PRACTICAL HINTS

ON

THE SELECTION AND USE

OF THE

MICROSCOPE

INTENDED FOR BEGINNERS.

BY JOHN PHIN,
\\

EDITOR OF "THE AMERICAN JOURNAL OF MICROSCOPY.'

SECOND EDITION.
Fully Illustrated and Greatly Enlarged.

NEW YORK:
THE INDUSTRIAL PUBLICATION COMPANY

1877.

Entered according to Act of Congress, in the year 1875, by

THE INDUSTRIAL PUBLICATION COMPANY,
In th'S Ofiice of the Librarian of Congress, at Washington, D. 0.

The reader is requested to correct the following errata:
Page 21. — Line 10 from bottom, strike out rest of paragraph
after "This is," and substitute: "This is found only in some
forms of the microscope." NOTE. — The Jackson Model, shown
in Plate II, has a collar, and in most of the microscopes made
in France and Germany, the body moves through a collar.

Page 36.— Line 7 from bottom, for "Student" read "Pro-
fessional."

Page 41. — Line 4 from top, before the words "The Instru-
ment " insert " Other forms of."

Page 62.— Line 16 from bottom, for "effect" read "affect."

Page 88. — Line 12 from top, after the word "power," insert
"the want of."

CONTENTS.

PREFACE. --v

INTRODUCTION. ...... jx

THE MICBOSCOPE.

What it Is ; What it Does ; Different Kinds of Microscopes ; Essen-
tial Parts of the Microscope ; Names of the Different Parts, - 13

SIMPLE MICBOSCOPES.

Hand Magnifiers ; Watch-Makers' Eye-Glasses ; Engravers' Glasses ;
Coddiugton Lens; Stanhope Lens ; Stands for Simple Micro-
scopes ; Raspail's Microscope ; Excelsior Microscope ; Twenty-five
Cent Microscopes ; Penny Microscopes ; Craig Microscope ; Fused
Globule Microscopes— Novelty, Globe, etc., - ... 23
COMPOUND MICBOSCOPES.

French Vertical Microscopes ; the Continental Form ; the Jackson
Model ; the Ross Model ; Browning's New Model ; the Inverted
Microscope ; the Binocular Microscope ; the Binocular Eye-Piece, 34

OBJECTIVES.

Spherical Aberration ; Chromatic Aberration ; Corrected Objectives ;
Defining Power ; Achromatism; Aberration of Form; Flatness of
Field ; Angular Aperture ; Penetrating Power ; \N orking Distan -e ;
Immersion Lenses; L^ns Systems; French TnpLts; Focal
Length of tUe Numbers used to Designate Objectives by Nache',
Hartuack and Guudlach, ....... -41

TESTING OBJECTIVES.

General Rules ; Accepted Standards— Diatoms, Ruled Lines, Arti-
ficial S ar, Podur* ; Methods of Testing the Various Qualities;
Nobert's Lines , Mover's Probe Platte, 58

SELECTION OF A MICROSCOPE FOR PRACTICAL PURPOSES.

Must b^ A lapte-l to Requirements and Skill of User ; Microscopes
for B »t my ; For Pnysicians ; For Students Magnifying .Power
Required ; i'he Staud ; Tne Stage Mechanical Stage Revolving
Stage; Stages for Special Purposes, Sub-Stage, Mirror. Body;
Draw-Cub^; Adjustment for Focussing; The Diaphragm; Ob-
jectives ; High versus Low Angles ; Eye-Pieces, 69

ACCESSORY APPARATUS.

Stage Forceps ; Forceps Carrier ; Object Holder ; Plain Slides ; Con-
cave Slides : Watcu-Gtasses ; Watch-Glass Holder ; Animalcule
Cage; Z >"phyte Troug'i Cornpressorium ; Gravity Compressor-
iuui ; Growing Slides ; Frog Pldte ; Table ; Double "Nose-Piece, - 93

ILLUMINATION — SOURCES OF LIGHT.

Sun-Light ; Artificial Light — Candles, Gas, Lamps, Magnesium,
Oxyhydiogen Lignt ; Parallel, Convergent and Divergent Rays, 107

ILLUMINATION OF OPAQUE OBJECTS.

Diffused Light ; Bulls-Eye Condenser ; Side Reflector ; The Lieber-
kuim ; Smith's liluuiuiator ; The Parabolic Reflector, - - 111

rr CONTENTS.

ILLUMINATION OF TBANSPABENT OBJECTS.

Direct and Reflected Light ; Axial or Central Light ; Oblique Light ;
The Achromatic Condenser; The Webster Condenser— How to
Use; The Spot Lens ; The Parabolic Illuminator ; Polarized Light, 114

How TO USE THE MICKOSCOPE.

General Hints ; Simple Hand Magnifiers ; Compound Microscopes
Practical Notes on Illumination ; Monochromatic Light; Care of
the Microscope, --118

COLLECTING OBJECTS.

Where to Find Objects ; What to Look for ; How to Capture Them
Nets; Bottle-Holders ; Spoons; Collecting Walking Cane; Water
Strainer ; Wright's Tollectiug Bottle ; Aquaria for Microscopic
Objects ; Dipping Tubes, 129

THE PBEPABATION AND EXAMINATION OF OBJECTS.

Cutting Thin Sections of Soft Substances; Sections of Wood and
Bone; Improved Section Cutter; Sections of Rock; Knives;
Scissors ; Needles ; Dissecting Pans and Dishes ; Dissecting Mi-
croscopes ; Separation ot Deposits from Liquids ; Preparing whole
Insects ; Feet, Eyes, Tongues, Wings, etc., of Insects ; Use of
Chemical Tests; Liquids for Moistening Objects; Refractive
Power of Liquids ; lod-Serum ; Artificial lod-Serum : Covers for
Keeping oui Dust ; Errors in Microscopic Observations, - - 139

PRESEBVATIVE PBOCESSES.

General Principles ; Preservative Media — Canada Balsam, Solution
of Balsam, Colophony, Damar Medium, Glycerine. Glycerine Jelly,
Hantzsch's Fiuid, Glycerine and Gum, Deane's Gelatiue, Alcohol,
Thwaite's Fluid, Beale's Liquid, Goadby's Fluids, Pacini's Fluid,
Castor Oil ; General Rules lor Applying Preservative Fluids, - 152

APPABATUS FOB MOUNTING OBJECTS.

Slides; Covers; Cells; Turn-Tables—Plain, Matthews, Kinne's,
Cox's ; Cards for Making Cells ; Hot-Plate ; Lamps ; Retort
Stand ; Centering Cards Mounting Needles ; Cover Forceps ;
Slide Holder ; Water Bath ; Simple Form of Spring Clip, - - 160

CEMENTS AND VABNISHES.

General Rules for Using ; Gold Size, Black Japan, Brunswick Black,
Shellac, Bell's Cement, Sealing Wax Varnish, Colored Shellac,
Damar Cement, Marine Glue, Liquid Glue, Dextrine, - - 171

MOUNTING OBJECTS.

Mounting Transparent Objects Dry ; Mounting in Balsam ; Mount-
ing in Liquids ; Mounting of Whole Insects ; How to Get Rid of
Air-Bubbles ; Mounting Opaque Objects ; Wooden Cells; Pierce's
Cell ; Prof. Smith's Plan ; Deep Cement Cell, - - - 174

FINISHING THE SLIDES.

Covering with Paper ; Varnishing for Preservation ; Labeling ; The
Maltwood Finder, 179

PREFACE TO SECOND EDITION.

The fact that an unusually large edition of thi£ work has been sold in a
comparatively short period, is, to the author, evidence that such a work
was needed, and that the present volume has, to a certain extent, supplied
the want. In the present edition, therefore, he has endeavored to
introduce several important improvements, while at the same time the
elementary character of the work remains unaltered. With a few very
slight and unimportant exceptions, the entire matter of the former edition
has been incorporated in the present, and in addition several important
subjects, particularly the chapter on objectives, have been greatly enlarged.

Many important points still remain untouched, but it is believed that in
its present form most beginners will find in it all the information that they
may require upon general topics.

As the want of all illustrations of the stands of different makers, and
of many accessories, has been urged as an objection to the first edition,
and as we have not deemed it advisable to fully supply this omission in
the present issue, a word of explanation may not be out of place. One
great object in view in the preparation of this book was the furnishing of a
cheap manual for those who cannot afford the more expensive books of
Carpenter, Beale, Frey, etc. To have given anything like a fair repre-
sentation of the products of the different makers of this country and of
Europe, would have nearly doubled the size and price of the volume. But
if the reader will examine the illustrations of this class in the books just
mentioned, he will find that, even in the best of them, the engravings are
mere reproductions of the figures found in the descriptive lists of the
various dealers. As new editions of these lists are being constantly
issued, and as they may in most cases be obtained without cost from those

VI PREFACE TO SECOND EDITION.

that issue them, we have thought it best to refer our readers to these
catalogues for information in regard to the construction of the instruments
of different makers, and to facilitate their doing so, we have given at the
end of the book a list of the prominent makers with their addresses.

In this, as in the previous edition, we have omitted all descriptions of
objects, believing that the proper aim of a book on the microscope should
not be to teach the general principles of botany, zoology or histology, but
simply the best methods of using the microscope in the pursuit of these
studies. The proper books in which to find a description of objects, are
those which treat of that department of science which takes cognizance
of the special subject under consideration. The present volume is
intended merely as a guide to the best general methods of using the
microscope.

It has been a source of great satisfaction to the author to be^assured by
those whom he deems good authority, that this little book has done much
to foster the use of the microscope in this country, and he hopes that the
present improved edition will tend to still further increase the deep
interest which is already felt in an instrument which has done more than
any other to extend our knowledge of organic nature.

New York, August, 1877.

PREFACE.

The Microscope and its applications in tbe Arts, and in general science,
having deservedly occupied a prominent place in the pages of THE TECH-
NOLOGIST, OR INDUSTRIAL MONTHLY, a very large number of enquiries in
regard to the best methods of using and applying this useful instrument
have been directed to us. It would have been easy to answer these en-
quiries by a reference to some one of the many treatises that have been
published on this subject, but as most of these works are expensive, and
as many of our correspondents desire an aivs^?r in a more concise and
simple shape, we have endeavored to give, in cheap and compact form, the
information that is most usually demanded.

It is an unfortunate fact that while the microscope is daily growing in
favor with those who know anything of its achievements, the operations
of certain parties, too weh" known to the public, have brought a certain
degree of suspicion upon all attempts to popularize this most valuable
instrument. Microscopes, varying in price from twenty-five cents to two
dollars and a half have been offered for sale, and the claim made for them
that they are capable of showing clearly the structure of the more min-
ute tissues, and that they may be used to advantage by physicians and
naturalists. To the young student whose means are limited, and to the
country practitioner, whose ability to supply himself with needed books
and instruments often falls far short of his desires, the offer of a service-
able microscope for a couple of dollars is a great temptation, and when
the instrument in question is endorsed by a long list of clergymen, law-
yers, and even editors, this temptation becomes irresistible. And if the
purchaser should happen to be unfamiliar with really good microscopes,
and unable to discriminate between a clear and accurately defined view
of any object and one that is distorted and incorrect, he may be led to use
it, and so fall into the most serious mistakes. That this, unfortunately,
does happen too often must be well known to all who are familiar with the
subject, and it is within our own knowledge that the most worthless cari-

YHI

cature of a microscope has been purchased and used under such circum-
stances.

We indulge a faint hope that the information conveyed in the following
pages will enable the inexperienced reader to avoid these mistakes, and to
assign a proper value to the certificates of clergymen and editors who
vouch for the excellence of articles concerning whose properties and uses
they are profoundly ignorant. These two classes we single out for repro-
bation, because— in this respect, at least — they seem to be sinners above
all other men.

As stated in the title page, it is intended for beginners, and not for
beginners in the use of the microscope only, but for those who have had
little or no experience in the use of instruments of any kind. Hence the
directions that are given are of the very simplest kind, and all theoretical
explanations have been avoided, for the reason that any person that is
desirous of studying the optical principles upon which the microscope is
constructed will find in the ordinary text books on natural philosophy all
the information he may want. Our object has been solely to impart such
information as will enable the reader to make a beginning in the practice
of microscopy, hoping that the start thus given will lead him to proceed
with his studies, and ultimately acquire that knowledge, skill and dexter-
ity -which will enable him to avail himself of the extraordinary powers and
advantages which the use of this instrument confers, both in scientific
pursuits and in everyday life. Above all things, therefore, we have en-
deavored to be accurate in our statements and judicious iii our directions,
and the reader is assured that no processes or methods are given which
we ourselves have not frequently and successfully put in practice.

JOHN PHIN.

New York, January, 1875.

INTRODUCTION.

Thousands of microscopes throughout the country are at the pres-
ent day lying idle, simply because their owners do not know how to
use them. If properly employed they might be made to afford an
incalculable amount of instruction and amusement; but. as it is, they
are a, drag upon the popularization of science, because they convey
the idea that the microscope is a difficult instrument to use, and
that it is not of much account after we have learned to use it. The
owners of these microscopes have examined all the mounted objects
at their commind, the entire number of which probably does not
exceed two or three dozen, and they have no information as to the
best methods of preparing common objects for examination or pre-
servation. Even the objects that they possess have never been ex-
plained to them, an I are merely pretty toys. The fly's eye is inter-
esting because it looks like a piece of netting, and the butterfly's
wing is attractive because it is probably a little more brilliant than
the most brilliant silk dress, but neither of these objects interests of
itself and because of its beautiful structure.

Moreover it often happens that an instrument which, when first
purchased, was of very fair quality, has, through ignorance and
carelessn- ss, become so soil d and dimmed that it no longer serves
the purpose intended. O a more than one occasion have we seen a
fine microscope leave the dealer's hauds in excellent order, and re-
turn in a week entirely uufu for use. Microscopes in this condition,
instead of being a source of instruction and pleasure, are an eyesore
aud an occasion of auuoyance. They continually serve as reminders
of a.vkwarduess and failure, of wasted time and ill-spent money.
And yet with proper instruction and a due amount of care all this
might have been avoided.

It is aUo a fact to be regretted that heretofore the microscope has
not been extensively employed iu the arts, aud in everyday life,
simply because practical men have not been taught how to use it,
aud consequently have be^n unable to avail themselves of the advan-
tages which it offers; but if carefully aud judiciously selected, and
properly handled, it is capable of affording an amount and kind of
assistance which cannot be safely neglected. It may be made to aid
in the examination of raw materials, and of the finer kinds of work;
it will enable us to measure spaces which would otherwise be inap-
preciable, aud this, in an age when even in ordinary machine shops
the thousandth part of an inch is frequently an important quantity,
readers it indispensable to the careful and skillful mechanic; on the

X INTRODUCTION'.

farm it will enable the agriculturist to examine closely and minutely
the various noxious insects and forms of fungi and blight, and thus
aid him in identifying them aud applying the proper remedy; and
in the examination of minute seeds, such as timothy, clover, etc., it
will prove a very valuable assistant, enabling him to detect any in-
feriority in the quality, or any impurity or adulteration. Frequently
the agricultural seeds offered in market contain minute seeds of of-
fensive weeds, many of which are so small that they are not easily dis-
covered by the naked eye.

Every farmer aud mechanic knows the value of a good pair of
eyes, and he also knows that an agent which doub es or trebles our
power in any given direction at once confers upon us in that respect
a superiority over our fellows. Very few men are twice as strong as
their comrades; still fewer have three times the strength of ordinary
men, and it may be safely affirmed that no man posst-sses the power
of ten ordinary men. But a microscope of very ordinary capacity at
once multiplies our powers of sight by ten, twenty, or even a hun-
dred times, while those of the better class enable us to see things
with a keenness and clearness which, when compared with that af-
forded by the naked eye, is as more than a thousand to one.

There are four dibtiuct and importaut directions in which a mi-
croscope may be made to serve us: 1. It is capable of affording the
most refined and elevating kind of pleasure by the exhibition of ob-
jects of extreme beauty and interest. There are few more splendid
sights than the gorgeous colors displayed by some objects when
viewed by polarized light, and even the tints of certain minerals,
and the brilliant scales of certain insects, when viewed as opaque
objects, by means of a good condenser, surpass anything that is
familiar to us in our ordinary experience. On the other hand the
exquisite beauty of form which is characteristic of most of the ob-
jects with which the microscopist concerns himself can be fully ap-
preciated only by those who have seen them. As a source of inno-
cent amusement and pleasure, therefore, the microscope has few or
no equals; for it may be safely affirmed that a five-dollar instrument
is capable of affording gratification of greater variety and intensity,
and of longer continuance, than that yielded by anything else of
the same cost. This arises chiefly from the fact that most other in-
struments, when once exhibited, with their slides or fixtures, lose
their freshness and interest, and become old. While for the micro-
scope, a few fibres of wool from the carpet, a few grains of sand from
the sea-shore, or a huidful of wild flowers Iroin the field, yield ob-
jects of surpassing beauty. Everything in nature and in art may bo

iNTRODtTCTlOK. XI

subjected to inspection by it, and will then disclose new beauties
and fresh sources of knowledge. Under it the point of the finest
cambric needle looks like a crow-bar, grooved and seamed with
scratches; the eye of the fly is seen to consist of thousands of eyes;
and the dust on the butterfly's wing appears to be what it really is,
scales laid on with all the regularity of shingles or slates on a house;
while to prepare and examine these simple objects requires no great
skill and no elaborate apparatus.

2. As a means of imparting instruction to the young, the micros-
cope has now become indispensable. The changes which of late
years have taken place in the views held by our ablest men in regard
to the best education are too well known to need even mention.
No education that docs not include a knowledge of natural science
is now regarded as complete, and there is a very wide range of the
most essential and practical knowledge that can be reached only
through the microscope. Thus, when we look at a leaf with the
naked eye, we see but a green mass of matter, possessing a certain
beautiful form, it is true, bat disclosing none of those organs which
render it more complicated and wonderful than anything ever pro-
duced by our most skilful mechanics. Looked at by the micro-
scope, however, this same leaf is found to be made up of innumer-
able parts, each one of which is highly complex and beautiful; it is
furnished with mouths for breathing, with cells for storing, digest-
ing and assimilating nutriment, and with ribs for strengthening its
structure; and all this, which is perfectly invisible to the unassisted
vision, becomes distinct and obvious when we call to our aid a
microscope of even moderate power. It is true that much of this
may be taught by means of books, engravings and verbal descrip-
tions, but every one knows that for distinctness and impressiveness
the very best engravings fall far short of a view of the real object.

3. As an instrument of research, the microscope now occupies a
position which is second to none. There is hardly any department
of science in which a student can hope to reach eminence without a
familiarity with the microscope.- Botany and Zoology have been
developed almost wholly by its aid, aud so necessary is it in the
study of these sciences, that Schleiden, one of the most successful
of investigators says of it: " He who expects to become a botanist
or a zoologist without using the microscope, is, to say the least of
him, as great a fool as he who wishes to study the heavens without a
telescope." In chemistry its services have been very important, and
in geology and mineralogy it has opened up new fields of research
which almost promise to revolutionize these sciences. Medicine has

Xll INTRODUCTION.

long acknowledged the microscope as one of its most efficient as-
sistants, and in the practice of the best physicians it is regarded as
an indispensable means of diagnosis in some diseases.

4. As an assistant in the arts. Its importance in this department
is but just beginning to be recognized, 'and in a former paragraph
we have endeavored to point out a few of the subjects to which it
may be applied with good hopes of success.

These important and obvious advantages are not difficult to secure,
provided we avoid two mistakes which are very commonly made by
beginners. One of these consists in supposing that it is only by
means of very expensive and complicated instruments that anything
of value can be accomplished in microscopy. Now while it is cer-
tain that, in some departments of study, none but the very best
microscopes are of any value at all, it is equally certain that a very
wide range of study and of practical work can be thoroughly culti-
vated by means of apparatus of very moderate cost, aud of great
simplicity of construction. The great discoveries of Ehrenberg,
which opened up entire new fields of research and of thought, were
made with a microscope which at the present day would not com-
mand $25. ludeed some of the French instruments that are sold
for $15 will show a very large proportion of the objects that are
figured in his earlier works. Most of the great anatomical and
botanical discoveries were made with simple microscopes of no great
power, and it is not many years since one of the most successful
workers in the field of botany gave it as his opinion that a power of
300 diameters is capable of showing everything that is of impor-
tance in this science.

The other error is of precisely the opposite kind. It is not at all
unusual to meet persons who seem to think that all that is necessary
in order to become a microscopist is to buy a microscope and place
objects under it! Such people always entertain an exaggerated idea
of the power of the microscope as an instrument of research. For
example, they think that in order to detect adulteration all that is
necessary is to place.a sample under the microscope, when all im-
purities will at once stand out conspicuously! To their imagination
every blood corpuscle is clearly marked with the name of the animal
from which it was obtained!

Truth lies between these extremes. No progress can be made
without steady application and persistent labor, but any person of
fair average ability and a moderate degree of perseverance can soon
learn to follow the beaten track at least, if not to branch out into
original research.

THE SELECTION AND USE

OP

THE MICROSCOPE.

What is a Microscope 7— The microscope is an instru-
ment which enables us to see either very minute objects or
very minute parts of large objects. It is a very popular idea
that the name microscope is applicable only to complex instru-
ments of considerable power; but this is clearly wrong. A ten
cent magnifying glass has as good a right to the name micro-
scope as has a complicated binocular instrument with all the
latest improvements. By common consent, however, the small
hand instruments, without stands, are generally called magni-
fiers. An attempt has been made to introduce the foreign word
loupe as an equivalent of magnifier. The word loupe is, how-
ever, superfluous, and is used only by ostentatious pedants, and
by foreigners who are ignorant of English.

What the Microscope Does.— It is well known that
the further off any object is, the less it appears. A house at a
distance appears less than a man who is close by, and the dis-
tinctness with which an object is seen depends largely upon its
apparent size. Thus, at a distance, a house not only appears
very small, but the windows cannot be distinguished from the
rest of the building. As we draw nearer it becomes apparently
larger, and the different parts become more distinct. First the
windows are seen clearly, then the individual panes of glass,
then the bricks, and finally the grains of the material of which
the bricks are made. When, however, we approach too closely
we again find it impossible to see distinctly, as may easily be

14 SELECTION AND USE

proved by a very simple experiment. Place some fine print,
such, for example, as the present page, at a distance of six feet
from the eye, and gradually move closer to it. At six feet the
letters will be indistinguishable; at two feet they will be quite
distinct; at one foot still more distinct; at three inches they
will be quite blurred. There is, therefore, a limit to the degree
of closeness with which we can approach auy object for the
purpose of examining it, and the object of a microscope is to
enable us to get close to it, as it were, without blurring our
view. Ir', without changing the distance of the eye from the
paper (three inches) we introduce between the two a lens of one
inch focus, and bring it into proper position, we will find that
the indistinctness formerly complained of disappears, and the
object is now not only seen clearly, but appears^very much
magnified. That objects appear large in proportion to their
nearness to the eye may be thus shown: Take two slips of
paper printed with type of the same size (two clippings from a
newspaper answer well) and place one at a distance of ten
inches from the eye and the other at a distance of five inches —
the edge of the upper slip being placed so as to lie about the
middle of the lower one. In this way we can readily compare
the apparent sizes of the type on the two slips, and one will be
found to appear just twice as large as the other, though, of
course, we have the evidence of our senses to prove that they
are precisely of the same size. Moreover, as the usual distance
for distinct vision is about ten inches, in persons of middle
age, it will be found that a lens which enables us to view any
object clearly and distinctly from a distance of one inch, will
enable us to see it just ten times larger and ten times more dis-
tinctly than we could do when looking at it from a distance of
ten inches. A consideration of these facts led the late Dr.
Goring to propose the name engiscope as a substitute for the
word microscope — the word engiscope signifying to see things
at a very short distance.

The facts which we have just detailed must, however, be re-
garded as illustrations, rather than explanations of the action of
the microscope. It is evident that the power of a lens to in-
crease the distinctness with which any object is seen, depends
not only upon the action of the lens upon the rays of light, but
upon the influence which such modified light exerts upon the

6P THE MICROSCOPE. 15

organs of vision. Now, the eye, considered merely as an optical
instrument, is in reality a small camera obscura in which the
cornea, crystalline lens, and other transparent portions, combine
to throw upon the retina an image of external objects. That
the transparent portions of the eye do in fact act as a lens, and
throw a real image upon the retina or posterior portion of the
eye, is easily shown by taking the fresh eye of an ox and grad-
ually shaving off the coating at the back until it becomes
transparent. If the eye, so prepared, be then held towards
a window or any very bright object, a distinct but inverted
image of the window or other object will be see on the coat
of the eye. *

The action of the eye in this case is the same as that of a
lens, and the general mode of action of lenses under such cir-
cumstances m;iy be easily illustrated by means of a common
hand magnifier or even a spectacle glass. If the reader will
hold before a window, at a distance of, say, six feet, a sheet of
white paper, and will place a magnifier in front of the paper,
then by properly adjusting the distance between the magnifier
and the paper, a picture of the window will be thrown on the
latter. If the magnifier and paper be now removed to a dis-
tance of twelve feet from the window, the picture of the latter
will be only half as large as it was in the first place, and it will
also be found that the distance of the lens from the paper will
have to be readjusted and made less.

That the eye possesses this power of adjustment we are all
conscious, for we feel that if, when the eye is adjusted for the
distinct vision of distant objects, we suddenly look at those
which are near, the condition of the eye requires to be changed
before a distinct view can be had, and to make this change
requires an effort of whicli we are perfectly conscious.

When a lens is held in front of a sheet of paper, so as to
throw on the latter a distinct image of the objects in front of
it, the distance between the paper and the lens is called the
focal distance or focal length of the latter. This, as we have
just seen, varies with the distance of the object which gives the
image. In order, therefore, to secure a standard in this
respect the object selected is always one whose distance is so
great that it may bs practically regarded as infinite.

16 SELECTION AND USE

When we examine an object, first at a distance, and then
close at hand, we see it through the medium of two different
sets of rays, those in the latter case entering the eye in such a
direction that the image thrown on the retina is larger than the
image produced when the object is more distant. The lens acts,
however, by bending the rays so that the same set, which, if
allowed to pursue their natural direction would not produce a
distinct image, are caused to enter the eye in such a direction that
the image is large and clear. The manner in which the lens acts
to produce these effects is not difficult to understand. It is true
that the ultimate causes which produce these phenomena are
beyond our knowledge, but in this respect the ablest philos-
opher has very little advantage over the veriest tyro. It may
be difficult also for the general reader to follow the mathe-
matical demonstrations of the action of lenses. There are,
however, a few simple facts which are easily understood, or at
least demonstrated and accepted as facts, and "which, when
clearly and firmly grasped by the mind, render the construction
of the microscope comparatively easy of comprehension.

There are two ways in which the subject may be studied.
We may examine the facts experimentally, by using lenses
and actual eyes in the way we have described, or we may fol-
low the course of the rays as laid down in any good book on
optics. A combination of both methods will of course give
the clearest views on the subject, and we would therefore ad-
vise the reader to provide himself with a few lenses of various
degrees of curvature, and consequently of various magnifying
powers, and test all the statements made in the text. He will
thus acquire such a practical knowledge of the action of lenses
as can be obtained in no ether way. For this purpose the
cheapest lenses are good enough. One or two cheap magni-
fiers and a few glasses from old spectacles will serve every pur-
pose. The simplest methods of arranging such lenses will be
found in a note on a subsequent page, and although very ac-
curately made tools are required for the construction of ser-
viceable optical instruments, it will be found that a very
large number of simple but valuable experiments may be
worked out with the aid of a few wooden rollers and a little
paper and paste.

Otf TflE MlCEOSCOPB. 17

While the magnifying power of lenses depends upon their
focal length, this in turn depends upon the material of which
the lens is made, and also upon the curvature given to its sur-
faces. Lenses of precisely the same form, and made respect-
ively of diamond, flint glass, crown glass and Canada balsam
would possess different magnifying powers; the diamond mag-
nifying most, the flint glass next, crown glass next, and Canada
balsam least of all. On the other hand, of two lenses composed
of the same material, that which has the sharpest curvature to
its surfaces will magnify most. Now, on reflection, it will be
evident to even the least mathematical mind that lenses which
have very sharp or quick curves must of necessity be small.
Suppose the curve which bounds the figure of a lens has a
radius of half an inch, it is evident that the largest lens which
could be made with this curve would be one inch in diameter,
and then it would be a perfect sphere. Most lenses, however,
resemble thin slices off the spheres, or in some cases two such
slices joined together, so that the diameter of the lens is in
general greatly less than the radius of the curves which form
its surface. Therefore, we see that all lenses of high power are
of necessity small, and when lenses are required of very high
power they become so minute as to be handled only with great
difficulty. Indeed, before the modern improvements in the
microscope, many of the lenses used by scientific men were
nothing more than little globules of glass, brought to a round
form by fusion.

We have made this lengthened explanation of a very simple
matter because we have found amongst beginners in micro-
scopy a very general idea that large lenses are the most power-
ful " Send me one of your largest and most powerful mag-
nifiers," is an order with which every optician is familiar, and
yet such an order contains a contradiction in terms. A lens
cannot possibly be large and magnify greatly at the same time.

The Different Kinds of Microscopes.— Microscopes
are divided into two classes — simple and compound — the dif-
ference between them being purely optical, and not mechanical;
for a simple microscope may be very complex and expensive,
while, on the other hand, a microscope may be compound and

lg SELECTION AND USE

yet contain very few parts. Thus the little vertical French
microscopes, which cost only $2,50, are compound, although
very simple in construction, while a simple microscope, if bin-
ocular, and provided with all desirable adjustments, might be
a very complicated affair. The difference between simple and
compound microscopes is this: in the simple microscope we
look at the object directly, while in the compound microscope
we look at a magnified image of the object. In the simple
microscope, objects are always seen in their natural position,
while in the compound microscope they are inverted, and right
becomes left, and left becomes right. This makes it very diffi-
cult for beginners to work upon objects under the compound
microscope; and hence simple microscopes are almost always
used for dissecting and botanizing.

It is true that by adding more lenses, and making the instru-
ment still more compound, we can again invert the image, and
thus bring it back to its original and natural position, and
almost all the very expensive microscopes are furnished with
these extra lenses arranged in a piece of accessory apparatus
technically known as an erector. The distinguishing feature of
the compound microscope remains, however, the same. Certain
forms of the microscope, in which concave lenses are substi-
tuted for the usual convex form, also give erect images, but
this does not affect the general truth of the statement just
made.

Simple microscopes frequently consist of more than one
lens. Thus, in using the ordinary pocket magnifiers with two
or three lenses, it is usual to employ all the lenses at once, look-
ing at the object through two or three lenses at the same time
when a high power is required. In this case, however, the two
or three lenses are placed close together and act in the same
way as a single lens, with surfaces more sharply curved than
those of any of the lenses forming the combination. Under
such circumstances the image is not inverted, but if we now
separate the lenses sufficiently, we will find that on again bring-
ing the object into focus, the image is inverted and greatly
enlarged. Moreover, it will be found that the magnifying
power may be greatly increased by increasing the distance be-
tween the two lenses, and it will also be found that as the dis-

OF THE MICROSCOPE. 19

tance of the two lenses from each other is increased, the dis-
tance at which the combination is placed from the object must
be made less and vice versa.*

The early forms of the compound microscope consisted of
little more than the two lenses we have just described, but the
modern instrument, even in its simplest form, is a vastly more
complicated arrangement. In the best forms, for the lens next
the eye there is substituted an eye-piece consisting of two lenses
with a diaphragm between them, while the objective, or lens next
the object, is composed of from four to ten different pieces of
glass, forming two or more lenses, which are so arranged that
each shall correct the defects of the others, and this optical
combination is mounted on a stand which is sometimes a mar-
vel of mechanical ingenuity.

*The student who possesses a little mechanical genius and a desire to
become experimentally acquainted with the properties of lenses and the
construction of the microscope, would do well to procure a couple of cheap
lenses, say one of half inch focus, and one of about two inches focus, and test
by actual trial the statements made in the text. Such lenses may be con-
veniently arranged in a tube formed of writing paper and gummed on the
edges. All the most important properties and detects of lenses may be thus
illustrated and studied. By means of a little extra care, two such lenses, ar-
ranged as we have described, in tubes blackened on the inside, and mounted
on a little wooden stand, the focus being adjusted by sliding the tube hold-
ing the lenses within another tube, also of paper, will give not only a very
fair view of such objects as the wing of a fly, the scales on a butterfly's wing,
and even the barbs on the sting of a bee, but it will show the globules of
blood quite distinctly, and we have even given a very interesting exhibi-
tion of the circulation of the blood in the foot of a frog by means of a
temporary arrangement of this kind, which we put together for the pur-
pose of explaining to a little girl the construction of the microscope. We
would not recommend any one to use such a microscope for purposes of
work or study, because the fallacies to which it may give rise are too
numerous and too serious. But any boy, or even girl, who will undertake
the construction of such an instrument, cannot fail to obtain thereby an
amount of information which the perusal of volumes would not give. As
hints towards aiding our young friends, we may remark that our tubes
were made of the best stiff paper, rolled up tight and pasted only along
the outer edge. The lenses were secured in their places by being attached
to the bottoms of pill-boxes, holes being punched through to admit the
light. Pill-boxes with holes were also used for diaphragms to reduce the
effects of aberration. A piece of mirror reflected the light, and the sides
etc., of an old cigar box furnished material for the stand. Fifty cents
covered all expenses.

26 SELECTION AND USE

Essential Parts of tlie Microscope. — When a good
lens is he id steadily at a certain distance from an object which
is properly illuminated, this distance depending upon the form
and material of the lens, we are enabled to see the object clearly
and distinctly. When, however, this distance is either in-
creased or diminished, the object becomes blurred and indis-
tinct. The point at which vision is most distinct is called the
focus* of the lens, and when we are able to see it clearly the
object is said to be in focus; when the distance is either in-
creased or diminished, it is said to be out of focus. An object
is said to be within the focus when the lens is too near it, and
beyond the focus when the lens is too far away.

The performance of any lens depends greatly upon the ac-
curacy with which it is adjusted to the correct focal distance,
and the steadiness with which it is held there. For all ordi-
nary purposes, lenses which do not magnify more than ten
diameters may be very conveniently held in the hand without
any special means of support; but when the power is much
greater than this, or where, as in the compound instrument,
the microscope is bulky and heavy, it becomes necessary to use
some mechanical contrivance which will hold the microscope
steadily in its position in relation to the object, otherwise the
view becomes indistinct. Thus a good lens, magnifying from
thirty to forty diameters, will very readily show the individual
corpuscles or globules in the blood of the frog, provided it is
arranged on a steady support and accurately adjusted for focus.
If merely held in the hand the corpuscles will probably be in-
visible. Hence the importance of providing efficient means for
adjusting the focus and holding and illuminating the object,
and the object of the stand is to furnish these means in a com-
pact and convenient form. Every microscope, therefore,

*It is scarcely necessary to inform the reader that the focus described in
the text is uot precisely the focus of the lens itself, but the focus of a
compound lens of which the eye forms one element. Henco the focal dis-
tance varies with different eyos, and so do< 8 the apparent size of objects.
To short sighted people objects appear of larger size than th< y do to
persons of ordinary eye-sight. In working with the compound microscope,
we frequently find that different people require a different focal adjust-
ment.

OP THE MICKOSCOPE. 21

whether simple or compound must possess: 1. Certain means
for supporting the object and placing and maintaining it in
proper position; 2. Means for illuminating the object, whether
it be opaque or transparent; 3. Means for transmitting to the
eye an enlarged image of the object.

NAMES OF THE DIFFEEENT PAKTS.

The different parts which are employed for securing these
several ends, have been constructed of an almost endless variety
of forms, according to the fancies of the different makers and
the requirments of different microscopists. The following are
the names of the essential parts of a compound microscope of
ordinary construction. The names of the different parts of
the simple microscope are the same as those of the compound
microscope, but the latter has several parts which do not exist
in the former.

The Stand is the term properly applied to the entire frame
used for supporting and illuminating the object and carrying
the optical part, the latter consisting of the eye-piece and the
objectives. Stands are frequently sold separately, or furnished
with eye-pieces only — the purchaser making such a selection of
objectives as may best suit his special needs.

The Base or Foot is that part which supports the rest of the
stand.

The Body is the tube to which the eye-piece and objectives
are attached.

The Arm is taat part which carries the body.

The Collar is the tube in which the body moves. This is
found only in microscopes of the Continental form (Plate I)
and similar models.

The Staae is the plate upon which the object is placed for
examination.

The Eye-piece is the short brass tube, with its lenses, which
is next the eye. The eye-piece contains an Eye-Glass, which is
that next the eye; a Field-Glass, placed next the objective, and
a DiapJiragm, consisting of a brass plate with a hole through
it, and so arranged as to cut off the outer rays of light. The

22 SELECTION AND USE

tube in which these lenses are secured is in general movable,
and the best microscopes are furnished with several eye-pieces
of different powers. We may here remark that where a micro-
scope is furnished with several eye-pieces, the shortest eye-piece
gives the greatest magnifying power.

The Object- Glass or Objective is the lens or lenses which is
placed next the object. The term is frequently applied to the
glass plate or slide upon which the objecb is placed, but this
use of the word is entirely wrong, and tends to produce con-
fusion.

A Draw-tube is a secondary body which receives the eye-piece,
and slides within the main body like the draw of a telescope.
It enables us to increase the distance between the eye-piece
and the objective, and thus to change the magnifying power, as
explained in a previous paragraph.

The objective is moved to or from the object by two methods,
which are calltd respectively the coarse and fine adjustments.

The Coarse Adjustment is used for bringing tl.e objective ap-
proximately but rapidly into focus. To effect this the body
either slides through a tubular collar or is attached by means
of dovetail t-lides to the arm, and in either case it is moved up
and down either by a rack and pinion (or some substitute there-
for) or directly by hand.

The Fine Adjustment is employed for bringing the object
exacty into focus. It usually consists of a fine screw, which
moves either the entire body or the lower part of it. In
some cheap stands, the fine adjustment is effected by moving
the stage towards the objective.

The Mirror reflects the light, and causes it to pass through
the object aud the body of the instrument.

A Sub-stage is furnished with some instruments. It is used
for holding and centering various means of illumination.

Clips are springs attached to the stage for the purpose of
holding in place the glass slide or plate carrying the object.

The Object is that which is subjected to examination. It is
usually mounted upon

A Slide, or plate of glass, which is laicl upon the stage.

OF THE MICROSCOPE. 23

DIFFEKENT FOEMS OF THE SIMPLE MICKOSCOPE.

To describe the different forms in market, either of simple or
of compound microscopes, would require a large volume. We
shall therefore content ourselves with a description of certain
typical models which afford variety enough for all practical
purposes.

Hand Magnifiers.— These are so generally useful and
applicable that no person who attempts to work much with the
microscope can possibly do without one. They are found in
market in a great variety of forms, styles of mounting, and
price, and are too well known to need minute description.
Large lenses, magnifying two or three times, are mounted
singly, and used chiefly for the examination of pictures, and as
reading glasses; the smaller sizes of the same style serve for
the examination of fine engravings. Very small lenses of con-
siderable power, and simply mounted in a frame, are also sold
by most opticians. They are known as "watch-charms," and
magnify about fifteen diameters. We have also seen a very
powerful magnifier mounted in a little ring attached to a pair
of eye-glasses.

For the purpose of the student and naturalist, a very
excellent form is that which is shown as attached to the Excel-
sior microscope. It consists of three lenses, mounted in frames,
and enclosed in a case so as to be perfectly protected. Each
lens has a different focal length, and the three, when combined,
give a magnifying power of twenty-five to thirty diameters.
Being very portable, and possessing a variety of powers, it is a
favorite form of pocket microscope.

Magnifiers composed of two or more lenses, are to be had of
two very distinct kinds. The lenses may either be simply united
in one frame, without any special adaptation to each other, or
the instrument may consist of two or more achromatic
lenses combined together in a fixed and accurately deter-
mined relation. Examples of the former are found in the
ordinary two and three lens magnifiers we have just described;
the latter are not so common, since they are somewhat expen-

24 SELECTION AND USE

sive when well made. They are known as doublets and triplets,'
and one maker in this country, Mr. Tolles, of Boston, has
become famous for the excellence of the simple achromatic
microscopes of this class, made by him. These doublets and
triplets are altogether the most satisfactory simple microscopes
in use, and several firms make a specialty of their manufacture.
In addition to Mr. Tolles, the Bausch & Lomb Optical Com-
pany, under the direction of Mr. Gundlach, make a very ex-
cellent lens of this kind, and in England, Mr. Browning makes
a very excellent achromatic magnifier under the name of the
Platyscopic Lens.

Where two or more simple lenses are used together (without
being combined so as to form a compound microscope) the
power of the combination is always equal to the sum of the
powers of the separate lenses. Thus if we have a lens of half
an inch focus and one of one inch focus, one magnifying ten and
the other twenty diameters, the resulting power is thirty and
and not two hundred times. In the compound microscope, on
the other hand, the combination of an objective magnifying
twenty diameters with an eyepiece magnifying ten diameters,
gives a magnifying power of two hundred diameters.

Two or more lenses, properly adapted to each other and used
together, give results greatly superior to anything that can be ob-
tained from a single lens, at least so far as clearness and accuracy
of definition is concerned. But when used as a working or dis-
secting microscope, they are open to the objection that the dis-
tance at which they must be placed from the object is very small,
and hence it is frequently inconvenient to use them for work-
ing upon objects. Thus if we have a plano-convex lens of a
quarter of an inch focus, and one of three quarters of an inch
focus, and place them at a distance of a sixteenth of an inch from
each other, we will have a very good magnifier which will
enlarge objects about thirty-five to forty times, but we must
place it at but a very short distance from the object. If we
separate the lenses a little, the definition will be improved, but
the working distance, as it is called, will be diminished. Those
who have studied optics are quite familiar with these facts, but
the ordinary reader does not always think of them, and yet they,

OF THE MICROSCOPE. 25

are very important when we come to choose a microscope for
working or dissecting purposes.

Watch-Makers' Eye-Glasses.— These are well known,
and may be obtained of almost any power within the useful
range of a single lens. The eye-glass ordinarily used by watch-
makers magnifies about eight times, but glasses magnifying
twenty diameters are not uncommon. Glasses of the latter
power are usually doublets, that is, they consist of two lenses,
arranged together, one being of much longer f.ocus than the
other. If well-made they give excellent definition and a large
field, and, when mounted on a stand, are very serviceable as
dissecting microscopes*, especially in working upon coarse
objects, and picking out shells, the larger foraminifera, etc.
Their form enables us to support them by means of a small
wire ring, arranged as in a retort stand, and the large bell-
mouth of the frame prevents any light from entering the eye,
except that which has passed through the lens. They are very
cheap, and any intelligent boy can make a tolerable stand for
one. The same stand will answer for several glasses of differ-
ent powers.

Engravers' Glasses.— These are mounted in frames, in
the same manner as the watch-maker's eye-glass, but as they
are larger, and are therefore not so readily held in the eye,
after the fashion of the latter, they are always used with a stand
of some kind. Those of the best quality are, in general,
doublets, which give a large field of view, with very good
definition, and they are altogether the best microscopes for
examining banK bills, fine engravings and similar objects.

*The term dissecting microscope is applied to all microscopes used for
working upon objects under moderate magnifying powers. They are used
not only for dissecting, properly so called, but for the study of botany,
mineralogy, etc., as well as for numerous investigations in the arts. A
good microscope of this kin.d is absolutely indispensable to those who
hope to do more than merely look at objects prepared by others. In sub-
sequent paragraphs we describe some of the best instruments of this
kind. A complete dissecting microscope should be furnished with stand,
mirror, etc., and if the student can afford it, there should be some good
mechanical means of adjusting the focus,

26 SELECTION AND USE

The Coddiiigton Lens.— Whenever a power greater
than twenty diameters is required for examining objects, a Cod-
dington, if well made, will be found to be the best lens in use,
always, of course, excepting the carefully corrected doublets
and triplets previously mentioned. The price of the latter,
however, is in general four to eight times that of a good Cod-
dington. It has this defect, however, that the working focus
is very short, and therefore for a dissecting microscope a
doublet is to be preferred. In using a Coddingtoii lens, great
care must be taken to secure good illumination of the object,
and the shortness of the focus makes this difficult to those
who have had no experience.

The Stanhope Lens is similar in form to the Codding-
ton, but is very different in construction. It consists of a
cylinder or rod of glass, one end of which is rounded so as to
form a lens, while the other end is either flat or slightly curved.
The distance between the lens and the flat surface is exactly
equal to the focal distance of the lens. : Transparent objects,
such as the scales of insects, animalculse in water, etc., are sim-
ply placed On the flat surface of the glass cylinder, and when
looked at through it, they appear greatly magnified. It is
easily used, but can not well be employed as a working micro-
scope. It is this kind of lens that is used in the construction
of those watch charms in which a large picture is seen on look-
ing through a very small hole. The picture is a photograph
attached to the flat end of a small glass rod, the other end of
the rod being formed into a lens of exactly the right focal
length required to show the picture clearly and considerably
magnified. Lenses and photographs of this kind are usually
mounted as miniature opera-glasses.

Stands for Simple Microscopes. -For ordinary pur-
poses of examination, the magnifiers we have just described
serve very well when merely held in the hand, but their per-
formance is greatly improved when they are mounted on appro-
priate stands, which not only enable us to adjust the focus with
great accuracy, but which hold the lens steadily in relation to
the object, and thus prevent any necessity for that constant ad-
justment of the eye itself , which always occurs wiien a lenstrem-

OF THE MICROSCOPE. 27

bles. Of the simple microscopes in use there are but two that re-
quire attention— Raspail's and the Excelsior. For a description
of the elaborate dissecting microscopes of the London micro-
scope makers, as well as those of Nachet, and others, we must
refer to the large works of Carpenter, Beale, Frey, etc.

Raspail's 3Iicroscope.— In this instrument the magni-
fying glasses are supported by an arm which projects horizon-
tally from an upright column that screws into the top of a box,
in which the entire instrument is packed when it is not in use.
This column also supports a stage which may be moved up and
down by rack work, and a mirror for reflecting the light up-
wards through the object. This was the instrument so largely
used by Kaspail in his investigations into the structure of plants,
and described by him in his works, and hence it has been called
by his name. It resembles very closely the instrument called
the Society of Arts Simple Microscope, which is manufactured
by Mr. Field of Birmingham.

The Excelsior Microscope.— The accompanying en-
graving gives a very clear view of this microscope, which is
constructed as follows:

To one end of the lid of a small wooden case or box, is at-
tached one of the ends of the box; and when the lid is reversed
and turned upside down, it may be slid into the groove which
usually receives it, and then forms a stand for the lenses and
glass stage, as is shown in engraving. The lenses and stage are
supported by a steel rod, D, the lower end of which is hinged
to the lid, so that it may be turned down and lie in a groove
provided for it. When raised into the position shown in the
figure, it is held very securely in place by means of the button,
E; and this button also serves to retain it in the groove when
it is turned down. The glass stage, G, which is fitted into a
frame of hard rubber, slides easily on the stem, D, so as to be
readily adjustable for focus, while at the same time it may be
firmly fixed, by means of a set-screw, at any desired height,
and will then serve as a stage for dissecting purposes. The
frame which holds the lenses fits on to the top of the stem. A
mirror, H, is fitted into the case, and is readily adjustable by
means of the button shown on the outside, so that light may

28

SELECTION AND USE

be reflected tip through the stage when the objects to be ex-
amined are transparent; and when they are to be viewed by re-
flected light there is a dark ground of hard rubber (not shown
in the engraving) which is also carried by the stem, D, and
may be turned under the stage, so as to cut off all transmitted
light. Dissecting needles (K and L), with neat handles, fit
into appropriate grooves. When the lenses and stage are re-
moved from the stem they are readily packed in the case; the

THE EXCELSIOR MICROSCOPE.

stem is then turned down and held in its groove by the button,
E; the lid is drawn out of the groove, turned over, and re-
placed so that the vertical piece (C) closes the open end of the
box, and the whole thing is packed into a compass which readily
admits of its being carried in the vest pocket.

The lenses are well made, and being provided with a proper
diaphragm, great clearness of definition is secured. Two styles

OF THE MICROSCOPE. 29

of frame are sold, one containing two, and the other three
lenses, the latter being altogether the cheapest, in proportion
to the power furnished. The magnifying powers are about as
follows: With the lens of longest focus, five diameters; with
the lens of medium focus, eight diameters; with the lens of
shortest focus, ten diameters. When the lenses of shortest and
medium foci are combined the magnifying power is about
eighteen diameters; all three lenses together give a power of
twenty-five to thirty diameters.

In using a combination of two or more lenses, the lens of
shortest focus should always be placed nearest to the object.

As a dissecting microscope for botanical, entomological, and
physiological work, this instrument is very efficient and con-
venient. The glass plate is fitted into the stage so as to form
a cell capable of holding water, so that dissections may be
carried on under that liquid, or aquatic animals may be kept
alive and examined at leisure. The stage may also be turned,
so that the flat side will be up when so desired, in which posi-
tion it is most convenient for some purposes, such as dissec-
tions and the teasing out of tissues by means of needles. The
only serious defect in the Excelsior microscope is that it is not
sufficiently steady for ordinary work, the case which forms the
base or foot being, for portability's sake, made quite small.
This difficulty is, however, easily remedied by screwing the
case to a piece of pine board six inches long, four inches wide,
and three-quarters of an inch thick. A single small screw,
which does not deface the instrument, is sufficient, and when
the microscope is to be carried in the pocket it is easily de-
tached from its temporary stand. Its low price, $2.75, is a
strong recommendation.

A very serviceable stand for a simple microscope is easily ex-
temporized as follows: Procure a good sound wine cork and
bore two holes through it, the holes being at right angles to
each other, and to the axis of the cork. The holes should be
of the right size to slide easily, but firmly, on a wire about the
sixteenth of an inch in diameter. One piece of such wire is
stuck perpendicularly in a wooden foot, and serves as a stand
upon which the cork slides up and down; another piece of wire
has a ring at one end for holding the magnifier, while the

30 SELECTION AND USE

other end is thrust through the second hole in the cork and is
supported by it.

Whenever a piece of apparatus is to be supported steadily,
while at the same time it is necessary that it should be easily
moved and adjusted, nothing serves so well as a fine cork
sliding on a smooth wire.

Twenty-five cent Microscopes.— Before leaving this
subject it may be well to say a few words about those very cheap
microscopes which have been so extensively advertised. We
frequently see in the papers an advertisement in which some
person offers to send for twenty-five cents a microscope which
will show animalcules in water, globules of blood etc., etc., and
the question naturally arises, Are these microscopes good for
anything, or is the advertisement a swindle — the advertiser
taking the money and sending nothing in return?

As a general rule, those who send to such advertisers, receive
in return, a plate of brass or lead, with a glass bead fastened
in a hole in the centre. The glass bead is formed by fusion and
is frequently ground flat and polished on the side by which it
was attached to the thread or rod of glass from which it was
made, forming in such cases a hemispherical lens. Such
lenses are very easily made by any one. Take a strip of flint
glass, such as a piece of flint glass tubing, or a piece of glass
rod, draw it out to a thread in the flame of a spirit lamp,
fuse the end and allow it to gather into a drop. Give plenty of
time and a good strong heat, so that the surface of the little
globe may become well-fused and truly round. The best re-
sults are always obtained by holding the thread perpendicularly,
as when held horizontally the globule is apt to become dis-
torted. Make one or two dozen of these, and in separating
them from the glass rod leave about an eighth of an inch of
the latter attached to each globule, to serve as a handle, in the
next step of the process, which consists in inserting them to
about half their depth in a plate of cement, consisting of
shellac thickened with very dry and finely powdered pumice-
stone. To form such a wax plate, melt some shellac in a ladle
or large iron spoon, mix it carefully with as much powdered
pumice-stone as can be conveniently stirred in, remove it from

the fire, stir well until it begins to stiffen, and then pour it out
on a flat metal plate — the surface of a smoothing iron answer-
ing very well. The plate of cement should be from one-half to
three-quarters of an inch thick, and the little globules are easily
fastened into it by seizing them by the small handles left ou
them, holding them by a pair of forceps in a lamp flame until
they are hot enough to melt the cement, and then pressing
them in to about half their depth or a little more. When quite
cold they will be very securely held. The little handles, or tails,
are now nipped off with a pair of cutting pliers, and the glo-
bules ground all at once on a fine grindstone, or still better on
a metal plate charged with emery. When they have been re-
duced nearly to the surface of the plate of cement, they should
be ground with emery of the finest kind, and as soon as all
coarse scratches have been removed they should be polished on
a buff leather with crocus rnartis or putty powder. When finely
polished they may be removed from the cement by means of a
small chisel, and any cement that adheres may be dissolved off
by means of alcohol. They are then mounted in thin plates of
lead, brass, or, what is better still, vulcanite. Out of two dozen
such globules, carefully made and well polished, three or four
may be obtained that will give satisfactory definition, and it
was with such lenses that the early microscopists made many of
their discoveries. These men, however, took great pains in
making and polishing them, and rejected hundreds as unfit for
use. The objections to the microscopes of this kind, that are
ordinarily sold, are that they are badly made, and that good
and bad are sold together without any selection being exercised.
But, even if well made, they are very difficult to use, and very
unsatisfactory in their results, even in the hands of persons of
great skill. The polish of a fused surface never equals that of
a surface finely cut and polished, as eveiy housekeeper that is
familiar with common, and with cut glass, very well knows.
The fused surface of these little globes is, therefore, always
more or less, covered with stria3 or very minute ridges which
interfere with their defining powers, and we have described
thus minutely the process of their manufacture, rather for the
purpose of giving our readers such information as will enable
them to understand how they can be sold so cheaply, than in
the hope that they will endeavor to make them for themselves.

82 SELECTION AND tTSfi

Peony Microscopes.— A few years ago a man in London
made a living by selling through the streets a microscope which
would show the eels in paste and vinegar, and of which the
price was only one penny, (equal to two cents. ) These micro-
scopes were thus made : In the bottom of a pill-box he punched
a small hole and then blackened the inside of the box. In this
hole was placed a drop of Canada balsam or damar varnish,
which was allowed to dry. When hard, the balsam formed a
very tolerable lens.

A drop of water, balsam, or varnish, laid on the under side
of a slip of clear glass will often enable us to extemporize a
microscope capable of doing good service in the hands of a
skillful observer. The outline of the drop should be perfectly
round, and the glass plate should be held as level as possible. We
have derived great assistance from such a lens, when better
could not be had.

The Craig Microscope.— This microscope at one time
attained an unprecedented degree of popularity, not on account
of its merits, but because of the extensive puffing and adver-
tising which it received. It consists of a vertical frame, some-
what like that of the cheap French microscopes, having a
mirror, but no sliding tube, as there is no occasion for any. The
slide which holds the object is slipped through a horizontal slit
cut in the stand, and the lens with its frame is laid on it.

The lens is a fused bead of glass set in a little frame, to the
under side of which is attached a thin plate of glass, whose
lower surface is exactly in the focus of the bead, so that when
a drop of water or vinegar is placed on the glass plfite, or such
objects as insects' scales, wings, etc., are laid on it, they are ex-
actly in focus. Hence, this microscope is said to require no
adjustment for focus. This is true when the objects to be ex-
amined are actually in contact with the glass plate, but when
we wish to examine objects that are covered with thin glass (as
all valuable preparations should be) or objects having a percep-
tible thickness, it is impossible to adjust it for focus, and hence
it is impossible to examine such objects satisfactorily. Besides
this, nine-tenths of the microscopes of this pattern in market,
are very badly made, and distort objects to such an extent that

OF THE MICROSCOPE. 33

one who has been accustomed to employ a good microscope
cannot recognize them. It has unquestionably done a great
great deal to impede the progress of microscopy in this country,
and we have been led to give this extended description of it,
chiefly because so many editors and clergymen have praised it
in the highest terms. It has even been patented, although the
principle upon which it is constructed is very old; but then we
must remember that under our present administration the
patent office seems to be conducted rather for the dtscourage-
ment than the encouragement of progress and invention. We
daily see patents issued for old and worthless devices, while it
is well-known that the author of a really meritorious inven-
tion will have the hardest work to obtain protection.

Of the Novelty, Globe, and other similar microscopes, it is
unnecessary to speak. In all the microscopes of this kind that
we have seen, the optical part is utterly worthless. The lenses
are mere fused globules of glass, and they distort beyond recog-
nition the image of any object.

Strange to say, however, even this fact has been used as an
argument to sell them. They have been sold chiefly by news-
dealers and stationers, and as the purchasers did not know how
any given object ought to appear, the fact that it looked so very
different from what they expected was considered an evidence
of the power of the microscope ! !

In regard to all microscopes in which fused globules are
used, it must be remembered that the lower the power of the
lens the more apt it is to be imperfect. No lens of this kind,
magnifying from 100 to 150 times, (according to the estimates
of those who deal in them, which, however, is in fact only from
ten to twelve times, as measured by proper methods), can be
good for anything. On the other hand, it must be borne in
mind that when we attempt to examine objects under high
powers, obtained by the use of very small single lenses, we
subject our eyes to an almost destructive strain.

34

SELECTION AND

DIFFERENT FORMS OF THE COMPOUND MICRO-
SCOPE.

The variety of styles or patterns which have been devised for
the stands and for the general arrangement of compound micro-
scopes, is almost infinite, and as they are continually changing,
it would be a hopeless task to attempt to give such a descrip-
tion as would be of real value to the reader, and since, from
motives of self-interest, the manufacturers of these instruments
promptly publish full descriptions and engravings of new styles
as soon as they bring them out, the best plan for those who
desire to make a judicious selection is to procure the catalogues
of as many manufacturers as possible, and carefully compare
the several advantages of the different forms. To facilitate
this, we give, at the end of the volume, the
addresses of most of the prominent makers.

There are, however, a few typical styles or
models to which it may be well to call at-
tention, and of these we give engravings
and such descriptions as will enable the
reader to form some idea of the stand best
suited to his special wants. In selecting
illustrations of the different types, we have
taken the cheaper forms in preference to the
more perfect, but more expensive models.

French Vertical Microscopes. —

This form, although modern when com-
pared with the microscopes of Adams,
Baker, etc., is one of the oldest forms in
use. It was, we believe, devised by Wol-
laston, as a stand for his doublet, and it
is now too well known to need elaborate
description, and as no microscopes of any
value are ever constructed upon this plan,
it is unnecessary to point out its defects.
The smaller sizes are still sold extensively;
and being manufactured in large quanti- VERTICAL MICROSCOPE.

Plate I.

CONTINENTAL MODEL.
As made by Geo Wale, Paterson. N. J.

[To face page 35.

Otf THE HICSOSCOPB. 35

ties, they are sold very cheaply, when the quality of the lenses
is taken into consideration. Therefore, until some manufacturer
concentrates his efforts upon the production of the more con-
venient forms, and turns them out in very large numbers, the
vertical microscope will probably maintain its place in the
market, and many beginners will be led into buying an instru-
ment which, even in its most complete and perfect form, will
almost certainly be a source of dissatisfaction. Whenever a
sum greater than three or four dollars is to be expended, some
form other than the vertical should be chosen.

Occasionally microscopes of this kind are furnished with
achromatic objectives of pretty fair quality. In such cases the
objectives and eye-pieces, if they could be applied to a better
stand, would be worth more than the whole microscope in its
original form.

The Continental Form.— Most of the stands made by
the better class of French, German and Austrian microscope
makers are characterized by a low, compact form, and great
simplicity and solidity of construction. Our engraving, Plate
I, shows a very serviceable instrument, manufactured by Mr.
George Wale, formerly of the Stevens' Institute, now of Pater-
son, New Jersey. The general form of this stand is very similar
to that of the large model of Nachet, of Paris, but the details of
construction have been changed considerably, and we think for
the better in many respects. The coarse movement is effected
by means of a pinion and chain, the latter "being kept tight by
means of a spring instead of by a screw, as is the common
method, and as the pinion round which the chain is wound is
milled so that the links fit into it, all slip is avoided, and there is
positively no "lost motion " whatever — the movement respond-
ing promptly in either direction even when suddenly reversed.
The fine movement consists of a lever which carries the entire
body (coarse movement included) downward by means of a
spring, and upward by means of a screw, the milled head of
which is placed below the arm which carries the body. This
brings both movements so close together that the hand may be
kept on both at the same time — the thumb and fore finger
operating the coarse movement, while the third and fourth
fingers make the fine adjustment.

SELECTION AND USE

A peculiarity of this microscope is found in the method by
which the pillars which support the arm are attached to the
foot. Instead of being screwed directly to the foot or cast on
it, as is sometimes the case, they are screwed to a circular plate
which rotates with a smooth and firm motion. This enables
the microscopist to place the horse-shoe either behind the
stage, when the instrument is used in a horizontal or greatly
inclined position (as shown in the engraving), or in front of it
when used in a vertical position. This confers great steadiness,
with a comparatively light and convenient base.

As our purpose is not to give a minute description of any of
these stands, but merely to call attention to their characteristic
features, we omit all mention of the very ingenious devices
which Mr. Wale lias introduced into its construction, such as
his new form of iris diaphragm, etc.

The advantages of this general form of stand are, that it is
low and compact, so that it can be easily used in a vertical
position — a favorite method with physicians and histologists.
And so generally has it become a favorite, that most of our best
makers have found themselves compelled to furnish micro-
scopes of this model in order to retain their customers. Gru-
now, McAllister, Zentmayer, and others in this country, and
even such English makers as Beck, Crouch, etc., have recently
begun to manufacture microscopes of the Continental model
in addition to their other styles.

The Jackson Model.— This model is a very general
favorite both with English and American makers. It has
been adopted by Beck, Crouch, Gundlach, McAllister, Boss,
Spencer, Tolles, Zentmayer and others. The new models
manufactured by the Bausch £ Lomb Optical Company, under
the superintendence of Mr. Gundlach, are of this pattern, and
as an illustration we have taken their Student stand, which is
shown in Plate II. The binocular microscope of Mr. Crouch,
(Plate IY),*is also constructed on this model. It will be observed
that the body of the microscope is supported along its whole
length by means of the arm, which is hung between two pillars,
so as to give great steadiness. To add to this steadiness, all
sharp angles are avoided, and the arm is gracefully curved in-

JACKSON MODEL.

As made by Bausch &. Lomb Optical Company.
Plate H. [To face page 36.

ROSS MODEL.

Latest Pattern as made by Ross & Co , of London.
Plate III. [ To face page 37.

OF THE MICROSCOPE. 3?

stead of joining the body at aright angle, as in the Ross mode),
and many of those made after the so-called Continental pattern.
The special advantages of this model are great steadiness and
the fact that in common with the form next to be described, it
affords abundant room underneath the stage for those accessory
methods of illumination which are indispensable in the highest
class of work. It is almost impossible to attach to the smaller
patterns of the Continental model a convenient sub-stage, carry-
ing polarizer, achromatic condenser, paraboloid, etc., while the
model under consideration is specially designed to receive these
important accessories.

Tiie Ross Model.— The distinguishing characteristic of
this model is the mode in which the body is supported. By re-
ferring to Plate III, it will be seen that the body is attached at
its lower end to a transverse arm, which in turn is supported
by a stout bar, which is moved up and down by means of a rack
and pinion. This movement constitutes the coarse adjustment,
the fine movement being effected by means of a lever which is
concealed in the transverse bar, and acts upon the nose-piece.

So far as mere questions of convenience and adaptability to
different kinds of work is concerned, this model is all that can
be desired, and as made by Ross & Co., the workmanship is so
perfect, and the finish so exquisite, that it has long maintained
a high position in public favor. It has, therefore, had numerous
imitators, and has probably been copied more extensively than
any other model in existence. At the recent Centennial Exhi-
bition there were microscopes on this model from the most
widely scattered localities. Canada was represented by two
microscopes made after this design. Unfortunately, however,
this model is one of the very worst that a poor workman can
attempt to imitate, for unless the workmanship is far above the
average, the results are execrable. The reason for this is
very obvious. The body, being supported only at the lower
end, every vibration causes the upper end to swing through
a comparatively large arc, and hence any motion arising from
looseness in the joints is multiplied a hundred fold. And even
when the joints are firm and without shake, any vibration com-
municated to the table on which the instrument stands, is

38 SELECTION AND USE

greatly increased in its effects \vlien it reaches the upper end of
the body. In addition to this, the unsupported part of the
body acquires, by each movement, a momentum which reacts
powerfully on the lower part, and consequently on the ob-
jective.

These defects have induced Messrs. Koss & Co. to bring out
a new pattern designed after the Jackson model. This design
has been carefully worked out by Mr. Wenham, and is certainly
very beautiful in appearance, and very efficient and convenient in
use. Our readers will therefore bear in mind, that all micro-
scopes made by Ross & Co., are not made on the "Boss
model."

It is not our purpose to enter here into the details of the con-
struction of the stage, and the various means of illumination
which accompany this microscope. For these we refer to the
extensive illustrated catalogues, which may be obtained from
the manufacturers.

Browning's New Model.— The importance of being
able to rotate the object in relation to the illumination is gen-
erally recognized, and in almost all the better class of stands,
full provision is made for this. It has always been difficult,
however, to secure perfect coincidence of the centre of rota-
tion with the optic axis. In the better class of microscopes,
made a few years ago, the makers depended upon perfect work-
manship to secure this end, and consequently a satisfactory rota-
ting stage was to be found only in the very best instruments.
After all, however, perfection is not to be found, and even the
best stages of this kind were very liable to get out of order. To
obviate this, Mr. Browning has constructed his stand with the
stage in two pieces, the lower part being connected with the
foot, and the means of illumination, while the upper part, which
rotates on the lower, is rigidly attached, to the arm which
carries the body. In this way the body, arm, and stage may
all be rotated in relation to the illumination, and for ordinary
purposes this answers very well. When polarized light is used,
however, it is obvious that it is impossible to rotate the object in
relation to the polarized ray, without also rotating the analyzer,
and, aa every one that has worked much in this direction

0* THE MICROSCOPE. 39

knows, it is often of great importance that both polarizer and
analyzer should be kept stationary, while the object itself
rotates between them.

The Inverted Microscope.— Some years ago, Professor
J. Lawrence Smith devised a microscope specially adapted to
chemical investigation. In this instrument the stage is placed
over the objective, which is inverted, and the body connected
with it at an angle. A prism reflects the image of the object,
and causes it to pass up the body to the eye of the observer.
For some purposes it is a very useful instrument. Nachet
seems to be the only maker that keeps this form in stock.

Tlie Binocular Microscope.— More than two hundred
years ago, attempts were made to construct binocular micro-
scopes, and yet a good and efficient binocular is a thing of
yesterday. Professor Riddel of this country, M. Nachet of
Paris, and Mr. Wenham of London, seem to have been most
prominent in the efforts that have been made of late years to
perfect this instrument, and to Mr. Wenham is due the credit
of having produced a method which is at once efficient and
easily made. Indeed, so little does the binocular feature now add
to the expense of even good microscopes, that a London made
stand, which in its monocular form costs $90, can be had as a
binocular of the most excellent quality for $100. Mr. Wenham
has, therefore, laid all microscopists under deep obligations,
not only by devising such simple and efficient means of accom-
plishing a most desirable result, but by giving the use of his
invention freely to the world.

Of the value of the binocular, there is a wide difference of
opinion, some regarding it as a mere toy, and altogether
beneath notice as an instrument of scientific research, while
others consider it a most important addition to our means of
investigation. Since, however, it will almost always be found
that those who place a high value on the binocular are those
who have used it most, while those who decry it know abso-
lutely nothing of its merits, and are even ignorant of the man-
ner of using it, the reader will have but little difficulty in de-
ciding on which side the truth lies. In England, where cheap

46 SELECTION ANb USE

and good binoculars are common, this form of instrument has
become a great favorite with all the noted microscopists, and
we refer not merely to men who own microscopes as a means
of amusement, but to those, who, like Carpenter and others,
have enlarged the boundaries of knowledge by their re-
searches.

There are, of course, certain limits to the range of usefulness
of the binocular microscope. As at present constructed, it is
most efficient in the use of the low powers, and hence, there
are certain classes of work to which it is peculiarly applicable,
while in other branches, particularly certain departments of
histology, it is of comparatively slight advantage. And it will
in general be found that the principal opposition to the binoc-
ular has come from continental histologists, who, because they
found it of little use to themselves, concluded that it could not
possibly be of use to any one else.

Attempts have been made, and with partial success, to apply
the binocular feature to instruments used with high powers.
Messrs. Powell & Lealand have produced an instrument of this
kind which has been highly praised by some. Not having seen
it, however, we cannot speak of its merits.

The advantages presented by binocular instruments are two
fold ; the relief to the observer arising from the ability to use
both eyes is very great, and the view which is obtained of
any object is so much clearer and more realistic, that we at
once perceive, by our mere sense of vision, those features which
we would otherwise have to work out by tedious mental pro-
cesses. It has been said by some, that the binocular is apt to
exaggerate the stereoscopic effects, and give false views. This
is certainly not the case under ordinary circumstances, and we
doubt much if it ever occurs.

We have chosen as an illustration of the binocular micro-
scope, one of moderate cost, made by Mr. Crouch, of London.
The stand is designed after the Jackson model, and the two
bodies are furnished with draw-tubes, which may be moved out
or in simultaneously by racks and pinions, the milled heads of
which are seen at the upper part of the instrument. The fitting
of the Wenham prism which produces the binocular effect, is
seen just above the nose-piece; by withdrawing it slightly, the

Otf Tttfi MICROSCOPE. 41

instrument is at once converted into a monocular, equal in
every respect to any that are made specially in that way. The
fine motion is by a lever, which moves the nose-piece, to which
the objective is attached. The instrument is furnished with a
well-made mechanical stage, and we would call the special
attention of the reader to a point, which we have already noted,
viz., the abundant room which is found below the stage for the
application of accessory illuminating apparatus. The sub-stage,
which has very delicate means of adjustment for focussing and
centering, has considerable range, and affords facilities which
cannot possibly be combined with what may be called the
"dumpy " models.

Binocular Eye-Piece— A very valuable and efficient
means of converting an ordinary monocular microscope into a
binocular, has been devised by Mr. Tolles, of Boston. But ite
price, $80, places it beyond the reach of "beginners," for
whom this work is specially intended.

OBJECTIVES.

The modern compound microscope owes almost all its value
to the high degree of perfection which has been attained in the
construction of the objectives used with it Some of the old
microscope stands were quite as elaborate, and quite as costly, as
anything that can be found in the workshops of our modern
opticians, but from the fact that the objectives were little else
than simple lenses, their value as instruments of research was
of a very low degree. This being the case, it is of some impor-
tance to the microscopist that he should have at least a fair
understanding of the causes to which the superiority of
modern objectives is due.

When we use a simple glass lens as an objective for a com-
pound microscope, we find on attempting to examine objects
under powers of more than one hundred diameters, the following
defects and difficulties: The field of view is so dimly illuminated
that objects are seen with difficulty; the outlines of the different
parts, instead of being sharp and clear, are thick and hazy;
several of the lines are fringed with brilliant colors, but colors

42 SELECTION AND USE

which do not belong to the objects, and finally, if the outlines
of the object should happen to be straight lines, and be known
to be such, it will be found that they will appear to be curved
and distorted. It is evident, therefore, that a simple lens can-
not be used as an objective in any important work; its indica-
tions are unreliable, and the imagination is allowed full scope,
so that the eye is enabled to see whatever the mind desires
to see.

The defects which we have just detailed, and which are found
in every simple glass lens, whose surfaces are bounded with
curves that are parts of circles, are largely due to what is called
spherical and chromatic aberration. As these terms are probably
not familiar to many of our readers, we will give as full and
simple an explanation of the subject as can be done without
the formal aid of mathematics.

Spherical Aberration.— The enlarged image formed at
the focus of any lens, and rendered visible on a screen or sheet,
is produced in this way: The rays proceeding from the object,
and passing through the lens, are, by the action of the glass,
bent from the path they would otherwise pursue. The object
may of course be supposed to consist of an infinite number of
points, and from these points rays proceed in every direc-
tion, and consequently through every part of the lens. If the
lens were perfect, all the rays from any one point would be
brought together at a second point corresponding with the
first. Unfortunately, however, the ordinary lens does not do
this; the central portions of the lens and the outer por-
tions act differently; the one brings the rays to a focus at
a point a little nearer to the lens than the other, and, con-
sequently, although we move the screen to a slightly greater
or less distance, we sfcill get an image of about the same
degree of distinctness. It is obvious, therefore, that when
placed at any distance within certain limits, the screen will
receive not one image, but a series of layers of images as it
were, and this consequently gives an indistinctness to the
resulting image.

Our readers will find no difficulty in thinking out this
matter for themselves, and when they have arrived at clear

OF THE ificEoscbPE. 43

ideas upon the subject, they will see that spherical aber-
ration is caused by the difference between the extent of the
refraction produced at different parts of the lens, and this
applies not only to all the rays proceeding from each in-
dividual point, but to the several pencils which proceed from
different points.

It is evident that if some parts of the lens bring the rays to a
focus at a shorter distance than others, these parts must magnify
more, and such is in reality found to be the case. But if one
part of an object is magnified more than another, the image
will be distorted, and hence we have what is sometimes known as
aberration of form. This distortion is easily seen by examining a
piece of muslin with a magnifier of high power and large
diameter. The threads in the centre of the field of view* will
appear to be straight, while those at the outside will appear to
be curved.f

Chromatic Aberration. —This is a defect of ordinary
or uncorrected lenses, whereby they not only act as magnifiers,
but as prisms, decomposing the light, and causing objects seen
through them to appear with a fringe of color. Common hand
magnifying glasses, used in the ordinary way, do not exhibit
this defect to a very marked degree, but when the images formed
by lenses of this kind are again magnified, as is done in the
compound microscope or telescope, the color becomes very dis-
agreeably perceptible.

* By field of view is meant that portion of the object which is visible
through the magnifier.

fin ordinary lenses and microscopes, in which this defect is not cor-
rected by the structure of the glasses themselves, the effects of spherical
aberration are lessened by contracting the field of view, so that only those
parts of the object which are seen through the centre of the lens or objec-
tive are looked at. This contraction is usually effected by means of dia-
phragms, or round plates of metal pierced with a central hole, which are
so placed as to cut off the rays which pass through the edge of the lens,
and leave only those that are central. This plan, however, is only the
substitution of one defect for another, for by lessening the field of view of
the lens, we are prevented from seeing more than a very small portion of
the object, and in addition to this the light is so much reduced that tho
object is seen only with very great difficulty, and not at all clearly.

44 SELECTION AND USE

Corrected Objectives.— The defects which we have just
described have been the chief difficulties in the way of perfect-
ing both the microscope and the telescope. In the case of the
latter, however, it was long ago found that very excellent re-
sults could be obtained by forming the lenses of two or more
pieces of glass of different kinds, and numerous attempts were
made to apply the same principles to the construction of the
microscope, but without marked success. The small lenses
used for the microscope seemed to defy the skill of the practi-
cal opticians of those days, and resort was had to such devices
as lenses made of precious stones, and the use of light which
could not be decomposed — mono- chromatic light as it was
called, or light of one color. Such light is readily procured by
the combustion of alcohol mixed with common salt, and when
illuminated solely by such a light, a brilliantly colored painting
looks exactly like a plain black and white engraving. But
although the use of such a light lessens the evils caused by
chromatic aberration, they introduce another which is quite
serious — objects which are really colored, appear in black and
white only. Moreover, such a light cannot easily be obtained
of a brilliancy sufficient to afford good illumination, and in
addition to this all the defects due to spherical aberration still
remain in full force.

The first attempts made to perfect the object-glasses of micro-
scopes, consisted in the use of doublets and triplets, it having
been found that the spherical aberration is greatly lessened,
when the total refraction is divided up amongst several surfaces
of moderate curvature, instead of one surface in which the curva-
ture is excessive, and this plan is still pursued in the construc-
tion of what are known as French triplets, which will be de-
scribed hereafter. About the year 1829, however, Mr. J. J.
Lister, of London, England, published an elaborate paper
upon the subject, and it was from the principles laid down in
this paper, that all the important improvements in the modern
objective took their rise. These principles were embodied in
the practical construction of objectives by Andrew Boss, who
suggested the important improvemeirt known as the adjust-
ment for thickness of cover. To Lister and Boss, therefore,
it maybe justly said that we owe that optical wonder, the

OF THE MICROSCOPE. 45

modern objective, for although great improvements have been
made within the past few years, it is upon the results of their
labors that these improvements have been based. And yet,
notwithstanding this well-known fact, the names of these
distinguished microscopists are not so much as mentioned
in this connection in the recent work of Dr. Frey, which has
been lately translated into English, and extensively circulated
in this country !

In estimating the quality of an objective, there are certain
features to which especial attention must be given. Aside
from magnifying power, which, of course, cannot be regarded as
affecting the quality of an objective, these points are: 1. Defin-
ing power; 2. Achromatism; 3. Freedom from aberration
of form; 4. Flatness of field; 5. Angular aperture; 6. Pene-
tration; 7. Working distance.*

Defining Power.— This is undoubtedly the most impor-
tant quality to be sought for in objectives. A glass that is
deficient in this point is absolutely worthless. Want of de-
fining power is shown by a general haziness and thickening of
the outlines, together with a want of clearness in the details.
It arises from the presence of either spherical or chromatic

*The authors of the Micrographic Dictionary enumerate the following
points as those in which object-glasses differ from each other: 1. Magni-
fying power. 2. Defining power. 3. Penetrating power. 4. Their cor-
rective adaptations. The functions attributed to " defining power " are
the same as those given by other writers; "penetrating power " seems to
be equivalent to what is generally called "resolving power;" "corrective
adaptation" is merely the presence of a means of adjusting for thickness
of glass cover. Frey distinguishes two attributes of object-glasses, viz.,
denning power, and penetrating or resolving power— penetrating power
and resolving power being considered by him to be the same thing.
Carpenter enumerates four distinct attributes of object-glasses, viz.,
" (1) Defining power, or the power of giving a clear and distinct image of
all well mai'ked features of an object, especially of its boundaries; (2) its
penetrating power or focal depth, by which the observer is enabled to look
into the structure of objects; (3) its resolving potcer, by which it enables
closely approximated markings to be distinguished; and (4) the flatness
of the field which it gives." We cannot regard any of these classifica-
tions, as strictly logical. Beale makes no formal statement, but gives
some very excellent practical directions in regard to the selection of ob,
jectives.

46 SELECTION AND USE

aberration, or both. It might be caused by a want of finish on
the surfaces of the lenses, but this is seldom the case in prac-
tice, except where the objective has been exposed to some cor-
roding fumes or liquids. Old objectives that have been very
excellent in their day, sometimes fail in defining power, from
the fact that the surface becomes covered with a greasy deposit,
very. slight, it is true, but just enough to destroy the efficiency
of the glass. Objectives in this condition should be returned
to the makers to be cleaned. In one case we found that in a
lens which failed to show anything clearly, the difficulty arose
from the fact that the cement used for uniting the glasses of
the combination had become affected. The objective was by a
well-known maker, but was over twenty years old

Achromatism. — When an objective shows much color, it
fails to define well except by monochromatic light, such as that
obtained by passing sunlight through a cell filled with the blue
solution of copper in ammonia. A very slight degree of color
is not regarded as objectionable, and indeed it has been found
almost impossible to secure the requisite angular aperture and
absence of spherical aberration without leaving a little color.
Some of the best objectives, therefore, show such objects as
the P. angulatum with decided colors, and yet well resolved.

Aberration of Form. —An objective may appear to de-
fine an object perfectly, and yet give a very distorted figure of
it, just as a cylindrical mirror gives a perfectly definite, though
very distorted, image of objects seen reflected in it. Aberra-
tion of form may arise either from over or under correction of
the spherical aberration, or from want of homogeniety in the
glass used for making the lenses, or from a want of perfection
in the workmanship — the surfaces of the lenses not being per-
fectly spherical. Sometimes this defect is shown very clearly
on one side of an objective, while the other side is not affected,
and this fact may give rise to very curious results when the
objective is tried on different stands, and with oblique light.
Owing to a variation in the point at which the screw threads
begin in the different stands, the objective, when fairly screwed
up, may have a different position in each, as regards the direction

OF THE MICROSCOPE. 4:7

from which the illumination comes. The consequence is, that
an objective which may give excellent results on one stand,
may fail on another. An easy way of testing this fact, is by
means of a rotating adapter. Of course the best test for aber-
ration of form is the artificial star, though in the hands of the
beginner, a micrometer, ruled into squares, is probably the
most available test. Any trace of the defect under consider-
ation will be shown by the lines being carved.

When the lines appear curved, from the fact that the spherical
aberration has not been properly corrected, the nature of the
error may be determined as follows: "When the micrometer
lines are widest apart at the centre (like the lines on a map of a
hemisphere) the spherical aberration has been over-corrected.
It is under-corrected when the reverse is the case.

Aberration of form is one of the worst faults with which a
lens can be affected, and experience has shown us that it is the
one ^vhich is least apt to be detected by a beginner. An objec-
tive may give a "beautiful" image, and yet be worthless
because affected with this defect.

Flatness of Field.— If, when we examine a perfectly flat
object, every part included in the field of view is clearly in
focus, the objective is said to have a flat field. Want of flat-
ness of field is shown by some parts of a flit object being
clear and well-defined, while other parts are out of focus. In
general it happens that where this defect exists, the centre and
circumference of the objective do not act together.

Angular Aperture. — This subject has given rise to some
of the most vexatious questions connected with microscopy, for
a discussion of which we must refer our readers to the pages of
the microscopical journals published during the past few years.
The views which have been promulgated by the two schools into
which microscopists have been divided on the questions affect-
ing angular aperture, have been of an extremely opposite
nature. Thus, the Boston school claim to have produced ob-
jectives whose angle of aperture is 180°, while the English
microscopists ridicule any such claims. Moreover, while what
may be called the English school lay it down as a law that the

48 SELECTION AND USE

angular aperture and penetration must of necessity be in in-
verse ratio to each other, the Boston school claim to have pro-
duced objectives of very high angles, which at the same time
possess great penetration. Even the definition of angular
aperture seems to be unsettled, all of which is, of course, very
puzzling to a beginner. In the present work we propose to
accept the old definition, and to confine ourselves to generally
acknowledged facts in regard to the other points.

Fig. 3.

It was Dr. Goring, we believe, that first pointed out the
special advantages of high angles, and suggested the use of test

OF THE MICROSCOPE. 49

objects, and the figures on the preceding page were used by him
to define and explain what he meant by angular aperture.

In these figures L L and L' L' are two lenses of the same
magnifying power, but different angular apertures. It will be
seen that the cone of rays proceeding from O, is substantially
the same as that from O', but that the lens L L takes in a larger
part of the cone from O, than the lens L' L' does of the cone
from O'. The angles L O L and L' O' L' are the respective
measures of the angular apertures of the two lenses.

The definition of angular aperture given by Goring, has
been followed by all subsequent writers, the accompanying figure
being that used by Dr. Carpenter for
the purpose of explaining and de-
fining the same thing. In this figure
<( b c is the angle of aperture.
Therefore, while it is not to be denied
that other angles may be of great im-
portance in considering the qualitic s
of an objective, it is altogether wrocg
to apply the term angular aperture to
any other angle than the one that has
been described.

Dr. Goring devised several practi-
cal methods of measuring the angular
aperture of different objectives, and

he gives a very full and clear description of an arrangement
adapted to his own instrument, in which the foot was made to
rotate on a carefully centered and graduated base — a very ex-
cellent plan, modifications of which have since been adopted
by Mr. Zentmayer, of Philadelphia, and by Mr. Bulloch, of
Chicago.

It is not difficult to demonstrate the importance of a compara-
tively wide angle of aperture, since object-glasses possessing
this feature are capable of giving important results which
lenses of lower angle cannot give. Thus, when we examine, by
means of a superior French triplet of one-sixth of an inch
focus, the silicious remains of certain very minute plants of the
species Pleurosigma Balticum, we are able to see certain lines or
markings which exist upon their surfaces. That we may be able

50

SELECTION AND USE

to see these lines, it is necessary that the
stand be a good one, and that the light
be very carefully managed, but, even
with the most perfect arrangements or-
dinarily used, we cannot, with such an
objective, discover similar markings upon
the Pleurosigma Angulatum, although
they exist there in great perfection. But
if for the French sixth we substitute a
first-class objective of less than half its
magnifying power, but of wider angular
aperture, we shall be able to see the lines
quite distinctly. We have now before us
an objective of four- tenths of an inch
focus, which does not correct for thick-
ness of cover, but which, with any ordi-
nary thickness of covering glass, is capa-
ble of resolving the lines on the Angu-
latmn perfectly, and we have seen objec-
tives of even lower magnifying power
which would accomplish the same thing.
That the effect depends here chiefly
upon angular aperture, was shown very
clearly by Dr, Goring, from whose work
we take the following figures, engraved
from seven drawings showing the ap-
pearances presented by the scale of a
butterfly's wing, viewed with the same
magnifying powers, but different angular
apertures. A well corrected lens of wide
aperture showed the scale as in G; re-
ducing the aperture, while all else re-
mained the same, the appearance was as
shown in F, and by successive reduc
tions the stages shown in E, D, C, B,
and A, were reached. The slightest ex-
amination shows that features which were
quite distinct under a high angle, became
invisible when the angle was reduced.

Fig. 5.

OF THE MICROSCOPE.

51

This quality of objectives of large angles, whereby they
are capable of showing distinctly delicate lines or dots placed
very closely together, is known && resolving poirer. In the early
days of microscopy, it was called penetrating power, the term
penetrating having been applied to that quality of the telescope
by which it is enabled to show separately the individual stars
of which the nebulae are composed. In the telescope this was
supposed to depend upon space-penetrating power as distin-
guished from mere magnifying power, and this space-penetrat-
ing power was found to depend very largely upon angular aper-
ture. In the case of the microscope, however, it is now gene-
rally agreed that what was called penetrating power in the
telescope, shall be called resolving power, while to the term
penetrating power an entirely different meaning has been given.

Mere resolving power, however, or the power of showing
separately lines placed very closely together, is not the only
valuable feature of well-corrected object-glasses of high angles.
They show delicate lines and fibres, and enable us to make out
differences of structure which are entirely invisible to lenses
of low angles. Thus, for example, it has been found during
recent researches, that the delicate flagella of certain monads
can be seen perfectly with high angle lenses, while with very
excellent glasses of low angular aperture they are quite invis-
ible. The same fact, probably, holds true in regard to the ulti-
mate fibres of nerves and similar objects.

The researches of Lister and Ross formed, as we have just
stated, the first great step in the direction of better correction and
increased angular aperture. "Whereas, 40° to 65° had previously
been regarded as very high angles, even in objectives of the
shortest focal distance, Boss, in his objectives, soon attained
an aperture of 132° to 135° and, working with the glass at that
time available, this was pronounced the highest attainable
angle. Attempts had previously been made to obtain a higher
angle by the use of the glass which Faraday devised for optical
purposes, and which is in fact a borate of lead. But this com-
pound is so easily tarnished and disintegrated, that it was
found impracticable to use it. It happened, however, that
a young American backwoodsman, Charles A. Spencer, of
Canastota, N. Y., a graduate of Hamilton College, had his

52 SELECTION AND T7SE

attention called to this subject, and after careful study
he concluded that if he could only procure a durable
glass of greater refracting power than that ordinarily at-
tainable, the angular aperture might be greatly increased.
He at once went to work, and after many experiments, he suc-
ceeded in producing a glass which enabled him to attain imme-
diately an angular aperture of 146°. As early as 1857 he had
produced a l-12th with an angular aperture of 178°. His objec-
tives had corresponding excellence in other directions, and
from that time forward this country has been noted for the
excellence of its objectives, and especially for their great resolv-
ing power. We may note in passing that glass of great re-
fractive power, combined with sufficient -hardness and dura-
bility, is now produced as a regular article of commerce.

Penetrating Power.— As previously stated, penetrating
power, in the early days of microscopy, meant precisely what is
now understood by resolving power. Now, however, penetrating
power means the extent to which an object-glass shows the depth
or thickness of an object. This is a very important feature for
some purposes, particularly in histological work, as by it we are
enabled to discover the relations between the different parts.

Until within a few years it has been accepted, as a thoroughly
demonstrated fact, that penetrating and resolving power always,
of necessity, exist in inverse ratio to each other, for it is always
found that, other things being equal, resolving power increases
with the angle of aperture, while penetrating power decreases.
Of late, however, it has been claimed that certain lenses of great
resolving power and high angle possess great penetrating power.

In attempting to reach a sound conclusion on this point, it
must be borne in mind that resolving power does not depend
wholly upon angular aperture. Two objectives of the same
angle may have very different powers of resolution, on account of
the degree of perfection to which the corrections have been
carried, and it is quite possible that a lens of great resolving
power may have a lower angle than another objective which it
excels in this respect. In this case the lens of greatest resolving
power might also have the greatest penetration. We confess,
however, that we cannot see how great penetration can be com-

OF THE MICROSCOPE. 53

bined with very high angular aperture, and in this view we believe
that we are in accord with the majority of our best microscopists.

Working Distance.— This is a very important feature in
all lenses, and good working distance is specially valuable to
beginners. There are many objectives in market that have to
be brought so close to the object that ordinary covering glass
cannot be used, and even with the thinnest glass, the distance
between the objective and the object is such that great skill
and care are required to avoid accidents. Such objectives do
excellent work in the hands of experienced microscopists, but
beginners should by all means avoid them. Objectives of very
high angular aperture have in general very short working dis-
tances, but there are great differences in this respect amongst
the products of different makers. Working distance does not
depend upon angular aperture alone.

Immersion .Lenses.— Objectives which require a drop of
liquid between the front lens and the covering-glass of the
object, are now familiar to most microscopists, and have come
into very general favor. The liquid employed serves two im-
portant purposes. In the first place, it partially extinguishes
two of the glass surfaces (the front surface of the objective and
the upper surface of the covering-glass) and thus it prevents, to
a considerable extent, the loss of light which always occurs at
these surfaces. In the second place, it decreases very slightly
the magnifying power, and lowers the angular aperture, but in-
creases the penetration and working distance; hence immer-
sion objectives do not require the same precision of adjustment
for thickness of cover that dry lenses need, and consequently
it is possible to produce non-adjusting lenses on the immersion
principle which are easily used and very effective. They are,
therefore, great favorites with most histologists. Some makers
have carried this principle so far, that the objectives con-
structed by them can be used only with liquids much more
dense than water — pure glycerine for example.

For ordinary work we prefer a dry lens, from the fact that it
is some trouble to apply the liquid and clean the lens, and also
the cover, and where a large number of observations are to be

54 SELECTION AND USE

made, even trifling delays must be considered. Some makers
have endeavored to avoid this difficulty by supplying double
fronts (a wet and a dry) to their objectives, while Mr. "Wenham
has elaborated a formula for a series of objectives which work
either wet or dry, according as the arrangement used for the
ordinary cover-adjustment is set to the one or the other. Not
having had an opportunity to examine objectives constructed
according to this plan, we cannot speak in regard to its success.
The origin of the immersion objective seems to be disputed.
So far as we can learn, the principle was first suggested by Sir
David Brewster, but the first really useful lenses of this kind
were brought out by Hartnack.

Lens Systems.— Formerly the term "system" was ap-
plied only to the entire combination forming the objective, and
we had "immersion systems," "correction systems," etc. At
present the word is used also to denote the individual combina-
tions of two or more pieces of glass, which, when arranged
together, form the whole objective, as will be understood from
Fig. 4, where 1, 2 and 3 form the separate systems, each com-
posed of two pieces of glass. Such a combination (the figure of
which is, of course, only diagramatic,) is said to form a three-
system lens. Very low powers, formed of two achromatic lenses,
are said to be two-system; four combinations, four-system, etc.

French Triplets. — A few years ago these objectives were
used quite extensively. They are so called because they ori-
ginated in the country after which they are named, while to
further distinguish between them and objectives constructed
according to the principles laid down by Lister, the latter were
known as the English form. Good makers of the English form
are now found in the United States, France, Germany, Austria
and Italy; and the French pattern is made in many of the
cities of Europe outside of France, although as yet neither the
English nor the American opticians have been able to manu-
facture them at prices which can compete with those of
continental Europe. The best of the so-called French ob-
jectives consist simply of lenses in which the chromatic aber-
ration is corrected by the usual plan of making each lens

OP THE MICROSCOPE. 55

of two different kinds of glass, while the spherical aberration
is corrected partly by the form of the lens, but chiefly by re-
ducing the aperture, and by properly combining a series of
single lenses, which, hovever, are never especially adjusted to
each other, as in the English forms. Each objective, in its
most perfect condition, consists of three lenses screwed to-
gether, and in the lower powers these lenses may be separated
and used either singly or in combinations of two or three. As
the magnifying power obtained with two lenses is less than
that obtained by three, the defects of the double combination
are not as obvious as they would be if the magnifying power
were equal to that of the triple combination. As, however, the
spherical aberration in the case of a single, lens, whether it be a
plain lens or an achromatic combination, is always greater than
that of a doublet, and the aberration in the doublet greater
than that in the triplet, it is never a good plan to attempt to
obtain a low or moderate power by separating the lenses of a
high power objective, and using them singly or in twos. Any
person having a few French objectives at hand who will try this
and attempt to secure the same magnifying power by the use of
two lenses, and also by the use of three, the latter being a regu-
larly adjusted combination, will find that the results obtained
by the use of the latter are far superior to those afforded by the
former.*

Considering their quality, these French objectives are re-
markably cheap. Thus a French No. 1, which is nearly equiv-
alent to the half-inch objective of the English and American
opticians, can be bought for $5, while the cheapest student's
objective of this power would cost at least double that sum. In
addition to this, the French objective may be divided so as to
afford two other objectives of about three-quarters and one
inch each, and although the performance of these is far inferior
to English or American objectives of the same power, they are

*In making such a trial, it is, of course, necessary to use lenses of equal
quality in both cases, since the quality of the professedly achromatic
French objectives in market varies very much. We have seen objectives
of this class of the same magnifying power, one of which would not resolve
the markings on the scales of the clothes-moth's wing, while the other
would resolve the Pleurosigma BaUicum.

56 SELfcCllON AND USE

capable of showing a great deal that is interesting and instruc-
tive. Two or three years ago these lenses were the only ones
furnished with microscopes costing less than $50, and in the
very cheap instruments the different powers were always
obtained by the division of one doublet or triplet, which
was thus made to yield two or three different objectives.
Those, however, who cannot afford American objectives, and who
wish to do work that is of some real value, are advised never to
separate their objectives, or at least never to separate any but
the very lowest — that is the No. 1, and against even this
we would protest were it not for the fact that cheap lenses
of lower power than the half -inch are seldom found in market ;
and therefore, no other course except the division of a No. 1
is left to us when we wish to use a lens of lower power. But
this system of dividing is often carried too far, and we find, mi-
croscopes in market which are furnished with No. 2 or No.
3 objectives which are divided when lower powers are needed.
This is decidedly wrong. If a power lower than No. 1 be needed,
it may be admissible to divide this number, because this is in
general the only course left to us, but a No. 2 should never
be divided for the purpose of obtaining an objective equivalent
to a No. 1.

The value of the numbers assigned to the different French ob-
jectives varies according to the fancy of the maker, but those
of the better class usually found in market are about as follows :

Number 1 2 3 4 5 6

Corresponding focus in parts of an inch 1-2 1-4 1-6 1-8 1-10 1-12

Frey, in his recent work on the microsccpa, regards the
English system, whereby the focus is expressed in inches, as
" peculiar." It certainly is "peculiarly " definite and positive,
instead of being indefinite and arbitrary, as is the system
adopted by the French and German opticians. According to
the English and American systems, an objective of an inch
focus ought to be the same, no matter by what maker it has
been constructed, but when designated after the plan which
Frey seems to prefer, it is impossible to tell what the focus of
the lens may be, and consequently what its power is. Thus—
a No. 2 of Nachet has a focus of half an inch, while a No.
2 of Hartnack has a focus of one inch, and a No. 2 of the

OF THE MICROSCOPE. 57

ordinary French objectives is about a quarter of an inch in
focal length. As it is often useful to the microscopist to know
the powers of the different objectives made by prominent con-
tinental makers, we give the focal lengths of the objectives of
Nachet, Hartnack, and Gundlach, premising, however, that
by so doing we by no means intend to class these objectives
with ordinary French triplets.
Nachet's ordinary objectives are as follows:

Number 0 1 2 3 4 5

Focus in inches 2 1 1-2 1-4 1-5 1-8

The immersion and correction objectives of the same maker
are as follows:

Number 6 7 8 9 10 11 12

Focus in inches 1-10 1-14 1-15 1-20 1-30 1-40 1-50

Hartnack's objectives of recent construction are as follows:

Number 123456789

Focus in inches.... 2 1 3-4 1-2 1-4 1-4 1-6 1-9 1-11

Hartnack's new objectives with immersion and correction are
as follows:

No.
9
10

Focus in inches.
1-12
1-16

No.
14

Focus in inches.
1-28

15
16

1-33
1-40

11

1-18

12 .

. 1-21

17

1-45

13
The

No.
I....

II

1-25
following is Gundlach's £

Focus in inches.
1

1-2

18

cale

No.
Via ....

1-50

Focus in inches.
1-12

VII)

1-12

III....
IV. . . .
V....

1-3
1-4
1-8

Vila

1-16

VII6

1-16

VIII
IX .

1-24
1-32

Via and Vila are not adjustable for thickness of cover,
while VI6 and VII6 are. Vila, VH6, VHI and IX are immer-
sion lenses.

Since taking up his residence in this country, Mr. Gundlach
has adopted the system of the English and American makers,
and designates his objectives by their focal length. The
table given above, however, will prove of service to those who
either possess or intend to purchase specimens of his earlier
work, some of which was very excellent.

58 SELECTION AND USE

Testing Objectives.— At first sight it would seem to be
the easiest thing in the world to test an objective, and find out
whether or not it is capable of doing certain work, but a little
experience soon teaches those who are not too self-conceited,
that it is the easiest thing in the world to be deceived. We
have seen those who considered themselves the most capable
of judges, condemn lenses that had received the approbation of
the ablest microscopists in the world — lenses too that had shown
their efficiency by doing really good work ; showing that even
those who consider themselves very expert, may sometimes arrive
at wrong conclusions. If this is the case, then, with men of
training and experience, how can a beginner, who has had no
experience, hope to be able to form a" correct judgment in re-
gard to the quality of an objective ?

But while it is difficult, or perhaps impossible, to pronounce
a positive opinion in regard to the quality of an objective,
especially those made for some of the higher departments of
microscopic work, it is in general easy for those who have had
experience, to form a judgment in regard to ordinary objectives,
or at least those designed for ordinary purposes. The ability
to form such a judgment depends rather upon experience and
a comparison with the work of other glasses than upon a refer-
ence to any special standard; and therefore, as a geneml rule,
we would advise beginners who are about to purchase objectives,
to obtain the advice and assistance of some skilful friend.
To those who cannot obtain such assistance, we offer the follow-
ing hints.

The great difficulty in the way of arriving at a decision in
regard to the quality of an objective, is the want of a standard
with which to judge its performance. When we examine the
image which an objective gives of any object, it is very difficult
to decide whether or not that image truly represents the ob-
ject. Take, for example, the podura scale: wide differences of
opinion exist as to its structure, and how it ought to look; sup-
pose, then, that two objectives show entirely different appear-
ances of this object, who shall decide which one is correct ?
And if, even in the case of expert microscopists, this holds
true absolutely, which it does, how shall a beginner determine
that the images which he sees throngh an objective are true or

OP THE MICROSCOPE. 59

false ? In some departments, the most earnest and long-con-
tinued discussions have been maintained in regard to the ac-
curacy or inaccuracy of certain images as seen by professional
microscopists, and, strange to say, these disputes affect the
very tests most commonly used, viz. , the Podura scale and the
test diatoms.

Makers of objectives, and skilful microscopists, being aware
of the fallacies which beset examinations of this kind, resort to
certain artificial atandards of which the construction is posi-
tively known, and which should therefore give appearances
conforming to this known structure. Numerous tests of
this kind have been suggested, but the only ones generally
accepted are the artificial star* and ruled glass plates. Of the
latter, ordinary micrometers answer a very good purpose, but
the most delicate tests are the famous ruled plates of M. Nobert.

In the examination of objectives, there are a few simple gene-
ral rules which must be observed by the microscopist if he
would secure accurate results.

The first important point, and one to which sufficient attention
is not generally given, is the health of the observer at the time of
making the trial. The eye is a very delicate organ, and the
slightest derangement of the stomach or nerves affects it to an
extent that few persons realize. We have an object-glass of
comparatively low power, with which, when in good personal
health, we find no difficulty in resolving the P. angulatum,
though a very slight disturbance of the digestive organs, ren-
ders the lines perfectly invisible.

It must also be remembered that in the case of such delicate
observations, personal peculiarities, irrespective of health or
sickness, exert a marked influence, so that it does not follow
that what one observer sees, all can see. We have frequently

*The artificial star is a very minute globule of mercury, obtained by
crushing a small drop by means of a smart tap with a flat slip of iron or
ivory. This globule is made to act as a small convex mirror, reflecting
the light of a lamp, candle or window. It is not mentioned by modern
writers on the microscope (Carpenter, Hogg, Beale, Frey, etc.), but is
used by some of our best opticians. Dr. Boyston Piggott, has recently
revived its use. Goring devoted considerable space to an account of the
oest methods of using it.

60 SELECTION AND USE

seen those who could not distinguish lines that were visible to
others, and we have also met those to whom an objective, in
which the chromatic errors were very obvious, seemed to be
perfect. This probably arose from a kind of color blind-
ness. We have also met eyes which distorted objects, and
those which saw fringes of color round objects viewed through
an objective of generally recognized excellence.

Attempts have been made to get rid of the errors arising
from personal peculiarities (or what may perhaps be called the
"personal equation") by employing photography, it being
assumed that if a lens will give an image which can be photo-
graphed, it must give an image that may be seen, and that
whatever is photographed must of necessity be a real image.
But from the known fact that the foci of the chemical and
visual rays do not coincide, and that the corrections required
in the one case are not those calculated to give the best results
in the other, we have little faith in photography as the best
test of the excellence of an objective, except, of course, in those
cases where photographic work is the chief purpose in view.
Lenses intended to transmit an image to the eye must be tested
by the eye, and if certain eyes show peculiarities not possessed by
the average eye, then lenses must be corrected specially for them.

It is scarcely necessary to say that when an objective is put
upon its trial, the stand and means of illumination ought to be
such as will do it justice. The best stand in the world cannot
make a good objective out of a poor one, but a poor stand will
give poor results even with the best objective. The eye-pieces
also should be of good quality, and if an objective, which the
microscopist has reason to believe is a good one, fails, it should
not be condemned until it has been tried with eye-pieces either
by the same maker, or of a known standard of excellence. And
we must also remember that it is not sufficient to examine an
objective in combination with a shallow eye-piece, or one giving
a low magnifying power. An objective may perform very well
if used with low eye-pieces, and utterly fail when a higher
power is applied. Most makers of objectives test their glasses
under eye-pieces of very high power — a quarter and even an
eighth of an inch focus, or what would be equivalent to H or K
on the usual scale.

01* THfi MICROSCOPE. 61

The room in which the test is made must also be a subject of
careful selection. Very many of our best microscopes are used
in our large cities; at least they are very generally examined
there with a view to purchase. Now, those who are familiar
with the subject, know that during the day time the buildings
along the principal thoroughfares in our large cities are in such
a state of constant vibration, that good results are rendered im-
possible, and therefore that an objective and stand which,
under such circumstances, fail to resolve difficult tests, or to
define clearly, should not on that account be condemned.

The illumination employed must also receive careful atten-
tion. An objective which readily resolves the P. anyulatum by
central illumination, when lamp light or good daylight is used,
may fail when poor daylight is employed. Special directions
on this point are given under the head of light and illumina-
tion, and therefore we would merely say here that an objective
which has been tested only by the dull blue light of a northern
sky, cannot be said to be inferior because it has failed either in
resolving or defining power.

On the other hand, we must not place too high an estimate
on an objective which, by the aid of monochromatic light, (the
blue-cell, for example,) has resolved certain difficult tests. It
is not uncommon to find that lenses of a quarter-inch focus
will, with blue light, resolve the Amphipleura pettucida, but
fail completely with ordinary light. Even eighths and tenths
by the same makers, and of a grade quite as good as the fourths
just mentioned, fail to resolve the Amphipleura by ordinary
illumination, even when well managed. The aid which is
derived from blue light in the resolution of difficult diatoms is
unquestionable, but it is not quite so clear that this kind of
light gives the same assistance in the matter of definition. Our
own experience leads us to believe that the real assistance de-
rived in the latter case is very slight. Therefore, we do not
regard it as a very high recommendation for ordinary work that
a lens can resolve the Amphipleura by blue light. We have,
however, seen a fourth which would resolve the Ampliipleura by
the light of an ordinary hand lamp, aided by Wenham's reflex
illuminator. The objective was made by Tolles, and matnipu-
lated by him.

62 SELECTION AND USE

To determine the quality of an objective it is best to take up
in succession the several features which we have just detailed,
and examine its efficiency in each of these directions. First of
all, the denning power should be carefully tried, this being the
most important quality that a glass can possess. No special
test can be named for this, and in fact the formation of a cor-
rect judgment in regard to it will depend more upon the
experience of the observer than upon any particular rules that
can be laid down. As Carpenter well says: " An experienced
microscopist will judge of the denning power of a lens by the
quality of the image which it gives of almost any object with
which he may be familiar." To which we may add that the
inexperienced microscopist will in general fail to detect a want
of denning power, no matter what object he may examine.

Tha chief points seem to be that the outlines should be sharp
and clear, the blacks black, and the other natural colors clear
and distinct. Frey compares the image given by a good lens
to a good copper plate, or a print with sharp letters, and no
illustration could be more to the point. He also states that an
objective which is deficient in this respect, is best tested with
a pretty strong eye-piece. In our own experience we have
found no surer test of excellence than this: An objective which
is deficient in defining power, is sure to " break down " under
a high eye-piece. Deep eye-piecing does not effect the resolv-
ing power of a lens to the same extent that the defining power
is lessened, and therefore, the fact that a glass shows lines
under a high eye-piece, is not an absolute demonstration of its
excellence as regards definition. At the same time, it will be
found that considerable angular aperture is absolutely neces-
sary to enable any glass to bear deep eye-piecing, because with-
out this, the loss of light is so great that nothing can be seen
clearly. Hence the truth of the somewhat paradoxical state-
ment, that an objective may be really good under a low eye-
piece, and yet fail under a high one.

With the English opticians a favorite test for definition is
the Podura scale. Unfortunately, however, the structure of
this scale and even the identification of the scale itself, seem
to be a matter of doubt. Page after page has been written for
the purpose of showing how the Podura scale ought to look, and

OF THE MICROSCOPE. 63

still the question seems to bs undecided. Carpenter, in his last
edition (page 702) says: "The sharp and distinct bringing-out
of the ' exclamation marks ' of the Podura scale, constitutes, when
it coexists with the greatest practicable freedom from color, and
with adequate * focal depth ' or 'penetrating power,' the most
valuable proof of the fitness of an Objective of high power for
the purpose of scientific work."

To give our readers an idea of

how the podura scale ought to look,
we give a figure copied from the
engraving published by the late
Richard Beck, in his work on the
microscope. The figure shows
" the appearance of the Podura
scale when the adjustment of the
object glass is correct and the mark-
ings are in focus." The objective
used was a one-eighth, giving a
Fig. 6. magnifying power of 1,300 diame-

ters.

It is, we believe, generally conceded that in the present state
of the art, perfect correction for color cannot be obtained, but
so long as the residuary chromatic abberration does not inter-
fere with the defining power of the objective, it cannot be re-
garded as an objection. And yet we have seen a would-be critic
reject a very excellent lens because it showed a little color,
while he was loud in his praise of another lens which, although
more perfect in this respect was almost worthless otherwise.
Like specks of dirt on an eye-piece, which do no harm and are
never even seen by experienced microscopists, slight color and
want of flatness of field are the betes noirs of beginners. They
are the defects which are most easily detected, and the detec-
tion gives the critic an air of knowledge which is to him a
source of great pride.

The best English and American opticians now slightly under-
correct their best objectives, so that the field shows a slightly
greenish hue, while any prominent markings on the object,
such as the dots on the Angulatum stand out clear and well de-
fined and of a very delicate ruby tint. According to Frey the

64 SELECTION AND USE

majority of continental makers adopt the opposite plan.
Their lenses are over- corrected, and objects show a bluish
border.

A want of correction for color is shown when thin objects with
many cross lines are examined, especially with slightly oblique
light. As a test for achromatism in low powers, Carpenter
prefers a section of coniferous wood, showing the glandular
dots. He also recommends the tracheae of insects, but almost
any lined object will answer the purpose.

The existence of aberration of form is best proved by the use
<)f a fine micrometer or a Nobert's plate. When this defect is
very marked, it is easily seen in the curved and distorted lines
of which the image consists, but such a state of things exists
only in extreme cases. Where this distortion is not very glar-
ing, it may be necessary to compare the magnified image of the
lines in the stage micrometer with straight lines ruled on a thin
plate of glass laid on the diaphragm of the eye-piece — in other
words with an eye-piece micrometer in which the ruled lines are
quite long.

For testing for flatness of field and aberration of form, Frey
recommends " a slide thickly smeared with India ink, in which
small circles or other figures are scratched with the point of a
fine needle. * * * If the instrument is adjusted with trans-
mitted light for such a circle, it should appear sharply
cut on the black ground, and not surrounded by a halo
of light. If the circle is then brought out of focus, it
gradually enlarges, while its sharp borders disappear, with-
out spreading a strong halo of light either inwards or outwards
over the black field."

The angular aperture of an objective can be determined
accurately only by measurement, and this is something that
beginners will hardly attempt. To measure accurately the
angular aperture of an objective, is a task requiring con-
siderable skill and knowledge, and most of the appli-
ances furnished by microscope-makers for this purpose, fail
to give accurate results. It must be remembered that in
measuring the angle of an objective, we must comply with
the same rules that govern the accurate measurement of
any other angle. Dr. Carpenter, in his work, gives a method

OF THE MICBOSCOPE. 65

which he calls "the simplest and most convenient." We ven-
ture to say, that none but an expert can obtain by it results
that are anywhere near accurate, especially with high powers.
We therefore consider that any directions upon this subject,
addressed to beginners, would be worse than useless.

Since, however, the resolving power of an objective depends
in a large measure upon its angular aperture, we may feel
pretty certain that an objective which fails to resolve tests
suited to its magnifying power, is deficient in angular aperture,
unless, indeed, its inefficiency should arise from want of defining
power, which may be tested by other means. Of ordinary work-
ing lenses, the half inch ought easily to resolve the Pleurosigma
Balticum; the quarter inch should resolve the P. angulatum by
oblique light, and those of a fifth or sixth inch focus should
resolve the latter test by axial or central light. An eighth,
tenth, or twelfth, ought to resolve all the diatoms on the Probe
Platte below the 17th. It is true that objectives of a quarter
inch focus have been made to resolve everything on the Probe
Platte, but such glasses cost too much to render it likely that
they will fall into the hands of ordinary students. Twelfths
and sixteenths should go through the Probe Pkitte easily. If
they cannot do this it would be better to take a lower power of
better quali by, and use it with a higher eye-piece.

We must also be on our guard against an old source of error —
the use of lined tests which vary from the accepted standard.*
Great differences exist in the different specimens of the var-
ious test objects that are used, some, owing to individual
characteristics and the methods employed in mounting, being
much more easily resolved or shown than others. Conse-

*"The proof objects [finely lined insect scales] originally discovered by
me, are sufficient for that purpose in honest hands, and when used with the
precautions I have pointed out. But it is well known that they have been
shamefully abused, owing to the various facilities of resolution which
exist between different specimens of lined objects, the external charac-
ters of which closely resemble each other; so that it may be said that
there are proof objects to suit the capacities of all microscopes; nay. they
are actually perverted to the purpose of deceiving the unscientific part of
the public in a much more effectual manner, than could possibly have
been done without them." — Goring. What is true of the scales used by
Goring is also true, though perhaps not to the same extent, of diatoms.

66 SELECTION AND USE

quently, because an objective resolves one specimen of the P.
Angulalum, it does not follow that it will resolve all others. One
of the most important steps in the direction of uniformity in
this respect, at least so far as testing the resolving power of
objectives is concerned, is furnished by the test-plate (Probe
Platte) of J. D. Moller. Upon a slide of the usual size, he ar-
ranges twenty diatoms, carefully selected as to cleanness, and
also as to resolvability. Those that he has chosen for the pur-
pose are named in the accompanying table. They are arranged
on the slide in a line which is about a quarter of an inch in
length, the beginning and end of the row being marked by a
specimen of Eupodiscus Argus, Ehrbg. The table on the oppo-
site page gives the closeness of the lines and the direction of
the markings in these diatoms according to the best authorities.
In this connection it must not be forgotten, however, that mere
closeness is not the only feature which makes a series of lines
easy or difficult of resolution. Every micrometer maker knows
that of two sets of lines, both ruled at 10,000 to the inch, one
may be much more difficult to resolve than the other. The
strength of the individual lines has as much to do with it as
the mere distance at which they are placed apart. Moller's
Probe Platte is furnished of two kinds, dry and in balsam, the
latter being, of course, by far the most difficult test. It is an
unfortunate fact, however, that even with all the care and skill
exercised, even the test-plates of Herr Moller do not always
conform to a standard; and, therefore, were it not for the facts
just stated, it would seem that the most trustworthy tests are
the ruled plates of M. Nobert.

It is not difficult to test an objective of moderate power for
flatness of field, provided we have on hand a suitable object.
For this purpose a thin section of wood, or of an echinus spine,
is generally chosen. For low powers a very excellent test is
one of those micro-photographs which are so common. One
showing a sentence or sentences should be chosen in prefer-
ence to a picture, since, unless the field of view be flat, the whole
of the letters will not be clearly readable at once, while in a
picture the effect known as aerial perspective may give rise to an
impression of want of flatness of field. In applying tests for
flatness of field, it is of course obvious that we must make sure

OP THE MICROSCOPE.

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68 SELECTION AND USE

that the test used is itself flat. Common glass slides ate not
flat, and as they are used for the cheaper kinds of micro-
photographs, this fact may give rise to errors if we are not care-
ful in our selections. The best slides are cut from glass plate
which has been ground and polished so that the sides are per-
fect planes, and it is this kind only that should be used. Care
should also be taken to see that the object lies flat on the slide,
and is not distorted by the cover. We have seen an objective
condemned because it did not show all the diatoms in the field
of view in focus at once, when the fact was that the diatoms
were attached to the cover which was slightly wavy as covers
often are. When it is suspected that the fault is not in the objec-
tive, but in the slide or cover, the object should be carefully
passed across the field of view, and the changes in focus noticed.
This will in general tell where the defect lies, for if the part
that is apparently foggy should move as the object moves, it
shows that the object itself is not flat. It has been recom-
mended by high authority to test objectives for flatness of field
by strewing some fine powder on a slide and seeing whether all
the grains are in focus at once. For obvious reasons this is a
very unreliable method.

Penetration in low powers is perhaps most readily deter-
mined by the examination of opaque objects of considerable
thickness. The round pollen grains of the hollyhock, and the
rounded forms of the polycystina are excellent tests for objec-
tives of an inch or inch and a half. Lower powers ought to
show coarser objects in all their dimensions, while for those of
medium power the coarser cellular tissue of plants answers very
well. It is more difficult to indicate a good test for penetra-
tion in the higher powers, in which, by the way, we have often
seen want of penetration mistaken for want of flatness of field.
This arose simply from the fact that scarcely any object is
absolutely flat, and hence, as explained under another head,
the curvature of the object is sometimes taken as an indica-
tion of a defect in the objective. Want of good working
distance makes itself obvious during the examination of any
object suited to the objective.

OF THB MICROSCOPE.

ON THE SELECTION OF A MICROSCOPE FOR PRAC-
TICAL PURPOSES.

The object of all the information given in the preceding
pages, is to enable the reader not only to understand and use
the microscope, but to select one judiciously; and, therefore, in
every section we have offered hints bearing in this direction.
"We now propose to give the reader such special instructions as
are necessary in addition to those previously offered.

In selecting a microscope, regard must be had, not only to
the excellence of the instrument, but to its adaptability to the
purpose for which it is intended, and to the person who is to
use it. A complicated and expensive compound microscope, if
placed in the hands of a person having little experience or skill,
would evidently be worse than wasted, while to attempt to con-
duct elaborate and delicate investigations by means of a cheap
non-achromatic instrument, would simply be to throw away time,
and wantonly incur the risk of serious errors. And yet no mis-
take is more frequently made. A microscope is wanted; the
purchaser is liberal with his means, and he is saddled with an ex-
pensive instrument entirely unsuited to his requirements. Or,
on the other hand, a physican or student of limited means re-
quires an instrument, and being unable to afford the price of a
really good one, he is induced to purchase a cheap affair, whose
indications, when applied to the subjects for which he requires
it, are entirely unreliable; whereas, he ought to be told that if
he cannot afford a microscope with good objectives, he ought to
leave microscopy in its applications to medicine and physio-
logy alone. So, too, we often see compound microscopes sold
to students of elementary botany, when a cheap dissecting mi-
croscope is really what they need.

It would be impossible to give anything like a list of special
nases in which the different styles of microscopes prove most

70 SELECTION AND USE

useful : the reader whose attention is called to this point will
have little difficulty in deciding the question for himself. We
merely give the general rule, that where dissections of plants
and animals are to be carried on, a simple microscope should
in general be chosen, while the compound microscope fur-
nished with good objectives, is indispensible whenever high
powers are required for the examination of objects.

Having decided upon the kind of microscope that is needed,
the next step is to determine the individual quality of the dif-
ferent instruments that may be offered to us. To do this
thoroughly, it will in every case be found a good plan to take
up, point by point, all those elements that are necessary or desir-
able in a microscope, and in this way subject the instrument
to the most careful scrutiny. Unless a microscope is made
specially to order, it will be difficult to find one that will com-
bine all desirable features, but the plan we suggest certainly
enables us to decide most readily and accurately as to the pres-
ence or absence of those points which are desirable for our pur-
poses. The following are the chief points that demand atten-
tion:

Magnifying Power. — We place this first, because usually
the first question in regard to a microscope that is asked by be-
ginners is, " What is its magnifying power? " Now magnifying
power, although an important element, is after all but a secondary
consideration. A microscope magnifying a thousand diame-
ters could easily be made and sold at a profit for five dollars,
and a few cents expended in paper and paste will at any time
double, or even treble, the magnifying power of an ordinary
compound instrument. The proper question is not how much
does a microscope magnify, but how much will it show. A
magnifying power of one hundred diameters, obtained by the
use of first-class objectives, will enable us to see more of the
true structure of an object than could be reached by a magni-
fying power of five hundred, the lenses in the latter case being
of inferior quality. But, although not the first consideration,
magnifying power is a feature of sufficient importance to deserve
careful deliberation, and without a knowledge of the powers
required, and the mode in which they are expressed, the begin-

/ OF THE MICROSCOPE. 71

ner will often encounter difficulty. Both these points being es-
sential, therefore, before discussing the magnifying powers
best suited to different purposes, it may be well to say a word in
regard to the mode in which magnifying power is always ex-
pressed by scientific men.

When we look at a small object through a microscope, and
see it magnified to twice its length, it is evident that its breadth
is also magnified twice, and consequently its surface, no matter
what the shape may be, is magnified four times. It might also
be said that as we only take cognizance of bodies having a sen-
sible thickness, this thickness must be magnified twice, and
therefore the object is magnified twice four, or eight times. The
latter, however, is a view which is never insisted upon, and
even those who claim the most for their microscopes, never do
more than express the magnifying power in surfaces. Scien-
tific men are, however, agreed that to express a magnifying
power in surfaces is to convey a wrong impression in regard
to the assistance rendered by the instrument to the natural
vision, for a careful study of the physiology of vision, teaches us
that our power to appreciate and distinguish the features of
any object depends upon the distances to which the characteris-
tic points of that object are separated, and this can be meas-
ured only by linear, and not by superficial units. There are
other considerations which lead to the same conclusion, but
for the beginner it is sufficient to know that all scientific mi-
croscopists are agreed that when the magnifying power of a
microscope is stated, it shall be stated in diameters, and not in
areas. By common consent, then, ten times means ten diame-
ters. And yet it is a very common thing for charlatans, and
those who wish to deceive the public, to say that a microscope
sold by them magnifies ten thousand times, or one hundred di-
ameters, and as " ten thousand times " is much more readily
appreciated by the popular mind than "one hundred diame-
ters," the majority of those who read such statements suppose
that they will be enabled to see ten thousand times more than
they could see with the naked eye, which assuredly is not the
case. In some instances these advertisers do not even state
the diameters. We have now before us, clipped from a journal
of deservedly good reputation, an advertisment which reads as

72 SELECTION AND USE

follows, omitting what printers call the " display " arrangement
of the words: " Microscopes constructed on scientific principles
magnifying 10,000 times." The microscope in question, as we
learned by personal examination, gives a magnifying power of
about one hundred diameters. Carpenter speaking upon this
point says: " The superficial magnifying power is of course es-
timated by squaring the linear ; but this mode of statement is
never adopted by scientific observers, although often em-
ployed to excite popular admiration, or attract customers, by
those whose interest is concerned in doing so. " We would,
therefore, advise our readers to look with suspicion upon any
concern advertising in this manner. Of course an advertisement
claiming a magnifying power of "10,000 areas or 100 diame-
ters " is unobjectionable, because both expressions are placed
upon an equal footing. It must also be borne in mind that
great though unintentional mistakes are often made by dealers
in stating the power of the microscopes they offer for sale.
Not long ago a friend told us that he had been offered a small
microscope having a magnifying power of 500 diameters, for
a moderate sum. We called to see it, taking the precaution to
put a micrometer and a foot rule in our pocket. By actual
measurement the highest magnifying power of this microscope
was 45 diameters ! Another instance occurs in the catalogue
of a well-known and honorable business house, who offer a
very neat and well made instrument, whose magnifying power
is claimed to be 350 diameters. Careful measurement of several
instruments, however, gave an average power of less than 200
diameters! Indeed it will in general be found that the magni-
fying power stated by dealers who do not devote their chief at-
tention to microscopes, is greatly over estimated.

So much, then, being clearly understood in each case, the
question naturally arises, What should be the magnifying powers
possessed by microscopes intended for certain specified purposes?
That a certain magnifying power is necessary, no matter what
the quality of the lenses may be, is true beyond a doubt.
Thus, for example, suppose we wish to see the lines on the
Pleurosigma Angulatum, which lines are about the one fifty-
thousandth of an inch apart; what magnifying power woult]
Ibe necessary?

OF THE MICKOSCOPE. 73

With the best illumination, the average human eye can just
clearly distinguish lines which are the two-hundredth of an inch
apart. Some eyes, under favorable circumstances, can see
lines placed as close together as 250 to the inch, but the aver-
age is as we have stated.* To be visible even to the best eyes,
therefore, the lines on the Angulatum, must be magnified so
that they will present the same appearance as lines spaced so as
to give at the very most, say, 200 to the inch. This requires a
magnifying power of 250 diameters, and with less than this
they cannot be seen, no matter how good the objective may be.
And when Dr. Frey says that they can be seen with a power of
80 or 100 times, while "weaker objectives, magnifying 40 or 50
times, should show something of the lines," he makes a state-
ment that we cannot accept.

In order, therefore, that an object may be distinctly seen, it
must be magnified to a certain extent, but the magnifying
power absolutely necessary in any given case, will also depend
upon whether the microscope is to be used for general purposes
of investigation, or merely for the recognition of known forms.
For the latter purpose a power of 100 may be sufficient, while
for the former, on the same class of objects, a power of 500
would be the least that would be serviceable. The following
are a few of the cases in which the power required can be stated
approximately:

For medical purposes (except for pocket instruments, intended
merely to enable the observer to recognize known forms) a
power of 400 is needed, and the objective should be of really
excellent quality.

Students of histology require a microscope with a wider
range of power. Low powers are more useful to them than
to the medical man, and if they push their researches in cer-
tain directions, there is no limit to the magnifying power needed.

*To test the statement in the text, place a glass micrometer, ruled 200
lines to the inch, ou the stage of a microscope, and by means of the
mirror throw a beam of light upon it, just as if for examination by trans-
mitted light in the usual way. If we now look at the lines, not through
the tube, but simply from one side, they will appear distinctly as well-de-
fined lines. Try the same with a micrometer ruled 250 to the inch; gome
oyeg will be able to distinguish the lines, but very many wfll fail to do so,

74 SELECTION AND USE

A good two-third, one-fifth, and one-tenth, giving magnifying
powers of from 50 to 1000 diameters, will, in general, answer
most requirements. It must be borne in mind, however, that
beginners can hardly be expected to use a one-tenth inch objec-
tive to great advantage, and, therefore, the purchase of this
item may safely be deferred.

For the study of botany, and the ordinary facts of vegetable
physiology, a power of 300 is sufficient; but the very minute
forms of vegetable life require a much higher power, and so
do certain of the higher points in the physiology of plants.

For the detection of adulteration, Hassal recommends the
inch and the quarter-inch objectives, giving a magnifying power
with No. 1 and No. 2 eye-pieces, of from 60 to 350 diameters.
For ordinary purposes of instruction and amusement in the
household, a microscope magnifying from 30 to 150 diameters
wiD be found most satisfactory, and for these reasons: Such an
instrument is easily managed; if well made it gives a power
amply sufficient for all ordinary objects, and it need not be ex-
pensive. Moreover, while it is an easy matter to prepare ob-
jects so that they maybe seen satisfactorily under low and me-
dium powers, it requires great skill and long practice to enable
the student to prepare objects so that they may be examined
with profit under a high power. And finally, under a high power,
but a very small portion of any ordinary object can be seen at
once, and consequently many of those things that are best
suited for popular examination can only be seen piecemeal —
a very unsatisfactory mode of proceeding. Thus, under a power
of 750 diameters, a fly's foot could not possibly be seen as a whole ;
we might examine a single claw or pad at a time, but not the
whole foot, and consequently would find great difficulty in ac-
quiring an idea of what the general structure of the foot is.
To give the reader clearer ideas upon this point, we have just
measured the diameters of the fields seen under French and
American objectives, with the following results: With a magni-
fying power of 25 diameters, the field is about a quarter of an
inch; with 50 diameters, it is one-eighth of an inch; with 100
diameters, one-sixteenth of an inch; with 500 diameters, one-
eightieth of an inch; and with 1000 diameters, the one-hundred-
and-fiftieth of an inch, a space which is ordinarily invisible

OF THE MICROSCOPE. 75

to the naked eye. Consequently, when these high powers are
used, it becomes very difficult for beginners to place the object
properly under the microscope, for, as will be readily seen, unless
it is adjusted with a variation less than the one-hundred-and-
fiftieth of an inch, it cannot be seen at all.

The lowest powers that will show satisfactorily certain well-
known objects, are about as follows: The scales, or so-called
feathers on the wings of most butterflies can be very well seen
with a power of 25 diameters; under the same power, the eye
of a fly shows very distinctly the several smaller eyes, or ocelli,
of which it is composed; the individual corpuscles or globules
of the frog's blood can be distinguished with a power of about
35 diameters, human blood requiring 40 to 50; to show dis-
tinctly the form, etc., of these same corpuscles requires a power
of 200 and upwards. The same may be said of starch granules.
Human hair and wool may be seen very satisfactorily under a
power of 100 diameters, the former appearing like a cord,
a quarter of an inch thick. In order to show the peculiar char-
acteristics of these fibres, however, the lenses must be good.
Cotton and flax can be readily distinguished under a power of
80 diameters.

A question very frequently asked in regard to cheap micro-
scopes is, Will they show the animalcules in water? And in
almost all the advertisements of cheap microscopes, we are told
that they will do this. Now, good well water does not contain
animalcules, that can be seen with ordinary microscopes. It
is only in stagnant water that they are found, and many of
them can be seen with the naked eye, without the use of any
microscope whatever. Others require the use of microscopes
having powers a hundred fold greater than that of the best
microscopes in ordinary use. It is evident, therefore, that
such statements are worthless as affording any indication of
the character of a microscope. A microscope magnifying fif-
teen to twenty diameters will show objects that are perfectly
invisible to the naked eye, and with fifty diameters, provided
the definition is good, we can obtain a very interesting view of
many of the most beautiful objects described in the books, and
sometimes called animalcules, such as the Volvox Globator, the
larger Vorticelli, etc., etc.

76 SELECTION AND USE

Tlie Stand. — This should be firm and substantial, with the
centre of gravity very low. Nothing detracts so much from the
performance of an objective as tremor and vibration, and a large
majority of the microscopes in market are very shaky, from the
fact that they are made tall and showy in order to command a
higher price. It is well, therefore, to bear in mind that size is
no criterion of the value of a microscope. Instrument makers
very properly give the size of their instruments, audit generally
happens that the largest instruments by the same maker bear
the highest prices. Other things being equal, however, small,
compact instruments are altogether to be preferred. Some
years ago the rage was for large, showy microscopes, which
made a fine appearance in the office of the physician, and the
study of the naturalist. It was found, however, that in this
case efficiency was sacrificed to show, and all our best makers
are now cutting down the sizes of their instruments, and making
them steady, substantial, durable and easily operated.

There is, of course, a limit to the extent to which stands may
be reduced in size without sacrificing their efficiency, and some
makers seem to forget this. There are stands in market that
are too small every way for anything but special classes of
work. The bodies are too small to secure efficiency in the eye-
pieces and objectives; the stage is too small to allow of the use
of slides of proper size, and there is no room beneath the stage
for the attachment of proper illuminating apparatus. All this
is as inconvenient as the three-feet-high microscopes of the end
of the last century.

The weight of the stand is a subject concerning which many
seem to differ in opinion. One writer goes so far as to say that
no stand weighing less than fifteen pounds can be steady enough
for the performance of good work. It will be found, however,
that a judicious distribution of the material, and a proper con-
struction of the different parts, will more effectually resist the
usual sources of unsteadiness than any increase of absolute
weight. Of course, if it is merely desired to make the micro-
scope steady, in the sense that an inkstand is steady — that is,
not liable to be tipped over — weight is everything. But the
stands that are most difficult to tip over are not those that
resist vibrations most perfectly. For the latter a tripod with a

OF THE MICROSCOPE. 77

small area of support is best; for the former a stand with a flat
base resting over its whole surface on the table should be pre-
ferred.

It is obvious that the causes of unsteadiness are either vibra-
tions transmitted from the floor, or movements caused by the
hand in performing the necessary manipulations and adjust-
ments. The first can never be stopped by weight, unless,
indeed, we make the stand so heavy that its weight will impart
rigidity to the table and floor, and this would require a good
deal more than fifteen pounds, or even twice that. For the
checking of vibrations transmitted from the floor, no device is
better than the stand or table described in a subsequent sec-
tion. So far as movements transmitted by the hand are con-
cerned, if a stand of three or four pounds will not resist them,
the observer should set himself about learning delicacy of
movement before he proceeds any further.

All microscopes made in this country and in England are
now constructed so that the body may be inclined to any angle,
thus giving the power of using the microscope in any position
— vertical, inclined or horizontal. The importance of this is
easily seen when we consider that on the one hand, when liquids
are to be examined, it is sometimes necessary, or at least desira-
ble to use the microscope in a vertical position, though this is
a very tiresome and inconvenient position, and one that is not
calculated to enable the observer to obtain the best possible re-
sults; and on the other, it is equally necessary that the body of
the microscope should be capable of assuming the horizon-
tal position when the camera lucida is to be employed for
making drawings, as will be hereafter explained. And yet
Frey actually gives the preference to microscopes that do not
incline, and which must always be used in a vertical position!
This, of course, necessitates the complicated and expensive ar-
rangement which he describes for adapting the camera lucida to
the vertical instrument, a singular instance of prejudice against
an obvious and successful improvement.

Tlie Stage.— In every case, a large, roomy stage is of the
utmost importance. One great objection to most French in-

78 SELECTION AND USE

struments is that the stages are too small. It should also be
firm and substantial, so that its position in regard to the other
parts of the stand cannot be varied by slight pressure.* The
most important points connected with the stage are the means
provided for holding and moving the object, and the facilities
afforded for attaching accessory apparatus.

In the most complete stands, the object is held between
sliding clips, which form a sort of clamp that is capable of
being moved in two directions, at right angles to each other,
by mechanical means, which generally consist of a screw for
one direction and a rack and pinion for the other. This form,
which is known as the mechanical stage, enables even a com-
paratively unskilled person to bring any part of the object into
the desired position in the field of view, and this with the
utmost precision. These mechanical stages may be said to be
characteristic of the higher classes of English microscopes, and
as they are expensive, they are not generally used. Neither
are they absolutely necessary for ordinary work with low or
medium powers, for with any objective lower than one-twelfth
of an inch focus, the object can be moved by hand quite as
readily as by the screws, and we hold it to be a well established
rule in all manipulations connected with scientific work, that
whenever any operation can be performed satisfactorily by
means of the hands alone, all special contrivances should be
dispensed with. For low and moderate powers, therefore, we
prefer the plain stage, on which the object is moved by means
of the hands alone. But when very high powers are used, and
especially when delicate micrometrical or goniometrical meas-
urements are to be made, a well-made mechanical stage becomes
a necessity. For while it is easy enough to bring an object
very near to a given point by means of the fingers alone, it is
almost impossible to secure perfect accuracy. In the effort to
attain this the mechanical stage is a great assistance, and
therefore when Frey utters a wholesale condemnation of the

*At the same time, however, it must be borne in mind that no stage
ever was made so firm that even a slight pressure would not affect it. If,
therefore, the reader is determined not to rest content with anything
short of a perfectly rigid stage, he will reject all the best microscopes in
market.

OF THE MICROSCOPE. 79

English microscopes, and asserts that they are unpleasantly
loaded with what he is pleased to call "screws and unessential
appurtenances," it seems to us that he commits a great error.
These costly and complex instruments are intended for the
highest class of work, and the most powerful objectives; per-
fection of the work to be done, and not simplicity in the means
by which it is to be done, is the end sought, and this can be
attained only by the complex means employed.

We have never found any of the so-called lever stages that
fulfilled the requirements of the highest class of work, and,
therefore, if a mechanical stage is to be chosen at all, the best
form should be procured.

A microscope fitted with a good mechanical stage leaves
nothing to be desired, but when other forms are used, it is evi-
dent that the chief points to be attained are these: 1. The
object should be held steadily, but at the same time perfect
freedom of motion should be allowed. 2. It should be possi-
ble to remove instantly from the surface of the stage, every-
thing in the shape of clips and holders, so that a clear field
should be left for the adjustment of very large slides, plates,
etc., or for the rotation of the object in relation to the light.
3. Even the simplest forms of the stage should be so constructed
that it may be possible to pass every part of the object under
the field of view, and this, without any risk of omitting even
the smallest portion. This point is of special importance to
physicians and naturalists. Thus, it not unfrequently happens
that it is desirable to know whether or not certain forms are
present in a given drop of liquid; unless we can subject every
part of that drop to microscopical examination, we cannot be
sure that the forms we are looking for are absent. There is
always a risk of omitting some portion of the slide, and conse-
quently doubt must always hang over the exhaustiveness of all
our examinations. The only certain means of avoiding all risk
of missing any portion of a given slide is to pass it across the
field of view in successive parallel bands, just as a plowman
plows a field. The process is clearly shown in the diagrams
on the following page, Fig. 7 showing the mode in which the
entire surface is completely covered with a series of parallel rib-
bons, the breadth of each of which is the diameter of the field

80 SELECTION AtfD tfSU

of view, while Fig. 8 shows the hap-hazard way in which ex-
aminations are usually made, abundant room being left (as
shown by the small crosses) for the escape of important fea-
tures. Now, with ordinary clips, or even with the glass stage,

Fig. 7.

it is impossible to effect this, and therefore we prefer those
stages in which a cross-bar moves over the plate in such a way
that it is always parallel to the front edge of the stage. Such
a contrivance also affords facilities for using the Maltwood
finder.

In the simpler forms of the stage, the object is held in place
by spring clips, which press it down, and under which it is
moved. These clips are frequently screwed to the stage, which
is a great mistake, as we are thus prevented from slipping them
off, and leaving the stage entirely clear. They should always
be held by pins, which merely slip into holes in the stage.

The so-called glass stage has recently come into very exten-
sive use, and is very much liked by some. We believe it is
now generally conceded that it was invented by Mr. Tilghman,
of Philadelphia, who placed it in the hands of Mr. Zsntmayer
to be manufactured. In this country it is generally known as
the Zentmayer stage; in Europe it has been called the Nachet
stage. It has assumed a great variety of forms, some of them
radically different from the original one. Dr. Carpenter seems
to prefer this form of stage to all others.

Revolving Stage. — It is often desirable to rotate an
object in the optic axis of the microscope, either for the pur-
pose of measuring angles or changing the direction of the
illumination in regard to the object. Means for effecting this

OF THE MICROSCOPE. 81

with perfect accuracy have been applied both to the mechanical
stage and the glass stage, though the latter is very frequently
so constructed that rotation is an impossibility. Mr. Brown-
ing's device for securing rotation in the optic axis has been
already described. Another ingenious device for securing
coincidence of the optic axis and the centre of rotation is the
centering nose-piece, by means of which the objective is moved
so as to bring it exactly over the centre of the stage.

Stages for Special Purposes.— It may be safely asserted
that there has never yet been constructed a stage which would
suit the requirements of every worker with the microscope.
Indeed, each investigator seems to require special modifications
of his own. Thus, it will be found that the ordinary stage,
with all its appurtenances, is too thick to admit the use of that
very oblique illumination which, is required by the worker on
diatoms,while if the stage be made thin enough it* loses the
necessary rigidity. Some makers have sought to obviate this
by supplying two stages — a stout one for common work, and a
thin one for diatoms. A microscope now in our possession is
furnished with an extra thin stage, which, by a very simple
and ingenious device, can be instantly substituted for the heavy
one. The microscope is said to have been made by Spencer or
Tolles, and must be over twenty years old. A thin stage on the
same principle, and called by' the maker a Diatom Stage, has
been recently brought out by Mr. Zentmayer.

The same object is also attained by means of the secondary
stage, invented by Mr. Lewis Rutherfurd. This is simply a
skeleton stage, which is placed on the ordinary stage, and is
raised so far above it that the illumination may be applied
between them. Rays of great obliquity may thus be passed
through the object. Rutherfurd's skeleton stage forms also an
admirable safety stage, since the object, being held against the
under side of the skeleton stage, yields to the slightest pressure
of the objective. Mr. Spencer has also taken advantage of this
principle, and so formed the under side of the stage in some of
his stands, that the object may be pressed against it by the
clips, which for this purpose are pushed through from below
upwards. In focussing, the objective is passed through the

82

SELECTION AND USE

stage if necessary. Great obliquity, and perfect safety against
breakage of the object by pressure of the objective, are secured.
When the microscopist is using valuable slides, costing from
fifty to two hundred dollars, the latter feature is one of great
importance.

Sub-Stage.— The sub-stage is a device for holding and
adjusting illuminating apparatus beneath the stage. It should
be moved by rack work, so that paraboloids, achromatic con-
densers, etc., may be brought accurately into focus, and it
should also have adjustments for accurately centering these
various pieces of apparatus. We regard the sub-stage as one of
the most important parts of the stand.

The Mirror. — In microscopes of modern construction, the
mirror is made of glass, coated with pure silver, (ordinary mir-
rors are coated with an amalgam of mercury and tin) and the
best instruments are provided with two mirrors, one plane and
the other concave. In both mirrors the surfaces of the glass
should be accurately ground and polished. Blown glass will
not answer. The plane mirror reflects the light just as it falls
on it, while the concave one causes parallel rays (such as those
from the sun) to converge and meet at a point, and renders
divergent rays (such as those from a lamp) either less
divergent, parallel, or convergent, as the case may be.
The concave mirror should be large, but the plane mirror is as
efficient when small as when large. Where but one mirror is
provided, the concave is the one usually selected.

In English and American microscopes of even the cheapest
construction, the mirror is so arranged that it may be made to
send a beam of light through the object very obliquely. This
is absolutely necessary for some purposes, but not for the ex-
amination of ordinary objects. The ability to use oblique light,
as it is called, is, however, a great advantage. It not only
enables us to resolve lined objects, but to secure important
changes in the illumination of common objects. A very fair
dark ground illumination may also be secured by placing the
mirror in such an oblique position that none of the light can
enter the object-glass directly.

OF TKE MICROSCOPE. ' 83

The Body. — The only point connected with the body of the
microscope which requires consideration is its size, and this
must of necessity vary so much according to the purposes to
which the microscope is to be applied, that no rule can be
laid down. Pocket microscopes are of necessity small; micro-
scopes intended to give very great magnifying power must be
large. A standard size is seven to eight inches in length. The
diameter is not of very great importance in bodies of moder-
ate length, but Beale says that in his long tubes, intended to
produce great magnifying power, a diameter of two to two-and-
a-half inches was found to be absolutely necessary. An inch and
an eighth is a good size for ordinary instruments. Since a very
long body is inconvenient when the microscope is used in a ver-
tical position, the best instruments are furnished with a

Draw Tube, whereby, the length of the body may be
varied at pleasure. As explained in a former paragraph, (page
15,) when the distance between the eye-piece and the object-
glass is increased, the magnifying power is increased also.
The draw tube, therefore, gives us the means of varying and
adjusting the magnifying power of the microscope, and this is
sometimes of great use. Thus, suppose it were required to
draw an object to a scale magnified exactly one hundred diame-
ters; it might be impossible to procure an eye-piece and an
objective that, with a fixed length of body, would give exactly
this amplification, but when we are able to vary the magnify-
ing power by changing the length of the body, it is easy to get
at it exactly. This, however, is but one of many advantages
afforded by the draw- tube. If the objective be good, and the
eye-piece not very high, an easy and very satisfactory way to
increase the magnifying power of the microscope is to lengthen
the body by means of an additional tube. Dr. Beale, who has
been a most successful worker with high magnifying powers,
tells us that in practice he has found this plan so much more
advantageous than the use of a deep eye-piece, that he never
uses the latter. In some cases he has extended the length of
the tube to two feet with good results. "We have frequently
adopted this method, and where brass tubing could not be had
we have used smooth writing paper, and a little paste with
very good effect.

84 SELECTION AND USE

The inside of all draw-tubes and bodies should be well
blackened. When bright or white the glare greatly injures the
defining power. When draw-tubes are so arranged that they
rub against the inside of the tube forming the body, they in-
variably make the latter bright by friction. They should,
therefore, always slide in a collar. It is always well to have the
lower end of the draw-tube furnished with the Society screw, as
by this means it is sometimes possible to use objectives of
greater working distance than could otherwise be employed.

Adjustments for Focussing.— In the cheaper forms of
the microscope the adjustment is made directly by hand, one
tube sliding within another. In a better class of instruments
the objective is brought nearly into position by sliding the
body through an outer tube, and then the final adjustment is
made by means of a screw or other mechanical means. But in
all the best microscopes, the coarse adjustment, as it is called,
is made by means of a rack and pinion, while the fine adjust-
ment is made in the manner just mentioned. Instead of a rack
and pinion, a chain is sometimes employed, and the coarse
adjustment is also made in some cases by screws of very wide
pitch, and similar devices. Nothing, however, can equal a
smoothly cut and well-fitted rack and pinion. It is sometimes
alleged that the chain is more delicate, but this is not so. We
have now in our possession a cheap, but well made microscope,
the rack and pinion of which is so delicate, that with it we can
focus an objective of an eighth of an inch focal distance with
sufficient accuracy for all ordinary purposes.

For ordinary purposes, especially the work of the physician
and medical student, the coarse adjustment may be more easily
dispensed with than the fine one, but at the same time it must
be remembered that any mode of adjustment in which the body
is liable to turn round, is incompatible with the use of many
important pieces of apparatus. Thus, for example, any turn-
ing of the body interferes with the use of the double nose-
piece, the polariscope in its higher applications, Prof. Smith's
opaque illuminator, etc. A rack and pinion, or its equivalent,
should, therefore, always be chosen, especially as it does not
add more than five or six dollars to the cost of the instrument.

OP THE MICKOSCOPE. 85

Of devices for fine movements the name is legion. An old
plan is to place the object upon a plate attached to the stage,
and move it towards the objective by means of a fine screw.
This is a cheap and convenient method. In general, however,
it is objectionable to have the object move, as it interferes with
many of the finer methods of illumination, etc. A very com-
mon plan is to make the nose-piece, which holds the objective,
movable. The only objection to this device is that it introduces
a change in the length of the body, and, consequently, a slight
change in the magnifying power, every time a change is made
in the focal adjustment. This change is too slight to be ob-
servable, but is sufficient to interfere with delicate micro-
metric measurements.*

To avoid this difficulty, various devices have been intro-
duced. The most common arrangement is a very old one, in
which the entire body, including the arm and coarse move-
ment, is carried by the fine adjustment. In its general features
this plan is very old, and has at different times been adapted by
Grunow, McAllister, George Wale, Zentmayer and others. One
of the best, however, is a recent invention by Mr. Gundlach, in
which the body is hung on two parallel springs, and moved by
a fine screw. This plan, although theoretically defective, is
practically perfect. The other method, just mentioned,
although theoretically perfect, is, in practice, rarely as good as
the Gundlach device — a curious, but not an unusual state of
things in mechanics.

In judging of the goodness of either a fine or coarse adjust-
ment, the points to be observed are the delicacy and accuracy
with which the objective may be moved to and from the stage,
and the freedom from twist or apparent displacement of the
object. In many microscopes, when a high power is used, and
the body is moved for the purpose of adjusting the focus, the

*It has been alleged that this increase or decrease of magnifying power
is more apparent with the higher powers than with the lower powers.
Indeed, it has been said that with high powers the change of magnifying
power is qnite perceptible. This, of course, is mere imagination, as any
one of an arithmetical turn of mind can see. Indeed, the facts would
seem to be rather the other way.

85 SELECTION AND USE

object is actually thrown out of the field of view. Such a
microscope should be at once condemned.

It is a common fault with instruments in which the coarse
adjustment is made by rack and pinion, that the body cannot
be raised high enough to admit the use of low power objec-
tives. This defect should be avoided. Avoid, also, all instru-
ments in which the coarse adjustment is made by moving the
stage by means of a rack and pinion.

Tlie Diaphragm.— Nothing tends so much to obscure our
view of the finer points of structure in any object as to have
them "drowned" in a superabundance of light, consequently
in order to regulate the amount of light which passes through
the object, a diaphragm is employed. As ordinarily con-
structed, it is simply a metal plate placed below the stage, and
pierced with holes of various sizes, which may be brought ex-
actly under the field of view, the small holes allowing but a
small amount of light to pass, while the large ones admit a full
stream. Considerable difference of opinion exists amongst
microscopisis in regard to the proper position of the dia-
phragm. Thus Carpenter says (page 133) that unless placed
half an inch below the object it is comparatively inoperative.
Continental histologists, on the other hand, allege that it is
useless unless placed close up under the object. Microscopes
constructed according to both these plans are to be found in
market. Where the microscope is furnished with a sub-stage,
the distance of the diaphragm from the object is variable at will.

It is obvious that when the diaphragm is placed at a consid-
erable distance below the object, the illumination is purified,
as it were, from all cross rays. When the diaphragm is placed
close to the object-slide, the illuminated field of view is con-
tracted. The action in this case, however, is somewhat com-
plex, owing to the action of the slide in modifying the course
of the rays.

Several very ingenious forms of Iris or graduating dia-
phragms have been devised, by which the size of the hole may
be changed without interrupting the observation. They are
exceedingly convenient, and present advantages which more
than counterbalance the cost.

OF THE MICROSCOPE. 87

ObjectiTes.— These are confessedly the most important
parts connected with the microscope; they therefore deserve
the greatest care in their selection. In a former section, we
fully explained the general characteristics of the different
kinds of objectives in market, and detailed the best methods of
of testing them. A careful study of that chapter will, we hope,
enable the beginner to avoid a glass that is absolutely bad,
though we must acknowledge that all experienced microscopists
are agreed that no amount of mere reading will enable a
novice to pronounce a correct judgment upon the quality of an
objective, unless its defects should be very glaring indeed. In
this place we shall confine ourselves to a few hints in re-
gard to those features which adapt objectives not only to
special kinds of work, but to the skill of different classes of
workers. For it is an undoubted fact that objectives which in
the hands of skillful microscopists, and on certain classes of
work, would give extraordinary results, would in other hands,
and for other purposes, prove of far less value than lenses of
what is commonly considered a greatly inferior grade.

We do not here propose to take part in what is called the
"battle of the object-glasses," such a discussion being out of
place in an elementary work like the present, but we think few
will be hardy enough to deny that one who has a taste for such
things, but has neither the money required to purchase a first
class glass, nor the time necessary to acquire the requisite skill
to use it, had better work with a cheap French triplet than not
work at all. Moreover, it is astonishing how far patience, skill
and experience will go to make up for a deficient instrument,
while at the same time, it is unfortunately true that some who
possess the very best glasses, and have done the most to
throw ridicule upon all work done with inferior lenses, have
never made a single contribution of the slightest importance
to any department of microscopical science.

In a former chapter we discussed at length the different
qualities of object-glasses, and showed how these various quali-
ties might exist in very different degrees in different ob-
jectives. It is, of course, obvious that the extent to
which any one quality should be sought in a particular
glass, must depend altogether upon the kind of work that is

88 SELECTION AND USE

to be done. To those who are addicted to what Holmes calls
"fighting objectives," resolution will be the quality to be
desired; others will prefer penetration, flatness of field, etc.
In our estimation, for the purposes of ordinary scientific work,
we would assign to these qualities values in the following
order: 1. Defining power; 2. Freedom from aberration of
form; 3-4. Resolution or Penetration; 5. Working distance;
6. Achromatism; 7. Flatness of field. The first quality that should
be secured in every lens is undoubtedly defining power, and this
whether its angular aperture be high or low. Achromatism we
place low in the scale, because unless so marked as to interfere
with the defining power, it does no harm._ Flatness of field we
place last, because it will be found that perfect flatness of field
is very seldom combined with first rate definition. Indeed, we
have heard one of the most celebrated makers of objectives
assert that the two qualities are to a certain extent antagonistic.
In giving advice in regard to the selection of an objective,
one of the points concerning which it is most difficult to arrive
at a decision, is that of angular aperture. Fortunately, however,
experienced microscopists may safely be left to decide this ques-
tion for themselves, and since those who have had no experience
will find it difficult to use objectives of very wide aperture, it will
certainly be prudent for them to choose those of moderate angle.
Objectives of very high angle are worthless, unless the illumina-
tion is well managed, and the adjustment for thickness of cover
properly regulated. On the other hand, a good non-adjusting
lens will give very fair results, with but a moderate amount of
skill on the part of the user. Almost all our best makers now
produce objectives of moderate angle, which do not adjust for
thickness of cover, but which have considerable resolving
power. We have now before us a one-fifth which costs but fif-
teen dollars, and which will easily resolve the Pleurosigma An-
gulatum by central light. An important point for consideration
will, of course, be, whether or not the glass is intended for
original work, or merely for the study or examination of well
known objects. The work of the physician is chiefly amongst
well known objects, and may be very satisfactorily accomplished
by means of good non-adjusting objectives, a great point in
favor of such glasses being that work may be done with them

Otf THE MICROSCOPE. 89

more rapidly than with glasses that require greater care and
skill. The same is true of the elementary studies of the bot-
anist and histologist, carried on under the guidance of a com-
petent teacher. And as in all such cases it is easy to find out
the special thickness of glass for which the object-glass has
been corrected, and to provide a supply of the proper thickness,
the absence of a means of adjustment for cover thickness is not
very important. But for all the higher class of studies, good
glasses, with well-made adjustments for thickness of cover, are
indispensable.*

Objectives of very low angular aperture, are, however, to be
equally avoided. There is a want of light, and an indistinct-
ness which renders them worthless. It is generally said that
the superiority of large angles is most marked in the objectives
of high power, and that for low powers the common objectives
do very well. In our judgment, however, the superiority of
the low powers is quite as marked as that of the higher ones,
and much more available to the beginner. It is true that the
superiority of a well made one-sixth of high angle, over any
triplet of whatever focal length, is immeasurable, but at the
same time it is equally true that the view of an opaque object
seen through an inch-and-a-half objective, carefully corrected, is
as much superior to the same as seen through a common triplet,
as it is possible to imagine. We have now before us a speci-
men of bone of very open structure, mounted as an opaque
object. Seen through a first class inch-and-a-half objective, it
presents almost a stereoscopic appearance, and the entire struc-
ture is easily made out. The view afforded by a very fair French
triplet (No. 0) is so markedly inferior, that any person who
should see the two would never again use a cheap objective, if
he could afford to get a good one. Moreover, the objection
which we have just urged against objectives of high power, and

*

*It is not long since a professional maker of microscopes, and one who
geem« to stand high in the favor of the medical profession, tried to per-
suade the author that the covering-glass exercised no influence on the
action of the objective, and that a non-adjusting glass could be made
with as great a resolving power, as one constructed so as to adjust for
different thicknesses of covering glass ! To such men, a famous micro-
scopist used to apply the term " shopticians," and they deserve it.

90 SELECTION AND USE

wide angle, viz. , that they are difficult to use by novices, does
not hold in the case of low powers. A good inch, of compara-
tively high angle, is more easily used than a poor triplet.

A question which has considerably occupied the attention of
inicroscopists, is the value of objectives of high power, and their
efficiency as compared with those of lower denominations.
That in many cases considerable amplification or magnifying
power is absolutely necessary, admits of no doubt; but the ques-
tion to be settled is: suppose that we wish a power of 2,000 diame-
ters, would it be better to get this by means of a tenth of an inch
objective, magnifying 100 times, and a half inch eye-piece mag-
fying 20 times, or by a twentieth of an inch objective magnify-
ing 200 times, and an inch eye-piece magnifying ten times ?

It is not very many years ago since one of our ablest Ameri-
can objective makers held that a lens of a quarter of an inch
focus might be made to do anything that a lens of any power
could be made to do, and the ground of this opinion was that
the individual lenses of objectives as low as a fourth, could be
made so much more perfect than the smaller lenses of higher
powers, that this perfection more than counterbalanced the
greater magnifying power of the objective of shorter focus.
The reasoning here seems sound and obvious, but it has been
found in practice that for everything except resolution, the limit
to which the power of objectives may be carried, is far beyond
a fourth. For resolution it has, we believe, been found that a
well made tenth is capable of doing anything that any lens can
do; for other kinds of work sixteenths and twenty-fifths, and
even fiftieths and eightieths have been declared to possess
advantages that are obvious. This, however, is one of those
points upon which authorities differ; Beale, for example, favors
high powers; Carpenter and Frey seem inclined to think that
very high powers show nothing that cannot be seen by means
of objectives of gleater focal length.

French objectives of the numbers 1, 2, 3 and 4, if carefully
selected, are capable of doing really serviceable work. A few
years ago, some of the best known makers of American micro-
scopes used nothing else, even in microscopes costing $150, but
this course we can scarcely regard as judicious, for whenever
the microscopist is prepared to expend $75 or more for a micro-

OP THE MICROSCOPE. 91

scope, a large part of this sum should be laid out in the pur-
chase of objectives of the better class, the one-inch and one-
fourth, or the three-fourths and one-fifth being those that are
usually selected by beginners.

French triplets are, however, going rapidly out of use, from
the fortunate circumstance that objectives of low price and
excellent quality are now produced by several makers of repute.
It is well, however, for the reader to be on his guard against
a fraud which has been but too common of late years. Some
so-called opticians go so far as to add a little brass- work and
engraving, and sell these French triplets as objectives of Ameri-
can make. We do not here refer to the mere operation of attach-
ing the objective to an adapter, and fitting it in a brass box, for
this adds greatly to the convenience with which such minute
objectives may be handled and preserved, but to a sort of
"making over," by which they are completely disguised and
made to resemble the objectives of English and American
makers. It is hardly necessary to characterize such a pro-
ceeding.

Eye-Pieces.— The eye-piece that is at present almost uni-
versally used is the Huyghenian, which, when well made, gives
very excellent results. In the use of low powers, wheie a very
flat and large field is desirable, the Huyghenian eye-piece fails,
and the same is also true in regard to very high magnifying
powers, where the enlargement is obtained in a great measure
by means of the eye-piece. The extent to which the definition
of really good objectives is deteriorated by the use of eye-pieces
of great magnifying power, and the loss of light which they
occasion, render them practically useless. For high powers,
the solid eye-pieces of Mr. Tolles are vastly superior, while for
low powers, where a large flat field is desired, Kelner's ortho-
scopic eye-piece presents important advantages.

Mr. Gundlach has recently brought out a new eye-piece,
which he has named the periscopic, and for which a large field
and excellent definition are claimed. They are much more ex-
pensive than the Huyghenian. We have not had an oppor-
tunity of examining them carefully.

In determining the quality of an eye-piece, attention is to be

92 SELECTION AND USE

paid not only to its general excellence, but to its adaptability
to the objectives that are to be used with it. In the higher
departments of microscopy, the latter is a most important point,
but one which is too frequently neglected. It does not, how-
ever, come within the scope assigned to the present work, and
we, therefore, content ourselves with a few general hints.

The lenses composing the eye-piece, should be of homogeneous
glass — that is, free from air-bubbles, specks and striae, and the
surfaces should be well polished. These points require atten-
tion, because we have in our possession a microscope in which —
though it cost enough money to be free from such defects —
they are glaringly apparent. On looking through the eye-
piece at a strongly and evenly illuminated surface, the entire
field of view — that is, the whole of the bright circle that is
seen, should have the light evenly diffused over its surface,
and the edges or border of this circle should be sharp and
black.

Eye-pieces intended for first-class objectives should give a
large field of view; but on the other hand, if French objectives
be used, the field of view should be small, otherwise the defi-
nition will be poor. This is a point that is frequently over-
looked, and we have seen very fair object-glasses condemned
as worthless when used with a stand and eye-piece intended
for objectives of an entirely different class. It is an easy thing
to contract the field of view, by means of a round piece of thin
sheet metal, having a hole of proper size in the centre. As pre-
viously explained, such a piece of metal is called a diaphragm,
and should always be well blackened.

The magnifying power of every microscope depends upon
three things: The focal length of the objective, the length of
the body, and the eye-piece. Most microscopes are, therefore,
furnished with several eye-pieces, whereby the magnifying
power may be varied. There is, however, a limit to the extent
to which this may be done. The image obtained by very deep
eye-pieces, as they are called, is rarely satisfactory.

The different eye-pieces are generally denoted by letters — A,
B, C, D, etc. A being the lowest, and B, C, D, etc. , successively
higher. Some makers use numbers — 1, 2, 3, 4, etc. These
letters and number, are, however, entirely arbitary, in this point

OF THE MICROSCOPE. 93

resembling the numbers assigned to objectives by continental
makers. A great improvement upon this arbitrary and uncer-
tain system would be to assign to each eye-piece its proper
power expressed in inches. Thus, an eye-piece magnifying the
same as a simple lens of two inches focus, should be called the
two-inch eye-piece.

And here let us call attention to the terms deep and shallow, as
applied to eye-pieces. By all authors of repute, a deep eye- piece
is one of great magnifying power, while a shallow eye-piece ib the
reverse. See the Micrographic Dictionary, and the works of Car-
penter, Beale, Larduer, Frey, etc., etc. It is, therefore, singu-
lar that Dr. Lankester, in his popular little work, " Half-Hours
with the Microscope," should have committed the mistake of
giving definitions exactly the opposite, upon the ground
that eye-pieces of great magnifying power are always short,
while low eye-pieces are always long. It is evident, however,
that the terms are liable to give rise to confusion, and we pre-
fer the words high and low — the meaning of which is so ob-
vious as to require no explanation, as every body knows what
high magnifying power is

While clearness of definition and resolving power are the
most important qualities of every good microscope, magnifying
power is also of considerable consequence, as explained in a
former section. Therefore, every good microscope should be
provided with at least one eye-piece of considerable power. It
often happens that with the objectives and eye-pieces at hand,
the amplification, as it is called, or, in other words, the extent to
which the object is magnified, is not sufficiently great to enable
us to make out its structure, while the objective has not by
any means reached the limit of its defining power. In this
case a high power eye-piece, which costs comparatively little,
will greatly extend our power of successful examination.

ACCESSOEY APPAKATUS.

Every microscope should be accompanied with certain pieces
of accessory apparatus, which are necessary for the convenient
and thorough examination of objects, but which do not form
part of the instrument itself. Some of these are intended for

SELECTION AND USE

the better illumination of the ob-
ject, and will be described in the
section on "Light;" others are used
for the procuring and preparation
of objects, and will be described in
the section devoted to that subject.
The following are employed chiefly
for holding and presenting objects
that have not been " mounted:"

Stage Forceps.— This little in-
strument accompanies the oldest
M microscopes. It consists of a pair
| of very delicate forceps, such as
^ those attached to the forceps-car-
jfc rier in Fig. 4, which close by the
w spring of the jaws, and hold any ob-
g ject that may be placed in their
*** grasp. They are opened by pressing
| on the pins which are seen at the
gj sides. They are in general fastened
| to the microscope by being stuck
M into a hole in the stage, and the
^ object may not only be moved back-
l ward and forward, but it can be
turned round. The better class of
•JP forceps carry a small brass tube
(shown in Fig. 4) which is filled
with cork, and which serves to re-
ceive pins, etc. , for holding insects,
and other objects.

Foi ceps-Carrier. — However
well made the forceps may be, it is
almost impossible to slide, with suf-
ficient delicacy, the rod through the
tube that holds it. Consequently,
it is exceedingly difficult to bring
into the field of view, the exact part
of the object, that we may wish to

OP THE MICROSCOPE. 95

examine. To avoid this difficulty, the author, instead of insert-
ing the piii of the forceps in the stage, provides a special for-
ceps-carrier like that shown in Fig 4. This consists simply of
a metal plate, the size of an ordinary slide, and having a hole in
one end to receive the pin of the forceps. A large hole is
pierced through the centre, to allow the passage of light from
the mirror when that is needed. This plate is placed on the
stage like a common slide, and it can be moved with as great
delicacy as any ordinary object. The mode of using it is too
obvious to require further explanation. We have found it ex-
ceedingly convenient.

Object-Holder.— The importance of being able to present
an object to the light in all directions is well-known to every
microscopist. Many years ago we devised an object-holder for
effecting this, the construction of which is very simple and in-
expensive. It consists of a slip of metal, the size of an or-
dinary slide — three inches by one — having a hole in the centre

Fig. 5. — OBJECT-HOUDEE.

and a short pillar rising from one end, as shown in the en-
graving, Fig. 5, which gives a sectional elevation of the
instrument. Through this pillar runs a wire, carrying at one
end a milled head by which it may be turned, and at the other
a ring which holds a perforated block. This perforated block
has a milled collar on the lower end, so that it can be readily
turned in the ring that carries it. The hole passing through
the block is just the size of a stout pin, so that a disk of card
or leather, with a pin through it, will be held steadily when
the pin is inserted in the hole. The object to be examined is
attached to the surface of the card, by means of balsam or mu-
cilage, and it is obvious that by the combined rotations that
may be produced by the two milled heads mentioned, it may
be exposed to the action of the light in any desired manner.

96 SELECTION AND USB

The changes -which are produced in some objects when the
light is made to fall on then in different directions are very
marked. Thus, for example, the mineral known as specular
iron ore, when illuminated by light falling on it in one direc-
tion, is brilliant in the extreme, while when the light falls in
other directions it is dead and lustreless. And as it is not
always convenient to change the position of the lamp, it is a
great advantage to be able to turn the object round. The sim-
ple contrivance just described enables us to do this perfectly.

A more perfect arrangement, intended for the same purpose,
has been devised by Mr. Beck, of London. Mr. Beck's is,
however, more expensive than ours.

Plain Slides.— The common plain slides serve very well
for examining ordinary deposits in liquids. This is particu-
larly the case where inanimate objects, vegetables and minerals
are to be examined. Active animals require some contrivance
for keeping them still.

Tlie Concave Slide, as it is called, is simply a thick
slide with a cup-like hollow ground in the centre. Such slides
are cheap, and very convenient. A drop of water placed in one
of these concaves, and covered with a thin glass, may be exam-
ined easily and thoroughly with moderate power. It is
sometimes desirable to employ a cell with a perfectly flat bot-
tom of very thin glass. Such cells may be easily and conve-
niently made out of a slide of metal, or preferably of vulcanite,
through which a hole the size of the proposed cell has been
pierced. A piece of thin glass may then be cemented to the
under side of the slide, so as to form a water-tight cup. The
hole in our slides is round, and has, on the under side, a seat
or rebate, a little larger than the hole itself. In this rebate a
round glass cover fits, so as to leave the under side of the slide
perfectly smooth. Such cells are very convenient, as they are
easily cleaned, and are not difficult to repair when the thin
glass gets broken. The liquid is also easily covered by means
of a thin glass cover, and when full, considerable inclination
may be given to the slide before the liquid shows a tendency to
run out. Various other devices of a simple kind may be con-
trived by the microscopist for similar purposes,

OF THE MICROSCOPE. 97

\Vatcli-Glasses.-Dr. Beale recommends small flat watch-
glasses for holding liquids that are to be examined, and we
have found them very excellent. The best kind for this pur-
pose are those known as lunette glasses, which are nearly flat on
the bottom. They are awkward things to manipulate, how-
ever, unless some means is provided for holding them steady,
and moving them about on the stage. We use for this purpose
a strip of wood, three inches long, and so wide that we can
easily bore in it a hole, about one-eighth of an inch less in
diameter than the watch-glass, of which the smallest size should
be chosen. The thickness of the strip should be such that
when laid on any flat surface, the watch-glass will not come in
contact with it. Glasses held in this way are very convenient.

Fig. 11.— WATCH-GLASS HOLDER.

Watch-glasses are very convenient for examining a "dip " from
a pond or stream, but for this purpose they require a holder.
The little instrument shown in the cut is formed of three pieces
of bright tin, which are hinged together. In the upper piece
js cut a hole just large enough to receive a watch-glass, A ring

98 SELECTION AND USE

of metal of proper height surrounds this hole, and forms
perfect protection to the glass when the instrument is carried !
in the pocket. The lower slip of tin may be adjusted to any
angle, and by turning towards the proper direction, the light
of any bright cloud may be reflected up through the liquid.
All the joints are made stiff enough to remain in position when
once adjusted. A watch-glass arranged in this manner holds a
liberal supply of liquid, so that an entire "dip" may be
readily examined at once. We have found this little contrivance
far superior to more expensive arrangements. It packs into
small compass, and is safely carried.

Animalcule Cage.— This forms a very excellent means
for holding animalcules that are too active to allow of observa-
tion on slides, or in watch-glasses. A good idea of its con-
struction may be ob-
tained from the en-
graving, where it will
be seen to consist of a
plate of metal, three
by one inches, to the
Fig. 12.— ANIMALCULE CAGE. centre of which is

fixed a short tube. In

the upper end of this tube is fastened a beveled piece of glass,
and a second tube fits over the first, and has a thin glass cover
secured in its upper end. The animalcule is securely held
between the two pieces of glass, and the lower glass being
beveled on the edge, a drop of liquid placed on it is held be-
tween the two glasses by capillary attraction, and cannot spread
over the inside of the cage. This point is generally neglected
in the cheaper forms of the cage, in which the lower glass
is simply a plain disc burnished into the upper end of the
inner tube. The consequence is that when the two glasses are
brought together the liquid flows over the entire inside of the
cage, and the objects are liable to be floated out and lost. As
it is important that the distance of the two glasses from each
other should be easily and accurately regulated, the outer tube
should be slit, so as to make it springy. In this way it may be,
made to move with a soft and equable motion.

OF THE MICROSCOPE.

99

Fig. 13. — ZOOPHYTE TROUGH.

The Zoopliyte Trough. -This little piece of apparatus
is almost indispensable to those who desire to watch the growth
and development of the larger animalcules and small aquatic

plants. Several forms are in
common use, the most com-
plete being that shown in Fig.
13. The trough itself is sim-
ply a glass tank, to which is
fitted a slip of thin plate glass
that acts as a division, and
enables the observer to keep
the objects close up to the
front plate. The distance of
the dividing plate from the
front plate is regulated by an
ivory wedge, and the dividing

plate is kept firmly up to its place by means of a spring. This
contrivance enables us to regulate the thickness or width of the
tank, so that the interior of the vessel may be made so large
that it can be easily cleaned.

A smaller and simpler form of the Zoophyte trough
is shown in Fig. 14. It consists of a simple glass box,
open at the top. The back
of the box is formed of a
stoutish piece of plate glass,
to which is cemented three
glass strips, forming the
bottom and ends. The front
is formed of glass as thin as
is compatible with durabil-
ity, and is also cemented to
the end pieces. The width
of the trough from front to

back is generally from an eighth to a quarter of an inch. When
the trough is filled with water, and living animals are placed in
it, their changes and movements may be very readily watched.
Small troughs, such as that just described, are not difficult
to make, though the very low price at which they are sold (60
cents to $1.00) renders it scarcely worth the while of ordinary

Fig. 14. — ZOOPHYTE THOUGH.

100 SELECTION AND USE

microscopists to construct them for themselves. Where this is
desirable, however, the best method of making them is as
follows: Select a piece of plate glass, of the thickness of an
ordinary slide, and cut it about three inches by one and a quar-
ter. Then select another piece of glass, as thick as the trough
is to be deep (from front to back), and cut it to the size of the
outside of the trough. From the bottom of this piece of glass
cut a strip a quarter of an inch -wide, and from the sides cut
also strips of the same width. The centre piece may now be
thrown aside, and the ends of the side strips will make a tight
joint with the bottom strip. The three strips should then be
cemented to the large plate, and over them should be cemented
a piece of the thin glass used for covers^ The strongest cement
is marine glue, but we are told by Prof. Starr, who is well
known for his success in keeping and exhibiting living micro-
scopic objects, that marine glue is very apt to destroy ani-
mal life. He, therefore, uses old Canada balsam, and we have
seen a large variety of minute forms of animal and vegetable
life which had been kept for months in a healthy condition in
such troughs or cages.

Compressorium.— In the examination of certain objects,
it is frequently necessary to flatten, and even to crush them,
in order to render their structure visible. This is best accom-
plished by means of a well-made compressorium, of which
there are several different kinds in use. The ordinary com-
pressorium consists of a metal plate, in the centre of which is
fixed the disc of glass upon which the object is laid. A second
disc, fastened in a ring which is hung at the end of a lever, by
means of two pivots, is pressed against the first by means of a
screw, which tilts the lever. In this way a very strong pressure
may be exerted, while, owing to the free movement of the ring on
the pivots, the plates of glass always remain parallel to each other.

A compressorium which may be used to examine either side
of an object is constructed by Messrs. Ross & Co. In this form
the two plates are brought together by means of a wedge,
which is moved by' a screw.

Gravity Compressorium.— The animalcule cage above
described was invented by Varley, many years ago, and is very

OF THE MICJROSCOPE. 101

excellent, but requires considerable skill in its use. It is diffi-
cult to adjust the pressure with sufficient delicacy, and we are
apt either to crush the animal or leave it too free in its move-
ments. These difficulties are avoided by the apparatus shown
in the engraving. Two plates of metal are provided, each with
a hole in the centre, which hole is covered with a piece of thin
glass. At one end the upper plate has two pins, which fit into
two holes in the lower plate, and serve to prevent all side
movements. A screw passes through the other end of the
upper plate, and serves to separate the two. A drop of water
containing an animalcule having been placed on the thin glass
attached to the lower plate, the upper plate, with the screw
projecting sufficiently from the under side, is laid on it. Then
by turning the screw, we can bring the two plates together to
any required degree of nearness, and with the utmost delicacy.

Fig. 15. — THE GRAVITY CO3IPEESSORIUM.

Any minute animal may thus be firmly grasped, without crush-
ing if, while the compressing power exerted by the mere weight
of the metal plate is in almost all cases sufficient, even for the
complete flattening out of small worms, etc. Even such crea-
tures as the larva of the common gnat or mosquito may be
completely crushed by the weight of a plate less than the eighth
of an inch thick; and, where greater force is required, it is of
course easy to apply the pressure of the finger. In the latter
case, no danger of exerting too great a pressure need be in-
curred, as the projecting screw prevents all that. The want of
parallelism between the plates does not prove a serious objec-
tion, as it is so very slight that it is hardly perceptible in the
short distances ordinarily under observation. Where, how-
ever, it is desirable to avoid this defect, screws may be substi-
tuted for the pins, and the points may be made to work in
holes bored half through the lower plate.

102 SELECTION AND USE

Where animalcule cages are not accessible, a small animal
may be held between a common slide and a thin cover. To
prevent crushing it, a hair or even a thread may be placed be-
tween the cover and the glass. A German author recommends
the use of fine gauze or netting, in the meshes of which an
animalcule may be held very conveniently. Acting on this
idea, we took a thin metal plate, and bored it full of holes of
various sizes. An animalcule placed in one of these holes may
be kept in the field of view for any length of time, and exhibited
to those who desire to see it, but it cannot be kept quiet for
scientific examination. We like a piece of fine wire-gauze,
better than cotton or linen netting.

Growing StideSo— Where it is desirable to keep the same
living object for a considerable time, so as to watch its changes,
it is necessary to use what is called a growing slide, by which it
may be regularly supplied with air and moisture. A large num-

Fig. 16. — GROWING SLIDE.

ber of complicated devices have been described for this pur-
pose, but the following simple contrivance answers the end
very well; we have used it for years. To one end of a common
slide with a concave centre, cement a small bottle, as shown in
the figure. This is easily done by means of a little marine
glue. The glue, cut in small pieces, should be laid on
the slide at -the point where the bottle is to be attached;
the slide is then to be gradually heated until the glue is
softened, when the bottle is laid on and moved back and
forth until it has been thoroughly imbedded in the cement.
The bottle is filled with water and corked, the upper side
of the cork having two notches cut in it, one for the en-
trance of air, and the other for the passage of a loose cotton

OP THE MICROSCOPE. 103

thread. The object is placed in the concavity, covered with a
piece of thin glass, and the end of the thread is carried under
the cover by means of a small notch cut in the slide with a
file. The bottle must be filled with very pure water, otherwise
the salts, etc., contained in it, become concentrated under the
thin cover, owing to the evaporation, and destroy the object.

Frog Plate. — The circulation of the blood in the capillaries
of living animals may be observed in the web of the frog's foot,
the tail of a small fish or water-lizard, the larvae of many insects,
the ear of a young mouse and the wing of the bat. The tongue
of the frog is also a favorite subject with some, and dissections
of the living animal have also been made, and the circulation ob-
served in the parts thus displayed. Except, however, for im-
portant investigations, we have no right thus to inflict torture
and destroy life, and, moreover, the obvious cruelty of the
means employed, will to most minds destroy nearly all the pleas-
ure arising from the beauty of the exhibition. Fortunately the
circulation of the blood in the foot of the frog may be witnessed
without subjecting the animal to any pain. For this purpose
the web of the hind foot is spread out over a piece of glass,
which is held in a frog-plate, as it is called, to which the little
animal is attached. The frog-plates usually sold, however, do
not lie conveniently on the stage of a small microscope; they
are apt to tip up, and there is no means of attaching them
firmly to the stage, so that it is impossible to incline the mi-
croscope. The annexed engravings represent a frog-plate, in
which these difficulties are avoided. As seen in the figure, it is
of the usual form, and has a large opening, into which is
burnished a piece of thinnish plate glass upon which the web
of the foot is laid. Around this opening is bored a number of
small holes, through which threads, tied to the frog's toes, are
passed and held firmly by small wooden pins. A series of holes
are also bored on each side and cut out at the edge, so that it is
unnecessary to pass the twine through the holes, as it may be
readily slipped into them. The frog may be enclosed in a bag,
one foot being left out, but a simpler and better plan is to
swathe him in a strip of muslin two inches wide and eight to
twelve inches long. The muslin is dipped in water, and the

104

SELECTION AND USE

Fig. 17.

FROG PLATE, PLAN AND SECTION. Fig. 18.

OF THE MICROSCOPE. 105

frog rolled up in it and laid on the plate, where he is held by a
few turns of light packing twine passed into the slits in the side
of the plate and carried from one to the other and over the
animal. Small frogs are best for this purpose, but when too
small they are not easily handled. The position of the animal
on the plate is so arranged that the foot may be spread over the
glass plate that fills the large opening.

The plate is attached to the stage as follows: A cylindrical
brass block (Fig. 10) is provided — this block having a milled belt,
which renders it more easily turned. The upper surface of
this block receives a screw which pas'ses
through a slot of considerable length, cut in
the frog plate, thus allowing a wide range of
motion on the part of the latter; the under
surface of the block receives a second screw,
which serves to secure it to the stage of the
microscope, as shown in Fig. 9. The holes
for these screws are not in the same line,
their axes being about a quarter of an inch
Fig. 19. apart, and the consequence is that when the

brass block is rotated on the stage, the screw
that passes through the plate acts like a crank in relation to
the plate, and moves it longitudinally, provided it (the plate) is
kept from rotating with the block. The upper screw is inserted
with sufficient tightness to keep the plate from shaking, but is
left so loose that the plate can be readily moved back and forth.
Hence, while the plate is attached to the stage in such a way
that it can not tip up or fall off, it may readily be moved in two
directions, one the arc of a comparatively large circle, and the
other a longitudinal motion at right angles to this.

This frog plate forms in fact a sort of mechanical stage which
admits of very delicate movements being very steadily made.
Where this plate is used, the microscope may be inclined to
any angle, and no jerking or starting of the animal can displace
the portion of the foot that is under observation. Different
parts of the same foot and different corresponding parts of dif-
ferent feet are more or less suited to purposes of observation,
according as they are more or less transparent and more
or less fully supplied with vessels. It is therefore of great ad-

106 SELECTION AND TJSE

vantage to be able to select that part which answers our pur-
pose most perfectly, and this plate affords peculiar facilities for
effecting this.

Table.— The table used for supporting the microscope
should be firm and substantial, so that all shake and vibration
may be avoided. Those who use very high powers, and desire
to avoid vibration as much as possible, will find that a barrel or
box, filled with sand, and resting on three feet, makes the best
support. Some years ago, having some rather delicate investi-
gations to make, we constructed a table in this way, and found
the results very gratifying. Our table was arranged as follows :
a common barrel, cut down a little, and filled with sand, was
supported on three stout blocks nailed to the bottom. The
table proper was made of plank, nearly square, and it entirely
covered the top of the barrel. It was supported by a -f- shaped
piece of wood, which was fastened to the centre of the table,
and descended into the sand. With such a table, walking 011
the floor, and the passage of heavy teams in the street, produce
no vibration, though, on an ordinary table, they render work
with high powers almost impossible.

Where several persons wish to look through the same micro-
scope, it is very awkward if each one has to get up and go to
the instrument. At the same time it is of course impossible to
move the microscope without moving the arrangement for illu-
mination also. This difficulty has been avoided by means of
revolving tables, around which the observers sit, each one in
turn examining the object, as the microscope is passed round to
him. This is a very excellent, but a somewhat expensive ar-
rangement. The same end may be attained by placing the mi-
croscope, lamp, etc., on a smooth board of suitable size and
shape, and passing this board to each observer in turn. The
board, carrying microscope, lamp, etc., may be made to slide
quite easily, and if placed on three feet, it is tolerably steady.
Such a support, however, is not to be chosen where the micro-
scope is used for scientific investigations.

OF THE MICEOSCOPE.

107

Fig. 20.— STBAIGHT NOSE-PIECE.

Double Nose-Piece.— This is one of the most useful
accessories that the microscopist can possess. The result to be
obtained, and the method of accomplishing it are obvious. The

nose-piece screws on to the
nose, or lower end of the body
of the microscope, and is
fitted to receive two objec-
tives of different powers,
either one of which maybe
brought into action by simply
turning the nose-piece. In

this way a low power may be used for finding objects and ex-
amining them as a whole, while the details may, without
trouble, be subjected to an
object-glass of much higher
power. Two forms of the
nose-piece are in use. The
older form is straight, as in
Fig. 20; the later form is
bent, as in Fig. 21. The lat-
ter form is altogether the most
convenient. Nose-pieces ca-
pable of receiving three or
four objectives have been con-
structed, and a very old mi-
croscope, at one time in our
possession, had a nose-piece
with eight objectives! The
modern nose-piece, so ar-
ranged as to be capable of

carrying the best objectives, is FiS- 21.— BENT NOSE-PIECE.

the invention of Mr. Brookes.

ILLUMINATION— SOUKCES OF LIGHT.

Sun Light.— It is generally acknowledged that the best
light for microscopical purposes is that of the sun; not direct
sunlight, however, for this is altogether too intense, but sun

108 SELECTION AND tJSE

light reflected from a white wall, or a light fleecy cloud. Sun-
light is something which we cannot command at will, and, there-
fore, the microscopist can do nothing more than select the lo-
cation of the room which he occupies. In general a room with
a northern aspect is to be preferred; if there should also be
windows looking towards the east or west, so much the better,
provided they can be completely darkened when not in use, as
cross lights produce a bad effect.

Artificial Liglit.— While good daylight is the best source
of illumination, poor daylight is one of the worst, and we have
frequently, during the day, obtained by the use of lamps and
candles, results which could not possibly be secured by natural
daylight. At the present time, gas-light, lamp-light and can-
dle light are the most available means of artificial illumination.
Candles are rarely used except when the microscopist is travel-
ing, or in a peculiar situation, but a good candle gives very fair
results, especially if the flame be protected from currents
of air, which may easily be clone by extemporizing a
shade out of a piece of glass tube or small lamp chim-
ney. Wax, paraffine or sperm candles should be chosen, as
they give a clear, white flame. Common tallow candles give a
dull yellow flame of inferior quality. Gas-light, as obtained
from the ordinary, flat, unprotected burner, is not sufficiently
steady; it flickers and changes, and for microscopy this is the
worst fault that an artificial light can have. Where gas is em-
ployed it is, therefore, necessary to use an argand burner, with
a glass chimney. Light obtained in this way is in general
very excellent. But the most convenient, as well as the best
means of illumination, is a good lamp, of which the ordinary
student's lamp is, on the whole, perhaps the best kind. It
gives a pure, steady and intense light; it is easily regulated,
both as regards brightness, and also position, and conse-
quently direction, and it may easily be procured almost any-
where. In default of a good student's lamp, any of the ordi-
nary lamps with circular, or flat wicks, may be made to answer.
Where a large quantity of light is required, as in the illumina-
tion of large opaque objects, the circular, hollow wick, from
the superior brightness and whiteness of the light, is always to

OF T3E MICKOSCOPE. 109

be preferred. But where a small light of great intensity is
needed, the common flat wick, turned edgewise to the mirror,
answers very well. It is a curious fact that flame is transparent
to light, and, therefore, the greater the depth of flame, the
more intense is the light. This is easily tested by looking at
the flame of a common hand lamp sidewise and edgewise. In
the latter case the eye receives the light from the entire flame
concentrated to a mere band.

Several varieties of lamps have been devised specially for the
use of microscopisbs, and some of them are very excellent,
the most perfect being that devised by Dr. Drysdale and Rev.
W. H. Dallinger, and described in the Monthly Microscopical
Journal for April, 1876.

It is hardly necessary to say that all kinds of oil have been
displaced by the mineral oils ordinarily called kerosene.

Very intense light, such as that from burning magnesium,
the calcium light, the Bude light and others, have been tried,
but without material advantage. Many years ago, we arranged
a common kerosene lamp, so that the air surrounding the flame
could be enriched with a supply of pure oxygen when neces-
sary. Dr. Beale describes the same thing in his work, but does
not seem to regard it as of any advantage. When used as a
source of direct light, however, we found that it more nearly
resembled sunlight than any other artificial source of illumina-
tion. A large diaphragm or shade, with an aperture of mod-
erate size, was placed close to the light, which was placed at
some distance from the microscope, and the rays passed di-
rectly through the object, not being reflected from a mirror.
The results in some cases were well worth the trouble incurred.
It is probable that in some cases very excellent results could be
obtained from the electric light if properly arranged. This,
however, is a department of microscopy which is certainly not
suited to beginners, and we, therefore, dismiss it.

The rays of light, from whatever source obtained, are either
parallel, convergent or divergent ; and in the illumination of
transparent objects the character of the light, as depending
upon these features, is of marked importance. This subject,
and the action of lenses and mirrors in changing the relative
direction of the rays, should be carefully studied by the stu-

110 SELECTION AND USE

dent, who will find it fully discussed in any work on optics.
The general principles may be best explained by a few experi-
mental illustrations.

Take a piece of cardboard about six inches square, and in it
punch a hole about half an inch in diameter. If this card be
held in front of a wall upon which the sun is shining strongly,
we will see the shadow of the card and a round spot of light
exactly the size of the hole. If the card be now moved away
from the wall, the shadow and the bright spot will still remain
of the same size, showing clearly that the rays proceeding from
the sun are sensibly parallel. The same holds true of a bright
cloud or a white wall placed at a great distance; but when the
wall or other reflecting object is very near, the rays no longer
possess this character to the same extent.

If in the first experiment the wall be illuminated by a candle
instead of by the sun, it will be found that as the card is moved
from the wall the shadow and the spot become larger, showing
that the rays are divergent instead of parallel. The same effect
is produced by fixing both the lamp and the card on a stand
and moving them away from the wall.

Convergent rays, that is rays that tend to meet at a point,
can be obtained only by passing parallel or divergent rays
through a lens, or reflecting them from a concave mirror. By
carefully arranging a large convex lens in the path of rays that
are divergent, it is easy to render them parallel. They are
known to be parallel when the bright spot which they make on
a fixed surface, after passing through a hole, is not varied in
size by changing the position of the hole.

The variations which are produced in the appearances of
objects when they are viewed by light possessing these different
characteristics can only be learned by practice, and the young
microscopist should experiment in every conceivable way.

Whatever be the source of light employed, most objects may
be viewed by means of any one of several very different
methods. Thus, an object, if transparent, may be viewed by
transmitted light, that is, by light reflected from the mirror, and
passing through the object. If opaque, it may be viewed by re-
flectedlight, in which case the light that passes to the eye through
the microscope is reflected from the surface of the object.

OF THE MICROSCOPE.

Ill

ILLUMINATION OF OPAQUE OBJECTS.

Diffused Light.— This term is applied to ordinary day-
light or lamp-light, allowed to fall on the object without the
intervention of any special means of concentration. That dif-
fused light may be available for the illumination of objects, it
is necessary that the objectives be good. Objects which, with
ordinary triplets of low angular aperture, are entirely invisible,
become beautifully distinct when a better class of objectives is
used. Under favorable circumstances the view obtained in this
way of any well marked object is very pleasant.

Bulls-Eye Condenser.— This is a large lens of compar-
atively short focus, which is made to condense the light on the
object in the same way that the common burning-glass acts, but
with effects greatly less marked, since the light is so much less
intense. In some cases the condensing lens is attached to the
microscope, and in some special cases this is very convenient,
but where there is only one condenser, it should be mounted on
a stand, as shown in Fig. 22, so that it may be
placed at any height and turned in any direc-
tion. Placed between the object and the
lamp, it collects the rays of the latter to a
focus which brightly illuminates any object
upon which it may fall. Opaque objects,
which by diffused light are barely visible
under the microscope, become very distinct
and clearly denned when thus illuminated,
and many of them, such as the wings of in-
sects and certain minerals, appear in the most
gorgeous colors, which, however, are perfectly
natural, and are not the result of chromatic
defects in the lenses.

In viewing an opaque object by reflected
light, it is evident that we are enabled to
judge of the irregularities of the surface
largely by means of the shadows cast by the
prominences. By raising or lowering the lamp, and also the con-

Fig. 22.— BTTLLSrEYE
COHDENSER.

1 SELECTION AND TJSE

denser, the direction and extent of these shadows may be greatly
varied. Hence one of the advantages of the students' lamp.

An important use of the condensing lens is to change the
direction or character of the rays employed. Thus, when a
lamp is in use the rays are divergent, and the easiest way to
render them parallel is to pass them through a condensing lens.
To effect this the distance of the lens from the lamp must be
exactly the same as that at which it brings parallel rays to a
focus. In other words, the lens must be at a distance from the
lamp which is exactly equal to its focal distance for parallel rays.

Condensing lenses are made of all sizes, and some of them
are quite expensive, but we have frequently obtained wonder-
fully fine results by means of a cheap lens of small size, but
good form. A condensing lens is, perhaps, the most important
accessory that can accompany a microscope.

Side Reflector. — This is a small silvered concave mirror,
which is used to throw the light on the object for the same pur-
pose as the condensing lens. The results which it gives are
slightly different, and it is a most valuable means of illumina-
tion. It has not been so generally introduced as it deserves to
be, and few microscopes are furnished with it unless to special
order. It should always be used in combination with a bulls-eye
condenser, as light of much greater intensity is thus obtained.

The Liefoerltiihii.— This was one of the first instruments
used for illuminating opaque objects. It consists of a small,
concave, spherical mirror, through the centre of which the ob-
jective passes, the focus of the mirror and objective coinciding.
The object must be small, and is generally mounted on a small
circular disc of leather or card, which stops out the central
rays, while the light which passes round it strikes against the
concave mirror, and is reflected back again upon the object.
The Lieberkuhn gives very brilliant effects with some objects, but
it is not very highly prized by modern scientific investigators.

Smith's Illuminator. — This admirable device is due to
Prof. H. L. Smith, now of Hobart College, Geneva, N. Y., and
is intended for use with objectives of such high power, as pre-

OP THE MICROSCOPE.

113

vents the use of the Lieberkuhn, condensing lenses, reflectors,
etc. Several different arrangements are used. The first was a
small annular silver reflector, placed just above the back lenses
of the objective. A hole in the side of the brass mounting
admitted the light, which was thus thrown down through the
lenses of the objective, on to the object, and back again to the
eye. For the silver reflector, some manufacturers substitute a
thin plate of glass, and with excellent results. This illuminator
has been used with immersion objectives of high power, and
with very good results.

The Parabolic Reflector was first made by Messrs.
Beck for Mr. Sorby, who employed it to examine the micro-
scopical structure of iron and
steel. As ordinarily constructed,
it consists of a parabolic mirror
attached to the end of a rod fur-
nished with universal joints, so
that it may be placed in any
position. It answers admirably
for condensing the light on the
surface of objects, and by throw-
ing the rays in any particular
direction across the surface, the
observer is enabled, by means of
the shadows, to determine the
nature of irregularities upon some
objects in a very satisfactory man-
ner. It is in general used in con-
nection with a large bulls-eye
condenser, which is thus made to throw parallel rays on it.

There are two ways of mounting the parabolic reflector. The
usual, and where applicable, the best method, is to attach it to
the arm of the microscope. Many microscopes, however, have
no arrangement for attaching this accessory, and to meet such
cases Mr. Crouch has devised the adapter shown in Fig. 23.
The jointed arm which carries the reflector is attached to an
adapter, which fits between the nose-piece and the objective,
and may thus be used on any stand.

Fig. 23. — PARABOLIC EEFLECTOB.

114 SEI/ECTION AND TTSE

ILLUMINATION OF TRANSPARENT OBJECTS.

The different methods which have been devised for viewing
transparent objects are quite as numerous as those available for
opaque ones, and require quite as much tact and study. A
skilful worker, who thoroughly understands the points essential
to good, or rather to appropriate and efficient illumination, will
attain results wonderfully superior to those achieved by persons
ignorant of the subject, and this, too, although the latter may
be working with far superior instruments. This is seen every
season at our microscopical exhibitions and conversaziones, and
although the work done on these occasions is chiefly for show,
the same principle holds good in regard to work done in the
direction of study and investigation.

Direct and Reflected Light.— When the microscope
is so arranged that the light from a lamp or other self-luminous
body shall pass directly through the object and into the micro-
scope without being first reflected from the mirror, the illumin-
ation is said to be direct, in distinction from light which has
been first reflected from a mirror or other surface. Light from
a cloud or a white wall can scarcely be regarded as direct.
Direct light gives results which are appreciably different from
those produced by reflected light, since light always suffers a
change in character by reflection. These two kinds of illumin-
ation may be either axial or oblique, and in the case of both
reflected and direct light, if the source of light be very distant,
the rays will be sensibly parallel, but if the source of light be
very near, the rays will be divergent, and, consequently, under
such circumstances, the illumination must in part be more or
less oblique.

Axial or Central Light.— When the mirror, either
plane or concave, is placed directly in the axis of the micro-
scope, and reflects the light through the tube, the illumination
is said to be axial or central. The same term also applies to
direct light, when the direction in which the rays pass through
the object coincides with the optical axis of the instrument.

OF THE MICROSCOPE. 115

The rays must, of course, be parallel. If either divergent or
convergent, some of the rays will be oblique. In the cheaper
forms of the microscope, axial illumination is the only kind
for which provision is made.

Oblique Light.— Many objects fail to show their peculi-
arities when illuminated by parallel rays of light passing
through them in the direction of the optic axis of the micro-
scope, but are seen very clearly when the light is sent through
them obliquely. To secure illumination by oblique light re-
flected from the mirror, the latter must be so suspended that it
can be turned to one side, and thus send a beam of light through
the object at an acute angle. Where direct light is employed,
the necessary degree of obliquity may be obtained by adjusting
the position of the lamp — a device to which we have resorted
when compelled to use a stand in which the mirror did not
swing to one side. In this way, also, oblique light may be em-
ployed to illuminate objects viewed through a pocket lens, and
very interesting effects obtained. For the resolution of fine
markings upon diatoms, etc., oblique illumination is a neces-
sity. When the angular aperture of the objective is low, and
the light is very oblique, the objects appear light on a dark
ground — in fact a sort of dark ground illumination is obtained.

The Achromatic Condenser.— The earlier forms of
the achromatic condenser consisted simply of an achromatic
lens, similar to an object-glass, so arranged that by means of it
the light from the mirror could be brought to a focus on the
object. With some objects, even this simple contrivance gave
very fine results. It was soon found, however, that great ad-
vantage was derived from cutting off portions of the pencil of
rays transmitted by the condenser, and by means of the proper
diaphragms, central, peripheral and one-sided or oblique illum-
ination was obtained. First-class achromatic condensers be-
came, therefore, quite complicated and expensive. Several
cheaper but very efficient forms are now made by opticians, a
favorite being the Webster condenser, shown in Fig. 24.

Of this accessory Carpenter gives the following very practical
description: "In its present form the arrrangement of the

116 SELECTION AND TTSE

lenses strongly resembles that used in the Kellner Eye-piece;
the field-glass of the latter serving as a condenser to receive
the cone of rays reflected upwards from the mirror, and to

make it converge upon
a smaller achromatic
combination, which
consists of a double-
convex lens of crown,
with a plano-convex
Fig. 24. — WEBSTER CONDENSER. lens of flint, the plane

side of the latter be-
ing next the object. These lenses are of large size and deep
curvature; so that when their central part is stopped out, the
rays transmitted from their peripheral portion meet at a wide
angle of convergence, and have the effect of those transmitted
through the peripheral portion of the ordinary achromatic
condenser. When, on the other hand, this combination is used
with a diaphragm that allows only the central rays to pass,
these rays meet at a small angle; and the illumination thus given
is very suitable for objects viewed with low powers. Again, by
stopping out the central portion of the combination, and re-
moving the condenser to a short distance beneath the object,
the effect of a black ground illumination can be very satisfac-
torily obtained with objectives of moderate angular aperture.
Further, by stopping out not only the central, but also a great
part of the peripheral rays, so as only to allow the light to enter
from a small portion or portions of the margin, oblique illumin-
ation can be most effectively obtained."

The Spot Lens.— This is a plano-convex lens of very high
curvature, (it is generally hemispherical,) so mounted that its
distance from the object may be properly adjusted, and in this
way the rays which pass through it may be brought to a focus
on the object. The central rays are stopped out by means of a
black spot, (hence the name,) so that the object is illuminated
wholly by rays which are of too great obliquity to enter the
object-glass, except when their direction is changed by the
object. The latter, therefore, appears brilliantly illuminated on
a dark ground, and in many cases features which could not
otherwise be seen are shown very distinctly.

OF THE MICROSCOPE.

117

The Parabolic Illuminator.— This is an instrument
iutended to accomplish the same end
as the spot lens, but in a far more
efficient manner. It consists of a
block of glass, the outer form of which
is a parabola with a cup-shaped de-
pression cut in the upper end. It is
mounted in a brass fitting, which
slides up and down in the sub-stage
of the microscope, and thus may be
readily adjusted, so as to throw the
light properly upon the object. The

results obtained by means of the parabolic illuminator are

wonderfully beautiful.

Polarized .Light.— The micro polariscope consists of two
distinct parts, a polarizer and an analyzer, each of which is
now generally formed of a Nichol prism properly mounted. A

Fig. 25. — THE PAKABOLIO
ILLUMINATOR .

Fig. 26. — POLARIZES.

Fl'J. 27.— ANALYZER.

common method of mounting the polarizer is shown in Fig. 26.
As there shown, the lower ring is intended to slip into the ring
of the sub-stage, the rack and pinion of which enables us to
place the end of the prism at a proper distance from the object.
"When the microscope is not provided with a sub-stage, the
polarizer is turned upside down, and the brass fitting slipped
into a ring, which is attached to the under side of the stage.
The milled ring, which is shown uppermost in the figure,
enables us to rotate the prism in both cases.

The analyzer may be arranged in either one of two ways. It
may be slipped over the eye-piece, or it may be mounted in a

118 SELECTION AND USE

brass tube, the upper end of which has an external Society
screw that attaches it to the body, while the lower end of the
tube has an internal Society screw for receiving the objective.
Fig. 27 shows the latter arrangement.

Polarized light, except for the mere beauty of its effects,
has not received the attention that it deserves. In some de-
partments of scientific investigation, especially mineralogy and
geology, its use has afforded very satisfactory and brilliant re-
sults. As regards its applications to medicine and physiology,
Dr. Frey says: " The examination of tissues by polarized
light has a high scientific value, as, by this means, molecular
relations become evident, which by investigation with ordinary
light, remain entirely concealed. The _ interpretation of what
is seen, is in many cases difficult, and generally lies within the
province of optics, with which the medical observer is usually
but little familiar."

To detail the method of using it, and the special features
which it discloses, would, however, far transcend the limits of
this work, and we must, therefore, refer the reader to some
special treatise on the subject.

HOW TO USE THE MICEOSCOPE.

The remarks which we are now about to offer, are intended
for the merest beginners — for those, in fact, who have never
used a microscope at all; and therefore they may, perhaps, to
some, appear childishly simple. And yet we have seen not
only teachers, but professors in colleges, v/ho might have de-
rived some benefit even from these simple hints. We remem-
ber on one occasion seeing a professor of botany attempt to
examine a minute plant with a common pocket magnifier with
three lenses. In the first place he turned the instrument wrong
side up, so that, although he could see through it, the results
attained were very inferior to what they would have been if the
instrument had been properly used; in the second place he
wore his hat in such a way as to cut off nearly all the light, and
in the third place he did not know how to hold his hands so as
to obtain the requisite degree of steadiness. If he had given
a few minutes thought to the subject, he could no doubt have

01' THE MICROSCOPE. 119

corrected his bad methods, but then he evidently had never
considered it worthy of earnest thought, although it formed
the very foundation of his powers of observation.

Simple Hand Magnifiers.— These are perhaps the most
important of all optical instruments, and yet we rarely find a
person who can use them efficiently. There are but three
points that require attention, viz: The proper position of the
magnifier itself, the perfection of the illumination, and the
steadiness with which the instrument is held at the exact focal
distance from the object. Many magnifiers are so constructed
that it is impossible to place them in a wrong position; the
side which should go next the eye, and the side which should
go next the object are so well marked that no mistake can be
made. The greatest liability to error exists where two or three
lenses of different powers are fixed in the same frame and used
together. This forms one of the most common and useful of
our magnifiers, and the rule is always to place the lens of great-
est power nearest to the object. Plano-convex lenses should be
placed with the plane or flat side next the object.

Hand magnifiers are, in the majority of cases, used for ex-
amining opaque objects, and one of the most important con-
ditions for perfect vision is that the object be well illuminated.
First of all, then, see that the light falls full and direct on the
object; then place the magnifier as nearly in focus as can be
done without actually looking through the lens, and, after this,
approach the eye to the magnifier. The errors most commonly
committed are: Turning the object away from the light; cut-
ting off the light by the projecting brim of a hat or cap;
shading the object by the hand or the lens itself; attempting to
examine an object in a room that is not sufficiently lighted.

Having secured a proper position for the magnifier and a
good illumination, the next step is to devise some means for
holding the lens steadily in focus during the examination. This
is most readily effected by resting the hand that holds the lens
upon the hand that holds the object. Lens and object then
move together, and the focussing remains unchanged.

Compound 3Iicroscopes.— We presume that the instru-
ment in hand is a very simple one, and that the magnifying

120

SELECTION AND USE

power to be used is not very great. No person should attempt
to use high powers and complicated instruments until he has
served an apprenticeship by using a microscope of simple con-
struction, and objectives of considerable length of focus.

Let the beginner commence by examining some transparent
object already mounted. To do this, set the microscope
on a firm table, in front of a window by day, or before
a lamp at night. Direct sunlight is to be avoided, the light
from a white cloud being usually preferred to any other
source of illumination. At night use a gaslight that does
not flicker, such as an argand burner, or a good kerosene
lamp, the German student lamp being very well suited to this
purpose. Good results may, howeveivbe obtained from any of
the ordinary lamps, especially those with a circular wick, which
are now so common. Very fair work may also be done by
means of a good candle. This subject has, however, already
been discussed at greater length in another section.

If the microscope be a cheap French one, the objectives will
be found attached to the body, there seldom being any special
provision made for holding them. But with all American and
English microscopes, and the better class of instruments from
the continent of Europe, special boxes are provided for holding
the objectives. These boxes are usually made of brass, and
are indispensable to the microscopist that endeavors to take
good care of his instrument. Where the objective is kept in a
separate box, the body of the microscope must be raised to a
sufficient height, and the objective screwed into its place. In
doing this be very careful not to let the objective fall and strike
against the stage. We have seen more than one good lens
spoiled through such an accident.

When the objective has been properly secured in its place,
move the body of the microscope up or down until the front
lens — that is, the lens which is nearest to the object, is about a
quarter of an inch above the stage. Then turn the mirror
until the light from the window or lamp is reflected through
the microscope, so that when looking through it a bright cir-
cle of light is seen.

Place on the stage some mounted object of large size, such
as a fly's wing or section of wood, If a low power objective b§

OF THE MICROSCOPE.

121

used, say one magnifying less than 100 diameters, move the
body of the microscope up, so as to increase the distance be-
tween the objective and the object. At the same time keep
your eye at the eye-piece and watch closely. At a certain point
the object will be seen with great distinctness; it is then in
focus, and is ready for examination. Always begin with low
powers. One of the greatest risks that the beginner runs is
that of breaking the objective by forcing it down on the object.
To avoid this with high powers, bring the objective down al-
most into contact with the slide; when doing this do not look
through the microscope, but watch the objective, and stop
whenever it is sufficiently near the object. Then apply the eye
to the eye-piece, slowly raise the body, and watch for the com-
ing of the object into focus. This is the only safe method with
high powers.

Before attempting to place an object on the stage, or to re-
move one from it, see that the objective is raised at least half
an inch above the stage. By attempting to introduce a new
elide without raising the objective, when using high powers,
you run great risk of injuring both the object and the objec-
tive. And in removing objects from the stage, never lift them
up; always slide them off. In lifting them up, great danger is
incurred of bringing them into contact with the objective, and
thus doing irreparable injury. Sliding entirely prevents this.

"Where the microscope is not provided with mechanical
means for adjusting the focus, such as a screw or rack and
pinion, a great deal may be accomplished by special methods
of manipulation. Thus if, instead of pushing the body directly
through the collar, it be moved with a slightly twisting motion,
the focus may be adjusted with considerable delicacy, and when
the microscope is not provided with a fine movement, a great
deal may be done by means of a slight pressure of the fingers
on the stage. Few stages are sufficiently rigid to resist even
the slightest pressure.

The chief points which the beginner should endeavor to
study are the variations which are made in the appearance of
the object by means of slight changes in the focussing and the
mode of illumination. Experienced microscopists constantly
keep their fingers on the fine adjustment of the microscope,

122 SELECTION AND USE

and watch the different appearances which are produced by
a change in the mode of illumination. Swinging the mirror to
one side, so as to send the light through the object in an
oblique direction, or, where the mirror cannot be turned to
one side, merely turning it on the trunnions which support it,
will often produce most important effects.

From what has previously been said in regard to the neces-
sity for clear and brilliant sources "of illumination, the young
microscopist may, perhaps, be led to suppose that the field of
view cannot be too brilliantly illuminated. Such, however, is
far from being the case. With ordinary powers (those below
500 diameters) it is almost always necessary to moderate the
light, even of a flat-wicked lamp, and still more that of a
students' lamp. The finer details of an object cannot possibly
be made out if the illumination be too strong; they are
" drowned out," and the whole object becomes what artists and
engravers call flat. The light may be regulated by the dia-
phragm which has been previously described. Where the mi-
croscope is not furnished with a diaphragm, increasing the dis-
tance of the lamp from the instrument is the best mode of
lessening the intensity of the light.

Very bright light is exceedingly trying to the eyes, and
therefore the student will find it advantageous to use lights of
moderate intensity, and to increase their efficiency in every
possible way. This may be done to a very great extent by
judicious management — chiefly by excluding from the eye all
unnecessary light. In a room very brilliantly lighted with a
number of powerful argand burners, it would be impossible to
secure the proper illumination of a microscopic object by
means of a candle, for the eye, accustomed to the bright light,
would fail to be impressed by the weaker one. Extinguish the
bright lights, give the eye a short time for rest, and the candle
will answer very well. The principle thus illustrated finds a
practical application in the use of pasteboard shades surround-
ing the eye-piece, and excluding from the eye all light except
that which passes through the microscope. Such a shade is
easily made and adapted to any microscope, and is of great
service. We have also in our own practice carried out the
same principle by means of extra diaphragms to our eye-

OF THE MICROSCOPE. 123

pieces, thus cutting off all the light which passes through the
microscope, except that which actually serves to illuminate the
object.

It will also be found of great importance to secure perfect
purity in the special illumination employed. Thus, if we are
examining an object by transmitted light, it always detracts
from the clearness and beauty of the image if light is reflected
from its surface. It is, therefore, of advantage to shade the
object by means of a small tin, brass or pasteboard shade, at-
tached to the stage so as to prevent any light from the lamp
from falling on the object.

A difficulty which frequently occurs to young microscopists
consists in the almost impossibility of securing a field of view
equally illuminated in all parts. Assuming that the mirror is
in proper position, and that there is nothing to shade any part,
it will in general be found that the difficulty arises from the
fact that the mirror throws images of the lamp, etc., upon the
object. Sometimes this is very distinctly seen; the shape of
the flame can be clearly distinguished, and the metal portions
of the lamp appear as dark shades. The cause is that the lamp
is at the exact distance at which the mirror forms an image of
it on the upper surface of the slide, just as a lens, held in
front of a white wall, will throw an inverted image of a lamp
or candle on the wall, provided the relative distances of the
wall, lens and candle are properly adjusted. The remedy is
very simple; move the lamp either towards the microscope or
away from it, as may be most convenient.

As previously stated, the character of the illumination af-
forded by a mirror, and by a white surface placed at a short
distance from the object, are appreciably different. A very
pleasant method of illuminating transparent objects consists iu
the use of a plate of plaster of paris. Its whiteness is probably
as pure as that of any other substance, and it is easily procured.
The plate we use was cast in the cover of an old tin box, half an
inch deep and three inches in diameter. A flat surface was
secured by casting it upon a board. If cast on glass or metal,
the surface is glazed and shiny, which is bad. Instead of plas-
ter, fine white paper or cardboard may be used. Such surfaces
must not be glazed, and they should be kept scrupulously clean,

124 SELECTION AND USE

The light is also sometimes modified by passing it through
ground or colored glass — blue being a special favorite. Such
light-modifiers, as they are called, produce a pleasant and
equable illumination, which is a great relief to the eyes, but,
except for the resolution of finely lined objects, we have not
found them otherwise of any special advantage. When it is
desired to obtain the greatest resolving power that a lens is
capable of affording, the blue cell, as it is called, is probably the
most efficient accessory. This is simply a glass tank, somewhat
like a zoophyte trough, filled with a solution of oxide of copper
in liquor ammonias. The solution is prepared by adding
liquor ammonite to a saturated solution of sulphate of cop-
per, until the precipitate which is -first formed is re-dis-
solved. The intensity of the blue may be regulated, either
by diluting the solution, or by varying the thickness of the
layer of liquid.

When it is desired to examine anything by light reflected from
it, instead of light transmitted through it, the object should be
placed before a dead-black surface, such as the dark part of the
diaphragm, or a blackened card, and at such a distance from it
that the surface of the background is not in focus. Then,
place the condensing lens in relation to the lamp, so that a
bright spot of light will fall on the object, and on bringing it
into focus it will be clearly seen. Low powers only can be
satisfactorily used for the examination of opaque objects by
beginners.

The beginner should commence with the simplest mounted
objects, and afterwards, when a little skill in the manipulation
of the instrument has been acquired, Le should proceed to the
examination of such simple unmounted objects as are easily
prepared. The latter course will prove altogether the most
valuable and instructive, for he who confines himself to the ex-
amination of mounted objects only can never hope to become a
microscopist. After a time, when a little skill has been
acquired in the preparation of objects, the student may proceed
to preserve and mount them. Most young people try to mount
before they have learned to prepare objects, and the conse-
quence is that they soon find themselves in possession of a
large collection of very poor slides.

OF THE MICROSCOPE. 125

Care of thie Microscope.— A microscope, when not in
use, should always be kept well covered, either in its case or
under a suitable cover. There is no more convenient mode of
keeping a microscope than to stand it upon a cloth mat, and
cover it with a glass shade. It is thus kept free from dust and
vapors, and is always ready for use; but when it is kept in its
case, and especially if it has to be screwed together, interesting,
valuable, or even important objects, will often fail to be ex-
amined, simply because too much time and labor are necessary
to prepare for the operation.

A good microscope should be so carefully protected, that it
shall rarely require to be cleaned or dusted, as this wears off
the lacquer, and exposes the metal, which, when thus uncov-
ered, soon begins to tarnish. "When dusting or cleaning becomes
absolutely necessary, chamois leather, or a very fine old linen
or silk handkerchief is most suitable. Never use coarse cloths,
or those that have been lying about exposed to dust and dirt.

The lenses should be kept in their boxes when not in use,
and when they are attached to the microscope, great care
should be taken to keep them from coming into contact with
liquids. In order to prevent the latter accident as far as pos-
sible, never examine liquids unless when they are covered with
thin glass. In the pursuit of micro- chemical studies, the
microscopist has frequently to deal with liquids that corrode
metals, and even glass. In well-appointed laboratories inverted
microscopes are used in such cases, but with ordinary instru-
ments, special means must be employed. The object should be
laid on a large piece of thin plate glass, and the objective
should be coated with oil. The rest of the metal work may
be protected with oiled silk or thin india-rubber.

When liquids which corrode glass are used, the front of the
objective should be protected by means of a very thin leaf of the
best mica, which may be attached either by glycerine or balsam.

These, however, are exceptional precautions. In ordinary
work it is sufficient to see that the lenses and metal work are
kept free from stains and finger marks.

Never touch with the fingers the surface of any lenses, either
eye-pieces or objectives, as this will be certain to soil them. Use
soft camel-hair brushes to remove particles of dust, etc. Where

126 SELECTION ANt> USE

dirt adheres more strongly, nse fine linen slightly moistened with
alcohol, and wipe dry with very fine chamois leather. Remem-
ber, that alcohol, if used profusely, will attack the lacquer of
the brass- work, and even dissolve the cement which holds the
lenses together. When objectives are smeared with balsam,
the best cleansing agent is said to be kerosene oil. The piece
of leather used for wiping lenses should be free from dust, and
is best kept in a small box by itself, and used for nothing else.
It must be remembered that the glass of which objectives are
made is easily scratched, being soft when compared with parti-
cles of sand and grit; consequently, when frequently wiped it
soon loses that exquisite polish upon which its excellence of
performance so much depends. What, 'then, are we to think
of the directions given by the author of a popular work on
the microscope, in which we are told to use a piece of leather,
slightly impregnated with brick dust ! ! No better method of
destroying an objective could possibly be devised. Therefore,
see that in wiping, the slightest possible pressure is used, lest
any particle of grit should make a scratch.

The exposed parts of all microscopes, as well as the objectives
and their cases, are lacquered, to protect them from being
soiled by handling, but the interior of the boxes which hold the
object-glasses are rarely so protected, and the black coating of
the interior of bodies, draw- tubes, etc., is frequently not very
firmly attached. Therefore, never touch them with the fingers.

After taking an objective out of its box, either screw on the
cover of the box, or place the latter with its open end down. Do
not stand it mouth up, so that it may catch all the dust.

When exhibiting the microscope to others, great care is neces-
sary to keep meddlesome fingers from soiling the glasses. Some
people are never content when merely allowed to look at things:
they insist upon handling them, and feeling them. To the
young microscopist, we would say that if any of your friends in-
sist upon handling your objectives, eye-pieces, etc., put up the
instrument and pack it away. A microscope carefully used is
as good after fifty years as when first made, but we have seen
an instrument suffer more injury in half an hour at the hands
of a thoughtless and dirty person, than it would have sus-
tained in twenty years in the hands of a careful microscopist.

OP THE MICROSCOPE. 127

COLLECTING OBJECTS.

Those who are engaged in special studies and researches re-
quire no directions for collecting objects; but to those who use
the microscope for purposes of general instruction or amuse-
ment, a few hints may not be out of place. Almost every text-
book on botany, physiology, mineralogy and kindred subjects,
will not only indicate a long list of objects, but will give di-
rections for procuring them. Plants yield a very large variety
of interesting subjects. Thus the cuticles of the leaves and
flowers; cellular tissue as shown by dissections, and by cross
and longitudinal sections; hairs, pollen, seeds, etc., all deserve
careful microscopical examination. Insects furnish an almost
unlimited field, and their wings, feet, eyes, mouth, scales, spira-
cles, hairs, etc., are all worthy of careful preparation and exam-
ination.

It is, however, amongst the more minute forms of animal and
vegetable life, as found in pools and running streams, that the
most interesting objects are to be found, and the number and var-
iety of these is so great that several large volumes would be re-
quired to describe them. Even the ponderous works of Ehren-
berg and Pritchard do not begin to exhaust the subject, and,
therefore, it will be obvious, that even if we were to devote the
whole of the present volume to this department, we could
but skim the surface. Thus far we have had to depend chiefly
npon foreign works for descriptions of these organisms, but it is
fortunate that while the higher classes of plants and animals
which inhabit Europe, and are described in European works,
are entirely different from their congeners on this continent, the
same does not hold true in regard to the lower forms. We have
found localities which teemed with the Volvox Globator and
various species of Closterium, Staurastrum, Pediastrum, etc.
Hydras are to be found in great abundance, and so nearly like
the described European species that the beginner will find it
difficult to detect the difference. "We have repeatedly found the
Stephanoceras, Melicerta and other beautiful microscopic objects,
and as for the more common ones, such as the Vorticetti, or

128 SELECTION AND USE

wheel animalcules and Entomoslraca, or water fleas, they are
to be found in every pool.

Every young microscopist that is desirous of pursuing his
studies in this direction, is met at the outset by two difficulties;
the first is to obtain the objects, the second is to find out what
they are after he has got them. The first is by no means a dif-
ficult task, but the second will often puzzle more experienced
students than those whom we expect to be readers of this book.
We know of but two ways to accomplish it; one is the laborious
plan of searching for them in the " Micrographic Diction-
ary," or the books of Carpenter or Pritchard; the other is
to obtain the desired information from some well-informed
friend.

The objects which are of most interest to the microscopist are
not difficult to obtain, if we know where to look for them, but
they are not to be found everywhere. Many stagnant pools
will be found to yield but a scanty supply, while others, which,
perhaps, to the uninitiated present a less promising appearance,
will yield a rich harvest. Beginners are very apt to entertain
the popular notion, that every drop of water teems with animal-
cules, and that when placed under the microscope, it will appear
to be literally filled with living things. This idea is fostered
by popular writers who describe a drop of water as a globe filled
with life, and by lecturers who exhibit pictures and enlarged im-
ages of what they call " a drop of water," but which is in reality
a considerable quantity of that liquid which has been artificially
supplied with inhabitants. Clear well water is almost free from
microscopic organisms, and the same is true of the water from
clear brooks, which flow swiftly over a pebbly bottom. Ordin-
ary rain water, as found in cisterns having free communication
with the air, usually contains large numbers of the larvae of
gnats and mosquitoes, and when exposed to the light it is almost
always rich in wheel animalcules, and some of the lower forms
of vegetable life. The water supplied to our cities is in gen-
eral very rich in microscopic vegetables. Thus in the Croton
water, which is comparatively pure, we have found a large
number of very beautiful species, amongst them the exquisite
Monachinus. The best way to secure a supply of the animal
and vegetable inhabitants of city water, is to pass a considera-

OF THE MICROSCOPE. 129

ble quantity of it through a filter, the surface of which will
then furnish a large amount of valuable matter.

But it is not in such fields that the microscopist will find his
best hunting grounds. Along the edges of quiet pools of clear
water is the best place for the finer vegetable forms, such as
the Volvox Globator, Closterium, etc. If the water is much con-
taminated with dead animal matter or with sewage, nothing will
be found but the coarser organisms and animalcules, such as
Paramecium. The same is true of small pools found in woods,
or very much shaded with trees, and filled with dead leaves.
Such places are, however, the favorite haunts of the larvae of
insects, and also of frogs and Tritons. The size of the pools is
not of much consequence. We remember on one occasion to
have found by the roadside in Centre County, Pennsylvania, a
little pool which was almost filled with the larvse of Tritons.
The gills, which were beautifully developed, would have formed
a splendid object under the microscope, but when we returned
next day, for the purpose of securing some, the water had dried
up, and the larvse were all gone.

The little pools formed in boggy ground by the footsteps of
cattle will often be found to contain large quantities of one or
two species of desmids or diatoms. It will not do to look for
these objects in similar pools formed in ordinary soft land, and
temporarily filled with rain water. The ground must be na-
turally and constantly wet, so that the pools are always kept
filled by the infiltration of water from the surrounding soil.
Such pools, however small, usually contain a large number of
specimens, and it is in such places that one is most likely to
find a supply of one variety unmixed with any others.

While many of the most interesting objects will be found
swimming freely about in the water, others of great beauty are
always attached to floating weeds, sticks, etc. We have gen-
erally been most successful in discovering specimens of this
kind when we have placed the gathering in a large glass jar, and
allowed it to stand quiet for some time. The water will then
settle, and the objects of which the microscopist is in search
will have time to expand, when they may be seen in a form
resembling light mould, or down, attached to the surfaces of
the solid matters.

130 SELECTION AND USE

The surface of the mud at the bottom of ponds of clear water,
is frequently very rich in microscopic vegetable organisms.
These minute plants seem to seek the light, and to rise through
the mud which would otherwise cover them, so that by care-
fully scraping the surface of the bottom, we are enabled to
procure them in large numbers.

It must, of course, be borne in mind, that while some species
are found in fresh water, others are marine, that is, they live
only in sea-water. The best locations for finding marine forms
are: 1, the pools of c1 ear water, found in salt marshes; 2,
the surface of the mud at the bottoms of harbors and quiet
coves; 3, the waters of the ocean itself, as well as that of the
bays and coves connected with it.

The apparatus required for capturing these various objects,
is neither bulky nor expensive. For larvae and the larger ani-
malcules, the most useful implement is a small net. Ours con-
sists of a ring of brass wire (iron wire would rust and destroy
the net) about six inches in diameter, soldered to a tin tube or
ferrule, which fits tightly on the end of a walking cane. To
the ring is attached a bag of any light, gauzy material, which
possesses the two qualities of letting water out rapidly, and
keeping small objects in. With this net it is easy to capture
anything from a small fish or a frog to the very smallest larva,'
and it is very portable, since an ordinary walking cane forms a
sort of universal handle for this and other implements. Next
to the net, we find the most useful articles to be bottles. They
should be of clear glass, so that any object contained in them
may be readily examined by means of a pocket lens. For this
reason we prefer what are called homoeopathic phials of large size
(half ounce and quarter ounce), and we generally carry a dozen
or two when out on a tramp. A fair sample of the contents of
a small pool is easily obtained by gently lowering the phial,
mouth downwards, under the water, and bringing it cautiously
to the place which is supposed to be richest in specimens. The
phial is then turned mouth upward, the air rushes out and the
objects are carried into the bottle by the force of the inrushing
current of water. For small, shallow pools, the phial is most
conveniently held in the hand, but when the water is deep a
handle is required, and for this we use the holder shown in

OP THE MICBOSCOPE. 131

Fig. 28, which is made to fit on the end of the walking stick. It
consists of a ferrule having a semi-cylindrical piece soldered at
right angles to it. The ferrule fits the cane,
and the bottle is fastened to the cross piece
by means of a rubber ring — the method of
arranging the latter being easily understood
from the engraving. A dozen or more bottles
I \ of proper size may be taken along, and they

Fig. 28. are so easily attached to the holder that there

is no necessity for transferring a "dip" to
another bottle. The contents are most easily carried in the
bottle in which they were first obtained.

"When the water is too deep for a walking cane, a fishing rod
or any long pole may be used, and where these prove too short,
as in harbors, etc. , a bottle may be lowered and raised properly
by means of strings. For this purpose the bottle must be
heavily loaded with lead round the neck, and two strings must
be attached to it, one fastened to the neck and the other to the
bottom. It is by the latter that the bottle must be lowered,
but it must be raised by the other. If properly managed it
will descend mouth downwards, but the tension of the string
attached to the neck will invert it, and when raised by this string
it will bring up its contents very perfectly.

For scraping the surface of the mud at the bottom of shallow
pools, we use the spoon shown in Fig. 29. It is simply a ring of
tin five inches in diameter and one
inch deep. The lower edge is
" wired " as the tinsmiths call it, and
there is a ferrule soldered to the side
so that it may be fixed to the same
cane that is used for the net. Over the Fig. 29.

bottom is stretched a piece of some

thin fabric, such as thin muslin, gauze or tarletan, which is held in
place by a rubber band that slips over the wire ring on the lower
edge It is best to make one side of the ring somewhat flat, so
as to adapt it better to the flat surface of the mud. When the
pieces of cloth get soiled, they are easily replaced, and, indeed,
in some cases it is not a bad plan to carry the mud home in the
wet cloths, a dozen or more of which, with their contents, may

132

SELECTION AND USE

Fig. 30.

be easily packed in a tin
box of small size. One of
the boxes used by school
children for lunch boxes
answers very well, but any
tin box with a lid or cover
will answer. As it is im-
portant that a record should
be kept of the locality from
which the dip was taken,
we carry a few slips of
parchment paper, one of
which is pinned to each
cloth, after the necessary
memoranda have been
written upon it with a hard
pencil. On returning home,
the contents of each cloth
may be transferred to a
separate bottle. This plan
saves the carrying of nu-
merous bottles, and the
water required to fill them,
An exceedingly coveni-
ent traveling companion
for those who are fond of
collecting, is shown in the
accompanying engraving,
Fig. 30. The main part
forms a very convenient
walking cane of ordinary
appearance. Like many
fishing rods, however, it is
hollow, and contains a sec-
ond rod by which it may be
extended to twice its length.
This enables the user to
reach the bottom of any
ordinary pond, and to reach
as far as is necessary from

OF THE MICROSCOPE.

133

the shore. Accompanying the cane, A, are the hooked knife,
B, and the ring and bottle, C. These are made with a double
screw, so that they may be attached either to the end of the
cane itself, or to the inner rod, and in this way we can have
either a short and stout handle, or a longer and more slender
one, as circumstances may require. The bottle is made so as
to screw into the brass ring, and the same screw enables us to
fit a wooden cap on it, which thus encloses the contents tightly.
The hook is made of fine steel, and has a sharp cutting edge, as
seen in the engraving, so that it is easy to cut off a piece of
weed, drag it out of the water and secure it.

Those who carry such a
cane do not attract attention
by any unusual parapher-
nalia, and at the same time
they are at all times ready to
secure any valuable material
that may present itself. Sev-
eral bottles may be carried in
the pocket, and screwed into
the ring as required.

The collector who desires
to make a thorough examina-
tion of the microscopic flora
and fauna of any pool or
stream, must not rest content
with infinitesimal quantities
of material. It is not neces-
sary, however, to lug home a
gallon of water for the sake
of the objects contained in it,
and so fully have microscop-
ists been impressed with this
idea, that the devices which
have been prepared for strain-
ing out the valuable portions
are almost endless. The best
and simplest that we have
seen is a modification of an Flg' SI-

134

SELECTION AND USE

arrangement, designed, we believe, by Mr. Highley, and
figured in Beale's work on the microscope. It consists, as
shown in the engraving (see Fig. 31), of a bag or net of
some light material, to the bottom of which is attached, by
means of twine, or a strong rubber ring, a wide-mouthed bottle.
Any quantity of water may be poured into the bag, and all the
objects which it contains will roll down the sides of the bag
and fall into the bottle, while the fluid escapes through the
sides. Delicate objects are consequently not exposed to
pressure, rubbing, or any other violence, as they would be in
an ordinary filter or bag, and the whole affair is so simple that
any one can make it.

A slight modification of this arrangement will be found ad-
mirably adapted to the microscopic examination of the water
supplied to cities. The bag may be attached to any faucet, and
and all the water that is used in the household may be caused
to pass through it. In this case, if the bag be made of some
tolerably stout material, it may be firmly tied to the faucet,
and then all the water that is consumed will be very thor-
oughly purified.

Another very excellent device is the bottle invented by
Mr. Wright, of which a modified form is shown in Fig.
32. The mouth of the bottle is closed by means of a
cork in which two funnels are inserted.
One of these funnels is placed in the bot-
tle, mouth down; the other projects above
the cork, as shown in the engraving. The
mouth of the funnel that is in the bottle
is covered with muslin or flannel, held in
place by a rubber band, which is prevented
by a wire ring from slipping along the con-
ical surface of the funnel. When water
is poured into the other funnel, it passes
into the bottle until the latter is full, and
then it flows out of the first funnel, and
is carried off" by means of a short piece of
rubber tubing. Meanwhile, all solid parti-
cles are held back by the filter, and as the
latter is horizontal and with the filtering Fig. 32.

OF THE MICBOSCOPE. 135

surface downwards, most objects of interest fall away from it,
and may be found in the water. A single bottle of this descrip-
tion is sufficient, as the cork is easily removed, so that the water
may be poured into other bottles. As ordinarily made and sold,
"Wright's collecting bottle is an expensive piece of apparatus,
costing four or five dollars, but as shown in the engraving it
may be made for a few cents by any tinsmith.

Where it is desirable to keep the specimens thus obtained so
that they may be examined, and their life-history studied,
bottles and jars of almost any kind may be used, but those which
we have found most convenient are what are known as "quin-
ine " bottles, and may be had at most druggists. For ordinary
objects they are just about the right size, and as they are made
of tolerably clear glass it is easy to examine the objects through
the sides of the bottle. A dozen or two of these little aquaria
occupy very little space, and are easily handled. Great care
must in general be taken to exclude from the vessels contain-
ing the finer organisms, such predatory animalcules as devour
them. Water fleas, the larvae of insects, etc. , will soon make
away with the finer specimens. On this account great diffi-
culty is found in keeping the Volvox Globator, since it is greedily
devoured by various rotifers, and these are exceedingly difficult
to exclude. We have succeeded best in this case by partially
filling a bottle with well-filtered water taken from the same
pool as the specimens, and transferring the objects to it singly
so as to avoid transferring their enemies too. For this purpose
the dipping tube should be used. Some authors caution us
against mixing the inhabitants of different pools, on the ground
that being strangers to each other they will fight. This is more
fanciful than accurate, though it has a basis of truth. It is not
the circumstance that they are strangers that causes the diffi-
culty, but the fact that the one is the natural prey of the other.
The same thing occurs between inhabitants of the same pool.
It must be remembered, however, that very slight changes in
the conditions in which they are placed will often cause the de-
struction of these objects. Thus, we have seen some very fine
gatherings totally destroyed by being removed from soft, boggy
water to clear, hard well water. Therefore, in transferring
either animals or vegetables to an aquariun, it is well to supply

136 SELECTION AND USE

them with the same water, mud, etc., in which they were origi-
nally found. It will sometimes, however, be- well to filter the
water so as to remove all such inhabitants as are apt to eat up
the others. Water, may be filtered through paper, or where fil-
tering paper is inaccessible, the neck of a funnel may be loosely
plugged with cotton. Even this does not quite free it from
noxious eggs or germs, and we have sometimes gone so far as
to boil it in a flask, the mouth of which we plugged with a loose
wad of cotton after introducing the objects we wished to pres-
serve.

When floating freely in these diminutive aquaria, many ob-
jects are as difficult to find and capture as would be a small fish
in a large pond. The microscopist, therefore, requires special
means for capturing them, and placing them on a slide. For
this purpose nothing serves so well as what are known as dip-
ping or fishing tubes. These are simply glass tubes of different
diameters (from one tenth to one quarter of an inch), and of
any convenient length. They are used by closing the upper
end with the finger, bringing the lower end near the object (un-
der the water), and then removing the fingers from the uppei
end. The water, in seeking to find its own level in the tube
rushes in with great force and carries the object with it. By
again placing the finger on the upper end of the tube, the latter
may be lifted from the bottle, and the water with it, and by a little
dexterous management it is easy to cause the object to flow out
on a slide without allowing too much water to go with it.
These tubes are made straight, curved, and with one end drawn
to a point, but for most purposes the straight tube answers best
as it is most easily kept clean. We prefer to grind the ends
rather than to make them smooth by fusion, as the latter pro-
cess generally contracts the opening, and renders the tube dif-
ficult to clean. The best plan, however, is to heat the upper
end strongly before the blowpipe, and turn the edge outward
like the mouth of a test-tube. It is then easily closed, and the
tube is very strong. The lower end should be ground.

1

OS* THE MICROSCOPE. 137

PBEPAEATION, PRESERVATION AND MOUNTING OF
OBJECTS.

These three operations are so frequently applied as a single
process to objects, that many writers have failed to make a suf-
ficient distinction between them. By keeping the proper dis-
tinction clearly in mind, however, the student will not only
save much valuable time, but he will secure vastly better re-
sults. Except by those who are more anxious to increase the
number of objects in their cabinet than the amount of know-
ledge which they possass, a very large proportion of the ob-
j ects examined will never be preserved or mounted at all. This
however, should not prevent the utmost care being given to the
process of preparing them for thorough examination. On the
other hand it often happens that objects which have been
carefully prepared and mounted, spoil because they have not
been subjected to a proper preserving process. Hence the
importance of treating these operations separately and fully.

The Preparation and Examination of Objects. —

It is a cqnimon but very erroneous idea that the only thing
that is necessary in order to examine any object under the mi-
croscope, is simply to place it on the stage, and get it into
focus. With the exception of mounted objects, a very few
transparent objects — such as the wings of insects — and some
things that are viewed by reflected light, every substance re-
quires to undergo careful preparation before it can be fit for
profitable examination. A good example of the necessity for
such preparation is seen in the common potato, a piece of which
when simply placed on the stage of the microscope, and brought
into focus, appears as a glistening mass, and reveals nothing of
its true structure. If we now cut from this lump, by means of
a very sharp knife, an exceedingly thin slice, place it on a
plate of glass, moisten it with a little spirit and water, or better
still, glycerine and water, and place over it a thin glass cover, it
will disclose to us a most wonderful and beautiful structure. The
entire mass will be seen to be composed of cells, these cells be-
ing filled with granules of starch of various sizes.

138 SELECTION AND USE

The operation which we have thus briefly described as appli-
cable to the potato, is required for a great many other materials ;
for whenever a substance is to be examined under any except
the very lowest powers, it is absolutely necessary to obtain it in
pieces as thin as possible, so that the light may readily pass
through them, and it is in general requisite to increase their
transparency either by immersing them in a fluid, or by some
other means. In preparing objects for the microscope, our
aim is in general to examine either the ultimate structure ot
the substance under investigation, or the arrangement of its
different parts; and the processes which are most available for
this purpose may be classed under three heads: 1, Mechanical,
such as section-cutting, dissection and: injection; 2, Chemical,
such as the use of iodine for detecting starch; of alcohol for
hardening certain structures; of coloring substances for stain-
ing germinal matter, etc.; 3, Optical, such as the action
whereby certain liquids change the transparency of some ob-
jects. Of some of these processes, such as injection, staining
and the extended use of chemical tests, elaborate descriptions
would be required in order to enable the student to carry them
out with success, and we must refer him to the works of Beale
and Frey, which are very complete on these points.

Thin sections of any soft substance are easily made with a
very sharp knife — a good razor being probably the best availa-
ble instrument. For work in the higher departments of micro-
scopy, and for the preparation of fine objects for sale, special
instruments known as section-cutters are employed, but for the
ordinary work of investigation, they are not absolutely neces-
sary though very convenient. Using a good sharp razor, it is
an easy matter to shave off any soft substance a wedge shaped
piece, the edge of which thins off to nothing, and which pre-
sents in its different parts all varieties of thickness, so as to afford
a perfect opportunity to study the object under examination. In
this way, which is known as the " free-hand " method, suitable
sections of most animal and vegetable substances may easily be
prepared, and the student will be surprised at the dexterity
which a little care and practice will confer.

For cutting sections of very soft tissues a special knife, known
as Valentin's knife, has been invented. It consists of two

OF THE MICROSCOPE. 139

blades so arranged in one handle that their distance from each
other may be easily regulated. When a cut is made with this
double-bladed knife, a thin slice of the tissue passes between
the blades, and constitutes the section. It is an instrument,
however, which will hardly be used by beginners. Sections
of substances of greater consistence, such as wood and soft
bones, are most easily made in a regular section cutter. The
patterns according to which these instruments are constructed
are very various, but they all act on the principle of raising
above the surface of a brass table, by means of a fine screw, the
substance to be cut, and then passing a very sharp razor or
knife over the table so as to shave off the projecting part of the
object. The table is usually of brass, ground and polished.
This gives rise to two serious defects. The metal is too soft in
the first place, so that it is impossible to press with sufficient
force on the razor without cutting into the table, and secondly,
when any soft metal has been ground on a grindstone or emery
wheel, the surface becomes so impregnated with gritty matter,
that it very rapidly destroys the edge of the cutting tool. We
avoid these difficulties by fitting to our section cutter a stout
plate of hardened steel, the surface of which has been highly
polished by means of buff leather. Quekett describes a cut-
ting machine in which the difficulties we have mentioned are
obviated by fixing the knife in a frame so that it is raised above
the table, and does not touch the metal. Its edge is thus pre-
served from injury, and the blade itself cannot be affected by
variations in the pressure exerted. Dr. Curtis, of this city, has
adopted the same principle in his section cutter, the details of
which are admirably carried out.

In making sections of wood and similar substances, the speci-
men is first well soaked in dilute alcohol, and is then fastened
securely into the tube of the section cutter, either by wedges or
by casting wax or paraffin around it. The process of raising it
by means of the screw and passing the knife over it, is simple
enough, and can easily be learned.

With the ordinary cutting machine, success in making thin
sections seems to depend upon the perfect sharpness of the
cutting edge, the thorough moistening of the knife and section,
and the rigidity of the blade. The latter point frequently fails

140 SELECTION AND USE

to receive the attention that it deserves. Where a thin, flexi-
ble blade is used, a moderate change in the amount of pressure
employed will make a great difference in the thickness of the
section, even so far as to double it. When the blade is stiff, a
change in the degree of pressure has but little effect.

Soft substances must first be hardened either by immersion
in alcohol or other means, and in general must be supported by
being surrounded with melted wax or paraffin. Where the
specimen is very slender (such as a hair) it must be carefully
supported between firm and rigid clamps. Corks and similar
yielding substances, which are recommended in most books,
never give a cross section accurately taken at right angles. The
same is true of the plan so much recommended for obtaining
sections of hair, viz. : to pass the razor over the face shortly
after shaving. We get sections it is true, but they are all
oblique. The best way to get true sections is to imbed the sub-
stances in glue, gum, paraffin, wax or some such material.

Sections of bone are prepared by salving off arthin slice in the
first place, and cementing it to a slide by means of thick or old
balsam; one side is then filed or ground flat, and polished on
buff leather, after which the section is transferred to another
slide so as to expose the other side, which is then filed down
and polished as before. Great care must be taken so as to hit
just the right thickness, and the operation of cementing to the
slide must be performed expeditiously, so that the balsam may
not saturate the section, and render it too transparent, as when
this occurs certain very important features become invisible.

Very hard substances require special apparatus, and consid-
erable skill. Still it is astonishing what may be accomplished
by means of good files, whetstones and grindstones in the way
of. preparing thin and transparent sections even of such sub-
stances as rocks and stones.

In order to acquire correct ideas in regard to the structure of
objects, of which sections are examined, the student should fa-
miliarize himself with the geometrical forms produced by cut-
ting cylinders, cones, spheroids, etc., in various directions.
Thus a cylindrical vessel, cut square across, shows a circle ; when
cut obliquely it shows an oval (ellipse) of greater or less length t
and when cut longitudinally it shows two lines which have no

OF THE MICROSCOPE. Ill

-^parent connection with each other. The truth is, however,
that we should never deduce the form of vessels from sections
alone. In every case it is necessary to examine carefully dis-
sected preparations as well as sectioi

The soft parts of animals and vegetables are frequently pre-
pared for examination by careful dissection, that is to say the
different parts are separated from each other, and freed from ex-
traneous matter by means of knives, scissors, forceps, needles,
camel hair pencils, etc. The knives used by the microscopist are
similar to the scalpels ordinarily employed by anatomists, but
smaller, and unless very finely tempered and well-sharpened,
they are worthless. The knives sent out with low priced micro-
scopes are in general the veriest trash, and the same is true of the
needles. There are three kinds of scissors which the microscop-
ist will find useful — plain, straight scissors, elbow scissors, and
curved scissors. They must be small, sharp and well made. But
the most useful, as well as the simplest instruments for dissecting
are a pair of fceedles, or, rather, a needle and a very fine
spatula. The needles used are those ordinarily employed by
seamstresses; they should be fixed in a light wooden handle and
carefully polished. The latter is a most important point, for
it will be found that ordinary needles are too rough for deli-
cate work, as may be easily seen by examining them under the
microscope. For microscopical purposes needles are made
both straight and curved — the latter being a very useful form.
In order to bend a needle, it must first be heated in the flame
of a candle, then bent by proper pliers, after which it must be
carefully re-tempered. There is little danger of getting it too
hard, provided it is not burned. After being hardened it must
be carefully re-polished. The handles should be light and
smooth. Ordinary penholders make good handles and cost but
a trifle, but in case of need any piece of straight-grained, light
wood will answer. Universal handles, handles with ferrules,
handles wound with thread, etc,, look as if they were not com-
mon articles, and are purchased by many, but no working mi-
croscopist would give them table-room. All the so-called uni-
versal handles in market are too clumsy and heavy.

In using needles or knives for dissection, they are generally
used in pairs, that in the right hand being used for teasing or

142 SELECTION AND USE

cutting, while the one in the left hand is used for holding the
object firmly in its place. For the latter purpose, however, we
prefer a very narrow spatula, curved and highly polished.
Curved needles, with the curve placed flat, answer very well,
however.

For the removal of loose matter, and for arranging parts
which have been dissected out, there is nothing more useful
than good camel hair pencils. Indeed, they are indispensable,
and with needles and pencils — two of the simplest and cheapest
articles — it is possible to do a"lmost everything.

During the process of dissection the object must be supported
upon a glass plate or a dissecting pan, according to its size. Some
of the finest preparations have been worked up on ordinary slides
three inches long by one wide, and as it is almost always neces-
sary to have the object covered with liquid, a single drop suf-
fices in this case. But where larger objects are to be dissected,
ordinary slides are not large enough, and besides there is no
provision made for holding a sufficient quantity of liquid.
Various kinds of dissecting dishes or pans have therefore been
devised. Those used by the author are exceedingly simple
and cheap, and are shown in Fig. 33. We use three kinds,
two with opaque bottoms, and one
in which the bottom is transparent.
The latter is used for objects which
are transparent, and is precisely like
the others, except that a portion
Fig. 33. of the metal bottom is cut away and

a piece of plate glass cemented over

the aperture. Those used for opaque objects are simply
oblong tin dishes, each two inches long, one and a quarter
wide and half an inch deep. The bottom plate extends on each
side, so as to form rests for the fingers, by which the pan may
be kept steady. Into this pan is poured a mixture of equal
parts of resin and beeswax, softened if necessary with a little
lard. It should be just so soft that a pin may be easily stuck
into it, and this affords us the means of pinning out the different
parts of a dissection as we progress. In one dish the wax is
colored black with lampblack, and this forms a wonderfully
effective back ground for most objects; the wax in the other

OF THE MICROSCOPE. 143

pan is white, chalk or sulphate of baryta being substituted for
lampblack. The pan with a transparent bottom is of precisely
the same size, except that the depth is but half as much — the
extra depth in the other pan being filled with wax. A quarter
of an inch is a sufficient depth of liquid for most objects, and
when the sides of the pan are higher than necessary they inter-
fere with the use of knives and needles.

Dissections may also be carried on in watch-glasses, though they
are not quite as convenient as pans with perfectly flat bottoms.
The kind known as lunette glasses should be chosen, as they
are flat in the centre. When a watch-glass is used for this pur-
pose, it is necessary to cement it into a hole cut in a thin piece
of wood about four inches long, and of a width which is rather
greater than the diameter of the glass.

Most of this work is, of course, done under a simple micro-
scope. The Excelsior, when screwed to a larger base, as de-
scribed on page 29, answers very well. Larger and more ex-
pensive dissecting microscopes are supplied by most opticians.

In addition to these general methods, which are applicable
to a great variety of subjects, there are a few special processes
which must be adopted in particular cases. In some instances,
as when the line of investigation is a new one, the microscop-
ist must work out his own processes, but the following special
cases will probably prove interesting to beginners.

It frequently happens that the objects for which the micro-
scopist is searching are found mixed with coarser materials,
and in this case it will be found possible to effect a separation
by the process known as elutriation or washing. Mix the mat-
ter thoroughly with water in a tall jar and allow it to settle.
In a short time — say one minute — the very coarse particles will
have fallen to the bottom, and if the liquid be now poured off
and allowed to settle, the finer portion will be found in the
second vessel. By graduating the time and carrying the pro-
cess out to its full extent, a wonderfully perfect separation may
be effected. Diatomaceous earth may frequently be treated in
this way to advantage.

In some cases separation must be effected by burning, or the
action of chemical agents. Guano and various organic matters
yield interesting residues after everything soluble has been

144 SELECTION AND USE

washed away and everything combustible has been burnt either
with fire or nitric acid. So too the siliceous cuticles of plants
may be procured by destroying all the other parts by chemical
means. The best way is to heat them in nitric acid, and add
to the hot liquid a small quantity of powdered chlorate of pot-
ash. The quantities used must be very small, and great care
must be exercised.

It is frequently necessary to separate a small quantity of
deposit from a large amount of liquid, filtering being inadmis-
sible. For this purpose use a conical glass or a large test tube,
allow plenty of time for the deposit to settle, and give occa-
sionally a slight stir, so as to detach the particles from the
sides of the vessel. Then pass a large, dipping tube (one quar-
ter of an inch in diameter) to the bottom, the upper end of the
tube being closed with the finger. On withdrawing the finger
the liquid and deposit rush in. Have ready a small ball of
soft cement (resin and beeswax equal parts, softened with oil)
and with it close the upper end of the tube, which may now be
withdrawn, carrying the liquid with it. Place the tube in a
vertical position, with its lower end on a slide or in a watch-
glass, and support it either by means of the ring of a small
retort stand or by a simple wire having a ring (horizontal) at
the upper end, and a small piece of board for a foot. Beale
directs us to cork the tube, but this is difficult unless the tube
is made specially for the purpose with a mouth like that of a
test tube. Tubes made in this way are, however, the most con-
venient, and a good velvet cork closes them perfectly.

There is a class of insect preparations, which are quite inter-
esting, though they are not as instructive as inferior prepara-
tions mide by the process of dissection. We refer to the whole
insects found in most collections. They are prepared by soak-
ing the insect in liquor potassse, which may be had from any
druggist; this renders the internal organs soluble and the outer
horny skeleton transparent. The viscera are then expelled by
pressure with a camel hair pencil, the insect well washed in pure
water, soaked first in alcohol, and then in turpentine, and
finally mounted in balsam. The points requiring attention aro
these: Soaking just the right length of time in the potash, for if
the insect remains too long in this liquid it will be destroyed ;

OF THE MICROSCOPE. 145

allowing plenty of time for the alcohol to displace the water,
and for the turpentine to displace the alcohol; and manipula-
ting the insect with great care, so as not to break any of the parts.
The eyes of insects are prepared by macerating them in very weak
potash, and, while still soft, pressing them between two slips of
glass. If allowed to harden before being pressed they will
split at the edges. The handsomest preparations of eyes are
obtained by taking a thin slice from a large eye, such as that
of a dragon fly, and treating it as directed.

The feet of insects are in general easily prepared. Moderate
soaking in potash, careful washing in water, thorough soaking
in alcohol and turpentine, and careful management in properly
displaying them on the slide, are the secrets of success. The
student who wishes to make a careful study of these objects,
however, should place them in glycerine, after soaking them in
potash and thoroughly washing them. They should of course
be deposited in a cell filled with liquid, and then covered with
thin glass, and examined. The so-called tongues, etc., of in-
sects require no potash, being sufficiently transparent without
it, and after being soaked successively in alcohol and turpen-
tine, they may be mounted in balsam. When wanted for exam-
ination merely, immerse them in dilute glycerine, and if the
student can succeed in mounting them in cells, in glycerine or
some of the gelatinous media hereafter described, they will show
their structure to far better advantage than in balsam.

In determining the character of what is brought into view by
the processes detailed, great aid will be derived from the use
of chemical tests. Thus, in the case of the potato, previously
described, most persons who had read anything at all upon such
subjects, would recognize the starch granules. All starch granules,
however, are not of the same form as those found in the potato;
indeed, some would hardly be recognized at all, except by those
having considerable experience. But if a little of the tincture
of iodine be brought into contact with them, they at once be-
come deeply blue. This subject is too extensive to be dis-
cussed here, but those who desire to become proficient in the
use of the microscope cannot safely neglect it.

In most cases after an object has been carefully brought into
proper mechanical condition, in one of the ways we have cle-

146 SELECTION AND TJSE

scribed, it is necessary to immerse it in some suitable medium,
so as to render it clear and transparent. The action of such
media may be very well illustrated by the following experiment:
Take a short piece of black human hair, place it on a slide,
bring it into focus and examine it. It will appear as a dark
cord with a light line running down the centre, and from this
circumstance has arisen the erroneous popular idea in regard to
the tubular structure of hair. Apply a drop of glycerine di-
lated with an equal bulk of water, and again examine it. The
appearance will have entirely changed, having become clearer
and more definite, so that the structure of the hair is more
easily made out. This effect depends upon the refracting power
of the liquid used. The following liquids are usually employed
for this purpose, their efficiency being in direct ratio to their
index of refraction, which we append to each. Water, 1.336;
glacial acetic acid, 1.38; alcohol, 1.372; vitreous humour, 1,340;
sea-water, 1.343; equal parts of glycerine and water, 1.40; pure
glycerine, 1.475; oil of turpentine, 1.478; Canada balsam,
1.532—1.549; bisulphide of carbon, 1.678; oil of annis, 1.811.
Alcohol and water, and solutions of various salts in water are
also very useful. When a pure article of glycerine is not avail-
able, a solution of white sugar may be used with good results.

Great care must be exercised lest the fluid that is added
should change the form or structure of the object. Upon this
subject the remarks of Frey are very judicious. He says : " Theory
requires that each constituent of the body should be examined
in a fluid medium which resembles in respect to quality and
quantity, the fluid which saturates the living tissue. Naturally
this requirement cannot be completely fulfilled in practice; our
aim should be to approach it as nearly as possible. Saliva,
vitreous humour, amniotic liquor, serum and diluted albumen
are generally recommended as suitable media for the investiga-
tion of delicate changeable tissues, and, in certain cases, they
accomplish their object in a satisfactory manner. But do not
expect them to suffice for every case. Not unfrequently one
and the same tissue of different species of animals reacts differ-
ently with the same fluid medium, as may be seen with the
blood corpuscles. M. Schultze has communicated to us an im-
portant and readily proved observation of Landolt's, that ani-

OF THE MICROSCOPE, 147

mal fluids may be preserved from decomposition for a long time
by the addition of a small piece of camphor."

Sehultze recommends as a neutral fluid, suitable for most
tissues, a liquid which he calls "lod-serum." It consists of the
amniotic fluid of the calf, to which has been added a concen-
trated tincture of iodine or a strong solution of iodine in the
proportion of six drops to the ounce. The color of the solution
is at first wine yellow, but after a few hours it becomes paler;
this paleness afterwards increases, and the subsequent addition
of a few drops of the iodine solution becomes necessary. As
the amniotic fluid is not always attainable, a good substitute
may be prepared by mixing 1 ounce white of egg, 9 ounces
water, and 40 grains chloride of sodium, with the proper pro-
portion of tincture of iodine.

During the entire process of preparation, the greatest atten-
tion must be paid to cleanliness. Particles of dust, which to the
unassisted vision are invisible, become offensively prominent
under the microscope. To exclude these, and to protect the
objects, it is important that the latter should be kept carefully
covered when not actually undergoing some operation. Small
bell glasses are recommended for this purpose by Dr. Carpenter,
and they answer admirably. We prefer, however, as being
cheaper and less bulky, watch glasses to which a handle has
been cemented as shown in Fig. 34. The
/^af^ffj^^^^ handle may be a little knob, turned out of
\^__ _J a piece of wood, or where this is not conveni-

i**^--T=TrT*^ enj. a gmau corj- wjn answer. A little sealing
Fig. 34. wax serves for a cement, the watcli glass be-

ing heated before the wax is applied. Flat
plates of glass answer well to cover the dissecting pans previ-
ously described.

When a number of objects are to be protected for some time,
we place them on a piece of plate glass eight inches square,
cover each with a watch-glass cover, and protect the whole by
means of a bell jar with ground edges. The latter fits closely
to the plate glass and excludes everything, while the small covers
protect the individual specimens when the large cover is raised
for the purpose of getting at them.

Singular mistakes have arisen from the fact that foreign

148 SELECTION AND USE

bodies which have accidentally found their way into a prepara-
tion have been mistaken for part of the specimen. The only
way to avoid similar errors is to exclude all such intruders by
means of proper covers, and to become familiar with them so
that they may be instantly recognized when present. Dr. Beale
gives the following list as those that are most apt to find their
way into the preparations of the microscopist : Oil globules;
milk; starch from the potato, wheat and rice; bread crumbs;
feathers; worsted; fibres of flax, cotton and silk of different
colors; human hair, cat's hair and hair from blankets; the scales
of butterflies and moths, particularly those from the common
clothes moth; fibres of wood, fragments of tea leaves, hairs
from plants, vegetable cellular tissue and spiral vessels; particles
of sand. The curious circumstances under which such bodies
will find their way into a specimen was recently illustrated in the
author's experience. In a liquid submitted for examination,
and said to be pure, he found foreign matter. It proved to be
brick dust, used to clean the tin funnel with which the vessel
was filled, and which had been washed in by the passage of
the fluid. The student can have no better exercise than to
examine these intruders and familiarize himself with their ap-
pearance.

4

Preservative Processes.— The object of all preserva-
tive processes is to prevent any change either in the structure
or composition of the object. An object may be most perfectly
prepared and beautifully mounted, but if it be not so treated
as to preserve it from change, the labor thus expended is wasted,
as regards the preservation of a permanent record. And yet
how many objects there are that we would like to keep for
future examination and comparison, or to show to friends.
This department of the treatment of objects is, therefore, of
great importance, and success in it can only be obtained
through a thorough understanding of the principles involved.

There are four methods in common use for the preservation
of perishable animal and vegetable substances: 1, Constant ex-
posure to temperature considerably below the freezing point of
water; 2, the perfect exclusion of air; 3, reduction to a state of
complete dryness; and 4, the employment of certain anti-septic

OP THE MICKOSCOPE. 149

compound. The third and fourth are the methods usually em-
ployed in microscopy, but the same principles which render the
second method so successful in the preservation of canned fruits
r.iiil meats, deserve the attention of the microscopist.

Drying, as a preservative process, can be applied to but few
specimens, chiefly transparent insect preparations, and opaque
objects. Blood and similar matters are also sometimes pre-
served by drying. Such preparations are so easily dried that
no special directions are needed. Warming them over a lamp,
or preferably on a water-bath, before applying the thin glass
cover (as directed in the section on mounting objects) is al-
most always sufficient. Where the specimen is liable to be in-
jured by heat it may be dried by placing it over sulphuric acid,
and covering both acid and preparation with a bell jar having
ground edges and resting on a perfectly flat plate of glass. The
acid soon absorbs all the moisture and renders the object perfectly
dry. Where a cell is used for an opaque object, and dryness is
essential, great care must be taken to make the cell impervious
to air, otherwise dampness will be sure to penetrate, and if the
object be of animal or vegetable origin, fungi will be very apt to
grow on it. We have found cells of cardboard peculiarly
liable to this defect, and such cells should always be thoroughly
saturated, and coated with varnish, such as gold size or Canada
balsam.

The great dependence of the microscopist, however, is in the
employment of certain preservative media, of the most impor-
tant of which, the following is a list:

CANADA BALSAM. — Of all the media employed for the mount-
ing and preservation of objects, Canada balsam is undoubtedly
the most generally useful, and it is probable that more objects are
mounted in this material than in all the other media put together.
As a preservative it is perfect, and its action in rendering many
objects transparent and clear is often of great value. Frey tells
us that " several sorts of Canada balsam occur in commerce.
To be good it should be of thick consistence, nearly colorless,
and thoroughly transparent." One difficulty, however, is that
much of the Canada balsam that is sold is factitious, being
made of cheap resins dissolved in impure turpentine. Such

150 SELECTION AND USii

balsam soon becomes cloudy, and is very apt to crack. Balsam
that is too highly colored may be bleached by exposure to sun-
light— a process applied by most opticians to the balsam used
by them for cementing the lenses of achromatic combinations.
Balsam when new is quite fluid, too much so, indeed, for the
mounting of most objects. On the other hand, old balsam is
thick, and is apt to crack. Microscopists generally keep balsam
in wide-mouthed bottles, and take out what is wanted by means
of a glass rod. As the process of evaporation, which makes
balsam thick and viscid, goes on more slowly in narrow-mouthed
bottles, we prefer the latter, and transfer the balsam to the glass
slide by means of a fine wire with a small loop at the end. The
wire is passed through a cork, or preferably a wooden stopper,
and descends to such a depth as to be just below the surface of
the balsam. As the latter is used up, the wire is pushed down,
and if cemented in its place by the balsam, a little heat soon frees
it. The latter remark applies also to the wooden stopper, which
is very apt to stick in the neck of the bottle. A very slight ex-
posure to the flame of a spirit lamp is sufficient to loosen it.

SOLUTION OF BALSAM. — When the objects that are to be pre-
served in balsam would be injured by the heat necessary to
melt it, it is advisable to use a solution of balsam in ether or
chloroform. The balsam used for making the solution should be
old and thick. This solution is frequently sold with the label,
" Balsam for use without heat."

COLOPHONY. — Thiersch recommends a solution of resin or
colophony in absolute alcohol. The advantage which this ma-
terial presents is that the preparation may be placed in it di-
rectly from the absolute alcohol, without becoming cloudy,
and without prejudice to the durability of the specimen. He
advises the microscopist to prepare the colophony himself from
Venice turpentine, which is done by dissolving it in an equal
volume of ether, filtering it through paper, and evaporating,
until, when cold, it breaks with a conchoidal fracture. The
material that remains is then to be dissolved in absolute alco-
hol until it is of a syrupy consistence.

OF THE MICKOSCOPE. 151

DAMAR MEDIUM. — Gum damar lias been recently introduced
amongst the materials used by microscopists, and with some it
has found great favor. Carpenter speaks highly of it. Dia-
toms are said to show better in it than in balsam, and for
delicate physiological preparations, especially transparent in-
jections, it is very excellent. It is thus prepared: Half an
ounce of gum damar is dissolved in one ounce of oil of turpen-
tine, and half an ounce of gum mastic in two ounces of chloro-
form. The solutions are filtered and mixed.

Ordinary damar varnish, such as is used by painters, is some-
times sold for microscopical purposes, but it does not give
satisfactory results.

Preparations which have been preserved and mounted in
balsam or damar are very durable, while those that are
mounted in fluids are a source of continual annoyance and loss.

Many microscopists, therefore, exclude from their cabinets
all preparations mounted in liquid on the ground that sooner
or later they will become worthless. And many of our best
dealers refuse to have anything to do with them. Neverthe-
less, as Frey well says, " the natural condition of the tissues is
completely represented only when mounted in a moist condi-
tion. This method permits of the most accurate recognition of
delicate textural relations, pale cells and fibres, etc. , and should
not be omitted with any tissue in the production of histologi-
cal collections. "

GLYCERINE. — At the head of the list of preservative media
for moist preparations stands glycerine. "Its strong refrac-
tive power, its property of combining with water, and of at-
tracting the same from the atmosphere, render it an invalua-
ble medium for mounting animal tissues containing water.
It may be truly said, that what Canada balsam is to dry tissues,
glycerine is to moist ones." — (Frey.) Much of the glycerine in
market is very impure, and although the impurities do not
show themselves very strongly at first, they soon become mani-
fest by the darkening of the liquid, (owing probably to the
presence of lead), and the formation of a cloudy precipitate.
Dr. Beale strongly recommends Price's glycerine, and we have
found it very excellent.

152 SELECTION AND tISE

When employed as a preservative, glycerine is used either
pure or diluted, according to circumstances. Equal parts of
glycerine and water form a very excellent medium for most
objects. It is alleged, however, that fungi are very apt to grow
in glycerine and its solutions. We are inclined to believe
that this may be avoided by adopting the precaution detailed
at the end of this section. We have now before us specimens
that were mounted in pure glycerine and water, eighteen years
ago, and they are still quite perfect. If, however, there should
be any danger in this direction, the addition of a little camphor
will prevent the evil. Glycerine exerts a powerfully solvent
action on many salts, particularly salts of lime, such as the car-
bonate, and hence it is employed for preventing scale in the
boilers of steam-engines. This property renders it dangerous
to use it for the preservation of structures containing com-
pounds of lime.

GLYCERINE JELLY. — The original directions given by Law-
ranee are as follows: " Take any quantity of Nelson's gelatine,
(any good gelatine will answer, however,) and let it soak for
two or three hours in cold water; pour off the superfluous
water, and heat the soaked gelatine until melted. To each
fluid ounce of the gelatine add one drachm of alcohol, and mix
well; then add a fluid drachm of the white of an egg. Mix
well while the gelatine is fluid but cool. Now boil until the
albumen coagulates, and the gelatine is quite clear. Filter
through fine flannel, and to each fluid ounce of the clarified
gelatine add six fluid drachms of Price's pure glycerine, and
mix well. For the six fluid drachms of glycerine a mixture of
two parts of glycerine to four of camphor water may be sub-
stituted."

Glycerine jelly is a very excellent medium, and is easily used.
At ordinary temperatures it is quite solid, but when slightly
heated it melts, and may be used like balsam, directions for
mounting in which will be found in the next section. Objects
that are to be mounted in glycerine jelly should be soaked
until thoroughly saturated with a mixture of 7 parts glycerine,
6 parts water, and 1 part alcohol. It is also well, after immers-
ing them in the melted jelly, to place the slide for a short time

Otf THE MICROSCOPE. 153

on a water bath heated to about 125° Fah. The jelly then pen-
etrates every part of the preparation.

When intended for use in very warm climates the proportion
of the gelatine to the other ingredients should be increased.

HANTZSCH'S FLUID. — Very beautiful preparations of delicate
vegetable forms have been prepared with this liquid, even the
coloring matter being left unaltered. It consists of 3 parts of
pure alcohol, 2 parts of distilled water and one part of glycer-
ine. The object, placed in a cell, is covered with a drop of
this liquid, and then set aside under a bell-glass. The alcohol
and water soon evaporate, so that the glycerine 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 glycerine is left to fill the cell,
which is then to be covered and closed in the usual manner.
We have used this liquid with gratifying success. It is easily
prepared, is not difficult to use, and it gives very excellent re-
sults.

GLYCERINE AND GUM. — Of this medium Carpenter says: " For
many objects that would be injured by the small amount of heat
required to melt Deane's gelatine or glycerine jelly, the gly-
cerine and gum medium of Mr. Farrants will be found very
useful. This is made by dissolving 4 parts by weight of picked
gum arabic in 4 parts of cold distilled water, and adding 2 parts
of glycerine. 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 clear
cold water; and it should be kept in a bottle closed with a glass
stopper or cap (not with cork), containing a small piece of
camphor. The great advantage of this medium is that it can
be used cold, and yet soon viscifies without cracking; it is well
suited to preserve delicate animal as well as vegetable tissues,
and in most cases it increases their transparency.

DEANE'S GELATINE. — Before the introduction of glycerine
jolly, Deane's gelatine was a favorite medium, and we still use

154 SELECTION AND USE

it with success. Take gelatine, 1 ounce; honey, 5 ounces;
water, 5 ounces; rectified spirit, £ ounce; creosote, 6 drops.
Soak the gelatine in water until soft, and then add it to the
honey, which has been previously raised to a boiling heat in
another vessel. Then boil the mixture, and when it has cooled
somewhat add the creosote mixed with the spirit. Lastly, ni-
ter through fine flannel. When required for use, the bottle
containing the mixture must be slightly warmed, and a drop
placed on the preparation upon the glass slide, which should
also be warmed a little. Next, the glass cover, after having
been breathed upon, is to be laid on with the usual precau-
tions. The edges may be covered with a coating of Bruns-
wick black. Care must be taken that the surface of the
drop does not become dry before the application of the
glass cover; and the inclusion of air-bubbles must be carefully
avoided.

ALCOHOL. — Mixed with water in various proportions, alcohol
forms one of our best preservative liquids, for both animal and
vegetable substances. The chief objection to it is the difficulty
with which it is retained in the cell.

THWAITE'S FLUID. — Take water, 16 ounces; alcohol, 1 ounce;
creosote, sufficient to saturate the spirit; chalk, as much as may
be necessary. Mix the creosote and spirit, stir in the chalk
with the aid of a pestle and'mortar, and let the water be added
gradually. Next add an equal quantity of water saturated with
camphor. Allow the mixture to stand for a few days and filter.
Used for preserving desmidise, and also animal substances.

BEALE'S LIQUID. — Creosote, 3 drachms; wood naphtha, 6
ounces; distilled water, 64 ounces; chalk, as much as necessary.
Mix the naphtha and creosote, then add as much prepared
chalk as may be sufficient to form a thick, smooth paste; after-
wards add, very gradually, a small quantity of the water, which
must be well mixed with the other ingredients in a mortar.
Add two or three small lumps of camphor, and allow the mix-
ture to stand in a lightly covered vessel for a fortnight or three
weeks with occasional stirring. The almost clear supernatant
fluid may then be poured off and filtered if necessary. It should
be kept in well-corked or stoppered bottles.

OF THE MICKOSCOPE. 155

GOADBY'S FLUIDS. — Goadby used two distinct fluids, desig-
nated by letters A and B, the difference being that alum was a
constituent of one and not of the other. Of both fluids there
were several degrees of strength, which were designated by
numbers. A fluid, as usually employed (A2), consisted of rock
salt, 4 ounces; alum, 2 ounces; corrosive sublimate, 4 grains;
boiling water, 2 l u rts. To make the B fluid take rock salt, 8
ounces; corrosive sublimate, 2 grains; boiling water, 1 quart

PACINI'S FLUID. — Take corrosive sublimate, 1 part; pure
chloride of sodium (common salt), 2 parts; glycerine, 13 parts;
distilled water, 113 parts. This mixture is allowed to stand for
tit least two months. After that time it is prepared for use by
mixing one part of it with three parts of distilled water, and
filtering it through filtering paper. This fluid is very strongly
recommended by Frey. It is used for blood globules, nerves
and ganglia, the retina, cancer cells, and especially delicate pro-
teinous tissues.

CASTOR OIL. — This is used for preserving certain crystals.
The best cold-drawn castor oil answers the purpose.

There are a few general rules which we have found essential
to the successful use of these media, but which are often
neglected, the result being the ultimate destruction of the
specimens. One of the most important points is the use of an
abundance of the medium (we are now talking of preserving,
not mounting] and the gradual saturation of the object with it.
A piece of fresh muscle, simply mounted in a shallow cell with
a drop or two of Goadby's fluid, will spoil in a very short time.
The same object, properly treated, may be preserved indefin-
itely. The proper course is to completely immerse the object
in a considerable quantity of the liquid, and if necessary
change the liquid several times until the substance to be pre-
served has been thoroughly subjected to the action of the
medium. For this purpose the quantity contained in ordinary
cells is altogether too little; small cups, basins, large watch-
glasses, etc. , are needed. It must be remembered that the sub-
stance acted upon generally absorbs certain constituents of the
preserving fluid, and hence the latter is left either very weak

15G SELECTION AND USE

or there is an unequal distribution of the constituents as re-
gards the substance itself and the surrounding fluid. Moreover
the fluids contained in many objects are displaced by the pre-
serving medium, and tend to dilute the latter. In most cases,
therefore, where the preserving medium is a liquid, the desired
result is best attained by soaking the substance in the fluid for
several days before mounting, changing the liquid two or three
times, and finally mounting in fresh fluid of regular strength.
We would lay great stress upon this point, having seen many
fine preparations spoiled by pursuing a different course. The
late Dr. Goadby, whose skill in this department was well
known, always insisted upon this course, and during a some-
what extended intercourse with him, and observation of his
methods and processes, we became fully convinced of its im-
portance.

With many preservative liquids, it is well to begin with a
diluted article, and gradually increase the strength at each
change of fluid until the proper strength has been reached.
This course is specially recommended with glycerine and saline
solutions.

Another point which demands attention is the entire exclu-
sion of air, especially of oxygen. Now air adheres with great
tenacity to most surfaces, such as those of glass or metal, and it
dissolves to a considerable extent in all watery solutions. To
get rid of it, the surface of the cell and cover should be either
well warmed, and then allowed to cool just before being filled,
or washed with alcohol (after which it may be dried). To ex-
pel the air from the liquids, they should be boiled, and to pre-
vent the absorbtion of a fresh dose of air, they should be kept
well stoppered. But as air will find access to the liquids so as
ultimately to saturate them, it is necessary to boil the fluids at
frequent intervals, so as to get rid of this element. Without
strict attention to these points it is almost impossible to pre-
serve animal substances for any length of time in saline fluids.

Mounting Objects.— For the purpose of conveniently
exhibiting and comparing objects, and arranging them in cab-
inets where they can be at all times accessible, it is necessary to
mount them securely in such a manner that they may be easily

OF THE MICROSCOPE. 157

handled. For purposes of mere examination and study,
mounting is unnecessary, but when the objects, are to be kept
for future reference it is indispensable. It is true that where
the specimens are large they might be kept in bottles in a pre-
servative fluid, and taken out when wanted. This would be
very inconvenient, however, and with very minute or delicate
objects it would be almost impracticable.

There are three modes in which objects are mounted: 1. Dry,
the object being simply attached to the slide and suitably pro-
tected. 2. In balsam, the object being immersed in Canada
balsam, damar medium, copal varnish, or some similar mate-
rial. 3. In fluid, the object being mounted in some of the pre-
servative liquids previously described. Specimens may be
mounted in any of these ways, so as to be viewed either as
transparent or opaque objects, and the instruments and mate-
rials required are neither numerous nor expensive. With those
named in the following list almost any ordinary object may be
neatly put up, though it is of course to be expected that occa-
sions will frequently arise when special instruments and
methods, which are not described by any author, will be
needed. Experience alone can enable the microscopist to treat
such cases successfully.

SLIDES. — Most objects are mounted between two pieces of
glass, one of which is called the slide and the other the cover.
As it is convenient to have these slides all the same size, so that
they may be easily arranged in cabinets, the Microscopical
Society of London has adopted a slide three inches long by one
inch wide as the standard size for use amongst their members,
and this size has been generally adopted by microscopists
throughout the world. All the best slides that are found in
market are of this size, and the microscopist who fails to adopt
it will be subject to great inconvenience when he desires to ex-
change objects with others who are pursuing similar studies.
Several other sizes are employed by the French, most of them
being quite small (2f by J and 2| by j), but as these small slides
are the only ones that can be used with some French micro-
scopes— the stages of which are too small to take a slide 3 by 1 —
they are usually kept in stock by dealers in microscopic appar-

158 SELECTION AND USE

atus. Small slides have this advantage, that they cost less, and
take up less room in a cabinet. Large slides look best, and af-
ford more room for descriptive labels, which is an important
point. But since slides 3 by 1 have been adopted by common
consent, the microscopist who mounts specimens, or who buys
objects mounted on slides of a different size, commits a mis-
take for which the advantages offered by the small slidas are
but a slight compensation. The only exceptions to this rule
are where the objects are too large to be mounted securely on a
slide of standard size, or where a large number are to be pre-
pared for the purpose of illustrating some special series of in-
vestigations. It is to be presumed that «uch a series will never
be broken up and separated, and as it will in all probability be
assigned to its own cabinet, it is sometimes of advantage to
have it upon slides of a size other than that in common use.
As the objects composing such a series will probably be num-
bered and catalogued, there is no necessity for extended de-
scriptions on the labels, and therefore slides of half the usual
size (l£ by 1) will serve very well. The cabinet may thus be
reduced in bulk by one-half. We have a special cabinet, illus-
trative of textile fibres, mounted upon slides of small size, and
find it quite convenient.

The glass from which slides are cut should be free from air-
bubbles, scratches and that wavy appearance which is due
either to inequalities in the surface or to irregularities in the
composition of the glass itself. Ordinary window glass is en-
tirely unfit for the purpose. The most suitable kind is plate
glass, the surface of which has been ground and polished, so as
to be perfectly even and smooth. Glass of this kind is used for
looking-glasses and by photographers, and when other material
could not be had, we have made very excellent slides out of
broken looking-glasses and photographer's plates, though it is
difficult to get the latter thin enough. Slides of good glass
are, however, manufactured in quantity and sold at a reason-
able price, so that under ordinary circumstances it will hardly
pay the microscopist to cut out his own slides. Moreover the
slides sold by the dealers have the edges neatly ground, an
operation which the microscopist will find tedious and trouble-
some.

OP THE MICROSCOPE. 159

As procured from the manufacturers, the slides are always
dirty, never having been washed after the process of grinding
and polishing the edges. If this dirt were soft it would not
matter so much, but it is in general hard and gritty — being in
fact the grinding sand — and the consequence is that the surfaces
of the slides are very apt to be scratched and injured. There
is but one firm that exports slides to this country, and they are
very careless in this respect. Out of a gross of slides it is often
difficult to find two dozen that are not so scratched as to be
worthless for the finest class of work. Having procured the
slides, however, the first thing to do is to clean and assort
them. They should be cleaned by being rinsed in water con-
taining a little washing soda; the dirt being removed if neces-
sary by the use of an old nail brush or tooth brush. Until
this has been done they should not be wiped with cloth or
leather, for by so doing the particles of sand are dragged along
the surface, making a deep mark. They should then be washed
in pure water, carefully wiped with a soft cloth, and assorted
for thickness and quality. It is in general best to sort them
into three classes — thick, medium and thin — the latter being
used for test and other very delicate objects. Elaborate instru-
ments have been devised for measuring the thickness of the
slides, so as to assort them accurately, but they are entirely un-
necessary; the eye is a sufficiently accurate guide. To deter-
mine their quality, they must be examined under the micro-
scope, and as it is only the central portion that is of any con-
sequence in this case, we place them on a brass plate, 3 by 1,
with the edges slightly turned up, and having a hole five-
eighths of an inch in diameter in the centre. That part which
lies over the hole is the only part which it is necessary to ex-
amine. Slides which contain air-bubbles, striae or scratches,
are at once laid aside to be used either for opaque objects or
those of a very coarse kind. Those that are perfect are care-
fully stored away where they will not be subject to injury.

COVERS. — After being properly arranged on the slide with a
suitable preservative medium, the objects must be covered with
a small piece of thin glass. Glass intended specially for this
purpose is made in England, and imported either in sheets or

1GO SELECTION AND TTSE

cut into squares and circles of suitable sizes. Directions for
cutting these covers would be out of place here. The beginner
will always find it most economical to buy them ready cut. Of
the two kinds — round and square — the former are, for all ordi-
nary purposes, the most convenient, as covers of this shape are
best suited to cells made with the turn-table, and they may also
be finished more easily and neatly than the square ones.

Covers should be carefully assorted for thickness, since the
thickness of the cover exerts a material influence on the per-
formance of all lenses except those of the lowest power or
quality. Where objectives which do not adjust for thickness of
cover are employed, the microscopist should find out the exact
thickness to which they have been corrected by the maker, and
use glass of this thickness in covering all objects that are to be
examined by means of these lenses.

The inexperienced student will be apt to find some difficulty
in cleaning these covers. They are so fragile that it is difficult
to rub them, so as to remove dirt, without breaking them.
The best method is to soak them in a weak solution of potash,
rinse them off carefully several times with clean water, and after
pouring the last water off, give them a final rinsing by taking
them up in a pair of forceps and moving them about in a tum-
bler of clean water. They should then be laid (singly, of
course) on a wiping block and wiped. Wiping blocks are made
by covering a flat block of wood with chamois leather or linen,
drawn tightly so as to present a flat but somewhat soft surface.
These blocks are generally made round and with handles, but we
prefer them oblong (4 by l£ inches) and without handles. One
of them is laid on the table face up ; upon this face the thin
glass is laid and wiped with the other block. In this way the
thinnest glass can be cleaned without risk of fracture.

CELLS — TUEN-TABLE. — All objects that are mounted dry or in
fluid should be placed in cells, as unless this is done it is dif-
ficult to arrange the object properly or to secure the thin cover
permanently. In the majority of cases these cells consist of
little more than a ring of cement laid on the glass slide and
allowed to harden, and their depth does not exceed the thick-
ness of a sheet of paper. Such cells are in constant demand,

OF THE MICROSCOPE. 161

and are almost always made by the microscopist himself by
means of a little instrument known as a turn-table or whirling
table, of which there are several different forms in market. A
cheap and efficient form is shown in Fig. 35. The table is sup-
ported by a spindle upon which it turns, motion being com-
municated by means of a milled ring. The slide is held in its
place by two spring clips, and it is brought to the centre by
means of a guide or bar, c, with a square projection. This is
carefully arranged, so that a slide 3 by 1 shall be accurately
centered. Hence it follows that the rings and cells on all the
slides put up by the owner may be instantly and accurately
C,

Fig. 35.— TURN-TABLE.

centered by simply placing them on the table and bringing
them up to a firm bearing against the guide. This bar or
guide may, however, be instantly removed when desired, and
when this is done, any cell may be truly centered by the usual
methods. This turn-table, therefore, enables us always to bring
cells of our own make instantly to a perfectly accurate centre,
while other cells can be centered at any time with very little
trouble.

To most turn-tables there lies the objection that the devices
for centering and holding the slide make one side heavier than
the other, and consequently, as every mechanic knows, irregu-
lar and eccentric moticn is the result. On many otherwise
well-made instruments it is, from this cause, impossible to make
a true cell, particularly if we attempt to work at a high speed.

162

SELECTION AND TJSE

In the turn-table just described, provision is made to obviate
this difficulty. A heavy-headed screw, of the precise weight
necessary, is screwed into the under surface of the table, and
gives a perfect balance to the wheel. It then runs smoothly
and truly.

Numerous attempts have been made to produce a self-center-
ing table, i. e. , one in which the
slides would be truly centered
without requiring care and skill
on the part of the operator. One
of the earliest forms was that of
Dr. Matthews, the centering part
of which is shown in Fig. 36.
Upon the surface of the table he
arranges two triangular plates
of brass, which rotate upon pins
placed at equal distances on each
side of the centre, and as the
plates are of the same size, when-
Fig. 36. — MATTHEW'S TURN-TABLE, ever their inner faces are par-
allel, these faces must be equi-
distant from the centre. Hence, when a slide with parallel
sides is placed between them, and the plates turned so as to
press upon the sides of the slide, the slide will be truly cen-
tered so far as its width is concerned. It is centered for length
by a stationary pin, against which the end is always brought.
Slides of irregular size are therefore centered only one way.

There are at present, however, before the public, two tables
which centre slides accurately in both directions. One was in-
vented by C. Mason Kinne, of San Francisco, who describes it
as follows: "As will be seen from the engravings, Figs. 37 and
38, which are reduced one-half, the slide will be grasped autom-
atically, upon removing the finger from the lever, the spiral
spring causing the clutches to instantly clasp the slide, and
retain it in a central position. One corner of either end of the
slide projects sufficiently for the purpose of taking hold with
one hand, while the other is pressing the lever, and can be
fixed or removed without pushing along a circular disc to its
edge. The slots are made to allow movement enough, so that

OP THE MICROSCOPE.

163

Fig. 37. — KDTjnt's TUKN-TABLE.
(Upper side.)

Fig. 38. — KINNE'S TURK-TABLE.
(Underside.)

the clutches can grasp
any piece of glass from
l£ to 3^ inches in diag-
onal length, and the
table is made of brass
about a quarter of an
inch thick, which gives
weight sufficient to se-
cure stability of move-
ment. The whole rests
on a small spindle 4 or
5 inches long, screwed
into the centre of the
brass stud, which is the
fulcrum of the lever,
and can be removed at
pleasure to pack away.
The pointed lower end
of the spindle is stepped
into a counter-sunk

metal rest, and with a collar placed at a suitable distance above
to allow of free movement of the hand, I find that a steady
motion can be obtained with the thumb and finger, of any re-
quired velocity, and is under greater control than with any
milled-head device."

Mr. Kinne suggests a very simple method of constructing a
home-made table on this plan: " The spindle can be fitted into
any appliance, primitive or expensive, at the option of the
worker, and I find that an old cigar box, with a portion of one
end removed, is just as useful as anything else, though if made
for sale, a cheap varnished box could be furnished, and in
which the table and spindle could be packed when desired. If
fitted up with the cast iron stand, the whole might present a
neater appearance, but the additional expense would not add to
its utility."

Slides which have been imperfectly centered on other tables,
are recentered for varnishing by the use of two rectangular tri-
angles and a little wedge. The inventor uses the corners of a
broken slide and a piece of match.

164 SELECTION AND USE

The other self-centering turn-table was invented by Mr. C.
F. Cox, of New York, and is shown in Fig. 39. The slide is

. 39. — COX TUBN-TABLE.

grasped by two angle-pieces, which are simultaneously moved
to and from the centre by means of a right and left hand
screw. When it is desired to re-varnish slides which have
not been accurately centered in the first place, a pair of spring
clips, attached to a stout bar, are fastened on. This can be
effected in an instant. The arrangement is shown in Eig. 40.

Fig. 40.— COX TTTKN-TABLE.

There is also a very ingenious device for placing a row of
small cells along the middle of a slide. This consists of two
equal right-angled triangles, the square corners of which fit into

OF THE MICROSCOPE. 165

the clutches, thus allowing the long sides to lie parallel to each
other, and at equal distances from the centre. A slide may
thus be grasped between them, and pushed along longitudin-
ally, as may be desired.

Those who once see a turn-table, will find no difficulty either
in understanding the method of using it, or in putting this
knowledge into practice. The slide, being held on the table
either by springs or clutches, is made to revolve rapidly, and a
brush, charged with cement or varnish, is held against its sur-
face so as to leave a ring. There is a slight knack about making
good cells, whicl^ it requires a little practice to acquire. The
brush must be held in the direction of a tangent to the ring —
that is, it must not point to the centre of the circle, but must
lie so that the ring, as it revolves, will draw the cement away
from the brush. Practice alone can give expertness in doing
this, and we would advise the beginner to work steadily for a
few hours at making cells on pieces of common window glass,
strips of which can be had for nothing from any glazier. The
chief points to be attended to are the position of the brush and
the consistence of the cement. If the latter be too fluid, it
spreads and does not form a well-defined circle. If too thick
it does not leave the brush as freely as is necessary. The
method of preparing the cement will be explained under the
proper head.

Fig. 41.

"Where a turn-table is not at hand, very good cells may be
made as follows: On a card draw the outlines of a slide with a
series of circles in the centre, as shown in Fig, 41 ; lay the
slide on the card so that the centre of the circles will be at the
centre of the slide, and then paint a circle of cement on the

166 SELECTION AND USE

wlide by hand, the rings beneath serving as a guide,, Very good
cells may be thus made, but the process is of course more
tedious than that with the tarn-table, and does not give as neat
results.

A few precautions are necessary in order to insure the per-
manent adhesion of the cells to the glass. In addition to pro-
viding cement of good quality, we must see that the slide is
dry aiid recently heated. It is difficult, with most cements, to
use hot slides, as the cement is apt to flow; but the slide should
have been recently heated, and after the cement has partially
hardened, the cells should be baked by exposure to a temper-
ature as high as they will stand. This is easily done by placing
them on a board or plate, and leaving" the latter for a short
time in an oven.

Where cells of greater depth are required, rings of various
materials are cemented to the slide. For objects mounted dry,
rings of leather or cardboard answer well, provided they are
carefully varnished so as to be impervious to air and moisture.
For liquids, rings of glass, tin, ebonite, etc., are used. Eings
of rubber and gutta-percha have been suggested, but they do
not answer, as they soon become rotten. Full directions for
making and using deep cells may be found in the works of
Quekett, Carpenter, Beale, Frey, etc.

HOT-PLATE. — This is simply a stout plate of brass or iron,
which is supported over a lamp by suitable means. The com-
mon plan is to insert four stout wires to serve as legs, but a
better mode is to support the plate on the ring of a retort
stand, as its distance from the lamp can thus be readily ad-
]' usted and the temperature regulated. The hot-plate serves to
distribute the heat, and thus to prevent the slides from becom-
ing suddenly and unequally heated. Moreover, by means of
it several slides can be heated at once, and thus much time
may be saved. It should be tolerably heavy. The one we use
is of cast iron, six inches long and three inches wide. The
upper surface has been ground so as to be tolerably smooth,
When a hot-plate is not at hand, a good substitute may be
found in a smooth brick, or, better still, a plate of soapstone.
These may be heated in the fire and will retain their heat for a
long time.

OF THE MICROSCOPE. 167

LAMP. — Any lamp, or even candle, will answer, but we prefer
a spirit lamp, the flame being free from smoke and easily man-
aged. At night the kerosene lamp used for giving light will
answer. Where gas is used, the Bunsen burner is a great con-
venience. Whatever lamp or burner be used, it should be sur-
rounded with a chimney or shade, so as to prevent the flicker-
ing of the flame by currents of air. The best shade is a tin
cylinder, with rows of holes at top and bottom for the admis-
sion and exit of air.

KETORT STAND. — ja. suitable retort stand is a very simple
affair, and is best made at home. Ours consists of a board of
hard wood, 5 inches by 4, into which is screwed a rod fourteen
inches long, and a quarter of an inch in diameter. The rings
have no screws, but are simply pieces of wire, one end of which
is twisted round the rod, while the other is formed into a ring
of the required size. Eings formed in this way are easily
moved on the upright rod, but no weight placed on them in
the usual manner can cause them to slip down.

CARDS FOR CENTERING THE OBJECTS. — Unless the objects
are placed on the centres of the slides, the latter have a very
awkward look. By drawing the outlines of a slide on a card,
and marking out the centre, this difficulty is easily overcome.
A card marked off in this way is shown in Fig. 42.

Fig. 42.

It is well to have two cards, one black with a white centre,
and the other white with a black centre, as some objects, when
immersed in the medium in which they are to be mounted,

168

SELECTION AND USE

show best against a dark ground, while others are most easily
seen against a light one. Those who use the self-centering
turn-tables may readily centre their slides by painting on them
a ring of some water-color, which is easily washed off. The
ring is, of course, laid on the side opposite to that which
receives the object.

MOUNTING NEEDLES. — These are similar to dissecting needles,
but being used in balsam, varnish and similar substances, they
cannot be used for dissection, and should be kept by them-
selves. They are most easily cleaned by being warmed over the
lamp, and wiped with a piece of soft leather. When the balsam
is burned on them, as recommended by some, it leaves a crust
which is not easily removed.

COVER FORCEPS. — In placing the cover on the object, the
ordinary forceps are very inconvenient. We
have long used a pair of forceps bent as in
Fig. 43, and with the points carefully
adjusted. The mode of using the instrument
will be obvious from the engraving. A very
ingenious device intended to answer the same
purpose has been invented by Dr. Fletcher.
These forceps are self-closing, so that the
thin glass cover is held without any effort.
After the cover is in position on the slide,
by pressing on the blades they open and allow it to slip out.
If the cover should
stick to the forceps
in the slightest de-
gree, it may be pre-
vented from moving
when the forceps
are removed by in- Fig. 44.

serting a common

pin in the slit seen in Fig. 44. When using the forceps
shown in Fig. 43, the same end may be attained by means of a
wire fork (a hair-pin is as good as anything), which may be
made to straddle the nose of the instrument.

Fig. 43.

OP THE MICROSCOPE.

169

SLIDE HOLDER. — The hot slides cannot be comfortably held
in the fingers, and therefore a pair of wooden forceps become
a necessity. Those usually sold are made by screwing together
two thin slips of wood with a piece of brass or lead inserted be-
tween them at one end. To admit the slide, the slips are forced
apart by pressing on pins arranged as in the stage forceps.
When placed on a table the
metal counter-balances the slide,
and keeps it from touching the
surface on which it is laid — a
very important point. The Eng-
lish forceps, being all wood, fre-
quently tip with a heavy slide.

A common spring clothes-pin
is frequently used, but when we
come to lay the slide* down, the
clothes-pin holds it in an awk-
ward manner. The end of the
hot slide is sure to lie on J;he table,
and if fluid balsam or other me-
dium should be present, the fact
that the slide is not level produces
bad results. By cutting off about
half an inch from one of the
limbs of the forceps part of the
pin, however, this difficulty is
avoided. The slide may then be
grasped in such a way that when
the clothes-pin is placed on the
table, the glass will be held in a
perfectly level position. A glance
at Fig. 45 will show what we
mean. A great advantage of this
form of holder is that it costs but a trifle, so that the micro-
scopist can supply himself with an abundance of them, and
thus several slides may be cooling, while work on others i ;
going on. When very heavy slides are used, it may beconi"
necessary to screw a plate of sheet lead to the under side of tlu
clothes-pin, so as to prevent tipping.

I

170 SELECTION AND USE

WATER BATH. — A water bath is indispensable in those cases
where a certain very moderate degree of heat is not to be ex-
ceeded. Few persons fully appreciate the difficulty of regu-
lating or even estimating the temperature of an object held over
a naked flame, and mischief is often done before the operator
is aware of it. A serviceable water bath is easily extemporized
out of an old fruit can and a small beaker glass. This serves
for exposing material and preparations to a temperature lower
than that of boiling water. Where slides are to be so heated,
the simplest contrivance is a flat tin box, with all the joints
(cover and all, of course,) tightly soldered. A small tube,
closed with a cork, serves to admit the water.

SPEING CLIPS. — One of the first of the needs which impress
themselves upon the mind of the beginner, is the necessity for
something to retain the thin cover in its place, until the ce-
ment, which is intended to hold it permanently, dries. An end-
less variety of spring clips have been invented for this purpose,
but we have never seen anything that we liked better than the
simple article shown in Fig. 46, and which we have used for

Fig. 4.5.

over fifteen years. It consists simply of a piece of brass wire
bent a* in the engraving. The slide being held in the left
hand, the clip, held by the upper wire, is brought so that the
projecting part of the ring is placed under the edge of the
slide. The upper part is then lifted up so as to open the clip,
which is then slid on to the slide until the vertical point is in
the right position. When a broader surface than the point of
the wire is needed, a piece of cork may be stuck on it, and if

OF THE MICROSCOPE. 171

there should be need for greater pressure than that which the
spring of the wire affords, this can be obtained by sliding a
small brass ring on to the clip.

Various other forms of spring clip have been invented, but
none that we consider more simple, or that we like better than
the above, which has this great merit, that any one can make it
for himself out of materials that may be obtained at any hard-
ware store. It must be borne in mind, however, that all clips
constructed upon this plan are apt to cause a slight displace-
ment of the object, from the fact that the movement of the
point is not quite perpendicular. With delicate objects this is
a matter of importance. The only remedy is to use the end
pressure of a rod moving in fixed guides.

CEMENTS AND VAKNISHES.

A supply of carefully selected cements and varnishes is indis-
pensable to the microscopist, and it is also well that he should
understand the nature and properties of the materials used,
otherwise he will be liable to make gross blunders. Thus, of
the different articles in use, some are easily mixed with each
other, while others separate as soon as left to themselves; some
dry in one way and some in another. It would require a vol-
ume to detail the properties of the different substances which
enter into the composition of the cements used by the micro-
scopist. We have space for only the following hints, which,
however, we hope will prove useful.

Cements become hard in three different ways — cooling, evap-
oration and oxidation. Shellac, sealing wax, electrical cement,
etc., when melted by heat, furnish examples of the first pro-
cess. Shellac and sealing-wax dissolved in alcohol, and asphalt
and damar dissolved in turpentine, dry by the second process —
the solvents evaporating and leaving behind the material which
they had dissolved. Drying oil in all its forms, such as gold
size, paint, etc. , becomes hard by oxidation — not, as is gener-
ally supposed, by evaporation.

In the case of varnishes which dry by the evaporation of
some of their constituents, it is obvious that if a fresh layer

172 SELECTION AND USE

be laid over an old one, the old layer will be softened, and
if there should be any tendency to a vacuum in the cell,
the softened cement will be unable to resist the outside
pressure, and will creep in and spoil the object. So, too,
with varnishes or cements formed chiefly of drying oil or gold
size. If the different coats be laid on too thickly or too
rapidly, the part that is beneath cannot easily harden, but will
remain for a very long time in a semi-liquid condition. We
have just removed some brass rings from slides to which they
were attached four months ago by means of gold size, and
although the outer surface of the cement was hard and dry, the
interior was quite liquid, freely soiling the fingers.

GOLD SIZE.— The most extraordinary recipes have been given
for the preparation of this cement, which is in reality nothing
but good linseed oil rendered very drying by the usual
methods. Gilders frequently make it into a semi-paint by
adding coloring matter, thus forming a ground of a shade
similar to the gold they use, and this seems to have misled some
of our best writers. There is no ochre, litharge, or anything
of the kind present in good gold size. It does not pay to pre-
pare gold size in small quantities, and it may be obtained from
any color dealer. The older it is the better, and it is well,
therefore, to lay in a good stock, which must be kept carefully
corked. The working supply should be kept in a small bottle.
This is the favorite cell making material employed by Dr. Car-
penter, and it is certainly the most reliable cement we have.
It adheres firmly to glass, and if laid on in very thin successive
layers, tolerably deep and very durable cells may be built
up, but the process requires considerable time, otherwise the
under layers will remain soft. It has this great advantage over
asphalt, damar, and other cements composed of solid materials
dissolved in some menstruum, that fresh coats have but very
slight action on the old layers on which they may be laid. It
mixes with turpentine, and consequently with most materials
soluble in turpentine, but when once dry and hard, turpentine,
alcohol, ether, etc., have little or no action on it. It does not
mix with alcohol, and therefore cannot be mixed with the
solution of shellac in alcohol in any of its forms.

OF THE MICROSCOPE. 173

BLACK JAPAN. — When this can be procured of good quality,
it makes a very excellent cell. It adheres very firmly to the
glass provided the latter be exposed to a moderate heat after
the cement has become dry. Black Japan dries up and thick-
ens when kept, but may be thinned with turpentine.

BRUNSWICK BLACK. — This is simply a solution of asphaltum
in turpentine. Occasionally it is rendered very black by the
addition of a little lampblack. When good, it makes a very
excellent cement. Its quality depends chiefly upon the char-
acter of the asphaltum that is used in its preparation. Now
there are several varieties of asphaltum in market, the most
common kind at the present day being that obtained from coal
tar. This seems to be entirely unfit for the purpose. The
proper kind is that which is found native in several parts of the
world. The two kinds are easily distinguished by their odors.

SHELLAC. — This well known substance, when dissolved in
alcohol, forms a varnish or cement of great value to the rnicro-
scopist, and is the proper one to be used when glycerine is em-
ployed. Much of the shellac in market is artificially made
from resin and wax, and makes a poor varnish. Heal shellac
must be employed if failure would be avoided.

BELL'S CEMENT. — Carpenter states that this cement is merely
shellac dissolved in alcohol. With us it has presented no ad-
vantage over other cements.

SEALING WAX VARNISH. — This is prepared by dissolving the
best sealing wax in alcohol. It unfortunately happens that all
the fancy colored sealing wax in market is of inferior quality.
Very excellent red wax may be obtained, but we have never
been able to obtain good blue, black, green or other colored
wax. We therefore make varnish of these colors by dissolv-
ing in alcohol the materials used for making the best red wax,
substituting some other color, however, for the vermillion.

r-

COLORED SHELLAC. — Bleached shellac, dissolved in alcohol,
and colored with aniline blue, red, etc., makes a very fine trans-
parent varnish.

174 SELECTION AND USE

DAJMAE CEMENT.— This is a mixture of equal parts of damar
varnish and gold size, mixed together. It should stand for
some time before being used. It is said to be very excellent.
It is very tough, and serves well as an outer coating over such
brittle cements as shellac and sealing wax varnish.

MAEINE GLUE. — This is undoubtedly the strongest cement
in use for joining pieces of glass or glass and metal together.
Skilful microscopists make great use of it; beginners do not
find it so easy to manage as some others. In using it, the
simplest method is to cut it in small pieces, lay it on one of the
surfaces that are to be joined, melt it by heat, and apply the
other surface, making sure of perfect contact by rubbing the
two pi cces upon each other, if they will allow of it. Marine
glue may be obtained from most dealers in microscopes.

The cement known as liquid glue, is simply a solution of
shellac in alcohol.

For attaching labels, paper covers, etc., to the slides, nothing
is better than good dextrine. After having mixed the dextrine
with water to the proper consistence, add six drops of glycer-
ine to the fluid ounce of dextrine. This will prevent the labels
or covers from cracking off.

Having provided himself with the necessary tools and mater-
ials, the next step is to learn how to use and apply them, and this
will probably be most easily taught by describing a few char-
acteristic examples. And first of all, selecting the most easily
mounted of all objects, we commence with the scales on the
butterfly's wings. Having prepared a cell of proper size, and
allowed it to dry, the first step is to select a cover to suit it,
and give a final cleaning to both slide and cover. When every
particle of dust has been removed, breathe gently on the slide,
and press the wing lightly against it, and within the cell. A
large number of scales will at once adhere to the slide, and
the next step is to attach the cover. Place the slide on the hot-
plate, (which must not be too hot, however,) and when it is
thorough ly-^dry, and the cement somewhat soft, lay the cover
on by means of the cover forceps. Press it into contact with
the cement, and the operation is completed. It is not difficult
to see when the cover and the cement are in perfect contact,

OP THE MICBOSCOPE. 175

and great care must be taken to close the cell all round in this
way. It is true, this point is not of so much consequence with
the particular object under consideration, but with some ob-
j ects it would be quite important. The scales are now mounted
dry, and may be kept for any length of time; no dust can soil
them, and they are not liable to be injured by contact with
other bodies. It only remains to label and " finish " the slide
as hereafter directed.

Next to the above in simplicity is the mounting of such ob-
j ects as the wings of insects in balsam. Suppose we wish to
mount one of the smaller wings of a bee or wasp, so as to show
the curious hooks with which it is armed: Place the warm slide
on the centering card, drop a little balsam on the centre, and
again warm the slide, so that any air that may be present may
collect in fine bubbles which can be removed by means of a
cold mounting needle. When the air-bubbles have been re-
moved, seize the wing (previously well cleaned with a camel
hair brush) with a pair of fine forceps, and lower the tip of it
i nto the warm balsam. Then slowly lower the wing until it is en-
tirely immersed. Drop, very little more balsam on it, warm
the slide again (slightly this time), and remove air bubbles if
there should be any. Then take a clean cover in the cover for-
ceps, make it quite warm, and place it over the object by allow-
ing it to first touch one edge of the balsam, and then to grad-
ually fall down so as to exclude all air bubbles. In the case
of the bee's wing it does not answer to apply much pressure as
this would tend to distort the hooks. Press the cover into
place as much as it will bear and no more, lay the slide in a
warm place for some time until the balsam hardens, and then
clean and finish the slide.

In mounting objects in balsam and fluids, the great difficulty
to be encountered is the presence of air bubbles. Careful and
j udicious management, however, readily enables us to avoid
them. In the first place see that they are entirely removed
from the balsam on the slide. This is much more easily done
before immersing the object in the balsam then afterwards.
Next see that the air is expelled from the object. In the case
of the wing, this is effected by slowly immersing the object in
the balsam. Lastly see that no air enters with the cover. To

17(5 SELECTION AND USE

do this see that the cover is hot, and that it is lowered on the
balsam slowly, and from one side. If in any case there should
be a vacant space under the cover as at a Fig. 47, and it should
be desired to fill it, do not apply the fresh balsam directly at a.
To do so would certainly be to inclose a large air bubble. Drop
the balsam at b, warm the slide, and the balsam will creep in by
capillary attraction, and expel the air.

Fig. 47.

Let us now suppose that we have some small insect which
we have prepared by soaking in potash, and which we desire
to mount in balsam. Such a preparation if immersed directly
in balsam, would be spoilt, since the balsam and watery solu-
tion would not mix. Therefore, proceed as follows: Wash the
insect in pure water, and drain off the water; wash with strong
alcohol, drain off the alcohol, and soak for twenty-four hours
in the strongest alcohol you can get. Pour off the alcohol and
soak for twenty-four hours in turpentine. The object may
now be immersed in balsam without difficulty.

Air pumps and similar contrivances are generally recom-
mended as the best means for removing air bubbles, but we
never use them. If the object be dry, we soak it in alcohol
until all the air has been expelled, then transfer to turpentine,
and finally to balsam. This requires time, it is true but it does not
occupy the time of the microscopist. The soaking process
goes on without any attention from him, and while it involves
far less labor, with us it has always given far better results,
though we have used very fine air pumps, and followed the
best published directions. Take tlie case of a dry shaving of
wood, many of which are well worth mounting. It would be

OF THE MICROSCOPE. 177

very laborious to get the air out of this by means of the air
pump, while by soaking successively in alcohol and turpentine,
it can be mounted with great ease.

Let us now take the case of an object mounted in fluid in a
cell. Suppose it is the so-called tongue of a fly, which of course
has been soaked for some time in the liquid in which it is to be
finally mounted, viz. , dilute glycerine. We make a cell of suita-
ble thickness, which in this case may be made with shellac
dissolved in alcohol. Several coats will be required, and as
shellac alone does not adhere well to glass, we prefer to lay on
first a coat of gold size or Japan, and when this is thoroughly
dry, to lay the shellac on it. No difficulty will be found in
making a cell of sufficient depth. The cell is now to be filled
with the liquid, the object placed in it, and the whole carefully
examined for air bubbles, which must be removed if they exist.
The cover is now applied, all superfluous fluid removed by
means of a camel hair pencil, which has been moistened and
then squeezed dry, and finally the edge of the cover is to be
coated with a thin layer of cement. After a day or so another
layer of cement should be laid on, and this process repeated
until at least three layers have been applied.

We give no directions for the construction and use of very
deep cells as this is work that will hardly be attempted by be-
ginners.

When opaque objects are to be mounted either in balsam, or
in fluid, the process required is the same as that employed for
transparent objects. Very many opaque objects are, however,
mounted dry, and in this case all that is needed is to attach
them to a slide, and see that they are properly protected.
When thin they may be readily mounted in cement cells, and
this is altogether the neatest and most secure plan where it can
be used. Thicker objects require deeper cells, which may be
made of card, ebonite or electrical cement. (3 parts resin, and
1 of wax, colored with ochre or any similar matter). Cells of
card are made by first punching out a disc like a gun wad,
and then punching a hole in this so as to leave a ring. The
ring is to be cemented to the glass slide and carefully var-
nished.

Wooden slides with a cell bored in the centre, are recom-

178 SELECTION AND USE

mended very highly, and seem to answer a very good purpose.
The cells are not bored quite through the wooden slip, and as
they are blackened on the inside, any small object that may be
cemented to the bottom of them shows very well. For seeds,
small shells, and similar objects, they answer admirably. In
most cases it will be found unnecessary to cover the cells with
thin glass. Several slides may be packed together face to face,
and if held in firm contact by means of a rubber ring, dust will
be entirely excluded. Or they may be arranged in the drawers
of an ordinary cabinet, face down, the labels being placed on
the backs. This will effectually exclude the dust.

Some years ago we mounted a large-number of specimens of
minerals on leather discs, which were cemented to glass slides.
These leather discs were three-quarters of an inch in diameter,
and we had a lot of pill-box covers which exactly fitted them.
These covers, when slipped on to the discs, protected the ob-
jects perfectly, and the whole formed a very cheap, convenient
and excellent mode of mounting.

A very ingenious cell for opaque objects, the invention of
Prof Pierce, of Provi-
dence, E. I. , is shown in
Fig. 48. It consists of a
metallic cell, having a
broad flange like the
rim of a hat, which is
cemented to an ordinary Fig. 48.

glass slide, as shown in

section in the lower figure. To this cell is fitted a metal cap,
which covers and protects the object. The object may be placed
directly on the glass, or raised by means of a disc of any re-
quired thickness, so as to be more easily illuminated. The
slide, with cell uncovered and containing an object, is shown
in the upper figure. Uncovered objects may in this way be
very perfectly protected from dust and mechanical violence.

Where it is desirable to cover objects with thin glass, the
method devised by Prof. H. L. Smith will be found very excel-
lent. He takes a circular disc of thin sheet wax, which is easily
cut with a punch from the sheet wax ordinarily used for mak-
ing flowers, and attaches it by means of heat to the centre of a

OP THE MICROSCOPE. 179

glass slide. A brass ring, of which the interior is the same size
as the disc, is then attached to the slide, and the object is fixed
to the wax by slightly moistening the surface of the latter by a
minute drop of turpentine. When dry, a cover, which exactly
fits into the bevel of the ring, is attached with a little cement,
and the whole may then be finished off on the turn-table. The
appearance is very elegant, and the specimens are perfectly pre-
served.

A cell which we have found very durable, easily and quickly
made, and very neat, is constructed as follows: Having pro-
cured some good gold size and pure litharge, grind the latter
to a very fine powder. Mix the litharge and gold size to the
thickness of cream, and color either black or dark olive by
adding lamp-black. With this paint, as it may be called, make
as many cells as are wanted, and when made, dust finely pow-
dered litharge over them until they are covered a sixteenth of
an inch deep ; allow them to stand a few minutes, and then
shake off all the loose litharge by means of a few smart taps.
The surface of the cell will now be quite rough. Allow it to
stand a few hours, and then press it against a plate of glass. If
this be done carefully, a smooth, solid ring will be left on the
slide. If the edges should not be as smooth as they ought to
be, it is easy to trim them off on the turn-table by means of a
small chisel. Such cells, after a few weeks, become very hard,
and may be finished so as to be very neat. By introducing a
few obvious and unimportant modifications, we have, in this
way, made cells of some depth which held liquids quite well.

Finishing the Slides.— After the objects have been
mounted, the slides may be finished in one of two ways: they
may be covered with paper, or they may be left without any
covering, the labels being attached directly to the slide. In
the latter case the edges of the slide must have been previously
ground and polished, and, as a general rule the thin cover is
circular and not square, and is finished with a neat coat of var-
nish on the edge. This varnish serves to do something more
than merely ornament the slide; it secures the cover in its
place, and prevents the drying up of the medium used for
mounting. Even in the case of Canada balsam it is of use, for

180

SELECTION AND USE

if gold size be used as the varnish, it prevents the evaporation
of the turpentine, and the ultimate drying and cracking of the
balsam. Where glycerine jelly, glycerine, or glycerine and
gum are used, it becomes indispensable.

The process employed for finishing slides in this way is as
follows: The objects having been mounted, the slides are laid
away until the balsam, cement, etc., have been hardened, when
all superfluous matters of this kind are easily removed with a
small chisel made out of a brad-awl ground thin and sharp. A
small chisel-pointed piece of hard wood, and a little water, will
remove the last traces of balsam or varnish, and if necessary a
final cleaning may be given with a rag moistened with alcohol.
The slide is then placed on the turntable, and a neat ring of
varnish, either plain or colored, is run around the edge.

The proper labeling of the slides is an important matter.
Oar system is as follows: As soon as the object is mounted, the
slide is labeled on the under side with a very thin gummed
label, and it is also numbered on the upper side with a writing
diamond. Of this number a record is kept, so that even if the
label should fall off or get soaked off, a new label may be pro-
vided. As soon as the slide is finished, the regular label is
attached, and the small label removed. As regards designs,
etc., for labels, the variety is -endless. Each microscopist will
probably select the one that accords most nearly with his own
taste.

The Mai t wood Finder.— This is a most important ac-
cessory to every microscope, as it not only facilitates inter-
change of notes between
microscopists living at a dis-
tance from each other, but
it enables observers to make
sure of the identity of the
objects under examination
at different times. It con-
sists of a glass slip, a little FiS- 49<
wider than an ordinary slide,

upon which is a photograph occupying a space 1 by 1 inch.
This space is divided into 2,500 squares (50 divisions on each

MALTWOOD'S
FINDER

IlilllS

**& STOP.

jllll:;

OF THE MICROSCOPE. 181

side) and each of these small squares is designated by two
numbers, one of which indicates its position from bottom to
top, while the other marks its position from right to left. Thus
the square which lies on the tenth line from the bottom, and
the fifteenth from the right hand side, would be ] £. Placing
on the stage an object mounted on an ordinary slide, with its
lower edge against a ledge of some kind, and its left hand edge
against a stop (the stop and ledge being movable as regards the
stage), we bring some particular spot into view. Removing the
slide, we now place the finder in its place, and read off the
double number. It is now evident that if at any future time
we should place the finder against the ledge and stop, and bring
the same number into view, then on removing the finder and
placing the slide on the stage, the precise spot originally under
examination will be in view. We can therefore easily register
anything of interest, and so be certain of finding it at any
future time.

The finder cannot be used with microscopes fitted with clips
only. An object-carrier, provided with a ledge and stop, must
be used. Such object-carrier may either be the usual form of
the glass stage, or, with the plain stage, a thin plate of brass
with a hole in the centre, a stop at one end, and a ledge at one
edge, may be made to answer.

Hitherto the Maltwood finders have been made only by one
firm in London. They are now made in this country, however,
very successfully. A dozen which we recently examined did
not vary as much amongst themselves as those obtained from
London.

FINIS.

iioOKS PUBLISHED BY

THE INDUSTRIAL PUBLICATION CO.:

J.T6 Broadivay, New York.

Plain Directions for Acquiring the Art of Shooting: on the

With Useful Hints concerning all that relates to Guns and Shooting, and par-
ticularly in regard to the Art of Loading so as to Kill. To which has been
added several valuable and hitherto secret Jiecipes, of great practical impor-
tance to the Sportsmaa. By an OLD GAMEKEEPEB. 12mo. Clotn. Illus-
, trated .............. 75 cents.

" The directions are so plain that they cannot well be mistaken, aud they are
expressed in the fewest possible words." — Turf, Field and Farm ...... "Facing

the title-page is one of the handsomest, best-executed wood-cuts, we Lave seen.
It is entitled " The Wounded Snipe," and almost equals a steel engraving." — The
Baptist Union ...... "A perusal of the copy sent us leads us to the conclusion that

there is in it much instruction even for old sportsmen, and tl^at it is generally an
improvement upon publications of this character." — Wabashaio Sentinel ......

" Much of the information we have never seen before in any shape." — Insurance
Monitor ...... "A little book full of counsel and instruction fa" sportsmen.'1 — The

Presbyterian ...... " From its pagts we should think even the mo.-t experienced

sportsman might derive pome new ideas, while the beginner wiL" find it an inval-
uable assistant."— Country Gentleman ...... "For concise instnicuons as to how

to shoot, to select, load, carry, and keep a gun in order, etc., it cannot be sur-
passed."— Western Rural ...... "A pleasantly written, and, it seems v^ us, correct

and practical treatise on the sportsman's art." — Appleton's Journal ...... "A prac-

tical and well-written handbook."— Scientific American ...... " This is jmuiently

the book for young sportsmen." — Painter and Builder.

What to Do in Case of Accident:

In the Household, the Workshop, the Factory and the Mine, and on the Farm,
the Railroad, and everywhere ......... 75 ct,Lf*

This is one of the most useful books ever published. It ought to be in every
house, for young and old are liable to accident, and the directions given in this
little book might be the means of saving many a vali able life.

The Chemical History of the Six Days of Creation.

By JOHN PHIN, C. E. 12mo. Cloth. 75 cents.

"A email book, but full of matter." — Presbyterian.. . . "A very candid and in-
genious essay." — Christian Union [Henry Ward Beecher's Paper.]. "Cannot

fail to interest even those who do not fully accept the theory it advocates." — Bos~
ton Journal of Chemistry .

Lightning-Rods, and How to Construct 'Them.

By JOHN PHIN, C. E. Second edition, enlarged and fully illustrated. 12mo.

Cloth 50 cents.

A work written, not in the interest of any patent or special manufacture, but
solely for the purpose of teaching any ordinarily skillful mechanic how to put up
a rod that will really insure safety, a^d of enabling every householder to decide
whether or not his home is perfectly protected.

Practical Hints on the Selection and Use of the Microscope.

By JOHN PHIN, Editor of " THE TECHNOLOGIST." 12mo. Cloth. 75 cents.

This book is intended for beginners, and explains in the simplest language the
best methods of using the microscope, at.d of preparing, examining, preserving
and mounting objects. At the same time it contains a great deal of original and
valuable information that is not to be found in other books. ^

THE ABOVE BOOKS MAY BE OBTAINED THROUGH JJff
BOOKSELLER OR NJEWS

THE AMERICAN
JOURNAL OF MICROSCOPY,

POPULAR SCIENCE.

PROSPECTUS.

The object of the JOURNAL OF MICROSCOPY is to diffuse a knowledge of the
best methods of using the Microscope; of all valuable improvements in the in-
strument and its accessories; of all new methods of microscopical investigation ,
and of the most recent results of microscopical research, 'ihe JOURNAL does
not addiess itself to those who have long pursued certain special liues of re-
search, and whose wants can be supplied only by elaborate papers, which, from
their thoroughness, are entitled to be called monographs rather than ruere arti-
cles. It is intended rather to meet the wants of those who use the microscope
for purposes of general instruction, ai d even amusement, ard who desire, in
addition to the infoimation afforded by text-books, such a knowledge of what
others are doing as can be derived only from a periodical. "With this object in
view, therefore, the publishers propose to make the JOURNAL so simple, practical
and trustworthy, that it will prove to the advantage of every one owning even a
pocket magnifier to take it.

ILLUSTRATIONS.— The JOURNAL will be freely illustrated by engravings
representiig either objects of natural history or apparatus connected with the
microscope.

EXCHANGES. — An important feature of the JOURNAL is our exchange column,
by means of which workers in different parts of the country are enabled, with-
out expense, except for postage, to exchange slides and materials with each other.

TZEIFLJVEJS.

In order to bring the AMERICAN JOURNAL OP MICROSCOPY within the reach of
every one, the subscription rate has been made very low —

FIFTY CENTS PER YEAR.

To those who send three subscribers we will Bend an extra copy free. Those who
desire to obtain the JOURNAL WITHOUT COST, can do BO by sending us the regu-
lar subscription price for any journal or magazine, in which case we will send the
periodical ordered and the JOURNAL OF MICROSCOPY too. There may be a few
cases in which we cannot do this, but they are very rare. The postage on copies
sent to Europe, however, is such that there are many cases in which we cannot
extend this privilege to our foreign subscribers.

All subscriptions are payable in advance, the rate being too low to warrant us in
opening book accounts.

SUBSCRIBERS IN GREAT BRITAIN.— The JOURNAL will be sent, postage paid,
for 2s. 6d- per year. English postage stamps, American currency or American
postage stamps taken in payment. In return for a postal order or draft for £1 Is.,
eleven copies of the JOURNAL will be furnished and mailed to one address. Make
all drafts and postal orders payable to John Phin.

VOLUME I.— BACK NUMBERS.— We have on hand a few copies of Tol. I., bound
in hai dsome cloth cases, which we offer for $1.25. We cannot eupp'y complete
sets in sheets, but can furnish any numbers, except NOP. 1 and 4, for four cents
each. For clean copies of Nos. 1 and 4 we are willing to pay 8 cents each.

The JOURNAL OF MICROSCOPY, from its very nature, is a visitor to the very
best families, and its value as an advertising medium has therefore proved to be
much above that of average periodicals. A few select advertisements will be
inserted at the rate of 30 cents per line, nonpariel measure, of which twelve
lines make an inch. Address

AMERICAN JOURNAL OF MICROSCOPY,

P. O. Box 4875, New York.

A Jtfew and Live Book on the Gun.

Just Published. Price 75 cents, in cloth.

PLAIN DIRECTIONS

FOB ACQUIKING THE ART OF

SHOOTING ON THE WING.

With Useful Hints concerning all that relates to Guns and Shoot-
ing, and particularly in regard to the art of Loading so as
to Kill. To which has been added several Valuable
and hitherto Secret Recipes, of Great Practical
Importance to the Sportsman.

BY AN OLD GAMEKEEPER.

Sent free by mail on receipt of price. Address

THE INDUSTRIAL PUBLICATION COMPANY,
P. 0. Box 4,875. 176 Broadway, New York.

Opinion g of the frett.

The directions are so plain that they cannot well be mistaken,
and they are expressed in the fewest possible words.— Turf, Field
and Farm.

Facing the title-page is one of the handsomest, best-executed
woodcuts, we have ever seen. It is entitled "The Wounded
Snipe," and almost equals a steel engraving.— Baptist Union.

From its pages we should think even the most experienced
sportsman might derive some new ideas, while the beginner will
find it an invaluable assistant. — Country Gentleman.

For concise instructions as to how to shoot, to select, load,
carry, and keep a gun in order, etc., it cannot be surpassed.—
Western Rural

A pleasantly written, and, it seems, to us, correct and practical
treatise on the sportsman's art; a modest little book, but one from
the reading of which a good deal of the right kind of knowledge
is to be gained. — Applelon's Journal

A practical and well-written handbook, especially adapted for
the use of young sportsmen, as it gives sensible advice on the
manipulation of firearms, and the rules and etiquette of the field.
Scientific American.

THE PISTOL

WEAPON OP DEFENCE,

In the House and on the Road.
12mo. Cloth. 50 cents.

This work aims to instruct the peaceable and law-abiding citizens in the
best means of protecting themselves from the attacks of the brutal and
the lawless. Its contents are as follows: The Pistol as a Weapon of De-
fence—The Carrying of Fire-Arms— Different kinds of Pistols in Market;
how to Choose a Pistol— Ammunition, different kinds; Powder, Caps,
Bullets, Copper Cartridges, etc.— Best form of Bullet— How to Load-
Best charge for Pistols— How to regulate the Charge— Care of the Pistol;
how to clean it — How to handle and carry the Pistol — How to Learn to
Shoot— Practical use of the Pistol; how to Protect yourself and how to
Disable your antagonist.

" No man is fit to keep house who is not fit to defend it."— Henry Ward
Beecher.

" So long as rogues cannot be prevented from carrying weapons, honest
men do not consult their own safety and the public good by totally dis-
carding them."— Recorder ffackett.

" Such I hold to be the genuine use of gunpowder; that it makes all men
alike tall (or strong.)"— Carlyle,.

For Sale by all Newsdealers, or Sent postpaid by Mail on receipt of price.
THE INDUSTKIAL PUBLICATION COMPANY,

176 Broadway, New York.

JUST PUBLISHED, inlvol, I2mo., Handsomely Bcnmd

'a. Cloth, Gilt Title. Price 50 cents.

PLAIN DIRECTIONS

FOR

fhe Construction and Erection

OF

LIGHTNING RODS.

By JOHN PHIN, C. E.,

Editor of THE TECHNOLOGIST. Author of THE CHEMICAL
HISTORY OF THE Six DAYS OF THE CREATION, etc.

SECOXD EDITION, ENLARGED V\D FILLY ILLUSTRATED.

This is a simple and practical little work, intended to
convey just such information as will enable every prop-
erty owner to decide whether or not his buildings are
thoroughly protected. It is not written in the interest
of any patent or particular article of manufacture, and
by following its directions, any ordinarily skilful me-
chanic can put up a rod that will afford perfect protec-
tion, and that will not infringe any patent. Every
owner of a house or barn ought to procure a copy.

May toe Ordered through any Bookseller,

or will be sent free by mail on receipt of price, by
The Industrial Publication Co.,

Post-Office Box 4,875. 176 Broadway, New York.

S3" AMY NEWS GOWPAKY WILL SUPPLY ITVS*

JUST PUBLISHED, in 1 vol., 12mov Handsomely Bound in Cloth,
Gilt Title. Price 75 cents.

A BOOK FOR JEVJEHYBODY.

"What to Do

to Do It

I

ACCIDENT.

IN CASE OF

This is one of the most useful books ever published. It tells
exactly WHAT TO Do IN CASE OF ACCIDENTS, such as Severe Cuts,
Sprains, Dislocations, Broken Bones, Burns with Fire, Scalds,
Burns with Corrosive Chemicals, Sunstroke, Suffocation by Foul
Air, Hanging, Drowning, Frost-Bite, Fainting, Stings, Bites,
Starvation, Lightning, Poisons, Accidents from Machinery, and
from the Falling of Scaffolding, Gunshot Wounds, etc., etc.

It ought to be in every house, for young and old are liable to
accident, and the directions given in this little book might be the
means of saving many a valuable life.

May foe Ordered through any Bookseller,
or will be sent free by mail on receipt of price, by

THE INDUSTRIAL PUBLICATION COMPANY,

P. 0. Box 4,875. 176 Broadway, \< \» York.

J9ST ANY NEWS COMPANY WILL SUPPLY IT. -©a

i]sr

The following Books are in active Preparation, and

as soon as ready will be sent by mail to any

address on receipt of price.

THE MICROSCOPIST'S ANNUAL

for 1878.

Containing Useful Tables, Kules, Formulae and Memoranda. List of
Microscopical Societies, with Officers, etc. Directory of the Prominent
Microscope Makers, Dealers and Importers in America and Europe. List
of New Books and Current Periodicals devoted to Microscopy. The whole
forming an indispensable handbook for all who have occasion to use the
Microscope.

Price — In United States, 25 cents. In Europe, 14 pence sterling. U. S.
or British Postage Slxjnps taken at futt value.

THE MICEOSCOPE.

By ANDREW BOSS. Fully Illustrated. With notes— chiefly Bibliographic.

This is the celebrated article contributed by Andrew Boss to the
" Penny Cyclopedia," and quoted so frequently by writers on the Micro-
scope. Carpenter, in the last edition, of his work on the Microscope, and
Brooke in his treatise on Natural Philosophy, both refer to this article as
the best source for full and clear information in regard to the principles
upon which the modern achromatic Microscope is constructed. It should
be in the library of every person to whom the Microscope is more than a
toy. It is written in simple language, free from abstruse technicalities.

1 Vol., I2mo, Cloth. Price 75 cents.

Directions for Collecting, Preserving, Transporting, Preparing and
Mounting Specimens of the Diatomaceae. By Prof. A. MEAD EDWAKDS,
M. D.; Prof. CHBISTOPHEB JOHNSTON, M. D.; Prof. HAMILTON L. SMITH,
LL. D.; and F. KITTON, Esq.

1 Vol., 12mo, Cloth. Price 75 cents.

THE INDUSTRIAL PUBLICATION COMPANY,

September 1, 1877. 176 Broadway, New York,

BOOKS ON THE MICROSCOPE.

The following books will be sent, post paid, to any address on re-
ceipt of price.

INDUSTRIAL PUBLICATION COMPANY,

176 Broadway, New York.

Airy? A. Geometrical Optics. 16mo. $1.25.

Be-ill, Lionel. M. D., F. E. S. How to Work with the Micro-
scope, Eighth thousand. With 70 plates, 4 colored. (Out of
print. Second-hand copies bring from $7.50 to $10).

-- The Use of the Microscope in Practical Medicine. For
Students and Practitioners, with full directions for examining
the various secretions, etc., in the Microscope. 4th edition.
500 illustrations. 8vo. $7.

- Kidney Diseases and Urinary Deposits. $7.50.

Ricliard. A Treatise on the Construction, Proper Use,

and Capabilities of Smith, Beck & Beck's Achromatic Micro-
scopes. London, 1865. 1 vol., royal 8vo. $8.75.

This is a practical and really valuable work, and not a mere advertise-

ment of Beck's microscopes. It contains a large number of original and

very fine plates.

Brewster. Sir David. A Treatise on Optics. New edition.
London, 1853. 520 pages, fully illustrated.

Carpenter^ TT. B. The Microscope and its Revelations. Fifth
edition. 25 plates and 449 wood engravings. 848 closely
printed pages. $5.50.

Cot>1>old9 T. Spencer. Entozoa: An Introduction to the
Study of Helminthology, with reference more particularly to the
Parasites of Man. With Supplement. 1 vol., royal 8vo.
Plates. $6.

Cooke, M. C. One Thousand Objects for the Microscope. 12

plates and over 500 figures. 1 vol., 12mo. 60 cents.

Tells where they may be found, and in many cases how they may be

prepared.

- A Manual of Structural Botany. 200 woodcuts. 12mo,
boards. 50 cents.

-- Manual of Botanic Terms. With over 300 illustrations. $1.25.

Davies, Tliomas. The Preparation and Mounting of Micro-
scopic Objects. Contents: Apparatus to Prepare and Mount
Objects Dry; Mounting in Canada Balsam; Preservative Liquids,
etc., particularly where Cells are used; Sections and how to cut
them, with some remarks on Dissection; Injection; Miscellane-
ous. New edition. 1 vol., 12mo. $1.25.

Duval et Leretooullet. Manuel du Microscope dans ses
Applications au Diagnostic et a, la Clinique. Second edition.
110 illustrations. Paris, 1877. 1 vol., 12mo. $2.25.

One of the very best hand-books on this department of microscopy.

Heather, J. F. Optical Instruments, including more especially
Telescopes, Microscopes and Apparatus for Producing Copies of
Maps and Plans by Photography. 1 vol., 12mo. London, 1871.

75 cents.

How to Choose a Microscope. By a Demonstrator.
London. 8 vo., paper. 50 cents.

King, John. The Microscopist's Companion: A Popular Manual
ot Practical Microscopy. Designed for those engaged in micro-
scopic investigation, schools, seminaries, colleges, etc., and com-
prising selections from the best writers on the microscope rela-
tive to its use, mode of management, preservation of objects,
etc. To which is added a Glossary of the principal terms used
in microscopic science. With 114 illustrations. 1 vol., 8vo. $2.

Martin, John H. Manual of Microscopic Mounting, with
Notes on the Collection and Examination of Objects. 1 vol.,
8vo. London, 1872. $3.

McDonald. J. D. A Guide to the Microscopical Examination
of Drinking Water. With 24 plates. 1 vol., 8vo. $3.

Micro graphic Dictionary. Third edition. Greatly en-
larged. 1 thick vol. Cloth. " $21.

Nageli und Schwendener. Das Mikroskop. Theorie
und Anwendung Desselhen. Leipzig, 1877. $5.50.

Nave, Johann. Handy Book to the Collection and Prepar-
ation of Fresh-Water and Marine Algae, Diatoms, Desmids,
Fungi, Lichens, Mosses, and other of the Lower Cryptogamia.
Translated by Kev. W. W. Spicer, A. M. London, 1869. 1 vol.,
16mo. $1.25.

Parkinson, S. A Treatise on Optics. 1 vol., 12mo. $3.50.
This is the standard English text-book on Optics.

Pelletan, Dr. J. Le Microscope. Son Emploi et ses Applica-
tions. Four plates and 278 wood engravings. Paris, 1876. 1
vol., 8vo. Paper. $5.50.

Rutherford, William. Outlines of Practical Histology. 1
vol., 12mo. $2.

Sachs, Julius. Text-Book of Botany, Morphological and
Physiological. Translated by Bennet & Dyer. One large vol-
ume, royal 8vo. London, 1875. $12.50.

Schafer, Edward Albert. A Course of Practical Histology.
Being an Introduction to the Use of the Microscope. Phila-
delphia, 1877. $2.

Spottiswoode, William. The Polarization of Light. Lon-
don, 1874. 1 vol., 12mo. $1.50.

Wythe, J. H. The Microscopist: A Manual of Microscopy and
Compendium of the Microscopic Sciences, Micro-Mineralogy.
Micro-Chemistry, Biology, Histology and Pathological Histologv,
Third edition. Rewritten and Greatly Enlarged. 205 illustra-
tions. Philadelphia, 1877- $4.50.

Deschanel, A. Privat. Elementary Treatise on OpticR5
(forming one of the volumes of liis Treatise 011 Natural Phil-
osopy.) 8vo., limp cloth. $1.75.

The best popular treatise on the general principles of optics.

Frey, IT. The Microscope and Microscopic Technology: A Text-
Book for Physicians and Students. Translated and Edited by
G. R. Cutter, M. D. One handsome octavo volume of about 760
pages, with several hundred fine wood engravings. $6

Indispensable to physicians and students of physiology.

Goadfty, Henry. Text-Book of Vegetable and Animal Phy-
siology. Embellished with upward of 450 illustrations (many
colored.) 8vo. $3.

GOSSC, Philip Henry. Evenings at the Microscope; or Re-
searches among the Minuter Organs and Forms of Animal
Life. Thick 12mo. Illustrated. $1.50.

, Jafoez. The Microscope: Its History, Construction, and
Application. Being a familiar Introduction to the Use of the
Instrument and the Study of Microscopical Science, with Direc-
tions for Collecting, Preserving, and Mounting Objects. Illus-
trated with upwards of 500 engravings and colored illustrations.
750 pages. Seventh edition. London, 1869. $3.

An excellent work for non-professionals. The best in market for the

price.

JLa ilk ester, Edwin, M. D. Half-Hours with the Microscope;
being a popular Guide to its use. New edition. Profusely illus-
trated. $1.
A very excellent little work, devoted chiefly to natural history.

Loo ill is, Alfred !>., M. D. Treatise on Physical Diagnosis as
illuhliated by the Microscope. $8.

Pereira, Jonathan, M. D. Lectures on Polarized Light;
together with a lecture on the Microscope. Second edition,
enlarged. Edited by Rev. Baden Powell, M. A. 12rno. Lon-
don, 1854. $1.25.

Richardson, J. G., M. D. Hand-Book of Medical Micro-
scopy. 40 illustrations. $2.25.

Suffolk, W. T., F.R.M.S. On Microscopical Manipulation.
Being the subject matter of a Course of Lectures delivered
before the Queckett Microscopical Club. With 49 Engravings
and 7 lithographs. $2.

Wood, J. G., M.A., F.L.S. Common Objects of the Micro-
scope, with upward of 400 • illustrations by the celebrated en-
graver, Tuffen Wei-t. 50 cents.
The best hand-book for beginners. Describes a large number of ob-
jects of popular interest that are easily procured.

Wormley, Theodore G., M.D. Micro-Chemistry of Poi-
sons, including their Physiological, Pathological, and Legal
Relations: adapted to the use of the Medical Jurist, Physician,
and General Chemist. With seventy-eight illustrations on
steel. $10.

A remarkably able and useful work. The illustrations are unequalled.

*•