liii).

AVhen the ray passes through the prism symmetrically the deviation

is at a minimum : (j) = yjr', ^/ = \^ = t, and

2

. i + D

sm „

/X = — 7" (liv>
sin

2

by which formula the refractive indices of media can be found, because
both l and D are capable of accurate measurement.

Formula relating to Conic Sections
Ellipse. — Let A = major axis ; a = minor axis. Then

T7 A — s/ A2 — a1 n ,

Focus = . — (lv>

2 v '

Parabola.— Let A = height ; a= ^ base. Then

Focus = (hi)

4 A

Hyperbola. — Let A = major axis ; a = minor axis. Then

Focus = — ■ — (lvn)

2

Works consulted: — Coddington,Camb. 1830; Parkinson, Cainb.; '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.

3050 APPENDICES AND TABLES

APPENDIX F

EXAMPLES USEFUL TO THE MICE0SC0PIST

Square £ inch . . . = 10-0804.") square millimetres.

i „ . . . = tV4.">143 „
n ^ » ... - 4480*21
» joo w . . . = "004. >1 ,,

i

1000

= 64514-8
= 645*148

V

I ooo

Square centimetre = 15*500;> square inch.

., millimetre «= l.Vf>008

„ ioo n i.v:>oo:%

10 n - -1550 „ .. ..

„ M m *00155 || || „

Multiples of the above nm\ lu> fuuiul l»y multiply in the values <;iven
by the s(puire of the multiplier.

Thus, square *, inch - ' • 4: the square of 4 -4 x 4 - 16, ami 8*46148
> 1*) UM-'l'i:>i>s square millimetres, the answer required.

Cubic i inch

»♦ a

r

♦» J..u

»' lOOl

B2 0050S nihil' inillimetres.

16*88668 ..

•I-4H.UH) „

•01688
ir.:tsti-i;*j „ u

it

GuMo centimetre r»i*o*2r>;'7 onUo ineh.

„ millimetre 01-02f)'J7 „ l(')n „

1^» 0l-02:»:i7 „ .J,

„ 10 /* mm; Ki-2",

,, ft 0000610*25 „

Multiples of the above ma*, he found by multiplying the values ^iven
by the cube of tin* multiplier.

ThOB, '2 cubic* millimetres: 2 cubed - 2 x 2 ■ '2 - H, and 01*02587 x 8
=. 4HH-2021MJ cubic / inch, tin* answer required.

. I rcnn <> f ( 'ircleg

£ inch diameter 1*22718 sq. '(i inch 7 - '. » 1 7 1 sip millimetres,

a m m *78589816 „ „ „ - 5*0670 „

[Cm h r '54541 h n = 8*5187 » »»

ria m n «■ "78540 „ /,„ „ - -iMM „

= 50670*0 H h-

rood » h - '78640 „ yu\w „ - 506*7

1 millimetre diam. = '78589816 tq. mm. 12*17'J7 nq. inch.

100 fi .... 7854*0 „ /x • 1*2*1787 „ |£ N

10 /x .... = 78*64 „ n = 'lt217 m M

f» = -7864 „ „ = -001217 „ „

USEFUL TO THE MICKOSCOPIST

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 = T!j- inch, then the square of 3 being
9 and -7854 x 9 - 7-0686 sq. ^ inch an<l '05067 x 9 = -45603 sq. millimetre,
are the areas required.

£ in. diameter
i

10 » "

J_

12 " »»

I

100 " "

1

looo " »

1 mm. diam. . ,
100 p
10 n

Volumes of Spheres
inch =

1-02266 cubic
•52360
•30301
•52360
•52360

i

10

J)

J I

3 00

1

looo

5J

16*758 cubic millimetres.
8-580
4-965
•00858

= 8580-0

= '52360 cubic mm. = 31-953 cubic inch.

= 523600-0 „ p. =31-953 „

523-60 „ „ = -03195,,

•52360 „ „ = -00003195

i_

looo

Multiples of any of the above follow the preceding example of cubic
measures. Thus, if the diameter of the sphere = 30 /z, then the cube of 3
being 27 and 523-6 x 27 = 14137*2 cubic and -03195 x 27 = '86265 cubic r^
inch, are the volumes required.

1052

APPENDICES AND TABLES

APPENDIX G

USEFUL NUMBEBS AND FOBMULM

Paris line = -088813783 inch.

Statute mile = 5280 ft. = 1609-330 metres.

Geographical mile = 6082*66 ft. = 1853-978 metres.

Nautical mile = 6080 ft. = 1853-167 metres.

Cubic foot of water weighs 62-2786 lb. avoirdupois at 62° Fahr.

Cubic inch of water weighs 252*286 grs. at 62° Fahr.

Gallon of water weighs 10 lb. avoirdupois at 62° Fahr.

1 gallon = 277-46288 cubic inches.

Cubic foot of sea water weighs 63-963 lb.

Weight of sea water = 1*027 weight of fresh water.

1 inch of rainfall = 100 tons per acre.

Pressure of water in lb. per sq. inch = -433 head of water.

Expansion of water between 32° Fahr. and 212° Fahr. = -04775.

Dip of horizon in nautical miles = 1*23 \/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 lb. 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 £ 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 = 187,272 statute miles per second.1

"Wave-length of yellow light = — - — inch.
5 J 8 50000

Number of vibrations per second 593,276,600,000,000.

Falling Bodies.

S, space fallen in feet ; V, velocity in feet per second ; g = 32*2 ; t, time-
in seconds.

S = V=V27 • VS= 8-025 Vs.

Arithmetical Progression

A, first term ; B, last term; S, sum; d, difference between terms;
number of terms.

1 Latest determinations by Young and Forbes with Fizeau's method.

USEFUL TO THE MICROSCOPIST

IOS3

A = B-<7 (rc-l). B = A + d(n-l). S = (A + B)™,

a

Geometrical Progression
m, multiplier or divisor.

B

m

B = mA(,-1\ S =

B m — A
m — 1

Properties of Circles and SpJieres <fc.

7T

3-14159265358979 +

log 7T . =0-4971499

tt2 = 9-86962 j„ . =1-77245 = '31831

7T

7T
7T

6

•10132

= -52360

x = -56419
\/7r

a^2 = 1-41421 V2# = 8-02496

Circumference, C. Area, A. Radius, r. Diameter, d. Vokune, V.
rface, S.

C = 2 7rr = 7r<Z. A = tt r\ S = n d2. V d = P.

O 7T

. <, r • 1 degrees in arc x area of circle

Area of sector 01 circle = — 2 — .

00O

Length of arc = number of degrees x -017453 r.

Unit of circular measure = 570,29578.

Side of square of equal area to a circle = r \/tt.

Side of inscribed square = r >/2

Area of ellipse = £ major axis x ^ minor axis x n.

Vohune of ellipsoid = major axis x (minor axis)2 x

6

Number of Threads in WhitwortWs Standard Screws
Sizes XV

" 8
1

» 4

3.

55 8
5) 2

. No. of threads 48
40
20
16
12

55

Convenient Approximations for rapid Calculations

6 knots = 7 miles, more correctly 13 knots = 15 miles.
5 kilometres = 3 ,, „ „ 50 kilometres = 31 „
1 metre = 3 ft. 3| in. „ „ 64 metres = 70 yards.
5 centimetres = 2 mches „ „ 33 centimetres = 13 inches.
3 milhmetres = § inch „ „ 5 millimetres = | inch.

1 pole = 5 metres ; 1 furlong = 2 hectometres.

5 /* = -g^jf inch ; T^ inch = \ mm. ; ttfitxttto' inch = \ fx.

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 ;
millilitres = 34 WL (minims) ; 2 decilitres = 7/3 (ounces) ; 4 litres

1054 APPENDICES AND TABLES USEFUL TO THE MICEOSCOPIST

= 7 pints (imperial) ; 2 grammes = 31 grains ; 4 grammes = 15 (drachm)-
(apothecaries') ; 7 grammes = 4 dr. (drachms) (avoirdupois).

5 kilogrammes = 11 lb. (avoirdupois).

50 kilogrammes = 1 centner = 1 cwt.

NoberVs 19 Band Test Plate

Band Lines per inch Band

1 11259-5 '15

5 33778-5 19

10 61927-3

Difference between each band = 5629*75.

Lines per inch
90076-1
112595-1

Nobert's last 20 Band Test Plate

Band Lines per inch Band
1 11259-5 15

5 56297-6 20

10 112595-1

Difference between each band = 11259*5.

Lines per inch.

168892*7
225190-3

Convenient Formula for Lantern Projection or Enlargement and

t Beduction.

Let D be the distance of the screen, and d the distance of the object
from the optical centre of the lens, F the equivalent focus of the lens, M
the magnifying power or ' number of times ' for enlargement or reduction,
then —

D = F(M + 1); # = F + US ;
v ; MM

F =

M + l

M =

D-F

F

Example: It is required to project by a lens of 6 inches equivalent
focus a slide having a 3-inch mask so that it may give a 10-ft. disc, what
must be the distance of the screen ? Here M the magnification will be
40 times. D = F (M + 1) = 6 (40 + 1) = 246 inches = 20^ feet.

Note, in a double combination the optical centre may be assumed to
be half way between the lenses. To reduce, interchange the object and the
screen.

INDEX

ABB
A

Abbe (Prof.), his compensation eye-piece,
42, 323 ; binocular eye-piece, 103 ;
stereoscopic eye-piece, 103 ; achromatic
condenser, 212, 256-259, 329; chro-
matic condenser, 212,256, 267 ; camera
lucida, 237 ; apertometer, 255, 337 ;
condenser, iris-diaphragm fitted to,
259 ; diffraction theory and homo-
geneous immersion, 312, 313 ; method
of testing object-glasses, 326-333 ; test
plate, 330, 331 ; experiments in dif-
fraction phenomena, 376

— on amplifying power of lens, 25 ; on
homogeneous immersion, 28 ; on im-
provement of optical glass, 31 ; on
classification of eye-pieces, 34 ; on
principle of microscopic vision, 43, 44,
45 ; on definition of aperture, 45 ; on
arjerture, 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, 93 ; on ' aplanatic system,'
94 ; on orthoscopic effect, 95 ; on Rams-
den's circles, 107 ; on solid cones of
light, 362

Aberration, 19 ; positive, 21, 309 note ;
negative, 21, 27, 309 note ; chromatic,
31 ; spherical, 31, 251, 254, 331 ; errors
of spherical and chromatic, corrected
by Ross, 306

Abies balsamea, 383

Abiogenesis, 686

Abraham's prism, 344

Absorption or dioptrical image, 64

— and diffraction images due to diffrac-
tion, 65 note

— of light rays, Angstrom's law, 273

— bands, 273, 274, 275, 277
Abstriction of spores, 562
Acalephce, sexual zoiiids of polypes, 786 ;

relationship to hydroids, 796 ; develop-
ment of, 798 ; medusan phase of, 801

Acantliowetrci xiphicantha, 774; echi-
noides, 111

AcanthometHna, 772, 776 ; central capsule
of, 776

ACT

| Acarina, eggs of, 928-929; anatomy of,.
933-936 ; larvae of, 933 ; nymph of, 933 ;
integument of, 934 ; legs of, 934 ; eyes
of, 935 ; classification of, 936
1 Accommodation, of the eye, 88 ; depth, 89-
; Acetabularia, 493 ; pileus of, 493
Acetic acid, as a test for nuclei, 440
Acheta, 911

— campestris, eggs of, 929

AchVya, zoospores of, 494; oospores of,..
495 ; zotisporanges of, 569

— prolifera, 493 and note, 494

[ Achnanthece, characters of, 545
Achnantlies, frustules of, 517, 544 ; stipe
of, 518,544; ' stauros ' of, 545; struc-
ture of frustule, 545
Achnanthes longipes, 545
Achromatic, comparison of, with chro-
matic and apochromatic lenses, 315

— condenser, Abbe's, 212, 256-259, 267 ;
Powell and Lealand's, 251, 258 ; for ob-
servation of pycnogonids, 883

— doublet, Fraunhofer's, 146 ; meniscus,
322

— lenses, Charles's, 146 ; Marzoli's, 302 ;
Selligue's, 303

— objectives, 19, 32 ; Powell & Lealand's
dry, 190 ; Tully's, 303 ; Wenham's, 310 ;
cover slips for use with, 380

— oil condenser, Powell & Lealand's, 267
Achromatism, 17, 19, 148; in photo-
micrography, 33 ; rise of, 145 ; in-
augurated, 313 ; imperfect, causing
yellowness, 360

Acineta, 697

' Acinetiform young ' of Ciliata,112 note
Acinetina, 696; food of, 697
' Acorn ' monad, 684
'Acorn-shells,' 891

Actinia, reproduction from fragments,
787, 801

— Candida, thread-cells of, 803

— crassicornis, thread-cells of, 803
Actinocyclus, 518, 539, 550
Actinommainerme, 114., 776
Actinophrys, 770

— form of Microgromia, 662

— sol, 662-665
Actinoptychus, 518, 540, 541

1056

INDEX

ACT

ActinosphcErium EJichornii, 665

Actinotrocha, 874

Actinozoa, 787, 801-806

Actius on myopy, 120

Actuarius on myopy, 120

Adams' variable microscope, 141 ; non-
achromatic microscope, 146
Adder's tongue ' fern, 604 ; sporanges of,
601

Adipose substance, 969
Adjusting objectives, Eoss's, 306, 309
Adjustment, coarse, 156, 157, 185 ; Wale's,
185

— fine, 157-164

•• long lever system, 151 ; Ross's, 151,

161 ; to Pritchard's microscope,
151 ; applied to stage, 153; Powell's,
161 ; short side lever, 162 ; Swift's
vertical side lever, 162, 181 ; Camp-
bell's differential screw, 164, 193 ;
to the sub-stage, Nelson's, 169 ; for
Powell & Lealand's sub-stage, 174;
in Beck's No. 1, 180 ; Wale's, 185 ; in
Beck's third-class microscope, 190

- — screw collar, 309

iEcidiospore generation of Puccinia, 566
JEcidium berberidis, relation to Puc-
■ cinia, 566

tussilaginis, 567
JEthalium septicum, plasmode of, 563
Agaricus, 575
■ — campestris, 576, 577
Agate, 1017

Agave, leaf of, 611 ; raphides of, 621
Agrion, 911

— pulchellum, wing of, as test for defi-
nition, 368

< — puella, pupa of, 918; wing of, 918
Air-angle, 50, 78

Air-bubbles, microscopic appearance of,
370, 371

Air-chamber of leaves, 641

Airy's modification of Huyghenian eye-
piece, 321

Alse of Siirirella, 535

* Alar prolongations' in Fusulina, 750 ;
in Nummulites, 752, 756; of Calca-
rina, 755

Albite, 1001, 1003

Albuminous substances, tests for, 440
Alburnum, 629, 633

Alcohol, as solvent for resins, &c, 441 ;
as hardening agent, 428

Alcyonaria, 801, 803; spines of, imi-
tated, 1022

Alcyonian, resembled by polyzoan, 832

Alcyonidium, 832 ; polyzoary of, 833

— gelatinosum, calcareous spicules in,
832 note

Alcyonium, 781

— digitatum, 803 ; spicules of, 804
Alder on branchial sac of Corella, 836

note

Alexander on myopy and presbyopy, 120
Algm, preparation of, 427 ; included
under general term of ' thallophytes,'
470 ; symbiotic in radiolarians, 773

AND

Algal constituents of lichens, 579
Alkaloids, micro- chemical examination of,
1024

Allman's experiments on luminosity of

Noctiluca, 694
Allman on Polyzoa, 833 ; on the ' Haus '

of Appendicularia, 842 ; on Myrio-

thela, 787 note
Aloe, raphides of, 621
Alternation of generations in Batracho-

spermum, 504 ; in Fungi, 563 ; in ferns,

605 ; in Medusce, 801
Althcea rosea, pollen-grains, 646
Alveolina, 729 ; resembled by Loftusia,

743; resembled by Fusulina, 750
Amaranthacece, pollen-grains, 646
Amaranthus hypochondriacus, seeds of,

648

Amaroucium proliferum, as example of

compound ascidian, 836
Amici suggests oil for immersion lenses,

29

Amici's invention of immersion system,
27; horizontal microscope, 146; camera
lucida, 235 ; objectives, 304 ; triple-
back objectives, 310 ; water-immersion
objectives, 310 ; oil-immersion objec-
tives, 312

Ammodiscus, 739

Ammotliea pycnogoiioides, 882

Amoeba, 658, 667-669, 942

^.mce&a-phase of Monas, 681

Amoeba proteus, 667

— radiosa, experiments on, 668
Amozbce, cells of sponges resembling, 779
Amoeboid phase of Tetramitus, 686
Amphibians, plates in skin of, 950
Amphibolites, 1000

Amphioxus, affinities with ascidians,
841 note

Amphipleura pellucida, with oblique
illumination, 59, 75 ; resolution of, 85 ;
markings measured, 230 ; markings
on, 521

Amphistegina, 752; internal cast of,
766

Amyliitetras, 543

Amphiuma, red blood-corpuscle of, 960
Amphonyx, haustellium of, 916
Amplification, 83

— linear, 25, 26, 39 ; of images, 45
Ampullaceous sacs of sponges, 780, 781
Anaboena, 491

Anacharis, 458

— alsinastrum, cyclosis in, 613, 615;
habitat, 614

Auagallis, raphides of, 621 ; seeds of
649

— arvensis, petals of, 644
Anal plate of Antedon, 827
Analgesince, 937, 938
Analyser, 269
Analysing nose-piece, 244
Anarapfiidecs, 527
Anchor-like plates of Synapta, 819
Andalusite, 1000

Androspore of (Edogonium, 503

I^DEX

IO57

ANE

Anemones, 787. See Actinozoa
Anemophilous flowers, (547
Anethnm graveolens, seeds of, 649
Angle of incidence, 3 ; of refraction, 8 ;

of aperture, 61
Angles of aperture, air, balsam, oil,

water, 83-87
Angstrom's law for the absorption of

light rays, 273
Anguillula aceti, 869

— fluviatilis, 869

— glutinis, 869
Anguillulce, 869
Angular aperture, 338

— — of dry objective, 334 ; of oil immer-

sion, 334

of aperture, resolution dependent

on, 44

of water immersion, 334

Angular distribution of rays, 56; grip,

61 ; semi-aperture, 77
Anguliferce, characters of, 542
Anilin dyes for blue and green stains,

436

Animal kingdom, two divisions of. 652

Animalcule cage, 294

Animalcules, 678. See Rotifera, Infu-
soria, Rhizopoda, &c.

Animals and plants, differences between,
461

Anisochehe of sponges, 783, 784
Anisonema, 690

Annelida, larvae of, collecting, 459; ma-
rine, 872 ; appendages of, 873 ; jaws of, I
873 ; development of, 873 ; eggs of, 874 ;
fresh- water, 879 ; luminosity of, 879 ;
bibliography, 880 ; ' liver ' of, 971

Annual layers in trees, 628

Annular cell, Weber's, 299

— ducts of Phanerogams, 623

— illumination and false images, 362

— illumination for examining perforated
membrane of diatom, 362

Annulus of sporange of fern, 601
Anodon, pearls in, 847 ; glochidia of,

857 ; for observation of ciliary motion,

864

Anomia, prismatic layer in, 848
Anopla (Nemertines), 875
Anoplophri/a circulans, 702
Anorthite, 1003

Antedon, food of, 696 ; pentacrinoid
larva of, 825, 826 ; pseudembryo of, 827

Antennae of insects, 911 : preparation of,
912, 913 note

Antherid of Vaucheria, 492; of Chara,
507, 508 ; of Fucacece, 556, 557; of Flo-
ridece, 561 ; of Peronosjwrece, 567 ; of
Marchantia, 590, 592 ; of mosses, 595 ;
of Sphagnacece, 598 ; of ferns, 602 ;
tapetal cells in, 603

Antherozoids, 467, 470 ; of VolvoX, 483 ;
of Vaucheria, 492 ; of Sphceroplea,
501 ; of CEdogonium, 503 ; of Batra-
chospermum, 504 ; of Chara, 507, 508;
of Ph&osporea, 556 ; of Fucacea*, 558 ;
of ferns, 603; of Bhizocarpece, 606

ARA

Anthers, 644

Anthony (Dr.) on pseudo-tracheae of fly's.

proboscis, 915 note
Anthophgsa, 690
Anthracite coals, 1006
I Antirrhinum met jus, seed of, 648
Apertometer, 174, 333 ; Abbe's, 255, 337

Tolles', 333 ; use of, 337
! Aperture, in microscopic objectives, 33,

43-47, 60-67 ; how obtained, 45 ; Abbe

on definition of, 45, 48 note

— relation of, to power, 82, 83, 311 ; as-
certained by vertical illumination,
286

— angular, 49 note, 53, 338

— numerical, 49 note, 53, 76, 333 ;
for dry objective 50 ; for oil immersion,
50 ; for water immersion, 50

— numerical, of Zeiss's apochromatic
series of objectives, 318

— of objective, 332, 333

— numerical, table of, 84-87
Apertures, relative, 49
Aphanizomenon, 491
Aphanocapsa, 477

Aphides, wings of, 922, 923 ; agamic re-
production in, 930
Aphodius, antennae of, 912
Apidce, 911

Apis mellifica, mouth-parts of, 915
Aplanatic system, 20, 23

— objective, use of, 21

— cone, 255

— aperture, 257, 262

— foci, Lister's discovery, 304
Apochromatic objectives, 19, 30, 32, 34,

80, 211; advantages of, 33, 34;
objective, Zeiss's, 314-320 ; dry, 315 ;
comparison of, with chromatic and
achromatic lenses, 315 ; homogeneous
objectives, value of, in study of
monads, 687 ; objective, use with
various test scales, 900

— condenser, Powell and Lealand's, 254
Apochromatism, 314

Apocgnacece, laticiferous tissue of, 620
Apogamy in ferns, 605
Apospory in ferns, 605
Apotheces of lichens, 578, 579
Apparent creation of structure, 68
Appendicularia, 835, 841 ; pharyngeal

sac of, 841 ; tail of, 842 ; notochord, 842 ;

'Haus' of, 842
Apple, raphides in bark of, 621
Apposition, growth by, 463

— mode of growth of starch, 620
Apus, 883, 886; parthenogenesis of, 888

note

— cancriformis, carapace of, 886
Aquarium microscopes, 219-225 ; Collins's,

221, 222 ; Schultze's, 222, 224
Aquatic microscope, 145
Arachnida, 982

— eggs of, 929 ; related to Pyoiogonida,
883 note ; reproductive organs of, 935

Arachnoidiscus, 518, 541
Arachnosphcera obligacantha, 774, 776

3 Y

io58

INDEX

ARA

Aralia papyrifera, parenchyme of, 611
Araneida, 932
Arcella, 670

Archegones of Vaucheria, 492 ; of Chara,
507, 508 ; of Marchantia, 590, 593 ; of
mosses, 595, 596 ; of Sphagnacece,
598; of ferns, 602, 603; of Lyco-
podiece, 606 ; of Bhizocarpece, 606

Archer, on amcebiform phase of Stepha-
nosphcera, 485 note ; on desmids, 509
note ; classification of, 515 ; on
Clathrulina, 666 note; on rhizopods,
677

Archerina Boltoni, 655
Arctium, stem of, 634
Arcyria flava, sporanges of, 564
Arenacea, 735-739

Arenaceous character of Textularinice,
748

— Foraminif era, varying size of particles
in test of, 743

— test of Foraminif era, 735
Arenicola, 872

Areolae of frustule of Coscinodiscus, 520

Areolar connective tissue, 964, 969

Argas, bite of, 936

Argasidce, 936

Argonauta, 853

Argosince, 932

Argulus folia ceus, 890

Aristolochia, stem of, 634

' Aristotle's lantern ' of echinids, 814

Aristotle on myopy and presbyopy, 120

Arragonite, 1017

— in shell of Pholas, 848

Arsenic, micro-chemical analysis of, 1023
Artemia, 886

— salina, movement of, 884 ; habitat of,
887

Arteries, 980

Arthrodesmus incus, 498
Arthbopoda, 881-940 ; smallest of, 932 ;
eye of, 907

— limbs of Pedalion compared with
those of, 718

Arthrosporous Bacteria, 582

Artificial light, 359

4 Artificial lightning,' 607

Ascaris lumbricoides, 868

Asci of Ascomycetes, 571 ; of lichens, 578

Ascidians, diatoms in stomach of, 544,
552 ; solitary, 835 ; branchial sac of,
836, 837, 839 ; circulation in, 836, 839 ;
compound, 836 ; cloaca of, 837 ; stolons
of, 838 ; bibliography of, 838 ; social,
838 ; general structure of, 840 ; de-
velopment of, 840 ; tadpole of, 841 ;
affinities with Amphioxus, 841 note

Asclepiadece, pollinium of, 647

Ascogone of Ascomycetes, 572 ; of lichens,
579

Ascomycetes, 571-574 ; as fungus-con-
stituents of lichens, 579

Ascopores of Ascomycetes, 571 ; of
lichens, 578

Asellus aquaticus, ciliated parasite in
blood of, 702

BAC

Asilus, eye of, 911

Aspergillus, fermentation by, 575

Asphalte for cells, 386

— varnish, 383

Aspidisca, a phase in development of

Trichoda, 709
Aspidium, indusium of, 600 ; sori of, 600 1
Asplanchna, in confinement, 458
Astasia, 475 ; mouth in, 690
Asteroidea, skeleton of, 815 ; spines of,. .

815 ; larva of, 821
Asterolamyra, 524, 539
Aster omphalus, 539
Astromma, 11±
Astrophyton, spines of, 815
Astrorhiza, 736, 740
Astrorhizida, 737
Athecata, 792

Athyrium Filix-fozmina, apospory in, 605 •
Atrium of Noctiluca, 691
Auditory vesicles of Mollusca, 865
Audouin on ' muscardine,' 574
Augite, 995 ; zonal structure in, 996
Aulacodiscus, 541

— Kittonii, markings on, 521

— Stitrtii, markings on, 521
Autofission of diatoms, 523
Auxospore, 523-530
Avanturine, 1016
Avicularia of Polyzoa, 834, 835
' Awns ' of Chcetocerece, 543
Axile body of tactile papilla, 977
Axinella paradox a, 782

Axis cylinders of nerve-tube, 975, 976

B

Bacillariacece of Kiitzing, 517
Bacillaria paradoxa, movements of } .

528, 531, 535
Bacilli, form of, 581

Bacillus, ' granular spheres ' of, 588 note

— anthracis, 582 ; spores live in absolute -
alcohol, 587

— megaterium, 582

— subtilis, 582, 583, 585 ; spores of, 587

— of anthrax, 961 note

— of tuberculosis, modes of staining, .
438, 439

' Bacon-beetle,' 904

Bacon (Roger), inventor of simple micro-
scope, 128

Bacteria, use of large and small cones in
examining, 363; photo-micrographs, 365;
as test for definition, 368 ; preserved
by osmic acid, 428 ; violet of methyl-
anilin as a stain for, 437; methyl-
blue as a stain for, 438 ; staining, 437,
438 ; (see Schizomycetes), 579 ; affinities
to AlgcB, 580 ; to Flagellata, 580 ; to ■
Nostocacece, 580 ; forms of, 581 ; move-
ments of, 581 ; mode of multiplication,
581 ; classification of, 582 ; nutrition
of, 585 ; flagella of, 587 ; germinating
power of, 587 ; spores of, 587

Bacteriastrum furcatum, 543

INDEX

1059

BAC

Bacteriology, 589

Bacterium hneola, compared with Cerco~
mutias, 580

— lineola, 586

— termo, flagellum of, 72 ; movement of,
581 ; zoogloea of, 585, 586 ; germination
of, 587

Bailey, on internal casts of Forarninifera,
753 note

Bailey's method of isolating diatoms, 553
Baker on Cuff's microscope, 140
Baker's students' microscope, 193 ; optic
axis of, 194

— lamp, 350
Balanidce, 891

Balanus balanoicles, 891 ; disc of, 892
Balbiani on supposed sexual reproduction

of Ciliata, 709
Balsam angle, 50, 78

— refractive index of, 77
Banksia, stomates of, 641
Barbadoes earth, 771, 774
Bark, 625, 627, 633

Barker's Gregorian telescope, 144
Bar movement, 215

' Barlow lens ' applied to a microscope,
147

Barnacles, 891. See Cirripedia

Ba'sals of Antedon, 825

Basidiomycetes, 575 ; as fungus-con-
stituent of lichens, 579

Basidiospores of Basidiomycetes, 576 ; of
Hymenomycetes, 576

Basids of Puccinia, 566 ; of Basidiomy-
cetes, 576

Bast, 635

Bat, parasite of, 936 ; hair of, 954 ; carti-
lage in ear of, 970
' Bathybius; 672

Batrachia, red blood-corpuscles, 959 ;

lungs of, 987
Batrachospermea , 503
Batrachospermum moniliforme, 504

— protoneme of, 505

' Battledore scale ' of Lyccenidce, 899

Bausch and Lomb's microscope, 185-188 ;
chemical microscopes, 217-220; ' labo-
ratory ' microscope, 218 ; ' University '
chemical microscope, 219, 220 ; neutral
tint camera lucida, 235

Bdella, maxillary palps of, 934

Bdellidce, 937

Bdelloida, 717, 718

Bead-moulds, 573

Beale's microscope for class demonstra-
tion, 225 ; camera, 234, 235, 239 ;
carmine, 435 ; bioplasm, 435 ; glycerin
method of preserving, 444
Beale on organic structure, 942
Beck's No. 1 microscope, 180, 182 ; sub-
stage in, 181, 182 ; small first-class mi-
croscope, 189 ; third-class microscope,
190-192 ; ' Star ' microscope, 194 ; ' eco-
nomic ' microscope, 194, 196; histological
dissecting microscope, 197, 198 ; port-
able microscope, 199, 202 ; binocular
dissecting microscope, 207; rotatory

BIP

nose-piece, 242 ; variable condenser,."
260; mode of using parabolic speculum,.
281 ; light modifier, 284 ; vertical illu-
minator, 285 ; disc-holder, 288 ; rings-
for locking coarse adjustment, 301 ;
lamp, 348, 349 ; achromatic binocular
magnifier, 396 note ; disc-holder for
examination of Forarninifera , 770
Beck (R.) on markings of Podura scale,*
902

Beck-Jackson model, 162

Bee, hairs of, 904; head of, 906; wing

of, 918, 922 ; sting of, 927
Beeldsnyder's achromatic objective, 145
Beetles. See Coleoptera
Beggiatoa, form of, 581

— alba, 583, 584
Begonia, seeds of, 649

Behrens' method of analysing minerals,.
1004

Bell (Jeffrey) on the spines of Cidarisy
813

Bell's cements, 383, 448

Beneden (Ed. Van), on Gregarina

gigantea, 674 note; on movement of

gregarines, 675
Benzol, uses of, 441
Bergh on Flaqellata, 689
' Bergmehl,' 551
Berkeleya, 528
Bermuda earth, 538, 540
Beroe, collecting, 459

— Forskalii, 805

— ovatus, Eimer on, 806 note
Bicellaria ciliata, 834
Bichromate of potash, 430
Biconvex lens, formulae relating to, 21
Biddulphia, 542

— cyclosis in, 517 ; chains of, 517, 525 ^
structure of frustule, 519 note

Biddulphiece, character of, 541
Biflagellate monad, 684
Bignonia, seed of, 648
Bignoniacece, winged seeds, 648
Biloculina, 727

Binary subdivision of cell, 465, 466
Binocular eye-piece, Tolles', 102 ; Abbe's,.
103

Binocular magnifier, Beck's achromatic^
396 note

Binocular microscope, 61, 97

Riddell's, 96 ; Nachet's, 98 ; stereo-
scopic, Wenham's, 98 ; Stephenson's,
100 ; Stephenson's erecting, 102 ;
stereoscopic, for study of opaque ob-
jects, 105, 107 ; use of, 105 ; non-
stereoscopic, 106 ; Powell & Lealand's
high-power, 107 ; portable, Rousselet's,.
200 ; body in Beck's portable, 200 ;
Stephenson's for dissection, 201, 203.,
344, 395 ; dissecting, Beck's, 207 ;
spectrum microscope, 276

Biology, 460

Bioplasm, 435

'Bipinnaria,' resemblance of Aoiino*

trocha to, 874
Bipinnaria asterigcra, 821

3 y 2

<io6o

INDEX

BIR

Birch, pollen-grains of, 647

Bird, parasite of, 936, 938; lacunae in

bone of, 946 ; epidermic appendages of,

953 ; red blood-corpuscles of, 958, 959 ;

lungs of, 988
Bird's egg, concretions on shell imitated,

1023

* Bird's head processes,' see Avicularia,
834

Bismarck brown, 437

Bivalves, structure of ligament in, 964

' Black dot,' Nelson's, 233

4 Bladderwrack,' 556

Blanchard (R.) on osmic acid, 428 ; on

Sporozoa, 674 note
Blatta, antennae of, 912

— orient alis, eggs of, 929
Blenny, scales of, 951

Blood, colourless corpuscles, 942 ; method
of mounting, 962 ; circulation of, 978 ;
flow of, 979 ; micro-chemical examina-
tion, 1024

<— of insects, circulation of, 917, 918 ;

of Vertebrata, 958
Blood-corpuscle, relation of size to that

of bone lacunae, 946
Blood-corpuscles of Vertebrata, 958
Blowfly's maxillary palpus, hairs on,

examination of, 365
Blowfly, proboscis of, examination with

apochromatic, 318 ; hairs on, as test

for definition, 368

— development of 931 ; ' imaginal discs '
of, 931

Blue-black, for nerve-cells, 437
Blue glass for softening light, 360
' Blue mould,' 571
Boclo, 475

Body of the microscope, 155
' Bog-mosses,' 598
Boletus, 575
Bombyx, 911

— mori, eggs of, 929

Bonanni's microscope, 134 ; his hori-
zontal microscope, 135 ; his compound
condensers, 248, 249

Bone, 943; structure of, 944; prepara-
tion of, 947 ; matrix of, 963 ; decalcifi-
cation of, 426

Bones, fossilised, 1012, 1013

' Bony pike,' scale of, 946

Borax carmine, 435

Bordered pits in the trachei'des of

conifers, 622, 628
Boscovich on chromatic dispersion, 42
Botryllians, 838
Botryllus violaceus, 839
Botryocystis, 475
Botrytis bassiana, 573
Botterill's growing slides, 289 ; his
i zoophyte trough, 298
Bouguet on uniform radiation, 51
Bowerbank on sponge spicules, 783

note\ on structure of molluscan

shells, 845

Bowerbankia, gizzard of, 829 ; stem of,
832; polyzoaries oi, 833

BUL

[ ' Box-mite,' 936 ;
•Brachinus, antennae of, 912
i Brachionus rubens, 714, 715, 718 ; male
of, 717

| Brachiopoda, shells of, 843, 849, 851;

relation of shell to mantle, 850 ;

affinities to Polyzoa, 851
Brachyourous decapods, young of, 894
Brady (H. B.) on Foraminifera, 735 ; on

arenaceous Foraminifera, 736; on

affinity of Carpenteria, 748
Brady and Carpenter on fossil Lituolce

742

Brain, Hill's method of preparation of,
434

' Brake-fern,' 600. See Aspidium
Bran, 649

Branchiae of annelids, 872, 873
Branchiopoda, 883 ; divisions of, 885
Branchipits, movement of, 884

— stagiialis, 886, 887
Branchiura, 889 note, 890

Brandt (K.) on artificial division of
Actinosplicerium, 666 note ; on
zooxanthellae, 773

Braun on Pediastrum, 497

Brewster, his hand magnifier, 37 ; on
modification of stereoscope, 91 ; oh
' lens ' from Sargon's palace, 121 ; his
' Treatise on the Microscope,' 122 ; on
achromatic condensers, 249

Bright-line spectro-micrometer, 274

Brightwell on Triceratium, 543 note ;
on Chcetocerece, 544 note

Brilliancy of image, 326

' Brimstone moth,' eggs of, 929

Brine, shrimp, 884, 887

' Brittle stars,' 815. See Ophiuroidea

Brooke's nose-piece, 241

Brownian movement, experiments, 373-
374

Briicke lens, 38

Brunswick black as a black ' ground,'
384 ; for cells, 386

Bryacece, 598
Bryobia, 937

Bryony, cells of pollen chambers, 645
Bryozoa, 828. See Polyzoa
Br yam intermedium, peristome of, 597
Bubbles in cavities of crystals, 997
Buccinum, 861

— undatum, palate of, 854, 856, 857;
nidamentum of, 858

Buchner's experiment on spores of

Bacteria, 587
Buckthorn, stem of, 628
Bug, mounting medium for, 897
Buyula, polyzoary of, 833

— avicularia, 834, 835
Built-up ' cells,' 389
Bulbils of Nitella, 507

Bulloch's modification of Zentmayer's

microscope, 184, 18(5, 187
Bull's-eye, 250 ; use of, 278, 279, 351, 857 ;

with high power, 280 ; for use in study

of saprophytic organisms, 280 ; Powell

and Lealand's, 280

INDEX

io6i-

BUL

Bull's-eye stand, 201 i

Bundle- sheath, 635

Burdock, stem of, 634

Biitschli on Bhizopoda and Sporozoa,

677; on mouth of Astasia, 690; on

Voxticellce, 701 note
— and Engelmann on conjugating vorti-

cellids, 711
Butterflies, wing of, 918
Butterfly. See Lepidoptera

C

Cabbage-butterfly, eye of, 907 ; number of

facets in, 907 ; eggs of, 929
Caberea Boryi, vibracula of, 831 note
Cabinet for slides, 454 ; arrangement of,

454

Cactus, cells of pollen- chambers, 645
Cactus senilis, raphides of, 621 ; brittle-

ness of, 621
Cacumaria crocea, development of, 824

note
Catamites, 1005
Calatkus, antenna? of, 912
Calcarina, 750, 755 ; compared with

Eozoon, 763
Calcispongice, spicules of, 783
Calcite in shells, 848
Calco-globuline, 1022
Callithamnion, 559

Calosanthes indica, winged seed of, 649
Calotte diaphragms, 247
Calycanthus, bark of, 634
Calycine monad, 685
Calycles of hydroids, 792
Calypter of mosses, 596
Calyx of Flagellata, 089
Cambium, 635

— in Exogens, 622

— layer, 633

Cambridge rocking microtome, 408
Camera lucida, 233 ; Beale's, 234, 235,
239 ; Soemmering's, 234 ; Wollaston's,

234 ; Amici's, 235 ; Bausch and Lomb's,

235 ; Schroder's, 236 ; Abbe's, 237
Campani's microscope, 130 ; eye-piece,

321

Campanula, pollen-grain of, 646

Campanularia, 794

— gelatinosa, 789

Campanulariida, 794 ; zoophytic stage j
of, 801

Campbell's differential screw, 158, 193 ;

adapted to the Continental model, 164 ;

fine adjustment, 164, 193 ; used in

photo-micrography, 194
Campylodiscus, 518, 524, 536 ; move-
■ ments of, 531 ; structure of frustule,

536

— clypeus, 536

— spiralis, cyclosis in, 517
Canada balsam, 383

as a preservative medium, 441 ; mode

of preparation, 441 , as mounting
medium, 444, 449 ; refractive index,

CAT

445 ; capped jars for, 447 ; for mount-
ing insects, 897
Canal system of Calcarina, 750; of
Polystomella, 752; of Nummulites
752

Canaliculi of bone, 943, 945

Cancellated structure of bone, 944

Cancer pagurus, skeleton of, 892

Canna, starch-grains of, 620

Cannel coals, 1006

Cannocchiale, 127

Capacity of object-glass, 326

Capillaries, 980, 986

Capillitium of Myxomycetes, 565

Capsule, central, of Badiolaria, 772

— of mosses, 595 ; of Purpura, 858

— silicious, of Clathrulina, 666
Carapace of Copepoda, 884; of Clado'

cera, 885

Carbolic acid : for mounting prepara-
tions, 442 ; for dehydration, 450

Carbon bisulphide as a solvent for oils,,
&c, 441

Carboniferous epoch, vegetation of, 606

— limestone, 1011
Carchesium, collecting, 457
Carcinus mcenas, metamorphosis of, 894
Carnallite, 998

Carnation, parenchyme of, 613
Carnivora, arrangement of enamel in,
949

Carp, scales of, 951

Carpenter (H. P.) on crinoids, 827 note
Carpenter (W. B.) on stereoscopic vision,
93 ; on classification of Foraminiferay
724 ; on Eozoon, 763 ; on alternation of
generations in Medusce, 801 ; on the so-
called excretory pores of Ctenophoray
806 note ; on development of Antedony
827 note ; on structure of molluscan
shells, 845

Carpenteria, 747 ; mode of growth com-
pared with Eozoon, 763

— rhaphidodendron, 748

Carpogone of Floridece, 561 ; of Ascomy-

cetes, 572
Carpospores of Floridece, 561
Carrot, seeds of, 649

Carter (H. J.) on affinity of Carpenteriat
748

Cartilage, 970 ; mounting, 971
Carum carui, seeds of, 649
Caryophyllia, lamella? of, 802

— Smithii, thread-cell of, 803

Cascarilla, raphides of, 621

Cassowary, egg-shell of, 1021

Castracane, on beaded structure of di-
atoms, 522; on Pfitzer's auxospores,
523, 524 ; on sporangial frustules of
diatoms, 524 ; on reproduction of dia*
toms, 526 ; on diatoms, 528

Cat, Pacinian corpuscles of, 977
Catadioptric illuminator, Stephenson's,

170, 263-265
Caterpillars, ' pro-legs ' of, 926 ; feet of,

926

Cathcart's freezing microtome, 412, 413 a

i662

INDEX

CAT

•Catoptric form of microscope, 144
Cattle-plague, 588
'Caulerpa, 493

Cauterisation by focussing the sun's

rays (Pliny), 119
Cedar, stem of, 630

Cell, contents of, 463-465; binary sub-
division of, 465
4 Cell ' of Polyzoa, 828
Cell-division and nucleus, 943 and note
Cell-sap, 464

Cell-structure, Strasburger on, 467
Cell-wall, 463 ; mode of growth of, 463 ;

apposition, 463 ; intussusception, 463
- — of Phanerogams, 617
Cells of plants, 462 ; multi-nucleated, 464 ;

primordial, 465 ; of vertebrates, 942
Celloidin as an imbedding mass, 417 ; as

congelation mass, 419 ; for freezing ,
' Hill's method, 419 ; clearing agents
* Cells ' for mounting Infusoria, &c, 299 ;
• for dry mounting, 385 ; sunk, 388 ; of

cement, 382 ; paraffin, 386 ; paper, 386 ;

of plate glass, for zoophytes, &c, 388 ;

built up, 389 ; mounting in, 451, 452 ;

of bone, 452 ; of tin, 452 .
; for, 419 ; Schering's, 419
Cellular cartilage, 970

— parenchyme, 613
Cellulose, 463

— tests for, 440 ; envelope of desmids,
509 ; in Dinoflagellata, 695 ; in zob-
cytium of Ophrydium, 706

Cement-cells, 386

Cements, 382 ; liquid, 382 ; Bell's, 383,
448; japanner's gold size, 383; Bruns-
wick black, 384 ; glue and honey, 384 ;
shellac, 384 ; Hollis's liquid glue, 384,
449 ; Venice turpentine, 384 ; marine
glue, 385 ; Heller's porcelain, 445

Cementum of teeth, 949, 950

Centipedes. See Myriopoda

Central capsule of Badiolaria, 659

Centring, 326

Centring nose-piece, 242 ; as sub-stage,
193

Centro-dorsal plate of Antedon, 825
Cephalolithis sylvina, 771
Cephalophorous mollusca, palates of,

854-857
Cephalopoda, 853

— organs of hearing in, 865 ; chromato-
phores of, 866

Ceramiacece, 559
Ceramium, 559
<Ceratium, 695
: — furca, 696

— tripos, 696
Ceratodus, 1013

Cercomonas typica, compared with Bac-
teria, 580
Cereals, seeds of, starch in, 619
Certes on osmic acid, 428
Centra vinula, eggs of, 929
Cestoid, 867

Cetonia, antennae of, 912
Chcctocerece, affinities of, 543

CHO

Chcetocerece, 1 awns ' of, 543

— occurrence in marine animals, 544
Chcetoceros Wighamii, 543
Chcetophoracece, 503, 589

' Chaff-scales,' silex in, 640

Chalk, metamorphism of, 1000 ; micro-
scopic constituents of, 1007, 1009 ; re-
semblance to Globigerina ooze, 1009 ;
mode of preparation for examination,
1010

Chama, prismatic layer in, 848
Chamberlets in Foraminifera, 723, 728,

729 ; of Parheria, 742 ; in Fusulina,

750 ; of Cycloclypeus, 760
Cha?nidce, Foraminifera attached to

shells of, 770
Changes of form of white corpuscles, 962
Chantransia and Batrachospermum,

504, 505

Chara, 505 ; antherozoids of, 593
Characece, 505-509

Charbon, due to Bacillus anthracis, 582

Charles's achromatic lenses, 146

Cheese-mite, 932, 937

Cheilostomata, characters of, 833 ; ex-
amples of, 834

Cheirocej)halus, 886, 887

Chemical tests for biological work, 439,
440

Cherry-stone, section of, 617

Cherubin d'Orleans/his binocular micro-
scope, 132, 133 ; his compound micro-
scope, 132

Chevalier on Charles's achromatic
lenses, 146

Chevalier's combination of lenses, 38 ;
achromatic microscope, 146, 148 ; mo-
difications of Selligue's lenses, 303 ;
objectives, Lister's note on, 304

Cheyleti, 937

Cheyletidce, tracheae of, 935

Cheyletus, hairs of, 934 ; legs of, 934 ;

mouth parts of, 934
Chickweed, petals of, 644
Chicory, adulteration of, 650 note
Chilodon, mouth of, 702

— cucullus, binary division of, 704, 706
Chiloynatha, 905

Chirodota violacea, 1 wheels ' of, 820
Chitin, in test of Arcella, 670; of insects'
skin, 898

Chitinous substances, mounting, 450
Chiton, shell structure, 852 ; eyes on

shells of, 865
Chlamydomonas, 473 note, 475
Chlamydomyxa, affinity with Monerozoa,

652

Chlamydospores, of Mucorini, 569 ; of
gregarines, 675

Chloral hydrate as a preservative me-
dium, 442

Chloride of gold, for histology, 437

Chloroform, uses of, 441

Chlorophyll corpuscles, 464, 465

Chlorosporece, 554

Chordata, 835. See Vertebrata

Choroid coat of eye, pigment-cells in, 966

INDEX

CHR

'Chromatic, comparison of, with achro-
matic and apochromatic lenses, 315

— aberration, 16, 17, 31

— condenser, Abbe's, 212, 256, 267, 329 ;
Powell and Lealand's oil, 258

— correction, test for, 331

dispersion, diminished by Huyghens'

objective, 42
Chromatophores of Peridinium, 695 ; of

Cephalopods, 866
•Chromatoplasm, 467
-Chromic acid as hardening agent, 428
Chroococcacece, characters of, 477
Chroococcus, 477; as gonid of lichen, 579
■Chroolepus, as gonid of lichen, 579
CJi rysaora, 798, 800 ; development of,

800

' Chyle, corpuscles in, 961
<Chytridiacea>, 565

— zoospores of, 555
Cicada?, wings of, 922, 923

C ichor iacece, pollen-grains of, 646
/Cicindela, 911
( 'idaris, spine of, 809, 812

— uietularia, mode of formation of spines
in, 813

Cienkowski on decaying cells of Nitella,
509 note ; on parasitic plasmode in
Xitella, 509 note ; on reproduction of
Noctiluca, 694

Cilia, 462, 968 ; of Infusoria, 699 ; use of,
in Ciliata, 701 ; of Turbellaria, 870

Ciliary action, 699

— motion on gills of Mollnsca, 864

— movement in protophytes, 465

i Ciliata, 699-712 ; ciliarv action of, 699,

701 ; ' shield ' of, 700 ; lorica of, 700 ;
myophan-layer, 701 ; trichocysts of,
701 ; ento-parasitic forms, 702 ; mouth

■ of, 702 ; foot-stalk in, 702 ; impression-
able organs of, 702 ; 1 eye-spots ' of,

702 ; food of, 703 ; artificial feeding,

703 ; contractile vesicles of, 704 ; mul-
tiplication of, 704 ; colonial forms, 705 ;
encystment of, 707-709 ; supposed
sexual reproduction, 705, 709-711 ; dis-
persion of, 709 ; desiccation of, 709 ;
conjugation of, 711 ; Stein on acineti-
form young of, 712 note

'•Ciliate Infusoria, general structure of,
679

Ciliated epithelium, 968
Ciliobrachiate zoophytes, 829

• Cilio-fiaqellata, 695
Ciliuni of Noctiluca, 691 note
Cimex lectularius, eggs of, 929

< Cinchona, raphides of, 621
Cinclidium arcticum, peristome of, 597
Cineraria, pollen-grains of, 647
Cineritious matter, 976
Circulation in ascidians, 836, 839

— of blood, 978

Circumambient chamber in Orbitolites,
731

Cirrhi of Cirripedia, 892
Cirripedia, 891
•Cladocera, 885

COH

Cladococcus viminalis, 774, 776

Cladonia furcata, 579

Cladophora glomerata, 499; cell division

of, 499, 504
Cladorhiza inversa, 784
Claparede and Lachmann on Lieber-

kuehnia, 656; on 'rolling' movement

of Amceba, 669
Clark (James) on Flagellata, 689
Clastic rocks, 998
Clathrulina elegans, 666
Clausius on emission of light, 54
Clavelinidce, gemmation of, 836; stolons

of, 838
Claviceps purpurea, 572
Clavicornes, antennae of, 911
Claws, 953, 957
Clay, 1014

Cleanliness, importance of, 453

Clematis, stem of, 627

' Closed ' bundles, 635

Closterium, cyclosis in, 510 ; 'swarming
of granules ' in, 511 ; binary division
in, 511 ; two zygospores in, 513 note ;
zygospore of, 514 ; form of cell, 515

Clostridia, form of, 581

' Clothes-moth,' 923

Clove-pink, seed of, 648

' Club-mosses,' 606

Clypeaster, spines of, 813

Coagulation, imbedding by, 418

Coal-plants, 1005

Coarse adjustment, 1 stepped ' diagonal
rackwork for, 157 ; of Ross model, 177 ;
Wale's, 185; arrangements for 'lock-
ing,' 301

Cobcea, testa of seeds of, 649

— scandens, pollen-grains of, 646
Coccidia, 676, 677
Coccidium oviform e, 676
Coccoliths, 672-674 ; in chalk, 1010
Cocconeidea?, characters of, 544
Cocconeis, 544

Cocconema, 528, 545, 551

— fusidiiun, 551

Coccospheres, 672, 674 ; in chalk, 1010
Cockchafer, antennas of, 898. See Melo-

lontha
' Cockle ' in wheat, 869
Cockroach. See Blatta
Cocoa-nut, 649

— shell of, 618
Cocos-wood, 629
Coddington lens, 37
C odium, 493

Codosiga umbeVata, fission of, 689 ;

arborescent colonies of, 690
Coclenterata, 786-807 ; bibliography of,

806 ; permanent gastrula-stage of, 652

— See Zoophytes
C&loplana, 806

Ccenosarc, of hydroids, 791, 794
Cosnurus, 868

Colin, on sexual generation of Volvox,
483 ; on movements in Oscillaria, 490 ;
on reproduction of Splueroplea, 500,.
501 ; on affinities of Bacteria, 519

1064

INDEX

COL

Cole (Arthur) on use of absolute alcohol,
428 ; on use of borax carmine for vege-
table substances, 436

Coleoptera, 897 ; dermo- skeleton of, 898;
scales of, 899 ; elytra of, 905 ; eyes of,
911; antennas of, 911, 912; mouth-
parts of, 913; wings of, 923; leg of,
924

Coleps, food of, 703
Collar correction, 307
Collared cells of sponges, 780
' Collars ' of Flagellata, 689
' Collateral ' bundles, 635
Collection of microscopic objects, appara-
tus for, 456
Collembola, 901
Colletonema, 528

Collins's aquarium microscope, 221-223;

condenser with rotating sub-stage, 329
Collomia, testa of seeds of, 649

— grandiflora, spiral fibres in seeds of,
818

Collozoa, 777

Colonial Acinetina, 698

Colonies, in Codosiga, 689 ; of Radiola-

rians, 773 ; of Polyzoa, 828
Columel of Sphagnacece, 599
Comatula, 824, 825; nerves of, 976
' Comb-bearers,' 805. See Ctenophora
Commensalism, in lichens, 598
Compensating eye-pieces, 34, 229, 323
Compositce, laticiferous tissue of, 623
Compound condenser, sub-stage, 135

— microscope, construction of, 39 ; inven-
tion of, Govi on, 122

Compression of light rays, 57
Compressor, 295 ; Rowland's reversible,

295 ; Powell and Lealand's, 296 ; De-

lage's, 29(5, 297
Compressorium, 295
1 Concentric' bundles, 635
Conceptacles of Fucacecc, 556 ; of Mar-

chantia, 591
Conchifera, shell of, 843
Concretionary spheroids, 1021
Condensers, 170, 248-263

— Kellner eye-piece used as, 177 ;
Gillett's, 184, 250 ; Hartsoeker's, '248 ;
Bonanni's compound, 248, 24'.); Swift's
low-power, 252; Webster's, 2.">f> ; Abbe's,
256-259; Beck's variable, 260 ; Powell
and Lealand's 262 ; Swift's, for use with
polariscope, 262 ; Collins's, with rotat-
ing sub-stage, 329

— total aperture of, 255

— tabular list of, 268

— achromatic, 198 ; Abbe's, 212 ; Powell
and Lealand's, 251-254, 203, 267 ; Brew-
ster on, 249

— chromatic, Abbe's, 212, 329

— sub-stage, Stephenson's, 101 ; com-
pound, 135

Cone of light, 170
Conferva, 486

Confervacea?, 478, 498-500 ; binary divi-
sion of, 499 ; zoospores of, 500 ; resem-
blance of Melosirece to, 537

COR

Conferva?, 869, 884

Congelation mass : gum arabic, 418 ; cel-
loidin, 419

Conical epithelium, 968

Conids, of Ascomycetes, 571 ; of Basidio-
mycetes, 576

'Coniferce, 609 ; woody cells of, 622

Coniferous wood fossilised, 630, 1005

Conjugate, affinities of, 477

Conjugate foci, 13 ; focus, 24 ; image, 24

Conjugating cells, 470

Conjugation, a sexual act, 467

Conjugation of Mesocarpus, 478; of
Spirogyra, 478 ; of Ulothrix, 486 ; of
Hydrodictyon, 495 ; of Desmidiacece,
513 ; of diatoms, 528 ; of Phceosporea?>
556; of Myxomycetes, 564 ; of Arcella,
671; (zygosis) of Gregarince, 676; of
Heteromita, 685; of Tetramitus, 686;
of Noctiluca, 694; of Glenodinium,
695; of Podophrya, 699; of Ciliata,
711 ; of Vorticella, 711

Connective tissue, 943 ; fibrous, 962 ;
areolar, 964 ; corpuscles of, 963, 964

Contact metamorphism, 1000

Continental correctional collar, 307

— microscopes, objections to, 158

— model, 208-213 ; criticism 011, 209
Contiguity of protoplasm in Florideeet

560

Contractile vacuole in Volvox, 481

— vesicle, of Actinophrys, 662; of
Microgromza, 661; of Amoeba, 668;
of Infusoria, function of, 679 ; in
Flagellata, 689 ; of Paramecium, 704 ;
of Stentor, 704; of Ciliata, 704; of
Botifcra, 716

Convergence of light, 18

Conversion of relief in spectroscope, 92 ;

shown by Araclinoidiscus, 541 note
Convolvidacecc, laticiferous tissue of, 620
Convolvulus, pollen-grains, (Mi;
Copepoda, 884; elassilication of, 889 note
( 'opens ccrbcrus, 718

Copper sulphate, crystallisation of, 1017
Coquilla-nut, 64'.)

— section of, (517

Coralline crag, microscopic constituents

of, 1011
Corallines, 884

— conceptacles of, 561 ; ostiole of, 561

— (sertularids), 794

Corals, section of hard and soft parts,
423

— red, 801 ; stony, 802 ; mushroom, 802
Cordierite ; pleochroism in, 1002
Corella parallelogra/mma, branchial sac

of, 830

Coreopsis tiuctoria, seeds of, 649
Cork, 633

Corky layer of bark, 633
Cormophytic type, 594
Cormorant, parasite of, 934
Corneules of arthropod eyes, 907
Corn-grains, husk of, 649
Comuspira , 726, 728
' Corpuscle ' of gynmospenns, (510-

INDEX

COR

Corpuscles, white, 901 ; change of form

of, 962 ; of connective tissue, 963, 965 ;

of blood, flow of, 980
Corrected lenses, 320
Correction collar, 21, 29, 50, 230
Correctional collar, Continental, 307 ; I

English method, 308
Corroded crystals, 995
Corrosive sublimate, as a preservative

medium, 443
Corynactis Allmanni, thread-cell of, 803
Coscinodiscece, characters of, 537
Coscinodiscas, 518, 550

— cyclosis in, 517 ; markings on frustule
of, 520 ; areolae of, 520 ; frustules of,
537, 538

— aster omphalus, for testing lenses, 333

— oculus iridis, 538

— punctatus, fossil, with embryonal
form, 527

Cosmarium, division of, 512; form of
cell, 515

— botrytis, zygospore of, 514
Cosmic dust, 1015

Costae of Cavipylodiscus, 536
Costonella, silicious shell of, 700
Cotyledons, 610
Cover-glass, 380

— consequence of using, 19 ; as section
lifter, 432

— tester, Zeiss's, 381 ; Ross's, 381, 382

— varying thicknesses of, 380 ; with
achromatic objectives, 380 ; cleaning
them. 382

Cox (J. D.) on structure of frustule in
Isthmia, 519 note

Crab, 881 ; metamorphosis, 893 ; blood-
corpuscles of, 962 ; ' liver ' of, 971

■Crabro, leg of, 898

Crane-fly. See Tipula

Crateriam pyriforme, 933

Crayfish, 881 ; young of, 893

Creation of structure by diaphragms, 68

Cribrilina figularis, 830

Cricket, gizzard of, 917 ; wings of, 923 ;
sound-producing apparatus, 923. See
Acheta

Crinoidea, skeleton of, 816; larva of,

822
Crista, 833

Crisp (F.) on ' aperture,' 45 ; on radia-
tion, 75 ; on collection of microscopes,
119

Cristatella, 833

Cristellaria, shell of, 723, 744

Critical angle, 6, 7 ; image, 30, 249 ;

images, 238 ; mode of obtaining, 352,

353

Crocus, pollen-grains of, 647
Crouch's adapter for parabolic speculum,
281

' Crow silk,' 499

Crown glass, refractive index of, 5 ;

composition of, 32
Crusta petrosa of teeth, 949, 950
Crustacea, 881-895

larvae of, collecting, 459

CYC

Crustacea, suctorial, 889

— collecting, 895 ; preserving, 895 ; com-
pound eyes of, 906 ; pigment-cells of,.
967 ; ' liver ' of, 971 ; concretionary
spheroids in shells of, 1021

Cryptogamia, 462-609

— preparation of, 427 ; structure of, 462-
465 ; reproduction of, 465-479 ; litera-
ture, 608 ; passage to Phanerogamia,
609

Cryptorapliidece , 527

Crystalline forms, list of, for microscope,.
1019

Crystallisation, microscopic examination
of, 1016, 1017

— effect of temperature on, 1017

— preservation of specimens of, 1020
Crystallites, 995

— in glass cavities, 997

Crystals, corroded, 995 ; in lava, 995 ;
zonal markings* in, 996 ; cavities in,
997 ; inclusions in, 997, 998 ; micro-
scopical structure of, 990 ; optical pro-
perties and chemical constitution,
1002; as microscopic objects, 1016;
of snow, 1016 ; as objects for polari-
scope, 1017

Ctenaria ctenophora, 801 note

Ctenoid scales, 951

Ctenophora, 801, 805, 806 ; excretory

pores of, 806 note
Ctenostomata, characters of, 833
Cucurbitacece, pollen-grains of, 646
Cuff's micrometer, 140 ; microscope, 140
Cidicidce, antenna? of, 912; larvae, blood

of, 918

Curculio, antennae of, 912

— imperialis, scales of, 899 ; elytra of, 905
Curculionidce, 905; foot of, 924 ; suckers.

on foot of, 926

Currant, parenchyme of fruit, 613 ; pollen-
tubes of, 648

Curvature of the field, 332

Curved scissors, for section cutting, 397

' Cushion-star, 815.' See Goniaster

Cuticle, 965, 966

— of leaves, 638 ; of Ciliata, 700
Cutin, 638

Cutis vera, 965

Cutleria, conjugation of, 556

Cuttle-fish, 853, 866. See Sepia

— ' sepiostaire ' of, structure, 853 ; irmV
tated, 1023

' Cuttle-fish bone,' structure of, 853

Cyancea capillata, ephyrse of, 799 ; scy-
phistoma of, 799 ; strobila, 799

Cyanthus minor, seed of, 649

Cyatholiths, 672-674 ; artificially pro-
duced, 1022

Cycadece, 609

Cycas, raphides of, 621

Cyclanimina cancellata, 741, 743

Cyclical mode of growth in shell of"
Foraminifera, 723

Cycloclypeus, 754 ; shell of, 723

— compared with Orbitolites, 726, 760
Cycloid seals, 952

io66

INDEX

CYC

'Cyclops, eye of, 884 ; larva, 892
• — quadricornis, 885 ; number of off-
spring of, 888
'Cyclosis, 464; in ' Char a, 506; in des-
mids, 510 ; in Diatomacece, 517 ; in
Phanerogam cells, 613 ; in plant hairs,
615 ; in Lieberkuehnia, 657 ; in Acine-
tina, 697

•Cyclostomata (Poly zoo), characters of,
'833

Cydippe, collecting, 459
— — pileus, 805
Cymbella, 528

■CymbellecB, affinities of, 545
Cynipidce, ovipositor of, 927
Cyprcea, shell of, 852
Cypris, 884

Cyst, of Protomyxa, 653 ; of gregarines,
675 ; of DalUngeria, 684 ; of Poly-
toma, 685 ; of Clathrulina, 667 ; of
Protoccccus, 480
Cystic Entozoa, relation to cestoids, 868
Cysticercus, relation to cestoids, 868
Cystids of Hymenomycetes, 576
•Cystocarp of Floridece, 561 ; of Batra-

chospermum, 504
Cystopus candidus, 568
Cythere, 884, 885

Cytherina, shells of, in chalk, 1009
•Cytodes, contrasted with plastid, 652
Cytoplasm, 467

D

Dallinger and Drysdale's moist stage,
289 ; tripod, 345 ; on life-history of
monads, 681-688 ; on effects of tempe-
rature on monads, 686
Dallinger (W. H.) on Navicula, &c, as
test objects, 530 note ; on nucleus of
monads, 687
Dallinger's thermo- static stage, 292-293
DalUngeria Drysdali, life-history and

structure of, 683 ; nucleus of, 687
Dalyell (J. G.j on Hydra tuba, 798
Damceus geniculatits, proven triculus of,
935

Dammar, as a preservative medium, 441 ;

as a mounting medium, 444 ; refractive

index of, 445
Dandelion, laticiferous tissue of, 620 ;

pollen-grains of, 647
Daphnia, eye of, 884 ; mouth of, 888,

889 ; eggs of, 888 ; ephippial eggs of,

888

Daphnia pulex, 886
Darwin (Charles) on Cirripedia, 891
Datura, seeds of, 649
Davis on desiccation of Potifera, 718
note

Dawson (W.) on foraminiferal nature of

Eozoon, 763
' Day-fly.' See Ephemera
' Dead-man's toes,' 803. See Alcyonium
Deane's medium for mounting insects,

897

DEV

De Bary on fungi, &c, 563 note', on
potato-disease, 569 ; on alternation of
generations in ferns, 605

Decalcification, 425 ; of echinoderms,
426 ; of bones, 426 ; of teeth, 426 ; of
Foraminifera, 426 ; of Eozoon, 426

Decapoda, 881 ; exoskeleton of, 892 ;
macrourous, 893 ; brachyourous, 894

Decomposition, produced by Bacteria,
588

— of rock-masses, 999
Defining power, 368 ; tests for, 368
Definition of image, 326
Degeneration in Tunicata, 835
Dehydration by carbolic acid, 450
Delage's parallel compressor, 296, 297
Dellebarre's microscope, 142
Delphinium, seeds of, 649
Demodex, legs of, 934

— folliculorum, 938

De Monconys, his compound microscope,
130

Dendritina, a varietal form of Peneroplis,
728

Dendrodus, teeth of, 1013
Dendrosoma, 698
Dentine, 944, 947

— resemblance of cuticle of crabs to,
893 ; in placoid scales, 952

Deparia, indusium of, 600

— prolifera, 601

Depth of focus, 83, 89 ; of vision, 88, 89,

90 ; perception of," 94,
Dermal skeleton of Vertebrata, 950
Dermaleichi, 932, 938
Dermanyssus, 936

— larva of, 933
Dermestes, hair of larva, 904
Descartes' simple microscope with reflec-

* tion, 128
Desiccation of rotifers, 718
Desiderata in a microscope, 215
Desilicification, 427

DesmidiacejE, 477, 509-516 ; connection
with Pediastrece, 496 ; sutural line of,
509 ; cellulose envelope, 509 ; mucila-
ginous sheath, 510 ; primordial utricle,
510 ; endochrome, 510 ; movements of,
510 ; cyclosis in, 510 ; binary division of,
511 ; sexual reproduction, 513 ; classi-
fication of, 515 ; habitat of, 515, 516 ;
mode of collecting, 516

— Hantzsch's glycerin method of pre-
serving, 444

Desmidiece, 869

— conjugation of, 514 ; zygospore of, 514

— preserved by osmic acid, 428
Desmidium, binary division, 511 ; fila-
ments of, 512, 515

Desmids. See Desmidiacece

Deutovium of Acarina, 932

Deutzia scabra, stellate hairs of, 639;

epiderm of, 640
Development of Hydra, 791 ; of hydroids,

792 ; of embryo in Gastropoda, 843 ;

of molluscs, 857, 864 ; of Annelida,

873 ; of Tomopteris, 878 ; of insects, 931

INDEX

IO67

DEV

Deviation, 9
"Diamond Beetle,' 899
Dianthus, seed of, 648

— caryophyllceus, parenchyme of, 613
Diaphragm, 213, 255, 288, 321

— with two openings for double illumina-
tion, 106

— use of, 261

— Zeiss's iris, 246, 248 ; calotte, 247 ;
in eye-pieces, 325 ; for use in testing
object-glasses, 329

— «- in Tully's microscope, 147
Diatoma, 517 ; frustules of, 517, 518 ;
girdle of, 518

— vulgare, chains of, 534
Diatomaceje, 477, 509, 516-554

— Mb'ller's type-slide, 286 ; perforated
membrane of, examined with annular
illumination, 362 ; mode of examina-
tion of, 363 ; preserved by osmic acid,
428 ; silicious coat, refractive index of,
445 ; mounting, 450 ; stipes of, 517,
•518 ; beaded appearance, 521, 522 ;
markings of, 522 ; binary division of,
523, 524 ; reproduction of, 523-527 ;
•classification of, 527, 532 ; placochro-
anatic, 527 ; coccochromatic, 527 ;
movements of, 528 ; conjugation of,
528 ; zygospores of, 528 ; gonids of,
529 ; habits of, 548, 549 ; habitats of,
549 ; distribution of, 549 ; fossil forms
of, 550, 551 ; used as food, 551 ; collect-
ing, 55] ; cleaning, 552, 553 note ;
mounting, 553 ; as food of Ciliata, 703 ;
in mud of Levant, 1007

Diatom-frustules in ooze, 1008
Diatomin, 517

Diatoms in stomach of .ascidians,

Holothurice, etc., 544, 552
Diatoms. See Diato\iace;e
Dichroism, 1019
Dickiea, 528

Dicotyledonous stems, fossilised, 1005
Dicotyledons, 625 ; stem of medullary

rays of, 627 ; epiderm of, 637
-Dictyocalyx ptumiceus, 785
Dictyochya fibula, 550
Dictyocysta, silicious shell of, 700
Dictyoloma peruviana, winged seed, 649
JDictyospyris clathrus, 771
.Dictyota, oospheres of, 556
Didemnians, 838

Didymium serpula, plasmode of, 564
Differential screw, Campbell's fine adjust-
ment, 164, 188-193
Differential staining, 439
Differentiation of cell, 463
Difflugia, 670; test of, 671
Diffraction, 62

— Abbe's theory of, and homogeneous
immersion, 312

— Fraunhofer's law, 57

— rays are image-forming, 59

— spectra, 28, 67 ; phenomena, 62, 64 ;
image, 64, 72; experiments, 66-70; fan
of isolated corpuscles, 72 ; problem, 73 ;
pencil, 74, 75 ; hypothesis of Abbe, 74 ;

DRE

fan, 75 ; theory, application of, 76, 78 ;
bands, 233 ; phenomena, Abbe's experi-
ments, 376 ; ghost, 377

Digestive vesicles of Ciliata, 703

Digitalis, seeds of, 649

Dimorphism in Foraminifera, 121

Dinobryon, 690

Dinofiagellata, 695

Dinomastigophora, 695 note

Dioptric investigations by Gauss, 108-112

Dioptrical image, 30, 72

Diorite, fluid inclusions in, 997

Dipping tubes, 299

Diptera, 897 ; eyes of, 911 ; antennae of,
912 ; mouth-parts of, 915 ; wings of,
922 ; ovipositor of, 927 ; imaginal discs
of, 931

Direct division of nucleus, 468
; ' Directive vesicles ' of egg of Purpura,
861

Disc-holder, Beck's, 288
Discuki, 111

Discoliths, 672-674 ; artificiallv produced,

1022
Discorbina, 749

— globularis, 723
Disintegration of rock-masses, 999
Dispersion, 9, 17 ; in glass, 31

— and desiccation of encysted Ciliatar
709

Dispersive power, 2, 9, 18 ; of flint glass,10
Dissecting apparatus, 394

— microscope, Beck's histological, 197,
198 ; Stephenson's binocular, 201, 203,
395; Huxley's, 204, 205; Zeiss's, 205,
206 ; Beck's binocular, 207

Distance of projection of image, 26, 27
Distinct vision, 26
Dtstoma, life-history of, 870

— hepaticum, 869
Divergence of light, 18

Divini's compound microscope, 131
Division, binarv, of cells, 465 ; of desmids
511

— artificial, of Actinosphcerium, 666 note

— of naiads, 880
Dobie's line, 973
Dog-fish, scales of, 252

D'Orbigny on plan of growth of Fora-
minifera, 724
i Doris, spicules in mantle, 852, 853 ; nida-
mentum of, 858; eggs of, 866; spines
of, imitated, 1022

— bilamellata, development of, 859-861

— pilosa, palate of, 855

— tuberculata, palate of, 855

Double illumination, Stephenson's me-
thod, 106
Doublet, Wollaston's, 36, 151
Dragmata, of sponges, 784
Dragon-flies, wings of, 922
Dragon-fly, facets in eyes of, 907

— See Libellula
Drajmrnaldia glomerata, 503
Draw-tube of microscope, 155
Drebbel's modification of Keplerian

telescope, 123

INDEX

DRE

Dredge, 458

Drepanidium ranarum, 677

Drone-fly. See Eristalis

Dropping-bottle, 446 ; German, 447 ; ex-
pansion, 447

Drosera, glands of, 639 ; seeds of, 649

Dry -mounting, Smith's ' cells ' for, 385

Ducts of Phanerogams, 623

Dudresnaya, fertilisation in, 561 ; fertilis-
ing tubes, 561

Dujardin, on ' sarcode,' 460 note

— separates Amoeba from Infusoria, 658
Dunning's zoophyte trough, 298
Duramen, 629

Dwarf-male of CEdogonium, 503
Dytiscus, eye of, 911; antennae of, 912;

spiracle of, 920 ; trachea of, 920 ; foot

of, 925, 926

E

Earth-stresses, 1000
Earwig. See Forficula
Eccremocarpus scabe?; winged seeds of,
648, 649

Echinoderm larvae, collecting, 824 ;
preparing, 824 ; mounting, 824

— skeletons in mud of Levant, 1007
Echinodermata, larvae of, collecting,

459

— 808-827 ; skeleton of, 808, 815, 816,
818; spines of, 809-813, 815; pedi-
cellariae of, 813 ; teeth in, 814, 816 ;
preparation of skeleton, spines, ttc,
816 ; internal skeleton, 818 ; larvae of,
820

Echinoderms, decalcification of, 426
Echinoidea, skeleton of, 808 ; spines of,
809, 813 ; pedicellariae of, 813 ; larva
of, 822 ; direct development in, 824

note

Echinometra, spine of, 810, 816 ; colour

of spines, 812
Echinus, shell of, 809, 810; spines of,

809 ; teeth of, 813

— lividus, coloured spines of, 811
Ectocarpacece, 555

Ectocarpus siliculosus, conjugation of,
556

Ectoderm, 651
Ectoplasm, 463
Ectoprocta, 833

Ectosarc, 464; in Bhizopoda, 658;

experiments on 668 ; of Ciliata,, 699
Edentata, cement in teeth of, 950
Edible crab, metamorphosis of, 894
Edmunds' immersion paraboloid, 269
Edwards (A. M.) on supposed ' swarm-
spores ' of Amoeba, 669
Eel, scales of, 951

' Egg without shell,' concretionary sphe-
roids in, 1021

Egg-capsule of Cyclops, 885

Egg-sacs of Lerncea, 890

Egg-shell membrane, 962

JEggs of Sepiola, Doris, 866 ; of Acarina,
928, 929 ; of insects, 928

ENT

Ehrenberg on eye-spot in ProtococcusT
473 ; on Volvox, 479 ; on structure of
frustules, 519 ; on rapidity of repro-
duction of Paramecium, 704; on
internal casts of Foraminifera, 752
note ; on fossil Badiolaria, 778 note

Elceagnus, raphides in pith of, 621 ;
peltate scales of, 639

Elastic ligament of bivalves, structure of,.
964

Elater, antennae of, 912

Elaters of Marchantia, 593; of Egui-

setacece, 605
Elatine, seeds of, 649
Elder, pith of, 611
Ellis's aquatic microscope, 145
Elm, raphides of, 621
Elodea canadensis, cyclosis in, 613
Elytra of Coleoptera, 905, 923
Embryo of Phanerogams, 648

— cell of fern, development of, 604
Embryo-sac, 610

— of ovule in Phanerogams, 464 ; free-
cell formation in, 466

— Strasburger's method for study of, 435'
Emission of light, power of, 51, 54 ;,

unequal, 52
Emitted light, unequal intensity of, 51
Empusa muscce, 571
Enamel of teeth, 949

— of teeth of Echinus, 815

— on ganoid scales, 952
Encephalartos, raphides of, 621
Encrinites, 816

End-bulbs, 977

Endochrome, 463 ; of Palmogloea, 471
of Spirogyra, 478; of Volvox, 479,.
482 ; of desmids, 510

Endoderm, 651

Endogenous spores of Mucorini, 569

— stems, 625-627
Endogens, spiral vessels of, 623
Endonema, 528
Endophloeum, 633
Endoplasm, 463

Endosarc, 463; in Bhizopoda, 658; of

Ciliata, 699
Endosperm, 610

Endospores of mosses, 597 ; in ferns, 602 ;
of Volvox, 484 ; of Hymenomycetes,.
576

Eudosporous Bacteria, 582

Enock's metallic ring for mounting, 451

Entomophilous flowers, 647

Entomophthorece, 571

Entomostraca, 881, 883, 885 ; desicca-
tion of, 887 ; agamic reproduction of,.
887 ; eggs of, 888 ; development of,
889 ; eye of, 906 ; non-sexual repro-
duction, 930

— collecting, 459

— Rotifer a upon, 713
Entomostracan eggs as food of Ciliata^

703

Entoprocta, 833
Entosphcerida, 776
Entozoa, 867

INDEX

IO69

EOL

JEolis, nidamentum of, 858
Eosin, red stain, 436

Eozoon, mounting, 450 ; mode of growth
of, compared with that of Polytrema,
749 ; canal system compared with Cal-
cnrina, 750; affinities of, 763; inter-
mediate skeleton, 764 ; nummuline
layer, 764; internal cast of, 765;
asbestiform layer, 766; pseudopodia
of, 766 ; young of, 767

— canadense, 762

— decalcification, 426

Epe'ira, foot of, 939 ; silk threads of, 939
Ephemera, branchiae of larva, 921

— marginata, larva of, 897 ; circulation
of blood in larva of, 917

Ephippial eggs of Rot if era, 717
Ephyrae of Cyancea, 799 ; of Chri/saora,
800

Epiblast, 651 note
Epiderm of leaves, 637
Epidermic appendages, 953
Epidermis, 965, 966 ; method of prepara-
tion, 967
Epidote, 1001

Epilobium, emission of pollen-tubes,
647

Epilucent zones of light, 315

Epipactis, pollen-tubes of, 648

Epiphloeum, 633

Epispore of Mucorini, 570

Epistome of Polyzoa, 833 ; of Aetino-

trocha, 874
Epistylis, collecting, 457
Epithelia, preservative for, 443
Epithelium, 967, 968

Epithemia, conjugation of 529 ; zygo-
• spores of, 529

— turgida, 533
Equiconcave lens, 22
EquisetacecB, 605 ; in coal, 1006
Equisetum, spores and elaters of, 606 ;

epiderm of, 639 ; silex in, 639
Equitant leaves of Iris, &c, 642
Erecting binocular, Stephenson's, 102

— prism, Stephenson's, 102
Ergot, 572

Erica, seeds of, 649

Eristalis, eye of, 911 ; antennae of, 912

Error of centring, 332

Errors in Tolles' mechanical stage, 166

Erythropsis agilis, eye-spot of, 702

Eschara, calcareous polyzoaries of, 833 ;

extension of perivisceral cavity, 851
Ether as a solvent, 441
Ether-spray microtome, 418; Rutherford

on, 419

Ethmosphmra siphonophora, 774, 776
Eucalyptra, vulgaris, 594
Eucopepoda, 889 note
Eucyrtidium elegans, 771-776

— Mongol fieri, 111

— tubulus, 771

Eudorina, sexual process of, 485
Euglena, 475, 690

Euqlypha alveolata, reproduction of,
671

FAR

[1 Euler's microscope, 146

Euler on achromatic microscopes, 145
Eunotia, 533

Eunotiece, characters of 553
Euphorbiacea* , laticiferous tissue of,
620

Euphrasia, micropyle of, 648
Euplectella aspergillum, 785 note
Eiqwdiscea>, characters of, 541
Enrotium repens, 572
Evening primrose, emission of pollen-
tubes, 647
' Exclamation markings ' on scales, 902
Excretory organ of Rotifera, 716 ; of

Oribatidce, 935
Exner (S.) on the image in eye of

Eampyris, 908
Exogenous stems, 625

— stem, structure of, 633

— and endogenous stems contrasted,
634, 635

Exogens, fibro-vascular bundles, 622, 623 ;
medullary sheath of, 623 ; spiral ves-
sels in, 623

Exoskeleton of decapods, 892
i Exospores of mosses, 597 ; of ferns, 602 ;
of Hymenomycetes, 576

Extine of pollen-grains, 644; markings
on, 645

Eye, accommodation of, 88

— of Pecten, 865 ; of Onchidium, 865 ;
of slug, 865 ; of snail, 865 ; of arthro-
pod, structure of, 907

Eye-glass of compound microscope, 36,
39

Eye-lens, 321

Eye-piece, Abbe's compensation, 42, 322 ;
Huyghenian, 42; Kellner's, 42, 322;
Ramsden's, 43, 323 ; Airy's, 321 ; Cam-
pani's, 321 ; Huyghens', 321

— binocular, Tolles', 102 ; Abbe's, 103

— Kellner's, as condenser, 177

— adapter, 228 ; multiplying power of,
240 ; orthoscopic, 322 ; projecting, 323;
micrometer, 323 ; field of, 323 ; pointer
in, 325 ; diaphragms in, 325 ; index, 325

— stereoscopic, Abbe's, 103
Eye-pieces, classification of, by Abbe, 34 ;

compensating, 34 ; positive, 321 ;
negative, 321-323 ; solid, 822 ; field of,
323 ; working, 323 ; searcher, 323
Eyes on Chiton shells, 865

— compound, of insects, 906, 907

— compound, 906-911 ; simple, 906, 910;
preparing, 910 ; mounting, 910

F

Faber, inventor of the name microscope,
126, 127

Falciform young of Coccidia, 677
False images, 362

Farrant's medium, 443, 449 ; for mount-
ing insects, 897

Farre (A.) on structure of Polyzoa, 832
note

1070

INDEX

FAR

Farrella, polyzoaries of, 833
Fat, 969

Fat-cells, 942, 964, 966, 969; capillary

network around, 986
Fats, solvents for, 441
Feathers, 953-956
' Feather-star,' 824. See Antedon
Feeding, mode of, in Actinophrys, 662 ;

in sponges, 780
Feet of insects, 924-926 ; of spiders, 938
Felspar, decomposition of, 999
Felspar-pyroxene rock, effect of dynamic

metamorphism on, 1000
Felspars, zonal structure in, 996
' Female ' plants of Polytrichum, 596
Fermentation of alcohol by yeast, 574 ;

by Penicillium, Mucor, &c, 575

— putrefactive, 588
Fermentative action of Fungi, 462
Ferns (see Filices), 599 ; in coal, 1005
Fertilisation of Phanerogams, 647
Fertilisation-tube of Peronosporece, 567
Fertilising tubes of Dudresnaya, 561
Festuca pratensis, paleae of, 640
Fibres and cells of Vertebrates, 942
Fibro-cartilage, 943, 970
Fibro-vascular bundles, 635

— of ferns, 599 ; in the ' veins ' of leaves,
622 ; of Exogens, 622, 623 ; of Phanero-
gams, 625

Fibrous tissues of Vertebrates, 943

— tissue, 962 ; white, 963, 964 ; yellow,
964

Field of eye-pieces, 323
Field-glass, 40

Field-lens, 321 ; applied to eye-lens by
Hooke, 321

Filices, 599-605 ; stem, structure of,
599 ; fructification of, 600 ; prothallium
of, 602 ; antherids of, 603 ; archegones
of, 603 ; development of, 604 ; apospory
in, 605 ; apogamy in, 605 ; alternation
of generations in, 605

' Filiferous capsules.' See Thread-cells

Finder, 166; Maltwood's, 246; Panto-
csek's, 246

Fine adjustment, 157-164

continental, 151 ; Ross's, 151 ; Ober-

hauser's spiral, 151; applied to the stage
by Powell, 153 ; by moving the whole
body, 158 ; by simply moving the nose-
piece, 158, 161 ; for Powell and Lealand's
sub-stage, 174 ; of Ross model, 177 ;
Wale's, 185 ; in Beck's students'
microscope, 190

Fire-fly, antennae of, 911

' Fire-fly,' 879. See Lampyris

Fish, circulation in tail of, 981 ; on yolk-
sac, 981

' Fish-louse,' 890

Fish-scales, concretions in, 1021

Fishes, lacunae in bone of, 946 ; dentine
of, 947 ; cement of teeth in, 950 ; plates
in skin of, 950 ; red blood-corpuscles
of, 958, 959 ; pigment-cells of, 967 ;
muscle fibre of, 973 ; gills of, 986, 987

Fission in Lieberkuehnia, 658 ; of Monas,

FOR

681 ; of Monosiga, 689 ; of Codosiga,,

689 ; of planarians, 871
Fissipennes, wings of, 923
Flabella of Licmophora, 534
Flagella, 462 ; of Bacteria, 581, 586, 587~
Flagellata, 680-696

— experiments on, 686 ; nucleus in, 687 y
karyokinesis in, 688 ; colonial forms, .
689

— collared, resembling cells of sponges, .
779

Flagellate chambers of sponges, 780, 781 .
Flagellum of Noctiluca, 691 note
Flat bottle for collecting, 457
Flatness of field, 368

Flea, presumed auditory organ of, 364 ; :
hairs on pygidium of, as a test, 364 ; .
mounting medium for, 897

' Flesh,' 972

Flint, derivation of, 550

— glass, refractive index of, 5 ; dispersive *
power of, 10 ; composition of, 32

— implements found with Orbitolince
749

Flints, preparation of, 1011
Floral envelope, 643

Floridece, 559, 560-561 ; affinities of, 559 '
Flosculariadce, 717
Floscules in confinement, 458
1 Flowering fern,' sporanges of, 601
' Flowering plants,' 609. See Phanero-
gamia

Flowers, 643-648 ; Inman's method of '

preparation, 644
' Flowers of tan,' 563
Fluid inclusions in crystals, 997
' Fluke,' 869

Fluorite lenses for apochromatic objec-
tive, 34, 35
Fluorspar, 1000

Flustra, mode of growth in, 828 ; gem-
mation in, 830 ; number of polypides, ,
832 ; polyzoaries of, 833 ; extensions-
of perivisceral cavity in, 851

Flustrella concentrica,, 111

Fly, various instructive organs to be ob-
tained from, 896 ; eye of, facets in, 907
proboscis of, 913; wing of, 918 ; spiracle^
of, 920 ; areolae on wings of, 922 ; foot
of, 924

Focal alteration and form of objects, 363 •

— depth, 38

— distances, by feeling, 167

— length of a plano-convex lens, 15
Focke on Navicula and Surirella, 532 :

note

Focus, virtual conjugate, 14, 25 ; principal, ,
16 ; mean, 17 ; virtual, 22 ; conjugate, .
24 ; depth of, 83, 89

— of lenses, 13, 21, 22 ; chromatic, 16
Focussing arrangements, 156-165
Fontinalis antipyretica, 596

Food of Hydra, 789
Foraminifera, 658, 720-770

— study of, by means of Beck's disc-holder, .
288 ; examination of, 365 ; woodert
slides for mounting, 390 ; method for-

INDEX

1071

FOR

sectionising, 421 note; decalcification I
of, 426 ; structure of, 720 ; chamberlets
in, 723, 728, 729, 730 ; cyclical mode of
growth in, 723 ; porcellanous shells,
724 ; vitreous shells, 724 ; tubulation of
shell in, 724, 725; plans of growth,
724, 729 ; rotaline type, 725 ; nummu-
line type, 725 ; Porcellanea, 726 ; inter-
mediate skeleton of, 726 ; canal system
of, 726; fossilised forms of, 726, 729,
737, 749, 762 ; dimorphism in, 727 ; se-
condary septa in, 728 ; Arenacea, 735 ;
sandy isomorphs, 739; nodosarine type,
740; Vitrea, 744; internal casts, 748,
752 note; nummuline series, 751; alar
prolongations, 755, 756 ; interseptal
canals, 755; marginal cord in, 755, 759 ;
collecting, 769; method of separating
from sand, &c, 769; mounting, 770;
tubuli of, compared with those of den-
tine, 944 ; in mud of Levant, 1007 ; in
rocks, 1007 ; internal casts of, 1012

Forbes, on reproduction of Sertulariida,
794

Forceps, 301

— slide, 393

— stage, 287

Forficula, antennae of, ^12
ForficulidcE, wings of, 923
Form of objects and focal alteration,
363

Formation of microscopic images, 43
' Formed material,' 942 ; of fibrous tissue,

943 ; of dentine, 944
Fossil coniferous wood, 630, 1005

— crinoids, 816 ; echinids, 816

— Cypridce, 884

— Foraminifera, 726, 749-750

— Lituolce, 741

— Badiolaria, 771, 778 note

— 'Saccammina, 737

— wood, 631
Fossilised Foraminifera (Eozoon), 762

— wood, sections of, 637
Fragilaria, 534
Fragilariece, characters of, 534
Fragmentation of nucleus, 468
Fraunhofer's law of diffraction, 57

— achromatic doublet, 146

— lines, 273, 274
Fredericella, collecting, 458
Free-cell formation, 465, 644

in embryo-sac, 464, 466

Freezing apparatus for Thoma's (Jung's)

microtome, 405, 406

— microtome, Hayes's, 411 ; Cathcart's,
412, 413

— imbedding by, 418

Fresnel on Selligue and Adams's micro-
scope, 146 ; on range of magnification,
147

Freyana heteropus, legs of, 934
Fripp's method of testing object-glasses,
330

Frog, blood - corpuscles of, 958, 959 ;
muscle fibre of, 973 ; papillae on tongue
of, 977 ; circulation in mesentery of,

GAN

980; circulation in tongue of, 980; lung
of, 987

Frog's bladder, histology of, as seen with
apochromatic, 318

— foot, epithelium of web of, 968 ; cir-
culation in web of, 979

Frond of Phceosjwrece, 555
Fructification, gonidial 470; sexual,
470

— of tballophytes, 470 ; of Ascomycetes,
571; of lichens, 578; of mosses, 595 ;.
of ferns, 600 ; of Fguisetacece, 605

Fruit juice as a preservative medium,
442

Frustules of Diatomacece, 517, 518 ;
structure of, 518, 519 note ; girdle, 518 ;
shapes of, 518, 519; ostioles in, 519;-
markings on, 520 ; character of, as basis,
of classification, 532; of Coscinodiscusy.
538

Fucacece, 556 ; conceptacles of, 556
Fuchsia, pollen-grains of, 647
Fucus, 555

Fucus platy carpus, 556-558

— vesiculosus, 556-558
Fulgoridce, wings of, 923
Funaria hygrometrica, 594

— sporange of, 596
Fungi, 470, 562r589

— preparation of, 427 ; zymotic action of, .
462 ; alternation of generations in
classification of, 563; parasitic on.
insects, 571

Fungia, lamellae of, 802

Fungiform papillae, 977

Fungin, 562

Fungus-cellulose, 562

Fusion in Dallingeria, 684

Fuss's description of a microscope, 145-

Fusulina, 750, 751, 1012

Fusulina-limestone, 750, 1007

G

Gabbro, 1016

— fluid inclusions in, 997
Gad-fly, ovipositor of, 927

— See Tabanus
Gaillonella procera, 551

— granulata, 551

— biseriata, 551

Galileo, inventor of the compound micro-
scope, 122-127 ; Viviani's life of, 122 ;
his invention of compound microscope,
Wodderborn on, 123 ; his occhialino
123, 126; his occhiale, 124, 125; bis
microscope, 129

1 Gall-flies,' ovipositor of, 927

Galley-worms. See Myriopoda

Gamasidce, legs of, 934 ; integument of,
934; Malpighian vessel of, 935; heart
of, 935 ; tracheae of, 935 ; characters of:.
936 ; reproductive organs of, 936

Gamasus terribilis, mandibles of, 933

Ganglion-globules (cells), 975

Ganglionic cells, 978

1072

INDEX

GAN

Ganoid scales, 952
Garlic, raphides of, 621
Garnets, 1000

Gas bubbles in glass cavities, 997
Gaseous inclusions in crystals, 998
Gastrcea, Haeckel on, 677
Gastropoda, palates of, mounting, 450 ;
, palate of, 843 ; development of, 843 ;

shell structure of, 852; embryonic
. development of, 858-864; organs of
: hearing in, 865

Gastrula, 651; -stage in Caelenterata,

, 651 ; formation of, 651 note ; of zoo-
phytes, 786; of Gastropoda, 859; of

. blowfly, 931

Gastrulse of sponges, 781

Gauss's optical investigations, 108-112 ;
his dioptric investigations, 108-112 ;

, his system, practical example of, 112-
118

Gelatinous nerve-fibres, 976

in sympathetic, 978

Gemellaria, polyzoary of, 833

Gemmae of Marchantia, 591, 592 ; of
Salpingo3ca, 689; of Stictoria, 698 ; in
Foraminifera, 723 ; of Polyzoa, 830

•Gemmation and shape of shell in Fora-
minifera, 721

Gemmulesof Noctiluca, 694 ; of sponges,
781

Gentiana, seeds of, 649

Geodia, spicules of, 784, 1008

Gephyrean worm, 875

Geranium, glandular hairs of, 639 ; cells

of pollen-chambers, 645 ; pollen-grains,

646

Germ-cells of Volvox, 483, 484 ; of ferns,
604 ; of Marchantia, 593 ; of mosses,
596 ; of Phanerogams, 609 ; of sponges,
781 ; of Hydra, 790

' Germinal matter,' 942 ; of fibrous tissue,
943 ; of dentine, 944

Gesneria, seeds of, 649

Ghostly diffraction image, Nelson on, 72
note

•Gibbes (Heneage) on multiple staining,
438 ; on staining Bacteria, 438

Gill (C. Haughton) on the ' decs ' of
Navicula, 522

Gillett's condenser, 184, 250

Gills of tadpole, 981, 983

Giraudia, conjugation of, 556

— Gizzard ' of insects, 917
Glanders, 588
Glands, structure of, 971

— of Drosera, 639

Glass-cavities in crystals, 997 ; gas
bubbles in, 997
Glass-crabs,' 892
Glass inclusions in crystals, 997

— rings for cells, 386, 387
Glaucium luteitm, cyclosis in, 616
■Glenodinium cinctum, conjugation of, 695
■■Globigerina, shell of, 723 ; mud, 736 ;

mode of life of, 746 ; Wyville Thom-
son's views on, 746 ; Carpenter's views
on, 747 ; pseudopodia of, 746

GRA

Globigerina bulloides, 745 ; in the ' ooze',
1007

— conglobata, 746

— ooze, 748, 1007 ; resemblance to chalk,
1009

— rubra,, colour of, 724

Globigerine shell, sandy isomorph of, 739

Globigerinida, 745

Globule of Chara, 507, 508

Glochidia of Anodon, 857

Glazocapsa, 477 ; as gonid of lichen, 579

Glow-worm, 879 ; antennas of, 911

Glue and honey cement, 384

Gluten of grass seeds, 650

Glycerin, as preservative medium, 441 ;
Hantzsch's method, 444 ; as a preserva-
tive medium, Beale's method, 444

— -jelly, Lawrence's mounting in, 443,
449 ; solvent for CaC05, 444 ; for mount-
ing insects, 497 ; for mounting cartilage,
971

G'yciphagus Krameri, 937

— paLmifer, 932

— platygaster, 937

— plumiger, 932 ; hairs of, 934
Gnathostomata. (Crustacean), 889 note
Goadby's solution for mounting cartilage,

971

Goes (Dr.) on affinity of Carpenteria, 748
Goette on development of Antedon, 827
Gold size, 383

Gomphonema, stipe of, 518, 544 ; move-
ments of, 531 ; attacked by Vampyrella,
655

— geminatum, 545, 546; stipe of, 545

— gracile, 551

Gomuhonemea^, characters of, 545
Goniaster equestris, spines of, 815
Gonidial cells, 470

— fructification, 470

— layer of lichens, 577
Gonidiophores of Peronosporea, 568
Gonids, or non-sexual spores of Crypto-
gams, 470 note ; of Vaucheria, 492 ;
of Podosphenia, 526 ; of Floridece, 561 ;
of Fungi, 562 ; of Peronosporea?, 568

Goniocidaris Jtorigera, spine of, 812
Gonium, 475

Gonothecae of Campanulariida, 794
Gonozoid of hydroids, 792; of Sijncoryne,

793; of Tubularia, 793
Gonozoids of Sertulariida, 794
Gordius, 868, 869
Gorgonia, spicules in, 853

— guttata, spicules of, 804
Gorqonice, 801 ; spicules of, in mud of

Levant, 1007
Goring (Dr.) on magnification of objects,
44

' Gory dew,' due to Palmella cruenta,
486

Govi on invention of microscope by

Galileo, 122
Graduated rotary stage, 338
Grammatophora, chains of, 517, 534

— angulosa, 550

— marina, 537

INDEX

I073

GRA

Grammatophora parallela, 550

— serpentina, 536

— subtilissima, 537
Granite, 1016

— fluid inclusions in, 997
Grantia, 781, 785; spicule of, 1008
Grasses, nodes of, 626 ; silex in epiderm

of, 629 ; paleae of, 640 ; seed of, 649
Grasshopper, gizzard of, 917 ; wings of,
923

Greensands, microscopic constituents of,
1012

Gregarina, characters of, 674 ; movement
of, 675

— gigantea, in lobster, 674 note

— Scenuridis, 676
Gregarinida, 674

Gregory (J. W.) on Eozoon, 768 note
Gregory (W.) on species of diatoms, 530
note

Greville on Spatangidium, 539 ; on

Triceratium, 543 note
Grey matter, 976
Griffith's turn-table, 391
Griffithsia, 559

Grinding sections of hard substances, 420
Grindl's microscope, 134
Gromia, 659, 660, 721

— and Arcella, pseudopodia of, con-
trasted, 671

Ground-mass of rocks, 995
Groundsel, pollen-grains of, 647
Growing slides, Botterill's, 289; Mad-

dox's, 289, 290 ; Lewis's, 289
Guard-cells, 640
' Gulf-weed,' 559

Gum and glycerin, 443 ; and syrup, as a
preservative medium, 443

— imbedding for vegetable substances,
427

— arabic, formula, 385; for freezing, 418

— resins, latex of, 620

— styrax, as a mounting medium, 444 ;
index of refraction, 445

Gyges, 475
Gymnochroa, 792
Gymnolcemata, 833
Gymnosperms, fossilised, 1005

— generative apparatus in, compared
with Cryptogams, 609

Gypsina, 749

H

Haddon on budding in Polyzoa, 831 note
Haeckel (E.) on Monera, 677 note

— on the Gastrcea theory, 677 note

— on Badiolaria , 772 ; on nature of
sponges, 789 ; on Hydrozoon affinity of
Ctenophora, 801 note

: — and Hertwig on classification of

radiolarians, 773 note
Hcematococcns, red phase of Proto-

coccus, 473

— sanguineus, 486

Hematoxylin, alcoholic solution, 433 ;

HEM

aqueous solution, 432 ; Weigert's, 433 ;

Hill's method, 433
Hcemionitis, sori of, 600
Haime (Jules) on development of Tri

choda, 707
1 Hair-moss,' 596
1 Hair-worm,' 868

Hairs of leaves, 639 ; of insects, 904 ; of

Acarina, 934 ; of mammals, 953
Halicaridce, 937
Haliomma Humboldtii, 77(1

— hystrix, 772
Haliotis (diatom), 542

— (mollusc), shell structure of, 852 ;
palate of, 855

Haliphysema, 739 ; sponge-spicules in,
747

Haller on auditory organs of Acarina,
934

Halteres of Diptera, 924
Hand-magnifier, Brewster's, 37
Hansgirg on movement of Oscillariacece r
490

Hantzsch's glycerin method for desmids,.
444

Haplophragmium, 739

— globigeriniforme, 738
Hardening agents, 427, 428

absolute alcohol, 428 ; chromic acid,
428 ; osmic acid, 428 ; picric acid, 428

Hardy's flat bottle for collecting, 457

Harpalus, antennae of, 912

Harting on Janssen's microscope, 122 ;
his experiments on formation of con-
cretions, 1022

Hartnack on immersion system, 27

Hartnack's model, 210 ; his stage, 211

Hartsoeker's simple microscope, 135 ; his
condenser, 248

' Hart's-tongue,' 600. See Scolopen-
drium

' Harvest-bug,' 937

'Haus' of Appendicularia. 842

Haustellate mouth, 916

Haustellium, 916

Haversian canals in bone, 946, 947

Hay craft (J. B.) on structure of striated

muscle fibre, 973
Hayes's ether freezing microtome, 411 ;
minimum thickness of sections there-
with, 412

Hazel, peculiar stem of, 628 ; pollen-
grains of, 647

Hearing, organs of, in Gastropoda, 865 ;
in Cephalopoda, 865

Heart of ascidians, 836 ; of Acarina, 935

Heartsease, pollen-tubes of, 648

' Heart-wood,' 629

Heating-bath, Mayer's, 393

Heliopelta, 518, 540

Heliozoa, characters of, 659 ; examples

of, 662-667 ; pseudopodia of, 770
Helix pomatia, teeth of, 854

— hortensis, palate of, 854
Heller's porcelain cement, 445
Helmholtz on aperture, 47
Hematite in carnallite, 998

3 z

io74

INDEX

HEM

Hemiaster cavemosus, development of,
824 note

Hemiptera, eyes of, 911 ; wings of, 923 ;

suctorial mouth of, 923
Hensen's stripe, 973

Hepaticce, 590 ; thalloid, 593 ; foliose,
593; elaters of, compared with, spiral
cells, &c, of pollen- chamber, 645

Herbivora, arrangement of enamel in
teeth of, 949 ; cement in teeth of, 950

Herring, scales of, 982

Herschellian doublet, 257

Hertel's compound microscope, 137, 138

Hertwig's research on Microgromia, 660
note ; on Actinia, 801 note

Heterocentrotus, spine of, 809

— ■ mammillatus, spine of, 811

Heterocysts of Nostoc, 491

Heteromita uncinata, life-history of, 685

Heterostegina, 759

Heurek (Van) on markings of diatoms,
522

Ilexarthra, 718

Hicks on amoebiform phase of Volvox,
485 ; on preparation of insect antennae,
913 note ; on structure of halteres and
elytra, 924

Hill's (A.) method of using "Weigert's hae-
matoxylin, 433

Himantidium, 533

Hipparchia janira, eggs of, 929

Hippopus, 542

Hippothoa, 833

Holland's triplet, 37

Hollis's liquid glue, 384

Hollyhock, pollen-grains of, 646, 647

Holothuria botellus, plates of, 819

— edulis, plates of, 819

— inhabilis, plates of, 819

— vagabunda, plates of, 819
Holothurice, diatoms in stomach of, 544,

552

Holothurioidea, skeleton of, 818; pharyn-
geal skeleton of, 819 note ; plates in
skin of, 819 ; preparation of calcareous
plates, 820 ; direct development in, 824

note

Holtenia Carpenteri, 785
Homeocladia, 528

Homogeneous immersion, 312, 313 ; Abbe's
combination, 313

— immersion lenses of Powell and Lea-
land, 29 ; of Zeiss, 29

— objectives, value of, in study of monads,
687

— system, 28

Homoptera, wings of, 922, 923
Hood of mosses, 596
Hoofs, 953, 957

— sections of, mounting, 450 ; for polari-
scope, 450

Hooke's airplication of field-lens to eye-
lens, 321

— compound microscope, 130
Hooked monad, 685

Hooker (J. D.) on diatoms of Antarctic
Circle, 549

HYD

Hooklets on wings of Hymenoptera, 923
Hoplophora, 936

— maxillae of, 934

Hormogones of Oscillariacece, 490; of
Rivnlariacece, 490 ; of Scytonemacece,
490 ; of Nostoc, 491

Hormosina globulifera, 738, 740

— Carpenteri, 740
Hornblende, 1001

— corroded crystals of, 995 ; pleochroism
in, 1002

Hornet, wing of, 923 ; sting of, 927
Horns, 953, 957

Horny substances, chemical treatment
of, 440

' Horse-tails,' 605. See Equisetacece
Hosts of parasitic plants, 462
House-fly. See Musca
Hudson on the functions of contractile

vesicle of rotifers, 716 note
Hudson and G-osse on classification Of

rotifers, 717
Human blood-corpuscles, 958

— hair, 954

Husk of corn-grains, 644

Huxley on the ectosarc of Amoeba, 668
note; on coccoliths, 672 ; on Batiiybius,
672 ; on Collozoa, 778 note ; on
structure of molluscan shells, 846 ;
on pul villus of cockroach, 924 note ;
on agamic reproduction of Aphis, 930

Huxley's simple dissecting microscope,
204, 205

Huyghenian eye-piece and spherical
aberration, 42

— Airy's modification of, 321
Hyacinth, raphides of, 621 ; cells of

pollen-chambers, 645 ; pollen-grains of,
647

Hyaline shells of Foraminifera, 724
Hyalinia cellaria, palate of, 855
Hyalodiscus subtilis, 537
Hyaloplasm, 468

Hydra, collecting, 457 ; cells of, 786 ;
intracellular digestion in, 787 ; struc-
ture of, 788; reproduction of, 790;
gemmation of, 930

— fusca, 787, 789

— viridis, 787

— vulgaris, 787

' Hydra tuba ' of Chrysaora, 798, 800
Hydrachnidce, 932 ; eyes of, 935 ;

mandible of, 933 ; reproductive organs

of, 936 ; characters of, 937
Hydrangea, number of stomates in,

641 ; seeds of, 649
Hydrodictyon, 486, 495

— utricalatum, 495
Hydroida, classification of, 792
Hydroids, compound, 791 ; habitats of,

795; Medusce of, 792; planulae of,
792, 795; structure of, 791 et seq.\
examination of, 795 ; mounting, 795 ;
polariscope with, 796 ; preparation of,
796

Hydrophilus, antennae of, 911, 912
Hydrozoa, 787-801

INDEX

HYD

Hydrozoa and marine mites, 937
Hyla, nerves of, 978

Hymene of Ascomycetes, 571 ; of Basidio-
mycetes, 576 ; of Uy menomy cetes, 576

Hymenomycetes, 576 ; pileus of, 576 ;
stipe of, 576

Hymenoptera, 897 ; eyes of, 911 ; mouth-
parts of, 915 ; wings of, 922 ; sting of,
926, 927 ; ovipositor of, 926, 927

Hyoscyamus, spiral cells of pollen-
chambers of, 645 ; seeds of, 649

Hypericum, seeds of, 649

Hyphae of fungi, 562

Hypnospore of Hydrodictyon, 495

Hypnospores, meaning of, 470 note

Hypoblast, 651 note

Hypopial stage of Tyroglypihidce, 937

Hypopus, 937

I

' Ice-plant,' epiderm of, 639
Ichneumonidce, ovipositor of, 927
Illuminating power, 367

— power of objectives, 54 ; compared
with penetrating power, 336

Illumination for dissection, 344

— for opaque objects, 147

— oblique, 170, 171, 331 ; in Zentmayer's
microscope, 184

— of objects, Ross on, 250 ; by reflexion,
278 ; opaque, 281 ; from the open sky,
355 ; by diffused daylight, 355 ; for
dark ground, 356 ; experiments in,
357 ; monochromatic, means of obtain-
ing, 360, 361 ; annular, 362 ; double,
objects for study with, 366; with
small cones, as cause of errors in
interpretation, 369

Illuminator, Stephenson's catadioptric,
170, 263-265; oblique, 170; white
cloud, 172 ; Wenham's reflex, 265,
266; parabolic, 267-269; Swift's sub-
stage, 271 ; Powell and Lealand's, 283 ;
Smith's vertical, 284, 285; Beck's,
285 ; Tolles' vertical, 285

Image, real, 14 note ; virtual, 14 note, 321 ;
conjugate, 24; inverted conjugate, 24 ;
aborption or dioptrical, 64 ; diffrac-
tion, 64 ; negative, 64 ; positive, 64 ;
solid, 95 ; real object, 321; definition
of, 326 ; formed by compound eye, 908,
909

Images, by diffraction, dioptric and

interference, 72
Imaginal discs m larva of blowfly, 931
Imbedding processes, 414 ; simple, 414 ;

in wax, 415 ; in paraffin, 415 ; metal

case for, 415

— masses, 416 ; paraffin, 417 ; wax, 417 ;
celloidin, 417

— by coagulation or freezing, 418
Immersion lenses and vertical illumina-
tors, 285, 286

homogeneous, outcome of Abbe's

theory of diffraction, 312, 313
water, Zeiss's, 317

INT

Immersion lenses, water, Amici's, 310 ;
Powell and Lealand's, 310, 313 ; Praz-
mowski and Hartnack's, 310 ; Tolles',
310

— objectives, 28 ; examination of, 331

— system, 27-29 ; invented by Amici, 27
Imperfect achromatism, cause of yellow-
ness, 360

' Impressionable organs ' in Ciliata, 702
Incidence, angle of, 3
Incident ray, 2
Incus of Botifera, 715
Index eye-piece, 325

— of visibility, 445

Indian corn, epiderm of, 637 ; stomates

of, 640
Indigo carmine, 437
Indirect division of nucleus, 468
Indusium in ferns, 600
Inflection of diverging rays, 62
Infusoria, preserved by osmic acid,

428 ; as food of Actinophrys, 663 ;

Ehrenberg's work on, 678 ; ciliate,

679 ; character of, 679 ; unicellular

nature of, 680 note
Infusorial earth, 536, 538, 540, 542, 546,

550, 552, 771 ; from Barbadoes, 771, 774
Injected preparations, 984
Inoceramus, portions of shell of, in chalk,

1009

Insects, 896-931

— parts of, wooden slides for mounting,
390

— parasitic fungi in, 573-574

— mounting media for, 897 ; integument
of, 898; tegumentary appendages of,
898 ; scales of, 899-904 ; hairs of, 904 ;
parts of head, 906; eyes, 906-911;
antennae of, 911 ; mouth-parts of, 913 ;
circulation of blood, 917 ; alimentary
canal, 917 ; wings of, 918, 922-924 ;
tracheae of, 918 ; stigmata of, 919 ;
sound-producing apparatus, 923 ; organ
of smell, 924 ; organ of taste, 924 ; feet
of, 924-926; stings of, 926, 927; ovi-
positors of, 926, 927 ; eggs of, 928 ;

. agamic reproduction of, 930 ; em-
bryonic development of, 931 ; 1 liver '
of, 971

Insect work, polarised light for, 366
Integument of insects, 898 ; of Acarina,
934

Integuments of ovule, 610
Intensity of light, necessaries for, 359
Intercellular substance, 943 ; in cartilage,
970

Intercostal points, Stephenson on, 73 ;

not revelation of real structure, 73
Interference, 62

— image, 72

Intermediate skeleton in Foraminifera,
726; of Globigerinida, 745; of Calca-
rina, 750 ; of Botalia, 750 ; of Xununu-
lites, 751 ; of Eozoon, 764

Internal casts of Botalia, 748 ; of Textu-
laria, 748 ; of Eozoon, 765 ; of wood,
1005 ; of shells in greensand, 1012

3 z 2

1076

INDEX

INT

Interpretation, errors of, 368

' Interseptal canals ' of Cdlcarina, 755

Intestine, cells of villi in, 968

Intine of pollen-grains, 640

Intracellular digestion in zoophytes, 787

Intussusception, 463

— mode of growth of starch, 620
Invagination, 651

Invertebrata, blood-corpuscles of, 962
Inverted conjugate image, 24

— microscope for chemical purposes
(Nachet's), 216

Iodin, as a test for starch, &c, 440
Ipomcea purpurea, pollen-grains of, 646
Iridescent scales of insects, 899
Iris, epiderm of, 637 ; leaves of, 642 ; cells

of pollen-chambers, 645
Iris-diaphragm, 229, 252, 253, 260 ; fitted

to Abbe's condenser, 259
Iris germanica, epiderm and stomates of,

640, 641

Irrationality of spectrum, 19, 314
Isochelae of sponges, 784
IsoetecB, 607

Isthmia, chains and frustules of, 517, 542 ;
structure of frustules, 519 note ; division
of, 525

— nervosa, 542

— areolations in, 521
Italian reed, stem of, 624
' Itch-mites,' 937

Ivory, 948

Ixodes, heart of, 935

Ixodidce, 932 ; integument of, 934 ; audi-
tory organ, 935 ; tracheae of, 935 ;
characters of, 936

J

Jackson's modification of Koss model,

78 ; his limb, 181, 215 ; his model, 190 ;

his eye-piece micrometer, 232
Janczewski on antherozoids of Spha-

celaria, 555
Janssen on invention of lens, 122; his

compound microscope, 122
Jars, capped, for Canada balsam, 447
Jelly-fish. See Acalephce and Medusce
Jones's compound microscope, 143, 146
Jinigermannia, 593
Jung's (Thoma's) microtome, 401

K

Kaolin, 999

Karop and Nelson on fine structure of
diatoms, 521 note

Karyokinesis in monads, 688

Kellner's eye-piece, 42, 322 ; as a conden-
ser, 177

Kent (Saville) on contractile vacuoles of
Volvox, 481 note ; on Flagellata, 689

Keplerian telescope, Drebbel's modifica-
tion as a microscope, 123

Keramosphcera Murray i, 735 note

LAR

Keratose network of sponges, preparation

of, 779, 781
Kidneys of Vertebrata, 971
King-crab, 881

Kirchner on the oospores of Volvox, 484
Klebs on mucilaginous sheath of des-

mids, 510; on movement of desmids,.

510

— and Biitschli on the 'cilia' of Dino-

flagellata, 695
Klein on Volvox, 484 note
Knife, two - bladed, Valentin's, 398 ;

special, for microtome, 399
Koch's method of sectionising corals,.

802

Kowalevsky on development of ascidiansr
841 note

Krukenberg on digestionin sea-anemones,.
787

Kiitzing on Palmodictyon, 487 ; on struc-
ture of frustules of diatoms, 519 ; his
classification of diatoms, 532

L

Labarraque's fluid for bleaching veget-
able substance, 427

Labels, permanent, 454

Labryinthic structure of Cyclammina,.
741 ; of Parkeria, 743

Labyrintliodon, tooth of, 1013

Lacunae and canaliculi of bone, misinter-
pretation of, 370

— of bone, 943-945 ; dimensions of, in
various animals, 946

— relation of size to that of blood-cor-
puscle, 946

Lagena, 721, 744
Lagenida, 744
Laguncula, 835, 874

— stolon of, 828 ; polypides of, compared
with ClavellinidcB, 839

— repens, anatomy of, 828, 829
' Lamellae ' of corals, 802

— of Hipnenomycetes, 576
Laniellibranchiata, shell of, 843
Lamellicornes, antennae of, 912
Laminaria, 555, 556

i Laminariacece, 556
Lamna, tooth of, 948
Lamp, Nelson's, 347 ; Beck's, 348, 349;

Baker's, 350
Lampyris, antennae of, 912

— splendidula, photograph through eye
of, 908

Land-crab, young of, 893

Langley on use of osmic vapour for
mucous glands, 429

Lankester (E. Ray) on Bacteria, 581 ; on
movement of gregarines, 675 ; on Pro-
tozoa, 677 note\ on intracellular di-
gestion in Limnocodium, 787

Lantern-flies, wings of, 923

Lapis lazuli, 1016

Larva of Ecliinodermata, 820 ; of As-
teroidea, 822 ; of Echinoidea, 822 ; of

INDEX

1077

LAT

Ophiuroidea, 822; of Crinoidea, 824 ;

of ascidians, 840 ; of fly, 931 ; of

Acarina, 933
Latex of Phanerogams, 620
Lathrcea squamaria, embryo of, (548
Latieiferous tubes, free-cell formation

in, 464

— tissue of Phanerogams, 620
Laurentian rocks, 762, 767

* Laver,' or green seaweed, 487

Lawrence's glycerin jelly, 443

Leaves, epiderm of, 637 ; internal struc-
ture of, 641 ; mode of preparation for
examination of, 642

Leech, 880

Leeuwenhoek's simple microscope, 134
Legg'smethod of selecting Foraminifera,
769

Legs of insects, 924, 926 ; of Acarina, 932,
934

Leguminosa, seeds of, 610
Leiosoma palmacinctum, 932; hairs of,
934

Leitz's objectives, 320

— semi-apochromatic objective, 320
Lens spherical, 12 ; biconvex, 12, 13 ;

plano-concave, 13 ; diverging meniscus,
13 ; plano-convex, 13, 15, 22, 37 ; con-
verging meniscus, 13 ; biconcave, 13 ;
piano - convex, focal length of, 15 ;
crossed biconcave, 16 ; crossed bicon-
vex, 16 ; equiconvex, 16, 22 ; Stanhope,
37 ; Coddington, 37 ; Briicke, 38

— from Sargon's palace, 121

— invention of, 121-122

— achromatic, Charles's, 146 ; Barlow's,
147

Lenses, refraction by, 10, 25

— homogeneous immersion, of Powell
and Lealand, 29 ; of Zeiss, 29

— fluorite ; for apochromatic objectives,
34, 35

— combination of, 37

— resolving power of, 64 ; amplifying
power of, 25, 26

— testing by Coscinodiscus, 333
Lepadidce, 891

Lepidium, seeds of, 649

Lepidocyrtus curvicollis, scales of, 903

Lepidodendra, 607, 1005

Lepidoptera, scales of, 899, 900; wings of,
905, 923 ; scales of, mounting, 906 ;
eyes of, 911 ; antenna? of, 912 ; mouth-
parts, 916 ; eggs of, 929

Lepidosteus, bony scale of, 946, 952

Lepidostrobi, 607

Lepisma saccharina, scales of, 900, 901
Lepismidie, 903

Lepralia, 833 ; extension of perivisceral
cavity of, 851 ; mode of growth in, 828
Leptodiscus (ally of Noctiluca), 694 note
Leptogonium scotinum, 578
Leptothrix, form of, 581
Leptus autumnalis, 937
Lern&a, 889 note, 890
Lessonia, 556

Lettuce, laticiferous tissue, 620

LIV

Leucite, mineral inclusions in, 998 ;

anomalies in, 1002
Lever of contact, Ross's, for testing

covers, 381
Libellula, 911 ; respiratory apparatus of

larva, 921
Liber, or inner bark, 633
Lichens, 576-579 ; fungus-constituents

of, 579

Licmophora, stipe of, 518-533, 534 ;
flabella of, 534

— flabellata, 517, 533
Licmoplwrece, 545

— characters of, 533 ; vittae of, 534
Liebcrkuehnia, movement of, 657

— paludosa, 658

— Wagneri, 656-658

Lieberkiihn's microscope, 138 ; his specu-
lum, 282-284
' Ligamentum nucha?,' structure of, 964
Light ; refraction of, 2 ; recomposition
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, 170; monochromatic,
271, 372 ; intensity of, necessaries for,
359

— modifiers, 284
Lignified tissue, test for, 440
Lignites, 1005

Lignum vitce, wood of, 629
Lilac, pith of, 611

Lilium, experiments with pollen-grains
of, 646

' Lily- stars,' 824. See Crinoidea
Limax maximus, palate of, 854

— shell of, imitated, 1023

— rufus, shell structure of, 852, 856
Lime, raphides of, 621
Limestone rocks, 1007

Limn&us stagnalis, nidamentum of,
858 ; velum of, 860

Limnocaridce , characters of. 937

Limnocharis, seeds of, 649

Limnocodium, intracellular digestion
■in, 787

Limpet. See Patella

Limulus, 881

Linaria, seeds of, 649

Lister's struts for support of body, 147 ;
his influence on improvement of Eng-
lish achromatic object-glasses, 148 ;
his zoophyte trough, 297 ; his discovery
of two aplanatic foci, 304 ; his note on
Chevalier's objectives, 304 ; his influ-
ence on microscopical optics, 305 ; his
triple-front combination, 309

Listrophorus, 932

Lithasteriscus radiatus, 550

Lithistid sponges, spicules of, 784

Lithocyclia ocellus, 771

Lituola, 739

Lituolce, large fossil forms of, 741
Lituolida, 739
Live-boxes, 294, 295
Liver, 971

INDEX

LIV

Liver-cells, 972

' Liverworts,' 590. See Hepaticce
Lobosa, characters of, 659 ; examples of,
667-672

Lobster, 881 ; metamorphosis of, 893
' Lob-worm,' 872
Loculi, of anthers, 644
Locust, gizzard of, 917 ; ovipositors of,
928

Locust a, eye of, 911

Loftusia, 743

Loligo, pigment-cells of, 866

Lomas (J.) on calcareous spicules in

Alcyonidium, 832 note
' London Pride,' parenchyme of, 613
Longicornes, antennae of, 912
Lophophore of Polyzoa, 829, 874 ; of

fresh-water Polyzoa, 833
Lophopus, collecting, 458
Lophospermum erubescens, winged seed

of, 649
Lopkyropoda, 883

Lorica of Acineta, 697 ; of Ciliata, 700 ;

of Botifera, 715
Loup-holders, 203

— for tank work, Steinheil's, 224
Loups, Reichert's, 38; Steinheil's, 38,

322, 457; Steinheil's aplanatic, 205;

Zeiss's, 261
Louse, mounting media for, 897
Loven on classificatory value of palates

in Gastropoda, 856
Loxosoma, lophophore of, 833
Lubbock on Thysanura, 901 ; on Podara

scale, 903
Lucanus, 911 ; antennas of, 912
Luminosity of Noctiluca, 690 ; of Cteno-

phora, 806 ; of annelids, 879
Lungs, circulation in, 980, 984
Lychnis, seeds of, 649
LycJmocanium falciferam, 111

— lucerna, 111

Lycoperdon, 575 ; hymene of, 576

Lycopodiacece, 606; in coal, 1006

Lycopodiece, 606

Lyminas, collecting, 457

Lymph corpuscles, 961

Lysigenous spaces in Phanerogams, 613

M

Maceration of vegetable tissues, 624 ;

Schultz's method, 625
Ma chilis polypoda, scale of, 902
Machines for cutting hard sections, 424,

425

Macrocystis, 556

Macrospores of Polytoma, 685 ; of

sponges, 781
Macrourous Decapoda, young of, 893, 894
Madder, cells of pollen-chambers, 645
' Madre,' Acanthometra occurring in, 777
Madrepores, 802

Magenta as a selective stain, 436
Magma, 996
Magnetite, 995

MEA

Magnification, range of, 147
Magnifying power, 367 ; determination
of, 238

Mahogany, size of ducts of, 624 ; stem of,
630

Malacostraca, 892

1 Male ' plants of Polytrichum, 596

Mallei of Botifera, 715

Mallow, pollen-grains of, 646, 647

Malpighian vessel of Gamasidce, 935

— layer of skin in mammals, 966

— bodies in vertebrate kidney, 971
Maltwood's finder, 246

Malva sylvestris, pollen-grains of, 646

Malvacece, pollen-grains of, 646

Mammalia: lacunae in bones of, 946;
plates in skin of, 950 ; epidermic ap-
pendages of, 953; red blood-corpuscles
of, 958, 959 ; epidermis of, 966 ; muscle
fibre of, 973; lungs of, 989

Mammary glands, 971

Man, arrangement of enamel in teeth of,
949 ; cement in teeth of, 950 ; hair of,
955 ; muscle fibre of, 973 ; lung of, 989

Mandibulate mouth, 913

' Mantle ' and growth of shell in Mollusca,
849

Marble derived from limestones, 1011
Marchantia, 590-593 ; archegones of,

596 ; prothallium of, 602 ; stomates of,

640 ; elaters of, 645

— androgyna, 590

— polymorpha, 590-593
Margaritacece, 843; nacreous layer of,

846 ; prismatic layer of, 847
' Marginal cord ' of Calcarina, 755

— of Nummulites, 759
Marine forms, collecting, 458

— glue for forming ' cells,' 385

— mites, 937

— work, tow-net for, 458 ; dredge for, 458 ;
stick-net for, 459

Marshall's compound microscope, 135,
136

Marsipella elongata, 738

Martin's ' pocket reflecting microscope,
138 ; his large microscope, 139 ; his
achromatic microscope, 145 ; his re-
flecting microscope, 145 ; his achro-
matic objective, 145

Marzoli's achromatic lenses, 302

Masonella, 736

Mastax of Botifera, 715

Mastigophora Hyndnianni, 830

Mastogloia, stipe of, 518, 548 ; gelatinous
sheath of, 518, 548 ; development of,
526 ; range of variation in, 547

— la?iceolata, 548

— Smithii, 548

Matthews's method of sectionising hard
substances, 420

Mayall on history of microscope, 119 ; on
Assyrian ' lens ' from Sargon's palace,
121 ; on Divini's microscope, 132

Mayall's removable mechanical stage, 194

Mayer's heating bath, 393

' Meadow-brown,' eggs of 929

INDEX

1079

MEA

' Measley pork,' due to Cijsticercus, 868
' Mechanical finger ' for selecting diatoms, \
554

— movements of the stage in Tully's j
microscope, 147

, g^fc^cr^ 215

T u'rrell's, 165 ; Tolles', 1G6 ; Zeiss's,

167 ; MayaH's removable, 194

— tube-length of microscope, 155

Continental, 156

Medullary rays, 629

in dicotyledons, 627

' Medullary sheath ' of Exogens, 623 ; of

dicotyledons, 627
Medusa of fresh water. 787
Medusa, mounting, 388 ; of Hydroids,

792 ; naked-eyed, 792 ; development

of, 798 ; alternation of generations in,

801 ; nerves of, 976
Medusoids, collecting, 459
Megalopa, 894
Megaloscleres, 783

Megasphere of certain Foraminifera,
727

Megaspores of Rhizocarpece, 606 ; of

carboniferous trees, 607 ; of Isoetece,

607 ; of Selaginellece, 607
Megatherium, teeth of, 950
Megatricha of Ehrenberg, a phase in

development of Suctoria, 698 ; Badcock

on, 698

Megazoospores of Ulothrix, 486; of TJlva,

489 ; of Scenedesmus, 496
Megerlia lima, shell of, 851
Melanosporea , 554
Meleagrina, 843, 846

— margaritifera, 847

Melicerta, collecting, 457 ; in confine-
ment, 458
Melicertadce, 717

Melolontha, eye of, 911 ; antennae of, 912 ;
spiracle of larva, 920

— vulgaris, eye of, 907

Melosira, frustules of, 517, 530 ; auxo-
spores of, 525, 526, 530 ; sporules of,
526 ; zygospore of, 530

— ochracea, 537

— subflexilis, 523, 524

— varians, 523, 524 ; endochrome of, 527
Melosirece, characters of, 537 ; resem-
blance to Confervacece, 537

Membrana putaminis, 962
Membranipora, 832, 833
Membraniporiche , 832
Mercury nitrate as a test for albuminous

substances, 440
Mereschkowski on movements of diatoms,

531

Meridiece, 545

— characters of, 533'
Meridiem circulare, oil, 532, 533
Merismopedia, 477

' Mermaid's fingers,' 803. See Alcyonium
Mesembryanthemum, seeds of, 649

— crystallinum, epiderm of, 639
Mesocarpus, conjugation of, 478; zygo-

spore of, 478

MIC

Mesoderm of sponges, 780

Mesoglcea of Hydra, &c, 788 note

Mesophlceum, 633

Metal case for imbedding, 415

Metamorphism of rock-masses, 999, 1000;

of limestones, 1011
Metamorphosis of Lerncea, 890 ; of

Cirripedia, 892 ; of Malacostraca, 892,

893

Metazoa, 652, 779

Meteorites in oceanic sediments, 1015
Metschnikoff on acinetan character of
Erythropsis, 702 ; on intracellular di-
gestion, 787; on phagocytes, 961 note
Mica, 1000, 1001

Michael's (A.) opalescent mirror, 172
Micrasterias denticulata, binary divi-
sion of, 512 ; form of cell of, 575
Micro-chemistry in Petrology, 1004 ; of

poisons, 1023
Micrococci, form of, 581
Microcysts of Myxomycetes, 565
Microgromia socialis, 660, 661
Microlites, 996 ; in glass-cavities, 997
Micrometer, Cuff's, 140

— use of, 231

— eye-piece, 323

Nelson's new, 227, 228, 229 ; Jack-
son's, 232
Micrometers, 226-233
Micron, a, 82 note, 400
Micro-petrology, 991

' Microplasts ' of Bacterium rubescens,
588 note

Micropyle in ovule, 610 ; of Euphrasia,
648 ; in orchids, Arc, 648

Microscleres, 783, 784

Microscope, Mayall on the, 119 ; history
and evolution of the, 119-225 ; inven-
tion of, 122 ; inventor of the name, 126 ;
essentials in, 154-172; adjustments in,
156-165 ; stage of, 165-168; desiderata
in, 215 ; preservation of, 278

— Galileo's, 129 ; Campani's, 130 ;
Pritchard's, with Continental fine ad-
justment, 150 ; Ross-Jackson model,

. 151 ; Powell's (H.), 153 ; Smith and
Beck's, 153

— achromatic, Euler on 145 ; Martin's,
145; Chevalier's, 146, 148; Selligue's,
146 ; Tullv's, 147 ; Ross's early form of,
150

— aquarium, 219-225

— binocular, 61 ; Riddell's, 97 ; Nachet's,
98; Che'rubin d'Orle'ans', 132; Wen-
ham's stereoscopic, 98 ; Stephenson's,
100, 395 ; Ross-Zentmayer's, 178 ;
Powell and Lealand's, 107, 175 ; Ross's,
177 ; Rousselet's, 200

— chemical, Nachet's, 216 ; Bausch and
Lomb's, 217-220

— compound, 36, 39, 123, 127 ; construc-
tion of, 39 ; path of light through, 40 ;
Rezzi on invention of, 127 ; Hooke's,
130 ; de Monconys', 130 ; Divini's, 131 ;
Janssen's, 122 ; Marshall's, 135 ;
Hertel's, 138 ; Martin's, 139 ; Adams's

io8o

INDEX

MIC

variable, 140, 146 ; Jones's, 143,
146

Microscope, concentric, 171, 178

— demonstration, 225

— dissecting, Beck's histological, 197 ;
Stephenson's binocular, 201 ; Ward's,
205 ; Baker's (Huxley's), 204 ; Zeiss's,
205 ; Beck's binocular, 207

— horizontal, Bonanni's, 135 ; Amici's,
146

— petrological, 992

— photographic, 211

— radial, 171, 178 ; Boss- Wenham's, 184

— reflecting, Newton's, 133; Martin's,
139, 145 ; Smith's, 144

— simple, 36, 128, 201 ; path of light
through, 25 ; inventor of, 128 ; Bacon's,
128 ; Descartes', 128 ; Bonanni's, 134 ;
Ward's, 205 ; Muschenbroek's, 134 ;
Leeuwenhoek's, 135 ; Hartsoeker's,
135

— spectrum binocular, 276

Microscopes, modern, Powell and Lea-
land's, 172, 190 ; Beck's, 180, 189,190, 194,
196 ; Swift's, 181, 190, 194, 197 ; Zent-
mayer's, 184 ; Baker's, 193 ; Continen-
tal models, 208 ; Zeiss's, 208-213

— portable, 198-200

Microscopic and macroscopic vision, 62

— determination of geological forma-
tions, 1012

— dissection, single lenses for, 38

— investigation of rocks, &c, 990

— vision, principles of, 43
Microscopical optics, principles of, 1
Microscopist's work-table, 341, 345
Microscopy, definition of, 340
Microsomes, 461, 468

Micro - spectroscope, Sorby - Browning,
272, 273

— use of, 277 ; in petrology, 1003
Microsphere of certain Foraminifera,

Til

Microspores of Sphagnacece, 599 ; of
Rhizocarpece, 606 ; in carboniferous
trees, 607; of Isoetece, 607 ; of Selagi-
nellece, 607 ; of Polytoma, 685 ; of
sponges, 781

Microtome, ether-spray, 48

— Byder's, 344 ; simple, 398 ; Rivet's
model, 401 ; Thoma's (Jung's), 401-408 ;
freezing apparatus. for, 405, 406

— Cambridge rocking, 408 ; advantages
of, 411

— freezing, Hayes', 411 ; minimum
thickness of sections with, 412 ; Cath-
cart's, 412

Microzoospores of Ulothrix, 486 ; of
Ulva, 489 ; of Hydrodictyon, 495

' Mildew,' 566. See Uredinece

Miliola, shell of, 724 ; encrusted with
sand, 735

Miholce, 121

Miliolida, 726 ; in limestone, 1011
Miliolina, 121

Milioline Foraminifera, fossils of, 726
Miliolite limestone, 1011

MOR

Millepore, resemblance of Polytrema to,
749

Millon's test for albuminous substances,
440

Mineral nature of Fozoon, 767
Minerals, analysis of, 1003, 1004 ; micro-
scopic testing, 1004
Minnow, circulation in tail of, 981
Mirror, 171, 172

— opalescent, as a substitute for polaris-
ing prism, 172

— replaced by rectangular prism, 172
Mites, 932. See Acarina

Mobius on mineral nature of Fozoon, 767
Mohl (Von) on protoplasm, 460 note
Moist-stage, Dallinger and Drysdale's,
289

Molecular coalescence, 1021
Molgida, development of, 841
Moller's diatom type-slide, 286
Mollusca, larvas of, collecting, 459

— shells of, 843 ; shell- structure of, 843-
849 ; colour of shell, 845 ; mantle and
shell-growth, 849 ; palate of, 854 ; de-
velopment of, 857 ; ciliation of gills,
864 ; organs of sense in, 864 ; biblio-
graphy, 866 ; resemblance of barnacles
to, 891 ; ' liver ' of, 971 ; muscle fibre of,
974 ; internal casts of, 1012 ; concre-
tionary spheroids in shells of, 1021

Molluscan shells in mud of Levant,
1007

Molybdate of ammonia as a general

stain, 437
Monad-form of Microgromia, 662
Monadince, life-histories of, 680-686 ;

saprophytic affinities of, 681 ; effect

of temperature on, 686; nucleus in

687

Monads, 680. See Monadince
Monas, 475

— Dattingeri, life-history of, 681

— lens, 680

Monaxonida, spicules of, 783
Monconys (De), inventor of field-lens, 130
Monera, Haeckel on, 677
Monerozoa, 552-658
Monocaulus, 795
Monochromatic light, 271, 372

— illumination, means of obtaining, 360,
361

Monocotyledons, 625 ; stem of, 625 ;

nodes of, 626 ; epiderm of, 637
Monocotyledonous stem, fossilised, 1005
Monocular, Powell and Lealand's, 173,

174

Monocystis agilis, cyst of,
Monophytes, digestion in, 787 note
Monosiga, fission of, 689
Monothalamous Foraminifera, 721
Monotropa, seeds of, 649
Moracecc, laticiferous tissue of, 620
Mordella beetle, eye of, facets in, 907
Mormo, scales of, 904
Morpho Menelaus, scales of, 900
Morula of sponges, 781

— compared to higher Protozoa, 651

INDEX

IO81

MOR

^Morula of Gastropoda, 859

Moseley (H. N.) on skeleton of pharynx

of holothurian, 819 note ; on Chiton's

eyes, 865
Mosses, 594-599

— " capsules of, wooden slides for

mounting, 390
* Mother-of-pearl,' 846
Moths. See Lepidoptera
Motion, spiral, 375
Motor nerves, 976
Motorial end-plates, 977
' Moulds,' 569, 571
Moults of Fntomostraca, 888, 889
' Mountain-flour,' 551
Mounted objects, keeping, 453; labelling,

453 ; arrangement of, 454
Mounting plate, 391, 392

— instrument, James Smith's, 394

— thin sections, 447

— in natural balsam, 449 ; in aqueous
liquids, 450; in deep cells, 451

— diatoms, 450 ; Ophinrida, 450 ; Poly-
cystince, 450 ; sponge - spicules, 450 ;
chitinous substances, 450 ; palates of
gastropods, 450 ; sections of horns, &c,
450 ; Lepidoptera scales, 906 ; wings
of Lepidoptera, 906 ; hairs of insects,
906 ; eyes of insects, 910 ; blood, 962

— media, Canada balsam, 444, 449
Mouse, hair of, 954-955 ; cartilage in ear

of, 970

Mouth, suctorial, of Hemiptera, 923

— of Acarina, 933
Mouth-parts of insects, 913
Movement, interpretation of, 374, 375

— of Lieberkuehnia, 657 ; of Amoeba,
668; of Dallingeria, 683; of plana-
rians, 870 ; of Artemia, 884 ; of Bran-
chipus, 884; of fly on smooth surface,
925 ; of white corpuscles, 961 ; of con-
nective tissue corpuscles, 965 ; of
Oscillariacece, 490; of desmids, 510;
of diatoms, 528 ; of Navicular, 531 ;
of Bacteria, 581 ; of Ciliata, 701

Mucilaginous sheath of desmids, 510
Mucor, fermentation by, 575

— mucedo, 570

Mucorini, 569 ; spores of, 569 ; epispores
of, 570

Mucous glands, Langley's method of
preparing, 429

— membrane, 965, 966 ; capillaries in, 986
Mud of Levant, microscopic constituents

of, 1007

Mulberry, laticiferous tissue of, 620
Mulberry-mass, 651

Miiller (J.) on the Badiolaria, 771; on
larva of Nemertines, 875

Muller's (Fr.) ' Common Nervous System '
in Bolyzoa, 831 and note

Muller's fluid, 430

Multicellular organisms, 651

Multiplication of Palmogloea, 472 ; of
Protococcus, 474 ; of Volvox, 483 ; of
Pahnella, 486; of Bacteria, 581; of
Microgromia, 661 ; of Amoeba, 669 ;

NAI

of Dallingeria, 683; of Heteromita,
685 ; of Tetramitus, 685; of Noctiluca,
694 ; of Peridinium, 695 ; of Suctoria,
698 ; of Ciliata, 704
Multiplying power of eye-piece, 240
Munier Chalmas and Schlumberger on

dimorphism of Foraminifera, 127
Munier- Charles on certain fossil Fora-
minifera, 493
Muricea elongata, spicules of, 804
Musca, eye of, 911 ; antennae of, 912

— vomitoria, eggs of, 930
' Muscardine,' 573
Musci, 594-599
Muscinece, 598
Muscle-cells, 975

Muscular fibre, 972 ; structure of, 973;

capillary network in, 986
Muscular tissue, preservative for, 443

preparation of, 974
Mushroom, 576

— spawn of, 575
Musk-deer, hair of, 954
Musschenbroek's simple microscope, 134
Mussels. See TJnionidce and Mytilacea
Mya arenaria, hinge tooth of, 848
Mycele of Fungi, 562 ; of TJstilaginea

565

■ Mycetozoa, 563
Myliobates, tooth of, 949
Myobia, 932 ; legs of, 934 ; maxillae of
934

Myobiida, 937
Myocoptes, legs of, 934
. ' Myophan-layer ' of Vorticella, 701
Myopy, 120

Myriophyllum, a good weed to collect,
458

Myriopoda, hairs of, 904
Myriothela, intracellular digestion in,
787

Mytilacece, sub-nacreous layer in, 848
Mytilus, for observation of ciliary motion,
864

Myxamozba, 564
Myxogastres, 563

Myxomycetes, 509 note, 563 ; develop-
ment of, 563, 566 ; spores of, 563, 565 ;
swarm-spores of, 563 ; affinity with
Monerozoa, 652

Myxosporidia, 674, 677

N

Nachet on ' immersion system,' 27 ; his
binocular, 95, 98 ; his stereo-pseudo-
scopic microscope, 208 ; his changing
nose-piece, 243

Nacreous layer in molluscan shells, 843,
846, 848

Naegeli and Schwendener on microscopi-
cal optics, 67

Niigeli's theory of formation of starch,
619

Nails, 953, 957
Nais, 879

io82

INDEX

NAP

Naphthalin, monobromide of, as amount-
ing medium, 445 ; refractive index of,
444

Narcissus, spiral cells of pollen-chambers

in, 645
Nassula, mouth of, 702
Nauplius, compared with Pedalionidce,

719

Nautiloid shell of Foraniinifera, 722
Nautilus, 858

Navicula, 520, 528, 546 ; markings on,
522 ; cysts of, 526 ; zygospores of, 526 ;
zoozygospores of, 526

— bifrons, presumed relation to Sicri-
rella microcora, 532 note

— in chalk, 1009

— lyra, as test for definition, 368

— rhoriiboides, markings on, 521 ; as test
for definition, 368

Naviculacece, frustule of, 518 ; ostioles in,
519

Naviculece, characters of, 546
Nebalia, carapace of, 886
Needles for dissection, their mode of use,
397

Negative aberration, 27, 309 note

— crystals, 997

— eye-pieces, 321, 322, 323

Nelson on the sub-stage condenser, 72 ;
his model, with Swift's fine-adjustment
screw, 163 ; his horse-shoe stage, 167,
190; his fine adjustment to the sub-
stage, 169 ; his screw micrometer eye-
piece, 227 ; his new micrometer eye-
piece, 228, 229 ; his ' black dot,' 233 ;
his plan for estimating edges of minute
objects, 233 ; his changing nose-piece,
244; his revolving nose-piece, 244; his
lamp, 347 ; his means of obtaining mono-
chromatic illumination, 361 ; his gelatin,
443

— on ghostly diffraction images, 72 note
Nelson and Karop on fine structure of

diatoms, 521 note
Nemalion multifidum, 560
Nematodes, desiccation of, 869
Nematoid worms, 868
Nemertine larva, 875
Nepa, tracheal system, 919 ; wings of, 924

— ranatra, eggs of, 929
Nepenthes, spiral fibre-cells of, 623
Nereidce, 872

Nereocystis, 556
Nerve-cells, 975

— staining with blue-black, 437
Nerve-fibres, 976

Nerve-substance, 975 ; mode of prepara-
tion, 978
Nerve-tubes, 975
Nerves, preservative for, 443
Nervures of wing of Agriou, 918
Nettle, hairs of, 639

Neuroptera, 897 ; eyes of, 911 ; circula-
tion in wings of pupa, 918 ; wings of,
922

Newt, red blood-corpuscles of, 959 ; cir-
culation in gills of larva, 981

NUC

Newton's reflecting microscope, 133

— suggestion of reflecting microscope,.
144

rings, 1018
Nicol prisms, 269

Nicol's analysing prism, 244 ; for re-
solving striae, 325
Nicotiana, seeds of, 649
' Nidamentum ' of Gastropoda, 858
Nitella, 505, 506

Nitrate of silver for tendon-cells, &c.,.
437

Nitric acid as a test for albuminous sub-
stances, 440
Nitrogenous substances, test for, 440
Nitzschia, 528

— scalaris, cyclosis in, 577

— sigmoidea, 535
Nitzschiece, 535
Nobert's test lines, 286

Noctiluca, collecting, 459 ; tentacle
(flagellum) of, 691, 692 ; cilium of, 691
note ; protoplasmic network of, 692 ;.
reproduction of, 694

— miliaris, 690-694
Noctuina, antennae of, 912
Nodes of monocotyledons, 620
Nodosaria, 744
Nodosarince, shell of, 722
Nodosarine shell, sandv isomorphs of,,

740

Nonionina, 754

— shell of, 722, 723

I Nonionine shell, sandy isomorph of, 739

Non-stereoscopic binoculars, 106
I Non-striated muscle, 972, 974
Nose-piece, centring, used as sub-stager
193 ; Brooke's, 241 ; centring, 242 ;
Zeiss's calotte, 242 ; Beck's rotating,
242; Powell and Lealand's, 242; Na-
chet's changing, 248 ; analysing, 244 ;
Nachet's changing, 244 ; Nelson's re-
volving, 244; Vogan's, 244
Nosema bombycis, cause of pebrine,.
588

Nostoc, 490, 491 ; as gonid of lichen, 579 ;

resemblance of Oplirydium to, 705
Nostocacece, 490; affinities with Bacteria,.

and Myxomycetes, 580
! Notochord in Tunicata, 835 ; of Appen-

dicularia, 842
Notonecta, 911 ; wings of, 924
Nucellus, 610
Nuclear stains, 430

— spindle, 468 ; plate, 468
Nuclein, 468

Nucleoli, 464
Nucleoplasm, 467
Nucleus, 464

— of minute organisms, 80

— action of acetic acid on, 440 ; its im-
portance to cell, 465 ; division of, 468 'r
fragmentation of, 468 ; presumed ab-
sence of, in some forms, 652 ; in monads,
687

— and cell division, 943 note
Nucule of Chara, 507, 508

INDEX

NUD

Xudibranchs, nidamentum of, 858 ; em-
bryos of, 860

Numerical aperture, 29, 53, 60, 333, 367 ;
formula for, 333 ; problems on, 334

of Zeiss's apochromatic series of

objectives, 318 ; of dry objective, 334 ;
of water-immersion, 334 ; of oil-im-
mersion, 334

and resolving power of objective, 336

— apertures, table of, 84-87
Nummuline layer of Eozoon, 764

— plan of growth, Parker and Rupert
Jones on, 752 note

Kummirfinidce, 751
Nummulites, 751, 752, 756

— distans, 757

— garansensis, 757

— laevigata, 757

— striata, internal cast of, 759

— tubuli in shell of, 725
Nummulitic limestone. 756, 760, 1007,

1011

Nuphar lutea, parenchyme, 612 ; stellate

cells of, 612
Nvmph of Acarina, 933 ; of Oribatidce,

933

O

Oak, size of ducts in, 624

— galls, 927

Oberhiiuser's spiral fine adjustment, 151
Object-glass of compound microscope,

36, 39 ; of long focus, 40 ; of short focus,

40; capacity of, 326
Object-glasses, power of, 44

— — testing, 325 ; Abbe's method of
testing, 326-333 ; diaphragms for use
in testing, 329 ; Tripp's method of
testing, 330

Object-holder for Thoma's (Jung's) mi-
crotome, 403, 404

— changer, Zeiss's, 243, 244
Objectives, achromatic, 19, 32 ; aplanatic,

19 ; apochromatic, 19, 30, 34, 80, 211 ;
corrected, 20, 21 ; immersion, 28, 34,
58; aperture of, 43, 65, 333 ; 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, 336 ; sliding plate with,
241 ; rotating disc with, 241 ; of wide
aperture, 316; of small aperture, ex-
animation of, 332 ; tests for, 332, 337 ;
readying power of, and numerical aper-
ture, 336

— triple-back, 310 ; "Wenhani's duplex
front, 311 ; Leitz's, 320 ; Reichert's,
321 ; adjusting, 306, 309

— achromatic, Martin's, 145 ; Marzoli's,
302: Tully's, 303; Selligue's, 303;
Amici's, 304 ; Ross's, 305, 309 ; Powell's,
305, 309 ; Smith's, 305, 309 ; Wenham's,
310 ; covers for use with, 3S0

OPE

Objectives, apochromatic, 314, 315, 320

— oil-immersion, Amici's, 312 ; Tolles',
312 ; Zeiss's, 313, 317

— water-immersion, Powell and Lea-
land's, 310, 313 ; Prazmowski and
Hartnack's, 310 ; Zeiss's, 317

Oblique illumination, 170, 171, 331

— illuminator, 170

Obliteration of structure bv diaphragms,
68

Occhiale, Galileo's, 124, 125

Occhialino, Galileo's, 123, 126

Oceanic sediments, microscopic examina-
tion of, 1014

Ocelli of planarians, 871 ; of insects,
906, 910

Ocellites of compound eye, 906

Ocular, 40, 321; spectral, 276

(Edogoniacea3, 502, 503

(Edogonium ciliatum, 502

(Enothera, pollen-grain, 646; emission
of pollen-tubes of, 647 ; embryo of, 648

Oil for immersion lenses, suggested by
Amici, 29

— of cassia, used with Stephenson's
illuminator, 265

— of cedar - wood, for immersion ob-
jectives, 29

Oil-globules, 370, 371
Oil-immersion, 29

objectives, Amici's, 312; Tolles',

312 ; Zeiss's, 317
Oils, solvents for, 441
Okeden on isolation of diatoms, 553

note

Oleander, epiderm of , 638 ; stomates of,
641

Olivine, alteration of, 1001

— corroded crystals of, 995
Onchidium, eyes of, 865
Oncidium, spiral cells of, 618
Onion, raphides of, 621

Oogones of Vaucheria, 492'; of Sph&ro-
plea, 501 ; of (Edogonium, 503 ; of
Chara, 507; of Fucacea, 556, 557;
of Peronosporece, 567

Oolitic rocks, structure of, 1011

Oophyte in ferns, 605

Oospheres, use of the term, 467 note ;
of Volvox, 484; of Vaucheria, 492 ; of
Sphceroplea, 501; of (Edogonium, 502 ;
of Chara, 507 ; of Phceosporece, 556; of
Fucace&, 557; of Marchantia, 593;
of ferns, 604

Oospores, 470; of Volvox, 484; of
Vaucheria, 493 ; of Achhja, 495 ;
of Sph&roplea, 500; of (Edogonium,
503 ; of Chara, 509 ; of Fucacea?, 558

Ooze, Globigerina, organisms in, 736,
745 ; compared with chalk, 1007

Opalescent mirror as a substitute for
polarising prism, 172

Opalina, 702

Opaque illumination by side reflector,
281

— mounts, 283

' Open ' bundles, 635

1084

INDEX

OPE

Operculina, 752, 755; and Nummitlites

compared, 759
Operculum of mosses, 596
Ophiacantha vivipara, development of,

824 note

Ophioglossacece, development of pro-
thallium of, 604

Ophioglossum, sporanges of, 601; pro-
thallium of, 606

Ophiothrix pentaphyllum, spines of,
815 ; teeth of, 816

Ophiurida, mounting, 450

Ophiuroidea, skeleton of, 815 ; spines of,
815; teeth of, 816; larva of, 822;
direct development in, 824 note

Ophrydia, quantities of, 706

Ophrydina, colonies in, 705

Ophrydium, cellulose in zob'cytium of,
706

— versatile, effect of light on, 702
Ophryodendron, 697

Opium poppy, latex of, 620

Optic axis of Powell and Lealand's No.

1, 174 ; of Baker's third-class, 194
Optical anomalies in petrology, 1002

— centre, 24

— tube-length of microscope, 155

— tube-length, Continental, 156
Orals of Antedon, 825
Orbiculina, 728, 729, 733

— shell of, 722

— compared with Heterostegina, 759
Orbitoides, 757

— Fortisii, 760
Orbitolina, 749

Orbitolince, occurring with flint instru-
ments, 749
Orbitolites, 729-735

— shell of, 723 ; range of variation in,
735 ; structure of Parkeria resembling,
742 ; deposits of, 1007

— and Cycloclypeus compared, 726,
760

— complanata, animal of, 732-734

— italiaca, 731 note, 733

— tenuissima, 733
Orbulina, 745

Orbuline shell, sandy isomorph of, 740
Orchidece, pollinium of, 647
Orchids, micropyle of, 648
Orchis, pollen-tubes of, 647 ; seeds of,
649

Organised structure and living action,
460

Organisms, minute, nucleus of, 80
Organs, 463

' Organs of sense ' in Ciliata, 702 note

Oribatidce, nymph of, 933 ; mouth-parts
of, 933 ; legs of, 934 ; integument of,
934 ; auditory organ, 935 ; reproductive
organs, 935 ; supercoxal glands of, 935 ;
tracheae of, 935 ; characters of, 936

Orienting small objects for sectionising,
416; Kingsley's method, 415

Origanum onites, seeds of, (549

Ornithorhyncus, hair of, 954

Orobanche seeds of 649

PAP

Orthoptera, eyes of, 911 ; antennae of,
912; wings of, 923; nymph of, 933

Orthoscopic effect, 95; with Eamsden's
circles, 107

— eye-piece, 322

Orthosira Dickiei, sporangial frustule of,
524

Oscillaria, movement of, 490
Oscillariacece, 490

— movements of, 375
Oscula of sponges, 780

Osmic acid and fatty structures, 429
Osmunda, sporanges of, 601

— regalis, prothallium of, 604 note
Ossein, of bone, 947

Ostiole of conceptacle of corallines, 561

Ostioles of Navicitlacece, 529 ; of Gym-
bellece, 519

Ostracoda, 884

Ostreacea, shell of, 847

Ostrich, egg-shell of, 1021

Otoliths compared with artificial concre-
tions, 1021

— of Mollusca, 865
Ovarium of Polyzoa, 831
Over-amplification, 88
Over-corrected objective, 20
Over-correction, 307, 308
Overton on Volvox, 484 note
Ovipositor of Oribatidce, 936
Ovipositors of insects, 926, 927
Ovule of Phanerogams, 609

— suspensor of, 464

— structure of, 609, 610 ; development of,
647

Ovum of Hydra, 790

Oxytricha, a phase in development of

Trichoda, 707
Oxyuris vermicularis, 868
Oysters, shell of, 847

P

Pacinian corpuscles, 977

Palaeontology, use of microscope in, 1005

' Palate ' of Gastropoda, 843, 854 ; classi-
ficatory value of, 856 ; preparation of,
856 ; viewed with polariscope, 857 ;
bibliography, 857

Paleae of grasses, silex in, 640

Palisade-parenchyma of leaves, 641

Palm, stem of, 626

Palmella, as gonid of lichen, 579

— cruenta, 486

Palmellacece, 486 ; frond of, 486
Palmodictyon, 487 ; zoospores of, 487
Palmoglaza macrococca, life-history of,

471-473
Palpicornes, antennae of, 911
Paludina, infested by Distoma, 870
Pancreas, 971
Pandorma, 475

— morum, generative process of, 485 ;
swarm- spores of, 485

Pantocsek's finder, 246

Papaveracecp, laticiferous tissue of, 620

PAP

Paper-cells, 38G

Parabolic illuminator, 267-269 ; reflector
(Sorby's), 281 ; speculum, 281

Paraboloid, 267-269 ; Edmunds', 269 ;
Wenham's flat-topped, 269

Paraffin, solvents for, 417

— for imbedding, melting point of, 417

— cells, 386

Paramecium, Cohn's experiments on,
668 ; contractile vesicles of, 704

— aurelia, supposed sexual reproduction
of, 710

Parapbyses of Puccinia, 567 ; of licbens,

578 ; of mosses, 596
Parasites, nourisbment of, 462
Parasitic Crustacea, 889

— Fungi, 562
Parietal utricle, 463

Parker (T. J.) on osmic acid for Ento-
mostraca, 428 ; on use of osmic acid for
vegetable structures, 428; on Hydra,
787

Parkeria, 742 ; a possible Stromato-

poroid, 742 note
Parnassia, seeds of, 649
Parthenogenesis, 931 note

— in Saprolegnice, 569

Passiflora car idea, pollen-grains of,
646

Passiflorea?, pollen-grains of, 646
Paste-worm, 869

Pasteur's solution for growing yeast, 574
note ; his experiments with Bacteria,
587, 588

Patella, shell structure, 852 ; palate of,
855

Path of ray of light through a compound

microscope, 40
Pathogenic bacteria, 585
Pavement epithelium, 968
Pear, constitution of fruit, 618
' Pearl oyster.' See Meleagrina
Pearls, 847

1 Pebrine ' in silkworms, 588
Peccary, hair of, 954

Pecten, prismatic layer in, 848 ; pallial
eyes of, 864 ; eye of, 865 ; fibres of
adductor muscle, 974

Pectinibranchiata, 861

Pectinidce, sub-nacreous layer in, 848

Pedalion, 718

Pedalionida, 718

Pedesis, 373 ; experiments in, 373-4
Pediastrece, 496 ; affinities of, 496
Pediastruvi, zoospores, 496, 497 ; micro-
zoospores, 497

— Ehrenbergii, 498

— granulatum, 496, 497

— pertusum, 497

— tetras, 498

Pedicelloriae of echinids and asterids,
813

Pedicellina, lophophore cf, 833
Pedicularis palustris, 648

— sylvatica, embryo of, 648
Peduncle of Lepas, 891
Pedunculated cirripeds, 891

cx 1085

1

PHI

Pelargonium, petal of, 643; pollen-grain,
646

Pelletan on osmic acid, 428
Pelomyxa palustris, 669
Peneroplis, 726

— variation in shape of shell in, 722 ;
shell of, 724 ; varietal forms of, 728

Penetrating power, 367
in objectives, 83 ; of objective, com-
pared with illuminating power, 336
Penetration, 38, 82, 83
Penicillium, fermentation by, 575

— glaucum, 571

; Pentacrinus asterius, skeleton of, 816

Pentatoma, wings of, 924

Peony, starch in cells of, 619

' Pepperworts,' 606

Perception of depth, 94

Perch, scales of, 952

Perforated shells of Brachiopoda, 850

Perforation of shell in Foraminiferar
724, 725

Perianth, 643
I Perichlamydium prcetextum, 775

Peridinium uberrimum, 695

Perigone of mosses, 595

Periodic structures, 74

Periostracum of molluscan shells, 846 ;
of brachiopod shells, 850

Peripatus, trachea? of, 935

Peritheces of lichens, 578

Peronosporecc, 567, 568

Perophora, respiratory sac of, 839 ; cir-
culation of, 839

' Perspicillum,' "Wodderborn's, 127

Petals, 643

Petrobia lapidum, eggs of, 933
Petrological microscope, Swift's, 992
Petrology: micro-spectroscope in, 1003;

micro-chemistrv in, 1004
Pettenkofer's test, 440
Petunia, seeds of, 649
Peziza, botrytis-iovm. of, 572
Pfitzer, on reproduction of diatoms, 523
Plia>odaria, 777
Pha>osp>orea>, 554, 555
Phagocytes, 961 note
Phakellia ventilabrum, 782
Phallus, 575

Phanerogamia, woody structures, pre-
paration of, 427

— embrvo-sac of, free-cell formation in,
464-466

— relation of, to Cryptogams. 607, 609 ;
structure of stems, &c, 610, 625 ; struc-
ture of cells, 612, 613 ; intermediate
lamella, 613 ; intercellular spaces, 613 ;
cell-wall of, 617 ; sclerogen, 617 ; spiral
cells in, 618 ; laticiferous tissue of, 620 ;
mineral deposits in cells of, 620, 621 ;
woody fibre in, 621 et seq.; fibro-vas-
cular bundles, 625 ; root, structure of,
625 ; flowers of, 643 ; pollen-grains of,
644 ; fertilisation of, 647 ; ovules of,
647 ; seeds of, 648

Phanerogams. See Phanerogamia
Philonthus, antennae of, 912

io86

INDEX

PHL

Phloem, 635

— of Exogens, 622
Pholas, shell of, 848
Phoronis, 875

Phosphorescence of sea, due to Noctiluca,
693

Phosphorus, as a mounting medium, 445
Photographic microscope, Zeiss's, 211,
212

Photometrical equivalent of different

apertures, 50
Photo-micrograph through eye of Lam-

pyris, 908
Photo-micrography, 174, 233, 324

— Campbell's differential screw used in,
194

— illumination for, 356
Phryganea, eye of, 907
Phycocyanin in Chroococcacece, 477
Phyco-erythrin, 560

Phy corny ces nit ens, 570

Phycophasin, 555

Phylactolcemata, 833

Phyllites, 1001

Phyllopoda, 886

Phyllosomata, skeleton of, 892

Physarum album, development of, 564

Physcia parietina, 579

Physma chalaganum, 579

Phytelephas, endosperm of seed of, 618

Phytophthora infestans, 568

Phytopti, mouth-parts of, 934

Phytoptidce, 932 ; characters of, 938

Phytoptus, larva of, 933

Picro-anilin, 437

Picro-carminate of ammonia, 436

Picro-carmine, 436

Piedmontite, 1017

Pieridce, scales of, 899

Pigment-cells of cuttles, 866; of ver-
tebrate skin, 966 ; of fishes, 967 ; of
Crustacea, 967

Pigmentum nigrum, of eye, 967

Pike, scales of, 952

Pileorhiza, 636

Pileus of Acetabularia, 493

Pilidium gyrans, 875

Pilulina Jeffrey sii, 737

Pimpernel, petals of, 644

Pines, pollen-grains, showers of, 646 note

Pinna, structure of shell of, 815, 843-
846; prisms of shell of, in Globigerina
ooze, 1008 ; prisms of, in chalk, 1009

— nigrina, colour of shell of, 845
Pinnularia, 546

— dactylus, 551

— nobilis, 551
Pinus canadensis, 383
Pipette, 447

Pistil, 647

Pitcher-plant, spiral fibre-cells of, 623

Pith, arrangement of, 625, 627

Pitted ducts of Phanerogams, 623

Placoid scales, 952

Plagioclase felspar, 1003

Planaria, stomach of, 890

Planarice, 869 ; movement of, 870 ; fis-

POL

sion of, 871 ; ocelli of, 871 ; intracellular
digestion in, 787
! Planarians. See Planarice

— allied to Gtenopliora, 806
Plano-concave lens, 13
Plano-convex lenses, 13, 15, 22, 37
Planorbulina, 749
Plantago, cyclosis in, 616

' Plantain,' cyclosis in, 616
Plants and animals, differences between,
461

Planulae, 792

Planularia hexas, in chalk, 1009
Plasmode in cells of Nitella, 509 note ;

of JEihalium, 563 ; of Myxomycetes,

564, 565

Plasmodium of Protomyxa, 654
Plastid, contrasted with cytode, 652
Plastidules, flagellated, of Protomyxa,
654

Plates, calcareous, of Holothicrioidea,
819

Pleochroism, 1002
Pleurosigma, 518, 546

— diffraction image of, 71

— angidatum, 69-71 ; as test for defini-
tion, 368; markings on, 521, 522

' — formosum, as test for definition, 368

— Spencerii, sporules of, 526

Pliny on cauterisation by focussing sun's

rays, 119 ; on sight, 120
Ploima, 111, 718
Plumatella, collecting, 458
' Plumed-moths,' wings of, 923
Plumule of Pieridce, 899
Plutarch on myopy, 120
Pluteus larva of echinoids, 821, 822, 823
Podocyrtis cothurnata, 111

— mitra, 111, 776

— Schomburgkii, lid, 776
Podophrya quadripartita, 697 ; imma-
ture form, 698

— elongata, 697
Podosphenia, sporules of, 526
Poditra scale as test for high powers

332

'Podura scales,' 900, 903
Poduridce, 903
Pointer in eye-piece, 325
Poisons, micro-chemistry of, 1023
Polariscope, condensers for use with,
262 ; for examination of gastropod
palates, 857 ; crystals for use with,
1017 ; list of objects for, 1020
I Polarised light for insect work, 366 ; use
of, in micro-petrology, 992
Polariser, 262, 269

Polarising prism, substitution of opales-
cent mirror for, 172
' Polierschiefer,' 546
Polishing ground sections, 424

— sections of hard substances, 420
slate, 546

stones, 421, 546

Polistes (wasp), with attached mould,
571

Pollen-chambers of anthers, 644

INDEX

POL

Pollen-grain and tube, 609

— grains, 644 ; form of, 645 ; experi-
ments with, 646

— mass, of orchids, 647

— tube, 645

— tubes, traced through the style, 647
Pollinium of orchids and asclepiads, 647
Pollinoids of Floriclece, 561 ; of lichens,

578

Polyaxial spicules, 783
Polycelis levigatus, 871
Poly clinic! ce, 837
Polycystina, 771, 772, 776

— skeleton of, 659

Poly cystines, as test for low powers,

332 ; mounting, 450
Polydesmidce, 905

* Polygastrica, Ehrenberg's erroneous

views on, 678
Polygonum, pollen-grains of, 646
Polymorphina, 745
Polyonunatus Argus, scales of, 900
Polyparies of zoophytes, 786
Polypary of hydroids, 791
Polypes, 787. See Hydrozoa
Polypide, of Polyzoa, 829 ; formation of

buds from, 830
Polypidom of zoophyte, 828
Polypite, of hydroids, 791
Poly Imodium, sori of, 600
Polyporus, 575

Polystichum angular e, apospory in, 605
Polystomella, shell of, 723

— craticulata, 752, 753

— crispa, 752, 754, 765
Polythalamous Foraminifera, 721
Polytoma nvella, life-history of, 684
Polytrema, 749 ; mode of growth com-
pared with Eozoon, 763

— miniaceum, colour of, 724
Polytrichuni commune, 595, 596
Polyxenus lagurus, hair of, 905

hair of, as test for objectives, 332;

as test for definition, 368

Polyzoa, collecting, 457, 458; keeping
alive, 458; 'cell' of, 828; structure
of, 828 ; gemmae of, 830 ; muscular
system, 831 ; sexual reproduction of,
831 ; ' colonial nervous system,' 831
and note ; fresh- water, lophophore of,
832 ; epistome of, 833 ; classification
of the group, 833 ; bibliography of,
834; relation to Brachiopoda, 851;
' liver ' of, 971

Polyzoaries in coralline crag, 1011

Polyzoary, 828

Pond-stick, 456

Poplar, pollen-grains of, 647

Poppv, laticiferous tissue, 620 ; seed of,
648

Porcellanea, 726-735
Porcellanous shells of Foraminifera,
724; of Gastropoda, 852

— and vitreous Foraminifera, difference
in, 725, 726

Porcupine, hair of, 954
Pores of sponges, 780

FBI

Porphyra, trichogyne of, 561
Porphyritic crystals, glass inclusions in,
997

1 Portable ' microscope, Powell and
Lealand's, 198, 199 ; Beck's, 199, 202 ;
Rousselet's binocular, 200; Swift's,
198, 200

Port una, skeleton of, 892

Positive aberration, 309 note

— eye-piece, 43

— eye-pieces, 321, 322, 323

Potash, caustic, action on horny sub-
stances, 440
Potato-disease, 568

— starch-grains of, 620

— tubers, starch in, 619

Powell, T., formula for objective, 34
Powell and Lealand's homogeneous im-
mersion objective, 29 ; fluorite lenses,
34, 35 ; high-power binocular, 107 ;
sub-stage, 170, 174 ; their microscopes,
173, 189, 190; binocular, 176; achro-
matic dry, 190; portable micron-ope,
198; rotating nose-pieces, 242; a hro-
matic condenser, 251, 267; new low-
power condenser, 252 ; apochromatic
condenser, 254; dry achromatic con-
denser, 258 ; chromatic oil condenser,
258 ; condenser for polariscope, 262 ;
achromatic oil condenser, 263, 267 ;
latest condenser, 267 ; bull's - eye,
280 ; vertical illuminator, 285 ; com-
pressor, 296 ; protecting ring for
coarse adjustment, 301 ; water-
immersion objectives, 310, 313 ; ^L-
inch objective, for observation "of
cyclosis, 614 ; objectives, for study of
monads, 687
Powell's (H.) microscope, 152, 153; fine
adjustment applied to the stage, 153

— fine adjustment, 161

j Prawn, skeleton of, pigment of, 893
Prazmowski and Hartnack's water-im-
mersion objectives, 310
Preparation of vegetable tissues, 427
Presbyopy, 120
Preservative media, 441-443
Primary tissues of Vertebrata, 941
Primordial cells, 465, 466

— utricle, 463 ; of desmids, 510 ; of Pha-
nerogam cells, 613

— chamber in Foraminifera, 723; of
Orbitolites, 731

Primrose, cells of pollen-chambers, 645
' Prince's feather,' seed of, 648
Principle of microscopic vision, 43
Principles of microscopical optics, 1
Pringsheim on generative process of

Pandorina, 485 ; on Vaucheria, 492
Prism, refraction by, 8, 9 ; Wenham's,

99 ; Stephenson's erecting, 102

— polarising, substitution of opalescent
mirror for, 172

— rectangular, in place of mirror, 172

— Nicol's, 244, 269; Nicol's analysing,
325 ; Abraham's, 344

— refracting angle of, 9, 18

io88

INDEX

PRI

Prismatic epithelium, 968

— layer in molluscan shells, 844, 845,
847, 848

— layer of shells compared with enamel,
949

— shell-substances imitated, 1022
Prisms, recomposition of light by, 18
Pristis, tooth of, 948
Pritchard's doublets, 249

— microscope with Continental fine ad-
justment, 150, 151

Privet hawk-moth, eggs of, 929
Problems on refractive index, 5
Procarp, of Floridece, 561
Projection eye-piece. 328
Promycele of Puccinia, 566
Prosenchymatous tissue, 621
Proteus, red blood-corpuscle of, 960
Prothallium of Sphagnacecs, 599; of
ferns, 602 ; of Equisetacece, 606 ; of
Bhizocarpece, 606 ; of Lycopodiacece,
607

Protococcus, as gonid of lichens, 579

— pluvialis, 473-480 ; life-history of,
473 ; multiplication of, 474 ; zoospores
of, 474, 475 ; mobile and still forms of,
475-477 ; encysted, 480

Protomyxa aurantiaca, 652-654
Protoneme of Batrachospermum, 505
Protophytes, 460, 580, 651

— mounting, 442 ; mode of nourishment
of, 462 ; movement by cilia and con-
tracting vacuoles of, 465

Protoplasm, 461 ; vital attributes of, 461 ;

continuity of, 469 ; of Bhizopoda, 658 ;

of Noctiluca, 692
Protoplasmic substance in Vertebrata,

941

Protoplasts, 435
Protozoa, 651-712

— mode of nourishment of, 462

— Lankester's papers on, 677

' Pseudembryo ' of Antedon, 827

Pseudo-navicellse, 675

Pseudo-parenchyme of Fungi, 562

Pseudopodia of Protomyxa, 653 ; of
Vampyrella, 655 ; of Lieberhuehnia,
656 ; of Bhizopoda, 658 ; of Beticu-
laria, 658 ; of Heliozoa, 659 ; of Gromia,
660 ; of Microgromia, 661 ; of Acti-
nophrys, 663 ; of Amoeba, 668 ; of
Arcella, &c, 671 ; in ^Lmce&a-phase of
monad, 682 ; of Eozoon, 766 ; of Glo-
bigerina, 746; of Badiolaria, 712,; of
endoderm cells in zoophytes, 787

Pseudoraphidece, 527

Pseudoscope, Wheatstone's, 92

Pseudoscopic effects, 95

— effect with Ramsden's circles, 107

— vision, 92
Pseudo-scorpions, 932

Pseudo-stigmata of Oribatidce, 935, 936 j
Pseudo-trachese, on fly's proboscis, 915
note

1 Psorosperms,' 677

Pteris, sori of, 600 ; indusium of, 600

— serrulata, apogamy in, 605

RAP

- Pterocanium, 772, 776

Pterodactylus, bones of, 1014
Pterophorus, wings of, 923
Pteroptus, 936
Ptilota, 560

Puccinia graminis, 566
Puff-ball, 576

Pulvilli of insects, 924 ; cockroach, 924

note
Pumice, 1014

Pupa of Neuroptera, circulation in, 918

— stage of fly, 931
' Purple laver,' 561

Purpura, method of examination of egg-
capsules of, 863 ; supplemental yolk of r
931

— lapillus, nidamentum of, 858 ; develop-
ment of yolk-segments of, 861, 862

1 Puss-moth,' eggs of, 929
j Pycnogonida, 881 ; related to Arachnida,.
883 note
Pyrola, seeds of, 649
Pyroxene, alteration of, 1001

— andesite, 999

Q

Quadrula symmetrica, 671, 672
Quartz-porphyries, 995
Quartzite, 1001

Quekett (E.) on Martin's microscope,.
189 ; cn production of raphides, 621 ;
on preparation of tracheae of insects,
921 ; on minute structure of bone,.
1013

Quekett's loup-holder, 204
' Quills ' of porcupine, 954
Quinqueloculina, 727

R

Radials of An ^edon, 825
Radiating crystallisation, 1017
Radiation of light in different media, 53-

58 ; in air and balsam, 55-57
Badiolaria, collecting, 459 ; skeleton of,

659, 773-777; fossilised forms of, 771,

HQ note; central capsule of, 772; zoo-

xanthellse in, 773 ; bibliography of, 778'
— colonies of, 773 ; distribution of, 778 ;

mounting, 778
Radiolarian shells in ' ooze,' 1008
Rainey, on presumed cause of cattle

plague, 677 ; on molecular coalescence,.

1021

Ralfs on British desmids, 509 note ;

classification, 515 ; on Nitzschia and

Bacillaria, 535
Ralph on dehydration by carbolic acid,

450

Ramsden circles, 107

Ramsden's eye-piece, 43 ; ' screw micro-
meter eye-piece,' 227, 228 ; positive eye-
piece, 323 ; micrometer eye-piece, 325

Baphidece, 527

Raphides of Phanerogams, 620, 621

INDEX

RAT

of plants and sponge- spicules compared,
784

Rat's intestine, villi of, 986
Rays, scales of, 952

Reagents, mode of labelling bottles, 345
Real image, 14 note ; formation of, 23, 24

— object image, 321
Recomposition of light by prisms, 18
Red ant, integument of, 898

— blood-corpuscles of Vertebrata, 958 ;
size of, in various Vertebrata, 959 ; re-
lative sizes of, in various Vertebrata,
960

— coral, 801

— corpuscles, flow of, 980

' Red snow,' due to Palmella cruenta,
486

' Red spider,' 937
Red spots in Infusoria, 7C2
Reflector, Sorby's parabolic, 281
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, 445 ; of Canada balsam, 445 ;
of monobromide of naphthalin, 445; of
phosphorus, 445

Refractive index of sihcious coat of dia-
toms, 445

— indices of air, of cedar oil, of water, 60
Regulator, Reichert's, 393
Reichert's loups, 38 ; his objectives, 321 ;

his thermo-regulator, 393

Reindeer, hair of, 954

Reproduction in Actinophrys, 664 ; of
Actinosph&rium , 666; of Clathrulina,
667 ; of Muglypha, 671 ; of sponges,
781 ; of Canipa)iulariida, 794 ; sexual,
of Polyzoa, 831 ; agamic, of Entomo-
straca, 887 ; agamic, 930 ; of Acarina,
932

Reproductive organs of Acarina, 935 ;
of Arachnida, 935

Reptiles, lacunae in bone of, 946 ; cement
in teeth of, 950, plates in skin of, 950 ;
epidermic appendages of, 953 ; red
blood-corpuscles of, 958, 959 ; muscle-
fibre of, 973 ; lungs of, 987

Reseda, seeds of, 649

Residuary secondary spectrum, 313

Resins, solvents for, 441

Resolving power of objectives, 367

of object-glasses, 44 ; of lenses, 64 ;

of objective and numerical aperture,
75, 336

Respiration of insects, apparatus of, 918
Respiratory organ of spiders, 938
Bete mucosum, 966

Retepora, calcareous polyzoaries of, 833
Reticularia, 720

ROS

Retic u laria, characters of, (558; examples

of, 659-662
Reticulated ducts of Phanerogams, 623
Retinuhe, 907

Revolving nose-piece, Nelson's, 244
Rezzi on invention of compound micro-
scope, 127
Rhabdamniina, 738

— abyssorum, 740

Rh abdolith us pipa, 111

— sceptrum, 771
Rhabdom, 907
RhabdopJeura, 833
Rhamnus, stem of, 628
Rheophax sabulosa, 740

— scorpiurus, 740
Rhinoceros, horn of, 957
Rhizocarpea0, 606
Rhizoids of mosses, 594
Rhizome of ferns, (500
Rhizopoda, 658-674, 770

— protoplasm of, 461 ; ectosarc of, 464 ;
Archer's papers on, 677 ; Biitschli on,
677 ; skeletons of, 720 ; sarcode of, 942 ;
pseudopodial network of, 977

Rhizosolenia, 543

— cyclosis in, 517
Rhizostoma, 798, 800
Rhizota, 717

Rhododendron, pollen-grains of, 647
Rlwdospermece, 503
Rhodospermin, 560
Rhodosporea>, 554
Rhopalocanium ornatum, 773, 776
Rhubarb, stellate raphides of, 621 ; spiral

ducts of, 623
Rhynchoflagellata, 694 note
Rhynchonellida?, shell structure of, 851
Ribbons of sections, 408
Ribes, pollen-tubes of, 648
Rice, silicified epiderm of, 640
' Rice-paper,' 611
Rice-starch, 620

Riddell's binocular microscope, 96, 97
Ring-cells, 386, 387

Rivalto (Giordano da) on invention of
spectacles, 120

Rivulariacece, hormogones of, 490

Roach, scales of, 952

Rocliea falcata, epiderm of, 639

Rock, ground-mass of, 995; fluxion-
structure of, 996

Rocks, method of making sections ofy
991 ; metamorphism of, 999

Rodents, hair of, 954

Roe-stone, structure of, 1011

Root of Phanerogams, structure of, 6257
636 et seq.

Root-cap, 635

Rosalina varians, 723

Rose, glandular hairs of, 689

Ross (Andrew) on correction of object-
glass, 19-21 ; his early form of achro-
matic microscope, 150 ; mechanical
movements of his stage, 151 ; his fine
adjustment, 151, 161 ; on illumination of
objects, 250 ; his arrangement for loek-

■1 A

1090

INDEX

ROS

ing coarse adjustment, 301 ; his achro-
matic objectives, 305-306 ; his lever of
contact for testing covers, 381
Boss, model, 177

— and Co.'s tank microscope, 221
Ross-Jackson model, 178
Ross's ' Jackson ' microscope, 151
Ross-Wenham's radial microscope, 178,

180

Ross-Zentmayer model, 178
Botalia, 749 ; intermediate skeleton of,
750

— aspera, in chalk, 1009

— Beccarii, shell of, 722

— Schroeteriana, 750
Rotalian series, 748
Botaliince, colour of shell, 724
Rotaline shells of Foramimfera, 722

— shell, sandy isomorph of, 739
Rotating disc of objectives, 241
Botatoria, 678. See Rotifera
Botifer vulgaris, 713

Rotifera, preserved by osmic acid, 428 ;
collecting, 457 ; keeping alive, 458 ; as
food of Actinophrys, 663 ; divisions
of, 678, 712-719, 867, 869 ; habitats of,
713 ; structure of, 714-717 ; mastax of,
715 ; lorica of, 715 ; contractile vesicle
of, 716; males of, 717; eggs of, 717;
classification of, 717 ; desiccation of,
718, 887 ; bibliography of, 719 ; wheel
apparatus of, compared with velum of
gastropods, 860, 863 ; winter eggs of,
888 ; non-sexual reproduction of, 930

Rotten- stone, 546

' Round worm,' 868

Rousselet's binocular portable micro-
scope, 200; his tank microscope, 224,
225 ; his compressorium, 295 ; his live-
box, 295

Rowland's reversible compressor, 295
Bugosa, 801

Bumia cratcegata, eggs of, 929
Rush, stellate tissue in, 612
Rutherford on freezing process, 419
Rutile in clastic rocks, 998
Ryder's microtome, 344

S

Sabellaria, tubes of, 872
Sable, hair of, 954
Saccammina in limestone, 1012

— Carteri, 737

— spherica, 737
Saccharoviyces cerevisice, 574

Sac char omycetes, 574; zymotic action of,

574 ; endospores of, 575
Saccolabium guttatum, spiral cells of

618

Sachs on Chara, 509 note
Sago, starch-grains of, 620
Salicylic acid, for mounting, 442
Salivary glands, 971
Salmon, scales of, 952

— disease, 569

SCH

Salpce, diatoms in stomach of, 544, 555

Salpidie, 835

Salpingazca, calyx of, 689

Salt solution as a preservative medium,

" 442

Salter (J.) on the 'teeth' of Echinus,
814

Salvia verbenaca, spiral fibres in seeds
of, 618

Sand-grains surrounded by silica, 999
' Sand-stars.' See Ophiuroidea
' Sand-wasp,' 898

Sandy isomorphs (Foraminifera), 739

— tests of Litnohda, 739
Santonine, crystallisation of, 1017
Sap-wood, 629

Saprolegnia, 493, 494 note

— ferox, 569
Saprolegnice, 569
Saprophytes, 575

Saprophytic organisms, study of, 280

— Bacteria, 585

— fungi, 562
Sarcocystids, 677

Sarcode, 460 note, 461 ; of Bhizopoda,
658

Sarcolemma, 973

Sarcoptes scabiei, 937

Sarcoptidce, mandibles of, 933 ; maxillae

of, 934 ; hairs of, 934 ; legs of, 934 ;

characters of, 937
Sa,rcoptiri(F, 937
Sarcosporidia, 674
Sargassmn bacciferum, 559
Sarsia, (Medusa of Syncoryne), 793
' Saw-flies,' ovipositor of, 927
Saxifraga, seeds of, 649

— umbrosa, parenchyme of, 613
Saxifrage, cells of pollen-chambers, 645
Scalariform ducts of ferns, 599 ; as

modified spiral ducts, 623
' Scales,' covering epiderm of leaves, 639 ;
of Ela'aqnus, 639

— of Lepidoptera, 899, 900 ; of Coleo-
ptera, 899 ; of Curcidio imperialis, 899 ;
of Lyccenidce, 899, 901 ; of Pieridce,
899 ; as tests for objectives, 900 ;
of insects, markings of, 900 ; of
Thysanura, 901 ; on wing of Bepido-
ptera, 923 ; of fishes, 950 ; of reptiles,
950, 953 ;

Scallops. See Pecten
Scarabcei, antennpe of, 912
' Scarfskin,' 965

Scatophaga stercoraria, eggs of, 930
Scencdcsm its, megazoospores of, 496
Schists, 1000, 1001

Schizogenous spaces in Phanerogams, 613
I Schizomycetes, 579-589
Schizone/ua, 528

— Grevillii, 547

— mucous sheath of, 518, 547
Schizonemece, character of, 547
Schnetzler, on movement of Oscillaria,

190

Schott (Dr.) on improvement of object-
glasses, 31

INDEX

1 09 I

SCH

Schroder on binocular vision, 107 ; his
fine adjustment, 100; his camera
lucida, 236

Schultz's method of macerating vege-
table tissues, 625

Schultze (Prof. E.), his aquarium micro-
scope, 222

Schultze (Prof. Max) on identity of

' sarcode ' and ' protoplasm,' 460 note;

on cyclosis in Diatomacece, 517 ; on

affinity of Carpenteria, 747
Schulze (Prof. F. E.) on soft parts of

Euplectella, 785 note
Schwendener on lichens, 577
Scirtopoda, 717, 718

Scissors, spring, 396 ; for section cutting,
397

Sclerenchyme of ferns, 600
Sclerogen, 621

Scleroses in Fungi, 562; of Myxomycetes,
565

Scolopendrium, indusium of, 600; sori

of, 600 ; sporanges of, 601
Scorpions, 881, 932
Screw-collar adjustment, 309
Scrophularia, seeds of, 649
* Scyphistoma ' of Cyanea, 799
Scytonema, as gonid of lichen, 579
Scytonemacece, 490 ; hormogones of,

490

Scytosiplion, conjugation of, 556
Sea-anemone. See Actinia
Sea-anemones, intracellular digestion in,
787

Sea-fans, 801. See Gorgonice
' Sea-jellies,' 777
Sealing-wax varnish, 384
4 Sea-mats,' 832. See Flustra and Mem-
■ branipora

Searcher eye-pieces, 323
4 Sea-slugs.' See Doris, Eolis
' Sea-urchin,' 808. See Echinus
Sea-weeds, 554

— continuity of protoplasm in, 469

— red, 503

Secondary minerals, 1001

— spectrum, 19, 31 ; overcome by Abbe's
objectives, 314

Section cutting, scissors for, 397

— lifters, 431, 432 ; cover glass as, 432

— mounting, 447

Sections, ribbons of, 408 ; of hard sub-
stances, 420 ; of bones, 420, 423 ; of
coral, 420, 423; of enamel, 420; of
fossils, 420; of shells, 420; of teeth,
420, 423 ; of hard and soft substances
together, 423 ; of Phanerogam tissues,
624

Sedum, pollen-grains of, 646; seeds of,
649

Seeds, 609, 648

Segmentation of Gastropoda egg, 859 ; of

annelid body, 872
Setter's solution for cleaning slides, 380
Selaginella, archegone of, homology of,

610

Selaginellecr. 607

SHR

Selective staining, 431

— stains, 436
Selenite, 270

— with mica film, 271

— stage, 270

Selenites, 262 ; blue and red, 271
Selligue's achromatic microscope, 146,

148 ; objectives, 303
Semi-apochromatic objective of Leitz,

320

Sempervivitm, seeds of, 649
Seneca, on magnifying by water, 120
Sense, organs of, in Mollusca, 864
Sensory nerves, 976

— organs of sponges, 780
Sepals, 643

Sepia, pigment-cells, 866
Sepiola, eggs of, 866

' Sepiostaire ' of cuttle-fish, structure of,

853 ; imitations of, 1023
Septa in shell of Foraminifera, 721, 728,

729

Serialaria, presumed nervous system in,
831

Serous membrane, 965, 966
Serpula, tubes of, 872
Serricomes, antennae of, 911
Sertularia cupressina, 795
Sertulariida, gonozoliids cf, 794 ; zoo-

phytic stage of, 801
Sessile cirripeds, 891
Seta of Tomopteris, 877
' Sewage fungus,' 583
Sexual fructification, 470

— generation of Volvox, 483

I Shadbolt on structure of Arachnoidiscus,
541

Shadbolt's turn-table, 386, 391
Shadow effects, 61
Shark, dentine of, 947
Sharks, scales of, 952
Sheep-pox, 588
Sheep-rot, 869

Shell, bivalve, of Ostracoda, 889

— calcareous, of Reticularia, 658 ; of
Microgromia, 661

— silicious, of Dictyocysta, Costonella,
700

— of Foraminifera, 721-726 ; oiLamel-
libranchiata, 843 ; of Brachiojjoda,
843

Shellac cement, protection against cedar
oil, 384

' Shell-fish,' 843. See Mollusca
Shells of Mollusca, nacreous layer of,
843, 846, 848 ; prismatic layer of, 844,
845,847, 848; colour of, 845; an ex-
cretory product, 846 ; sub-nacreous
layer of, 847, 848

— of Brachiopoda, 849 : periostracum of,
850 ; perforations of, 850

— of Gastropoda, structure of, 852

— of Cirnpedia, 892

' Shield' of Ciliata, 700

Shrimp, concretionarv spheroids in skin

of, 1021
Shrimps, skeleton of, 893

4 a 2

1 092

INDEX

SID

Side reflector, 281

— lever, short, fine adjustment, 162
Swift's vertical fine adjustment,

162, 181

Siebold on agamic reproduction in bees,
930

Sieve-plates, 635

Sieve-tubes, 635 ; in Exogens, 622
Sigillarice, 607, 1005
Silene, seeds of, 649

Silex in Equisetacece, 605 ; in epiderm of

grasses, 639
Silk glands of spiders, 939
' Silk-weeds,' 499
' Silkworm,' eggs of, 929
Silkworm disease, 573
Silpha, antennae of, 912
Simple magnifier, 37

— microscope, 201
Sines, law of, 3

Siphonacece, 491-493 ; Munier-Charles

on fossil forms of, 493
Siphonostomata, 889 note
Siricidce, ovipositor of, 927
Sirodot on alternation of generations in

Batrachospermnm, 504
Skate, muscle fibre, 973
Skeleton, dermal, of Vertebrata, 950 ;

fossilised, 1012

— fibrous, of sponges, 781

— silicious, of Heliozoa, 659 ; of Badio-
laria, 771

— of sponges, 779; of zoophytes, 786;
of Echinoidea, 808 ; of Asteroidea,
815; of Ophiuroide a, 815; of Crinoidea,
816; of Holothurioidea, 818; of Ante-
don, 825; of Vertebrata, structure of, 944

Skin, 965 ; pigment-cells in, 966 ; capil-
laries in, 986
Skip-jack, antennae of, 911
Slack on the costae of Pinnularia, 546
Slack's optical illusion, 370
Slide-forceps, 393
Slide-glass, 379
Slides for cultures, 288, 289

— Seiler's solution for cleansing, 380
Sliding-plate of objectives, 241
Sloths, fossil, teeth of, 948

Slug. See Limax

Slug's eye, 865

Slugs, Botifera in, 713

Smell, organ of, in insects, 924

Smith and Beck's microscope, 153, 154

Smith's Cassegrainian microscope, 144 ;
his reflecting microscope, 144

Smith (H. L.) on Tolles' binocular eye-
piece, 103 ; his vertical illuminator,
284, 285 ; on classification of diatoms,
527

Smith (James) on use of bull's-eye with
high powers, 280 ; his separating lenses,
309; his mounting instrument, 394

Smith (T. F.) on markings of diatoms,
522

Smith (W.) on cyclosis in Diatomacea*,
517; on species of diatoms, 530 note;
on habits of diatoms, 548

SPH

Smith (W. H.) on structure of frustules,
519 note ; on movements of diatoms,,
531

Snail, 854 ; eye of, 865. See Helix

— muscle of odontophore, 974
Snake, lung of, 987
Snapdragon, seed of, 648
Snell's ' Law of Sines,' 49
Snow, crystals of, 1016

Snowberry, parenchyme of fruit of, 613
Snowdrop, pollen-grains of, 647
Soda, caustic, action on horny substances,
440

Soemmering's simple camera, 234
Sole, scales of, 950, 951, 952
Solen, prismatic layer in, 848
Solid cones of light for minute observa-
tion, 362

— eye-pieces, 42, 322

— image, 95

- — objects, delineation of, 83 ; correct
appreciation of, 88

— vision, the consequence of oblique
illumination, 61

Sollas on sponges, 783 note ; on the ex-
tensions of the perivisceral cavity in
Polyzoa, 851

Sorby (H. C.) on microscopic structure
of crystals, 990

Sorby's parabolic reflector, 281

Sorby-Browning's micro- spectroscope,
272, 273

Soredes of lichens, 577

Sori of ferns, 600

Sound-producing apparatus of crickets,
923

Spatangidium, 539
Spatangus, spines of, 813
' Spawn ' of mushroom, 575
Spectacles, invention of, 120
Spectra, diffraction, 67

— artificial, 274
Spectral ocular, Zeiss's, 276
Spectro-micrometer, bright-line, 274
Spectroscope in micro-chemical opera-
tions, 1024

Spectroscopic test, 273
Spectrum, 19 ; irrationality of, 19

— binocular, microscope, 276
j — map, 275

— natural. 274

— of dark lines, 273 ; of bright lines, 273
Speculum, parabolic, 281 ; Lieberkiihn's,

282-284 ; in Smith's illuminator, 284,
285

Spermathecae of Gamasida1, 936 ; of

Tip 'og lyp h idee, 9 3 I '>
Spermatia of Puccinia, 567 ; of lichens,

578

Spermatic fluid, preservative for, 443
Sperm-cells, 467 ; of Volvox, 483 ; of

ferns, 603 ; of sponges, 781 ; of Hydra,

790 ; of Polyzoa, 831
Spermogones of Puccinia, 567 ; of

lichens, 578
Sphacelaria, 555
Sphacele, 555

INDEX

1093

SPH

Sphcpria in caterpillars, 574
Splueroplea annulina, 500, 501
Sphcei-ozosma, rows of cells in, 512
Sphcerozoum ovodimare, 777
Sphagnacece, 598, 599
Sphagnum, leaf of, 598
Sphenogyne speciosa, winged seed of,
649

Spherical aberration, 14, 15, 31, 249,

251, 254, 331
diminished by Huvghens' objective,

42

Spheroidal concretions of carbonate of

lime, 1021
SphingidcF, antennae of, 912
Sphinx, eye of, 911; antennas of, 912

— ligustri, eggs of, 929
Spicules of alcyonarians, 804

— of sponges, 772 ; their names, 783-784

— silicious, of sponges, 781

— calcareous, of sponges, 781
Spiders, 881, 932, 938; microscopic objects

furnished bv, 938 ; spinning apparatus,
939

Spinal cord, Hill's method of preparation

of, 434
Spindle fibres, 468
Spinnerets of spiders, 939
Spiny lobster, metamorphosis, 893
Spiracles of insects, 919, 920
Spiral cells in Phanerogams, 618 ; mode

of preparation of, 619

— crystallisation, 1018

— focussing for projection-lens, 324

— vessels of Phanerogams, 622, 623 ;
observation of, in situ, 644 ; of plants
compared with tracheae of insects, 919

SpiriferidcB, perforation in shells of, 851
Spiriferina rostrata, shell of, 851
Spirillina, 744

— sandy isomorph of, 739
Spirillum, movement of, 375; granular

spheres of, 588 note

— undula, 586

— volutans, movement of, 581, 583, 586
Spirit, dilute, as a preservative medium.

442

Spirochete, 581

Sprogyra, 478 ; attacked by Vampy-
rella, 654

Spirolina, a varietal form of Peneroplis,
728

Spiroloculina, 727
Spinda, 853

— shells of, bearing Protomyxa, 652
Spirulina, movement of, 490
Splachnum, sporange of, 594
Splenic fever, 588

— due to Bacillus anthracis, 582
Sponge-spicules, 781-784

— mounting, 450

— in Carpenteria, 747 ; in mud of Levant,
1007

Sponges, 779-786 ; skeleton of, structure
of, 779, 780 ; reproduction of, 781 ;
habitat of, 785 ; preparation of, 785,
786 ; bibliography of, 786 ; pseudopodia

STA

of cells in, 786; intracellular digestion
in, 787 ; fresh-water form of, 787
Spongilla, 785
Spongolithis acicularis, 550
Spongy parenchyma of leaves, 641
' Spontaneous generation, 686
; Sporange of Fungi, 562 ; of Marchantia,
590, 593; of mosses, 596; of Sphag-
nacece, 599; of ferns, 600 ; of Equise-
tacece, 605 ; of Myxomycetes, 565
Sporangia of Lycopodiacece in coal, 1006
Sporangiophores of Mucorini, 569
Spore, use of the term, 467 note
Spores of Nostoc, 491 ; of Myxomycetes,
563, 565 ; of Peronosporea, 568 ; of
Bacteria, 587; of Marchantia, 593;
of mosses, 597 ; of ferns, 601 ; of ferns,
method for studying development of,
604 note ; of Equisetacea, 605 ; of
Lycopodiea?, 606 ; of gregarines, 675 ;
of Monas L-allingeri, 682; of Lycopo-
diacea? in coal, 1006

— different kinds of, 470 note

— resting, of Cha>tophorace&, 583
Sporidsof JJstilaginea, 565; oiPuccinia,

566

Sporocarp of Ascomycetes, 572

Sporogone of mosses, 597

Sporophores of Myxomycetes, 565 ; of

Peronosporea?, 568 ; of Ascomycetes,

571

Sporophyte in ferns, 605
Sporozoa, 674-677

Sporules of Melosira, 526 ; of Pleuro-

sigma, 526; of Podosphenia, 526
Spot-lens, 267
Spring-clip, 394

— press, 394

— scissors, 396

' Spring-tails,' 903. See Podurida
Squid, 866

Squirrel, hair of, 954, 955

Stag-beetle, antennas of, 912

Stage, horse-shoe, Nelson's, 163, 190; of
the microscope, 165-168 ; concentric,

■ rotatory motion of, 167 ; qualities need-
ful in a, 167 ; in Hartnack's model,
211 ; mechanical, 215 ; graduated
rotary, 338

forceps, 287

micrometer, 226, 230, 239, 240

— moist, 290

plate, glass, 288

— thermostatic, 292, 293

— Turrell's, 165, 189 ; Tolles', 166, 184 ;
Zeiss's, 167

vice, 287

'Staggers' of sheep, due to Ccenurus, 868
Stahl on movement of desmids, 510
Staining, process of, on glass slides, 430,
431 ; multiple, 438 ; double, 438 ; me-
thods, 439 ; differential, 439

— Bacteria, 437, 438

— fluids, 432-437

— processes, 430

Stains, violet of methanilin for Bacteria,
437 ; methyl-blue for Bacteria, 638

1094

INDEX

STA

Stains, Nicholson's blue, 489

Stanhope lens, 37

Stanhoscope, 38

Stcyphylinus, antennae of, 912

Star-anise, tissue of testa of, 617 ; testa j

of seeds of, 649
Starch, tests for, 440 ; formation of, 619

— grains, 464, 465 ; mode of growth, 620 ;
hilum of, 619 ; in Carina, 620 ; in
potato, 620 ; in wheat, 620 ; in rice,
620

' Star-fish,' 815. See Asteroidea
Statospore of Protomyxa, 653
Staurastrum, binary division of, 512;
form of cell, 515

— dejectum, 498
Stauroneis, 546

' Stauros ' of Achnanthes, 545
Steenstrup on alternation of generation s,
801

Stein on affinities of Volvox, 479 ; on
contractile vacuoles of Volvox, 481
note; on Flagellata, 689; on Nocti-
luca, 694 note ; on Acinetina, 699 note

Steinheil's loups. 38 ; his combination of
lenses, 38 ; his aplanatic loup, 205 ; his
loup for tank work, 234 ; his formula
for combination of lenses, 316 ; his
triple loups, 322

Stellaria, seeds of, 649

— media, petals of, 644

Stem of mosses, 594 ; of Bryacece, 598 ;
of Sphagna eece, 598; structure of, in
Phanerogams, 625 ; of Phanerogams,
development of, 634, 635; treatment of,
for examination of their structure, 636,
637

Stemmata of insects, 910 ; of spiders, 938

Stentor, collecting, 457 ; impressionable
organs of, 702 ; contractile vesicle of,
704 ; conjugation of, 711

Stephanoceros, collecting, 457 ; in con-
finement, 458

Stephanolithis spinescens, 771

— nodosa, 111

Stepha uvspJt cera phi rial is, amcebiform

phase of, 485 note
Stephenson on Pleurosigma. angulatinn,

70 ; on ' intercostal points,' 73

— his suggestion on homogeneous im-
mersion, 28

— on Coscinodiscus, 538

Stephenson's binocular, 100, 844 ; sub-
stage condenser, 101 ; erecting binocu-
lar, 102 ; erecting prism, 102 ; cata-
dioptric illuminator, 170, 268; binocular
dissecting microscope, 201, 203, 395 ;
tank microscope, 220

Stereocaulon ramvlosus, 579
Stereo-pseudoscopic microscope, Na-
chet's, 208

Stereoscope, 90 ; Brewster's modification
of, 91

Stereoscopic binocular, Wenham's, 98;
for study of opaque objects, 105, 107

— eye-piece, Abbe's, 103

— vision, 89-98

SUN

Sterigmata of Puccinia, 566
Sterile cells of Volvox, 483
Stibbite, 998

Sticliopms Kefersteinii, 819
Stick-net for marine work, 459
Stickleback, parasite of, 890 ; circulation

in tail of, 981
Stigmata of insects, 919, 920
' Stinging hairs ' of nettle, 689
Stings of insects, 926, 927
Stipe of diatoms, 517, 518 ; of Licmo-

phora, 584 ; of Gomplionema, 545
Stolon of Foraminifera, 721 ; of Eozoon,

764 ; of Laguncula, 828; of ascidians..

838

Stomach, follicles of, 971
Stomates, 640

— of Marchantia, 591
Stomopneustes variolaris, spines of, 812
Stone-cavities in crystals, 997
Stone-mit3, eggs of, 938

Stones of fruit, preparing sections of, 624

— of stone fruit, constitution of, 618
Stone-wort, 505

Stony corals, resembled by polyzoaries..
828

Stop, introduction of, 87 ; in the eye-
piece, 42 ; use of, 261, 263

Strasburger's borax carmine, for study of
embryo-sac, 435

Strawberry, parenchyme of fruit, 618

Streptocauhis pulcherrimus, 795

Striated muscle, 972 ; size of fibres in
different groups, 973

Striatella unipuiictata, 527

Striatellece, characters of, 536

' Strobila ' of Cyanea, 799

Stromatopora, doubtful character of,
767

Stromatoporoids, 742 note
StrophoincnidiC, perforations in shells of,,
851

Stylodyctya gracilis, 775
Suberous layer of bark, 633
Sub-nacreous layer in molluscan shells,
847, 848

Sub-stage, 169-171, 215; Nelson's fine
adjustment to, 169; Powell and Lea-
land's, 170, 174 ; swinging, 170, 171,
184 ; in Ross-Zentmayer's model, 178 ;
in Beck's microscope, 181 ; centring-
nose-piece used as, 11)8

' Sub-stage condenser,' Nelson's, 72 ;
Stephenson's, 101 ; compound, 135

— illumination, 248

— simplest form of, 261
Succulent plants, stomates in, 640
Sucker on legs of Sarcoptidce, 934
Suckers on foot of Dytiscus, 925 ; of Cur-

cnlionidce, 926
Suctoria {Protozoa), 696-699

— {Crustacea), 890, 891

' Sugar-louse,' 901. See Lepisma
Sulphuric acid, as a test, 440
' Sun-animalcule,' 662
' Sundew,' glands of, 639
Sunk-cells, 388

INDEX

SUP

Super-amplification, 33
Super- stage, 169

Supplemental yolk in Purpura, 862, 863,
931

Surirella, 518, 535; conjugation of, 528 ;
zygospores of, 529 ; movements of, 531 ;
frustule of, 535

— biseriata, cyclosis in, 517

— caledonica, 551

— constricta, 536

— craticula, 551

— plica ta, 551
Surirellece, 535

Suspensor of ovule of Phanerogams, 464

Sutural line of desmids, 590

Swarm- spores, 466 ; not a new genera-
tion, 467 ; meaning of term, 470 note ;
of Pandorina, 485 ; of Hydro diet yon,
495; of Cutleria, 556; of Clathrulina,
667 ; presumed, of Pelomyxa, 670

Sweat-glands, 966

' Sweetbread,' 971

Swift's side-lever, 158 ; vertical side-
lever fine adjustment, 162, 181 ; micro-
scopes, 181, 190, 194, 197; portable
microscope, 198 ; low-power condenser,
252 ; condenser for polariscope, 262 ;
sub-stage illuminator, 271 ; micro-
spectroscope, 275; live-box, 295; petro-
logical microscope, 992
Symbiosis in lichens, 578
Synibiotes trvpilis, hairs of, 934
Symbiotic algae in radiolarians, 773
Sympathetic nerves, 978
Symphytum asperrimum, seeds of, 649
Synalissa symphorea, 579
Synapta digit at a, ' anchors ' of, 819

— i/nhcerens, 'anchors ' of, 819
Synapta?, rotifers in, 713
Syncoryne Sarsii,- gonozob'ids of, 792
Syncrypta, 475

Synedra, 535
Syringamm ina , 736

Syringe for catching minute aquatic

objects, 300
Syrup, as a preservative medium, 442

— and gum, as a preservative medium,
443

T

Tabanus, 911 ; ovipositor of, 927

Tabellaria vulgaris, 551

Table of numerical apertures, 84-87

— for microscopists, 341-345 ; for dis-
secting and mounting, 342

Tactile papillse of skin, 966 ; nerve to,
977

Tadpole, pigment-cells of, 967 ; circula-
tion in tail of, 980 ; general circulation
in, 981 ; blood-vessels of, 983, 984

— of ascidians, 841
Tadpole's tail, epithelium of, 968
Taenia, 867

Tank microscopes, 219-225

Tannin, test for, 440

Tapetal cells in fern antherid, 603

THA

' Tape-worm,' 8(57

Tardigrada, desiccation of, 8t>9

Tarsonemida;, 937

Taste, organs of, in insects, 917, 924

Teeth, decalcification of, 426

— fossilised, 1012

— in palate of Helix, 854 ; of Limax,
854 ; of Buccinum, 854 ; of Mollusca,
854

— preparation of, 947

— of Echinus, 814 ; of Ophiothrix, 816 ;
of Vertebrata, 947

— of elephant, Rolleston on enamel in,
852 ; of Podentia, Tomes on enamel in,
852

Tegeocranus cepheiformis, 932

— dent at us, 932

Tegumentary appendages of insects, 898
Telescope, Barker's Gregorian, 144
Teleutospore generation of Puccinia,
566

Temperature, effect of, on various

monads, 686
Tendon, 943

Tentacle of Noctiluca, 691, 692

' Tentacles ' of Drosera, 639 ; of Suctoria,

697 ; of Hydra, 788 ; of annelids, 873
Tenthredinida*, ovipositor of, 927
Terebella, tubes of, 872 ; gills of, 873

— couchilega, 872
Terebratula bullata, shell of, 851
Terebrutuhe, shells of, 849, 850
Terpsinoe musica, 537
Terpsinoece, character of, 537
Tertiary tints in crystalline bodies, 1018
Tesselated epithelium, 968

Test of Gromia, 660; of Arcella, 670;

of Difflugia, 671
Testa of seeds, 649
Testaceous amcebans, 670, 671
Testing object-glasses, 325 ; diaphragm

for use in, 329 ; Fripp's method, 330 ;

Abbe's method, 326-333 $ £ f
Test-plate, Abbe's, 330, 331
Tests, sandy, of Lituolida, 739
.Tethya, spicules of, 1008
Tetramitus rostratus, life-history of,

685 ; nucleus of, 688
Tetranychi, 937
Tetranychus, mandibles of, 933
Tetraspores of Floridea*, 561; of Vam-

pyrella, 655
Textularia, 24.H

— aculeata, in chalk, 1009

— glolndosa, in chalk, 1009
Textularian form of shell, 723

— series, 748
Textulariida, 736

Textularinice, arenaceous character of,
748

Tlialassicolla, 772, 777

Thallophytes, 467, 470

Thallophytic type, passage to cormo-

phytic, 594
Thallus of Ulva, 488; of Ph&osporccr,

555 ; of lichens, 577
Thaumantias Esclischoltzii, 797

1096

INDEX

THA

Thaumantias pilosella, 797
' Theca ' of mosses, 596
Thecaphora, 792
Thecata, 792, 794

— zoophytic stage of, 801
Thermo-regtilator, Reichert's, 393
Thermostatic stage, Dallinger's, 292-293
Thoma's (Jung) microtome, 401
Thompson (J. Vaughan) on pentacrinoid

larva of Anteclon, 825 ; on Cirripedia,
891

Thomson (Wyville) on development of
Anteclon, 827

Thread-cells of zoophytes, 701 ; of Hydra,
788 ; of Zoantharia, 801, 802 ; of pla-
narians, 871

' Thread-worm,' 868

Threads of spiders' webs, 939

Thurammina papillata, 738, 740

Thwaites on conjugation of Epithemia,
529 ; of Melosira, 530

Thysanura, scales of, 901

Ticks, 932. See Acarina

Tineidce, wings of, 923

Tinoporus baculatus, 749

Tipula, eye of, 911; antenna? of, 912;
spiracle of, 920

Tolles' binocular eye-piece, 102 ; his me-
chanical stage, 166, 184; his vertical
illuminator, 285 ; his water-immersion
objectives, 310 ; his apertometer, 333

Tomes (Charles) on teeth, 949

Tomopteris onisciformis, 876, 877 ; de-
velopment of, 878

— quadricornis, 878

' Tongue ' of Gastropoda. See Palate
' Tortoise-shell butterfly,' eggs of, 929
Torula cerevisice, 574
Total reflexion, 6, 7
Tourmaline, pleochroism in, 1002
Tow-net, 458

Tow-nets of Challenger Expedition, 459

note

Tracheae of insects, 918 ; of Acarina,
935

Trachei'desof ferns, 600; of conifers, 622,
628

Trachelomonas, 475

Tradescantia virginica, cyclosis in hairs

of, 615, 616
Tragulus javanicus, red blood-corpuscle

of, 960
Trematodes, 8(59
Triceratium, 518, 520

— experiments with, 357, 358
— favus, 542

— markings on, 522

— fimbriatum, as test for higher powers,
332

Trichocysts of Ciliata, 701

Trichoda lynceus, crawling of, 702 ; re-
production of, 707-709

Trichodina grandinella, a phase in de-
velopment of Vorticella, 707

Trichogyne of Floridece, 561 ; in lichens,
578

Trichonympha, 702

UND

Trichophore of Floridece, 561
Trichophrya, a phase in development of

Suctoria, 698
Trigonia, prismatic layer in, 848
Triloculina, 727
Triple-backed objectives, 310
Triplet, Holland's, 37
I Triplex front to objectives, 317
Tripoli stone, 546

Trochus zizyphinus, palate of, 855
Trombidiiclce, 932 ; legs of, 934 ; hairs

of, 934 ; eyes of, 935 ; tracheae of, 935 ;

characters of, 936
Tronibidium, maxillae of, 934 ; larvae of,

937

— holosericum, 937
Trophi of Botifera, 715
Truncatulina rosea, colour of, 724
' Tube-cells,' 382

Tube-length, English and Continental,
155

Tuberculosis, bacillus of, 437 ; Pittion

and Roux's method of staining, 439
Tubifex rivulorum, gregarine of, 676
Tubipora, 801

Tubularia, gonozooids of, 793

— indivisa, 793

Tubuli in Nummulites, 751; of dentine,
948

Tubitlipora, 833

Tulip, r aphides of, 621

Tully's achromatic microscope, 147 ; his
live-box, 294 ; his triplet, 303 ; his
achromatic objective, 303

' Tunic ' of Tunieata, 835

Tunicata, 828, 835-842 ; zoological posi-
tion of, 835 ; bibliography of, 842 ;
' liver' of, 971

Turbellaria, 867, 870

— larvae of, collecting, 459
Turbinoid shell of Foraminifera, 722
Turbo, shell structure of, 852
Turkey-stone, use of, 421 ; constituents of,

546

Turn-table, Shadbolt's, 386, 391 ;

Griffith's, 391
Turpentine, uses of, 441
TurreH's mechanical stage, 165, 189
Twin lamellae in leucite, 1002
Tylenchus tritici, 869
Tympanum of cricket, 923
Typhoid of the pig, due to Bacillus, 588
Tyroglyphi, nymph of, 933 ; legs of,

'934

T yroglyphldce, reproductive organs of,
936; characters of, 937

U

Ulothrix, conjugation of, 486
Ulva, 488, 489
TJlvacea;, 487

Umbelliferous plants, seeds of, 649
Umbonula verrucosa, 830
Under-corrected objective, 20, 21
Under-correction, 255, 307, 308

INDEX

IO97

UNG

Unger on the zoospores of Vaucheria,
492

Unicellular plants, 469

TJnio, pearls in, 847 ; glochidia of, 857

— occidens, formation of shell in, 849
UnionidcB, nacreous layer of, 847
Unit (standard) for microscopy, 400
TJredinece, 565 ; alternation of genera-
tions in, 565

Uredo-form of Puccinia, 567
Uredospores of Puccinia, 567
Urinary calculi and molecular coalescence,
1023

Urine, micro-chemical examination, 1024

Urochordata, 835

JJropoda, tracheae of, 935

' Urticating organs.' See Thread-cells

TJstilaginece, 565

JJvella, 475

V

Vacuoles in cell, 464

— contractile, in protophvtes, 465 ; of
Volvox, 481

— of Actinophrys, 662
Vagine of mosses, 596
Valentin's two-bladed knife, 398
Vallisneria, habitat, 613, 614 ; mode of

demonstration of cyclosis, 613, 614
Valvulina, shell of, 723
Vampyrella, 654, 655

— gomj)honemaiis, 655, 656

— spirogyrce, 654, 655
Vanessa, 911 ; haustellium of, 916

— urticce, eggs of, 929

Variation, range of, in Astromma, 774

Varley's live-box, 294

Varnish, test for, 383 ; asphalte, 383

Varnishes, 382; sealing-wax in alcohol,
384 ; red, 385 ; white, 385 ; various
colours, 385

' Vascular Cryptogams,' links with Pha-
nerogams, 607

Vascular papillae of skin, 966

Vaucher, on Siphonacece, 492

Vaucheria, 491-493

— Botifera in, 713

' Vegetable ivory,' endosperm of, 618
Vegetable substance, preparation of,
427 ; gum-imbedding for, 427 ; bleach-
ing of, 427 ; Cole's staining method, 436

— structures, hardened in osmic acid, 428
Veins of vertebrates, 980

Velum,' in gastropod larva, 860

Venice turpentine cement, for glycerin
mounts, 384

Ventriculites, 785, 1010

Venus' flower basket, 783, 784 ; spicules
of, 784

Verbena, seeds of, 649

Vertebrata, 835 ; bone of, 944 ; teeth of,
947 ; dermal skeleton of, 950 ; blood
of, 958 ; red blood-corpuscles, 958 ;
white blood-corpuscles, 960 ; distribu-
tion of ciliated epithelium, 968 ; kidney
of, 971

WAS

Vertebrated animals, 941

Vertical illuminator for ascertaining

' aperture,' 206
Vespida*, 911

Vibracula of Polyzoa, 834, 835
Vibrio, movement of, 375

— rugula, 586

' Vibriones,' as applied to certain nema-
todes, 869

Vibriones, form of, 581, 586

Vigelius on tentacular cavity of Polyzoa,
829 note

Vignal on osmic acid for Noctiluca, 428
Vine, size of ducts of, 623
Viola tricolor, pollen-tubes of, 648
Violet, cells of pollen-chamber, 645

— of methanilin, for staining Bacteria,
437

Virginian spiderwort, cyclosis in, 615,
616

Virtual image, 14 note, 24, 25, 321
Vision, depth of, 88, 89, 90 ; stereoscopic,
89

Visual angle, 27

Vitrea {Foraminifera), 744

Vitreous cells (arthropod eye), 907

— optical compounds, 31

— shells of Foraminifera, 724

' Vittae ' of Licmophorecs, 534 ; of seeds

of umbellifers, 649
Vocal cords, structure of, 964
Vegan's changing nose-piece, 244
Volcanic ashes, microscopical examina-
tion of, 101
. — dust, examination of, 999
Volvocinea*, 479-485
Volvox associated with Astasia, 690

— vegetable nature of, 484 note ; amce-
biform phase of, 485 ; Botifera in, 713

— aureus, cellulose in, 481 ; starch in, 481

— globator, 479-485; flagellate affinities
of, 479; contractile vacuoles in, 481 ;
endochrome of, 482 ; multiplication of,
483 ; reproductive cells of, 483, 484

Vorticella, foot-stalk of, 701; contrac-
tion of foot-stalk, 702 ; fission of, 704 ;
' gemmiparous reproduction of, 711 ;
conjugation of, 711

— microstoma, encystment of, 706, 707
Vorticellina, encystment of, 706

W

Waldheimia australis, shell of, 850
Wale's coarse adjustment, 185 ; his fine

adjustment, 185 ; his limb, 185, 189
Wallflower, pollen-grams of, 647
Wall-lichens, 577

Wallich, on structure of diatom frustule,
519 note ; on Triceratium, 543 note ;
on Cha>tocerece, 544 note; on cocco-
spheres, 672 ; on Polycystina, 776 note

Wallich's plan for sectionising a number
of hard objects, 421 note

' Wanghie cane,' stem of, 626

Ward's simple microscope, 205

1098

INDEX

WAR

' Warm-stage' for observing blood-cor-
puscles, 958

Warmth, mode of applying, for cyclosis,
616

Wasps, wings of, 922, 923 ; sting of, 927
Water, refractive index of, 3, 7

— distilled, for mounting Brotojplujtes,
442

— milfoil, 458
Water-angle, 50
Water-bath, 392
Water-boatman, wings of, 924

< Water-fleas,' 883, 886
Water-globules in oil, 370, 871
Water-immersion objectives, 310 ; Zeiss's,

317

Water-lily, leaf • structure of, 642; cells
of pollen-chambers, 645

< Water-mites,' 937

' Water-net,' or Hydrodictyon, 495
Water-of-Ayr stone, 421
Water- scorpion, 919. See Nepa
' Water- snail.' See Limnceus
Water- vascular system of Tcenia, 867
Wavellite in Mija, 848
Wax and olive oil for imbedding, 417
Web of spiders, 939
Weber's annular cells, 299
Webster condenser, 256
Weismann on development of Dijjtera,
931

Wenham on binocular vision, 107 ; on
cyclosis of Vallisneria, 615

Wenham's suggestion of homogene-
ous immersion, 29 ; his stereoscopic
binocular, 95-98 ; his prism, 99 ; his
reflex illuminator, 265, 266 ; his disc
and button illuminators, 266 ; his
paraboloid, 267-269 ; his achromatic
objective with single front, 810; his
duplex front objective 311

West on Chcetocerece, 544 note

' AVhalebone,' 957

Wheat, starch -grains of, 620

Wheatstone's stereoscope, 90 ; his
pseudoscope, 92

' Wheel- animalcules,' 678, 712. See

RoTIFEBA

Wheel-like plates of Chirodota, 820
' Wheels ' of Botifera, 713
Whelk. See Buccinum
' AVhite ant,' ciliate parasite of, 702
White blood-corpuscles of Vertebrata ,
960 ; flow of, 980

— fibrous tissue, 963, 964

— of egg, as a preservative medium, 44 2
Whitney's directions for examination of

frog's circulation, 984
Wild clary, spiral fibres of, 618
Williamson (W. C.) on Volvox, 484 note;

on structure of fish-scales, 951 ; on

structure of coal-plants, 1006
Willow-herb, emission of pollen-tubes,

647

Wing of Agrion, 918
Winged seeds, 648

Wings of insects, 922-924 ; of Btero-

ZOO

2)horus, 923 ; venation of, in Neuro-
ptera, 922

Wodderborn on Galileo's invention of
.compound microscope, 123

Wodderborn's ' perspicillum,' 127

Wollaston's doublets, 36, 151 ; his
camera lucida, 234

Wood, arrangement of, 625, 627 ; concen-
tric rings of, 628 ; fossilised, 630, 1005

Wooden slides for opaque objects, 390

Woody fibre, 621

— tissue of ferns, 599

Working eye-pieces, 323

Worms, 867-880

X

Xylem of Exogens, 622, 623, 685
Xylol-balsam as a preservative medium,
442

Yeast, 574; fermentation due to, 574
Yellow cells, in Actiuice, 773 ; in radio-
larians, 773

— fibrous tissue, 968, 964

Yolk-bag of young fl,sh, circulation on,
981

Yucca, epiderm of, 687 ; guard-cells of
stomates in, 640, 641

■

Z

Zanardinia, swarm-spores of, 556
Zea Mais, epiderm of, 637 ; stomates of,
640

Zeiss's oil-immersion objectives, 29 ; his
eye-pieces and objectives, 34 ; his
mechanical stage, 167 ; his dissecting
microscope, 205 ; his photographic
microscope, 211, 212 ; his latest mi-
croscope, 218 ; his calotte nose-piece,
242 ; his sliding objective changer, 243 ;
his iris-diaphragm, 246, 248 ; his apla-
natic loup, 201 ; his apparatus for mono-
chromatic light, 272 ; his spectral
ocular, 276 ; his apochromatic objective,
314-320 ; his water-immersion, 317 ; his
apochromatic, for resolving diatom
markings, 521 ; his apochromatic for
study of monads, 687

Zeiss-Steinheil's loups, 207

Zentmayer's microscope, 184 ; swinging
sub-stage in, 184

Zeolite, 1017

Zinc, chlor-iodide of, as a test, 440

— cement, Cole's, 385 ; Zeigler's, 385
Zoantliaria, 801

Zoea, 894

Zonal structure in crystals, 996
Xndchlorellaj of Heliozoa, 65!)
Zob'cytiumof Ojphrydium, chemical com-
position of, 700
Zob'glcea of Beggiatoa, 588

INDEX

IO99

zoo

5oogloeaB, 581
Zoophytes, 786-807

— mounting, 388, 08!)

— non- sexual reproduction of, 930
Zoophyte troughs, 297, 298
Soosporange of Vol vox, 483, 485
Soosporanges of Phceosporece, 556
Soijspores, 466 ; of Palmogloea, 472 ; of

Protococcus, 474, 475 ; of Palmodic-
tyon,-i87 ; of TJlva, 488 ; of Vaucheria,
492 ; of Achlya, 494 ; development of,
494 ; of Hijdrodictyon, 495 ; of Con-
fervacece, 500; of (Edogo)iium, 502;
of Cli'cetophoracece, 503 ; of Chytri-
diacece, 555 ; of Phceosporece, 556 ; of
Floridece, 561; of Fungi, 562; of
radiolarians, 773
Zoothamium, collecting, 457

ZYM

Zooxantlielloe in radiolarians, 773
Zouzygospores of Navicula, 526
Zukal on movement of Spirulina, 490
Zygnemacece, characters of, 477 ; habitats

of, 477 ; conjugation of, 478
Zygosis in Actinophrys, 665; of Amoeba,

669 ; of gregarines, 677
Zygospore, 467 ; formation of, 470 ; of

Hijdrodictyon, 495 ; in Desmidiacece,

513, 514

Zygospores of Palmogloea, 472 ; of Meso-
carpus, 478; of Spirogyra, 478; of
Pandorina, 485 ; of Ulva, 490 ; of
Navicula, 526; of diatoms, 528; of
Mucorini, 569

Zygote of Glenodiuium, 695

Zymotic or fermentative action of Fungi,
i 62

Date Due

In

10

- —

L. B. CAT. NO. !187

L