i§&

%

rt^wSaSffiElBltrf ig'Mi
«>5A •KS:A}<i w-.-ife 1 if;

L?**t>cffi V * <ry>4

^.ojS S .k» HrO > Wrvi}t

Library
of the

University of Toronto

I

Digitized by the Internet Archive
in 2018 with funding from
University of Toronto

https://archive.org/details/popularhandbookt00wrig_0

Mungo Ponton, del.

Fossil Radiolabia (Polycystina) x 100

A POPULAR HANDBOOK

TO

THE MICROSCOPE

BY

LEWIS WRIGHT

AUTHOR OF ‘LIGHT: A COURSE OF EXPERIMENTAL OPTICS’
* OPTICAL PROJECTION ’ ETC.

LONDON

THE RELIGIOUS TRACT SOCIETY

56 Paternoster Row, and 65 St Paul’s Churchyard

1895

‘ The wisdom of God receives small honour from those that
stare about and with gross rusticity admire His works : those
highly magnify Him, whose judicious inquiry into His acts, and
deliberate research into His creations, return the duty of a
devout and learned [i.e. intelligent] admiration.’

Religio Medici, a.d. 1G8G.

INTRODUCTION

No excuse is needed for any attempt to promote the better
study of Nature; and if it be thought that the Society for
which this book is written requires such, it may be found in
the passage I have chosen to preface it.

But the Microscope has special claims upon such a
Society. Chalmers long ago pointed out 1 that whereas the
stupendous revelations of the Telescope had been used to
support a class of doubts and negations, which we have seen
repeated only yesterday in sneers at * the anthropocentric
theory ’ of the universe, the Microscope had even then
furnished reply to such arguments, by enabling us to ‘ see a
world in every atom.’ So also in our own day, when
Haeckel and Spencer had taught that Life began in chemical
combination into more and more ‘ complex molecules,’ whose
mere complexity 2 added Life to previous properties of
Matter, in either ‘ primeval slime ’ 3 or lowly living beings
‘ without organisation or structure,’ it was the Microscope

1 Astronomical Discourses, No. III.

2 See it all summarised in The Story of Creation, by Mr. Edward Clodd.
I have discussed this ignorant dogmatism at length in Contemporary Beview,
August, 1888.

3 Huxley’s Bathybius. This was soon exploded : a simple chemical
analysis of the stuff did that. But it was distinctly the biassed (and therefore
unscientific) predisposition to this view of Life which led so able a man to
believe in it without the slightest experimental investigation, and leaving to
others such simple analysis as finally disposed of it.

6

THE MICROSCOPE

that proved such teaching due to sheer ignorance and not
to superior knowledge, and, if it could not reveal the Divine
mystery of living existence, at least manifested it to us as a
greater Mystery than ever.

The Microscope, then, has deserved well of the Christian
believer ; and it is to be hoped that this may not be the last
work the Religious Tract Society may see fit to publish con¬
cerning the marvels it unfolds to us, upon which part of the
subject very limited space has prevented any enlargement in
this little volume. For space has made my task a difficult
one, every page requiring anxious consideration as to how
best to use it.

In those devoted to apparatus, it seemed most useful to
follow the example of my friend the late Dr. Carpenter, and
show each reader straightforwardly, what he could get for a
given expenditure. In presenting such types of instrument
as alone fall within the plan of a book only elementary and
introductory, there has been absolute impartiality. There is
indeed hardly a manufacturing firm of standing, from which
I have not received some personal kindness, and there are
several to whose workshops I have been frankly admitted,
long before this book was proposed to me. As I knew enough
of tools and workmanship in my youth to have made a fair
microscope myself had I cared, such means of forming a
practical judgment have been valuable ; and since every
stand mentioned has been carefully examined, confidence
may be placed in the opinions expressed, even where (as in
some cases) they differ from others as to the merits of certain
details. Of objectives also I have had unusual opportunities
for judging, a very large number having passed through my
hands, including I believe nearly all of any repute, and in
many cases the very first specimens of lenses now well known

INTRODUCTION

7

and valued. Every lens named is familiar to me, and the list
(again necessarily almost confined to the more moderate
prices) will be found both catholic and trustworthy.

As to manipulation, endeavour was made chiefly to
recall from my own early experience what the beginner most
needs to know, and to tell him about it so simply that he
may get to know it, for practical purposes. The experienced
worker is not pretended to be instructed in this book ; and
the beginner who uses it will be the best judge how far its
more modest objects have been attained.

The later chapters are necessarily only outlines : opinions
may well differ as to how these have been drawn, though no
part received more anxious consideration. A miscellaneous
illustrated list of * objects ’ did not promise to be of any
particular use : any really systematic treatment was forbidden
by the narrow limits : what was it best to do ? The idea
finally adopted was to endeavour to make students, rather
than to teach them. For this, it seemed best to blend into
such broad and popular descriptions as might perhaps
interest even a general reader, hints and suggestions as to
personal courses of study, or lines of investigation, or even
systematic and intelligent in place of random collection of
objects — to suggest, in fact, definite ivork to do, which might
maintain the Microscope in its place as a valued companion
through life. Some attempt is at the same time made to
convey a sense of that unity of Nature , through all its
diversity, in which the present writer profoundly believes.

My sincere thanks are due to Messrs. Beck, Swift,
Baker, and Watson, for having, besides their own stands
(which makers are naturally willing to advertise) placed at
my disposal numerous illustrations of those subsidiary
appliances common to all. They are more specially due to

8

THE MICROSCOPE

Messrs. Beck for allowing me to photograph the late Mr.
B. Beck’s unrivalled original drawings of Arachnoidiscus
and of the ‘ test ’ Podura scale ; and to Mr. C. L. Curties
for negatives of N. lyra and Heliopclta. To Messrs.
Partridge & Co. and Dr. Dallinger I owe the two fine
drawings by the latter referred to in their place ; and in the
descriptions of Water Fleas and Insects, I have drawn freely
upon a series of excellent microscopical drawings by Mr.
Arthur Hammond, F.L.S. The Frontispiece is a selection
taken entirely from an exquisite set of illustrations 1 drawn
under the microscope with his own hand by the late Mr.
Mungo Ponton, who will be remembered amongst photo¬
graphers for his discovery of the photographic sensitiveness
of potassic bichromate, and from whom I received when a
lad the kindest personal assistance regarding my earlier
experiments in optics and physical science. Some of the
specimens shown are almost unique ; and the beauty of the
drawings will speak for itself.

September , 1895.

1 For The Beginning , its When ancl its How (now out of print).

CONTENTS

- •<>« -

CHAPTER PAGE

I. Rays, Images, and Lenses . 11

II. Practical Optics of the Microscope . . . 19

III. The Simple Microscope and its Use . . . 37

IV. The Compound Microscope and Accessories . . 44

V. Microscope Manipulation . 81

VI. Microscopical Drawing, Measurement, and Pho¬
tography . 109

VII. Manipulation, Preparation, Mounting, and Selec¬
tion of Objects . 117

VIII. Pond and Marine Life . 137

IX. The Insect World . 172

X. The Vegetable World . 200

XI. Vertebrate Physiology . 218

XII. Miscellaneous Microscopic Objects . . . . 233

XIII. Microscopical Exhibitions . 247

Index . . 253

A POPULAR HANDBOOK

TO

THE MICEOSCOPE

- KX -

CHAPTER I

EAYS, IMAGES, AND LENSES

It will help the reader, both in the selection and use of his
microscope and accessories, to comprehend the main prin¬
ciples upon which optical images depend.

1. Images and Magnification. — For anything to be ‘ seen,’
an image, or picture of it has to be thrown upon the sensitive
surface at the back of the eye-ball, known as the retina, as
really as a magic-lantern pic¬
ture upon the white sheet. The
apparent size of a thing seen
depends upon that of this
image ; and to ‘ magnify ’ an
object is to make the picture
of it on the retina larger than
it would be otherwise.

As regards the eye itself,
which in all essentials is like a
camera, with its lens in front and screen at the back, the
apparent size of a thing depends upon the visual angle , or
angle subtended by straight lines from its boundaries a r or
o w (fig. 1), and the pupil of the eye. So the simplest way of
magnifying anything is to bring it nearer the eye : the nearer

Fig. 1.— Visual Angle

12

THE MICROSCOPE

it is, the larger it looks. We are not always fully conscious of
this, because we allow for its distance, and thus think its size
as well as see it ; but direct comparison will show it. Hold
a piece of money at arm’s length, and see how little of the
landscape it covers. Now bring it nearer — we think it looks
the same size ; but, looking beyond, we see how much more
of the view it covers. Bring it close to the eye, and it blots
out almost all we can see of earth and heaven. There are
people who look at money this way every day of their lives.

We can literally ‘ magnify ’ tiny objects in this way.
The distance of sharpest vision for small objects with most
people (till from age they get more long-sighted) is about
10 inches ; and this is taken as the standard for judging all
magnitudes in microscopy. Much closer, very few can see
sharply or ‘ in focus.’ But by looking through a small pin¬
hole in a card we can see a thing almost close. Put a few
cheese-mites within a small ring of card (to keep them from
being crushed) between two bits of glass pinched together
with finger and thumb, and look at them as closely as pos¬
sible through the pin-hole. There will he no mistake about
the magnification so obtained, which is measured by the ratio
of the distance to 10 inches ; at 1 inch distance it is ten
times, which is expressed as ten diameters, or x 10. 1

2. Images formed by Rays. — Let us now think a minute
about the ‘ image ’ on the retina. We say it is formed by
the lens-front of the eye, just as the image in a telescope or
microscope is by its lenses. We must clearly grasp the simple
hut wonderful fact, that only because every ray of light has
the power of forming an image, are lenses able to do so.

It is easy to prove this. Light a candle in a dark room,
and support a sheet of white card or paper 2 feet away. Take
also a large blackened card, and prick a pin-hole in the

1 Foolish statements often made about magnifications of ‘ millions ’ reckon
the surface of the object, or the squares of the diameters. The so-called
‘ million times ’ is really 1,000 diameters, or x 1,000. Microscopists never use
any other measure than the latter.

RAYS, IMAGES , AND LENSES

V

o

Sf-*

Fig. 2. — Inverted Image

middle, and hold this about midway between candle and
white card. We have only just separated off a few rays by
themselves, to see what they do ; but we see unmistakably
an image or real picture of the flame on the white card.

3. Inversion and Size of Image. — This image is upside
down, and the reason is evident from fig. 2. Each point in
the object o sends its rays straight through the pin-hole a
to the image i ; so the ray from the top goes to the bottom,
and vice versa. It
will also be observed
that the size of the
image must be in
proportion to its dis¬
tance from the pin¬
hole compared with
that of the object.

Move the white card
more away, or the
black one nearer the candle, and the image is larger, and of
course more faint also.

This simple fact about rays is true about all images, in
the most complicated apparatus. No matter for the moment
what ‘rays’ are; just now we need only remember that
(1) they travel in straight lines so long as they traverse un¬
disturbed the same medium (as air or glass) ; and (2) that
objects are visible by rays from each point, which diverge or
proceed from that point in all directions (where not stopped
by something, or by the object itself) into any transparent
medium.

4. Refraction of Rays. — The rays can, however, lie readily
reflected into some other direction, as by a mirror ( image
reflection), or by other objects (which we then see by this
‘ scattered reflection,’ when not self-luminous). And what is
more important to us, they can be bent aside, or refracted ,
by the surface of any fresh transparent medium of different

14

THE MICROSCOPE

density, through which they then travel in some other straight
line. In such cases we must remember that we appear to
‘ see ’ the object in the final direction from which the rays
reach the eye. Thus wTe ‘ see ’ anything in the looking-glass,
and not in its real direction ; and thus a stick half in the
water appears bent, and the bottom much higher up than it
really is.

5. The Law of Refraction. — The law of this refraction is
very important. Let us take water and air, and let w Wj
represent the surface of the water, and draw a perpendicular,

p p, through some point, c,
where every ray is to strike
the water, and also describe
a circle round c. Any ray,
p c, striking the surface per¬
pendicularly, goes straight
on to p. Take some other
ray, a c, striking the water
at an angle acp with the
perpendicular.1 Part of it
will be reflected at an equal
angle, as if the surface
were a mirror ; but the part

P

O p

Fig. 3. — Law of refraction

that enters the water is refracted at c to a , down towards
the perpendicular, because the water is denser than the air.2
That is the first law. In the denser medium the rays are
refracted towards the perpendicular. Conversely, on enter¬
ing a rarer medium, they are refracted from the perpendicular,
so that a ray from a to c in the water would be refracted to a.
This involves another important fact, that if we trace a ray
the other way, its path is exactly the same, or it is reversible.

1 Optical angles are measured from the perpendicular, called the 1 normal,’
and not with the surface.

2 The optical density is meant. As a rule heavier fluids or glasses refract
more, but not always. Thus oil of turpentine floats on water, but has more
optical refractive power.

i5

RAYS, IMAGES, AND LENSES

But the measure of the refraction is most important to
us. Draw the line a s from where the ray cuts the circle,
at right angles to the perpendicular p c ; then a s is the
geometrical sine of the angle acp. Divide c w and c wt
each into six parts numbered, and let the ray be such that a
perpendicular from a meets c wt at the fourth division : this
gives the measure of a s on c w, as on a scale. Treat the
sine a s of the refracted ray c a in the same way ; we shall
find a s (in water) measures three divisions, a s and a s are
in the proportion of 4:3. Now take some other inclined
ray b c — for convenience we take it here so that the sine b s
of its angle is exactly half a s, or two divisions. Then the
refracted ray c h will be such that its sine h s will also
measure half of a s. And so with any other rays ; the sines
of the incident and of the refracted rays are always in the
same proportion of 4 : 3.

It is the same with all transparent media, including glass.
The proportion differs in each, according to its density, so
that in crown glass the ratio is about 3 : 2. But the ratio
is the same for all rays , and that enables us to put the ratio
into a simple number compared with air as 1, and call that
number the ‘ refractive index ’ of the fluid or glass. Thus
the index of ordinary optical crown glass is 1*516. And
when we know the index, it is easy to calculate the direction
in which a ray entering at any angle will go.

6. Total Reflection : Critical Angle. — The diagram
shows how fast refraction increases with the angle, and
this fact brings us to a curious limit. Let the air-ray just
graze the surface Wj c, then this surface itself is the sine, or
6 divisions. The refracted ray (in water) must be c r, sine
4| divisions. Conversely, the ray r c will just escape from
the water in the path c w^ If, then, we take some ray in
the water of greater angle than r c p, what is to happen to
that, since it cannot get out of the water ? It does not do
so, but is totally reflected back at the same angle, with a

i6

THE MICROSCOPE

brilliance no mirror can equal ; as can be seen by holding
high a tumbler of water, and observing the bright reflection
from under the surface. This reflection at the surface of a
rarer medium is the only case of total reflection of rays in
nature ; but the very important point to us is that rays
cannot get out of a denser medium, beyond what is called
the ‘ critical angle,’ ecr. This angle is plainly less, the
denser the medium. In water the critical angle is 974°,
in crown glass 82J°.

7. Action of a Lens. — We see things undisplaced through
a window, because the rays which are bent towards the
perpendicular on entering the pane of glass, are bent just as
much back again on coming out, and so resume their

original direction. But if
the glass has inclined sur¬
faces (called a prism), the
rays take quite another
R direction on emerging, and
are permanently refracted.
Fig. 4 is the section of a
prism. The ray ei is refracted towards the normal in
the glass to i e, and on emergence it is refracted again
away from the normal in the line e r, and an object
at r would be ‘ seen ’ in the direction r o. The ray is
refracted towards the thick side of the wedge or prism. A
three-cornered glass ‘ lustre ’ will show this excellently ;
and, if a candle flame be the object, will also show vividly
that the refracted beam of light is widened out into a band
of colours called the spectrum. This however we leave to
the next chapter.

Now a lens with its curved surfaces is practically a
circular prism, whose faces become more and more inclined
towards the edges. There are six possible forms of a single
lens, shown in fig. 5, and it will be observed that the first
three are thickest in the middle, which are called generally

RAYS, /MAGES, AND LENSES

1 7

while the last three are thinnest in the

lenses. Suppose

or

negative

B

Fig. 5. — Forms of Lenses

‘ convex ’ lenses .
middle, and are ‘ concave
we take a double convex
lens — the usual form of a
common magnifying or
reading glass — mid hold it
in the parallel rays of the
sun as in fig. 6. The
centre ray strikes perpen¬
dicularly and is not re¬
fracted. Rays a little way from the centre strike the surface
at an angle, and therefore are refracted towards the thicker
part or centre, as in
fig. 6. Rays farther out
strike at a greater angle
and are more refracted,
and so on ; and the result
is that all meet pretty
nearly in the same point f, which is the ‘ focus for parallel
rays ’ (known also as the focus), and where the collected
sun-rays will burn a hole in a black coat.

The rays being reversible (§ 5) if the rays proceeded from
a very bright point (such as an arc lamp) at f, they would
emerge on the other side of the lens as a beam of parallel
rays like the sunbeam. If the arc lamp were placed nearer

V///A —

Fig. 6

the lens than f, the rays would still be refracted inwards,
but evidently not enough so to be parallel : they would still
be divergent, but less so. On the other hand, let the arc

B

i8

THE MICROSCOPE

lamp be drawn back farther than f, to some other point /
(fig. 7). Again, they will not issue parallel, but this time
must meet again in some other point f2, which must have
some fixed distance relative to /, according to the curves
and density of the lens. So f2 is called the conjugate focus
of /, because such foci always are in pairs ; and according
to the law of reversibility, if the arc was at f2 the other focus
must be at /.

Of course concave lenses, being thickest at the edges,
cause rays to diverge, and therefore have no actual focus.

y What is called their
‘ negative focus ’ is,
however, measured
from parallel rays
in the same way.
If the parallel rays
falling on the lens
are refracted as at
a b, these rays are
produced back-
^ wards on the other
side of the lens to

F

Fig. 8

f, and the distance from f to the lens is called its negative
focus (fig. 8).

8. Lenses and Images. — If a reading glass of about 3
inches diameter be at hand, a simple experiment will now
show us exactly what a lens really has to do with images.
Light the candle again in the dark room, and set up the
white card picture screen, but this time prick four more pin¬
holes in the black card round the first one, an inch or less
from it.1 If the reading glass be about the usual 10 inches
focus, put the black card about 8 inches from the candle, with
the centre pin-hole about level with the flame, and the white
card about 3 feet beyond. There will now be seen five

1 The lens must considerably more than cover over all the holes made

RAYS , IMAGES, AND LENSES

19

inverted images of the flame on the white card, showing that
each ray (or practically each tiny pencil of rays) forms its
own separate image. First, hold the reading glass close to
the holes, and it will be seen how the refraction of the lens
bends in the outer four images, very much nearer the middle
one. Gradually move the lens towards the white card, and
they approach more and more ; till a point is found where all
the five images unite in one , and this is five times as bright
as each was by itself. The lens is now ‘ focussed ’ on the
white card ; and we may now prick more holes, and finally
remove the black card, and the one image is still there, but
much brighter still, because the lens converges every ray
that falls on its surface to the same spot on the white card.
So far as the lens gathers them up, all the ray images
combine into one.

This is the true action of a convex or image-forming lens.
It determines a place where a cone of rays diverging from
any point in an object, shall recombine at another point, in
one image ; and as its position marks the place where the rays
are refracted so as to do this, it marks also the place where
the rays from opposite points of an object cross and invert
the image. Thus the relative sizes of image and object
measure from the optical centre of the lens, according to
the ratio of the two conjugate foci. But the lens does no
more than thus converge and combine into one, veritable
images potentially formed by the mere bare rays.

CHAPTER II

PRACTICAL OPTICS OF THE MICROSCOPE

We come now to certain optical principles belonging to the
microscope specially, which should be understood if the best
choice is to be made amongst the instruments and accessories
offered to our notice.

20

THE MICROSCOPE

9. The Simple Microscope. — The action of this instru¬
ment, or a simple ‘ magnifying glass ’ in any form, will be
easily understood after the last chapter. Let the arrow a cl
represent an object o, and l a lens held in front of the eye e
(fig. 9). From one point a in the object, diverging rays
a b and a c proceed, and are so coyiverged by the lens as to
enter the pupil of the eye almost parallel, which is required
for distinct vision. The rays thus parallelised are brought
to a focus at z on the retina r, forming there the visual

A

D

Fig. 9. — Action of Magnifying Glass

image of a. The image of cl is imaged at x in the same way.
But the object being ‘ seen ’ in the direction of the rays which
last enter the eye (§ 4) if we produce these till they meet,
they appear to proceed from the points ad, which is the
apparent image i of the object ad, as apparently seen at the
distance of 10 inches. It is never to be forgotten, that the
last stage in every optical combination is the formation of
this image on the retina at b, and that the final rays which
must enter the pupil in order to this, have to be rendered so
slightly divergent as to be practically parallel.

PRACTICAL OPTICS OF THE MICROSCOPE 21

10. The Compound Microscope. — In this instrument, known
in general as the microscope, and the general plan of which
is shown in fig. 10, a real enlarged image of the object 0 is
formed by one lens (called the object glass or objective) in the
air. This enlarged image is looked at
through another lens called the ocular or
eye-piece, and by it further enlarged in
the manner of the simple microscope.

For reasons to be seen immediately, each a
of these lenses is always compound, and

may be of various forms ; but it is well to

" « -

notice at once the form of eye-piece shown |
in the diagram, and called the Huygenian
or ‘ negative ’ eye-piece. It is called nega¬
tive because the image looked at is formed
within it, or between its two lenses ; and
some form of it is more used than any
other.

It consists of a larger plano-convex
lens //called the field-lens, a smaller one
e e called the eye-lens, and a diaphragm
or stop at b b, in the focus of the eye-
lens. Three pencils of rays are drawn
from the centre and each end of the
object 0, which if let alone would form an
inverted image at a a. This aerial image
‘ in space ’ behaves in one respect differ¬
ently from a real object, which sends out
wide cones of rays in all directions (§ 3).

The image, on the contrary, is formed
by the narrow convergent pencils oa, which continue on
after meeting or focussing. So if we looked at the image
by the lens e e alone, only rays from the centre of the image
could pass through this lens and reach the eye. But the
field-lens bends in the pencils fa to/ b, also shortening the

li

Fig. 10

22

THE MICROSCOPE

focus a little, forming a rather smaller image at b b, and so
altering matters that the rays fa are now all converged to
pass through the eye-lens e e. Besides this, the ‘ aberrations ’
presently mentioned are nearly corrected by this simple form
of eye-piece. And lastly, as the rays begin to be converged
later by the edges of the lens //than by the centre, the
conjugate focus is rather extended there, and so this eye¬
piece helps to 1 flatten the field’ (§ 36).

It will be obvious that the field-lens //belongs in principle
to the objective, taking a share in the focal convergence of
the image-forming rays. But it is always reckoned as part
of the eye-piece.

11. Power of a Microscope. — This is calculated by multi¬
plying the original magnification of the image a a, by the
further magnifying power of the eye-piece. If an objective
of \ -inch focus forms its image at 10 inches, this will be
forty times as large as the object o. Then if the eye-piece
magnifies ten times, the power (with that objective and eye¬
piece) will be 400 diameters.

12. Spherical Aberration of the Lenses. — If single lenses
did perfectly the refracting and image-forming work described
in Chapter I, a microscope would be simple indeed ; unfortu¬
nately they do not, failing in three ways.

Firstly, they are ground to spherical surfaces, and these
refract too much in proportion as we approach the edges; thus
the rays falling on the edge of the lens come to a focus nearer
the lens than the central rays. The image formed by the
large reading glass in § 8, on close examination will be found
rather blurred from this cause, which is called ‘ spherical
aberration.’

Secondly, it will be noticed that print seen through such
a glass appears distorted in the marginal parts. And the
image on a card of a flat surface like a lantern slide is not
itself flat, but curved hollow towards the lens. These are
other effects of spherical aberration.

PRACTICAL OPTICS OF THE MICROSCOPE 23

13. Chromatic Aberration. — Thirdly, in looking through
the glass lustre or prism at the candle-flame (Chapter I, § 7)
the image was not only refracted, but spread into a band
showing all the colours of the rainbow — the coloured band
called the spectrum. That is because different colours are
differently refracted, blue being the most so and red the
least.1 This being so, clearly a simple lens must bring the
blue rays to a focus nearer to it than the red rays ; and this
can also be seen, with care, in any large lens. It is called
chromatic aberration.

These faults do not prevent much rough useful work with
simple lenses, as in a magnifying glass. But they are fatal
to precise work, and most of all to the perfection of images
magnified many hundreds of times.

14. Correction of Aberrations. — Such defects can be
more or less corrected in several ways.

(a) All of them are worst at the edge of a lens. Hence,
by stopping all but a portion in the centre, the image is
greatly improved in sharpness. But it is at the expense of
light and of other functions of large aperture (§§ 17, 22).
Hence this plan can only be used to some extent, and as
the cheapest mode of improvement.

( b ) Both aberrations differ greatly in amount, according
to the inclination of the surfaces to the rays. The spherical
aberration of a plano-convex lens in parallel rays is less
than a third when the flat side is turned towards the focus,
of what it is the other way. And two inclinations may be
so disposed that the aberration of one largely corrects that
of another. Thus, by using two or more lenses, amongst
which the power is divided, instead of one, and proper com¬
bination of curves, much may be done. This method is
largely employed, and is entirely depended upon in the
Huygenian eye-piece.

1 The law of proportional sines holds good ; but each colour has a some¬
what different refractive index (§ 5).

24

THE MICROSCOPE

(c) Chiefly of all, while different sorts of glass differ
largely in both their refractive power (or optical density) and
also in their dispersive power (or degree to which the spec¬
trum is spread out by a prism of given angle), the two
qualities do not preserve the same 'proportion. So it is
possible to make, let us say, a plano-convex lens (fig. 11) of
two glasses, of which the crown-glass lens, c, has
double the curvature of the flint-glass concave
lens, f, and so has a large surplus of refractive
power, while yet the colour-dispersion of f is enough
to counteract all that of c, and so brings all colours
nearly to the same focus. Such a lens is called
achromatic, or ‘ corrected for colour.’ Fortunately
it also happens that the concave curve of the flint
fig. ii lens largely corrects the spherical aberration of c as
well ; and so we get a more perfect lens every way.

15. Microscope Objectives. — But even such a lens is not
generally good enough for the microscope. Though much
of the spherical aberration can be so corrected, all of it can¬
not be, except in the one case where all the rays proceed
from one point in the optic axis, and the conjugate focus is
at another point on the axis ; hence this simple form is
much used for astronomical telescopes. More can be cor¬
rected by a single triple combination — a form often employed
in microscopes for low powers, such as 3-inch, or cheap
powers up to 1-inch. But very much more is possible from
combinations of two achromatic lenses, separated by an
interval, or by additional lenses still. By combining different
glasses, with appropriate curves presenting various angles to
the rays, the modern microscope objective obtains its excel¬
lent results.

It will be pretty evident that very similar results may be
produced by quite different combinations. For a half-inch
lens of the same aperture (§ 17) three opticians may use
different glasses, and different combinations and curves ;

PRACTICAL OPTICS OF THE MICROSCOPE 25

then each man’s particular lens would be known as his
‘ formula,’ and the number of possible constructions is almost
endless — three are shown at d e f in fig. 12, suitable for
gradually increasing or higher powers. It is easy to see
that some are more complicated, and hence more costly,
than others ; but that cost may be well spent. On the other

1 4

Fig. 12. — Forms of objectives

hand it may not. The best constructor is he who corrects to
the utmost, but adds 110 complication unless it distinctly
improves the image.1

16. Effect of Cover-glass. — An objective whose spherical
aberration is so corrected that all the rays are converged to a
sharp and true focus is called aplanatic. If the correction
still leaves the focus of its marginal rays shorter than for
the central rays (§ 12), it is called ‘under-corrected.’ If
(as is quite possible) the correction is so great that the central
rays are actually shorter in focus, it is ‘ over-corrected.’

As soon as the late Mr. Andrew Eoss had constructed
good achromatic lenses, he discovered that if one was made
aplanatic for an uncovered object, the interposition of a
cover-glass upset his corrections. It is easy to see how
Let o, in fig. 13, be an object in balsam b under the cover-
glass c, and o p p and oqq twTo pairs of rays proceeding at
large and small angles from o. At p p and Q Q they are

The last improvements in lenses are described in § 24.

26

THE MICROSCOPE

O

Fig. 13. — Effect of cover-glass

refracted away from the normal, as shown. Producing back
these rays as they enter the lens l by the dotted lines, it is
as if one pair really proceeded from a, and the more mar¬
ginal rays, p p, as if
they proceeded from
b, nearer the lens.
But such foci for
marginal and central
rays represent under -
correction ; and hence
the objective, to
focus truly under
these circumstances, must itself be under-corrected for rays
proceeding from o in air . Also, the thicker the cover and
balsam above the object, the more of this effect is produced.

Mr. Ross knew that in any objective with two or more
parts, properly adjusted, under-correction could be produced
by causing the front and back components to approach each
other, and over-correction by more separating them. Hence
the best high powers, except oil-lenses (see § 20), are furnished
with a screw-collar, called the collar-correction , to effect this
as required. Under-correction for thicker covers can also
be effected by lessening the conjugate focus, or distance
between objective and eye-piece ; and this method is much
used for cheap, and for low or moderate powers. A high-class
lens can only b e perfectly corrected for one thickness of cover,
with one tube-length or focus ; and the more perfect the lens,
the more sensitive it is to these conditions.

17. Angle and Aperture. — It is plain that two objectives
of the same focus may utilise in their images very different
cones of rays (§ 8) from the object. In fig. 15 the lens
takes up a much wider cone of rays bob from the object o
than the similar cone bob in fig. 14. These lenses are single,
but it is just the same with compound lenses.

The most obvious gain from wide aperture is brightness,

PRACTICAL OPTICS OF THE MICROSCOPE 27

which in itself may be needed for very high magnifications.
But as long ago as 1830 Dr. Goring pointed out that there
was more than this ; and that in examining certain insect
scales with minute markings, which he used as tests, wide
aperture was so necessary, that even an unachromatised com¬
bination of wide aperture had often more of this separating
or resolving power,1 than achromatic lenses of far better
definition, but less ‘ angle.’ This led to gradual increase in
the angle of achromatic
lenses; with which, how¬
ever, the aberrations
became more difficult to
correct, and the working
distance also (§ 36)
became less. For the
greatest theoretical angle
of the cone of rays which
can enter the front of ^
an ordinary lens is 180°

(§ 6), and such is only
possible when the object
touches the plane face of the lens, which gives no working
distance at all. Even with less angles than this, the loss of
light by reflection is (near the edges) very great.

18. Immersion Objectives. — About 1840, Amici proposed
to prevent this loss of light by interposing water or oil
between the lens and the cover-glass ; and he used oil in
this way about 1844. Such a medium, however, quite de¬
stroys accurate ‘ correction ’ in a lens calculated for air, and
needs entire re-computation, which Amici never effected.
The idea was, however, taken up again by Hartnack and
Prazmowski of Paris, whose high-power ‘ water immersion ’

1 Then called ‘ penetration,’ but now termed ‘ resolution ’ ; while the term
‘penetration’ is now applied to depth of focus, or the power of imaging
points not in the precise focal plane.

28

THE MICROSCOPE

glasses were found far superior to all others of similar power
at the Paris Exhibition of 1867. These lenses were much
improved by Powell and Lealand, until about 1877 an angle
in water of 114° was grasped by their lenses. In 1870 the
use of oil was again suggested by Mr. Wenham, but only
suggested. In 1873 Mr. Tolies, of Boston, constructed
objectives which used soft balsam as the immersion fluid,
and took up an angular cone of 110° in that medium. This
medium is so very disagreeable and difficult to use, that it
is nearly certain Tolies must have had a clear apprehension
of the essential nature of the connection between greater
aperture and greater resolution, outlined in § 22.

Microscopists owe to Mr. H. Stephenson, however, the
suggestion of the final improvement in immersion lenses :
viz. the adoption of some fluid which should resemble, both
in its refractive index and dispersive power, that of the cover-
glass and the front lens of the objective. Such a system
is termed ‘ homogeneous ’ immersion, and its optical ad¬
vantages were clearly pointed out by Mr. Stephenson. A
suitable fluid was systematically searched for by Prof. Abbe,
of Jena ; and that in universal use now, as the result of such
search, is cedar-oil, carefully reduced to the refractive index
of about P516.

19. Immersion, Angle, and Aperture. — Amici had evi¬
dently no idea of the great gain in angular aperture from the

immersion system ; but this
necessarily follows from
what was said about the

r 70° IN AIR

law of refraction, and total

reflection, in § 5, 6, and fig. 3.
For in fig. 16 let the upper

Fig. 16

figure represent a cone of 170° from an uncovered object on
the face of a thin glass — this is the extreme angle (in practice)
which can enter the face of a ‘ dry ’ lens. But in the lower
figure the object emitting the cone is on the face of a slip

PRACTICAL OPTICS OF THE MICROSCOPE 29

beneath, and covered by balsam and a thin glass ; then, as in
§ 6, the black cone of 82° is the utmost angle that can emerge
from the balsam, and becomes 170° in air. The rays outside
this, as dotted, are totally reflected back into the slide. But
if the space between slide and lens be also filled with balsam,
or cedar-oil, there is no refraction , and a far wider cone of
the rays in the balsam can enter the lens.

All this is clear gain, and an oil angle of even 110° is
greater than an air angle of 180° from an uncovered object.
For it was established by Clausius, in 1864, that all radiant
energy (whether light, heat, or photographic effect), emitted
by any substance, increases as the square of the refractive
index of the medium. So that if we lived in air that had an
index of 1*516 instead of 1*0, we should £ see ’ all things
brighter in the proportion of 9 : 4. So a glass or oil angle of
82^° contains all the brightness of the air angle of 180°, and
all above 82^° is clear gain, even in light ; while all the
‘ resolving ’ power presently mentioned, of greater cones of
rays, being similarly compressed in water or oil, increases in
the same proportion.

20. Homogeneous Immersion. — Figs. 17 and 18 show the
peculiar advantages of homogeneous immersion as pointed out
by Mr. Stephenson. Fig. 17 shows a water-immersion lens
with an angle of 140° in water, or 70° on each side of
the axis. This angle is filled by the balsam semi-angle of
56°, as shown, refracted into the water-angle of 140°, which
at a is again refracted back to the glass semi-angle of 56°.
Thus there is great spherical aberration at this flat surface
a, before the cone can be further refracted at the back spheri¬
cal surface into an angle of 70° to 90°, for entering the back
portions of the objective. But with oil, as in fig. 18, there
is none of this — there is no refraction at all at a ; none of any
kind till the back spherical surface of the lens, which, pre¬
senting a concave curve towards the cone, occasions scarcely
any. Thus the actual high angle from f is reduced to the

THE MICROSCOPE

3°

angle of 70° or 90° behind the lens (as if the cone proceeded
from f') without any appreciable spherical aberration, and the
difficulty of ‘ correction ’ is reduced accordingly.

Fig. 17 “ Fig. 18

It will also be seen that there is no need, with this system,
of any collar-correction for cover-glass.

For very high-power work, the best lenses are now, there¬
fore, always constructed on the ‘ homogeneous ’ system ; such
lenses being also termed ‘ oil ’ lenses. Equal results can be
obtained in no other way.

21. Measure of Aperture: Numerical Aperture. — We

cannot therefore measure the effective aperture of objectives
by angle alone ; we must take account of the medium in
which the angle of rays is reckoned. The measurement
now universally followed is due to Professor Abbe. He
showed that, even in lenses for the same medium (as air),
their comparative aperture, as compared with their focus,
was not correctly measured by the angle of the rays grasped,
but by the actual diameters of the pencils of rays transmitted,
which depended more upon the bach of the lens than its
front. To get a geometric measure for comparison, he took
the radii, or half diameters (whose relative proportions
would be the same), and which, geometrically, are the
sines of the semi-angle of the outermost rays grasped. He
further showed that, if this sine of half the outside angle

PRACTICAL OPTICS OF THE MICROSCOPE 31

were multiplied by the refractive index of the medium used,
we should have a number which would give us the compara¬
tive ‘ aperture ’ of any lens , ivhatever the medium. This
number is called ‘ numerical aperture,’ or n. a. Calling the
semi-angle u, and the refractive index n, it is expressed by
the formula :

Sin u x n — n. a.

The following is a table of some numerical apertures,
showing the respective angular pencils which they express
in air, water, and cedar-oil or glass. Of the higher powers
it is most convenient to take round numbers in n. a., and
their angles can only be stated in fluid media ; of lower
powers it will be most serviceable to give the nearest n. a.
equivalents of the more usual angles used in ‘ dry ’ lenses.

N. A.

Air angle

Water angle

Oil angle

1-40

-

_

134° 10'

1-30

—

155° 38'

117° 35'

1-25

140° 3'

110° 39'

1-20

128° 55'

104° 15'

1- 0

180°

97° 31'

82° 17'

0-95

143° 36'

91° 10'

77° 22'

0-87

120° 55'

81° 42'

69° 49'

0-85

116° 25'

79° 37'

68°

0-82

110°

0-77

100°

0-71

90°

0-64

80°

0-50

60°

0-42

50°

0-35

40°

0-30

35°

0-26

30°

0-22

25°

0-18

20°

22. Aperture and Resolution. — It has already been hinted
v§ 17) that mere poiver, or magnification, however well
corrected the lens and powerful the light, will not separate

32

THE MICROSCOPE

or 1 resolve ’ markings of a certain minuteness, unless there
be a certain aperture as well. A simple experiment will
illustrate this. Get a bit of wire gauze with meshes from
50 to 100 per inch, and in a bit of black card prick a small
pin-hole, and another pretty large one. With the card close
to the eye, look through the small hole at the gauze, and
move this gradually away, against the light of the sky. A
distance will soon be found at which the separate wires can
no longer be seen. Now move the card so as to look through
the larger pin-hole (i.e. more aperture to the pupil of the eye)
and the meshes appear again quite distinctly.

Space forbids explanation of this in detail, but a general
idea of the reason should be grasped. In § 3 we took no
note of what a ray of light really was. In reality it is simply
the propagation of a disturbance in the ether, of the nature
of a wave. These waves have w&ve-lengths, each colour of
light having its own wave-length, varying from about the
?pMth to the of an inch. As it is the convergence of

these lines of wave- disturbance that has got to make our
image, it is easy to see that when we have to do with
Very small details, the absolute size of the loaves limits the
size of the smallest possible image we can get of such a
detail. For instance, the waves in a pond have a certain
length. If we suppose them set up by a small stone like a
marble, much less than one of these wave-lengths in size,
we see at once that however such waves might be converged
on one point, they could never be reconverged into a space
so small as the marble — the waves themselves are too big
for this to occur. Now many microscopic details are far less
in size than half a wave-length of light ; and so this limit
comes in. But it can be shown mathematically, and is
found in practice, that the wider the aperture of a lens, and
consequently the more oblique the outside rays which form
the image, the smaller (short of the final absolute limit) the
image of a ‘ point ’ becomes. This law affects all lenses,

PRACTICAL OPTICS OF THE MICROSCOPE 33

from a 36-inch telescope-lens of 20 feet focus, to a oil-
immersion objective.

Of course when we have two small points close together,
the law determines whether they can be imaged small
enough to be separated. It has been calculated 1 that ‘ two
lines cannot be fairly resolved unless its components subtend
an angle exceeding that subtended by the wave-length of
light at a distance equal to the aperture.’ An -3- -inch dry
objective of n. a. F0 has a diameter or aperture of ^ inch.
If therefore we are using light of a wave-length of ()T of an
inch, this at a distance of \ of an inch (the aperture), will
subtend the same angle as two lines 9-g^ro °f an inch apart
at inch (the focal distance) of the lens — hence this lens
will ‘resolve’ something less than 96,410 lines per inch.2
Here, therefore, we find the great gain in optical power of
homogeneous-immersion lenses.

This discovery of the function of ‘ aperture ’ has revolu¬
tionised microscopy. Formerly objectives were made of ^
of an inch focus. These cannot be made of large apertures,
and will not therefore exhibit such minute detail as can be
easily seen with | andyV oil objectives of large aperture, with
which all modern work is done, and which are far less
expensive. Mere ‘ power ’ is not of the slightest use, beyond
that limit of visual separation which the ‘ aperture ’ permits.

23. Diffraction Spectra. — The above law supposes our
objective to be filled by an angular cone of rays — else its
aperture is not all used. Owing to special difficulties in
many objects, and to imperfections in the best objectives and
condensers (§ 39 ), we can, however, scarcely ever use such a

1 Encyclopcedia Brit., art. ‘ Wave Theory.’

2 In the same article Lord Rayleigh shows that the above measure of
resolution is only possible with a square aperture, or one bounded by straight
lines parallel to the lines resolved. He calculates that with circular apertures
such as those of objectives, the aperture must be increased, or the resolution
will be decreased, by about one-tenth. Actual measurements have verified
this as exactly as possible. (Phil. Mag., Aug. 1880.)

C

34

THE MICROSCOPE

full cone of rays, though advances in this respect are being
made (§ 24 ). Objects displaying a series of equi-distant
minute details, however, when illuminated by a narrow
pencil or cone of rays, are seen to throw out on each side of
that central pencil a series of diffraction- spectra, such as are
risible when looking at a distant candle through a feather,
or piece of fine cambric.1 It is found that, although a much
narrower cone of rays be used than the lens will grasp (§ 17),
if such outside spectra also enter the objective, their rays
also unite in the image, and give additional resolving power
due to their greater angle. Professor Abbe was the first to
study the effect of these spectra, and by using very narrow
pencils many curious results and false images were ob¬
tained ; from which he deduced a theory that interference
(and not union) of the rays in the spectra was the essential
element in the image of minute structure.2 Much has been
very ignorantly written on this ‘ Diffraction Theory.’ There
is no doubt that it has done invaluable work in enforcing the
necessity for aperture in all objectives of high power. But
many practical errors in it were gradually pointed out ; and
there can be no real doubt of the underlying truth in it being
simply and solely, that the spectra are of service in furnish¬
ing rays which utilise the aperture, in cases where the cone

1 These spectra can readily be seen in the microscope by using a narrow
pencil of light to illuminate P. angulatum, with a 5 inch of 100° air angle.
Removing the eye-piece and looking down the tube, the spectra will be seen
surrounding the central pencil, in the back of the objective. It is these
spectra which utilise the marginal zone of the objective when a small cone is
used (see § 60).

2 When a small pencil or illuminating cone is used, marked ‘ interference ’
bands and fringes are produced, which in some cases correspond, in mere
number of details per inch, with the reality. These, however, vary with the
slightest alterations of focus, and produce the most marvellous and baffling
changes in the appearances, like all other interference bands and fringes.
No one of them can be selected as a true dioptric image, and so far from being
of real use in microscopical study, they are a constant source of false inter¬
pretation and error. They are especially liable to double the apparent amount
of detail.

PRACTICAL OPTICS OF THE MICROSCOPE 35

of direct rays which can be employed on an object is
insufficient to do so.1

24. Apochromatic Objectives. — The last great advance in
the optics of the microscope is also due to Professor Abbe.
Achromatism has been briefly explained (§ 14), but previously
to 1881 the best objectives only approximated to colour
correction. The colour produced by the convex crown-glass
lenses was easily corrected by the contrary and stronger
dispersion of the concave flint-glass lenses, so far as regarded
the two ends of the spectrum, or any other two coloured
rays ; but the spectra of the two glasses never corresponded
in all rays, and consequently there was always some residual
colour left, known as the ‘ secondary spectrum.’ The same
discrepancy affected spherical aberration also, so that only
one colour could be truly corrected for that. Professor
Abbe pointed out in 1876 that removal of these residual
errors must depend upon the manufacture of new optical
glasses with certain definite properties of refraction and
dispersion not then obtainable. In 1881 he and Dr. Schott,
aided by a subsidy from the German Government, com¬
menced experiments in conjunction with the optical manu¬
facturing firm of Zeiss, in Jena, and produced various new
glasses. Some of these glasses effected high dispersion with
low refraction, and some possessed low dispersive power
combined with high refractive index ; and pairs were produced
whose dispersion 1 vas in almost similar relative proportion.
With these glasses lenses were calculated which brought to
the same focus three coloured rays, and corrected spherical
aberration for two, leaving scarcely any residual error.
Such objectives are now made by all the best English and

1 I have discussed this matter at length in The English Mechanic and
World of Science , September, October, November, 1894. The conclusions
there arrived at have been accepted in all main points by foremost mathe¬
matical physicists in this (or any other) country ; and substantially by Mr.
E. M. Nelson amongst microscopists (see Qaekett Club Journal, March
1895).

C 2

36

THE MICROSCOPE

Continental opticians, and are termed apochromatic. They
have enabled results to be obtained in biological research
which were previously altogether impossible. Some of the
best, optically, of the new glasses were found to oxidise so
rapidly on the surface that they had to be discarded ; and
one or two of those still employed are not so durable as the
crowns and flints formerly used ; but the optical improve¬
ment has been so great, that the highest class of workers
would rather, at the worst, have to procure fresh lenses now
and then, than go back to the older ones.

Besides new glasses, the best class of apochromatic
objectives contain one or more lenses of fluorspar or fluorite.
This is not necessary to apochromatism ; but its low density
and dispersion (little exceeding that of water) enables flatter
curves to give the same effect, and so facilitates the spherical
correction.1 Unhappily clear fluorite is scarce, and the spar
is rather difficult to work, which makes such lenses neces¬
sarily costly ; but the new glasses are used in nearly all
modern objectives, and effect far better ‘ correction ’ than
formerly, even in those objectives falling short of strict
apochromatism.

25. Compensating Eye-pieces. — The more perfect cor¬
rection of objectives made more evident some aberrations
in those with thick plano-convex fronts (including all high
powers) as regards the margin of the field, which were
almost beyond correction in the objective itself. These also
Professor Abbe got rid of by contrary ‘ over-correction ’ in
the eye-piece ; the plano-convex ocular lens of a Huygenian
eye-piece being replaced by an achromatic with rather too
much flint correction. Such ‘ compensating ’ eye-pieces so
greatly improved all high powers, that corresponding aberra¬
tion was purposely introduced into the lower apochromatic
objectives also, in order that the same eye-pieces might be
used throughout.

1 Mr. Spencer used fluorite in America in 1860.

PRACTICAL OPTICS OF THE MICROSCOPE 37

The result of these modern discoveries and improvements
— chiefly due to Professor Abbe of Jena — has been to abolish
the terribly expensive and very high powers of former days.
It has been shown that all the * resolution ’ possible can be
obtained with one-eighth or one-twelfth inch oil immersions ;
and that such perfect images can be obtained with these, as
will bear an amount of further eye-piece magnification
formerly unknown. In all the older works on microscopy,
great magnifying power is recommended to be got by high-
power objectives and low-power eye-pieces. Now, the
compensating eye-piece in most general use magnifies twelve
times, and a power of twenty-seven times is used very
frequently. I have seen the phenomena of cyclosis in
vallisneria (§ 114) shown by Mr. T. H. Powell most
beautifully, sharp and clear beyond criticism, under a
magnification of x 1,000, of which x 25 was obtained by a
T4o-inch apochromatic, and x 40 by a quarter-inch eye¬
piece. Such a proportion in the respective magnifications,
and such definition of such an object under such a high
power, would have been incredible to any microscopist so
lately as 1880.

CHAPTER III

THE SIMPLE MICROSCOPE AND ITS USE

The optical action of a simple microscope was explained in
§ 8. Here we have only to deal briefly with its forms and
its practical use.

26. Single and Compound Lenses. — Single lenses are
constantly used by engravers and others, but the well-known
eye-cell which these employ is only suitable for their work
and with low powers. For general use the most convenient

38 THE MICROSCOPE

plan is to have the lens or lenses in an ebonite or horn
mount, into which the glasses fold back. Such can be bought
in great variety. A useful form in some respects is shown

in fig. 19, where two lenses are
shown thus mounted. In the
handle or case end a hole is
bored, by which the lens can
be fixed on a slightly tapered
rod in a stand for holding the
lens when both hands are
needed for other work. Such a
double lens may cost about
2s. Gel., and the stand 2s. The
hole in the handle, however,
allows dust to get to the lenses.
About the most generally use¬
ful powers for the two will be
2 inches and lj inch focus,
which gives three nicely gradu¬
ated powers. A single lens in a handle can often be got
for Is.

Higher powers are smaller, and are often fitted up two at
each end of one handle. Two modifications of the simple
lens are also made for rather high powers, known as the
Stanhope lens and the Coddington lens. The latter is a

spherical lens, which has a
deep groove ground out all
round its diameter and filled
with black material, so that
light only passes through a
small aperture at the centre of the sphere. Fig. 20 shows
a Coddington lens in one end of a mount, and three plain
lenses at the other, with two stops that can be used
between them. The Coddington lens is hardly worth its
cost, however.

Fig. 19

THE SIMPLE MICROSCOPE AND ITS USE 39

The Stanhope lens is really a short cylinder of glass with
spherical surfaces at the two ends, the cylinder being such
a length that the farthest spherical surface is in focus. This
lens also is of no utility except for the one sole purpose of
examining rotifers, diatoms, or other minute objects in a
drop of water. For this it is handy, a drop being placed on
the lens itself, and being in focus when the eye is applied to
the other end of the lens.

The best lens of all is an achromatic form introduced by
Steinheil, and known by his name. It is a triple combination
of the form shown in fig. 21, and gives beautiful definition,
with a wide and flat field. Mounted to fold in a handle,
these are known on the Continent as Steinheil loups (fig. 22),

and can be obtained of any optician for 14s. to 20s. each, of
foci varying from 2 inches (x5) to \ inch (x20). Only
quite lately these beautiful lenses have been made by Leitz
of Wetzlar at the low price of 10s. each, either in a handle,
or in a plain collar for use in the dissecting microscope
shown farther on. The best plan is to combine both uses,
and have two or three powers in collars, with a spring ring
folding into a handle, which will carry any one of them in
that manner. A Steinheil lens at this low price costs little
more than a Coddington, while its performance is infinitely
superior. Browning’s Platyscopic lens is also a good pocket
magnifier.

27. Dissecting Microscopes. — If a single lens is so
mounted that it can be focussed by a rack and pinion, over

40

THE MICROSCOPE

a convenient stage on which the slide or object can be placed,
and with a mirror underneath which moves freely all ways
so as to throw light upon the object through an aperture in
the stage, we have converted the single magnifying glass
into a real instrument, with which real work can be done.
Well made, with a firm stage and side rests attached to the

stage, to support and
steady the two wrists,
this becomes what is
generally known as a
dissecting microscope.
The best combination
of cheapness with effi¬
ciency in such an in¬
strument which I have
been able to find after
extensive and system¬
atic search, is Leitz’s,
shown in fig. 23. The
side-rests for the hands
or wrists on each side
of the stage are detach¬
able, and not shown in
the figure. There is a
very good stage and
mirror and rack motion ;
and the instrument is

Fig. 23. — Dissecting Microscope -it • ,i , 7

sold, with two achro¬
matic Steinhcil lenses (of either focus preferred), for the sum
of 38s. To a real working microscopist such an instrument
is invaluable, and will be in use for a lifetime.

28. Use of the Simple Microscope. — No microscopist who
has the root of the matter in him ever goes out without a
pocket lens of some kind ; and a person ignorant of its use
has no idea of what can be seen with it, or of the additional

THE SIMPLE MICROSCOPE AND ITS USE 41

interest it is capable of imparting to the world of nature
around him. Did I wish to awaken in a young person of
either sex such an interest in that world as might probably
make the microscope a means of enjoyment and improve¬
ment for life, my first present would not be a compound
instrument, which might very probably be ‘ played with ’ a
few times and then tired of, but rather a good pocket lens,
with a little practical teaching of what could be seen with it
during any walk in the fields.

Take it, for instance, as a friend who will reveal countless
hidden beauties in flowers. First, use it to examine the very
smallest, which to the naked eye exhibit practically nothing :
such as the tiny white florets of fool’s parsley, wild carrots,
and others of similar magnitude, not forgetting in the season
the exquisite ‘ flowering ’ on 1 tail ’ and other grasses. It is
wonderful at first to see how, under a simple magnifier,
exquisite lilies start into being. Examine also the parts of
larger ‘ composite ’ flowers ; see how each so-called ‘ petal ’
of a daisy or feverfew is in truth a perfect little floret by
itself ; how a head of clover is ‘ made ’ ; a thistle flower ;
and others of that sort. Then let larger objects have
attention ; it will be marvellous to see the added beauties
which appear in such familiar flowers as the forget-me-not,
pimpernel, agrimony, heaths of all sorts, valerian, sainfoin ;
and even in flowers so large as lily of the valley, the hairy
yellow St. John’s wort, or the various forms of orchis. This
is no attempt at a list ; let the owner of a pocket lens,
during one walk, simply make a point of looking at every
smallish flower or grass in flower that he can find. The
small flower thus magnified is not merely like a larger one ;
there will be seen an exquisite beauty and waxy texture, in
most, that only the finest lilies resemble on a larger scale ;
and many a common flower — the hedge stachys is another
good example — puts on a magnificence of decoration abso¬
lutely enchanting.

42 THE MICROSCOPE

Then the reproductive organs of larger blossoms will
repay examination. Though but a portion can be seen of the
beauties of anthers, stigmas, and pollen grains, enough will
be glimpsed to awaken interest and curiosity to know more ;
and it is such living interest, and desire to look into things ,
which makes the real microscopist. Yet another world of
interest will begin to open as various mosses and lichens are
examined ; and something of the lovely miniature forest con¬
tained in a spot of ‘ mould ’ may be understood, even from a
pocket lens.

A second class of objects likely to interest an intelligent
person will be found in the insect world. Tiny insects no
larger than a pin’s head, only specks to the unaided eye,
grow into wonder and beauty under the lens. Little iri¬
descent beetles and flies will exhibit a magnificent wealth
of colour and ornament never imagined. With the higher
powers, the curious variety of insect organs can be made
out, and will add tremendous interest to any popular book
on the subject. The heads of insects alone, with their many
organs, so totally different from those of all other animals,
will furnish interesting study for months. They are easily
examined by preparing a few cones of gummed paper wrapped
round and round, and trimmed evenly at the ends, of various
sizes, as fig. 24. The insect being inserted head

Jl'j 111 first in one of suitable size, so that it can get its

f In It head outside, and no more, a little tuft of cotton-
Ijjl i j j || wool is put in behind and held by the end of a
v... —J finger, when the head can be examined com-
p 24 fortably. For weevils and beetles the cone is first
pinched oval, or nearly flat, so as to fit roughly the
section of the body. From personal experience, I can affirm
that even experienced microscopists, previously unfamiliar
with it, have found absorbing interest in this method of exami¬
nation, which is within the reach of any boy with a shilling
magnifier. Then the insects, killed as usual in a chloroform

THE SIMPLE MICROSCOPE AND ITS USE 43

or laurel bottle, may be cut up and their organs examined
in detail.

Something can be seen with a pocket lens even of micro¬
scopic pond-life, but this mostly requires more or less training
both of eye and search (see Chapter VIII). Such is not,
however, the case with sea-life, and the lens will add great
interest to a sea-side holiday. Take a few glass tubes, with
corks, and in these place for examination, with clean sea-
water, bits of weed or rock that seem to have anything on
them different from the mere weed, especially anything in
the shape of slender white or creamy branches, no thicker
than small or large pins. After a gale, even at places where
there are no rocks, such can be found in great variety on
weed washed up by the waves ; and in the tube, under the
lens, they will be seen to be lovely Hydrozoa. Abundance
of small crustaceans, and curious sea- worms, &c., will also
be found, which cannot be detailed here.

Something might be added about examining even bits of
stones, fresh broken ; a bit of granite, for instance, will show
plainly how complex is its structure. Seeds, spores, insect
eggs, &c., may also receive attention. But more is unneces¬
sary. It is only desired here to indicate how to use a simple
microscope. Anyone who has so used it, more or less con¬
stantly, through one summer season, will have a far better idea
of the world of minute nature, a truer conception of its
wonders, and most of all, a more vivid idea of how much
there is to repay further examination and study, than many
an one who has had presented to him a costly instrument
and box of objects, and who having looked over those objects,
has perhaps laid the whole aside. He may probably have
learnt to value the microscope for what it really is, and be
fairly on the way to become a microscopist.

The use of the simple microscope in dissecting belongs
chiefly to a later stage.

44

THE MICROSCOPE

CHAPTER IY

THE COMPOUND MICROSCOPE AND ACCESSORIES

Great changes have taken place in the microscope since the
late Rev. J. G. Wood wrote that a first-rate one cost ‘ forty
or fifty pounds and upwards,’ and recommended, as all that
could be desired, a three-guinea instrument—' not half the
price of a single object-glass belonging to the larger micro¬
scopes.’ From about the year 1880 especially, development
began to settle into one definite line ; the result of which,
with the improvement in lenses already sketched in Chap¬
ter II, has been great economy, combined with so much
greater excellence, that better results can now be accom¬
plished with 1 61. or 201. than with the 50 l. above quoted in
former years. Even the very cheapest instruments have
shared in this progress ; but the greatest gains have been in
those of a medium quality. Nearly all the old patterns have
disappeared, replaced by new ones, which at first sight
seem so various, that a novice may not perceive the one
general line of development, unless it is pointed out to him.

29. The Modern Microscope. — Comparing older and
modern stands, the modern stand of any good maker pro¬
vides a steadier ‘ fine adjustment,’ wherewith to precisely
focus the much better lenses now used ; in fact, the entire
microscope is steadier than the bulk of older instruments.
But chiefly, whilst in the older text-books all sorts of sub¬
sidiary apparatus, such as spot-lenses, paraboloids, and
various ‘ illuminators ’ and devices for using oblique light are
mentioned ; the modern instrument supersedes all these by
focussing on the object an image of the light, by a lens or
combination of lenses under the stage called the substage
condenser. In the cheapest instruments this slides into a

COMPOUND MICROSCOPE AND ACCESSORIES 45

simple tube-fitting under the stage ; but in really good ones
the condenser lenses fit into a ring, which can be focussed
up and down by a rack and pinion, and centered by two
screws. A proper condenser thus mounted does all that is
required (for transparent objects) far more efficiently, and
the best results cannot be obtained without it.

30. Choice of a Microscope.— The best microscopes are
made by a comparatively few London houses, who are in
constant touch with the two principal microscopical societies,
and keep up with the march of improvement. The same
real value or cheapness cannot be obtained elsewhere.1

The advice of a friend who really is experienced is
valuable ; but some profess knowledge on very small founda¬
tion. It is also to be regretted that even acknowledged
authorities, of high reputation, have on various occasions
enforced as positive canons what are neither more nor less
than dogmatic prejudices, likely to prevent many persons
from obtaining what to them would be the best value for
their money.2 Some details of many stands have weak

1 Many other paragraphs will show, it is hoped, total absence of any anti-
German prejudice ; but in face of the persistent pushing of German stands by
professors in colleges, it is well to state that these are neither better nor
cheaper than English. No doubt it has been largely done in simple ignorance
of practical microscopy, though when it is carried so far as to direct students

by notice to procure certain stands, ‘ which can be obtained of - ,’ it is

difficult sometimes not to suspect other reasons. I was personally informed of
a case in which a student, having previously purchased an excellent stand
with a substage, and declining to buy another for the study of Bacteriology,
the Professor manifested the greatest irritation, and finally told him he ought
to take it back, and insist on exchanging it for one without that invaluable
appliance, demanding the difference in price ! Without discussing motive,
or giving any clue which can cause pain, when such things are done to the at
tempted prejudice of English makers of the highest class, it is time to allude
to them and state the facts. German makers are only now adopting accepted
improvements in which England has always led the way ; and though some
of their stands are now very good, for equal work they are quite as costly.
Excellent stands and objectives are made in America, but are much more
expensive.

2 Three examples maybe given. 1. I remember that at a certain meeting
of the Royal Microscopical Society a certain maker’s lever fine adjustment

46

THE MICROSCOPE

)

points ; but every detail in extensive use has some special
advantage which has gained it that position, and which a
wider knowledge and more dispassionate judgment would
have recognised.

Leaving specific details and stands for the moment, one
or two points should be looked to in choosing any stand.
That of any good maker will have scarcely any grease about
its sliding parts : much should arouse suspicion. The stage
should be firm. Chief of all, however, both the coarse and
fine adjustments (see next section) should work with perfect
delicacy and steadiness, without any play or back-lash, and
yet without needing force. Particularly, if the rack-work of
either the focussing motion, or of the substage, needs force to
move it, any such stand should be refused, as should any
which gives a feeling of jerkiness or grittiness in work. The
slightest motion of finger and thumb should turn the milled
heads, else precise focussing cannot be done ; and a good
microscope (even at five guineas) should focus a ‘ dry ’ \ or
■g- easily. The vendor may say that a tight or hard rack will
‘ wear easy.’ It has no business to do so ; and the process
means a wear which is destructive, and ruinous to good work.

Lastly, up to a certain point get the best stand you can, if
only with two good objectives and one eye-piece. Keep in
mind, if possible, the addition of any important extras, as
small additional sums may be afforded, which cannot be
afforded at first. Chief among these regard a focussing and

was most unmercifully condemned as bad by a leading authority. This same
adjustment is now generally admitted to be one of the best. 2. The horse¬
shoe foot has been condemned in the most unqualified terms ; for large classes
of practical workers it is the most convenient and useful. It sells more
largely than almost any other ; and this would not be without reason. 8. The
micrometer screw adjustment has been similarly condemned, as ‘wearing out.’
I have known it do so when badly made. But this form of adjustment has
done more than any other to cheapen good instruments ; I have come across
several which have been in constant use for years without wearing out ; and
if it does wear out the cost of renewal is only a few shillings. It is not the
best ; but such considerations are not to be lost sight of.

COMPOUND MICROSCOPE AND ACCESSORIES 47

centering substage. In the old days the first ‘ extra ’ added
to other than the cheapest microscopes was a ‘ mechanical
stage,’ which moves the slide about by screw motions. Many
such microscopes are made still by anonymous wholesale
houses, for country sale. Some of the best working micro-
scopists think little of a mechanical stage at best, and some
actually prefer a plain stage ; but not one such will dispense
with the substage if he can possibly afford one, and the
additional cost is only about 21.

31. Parts of a Microscope. — Let us now briefly consider
the essential parts of a microscope stand, from a lettered
drawing (fig. 25) of one referred to in the next section, in
which section various stands suitable for given purposes or
expenditure will be found.

The Foot (f) is the base on which the whole stands. Its
best form, for mere working and steadiness, is a wide ‘tripod,’
as in figs. 33, 34. This form was first carried out in Powell
& Lealand’s best stands, and is now widely made ; but it
has the drawback of requiring the largest and heaviest case.
It is the best for constant critical work upon the table. The
foot shown in the figure is the ‘ claw ’ foot, also a kind of
tripod, but covering a smaller triangle, and hence less steady.
On that very account it is more handy for packing into the
case, or standing under a shade, and is very widely used.
The most portable form of all was first used on the Continent,
and is known as the ‘ horse-shoe ’ foot (fig. 30). As often
made it is far from steady ; but a rather larger scale than
formerly is now usual, assisted by a spur at the back of the
horse-shoe, as shown in figs. 30, 31. Such feet are steady
enough for all ordinary practical purposes, even without the
additional stability given by the reversing pillar described
on p. 57 ; and the smallness of the case which will contain
a stand of this kind, and the rapidity with which it is packed
or set up, make it the most handy form for medical students
and others who constantly carry their instruments about.

48

THE MICROSCOPE

Stands are also made, as Messrs. Beck’s ‘ Economic ’ and
‘ Pathological ’ models, supported on a central pillar from a
true flat tripod foot ; and the ‘ four-legged ’ microscope

shown in fig. 32 is a modification of the tripod form. A
double-pillar on a tripod foot is also used.

The Stage (s) is for the support and manipulation of the
slide or other object. It must be pierced in the centre for

COMPOUND MICROSCOPE AND ACCESSORIES 49

illumination from below, and the aperture should not be less
than an inch in diameter. It is better more, and a convenient
form for simple stands is that shown here and in fig. 32,
suggested by Mr. Nelson, and called a horse-shoe stage, the
opening being carried out to the further edge. A plain stage
may be fitted with two plain springs, as in fig. 28 ; but it is
much better as shown here, with a ‘ sliding bar ’ e carrying
springs which can be turned aside. The great advantage of
this is, that by pushing the slide along the bar every point
in the slide can be systematically examined (§50). Another
great advantage is in using high powers (p. 90).

The sliding carrier with two handles, seen on some stages,
I regard as useless ; and a mechanical stage is only needed
by few. For study of minerals, and of other objects by
polarised light (§ 40) a rotating circular stage is useful,
which may be either quite plain or combined with a
mechanical stage.

The Tube, or Body (t) carries and connects the lenses of
the microscope. It is of variable length, Continental objec¬
tives being corrected for a focal length of about 64 inches, and
some best English objectives to one of 10 inches ; of late,
however, most cheaper lenses and instruments are made in
England also for 6| inches.1 As microscopists wish to use
such lenses as they prefer, it is usual in all but the cheapest
stands to have a sliding draiu-tubc (b), or in the best stands
even a double draw-tube, so that all lengths can be obtained.
The bottom of the tube has a screw at c (called the ‘ society
screw,’ and adopted as a standard by every maker), into
which the objectives screw; and into the top of the draw-

1 Messrs. Zeiss have done an ill service to general microscopy in adopting
the mechanical ‘ long ’ tube-length of 10 inches. That was not the average
English length, which was about 8^ inches ; and this latter would have been
far better and more convenient for obtaining both by one draw-tube. More¬
over nearly all good objectives perform better when adjusted or corrected for
the longer length, besides giving higher power ; and it is distinctly to be
regretted that such length has been so heavily discouraged.

D

5o

THE MICROSCOPE

tube fits the eye-piece a. The eye-pieces should fit in quite
easily, and not have to be pushed, or hardly at all.

The coarse adjustment (d) is by rack-work. Some very
cheap stands are made with only a sliding movement ; but
there can be no pleasure or even comfort in the use of such
an instrument. As already hinted, the rack-work should
work smoothly and easily enough, to focus accurately a ‘ dry ’
objective.

The fine adjustment (e) is made most variously of all the
points in a modern microscope. That preferred by Dr.
Dallinger, and used in Messrs. Powell & Lealand’s best
stands, is in a much rougher form shown in fig. 27, the
screw acting on the end of a lever of the first order, the
pivot of which is seen further along the hollowed bar, while
the other end moves a nose-piece only, with the objective,
inside the body. The same movement, but with a longer
arm to the lever, and lifting the entire body, is Messrs.
Watson’s adjustment. Swift’s (wherever the direct screw
is not used) is essentially the same, but with the long arm
of the lever bent down at right angles, parallel with the
body-tube. Figs. 30 and 31 show a direct-acting micrometer
screw, which is general in all Continental stands ; and our pre¬
sent figure (25) shows a double or ‘ differential ’ screw, acting
on the body in the same general way, but with the advantage
of securing a finer motion with much coarser threads, as
only the difference between the two screws is effective.

As already hinted (p. 46, note) I consider the condemna¬
tion passed upon all these later forms (i.e. those moving the
entire body) by a high authority, uncalled for. The disas¬
trous wear predicted, as a matter of fact does not take place in
well-made instruments (I have personally examined several
which have had long and really hard wear). Hasty dicta of
this kind are to be regretted, as condemning the very
methods to which we owe such excellent instruments at such
moderate prices. Friction is strictly calculable, and is not in-

COMPOUND MICROSCOPE AND ACCESSORIES 51

creased to nearly the extent supposed by a few ounces more
weight, if fitting is really good ; and by some strange mistake
it has been overlooked by one writer, that instead of the pres¬
sure of the spring giving additional friction, the weight may be
balanced by the spring as nearly as desired, and generally is
so. Moreover, the screw portion can, if necessary, be
renewed for a very few shillings ; or its wear may be com¬
pensated, as in Swift’s stand (fig. 32). A good differential
screw is undoubtedly the best form of direct screw adjust¬
ment, and costs a few shillings more than the plain screw.

There is one antiquated form of fine adjustment which
it is so important should not be purchased, that it is here
represented (fig. 26). A screw work¬
ing in a bracket fixed on the outer
tube, moves an inner tube bearing the
objective, sliding in the outer tube.

There are hundreds of stands in shops
(especially provincial shops) with
this, and quite recently I saw a 20/.
binocular with mechanical stage, so
fitted. But it is to be absolutely re¬
jected, whether the screw be at the
back as here shown, or in the front,
or at the side. It cannot focus any¬
thing steadily, better than those
cheap dividing objectives sold with
the very cheapest microscopes ; and
is discarded by every house of the slightest reputation
at the present day. Its utter supersession was the great
work done by the micrometer movement.

The mirror (k) is flat on one side and concave on the
other. If possible a stand should be chosen with a mirror
of good size.

The substage (j) has already been referred to. Only the
cheapest instruments have nothing of this kind. A really

p 3

Fig. 26

52

THE MICROSCOPE

efficient microscope has the substage tube (which also is a
standard gauge of If inch in all instruments) capable of
being centred by two screws h and i acting at right angles,
and focussed by a rack and the milled head g. Such is de¬
scribed in catalogues as a ‘ focussing and centering substage. ’
A plain spring tube of the same gauge is screwed to the

bottom of the better cheap
microscopes. A fairly good
focussing and centering ap¬
pliance costs about 38s., and
centering movements only
are supplied by Messrs.
Watson for 10s.

32. Microscope Stands.
We will now look at some
representative stands at
various prices, beginning
with the cheapest.1

In 1855 the Society of
Arts offered a prize for the
best microscope that could
he made for three guineas,
which was won by Messrs.
Field & Co., of Birmingham.
The pattern has been made
since by many opticians ;
but I think their own make,
shown in fig. 27, still the
best, as having the best
met with, and having two
objectives, a separate If -inch, and a J-inch which separates

1 All tlie instruments named are personally known to me, and I have
made a thoroughly catholic though typical selection. Obviously a complete
list is quite impossible in the space at disposal; and many instruments,
including the whole made by Powell & Lealand, are omitted as being too
expensive for most of the readers here in view.

Fig. 27

fine adjustment of those I have

COMPOUND MICROSCOPE AND ACCESSORIES 53

so as to make a ^-inch. Other makes have generally one
lens separating into three, and the lowest power is in
these cases the worst of all. The instrument has two eye¬
pieces, a wheel of
diaphragms, and
stage forceps, and
is packed in a case
with a stand con¬
denser or bull’s-eye.

Such a type of
instrument wTill do
a great deal, show¬
ing nearly all the
objects named in
most popular works.

It has introduced
many into this
fascinating region of
study, and is able to
reveal a great part
of those wonderful
works of the Creator
which the micro¬
scope only can dis¬
play. Where it fails
to justify such high
praise as the writer
already named
formerly bestowed
upon it, is in show¬

ing the exquisite delicacy of those works in their minuter de¬
tails. The details often appear, and are ‘ magnified ’ and made
amply apparent ; but what appears a thick and coarse cord
under objectives of this class, may probably become a filament
or line of inconceivable fineness under better powers.

54

THE MICROSCOPE

Let us next suppose that real work is intended from the
first, but that only a very little more can be afforded ; what
is the best that can be done ? Probably one of Beck’s ‘ Star ’
microscopes. This will not he so nice-looking on the mantel¬
piece as even the preceding, everything being spent upon
working parts, and not a penny upon bright brass or outer
finish. It can even be obtained for three guineas, in a cheap
case with two eye-pieces and one objective — an inch. For
another guinea a quarter-inch will be added, or if the pur¬
chaser be a medical student it will be better to have two
powers, of § and instead, at a little more. What is gained
will be a steadier fine adjustment, two much better objectives,
an iris or contracting diaphragm, and the power to add in the
fitting of the latter, for another 10s. 6d., a simple form of
condenser. This stand will focus satisfactorily an oil-im¬
mersion objective of ^ focus.

• The next step upwards is a model very generally made,
with but small variation, under the name of the ‘ histological ’
pattern, our typical example in fig. 29, being from Messrs.
Watson. In a case without objectives, but one eye-piece, this
will cost 4:1. 10s., or with really good inch and powers 61. 5s.
Closely similar instruments, but mostly with direct micro¬
meter screw fine-adjustments, as in the succeeding stands, on
either claw or horse-shoe feet, are made by Messrs. Baker,
Swift, Johnson, and others ; prices being calculated so closely
as only to vary a few shillings, for which there is generally
sound reason. This type has in fact become so general,
that (almost the only stand of which such can be said) it
may often be found of good quality in provincial towns, sup¬
plied to the shops by anonymous London wholesale makers.
It is a handy, neat, well-looking pattern, and the under-stage
fitting is of standard 1^-inch gauge, which allows of a wide
range of condensers being used. The stands following,
without their focussing sub-stages, also belong to the same
rank of instruments.

COMPOUND MICROSCOPE AND ACCESSORIES 55

But every one who really means to pursue microscopic
study of any sort, is again strongly urged to purchase, if
possible, an instrument with a focussing and centering sub¬
stage for the condenser, or
to which one can afterwards
be added. Since the value
of this appliance has been
understood, most (not all)
of the great London makers
have set themselves to pro¬
vide such instruments at the

Fig. 29

lowest possible prices for sound workmanship ; and it is worth
noticing again how closely the cost of similar work agrees
in their hands. Let us see what those who have specially

56

THE MICROSCOPE

studied this class of microscope can give us for our money,
especially as it is perhaps in this particular grade that we
find the best real value compared with outlay.

Fig. 25, already given on p. 48, represents Messrs.
Baker & Co.’s ‘ Advanced Student’s ’ pattern, giving a good

racked and centered sub¬
stage, draw-tube, gradu¬
ated stage with sliding-
bar, and differential fine
adjustment, at the cost
of 61. 15s. for the stand
alone in case ; that is,
not including either ob¬
jectives or eye-pieces.

Fig. 30 shows the
same essentials in Beck’s
‘ large model Continen¬
tal ’ 1 pattern, with a
horse-shoe foot. The cost
of this stand alone in case
is 6Z. 10s., the stage being
plain with spring clips ;
the sliding-bar will cost
a few shillings more. A
very similar pattern is
obtainable of other
makers, and is also made
by Leitz of Wetzlar.

Another horse-shoe
stand by Newton & Co. is
represented in fig. 31, on
account of the pillar being made reversible , or rather, capable
of being turned round. This gives the greatest possible

1 The name is a misnomer, except as regards the foot and fine adjustment.
The focussing sub- stage and inclining limb are English details.

COMPOUND MICROSCOPE AND ACCESSORIES 57

screw.

Curiously enough, just
after meeting the above
stand it appeared that
Messrs. Ross & Co. had
adopted the very same
principle of rotating the
pillar in a moderate-priced
stand — the first time they
have catered for this class
of purchasers. Their stand
is called the ‘ Eclipse,’ and
the foot is a turned circu¬
lar ring, the pillar rising
out of one side. It is a
firm solid stand, with ex¬
cellently-protected micro¬
meter fine-motion. They
do not quote for the stand
alone, but it is sold in case
with two eye-pieces, two
objectives (1 inch and or §and^), and double nose-piece
with the objectives adjusted to same focal plane, for 9/. 18s.
including the racked substage, or SI. without.

Still seeking really distinctive types, fig. 32 represents
a stand by Messrs. Swift & Son, which has several peculiar

steadiness when the instrument is much inclined, and
enables this simple form of stand to be used in a horizontal
position for photography. The device was first employed

by Nachet many years ago;
till lately. This stand is
sold in case for 61. 5s. with
single-screw micrometer
fine adjustment, or 10s.
more with differential

but I have not seen it again

53

THE MICROSCOPE

/

features. The most conspicuous is the four -legged form of
‘ tripod,’ for it is really a tripod, the back pair swinging on a
pivot compensated for wear, so as to adjust on the table like

Fig. 32

a single leg. This is therefore an extremely steady stand,
yet light and compact ; and when the hind legs are swung
upwards, the whole actually packs into a box two inches
smaller than the three-legged tripod (which is also made,

COMPOUND MICROSCOPE AND ACCESSORIES 59

exactly similar in other respects). The second feature is,
that the micrometer screw works in a half nut, which can
be adjusted for wear by the set screw seen at the back under
the milled head. A third feature is that a racked and
centering sub-stage is so made, to fixed gauges, that the
stand may be bought with only a plain sub-stage ring as in
the previous lower class of instruments, while the racked
appliance can be purchased later for the sum of 38s. and be
affixed in place by any intelligent person, with no tool
except a good small screw-driver. This is a most desirable
system* which I am glad to know is gradually extending
amongst other cheap microscope stands. The sliding-bar
on a divided stage is well seen in this figure, and it will be
understood how, by pushing the bar up one division at a
time and moving the slide along, every point can be syste¬
matically gone over. This stand alone in case will cost
71. 2s. 6 cl., with three-legged tripod 5s. less, or with differen¬
tial screw 5s. more.

Our last figure of this class of instruments (fig. 33) is given
for its distinctive feature of a swinging though racked sub¬
stage, and is the H pattern of Messrs. Watson’s ‘ Edinburgh ’
microscope. It is selected also to show a * mechanical ’
stage applied to this class of instruments, which costs from
21. 10s. to 3 1. 10s. extra. This particular stage rotates as
well. Such an addition makes a really fine instrument of
any of this class. The racked and centering sub-stage here
swings upon a pivot under the main stage, so as to move
quite out of the way when direct light from the mirror is
required, and being supported by a catch on the other side
when in position. Opinion differs about this method of
attachment, which has been condemned ; but after repeated
examination at intervals of instruments which have been
long in constant use, I can find no solid ground for such a
judgment, and prefer such practical tests to forming any a
'priori opinions. In other microscopes such sub-stages as are

6o

THE MICROSCOPE

plain stage for 71., or with horse-shoe foot for 6/. 6s. For an
extra cost of 11. 5s. an extra draw-tube worked by a rack
can be added to the body. This extends the range of tube
length from about 5| inches to 12 inches. It is very

removable (some are not) slide entirely out in dovetailed
grooves. This instrument is sold as here shown, with the
mechanical stage, in case without accessories for 10 1., with

FlU. 33

COMPOUND MICROSCOPE AND ACCESSORIES 61

difficult to get the full range from 61> to 10 inches with a
single draw-tube.

Any of the plain-stage microscopes of this grade can be
had furnished with two eye-pieces, two objectives varying

Fig. 34

from 1 inch and £ inch to f inch and ^ inch, and an Abbe
sub-condenser with iris diaphragm (§ 39) for an average
price of about 11/., varying only a few shillings more or less.
With the addition of an oil-immersion objective of high

62

THE MICROSCOPE

power, costing from 5k to 8k, according to aperture and
quality, and a couple of ‘ compensating ’ eye-pieces, the
microscopist of the present day finds himself in all essential
respects better equipped than with any of the older instru¬
ments costing 50 1. or 60 1., referred to by the Rev. J. G.
Wood in a passage already quoted.

Beck’s ‘ Economic ’ pattern, on a true flat tripod with
pillar in the centre, with the addition of the racked sub-stage
(this instrument having been originally made without one)
also belongs to this class, costing about 11. more, owing to
the work in the tripod.

Few readers of these pages will wish for better stands
than the foregoing. Any who do so should obtain cata¬
logues of the noble instruments of Messrs. Powell & Lealand
and Messrs. Ross, as well as of the higher types made by
the opticians already mentioned, such as Swift’s ‘ Paragon ’
model, Watson’s Van Heurck pattern (an elaborated ‘ Edin¬
burgh ’ made to the order of the celebrated Continental
microscopist), Beck’s ‘ first-class’ stands, or Baker’s higher
forms of the ‘ Nelson ’ model. Let us take one illustration,
of the last-mentioned in its smaller form, just to show the
directions in which additional refinement is carried in other
good instruments as well as that here cited (fig. 34). The
whole is carried upon a wide tripod. The mechanical stage
is further elaborated, and there is a double draw-tube, one of
them divided and with rack- work adjustment, giving lengths
from 5k to 12 inches ; the sub-stage is more substantial and
with greater length of focussing adjustment ; and there is
often added to the sub-stage a fine adjustment by screw
motion. The whole instrument is larger and more massive.
The cost as figured, in case, is 16Z. 16s., which will be two
guineas more with fine adjustment to sub-stage (only useful
for very critical work with oil-immersion lenses), or four
guineas less with a plain stage. Watson’s Van Heurck
model is of about the same rank in points and price.

COMPOUND MICROSCOPE AND ACCESSORIES 63

33. Binocular Microscopes. — All the foregoing stands are
shown for use with one eye, or are ‘ monocular ’ stands. But
many microscopes are also made for use with both eyes, or
‘ binocular.’ One reason for this is to obtain somewhat of a
stereoscopic effect in examining an object; but a far more
important object is to divide the work between the two eyes.
Wherever, as in my own case, the eyes are weak or specially
sensitive to glare, the relief
to both from this is incalcu¬
lable.

The most frequent bino¬
cular arrangement is due to
Mr. F. W. Wenham, who
made a free gift of it to
the microscopical world. A

Fig. 35

prism of the form shown in fig. 35 is placed behind the
objective, with the lower edge across the centre. This is
of such a form that the part of the pencil a which enters
the prism is reflected internally in the path b c cl. The
arrangement in the microscope is shown in fig. 36, one half
the pencil proceeding direct up the primary tube b, while the
other half is directed up the inclined tube c. The prism is so
mounted at a (fig. 36) that it can be drawn out of the way at

64

THE MICROSCOPE

pleasure, when the instrument becomes monocular. This is
its great advantage ; the next being cheapness and simplicity,
so that a binocular arrangement can be had for an extra cost
of 2 l. 10s. to 3 l. 10s. The disadvantages are, however,
several. The system is not well adapted for objectives
higher in power than about t4q, and of angles above 50°
in air, though higher powers can be used by having them
specially mounted in shorter cells, so that the back lens
comes close to the Wenham prism. With high angles, how¬
ever, the stereoscopic effect becomes painfully exaggerated.
The most serious defect is that a long, extending draw-tube
becomes impossible, as the tubes must be cut away on their
inner sides. Some draw-adjustment is necessary, in order
to provide for the varying distance between people’s eyes ;
but it can hardly exceed at the most 1\ inch. Hence a
Wenham binocular is difficult to use with objectives of both
Continental and English tube-lengths. The best compromise
is to have a length that will rack down to 7 inches, which
most short-tube objectives will bear fairly well ; this will
rack out to 8^- inches, and then a ‘ lengthening adapter ’ can
be inserted in the monocular body for use on critical work
with long-tube objectives.

Both Messrs. Ross, Watson, and Powell & Lealand
supply other prism arrangements which will work up to

immersions ; but all of them are necessarily much more
expensive that the Wenham prism. Some are non-stereo-
scopic, others stereoscopic. The average cost varies from
51. to 8 L, or more. The non-stereoscopic is, I think, to be
preferred, the relief to the eyes being the main point, and
well worth the cost whenever a long microscopic career is in
view and the eyes are specially sensitive to glare.

There are also binocular eye-pieces constructed, which fit
into a single body-tube. The best is probably Abbe’s, made
by Zeiss of Jena, and costs about 81. This requires a tube
not more than 170 mm., and is therefore preferable for short-

COMPOUND MICROSCOPE AND ACCESSORIES 65

length microscopes. Its weak point is that the eye-pieces
have to be specially constructed for it ; an ordinary pair can¬
not be used.

34. Objectives. — The advance in these is even greater
than in stands, both as regards the highest class and those
sold at a cheap rate. A few hints may assist the reader to
get value for his money.

Except under first-rate advice, none should be purchased
but such as bear the name of a maker with a present reputa¬
tion. Many lenses, good for their day, may be offered for a
third of their cost, and yet be dear now.

Most of such stands as have been described are habitually
sold with pairs of objectives, usually either 1 inch and \ inch,
or § and Often both pairs are made, the first being pre¬
ferred by ordinary users, and the higher powers by medical
students. Such pairs of objectives being required in numbers,
special pains have been bestowed in working them out ; and
almost without exception they will be found sound and good
lenses, of a defining power unobtainable at twice the cost
years ago. To buy a stand thus equipped usually saves
about 10s. on the total, and the lenses will never cease to be
useful, whatever may be acquired later on. The beginner
should, however, also have a lower power, say 2 inches or
thereabouts. These can be had cheap, of German make ;
English powers may cost from 25s. upwards, but are much
better.

The German makers — Zeiss, Reichert, Leitz, and Seibert
— also make really fine but cheap lenses of the usual
powers. Every maker, without exception, is found to have
certain powers in his series better than the rest, which
are soon recognised by workers and become favourites.
Of Seibert’s I have found a dry at 21s. and a water im¬
mersion -jig- at 31. the best. Reichert’s No. 3 (§) at 21s., No. 6
(1) of 077 n. a. at 30s., and No. 7 a (j) of n. a. 0-82 at
36s. are very fine. The same maker supplies a good oil

E

66

THE MICROSCOPE

y1^ for 51., but the best of his ordinary series is an oil-
immersion of \ real focus, and 1‘30 n. a., sold for 81, which
no eye-piecing has broken down. Twenty years ago, no lens
then made, at any price, would have borne such a test. A
new oil of same aperture and price is another excellent
objective. A semi-apochromatic of 8 mm. focus and 0’50 n. a.,
sold for 32s., is also an admirable and well-known lens.

Leitz makes a ‘ pantochromatic ’ (lettered p) as well as
achromatic series. His ordinary No. 2 (1^) and No. 3 (§) at
15s. each are as good of their power as it is easy to find ;
No. 7 (j-) at 30s. is also good, and the oil-immersion ^ at 51.
is an excellent lens. The pantachromatics, though more
costly, I did not find so good for general work, with two
exceptions.1 These are the p 3, a dry with correction-collar
087 n. a., sold at 31., and an oil -^2 °f 1'30 at 81 The former
was one of the best dry §■ objectives I ever met with.

Zeiss’s series are rather more costly than the above.
The best of them, I think, are the aa, at 27s., which is a
very fine inch power, and the d of O’ 65 n. a.) at 21. 2s.
Their quality is very uniform, and definition admirable, but
they are not so flat in field as the above.

Of English lenses, the ‘ pairs ’ most in demand have
already been named ; all such powers are excellent, from any
of the recognised makers. Watson’s dry at 21., is a well-
known lens. So also is Swifts’ dry at 36s., and two oil ^
at 51. and 71. : of their general series the most noteworthy are
the 2-inch, 33s., ^ at 21. 12s., ^ at 32s., and a } of n. a. 087
at 3 1.2 The cheapest really good 1-inch made, up to the date
this is written, is probably Beck’s, of 80° air angle, at 25s.
One of these (taken at random) bore a x 12 eye-piece well,
and a x 18 one fairly, on the usual tests. Their 3% of ‘ air ’

1 Tlie series examined was the first sent to this country.

2 After above was in type I received from them a neiv § of n. a. O’SO and
h of 0-50, at 80s. and 40s., both flat in field, and on most objects bearing a x 27
ocular. The Ij-inch is the best of that power, for general work, which I have
yet met with.

COMPOUND MICROSCOPE AND ACCESSORIES 67

angle 95°, with a correction collar, at 51, is the cheapest
objective of such wide aperture obtainable, and a really
magnificent lens, which it is not so very easy to distinguish
in performance from an apochromatic. As a rule, however,
Messrs. Beck devote attention to smaller apertures, for histo¬
logical students, and while this is written are preparing a
low-aperture oil yb to be sold for 4 l.

Messrs. Boss & Co. have very recently indeed brought
out a new set of lenses from Dr. Schrceder’s calculations.
An inch of 35° at 3 l. 3s. is very fine ; but the most notice¬
able value was in an oil §- of 1*20 at 11., and an oil Tb of 1*25
at 11. 10s. The §- proved to have a little more aperture than
was stated, and was if anything the best, bearing the highest
ocular well on Podura with unusually little residual colour.
Both were however very fine powers for their apertures, and
Hatter than usual in field.

35. Apochromatic Objectives. — These are so high in
price that probably few readers will procure them, and a
few words must suffice, as they are only needed for the
highest class of serious work. In flatness of field the Zeiss
series are (or have been up to the date this is written)
defective, but in definition and in uniformity of quality they
excel most of those made by other opticians. The l-inch of
n. a. *65, and the oil-immersion ^ of n. a. 1*40, are probably
the finest objectives yet produced, as regards definition over
a small field ; and as their power to bear high eye-piecing
enables the Ir-inch to give as much magnification as the
5-inch with a lower power, two such lenses alone will cover
an immense range of work. The present prices of these
two powers (reduced 20 per cent, since the early lists)
are 11. for the ^-inch and 20 1. for the §• immersion, others
being in proportion. The cheapest high-power apochro-
matics are the rb oil-immersions by Powell & Zealand, at
10k, most of which are of high quality, though not I think
strictly free from residual colour. Powell & Lealand’s I

68

THE MICROSCOPE

have found generally flatter in field than those by Zeiss, but
not quite so uniform : the best of them are certainly amongst
the finest in the world.

36. dualities of Objectives. — The various points desirable
in an object-glass have not the same relative value for all
workers; and some being more or less inconsistent with
others, a few words may be of service respecting them.

Definition, or that quality by which a fine line is repre¬
sented in the image by a fine sharp line, and not a blurred
or 1 woolly ’ line, is, however, in all cases the first property
necessary. Nothing whatever can compensate for the want
of this, under the eye-piece power required. In the extent of
the eye-piece power it will bear, lenses vary. Thus a cheap
lens, which remains sharp under an a or b eye-piece, may
suffice if the work does not require more. Higher powers
especially, however, should bear high-power eye-pieces.

Aperture. — The bearing of angular aperture upon resolu¬
tion, or separation of minute detail, has been referred to in
§ 22. For a certain number of markings per inch, the lens
must have a certain aperture. Hence magnification without
corresponding aperture is of no account ; and, on the other
hand, it is important to know that, if we have the aperture,
a lower power is as good as a higher one, provided it will
bear eye-piecing up to the needed magnification. No one
now uses and powers. But the value of great aperture
differs with the work. The student of diatom-marking, or
of bacteriology, must have it ; for ordinary current use
moderate apertures will suffice, and will suit better, as giving
more working distance, and entailing less cost, and giving
more of what is called

Penetration, or depth of focus. — This power to see some¬
thing in more than the exact focal plane, decreases with the
aperture. Hence high apertures are disliked by biological
students. It is true that an apochromatic objective of
lower poiver will have still more penetration though of high

COMPOUND MICROSCOPE AND ACCESSORIES 69

aperture, while it can be brought up in power by eye-piecing.
Better results can therefore be got in that way, if cost be no
object. But the student can rarely afford such high-class
lenses, and hence he prefers moderate aperture.

Working distance. — Objectives being compound, the opti¬
cal centre from which focus measures is within the system,
and the clear space between the front surface and the cover-
glass, called ‘ working distance,’ is much less than the focal
length. This also decreases with aperture. To a great ex¬
tent the same remarks apply as in the last paragraph, and suf¬
ficient but moderate apertures with good working distance,
will best suit general purposes and most present readers.

Flatness of field means that the image of a really flat
object should be in focus over the entire field of the eye¬
piece. The necessity for this quality differs with the work
and the objectives and oculars used. Formerly, with low
eye-pieces, it was of very great importance ; and for mis¬
cellaneous work it is still so, especially in examining small
forms of life, such as rotifers or infusoria. To work on
these with comfort, they must be comfortably in focus over
the large field of a low eye-piece. Unfortunately, the ten¬
dency to use apochromatic and other high-class lenses with
high-power compensating eye-pieces which have a small
field, has led to great and careless disregard of this useful
quality. Hence some of the most costly lenses are really
unfit for certain classes of work, for which, if only flat in
the field, they would be perfectly suitable.

To be practical, an inch power should, if it can be
afforded, be about 25° air angle ; a Uinch, not less than 40°,
or t47 than 50° ; a | or -1 from 90° to 100° ; a ^ from 110° to
120°, for general purposes. These angles allow of good
working distance. For bacteria, higher apertures are better
if they are within the means of the purchaser ; for syste¬
matic study in cell life, such as Dr. Ballinger occupies himself
with, they are necessary.

?o

THE MICROSCOPE

37. Testing Objectives. — The novice cannot possibly test
objectives ; and the highest class can only be exhaustively
tested by methods which require long experience to master.
Should however even a novice purchase higher-priced

‘ pairs ’ usually sold with a first micro¬
scope, he may reasonably ask to be
shown certain things ; and the few hints
here given must be confined to these,
and to such tests as can be grasped
with moderate experience, and yet will
prove a lens to be of very fairly good
quality. They must be read in connec¬
tion with §§ 53 and 60.

Good powers from inch to half-inch
or yty, should stand a two-thirds to three-
quarter cone easily (§ 53), and with
this can be very fairly tested by the
numerous small hairs which cover the
whole surface of the tip of a blow¬
fly’s proboscis. With such cones, all
these which are in focus should be
shown as sharp as needle-points, with
no nebulousness or fringes.

For ^ and even immersions,
there is no better test of definition than
a ‘ test ’ Podura-scale, known as Lcpi-
docyrtis curvicollisd Fig. 37 is a re¬
production by photography of the late
Mr. E. Beck’s original drawing (which
has never been equalled) of this scale, as seen under a
J-inch. With even a good lens it should be observed with

1 A slide of tlie 4 test, coarse Podura ’ should be asked for. This and the
other tests named can usually be obtained of Messrs. Watson or Baker:
the scales often of Messrs. Beck. All microscope houses do not keep them.
Test diatoms should be by Muller or Tlium of Germany, either 4 dry,’ or
mounted in dense media, which shows up their details more strongly.

objectives than the

Fig. 37.— Test Podura Scale

COMPOUND MICROSCOPE AND ACCESSORIES 71

a 1 cone ’ of only half the objective aperture (§ 53). Then, if
the lens is well corrected, the scale will be covered with
distinct * exclamation marks ’ as in the figure, each black
mark with a light streak in the centre. The width of the
white streak will differ with the precise focus, and the
markings may vary a little. The essential point is the
sharpness of this black and white marking ; and the test of
the lens is the eye-piecing which it will stand without
breaking down. A first -class dry lens will stand the
Zeiss x 27 compensating ocular ; but it is a very fair glass
that will bear x 18.

We also need tests, however, which embrace more of the
aperture. For a or the Pleurosigma angulatum

mounted dry is still a good test, though the old so-called
‘ resolution ’ is worthless. But if with a full three-quarter
cone (§ 53) a lens will give, with the x 27 ocular, either
sharp white discs, or clean black dots, with precision,
according to the focus, the lens is not a bad one. A first-
class ^ ought to do a little more, and show at least indica¬
tions of the N. rhomboides next mentioned.

Ground-glass illumination (§ 54) will much diminish a
beginner’s difficulty in illuminating some of the tests here
mentioned.

Immersions must be tested more highly ; but the mere
‘ resolution ’ of A. pcllucida, once so much talked of, is worth¬
less. It is a mere test of aperture, and of the operator, and of
the slide ; in fact A. pellucida may be ‘ resolved ’ by a really
bad lens. The following two diatoms interpose no great diffi¬
culty, and yet the lens cannot be a bad one that succeeds.
They are Navicula rhomboides and Amphipleura Lind-
heimeri, and should be procured mounted in styrax or
equivalent media : an extra one of each, finely ground on
the bach, will also be useful. With a large central cone, the
dots round the ends of the Navicula should appear distinctly
black or white, with a high-power ocular. (I never found

7 2

THE MICROSCOPE

these dots sharp, however, like others : there is always some
woolliness about them.) And the Amphipleura should also
be capable of being dotted black or white, at least on a
portion of any valve preserving the outer membrane, where
the marking is always bolder ; but even the finer marking
ought to be clearly dotted. The last is the severer test, and
will need care ; but both interpose no real difficulty with a
really fine lens, and for that very reason are selected. But
though they put no great demand upon the operator, the
objective which will pass them, with Zeiss’s highest-power
compensating ocular, is a very good one indeed. Care must
be taken to choose a valve which lies flat on the slide.

Slides of some bacteria — of leprosy, or Koch’s ‘ comma ’
cholera bacillus, or tubercle — are also exceedingly good
tests. They should be examined with a large cone — from
two-thirds to three-quarters — and should then appear sharp
and thin. But they will almost never bear such high-power
oculars as the preceding : the fact is their exceeding fineness
severely tests the correction of the ocular as well as of the
objectives, and all compensating oculars are by no means
alike good.

38. Eye-pieces. — The construction of the Huygenian eye¬
piece was described on p. 21. This is the most usual form,
and costs from 7s. 6d. each for the small size used on smaller
stands, to 11. or more for those with caps, for larger tubes.
There is an accepted gauge for the small ones ; unfortunately
there is none such for the larger, which is to be regretted.
The ocular lens, instead of being simple, is sometimes made
achromatic, which gives better definition with the higher
powers.

Compensating Eye-pieces have correction carried to an
excess in the ocular lens, to counteract certain errors it is
almost impossible to correct in high-power objectives, or
others made with single hemispherical fronts. The per¬
formance of all such lenses, whether ‘ apochromatic ’ or not,

COMPOUND MICROSCOPE AND ACCESSORIES 73

is greatly improved by this compensating error introduced
into the eye-piece ; but ordinary lower powers are deterio¬
rated. To allow the same eye-pieces to be used throughout,
the Abbe-Zeiss apochromatics have therefore similar errors
artificially introduced into the lower powers as well ; and it
is to be hoped that this example may be followed in future
high-class objectives. The lower power compensating eye¬
pieces are Huygenian ; the higher are combinations with no
field lens, working as a single or positive eye-piece.

The Kellner Eye-piece has an achromatic meniscus for the
ocular lens, and a double-convex field lens in its focus. It
gives a very large and flat field, but of bad definition ; and
the least speck on the field lens is, of course, in focus, and
unpleasantly conspicuous.

Bamsdcns eye-piece consists of two plano-convex lenses
with their convex surfaces towards each other, at such a
distance that the focal plane is a little outside the field lens ;
hence this eye-piece is ‘positive.’ It is chiefly used for
micrometry, but it is more usual now to insert a micrometer
in the focal plane of a Huygenian eye-piece.

English oculars are generally called either a, b, c, d, e,
or No. 1, 2, 3, 4, 5, and the magnification of either is not
uniform. Zeiss professes to remedy this indefiniteness in
his ‘ compensating ’ series, by marking each with its actual
magnifying power : e.g. 18 or 27 is supposed to mean
that the eye-piece magnifies the original image eighteen
or twenty-seven times. Some other makers follow this plan
in their ‘ compensating ’ eye-pieces, and it is to be hoped
it will spread to all. Unfortunately, however, Zeiss does
not carry out the plan consistently, but only for the long
or 10-inch tube. For this the stated magnifications are
correct. But the eye-piece marked 18 for the 6-inch tube is
the same power as the 27 for the 10-inch tube ; being in this
case marked 18 on the fictitious supposition that the image
is the same size with the short tube. Thus, a J-inch gives

74

THE MICROSCOPE

an original image x 40 at 10 inches, and a x 12 eye-piece
gives x 480. With the short tube the same lens and eye¬
piece give only x 320. By the absurd fiction referred to
(doubly absurd when ‘ accuracy ’ is specially claimed) this is
reckoned as made up of objective still x 40 and eye-piece
now marked x 8 ; whereas in fact the figures are, objective
about x 26, eye-piece as before x 12. It is to be wished that
this foolish pretence should be done away.

Flatness of field partly depends upon the eye-piece, as
intimated on p. 22. It can be increased in the Huygenian
forms, by bringing the field lens slightly nearer the field
diaphragm, and also by substituting for the plano-convex
a slightly meniscus field lens. With achromatic or ‘ compen¬
sated ’ ocular lenses, there need be no loss of definition from
this, and my own best compensating ‘projection’ eye-pieces
are specially constructed upon this plan.

39. Illuminating Apparatus. — The mode of illumination
will vary, as the object or slide examined is transparent or
opaque. The vast majority are transparent, or examined by
light sent through them from the under side. For a large
number of ordinary subjects the concave side of the mirror
is sufficient ; but for high powers or critical work this is not
sufficient. Formerly a large number of appliances were in
use, called condenser, spot-lens, paraboloid, and many
‘ illuminators.’ The most valuable modern improvement and
simplification of the microscope is to make an improved
‘condenser’ alone, properly mounted, answer all purposes.

Fig. 38 is a section of the condenser in most general use
amongst medical students, known as the Abbe model, and
which when racked up to the under side of the slide, with
a large drop of cedar oil 1 between (else rays above n. a. 1*0
could not emerge, as explained in § 6), will give rays of
n. a. 1-40 at the margin of the cone of light. The chromatic
and spherical aberrations are however enormous, the focus
1 A slip of cover-glass is also used, to fill up the interval.

COMPOUND MICROSCOPE AND ACCESSORIES 75

of the marginal rays being much shorter than of the central
part of the cone ; hence the solid cone of focussed light,
which alone can be used in critical work, does not exceed
n. a. 0‘5. For the simple under-stage tube shown in fig. 29
this condenser is mounted as in fig. 39, to push in from
beneath, with an iris diaphragm underneath which contracts,
on moving a projecting arm, to an aperture as small as
desired, and a flanged circle which swings outwards, to
carry various central stops for manipulating the light. The
condenser mounted either thus, or to fit from above into the
substage as in fig. 40, will cost about 21. 2s. A cheaper form
of two lenses, of n. a. 1*2, will be about 7s. Gd. less. The
best condenser of this form is Powell’s, made on the same

FtG. 38 Fjg. 39

system but of much smaller diameter and focus ; the optical
part alone of this costs 21., and has to be mounted over the
iris and stop mount, which will cost about 25s. besides.

A much better condenser is the achromatic Abbe form,
shown in fig. 40 mounted over the iris diaphragm and stop-
carrier, and so as to fit from above into substage, but it can
also be had reversed as in fig. 39. This gives an n. a. of P0
only ; that is, the largest cone of light which can emerge
from ‘ dry ’ lenses ; but the ‘ solid ’ cone available for critical
work (see § 53) is 0-65 as made in Germany. Some English
ones are rather better, and I met with one giving nearly
n. a. O' 70 of solid cone. The cost, with mount and a set of
stops complete, will be about 31. 15s. This condenser does

76

THE MICROSCOPE

excellent work all round, and also in photography, and is
probably the most generally useful.

Condensers giving far higher aplanatic cones of light are
made by the great English opticians, but are only needed for
high-class work, with the best objectives. Powell’s dry apo-
chromatic gives nearly 0'9 of aplanatic cone, but the optical
part alone costs 6/., while his dry achromatic (mounted) is 7 l.
Messrs. Swift make an excellent dry apochromatic for 4 1. 10s.,
without the iris, and an achromatic at 3 l. This latter, with

iris and stop mount,
will cost about the
same as the Abbe
achromatic, but
gives a far larger
aplanatic cone for
use with high
powers. Messrs.
Beck also make a
good achromatic
condenser of about
the same figures ;
and Messrs. Watson are working out an apochromatic whilst

these pages are in the press.

No expensive condenser will be needed at first to utilise
the substage. Any lens of suitable size and focus, mounted
in a plain wooden ring with a flange, which can be dropped
into the substage, and focussed on the object so that the
image of the flame is focussed by the objective together with
the object, will act as a condenser, and on all but coarse objects
improve the image. One of the achromatic Steinheil lenses
shown in fig. 21, thus mounted and used, will make an
excellent condenser of moderate angle, sufficient for all early
work, up to a J-inch objective. Various condensers of
moderate angles have long been sold under various forms, as
Webster’s, Gillett’s, &c. If such a one is attainable at a low

COMPOUND MICROSCOPE AND ACCESSORIES 77

price, it is not to be despised ; but for purchase as new
apparatus, those above named are to be preferred.

The forms known as a ‘ spot-lens ’ and * paraboloid,’ for
exhibiting transparent objects as if self-luminous on a dark
field of view, are referred to in the next chapter. A really
good condenser, however, does for all.

Opaque objects have to be illuminated quite differently ;
the necessary appliances will also be best dealt with when
treating of their management.

40. Polarising Apparatus.— It is impossible here to ex¬
plain in detail the nature and effects of polarised light. It
must suffice to say that the vibrations in light-waves are across

Fig. 41

A

' ! 1 1 1 1 , 1 1 1 1 ;
!'• .

! 1 ! 1 1 ! ,' 1 ! I

M 'J1 i'i1,1
:;;!!! !!!!

i'll'!;"1

i ' ! ' i 1 1 1 1 ;

| |! 1 1 1 | 1 | 1
hi 1 1 1 1 1 1 1 1

hi 1 1 1 1 ' 1 1 1

1 1 1 ! 1 1 1 1 ! 1 1 1

1 1 1 'ii'

!! l iiljl!!

! ' 1 1 ' i ' ! ! ' ' '

1 1 1 1 1 1 1 1 1 1 1 1

"H"l • I | 1 1 1 f •

1 1 1 1 1 l i i 1 1 | 1

1 1 1 1 1 I 1 1 1 l 1 1
l'1'l 1 " 1 1
I'll , "||

1 1 1 1 " 1 1 1

1 1 1 1 1 1 1 i l i i i

B

Fig. 42

the line of the ray, as when a stretched cord is vibrated.
In common light this vibration is in all planes across the
ray, so that in a beam of light coming through a pane of
homogeneous substance, like glass, the vibrations may be
considered as in fig. 41. But if the substance be con¬
sidered to have a decided grain , as in fig. 42, like a deal
board, or to be unequally elastic in different directions,
like many crystals (not all), then these vibrations are all
‘ resolved ’ into the two directions at right angles , a b, c d,
(fig. 42), which are the directions of greatest and least
elasticity. In a single thin film of the crystal we can see

73

THE MICROSCOPE

nothing of this, but it is so ; 1 and as the two parts of the
beam of light, now termed plane-polarised (for the vibra¬
tions can be by more complicated arrangements also
polarised in a circular or elliptical path), are differently re¬
fracted, or the same crystal has different indices of refrac¬
tion (§ 5) for the two polarised rays, consequently the
two parts are ‘ totally reflected ’ at different angles. Hence,
by cutting a piece of Iceland spar in two at a particular
angle and joining together again, one of the two polarised
pencils can be totally reflected to one side, while the other
is transmitted, and we now get, through the ‘ Nicol prism ’
as it is called, only half the beam of light, polarised only
in one plane. Still we see no difference except loss of
half the brightness ; but if, holding one Nicol prism against
the sky, we look at it through a second which we turn
round, we find when the two are ‘ crossed ’ that we can¬
not see through at all : the two crossed are as opaque to
light as a brick wall, though both are as clear as glass !
The beam transmitted by the first prism, is the one
totally reflected out at the side by the second. At inter¬
mediate angles, part of each is transmitted.

Such a pair of Nicol prisms constitutes ‘polarising appa¬
ratus.’ One is mounted so as to go under the stage, either
in a plain fitting or in the substage, and so that it can be
rotated. The second one has to be somewhere above the
slide. It is by some preferred mounted so as to be inserted,
by a slot in the tube, over the objective, so that it can be
rotated by a milled ring ; by others it is preferred above the
eye-piece. Both plans have advantages and disadvantages.

Stopping the light, by crossing the top prism, is only
interesting. But if between the prisms we place a thin slice
(such as most micro-slides or objects consist of) of any trans-

1 In a thick piece of Iceland spar we can see the two beams, (doubly
refracted as immediately explained) by a spot on paper appearing double when
looked at through the spar.

COMPOUND MICROSCOPE AND ACCESSORIES 79

parent substance which is doubly refracting (because more
elastic in some directions than in others), there is an ad¬
ditional splitting up of the light, which produces appearances
both beautiful and useful. The plane-polarised vibrations
which pass the first Nicol, are now split up by the object
into two, nearly equal if the two planes of vibration in the
object are at angles of 45° with the plane passing the Nicol.
These are differently refracted, one being more retarded than
the other in passing through the slide, but not visibly
separated as in a piece of Iceland spar, because the object
is so thin. Each of these two is, however, again resolved or
split up by the first piece of spar in the second Nicol, or
‘ analyser ’ as it is called. So that we have now four beams,
two of them vibrating together in each of the two analyser
planes. The two in one of the planes, are totally reflected
out to the side by the cut in the spar ; the other two alone
pass. Now, of these two pencils transmitted, and vibrating
in the same plane, o?ie is retarded more or less behind the
other by the double refraction in the object. The discordance
caused by this retardation causes interference between the
two sets of waves thus forced to vibrate together in the same
plane. The usual consequence of such interference, by re¬
tardation of one of two waves, is colour , as in a soap-bubble,1
sometimes only light and shade. Thus objects which
‘ polarise ’ display either the most gorgeous colours, or strong
contrasts of light and shade, when examined by polarised
light.

When, from excessive thinness, or weak doubly-refract¬
ing power, the phenomena consist of only shades of grey, it
is customary to lay on the stage first a thin film of some
crystal, usually selenite or mica, which itself shows a field
of uniform colour by polarised light. Then a very slight

1 For full detailed treatment of the beautiful phenomena of Interference
and Polarisation of Light, see my work on Light : A Course of Experimental
Optics, 2nd ed. Macmillan & Co.

So

THE MICROSCOPE

effect in the object itself, superposed above this, comes out
more conspicuously on the coloured ground. Simple slides
with one plain sheet of selenite are sufficient for most pur¬
poses, costing a couple of shillings. More complicated
‘ selenite stages,’ which give a wide range of ground-colours,
are prepared at higher cost.

The significance of polarised light lies in the fact, that
nearly everything with any structure, not being homogeneous,
is doubly-refracting. Glass itself becomes so if squeezed in
a vice, the elasticity in one direction being affected ; and by
such compression alone gorgeous colour can be produced.
Hence this appliance brings out or reveals structure, both in
organic substances and in crystals, which in some cases
cannot be so seen in any other way. In the study of rocks
especially, polarised light is simply indispensable ; all the
different fragments of different minerals in such a con¬
glomerate as granite, for instance, being brought out sharply
in various colours. For study of rock-sections special
patterns of polarising microscopes are constructed, always
described in the catalogues as ‘ petrological ’ microscopes ;
and to such catalogues students intending to take up this
branch of study must be referred.

Others add polarising apparatus for the beauty of the
colour-phenomena alone. For such, a pair of prisms and a
couple of selenites will be sufficient, and can be added from
11. upwards, depending chiefly upon size and mounting of
the prisms, Iceland spar being now very scarce and dear.

41. Projecting or Lantern Microscopes.— These are con¬
structed for the purpose of throwing upon a white screen
immensely magnified images of microscopical slides, to be
seen by an audience in the same way as ordinary lantern
slides. At the suggestion of several members of the Royal
Microscopical Society, about 1881 I began several years’
special study and experiment with a view of improving such
instruments (of which nothing efficient was then in exist-

COMPOUND MICROSCOPE AND ACCESSORIES 81

ence) in conjunction with Messrs. Newton & Co. There
were great difficulties, but the ultimate result was so suc¬
cessful, that the instrument thus worked out is now in use
in all the principal scientific institutions in Great Britain,
besides many in America and on the Continent, no other
having as yet proved capable of similar demonstrations.
Details would be of no general service in such a book as this,
but it may be of some use simply to state generally what can
be done. With only the oxy-hydrogen light, such an object
as a flea or the tongue of a blow-fly can be shown crisply
and brightly 12 feet long or more ; and on smaller objects,
such as certain scales of insects, the mouth-organs of a flea,
or the barbed teeth at the end of an insect’s sting, I am in
the habit of using a power of 2,500 diameters, with a 3-
Reichert objective. With the electric light, which is far the
best for this instrument, immersions are easily used, and
5,000 diameters and upwards obtained ; but some practice
and acquaintance with ordinary microscopy is required to do
such powers justice, besides properly constructed compensa¬
ting eye-pieces.

Smaller accessories will be best described in the next
chapter.

CHAPTER Y

MICROSCOPE MANIPULATION

Having got our microscope, according to our pecuniary
means or general purpose, let us now consider how to use it.

42. Care of the Eyes. — This should be borne in mind, as
injury might result from much work under certain conditions.
The causes of injury are either (1) too fierce a glare of light,
or (2) the vicious habit of always using one eye whilst screw-

82

THE MICROSCOPE

ing up the other. Too little light might also strain the eyes,
but rarely occurs.

The light should always be subdued to what is sufficient
and agreeable, either by contracting the iris diaphragm ; or
in some cases turning down the lamp ; or by interposing
coloured glass screens as mentioned presently.

It is much better to use each eye in turn if possible,
keeping the other open ; and this can usually be done after a
little practice, the mincl, as it were, not ‘ seeing ’ what is
before the eye not in use. But if there is difficulty, it can be
overcome by bending a piece of wire into two rings connected
by a straight bit, as in fig. 43, the two rings being the distance
of the eyes apart. One ring is covered with black paper, the
other is made to ‘ spring ’ tightly on the neck or cap of the

eye-piece. The black screen will turn to either side for the
unused eye. Screwing up the eye is most injurious.

A good binocular arrangement is principally valuable for
its relief to the eyes, if expense be no object.

43. Three Great Principles. — From the beginning let a
new student accept on the authority of all microscopists of
experience, and keep present to his mind, three cardinal
principles.

(a) Use no greater power than required by an object.
Every novice has a tendency to magnify as much as possible ;
the experienced worker does the contrary. If we can see all
that is to be seen with a given power, we see it worse, and
not better, by increasing. Of course a certain scale is neces¬
sary, and one observer can see much finer detail than

MICROSCOPE MANIPULATION

33

another ; I am personally heavily handicapped in this
respect. But a beginner who possesses an inch and a J-inch
will learn more and get on faster, if he does not use the
J-inch at all till he has had several weeks’ practice with the
other ; and in miscellaneous study, and even amusement,
more work can be done with inch and half-inch than with
any other powers.

( b ) The eye needs educating as well as the hands. The
microscopist will not at first see half there is to be seen with
a given power, in a great many objects. If it were not so,
an object would only need to be seen, and not to be studied.

(c) As much depends upon proper illumination as upon
having the finest lenses and most sensitive focussing ; in
many cases far more.

44. Care of the Apparatus. — It is tiresome to be packing
and unpacking, if much work is done ; it is better to put the
whole away under a glass shade, with velvet or cloth under
the rim to exclude dust. The instrument itself should be
dusted pretty often (gently and not flapped about) with a
wash-leather. If working paits become stiff from cold or
disuse, the very least touch of vaseline will generally suffice ;
if not, or if anything appears loose, an average worker will
do best to take it to an optician to be put right. When he
has become familiar with every movement, he will probably
be able himself to do this with the adjustments provided ; but
most novices had better let their instruments alone.

In regard to all lenses, take especial care never to touch
a glass surface with the finger, or to breathe on it ; either
leaves a film of grease. If dew condenses on a lens on
taking from the case, or in a warmer place, do not wipe it
off, but pass the lens to and fro in the air to promote evapo¬
ration. An objective out of its box should be stood on its
screwed end, with the front upwards, that no dust may enter
the barrel ; and for the same reason, when objectives are
left on the instrument, an eye-piece should always be left

34

THE MICROSCOPE

on also, that dust may not fall down the tube. Objectives
should hardly ever need attention, but eye-pieces must be
cleansed every now and then from the dust that falls on them,
whenever it begins to appear as specks in the field. Most
microscope opticians will supply a chamois leather, carefully
washed, and then cleaned with methylated spirit from soap
and alkali. This should he cut in pieces and kept in a box,
a piece being used when necessary, for glass only. The front
lens of an objective may be cleaned with the same, and dust
at the back can be removed with a clean camel-hair pencil
cut square at the end, or a piece of the washed leather may be
rolled up tightly and tied with thread, and then cut square
across the roll with a sharp knife, to clean the back of a lens.
If more be needed, the lens should be sent to the optician.

A water-immersion lens is dried with a bit of soft linen,
washed and then soaked free from soap. An oil-immersion
is first nearly cleansed with a dry bit of the same ; then a
piece is moistened with the tongue, and the moist linen will
remove every trace. A handkerchief is often used at a
pinch ; but it is better to keep some cut pieces of clean soft
linen in a collar-box, when such objectives are used. Care
should be taken not to use too much of either immersion
fluid ; a single drop is sufficient. Lenses or slides should
never be put away with oil on them, or it will dry into a
hard varnish, which it is troublesome to remove.

45. Light. — This will be either daylight or a lamp. For
moderate powers the first is most comfortable ; for critical
work with high powers the lamp must be employed, and
often will be a necessity for the only leisure time available.

For daylight work, get near the window, so as to have
the open sky and not an image of window-bars. Direct
sunlight is, of course, to be avoided.

The precise kind of lamp is not material, unless ‘ critical *
work has to be done, or unless photography is in question.
The chief thing is a steady flame. An Argand gas-burner,

MICROSCOPE MA NIP ULA TION

35

I have found, will easily resolve the diatom Amphipleura
Lindheimeri, or give excellent images of bacteria. A shilling
paraffin-oil lamp will do better in higher resolutions, and add
intensity to even the above, using the edge of the flame for
such work. A half-inch wick
should be chosen. But for
steady work and dense and
crisp images, a proper micro¬
scope lamp is certainly better,
and, for the highest type of
demonstration, indispensable.

Fig. 44 shows one of Baker’s,
sold for 17s. 6<A, or in pine
case for 21s., very similar
ones being supplied by other
opticians. The points about
lamps of this type are, that
the cistern is shallow , so that
the flame can be lowered near
the table to be focussed direct
(i.e. without using the mirror)
into the condenser of an in¬
clined microscope ; and,
secondly, that the common
glass chimney is replaced by
one of sheet iron, in the side
of which slides loosely a flat
piece of glass, either white or

° Fig. 44

coloured. Through this flat

glass, easily replaced if cracked, or toned to any colour,
there is no distortion, and we have a sharper image of the
edge of the flame. A metal reservoir is often used to bring
the flame still lower, and rackwork to raise and lower it, and
other refinements may be added ; but a lamp as figured will
answer all usual purposes well.

86

THE MICROSCOPE

46. Bull’s-eye Condenser. — To avoid confusion with the
substage condenser, or ‘ condenser,’ or ‘ illuminator,’ this is
now generally called the ‘ bull’s-eye.’ It is a thick plano¬
convex lens, nearly or quite hemispherical, mounted with a

universal joint on a sliding pillar,
somewhat as in fig. 45 ; but there
are many patterns. It is better
not so thick, i.e. less than a hemi¬
sphere, there being enormous
aberration at the edge of a hemi¬
spherical lens. To diminish this
a more expensive form is some¬
times used, comprising two or
even three lenses.

The bull’s-eye is used chiefly
for two purposes. One is to
focus the light of the lamp or of
a window (in this case some dis¬
tance from the window is better)
upon a solid object to be examined
by reflected light. The more usual
purpose is to obtain from the edge
of the lamp-flame a nearly parallel
beam of light about equal to the
diameter of the bull’s-eye and of
the mirror of the microscope. It
will be seen from fig. 6, p. 17,
that this can be done by adjusting
the bull’s-eye at its focal distance
from the flame. The flat side is
turned towards the flame, and by
covering the mirror with a piece of card or paper, the lens
is easily adjusted so that a nearly parallel beam falls upon
and fills the space occupied by the mirror. It is used in
the same way when it is desired to throw a parallel beam
of rays direct into the sub-stage condenser.

i? l(i. 45

MICROSCOPE MANIP ULA TION

37

47. Arrangements for Work. — The usual height of a
table is convenient for most people, and enables one to look
straight down a small-model instrument, when used upright
and the stage flat, which is often necessary for ‘ wet ’ work. It
may be rather low for the more comfortable inclined position,
in which case a little wooden stage should be provided, like
the lid of a box, of the convenient height ; this will also be
convenient for using light direct from a lamp on the table.
A little bead round the edge of this, to prevent the foot of the
microscope from being pushed off, is as well. When a lamp
is in use, unless focussed direct, it is almost always most
convenient to place it a foot or more to the left side ; and if
the chimney itself does not do so, all direct light from it
should be screened from the eyes, and (unless wanted there)
from the upper side of the stage.

For examination by several people, it is convenient to
have a flat tray with baize on both sides, just large enough
for the microscope to stand on with a small lamp in one
corner. Then when an object is adjusted, the tray with all
upon it can be pushed round to each person without dis¬
turbance.

48. Elementary Manipulation. — We will now suppose the
simple Society of Arts microscope of fig. 27, with nothing
but the mirror and wheel of apertures, with the inch (or
lower power) screwed on, and some ordinary transparent
slide on the stage, such as the proboscis of a blow-fly. How
shall we attack it ?

If near the window by daylight, with free sky or cloud
illumination, the proper plan will be, after first inclining the
instrument comfortably, to adjust the plane (flat) side of the
mirror so as to throw the light straight up the optic axis.
There is no particular focus to the mirror so used ; but take
the average distance. The sliding ledge on the stage being
placed aright, get the object in the centre, and focus the
objective with the coarse adjustment only; the fine adjust¬
ment should from the first be ‘ saved ’ as much as possible.

88

THE MICROSCOPE

Remember another point from the first. In working the
milled heads of either the coarse or fine adjustment, or of a
racked substage, take care to balance the pressure between
finger and thumb. Never turn the head by bearing on one
side, as one sees many novices do ; this would soon ruin any
movement. Avoid also downward pressure of the whole
hand, either on the tube or the stage, studying from the first
a controlled, delicate, sensitive, balanced handling, free from
force, or any shaking when the hand is removed. This
delicate touch is essential to fine microscopy , and though it
only developes by practice, and needs a good instrument to
fully bring it out, it should be cultivated on the simplest stand
from the first. There are many who can focus a high power
with the coarse adjustment, more accurately than some do
with the fine.

With daylight, the plane mirror will generally be about
right as regards amount of light. If there is too much, try
the smallest aperture in the wheel which fully embraces the
object ; if still too bright, a piece of white card laid on the
mirror (preserving the same angle) will diffuse and subdue
the light still more. On a dull day light is more likely to be
deficient, in which case use the concave mirror. This latter,
however, must be used at about its focal distance, which will
be known nearly enough by its position when used with
lamp and bull’s-eye. If the light be now too great (there
should be nice, soft, grateful light, enough to show easily
what is sought, but no more), use the means mentioned on
the next page. Never endure a disagreeable glare.

Suppose next that lamp-light is employed. For low
powers, the flat of the flame should be turned towards the
instrument ; for the J-inch the edge will be better. The
image of the flame should be focussed on the slide by
the concave mirror, so that it is seen by the microscope in
the same plane as the object, except with the inch and lower
powers, when better effect is sometimes produced by bring-

MICE 0 SCOPE MA NIP ULA TION

89

ing the mirror higher up, so that it focusses rather beyond
the front of the low-power objective. There may very pro¬
bably be excess of light from the lamp. This can be toned
down by a blue glass shade, or using a smaller diaphragm ;
for often it is rather glare in the outer ‘ field,’ than in the
object itself, which tries the eyes.

But the pleasantest method of subduing the light is to
have a circular piece of finely ground glass cut to drop loosely
in the stage aperture, upon the diaphragm plate just below,
so as to lie slightly below the stage level. If a piece each be
provided of plain, light blue, darker blue and neutral-tint
glass, all the better, or the same effects can be got from one
plain ground glass and a piece of coloured gelatine laid
underneath. The light from the concave mirror sent through
such a glass, suitably toned, will be found most grateful, and
to give the sharpest image any ordinary objective admits of.
It adds great comfort and pleasure to a vast range of work
with the microscope. Even a piece of tracing-paper, dropped
in the stage aperture, will give a great deal of the same
effect, if nothing better be at hand. For the use of ground
glass with higher apertures see § 54.

We will take next the j-inch power. With this it
will be best to use the bull’s-eye stand condenser to throw
a parallel beam of light upon the concave mirror, which
must be carefully adjusted to focus the edge of the flame
upon the slide. Then bring underneath the diaphragm-
aperture which gives the best effect. Finally, with many
objects sharper relief will be obtained by tilting the mirror
a little, so as to get a slight obliquity of the light ; but what
is called ‘ central ’ light is to be preferred as a general rule.
The direct cone of light from the mirror will sometimes be
best with a good lens ; but the performance (especially of
cheap quarters) is generally improved by the ground-glass
method just mentioned. Even with good lenses, many
classes of objects are better shown this way than in any

90

THE MICROSCOPE

other, except with a first-class condenser ; but with |-inch
powers and upwards it is best to place the ground surface
of a slip of glass in contact with the bottom of the slide itself.

49. Precautions with High Powers.— It is only too easy
for the novice, especially if he hurries to use his ^-inch
at the beginning, to rack down a high power upon the
slide, cracking the latter and perhaps injuring the lens.
Even a cheap quarter will not be more than the thickness
of a thick card from the cover-glass, and a better power will
he nearer still. But there need not be the least danger.
This is one chief merit of the sliding-bar shown on the stage
in figs. 25, 32. It can be seen by the eye when a part of
the slide is in the centre of the stage. Then with the nail of
the left forefinger lift the front or top edge of the slide about
the tenth of an inch. The objective can now be boldly but
steadily racked down, and will be felt the instant it touches
the slide thus slanted towards it. Moving the slide a little
to and fro, some portion of the object will be found in the
field, if only as a nebulous mass ; and the slide, being gently
let down flat upon the stage, is readily focussed without
danger. The same procedure is followed on a mechanical
stage, which has usually a free ledge for the purpose. It is
to facilitate this use of the forefinger that the horseshoe
stage (fig. 25) is preferred by some ; but there is no real
difficulty in lifting by the finger-nail on a solid stage. It is
more difficult when the slide is pressed down by spring clips,
as in fig. 28 ; but with a little careful practice at first it can
be done even then. I, however, most strongly advise the
sliding ledge for all plain stages, as an enormous facility and
comfort in general work ; while the spring clips on it, though
usually turned aside, can he brought into play whenever
required.

50. Searching a Slide. — In this, too, the sliding bar or
ledge is of the greatest use. A slide often has many points
of interest, which are by no means easy to find ‘ at random ’

MICR O SCO PE MANIP ULA TI ON

9i

under a high power. With the sliding ledge it can be gone
over as systematically as with a mechanical stage. Arranged
first so that the top or bottom of the preparation is in the
field of the lens, the slide is gently pushed, so as for the
subject to pass completely from side to side. Then the ledge
is pushed as much farther up or down the stage as will just
bring another area the size of the field under the lens, and
this stripe of the subject is again all passed under review,
and so on ; at the end all has been gone over, and nothing
can be missed. If the stage and bar be ‘ divided ’ by lines
(as in fig. 32), the situation of any point of interest in a slide,
for that microscope , may be marked on the label by two
figures, denoting the divisions, to which if the bar and end
of the slide be again brought, the point sought will be in the
field.

51. Nose-piece. — This appliance is of the greatest use
for finding objects to be examined by ^-inch or higher
powers, or for rapidly exchanging
the power ; in fact, to the real
worker (i.e. who uses his micro¬
scope to cxa?ninc objects and not
to exhibit his skill in producing
superlative images of certain

selected subjects) it is indispensable. The double nose-
piece (fig. 46) is the most useful, and will probably cost
15s. A triple one costs, say, 30s., but three lenses are
rather heavy and rather in the way. The double one is
shown on an instrument in fig. 32, and it will be seen at
once how, the object being found and centred under the
lower power, the higher can be rotated into place instan¬
taneously. It is only necessary, when first using a nose-
piece, to notice carefully how much (if any) the power should
be racked back from the slide to make the movement safely.
Years ago nose-pieces were very unsatisfactory, but are now
made of very great accuracy and durability.

92

THE MICROSCOPE

Messrs. Baker have just introduced another and single
nose-piece of great value. The female screw is divided into
three jaws, worked by a small side-screw just like a centering
chuck. When open, the objective pushes straight in up to
the collar : then a turn of the side-screw grips the thread
in an instant, and the least fractional turn of the objective
makes all tight. It appears to work exceedingly well, and
is sold for 125. 6d.

52. Use of the Condenser. — The reader will however, we
hope, have an instrument with at least an under-stage
fitting, or perhaps a substage, and somewhat better objectives
than were considered in the preceding paragraphs. Even
with the plain tube-fitting a condenser may be fitted, more
or less like fig. 39. The condenser, or a plain lens or two,
should be fitted to slide in the tube without great force, but
so as not to fall down. The Abbe form is usually fitted so
as to be in focus when pushed full up. A piece or pieces of
finely ground glass, as already mentioned, may also be
mounted so as to be capable of pushing up close to the
slides for low-power work ; the great comfort of this I know
from an experience of years, since adopting it under the
personal advice of the late Dr. Carpenter. With a proper sub¬
stage there will be no difficulty ; and, as already mentioned,
any lens may be mounted for this in a plain boxwood
flanged ring, until means permit of a regular condenser
being procured. (Concerning this see § 39.)

Let us understand the principle of the substage con¬
denser, which appliance we owe entirely to English micro-
scopists and opticians. We have seen (§ 8) that objects
are seen or imaged by diverging cones of rays proceeding
from every point in them, which are re-converged to
corresponding points in the image. In objects seen by
reflected light, or self-luminous, we have nothing beyond to
consider; but in the case of transparent objects like most
microscopic slides, examined by rays of light sent through

MICROSCOPE MANIPULATION

93

them, most of which pass on straight after traversing them,
it is different. The rays are now focussed by the objective
in two characters : (a) as image rays proceeding from the
object ; and (b) as original luminous rays from the lamp.
The two must always have independent foci. The source of
light (when used without mirror or condenser) is focussed
near the back of the objective, while the object is focussed
in the field of the eye-piece. This may not necessarily
affect definition. But let us now suppose the rays from the
lamp-flame focussed by mirror or condenser to a point a
quarter-inch below the slide. As luminous rays, the rays
then cross and diverge again in cones from that point ; while
as imag e-ioiming rays from the object, they diverge from it
a quarter-inch higher up. It will be seen in a moment that
such violent ‘ crossing ’ of the rays in these two characters
must blur the image and ‘ flood ’ it with confused light. It
is therefore necessary to make light-rays ancl image-rays
coincide.

One way is to use the ground glass already mentioned.
This is strictly correct and scientific ; for the ground surface
so ‘ scatters ’ the light, that there are no definite cones of
luminous rays to be focussed as such, but the infinitely
broken-up rays diverge from every point in the object, with
no others to overpower them (§ 54).

A second way, with the condenser, is to use this as in an
optical lantern, so that rays from it (or from the concave
mirror in the same way) converge to a point beyond the
objective. The rays then never cross in their two characters ;
every ray sent through the object goes from it direct to some
point in the objective, and we get a good image. This method
is necessarily confined to low or moderate powers.

The third way is to focus an image of the light upon the
object. The converging cones then become diverging cones,
diverging from the same point in both characters ; and in this
way we get both the whole illumination and good definition

94

THE MICROSCOPE

with very high powers. This is the principal function of the
substage condenser.

There are two methods of work, each of wdiich may be
carried out either direct or with the plane mirror. The direct
method is simplest and easiest ; the use of the mirror is more
troublesome, but is often more convenient.

For direct work, the lamp is adjusted as low on the table
as possible (unless the microscope tube for some reason is
wanted to be horizontal, or nearly so), and about 10 inches
usually (precise distance not material) from the back of the
condenser, using the flat of the flame for larger objects and
more moderate powers, and the edge for anything special
with high powers. The flame must be in the centre, or
optic axis ; and the easiest way to get this, at first, is to look
through the empty tube, with nothing whatever between eye
and flame. With a very little experience it will be easily
done with all the lenses in place ; but it is well for the learner
to get from the first accustomed to the proper results of true
central rays. It is very easy, with the rays at a small angle,
to bring the image of the flame back into the centre of the field
by the centering screws of the substage, and for some purposes
this may be even better, but such will be slightly oblique
illumination : at present the light is to be central. Then the
condenser is to be racked up to focus on the object, itself
focussed and centered on the stage, with the low eye-piece,
so as to show the flame in a large field. Then the flame
image is brought into centre by the substage centering screws.

Should the plane mirror be used, see that its centre is
in the optic axis, and then bring the reflection into the centre
of the open tube as before. It is the plane mirror which is
always to be used with the condenser.

The other way is to interpose the bull’s-eye, and thus to
supply the condenser with a beam of parallel rays. The
plane side of the bull’s-eye is turned to the lamp, and care
must be taken that the lamp, bull’s-eye, sub-condenser, and

MICROSCOPE MANIPULATION

95

objective are all in one line, and the bull’s-eye truly at right
angles to this line. A piece of paper or card held across the
bottom of the substage will show if a beam of fairly parallel
rays is obtained, and the bull’s-eye must be adjusted till this
is the case. Then the object is brought into view and
focussed ; next the sub-condenser is focussed on the slide,
and finally the centering screws, or perhaps a little move¬
ment of the lamp or bull’s-eye, will centre the image of the
flame. With the plane mirror there may be more trouble,
but a little patience will soon effect the centering.

Working with parallel rays, the focus of the sub-condenser
is somewhat shortened, and the angle of the outside of the
cone increased. The illumination is also largely increased,
and is too great except for high powers. The definition is
not so good in delicate cases, owing to the uncompensated
aberrations of the stand condenser.

53. The Illuminating Cone. — We have next to attend to
the cone of light from the condenser. Suppose we have a
quarter objective of 100° air-angle, or n. a. O’ 77, and are
using the ‘ achromatic Abbe ’ condenser of n. a. 1*0. That
means an outside illuminating cone of 180° in air, only
obtained, however, when the condenser is in actual contact
with the slide or has no focal distance. Therefore, no dry
condenser can be aplanatic up to that angle, but the central
rays will focus the flame on the slide before the angle 180°
is reached. Place a dry ‘ test ’ slide of P. angulatum on the
stage, and focus on it first the objective and then the con¬
denser, centering the edge of the flame, and with the iris
diaphragm closed to a small cone. Push aside the object and
remove the eye-piece, and look down the open tube at the
back of the objective ; the iris aperture will show as a bright
circle in the centre of the back lens (fig. 47). Open the iris
and the bright circle will increase (fig. 48). This circle,
when increased as much as will remain unbroken, will give
the aplanatic cone of the condenser, and in this case should

96

THE MICROSCOPE

fill beyond three-fourths the diameter of the back of such an
objective. In some cases the unbroken disc will be smaller,
but may be increased a little by racking up the condenser ;
such enlargement will be a solid cone, but is not a truly
corrected aplanatic cone. Racking up further enlarges the
outer circle of light, which will enlarge till it far surpasses
the back of the 100° angle lens, but in doing so the bright
disc ‘ breaks,’ as in fig. 49, and only the largest unbroken disc
of light at the back of the objective, as fig. 48, can be used
for critical work. In this particular condenser it will be
about n. a. '65, or an air-angle of 80°. If we were using an
objective of only that aperture, its back would be just filled
by such a cone, as in fig. 50. This is known as a full cone,

and any greater angle than this cannot enter the objective
at all to any good purpose, but can only blur the image by
possible false reflection from the inside of the brass mount.1

In the present state of microscope optics, there is hardly
a lens made that will bear a ‘ full ’ direct cone, even with a
first-rate condenser : scarcely one will bear a cone over three-
fourths of the ‘ aperture,’ and heaps of really good lenses will
only stand two-thirds. The better the lens and the condenser,
the more cone can be borne with advantage, provided the
object itself will also bear it ; but most objects will not. This
depends chiefly upon thinness and opacity. With very trans¬
parent objects the cone may have to be cut down more and
more, simply in order that the details may give (by their

1 The full elucidation of these details of the cone is mainly due to Mr.
E. M. Nelson.

MICROSCOPE MANIPULATION

97

refractive differences) sharper shadows , which can be
imaged. By the smaller cone we get this necessary light
and shade, but at the same time we more or less thicken and
blur the image, or get diffraction-fringes round it.

Thus the microscopist has to employ different expedients,
and sometimes to put up with one imperfection in order to
fight another which is a still greater difficulty, and so on.
This applies chiefly to higher problems, some of which have
to be attacked in various ways before a result can be relied
upon. It will suffice for most present readers to make some
general study of the ‘ cone ’ as seen at the back of his ob¬
jectives when the eye-piece is removed, and gradually aim,
by management of the illumination, at the best images,
especially under high powers, which his apparatus admits of.

54. Full Cones from Ground Glass. — With good objectives,
on all but thick or very transparent objects practically full
cones may be employed, without the ‘ glare ’ of a full direct
cone, by the use of ground glass combined with the con¬
denser. The rays, focussed in this case upon the ground
surface, are by it so scattered in every direction as to produce
full cones from every point in the object, while there are no
glaring oblique rays to blur the effect. Also there are no
diffraction-fringes or spectra. Thus it was that, with moderate
power, the fine hairs of the blowfly’s proboscis were shown
as fine at the tips as the lens was capable of (p. 89). Laying
now a ground-glass slip immediately under a test slide of
P. angulatum, with any J to ^ dry lens good enough to bear
Zeiss’s highest compensating ocular, the diatom gives either
black or white dot (§ 60) with precision and ease. By grind¬
ing the back of the slide itself, rays enter the glass in cones
far above n. a. 1-0, and (with a Reichert oil-immersion of
L30 and Zeiss x 27 ocular) Ampldpleura Lindheimeri can
also be dotted black or white quickly and certainly, with
only an Argand gas-burner as illuminant. And bacteria ex¬
hibit beautifully.

Q

98

THE MICROSCOPE

Such practical results cannot be against any sound
optical theory, and their facility makes them advantageous
in a great deal of work.1 At the same time, owing to imper¬
fections still remaining in the outer zones of even the best
lenses, and to the fact that many objects cannot yield light
and shade with full cones (§ 53), the carefully adjusted
direct cone from the condenser will generally yield the best
results in very critical problems.

55. Oblique Illumination.- — On racking up, a high-angled
condenser gives its widest cone in a bright ring of light,
the central pencil being of longer focus, and the rest worse
than lost through spherical aberration. By placing various
‘ stops ’ in the fitting beneath, either the outer ring, and
that alone, may be transmitted, or one or more pencils of
light at extreme obliquity. This is often useful in two ways.

( a ) When rays are sent obliquely across fine lines or
striations in a very transparent object, the shadows are so
increased as to give much more effect of light and shade in
the image.

(5) When the object is so transparent that only a small
angle or cone of light will give sufficient light and shade, this
cone may not fill sufficient aperture of the objective to give
‘ resolution ’ (§ 22). In this case we are dependent upon the
diffraction spectra caused by the fine structure, to occupy and
so use that aperture. But very fine striation may throw
even the nearest pair of spectra from a central pencil quite
outside the objective ; if so, by using instead an oblique
pencil, this pencil and one diffraction spectrum may come
within the objective, and thereby using the outer portions of
the aperture, give us resolution.

1 I was led to these results with ground glass used for full cones, in the
course of a theoretical and experimental investigation of Abbe’s Diffraction
Theory (§ 23). Ground glass had been personally recommended to me years
before for lower powers by the late Dr. Carpenter, but I do not think anyone
had previously tested the results with high apertures, and especially with
immersion lenses,

MICROSCOPE MANIPULATION

99

Generally both modes of action come into play ; and
very often quite a slight degree of obliquity will marvellously
add to the resolving power. With an ordinary \ objective,
place a dry P. cingulatum on the stage, and contract the cone
with the iris till resolution just fails. The slightest tilting
of the mirror will generally show lines. With an immersion
lens the same may be easily seen with A. Lindheimeri. The
resolution of A. pellucicla in balsam is most difficult to effect
with any other than a strong and very oblique pencil from an
oil-immersion condenser and an oil-immersion objective. It
is not alone the fineness of the striation — 92,000 per inch— as
ruled lines on glass are resolved far more easily ; it is the fact
that the striae are in clear transparent silex, and such phan¬
tom-like differences require artificial relief. No one need
attempt this diatom in balsam till after long experience with
the microscope.

But while smaller cones, and also oblique rays, are
often necessary for the relief or contrast needful for resolu¬
tion, this is never to be confounded with real truth of image.
On the very contrary. Those black and white lines which
a practised microscopist shows on Ampliipleura pellucida,
after infinite skill and pains, are utterly unlike the reality.
He has produced by artificial shadow-intensification of
almost phantom striae, something he can see. A first-rate
manipulator like Mr. Nelson can also produce its resolution
with a central wide cone. The striation thus seen is far
weaker, and by comparison nebulous and indistinct ; hence it
is often called ‘ inferior ’ resolution. But these very points
make it probably a far truer image of the reality than the
other.1

Fig. 51 represents some of the various stops used with a
condenser for oblique illumination when used to give

1 These remarks refer to ordinary mounts. One of Dr. Van Heurck’s
mounts in arsenic (refractive index 2-0 or over) resolves easily, the great
difference between the arsenic and the silex showing up the marking strongly.

100

THE MICROSCOPE

either relief , or resolution. The plain central stops shown in
fig. 54 are often used, giving oblique rays all round the axis ;
but the transmission of a central pencil as well, as in fig.
51, a, often strengthens the resolution enormously ; hence it

A B C D

Fig. 51. — Stops for Condenser

is better to have most of the central stops pierced, with one
central solid one which will cover the central pencil when
required. The stop b is largely used when one strong
oblique pencil is needed, as in resolving A. pellucida: and d
is another form which gives a single oblique pencil of greater

width but less depth. The c
stop gives two strong resolu¬
tions at right angles ; and three
similar notches equi-distant are
also used.

56. Dark-ground Illumina¬
tion. — This is another important
use of the condenser, and is a
very beautiful mode of showing
suitable objects. The principle
is best shown by the sjpot-lens
(fig. 52), which was formerly
used for low powers, and is
often used still. It is a large
condensing lens with central opaque spots ab, ab, ground out
and blackened on front and back. Parallel rays of light n r
being focussed on the object o, it is obvious that the
central cone a o b is destitute of rays. A similar cone from

i ' i > i i i i i

RRRR RRRRR

Fig. 52

MICROSCOPE MANIPULATION

IOI

the object after crossing at o is also destitute of direct rays,
and if this dark cone is as wide as the front of the objective
lens at l, no rays can pass into this lens through a plain
slip of glass : there is a ‘dark field.’ But the object is further
illuminated by the outer zones of rays c oa, bod. These
rays would not of course pass direct into the objective ; but
the details of the object scatter and reflect them to a great
extent in all directions, and by these scattered rays the object,
if adapted for this method, appears as if self-luminous on
the black ground. The small silicious shells known as
Polycystma ( see Frontispiece) are best seen in this way; and
some delicate details, such as the infinitely thin setae of
Floscules, can scarcely be displayed in any other.

It will be seen that a single lens can only suffice for
objectives of moderate angle : for higher angles it was usual
to employ Wenham’s paraboloid (fig. 53).

In this case parallel rays are not refracted
as in a lens, but ‘ totally reflected ’ from
the internal surface of a parabolic cone
of glass p, the truncated apex of which
is ground and polished into a hollow
semi-sphere, so that all the emerging
rays pass through this at right angles.

A central stop is mounted on a wire, which slides up or
down through a hole drilled along the centre of the cone,
and as this is pushed up towards the object it cuts off
more and more of the central rays, or gives more of dark
field, for a wider-angled objective. A good paraboloid will
work up to an objective of £ or even -5. Other devices
have been employed. But a good condenser answers
most purposes,1 inserting into the ring underneath it central

1 After careful comparative trials I do not consider it in all respects
equal to a carefully worked paraboloid. Of the marginal zone of rays which
alone are used in either case, much is lost in the condenser by reflection at the
high angles. With the paraboloid, the same zone, being totally reflected,
suffers no loss.

Fig. 53

102

THE MICROSCOPE

1 stops ’ of various diameters (fig. 54) which can be cut out
of blackened card if any special size be required. The con¬
denser so modified becomes a compound spot-lens of high
angle.

Suppose we have a few Polycystina to be shown in this
way, we should proceed as follows. Arrange and focus the
object in the ordinary manner, taking special care that lamp,
condenser, object, and objective are all truly centered; and
the same with stand-condenser if it is used. (The stand
condenser will give a larger black field, and more light.) It
only now remains to put in the smallest stop that seems
likely to suffice, and rack the condenser up or down till the
best effect is obtained ; if that stop does not suffice, a larger

Fig. 54. — Stops for Dark Ground

one must be tried, the size of stop increasing with the angle
of the objective. It is very difficult to get a dark ground
with an objective over n. a. '65 or air-angle 80°, but powers of
more than this can be used by stopping down their back
lens. That angle will, however, do all that any reader of
this book is likely to require.

With low-power objectives, very fair dark-ground illumi¬
nation can be had by simply swinging the concave mirror far
enough to one side to fling all direct rays outside of the
objective. These oblique rays act in exactly the same way,
with the sole difference that, as they all have the same general
direction, they only throw into very strong relief any striae or
lines or rows of dots at right angles to that direction. This
may be useful, but such very partial effect has to be allowed

MICROSCOPE MANIP ELATION

103

for, and the interpretation corrected by turning round the
object or rotating the stage.

57. Opaque Illumination. — A great many objects are best
observed in this way, i.e. by reflected light. There are
various methods available.

The simplest is to turn the microscope away from the
window or lamp, and then to focus rays upon the object
obliquely by the stand-condenser ; or a bull’s-eye condenser
is often mounted so as to attach to the stage, or limb, of
the instrument itself. Often no condenser is needed, but
the direct light alone from behind the observer may be

Fig. 55. — Side-reflector

sufficient. In most cases care should be taken that no light
is thrown up through the slide from beneath.

The most general appliance used when more light is
required is the parabolic side-reflector shown in fig. 55. It
will be readily seen how the two ball- joints allow of adjust¬
ment : the short stem fits into a hole in the stage or limb.
The lamp being placed at one side, the rays are collected by
the reflector and focussed strongly upon the object, obliquely
from one side.

Another method, now too much neglected, is the Lieber-
kilhn reflector, fitted so as entirely to surround the objective.
The action will be seen from fig. 56, where the parallel rays
from beneath the stage (parallelised by the stand condenser,
and reflected by the plane mirror) are focussed by the

104

THE MICROSCOPE

reflector c (here represented as screwed on the objective b)
upon the object d. This object is here shown as held in a
stage forceps /. No light should pass through the object;

by mounting it upon an opaque

and this is often secured
black disc on the slide.

These are generally much too
large : the disc should stop
any rays from entering the
lens, but as few other rays as
possible. A transparent slide
may have a disc of black paper
temporarily fixed at the back ;

cr a better plan (necessary for objects held in forceps as in
the figure) is to have one or two stops e of different sizes
(often called ‘dark wells’) mounted in the substage.

By having the barrels of several lenses the same size, a
Lieberkuhn made as fig. 57 may be fitted to slide on them,
and be thus ‘ focussed ’ on a slide so as to work with all. Of
course this is only possible within certain limits of focus.

58. Polarised Light. — Very few words are needed under
this head ; with the polariser and analyser in place, any
object on the stage is at once seen ‘ by polarised light.’ It
is only necessary to say that the best effect is always with
the prisms crossed, giving (apart from the object) the ‘dark
field.’ The position on the stage will, however, affect the
phenomena remarkably. It is best not to use a selenite
beneath, unless it is needed ; but if the effect is very faint,
though visible, the selenite will often bring it out much
more prominently.

MICROSCOPE MANIPULATION

105

59. Combination of Methods. — Many objects can only be
understood after examination in different ways. A large
diatom, for instance, should be examined in the ordinary
way, again with dark ground illumination, and also (if
possible) with the binocular arrangement. Every one of
these methods will add to our conception of the beautiful
object. Two methods may also be combined at once.
Thus, an object may yield further information when illu¬
minated as opaque, yet with some amount of transmitted
light as well. Dark ground illumination with polarised
light, again, produces sometimes very striking effects. Mr.
Nelson has pointed out the remarkable fact, that when a
difficult diatom like A. pellucida is resolved, placing the
analysing Nicol over the eye-piece in most cases appreci¬
ably intensifies the resolution. Some of these peculiar
effects can hardly be said to be fully understood.

60. A Critical Image. — This is a very expressive term
invented by Mr. Nelson to describe a sharp, perfect image of
minute structure ; and every micro scopist should try as soon
as possible to educate himself to distinguish between such a
one, only possible with good lenses and large cones, and the
coarser images from inferior lenses and narrow cones. If
he possesses good lenses, his own will teach him ; if he
only has the cheap J sold with a three-guinea microscope,
he should still try to learn this by the aid of some friend
who possesses better. Two objects will suffice to mention
here : a blow-fly’s proboscis and a test slide (dry or in styrax)
of P. angulatum.

The common narrow-angled \ will show all the main
details of the proboscis, as described by the late Eev. J. G.
Wood, very plainly. Unquestionably there is a high degree
of pleasure and instruction to be obtained from what is so
seen and known. The details will in fact appear thicker
and blacker, and in some respects ‘ more magnified,’ with
such a lens. Fix now the attention specially upon the little

io6

THE MICROSCOPE

pointed hairs that thickly stud the whole membrane near
the tip. They will look, by the cheap lens, when we have
done our best, quite plain, black, and thick. We see
their number and that they are there. But such an image
has missed the beautiful and delicate perfection of the
minute works of God. Put on the better lens, of higher
angle, and a good condenser, nicely adjusting all (or if a
common condenser, using the ground-glass slip), and all is
changed. The hair is now not thick, but has become thin ,
and goes off to a perfect point. The diffraction-fringes, or
curious white nebulous lines, seen round the hairs before,
have also vanished. We see the hair now as it really is.
The fine, absolutely pointed little structure now before our
eyes, is a ‘ critical ’ image.

Now take the diatom. The dividing cheap J-inch
will not resolve this at all, but we suppose an objective
that will do it easily, which can be had for 25s. Use
light from the plane mirror only, with a small stop in the
substage, or as small cone with the condenser as will do
it. The diatom, now, looks very ‘ black and white ’ indeed.
But it will be found that all sorts of images appear as
the objective is focussed up or down ; it is hard to judge
which is right. Try it now with a better lens, and with
a large cone carefully adjusted, or a fairly large cone and
the ground glass. There are now but two appearances
possible— either white spots or black dots arranged in sixes
and sharply focussed, the one a shade nearer in focus
than the other. These are true and ‘ critical ’ images, the two
effects being accounted for by well-known laws of physical
optics. The black shadow contrast is not nearly so strong;
but the image displays the delicate details, sharp and
true.

There are higher lessons to be learnt with experience ;
but these will suffice for the present reader, with the
remarks in § 61.

MICROSCOPE MANIPULATION

107

61. Collar Correction. — The principle of this has been
explained in Chapter II, § 16. The student will only need
it as he begins to use rather wide-angled dry lenses, and to
understand the difference between a blurred and critical
image. When that time arrives, a few simple remarks here
may save him some time and trouble.

First recall the principles concerned. An objective
requires ‘ correction ’ ; and if any objective is perfectly
corrected for an uncovered object, it becomes over- corrected
when the object is covered by a glass ; the more over¬
corrected the thicker the cover-glass, or the glass and layer
of balsam or fluid (if mounted in such). To compensate or
counteract this, the objective itself, or else the whole optical
combination, has to be made more or less under -corrected for
the uncovered condition. In less degree, if an objective is
corrected for a given cover-glass, it will need to be made
more under-corrected for a thicker cover-glass.

This under-correction may be effected in two ways, but
which are really the same in principle. The first is, by the
‘ collar,’ to bring the front and back combinations of the ob¬
jective itself nearer together, which in modern objectives
means turning the collar to the right or to the higher numbers.
The other is to shorten the tube-length, or bring the eye-piece
nearer the objective ; this, to be effective, has to be done to
a far greater extent. But it is really the same thing in prin¬
ciple, since (as has been pointed out in § 10) the front
portion of the eye-piece in principle belongs to the objective.

Thus fixed objectives of high class, such as Zeiss’s J-inch
apochromatic, have to be ‘ corrected ’ by lengthening or
shortening the draw-tube. The old directions about getting
more power by * extending the draw-tube,’ can only apply to
lenses very moderate in angle and quality ; a ‘ critical ’
image can only be had, with a fixed objective and given
cover-glass, within very narrow limits indeed as regards
length of tube. Hence, again, collar correction may come

io8

THE MICROSCOPE

into play on account of tube length ; and though it is not
usual to provide so much of it, an apochromatic J was made
for me by Mr. Powell which could be used for either the
6-inch or 10-inch tube.

Next for collar-correction in practice. Good slides to
practise upon are a dry P. angulation and a Podura scale.
Suppose it is the diatom, and the lens a really good ^-inch.
First of all, see that the collar works easily. Secondly, get
thoroughly into the mind the fact, as an instinct, that to
turn the collar to the right (looking up the tube ; from the
operator’s position behind the instrument it will be seen to
be to the left) is to close up the lenses for more under-correc¬
tion. Thirdly, without any attention to correction, practise
a little in working the collar a bit, whilst keeping in focus on
the slide with the other hand.

This is just the mere handling of the thing; and now we
can attack the diatom. Find one of the flattest, and focus
one edge. Then alter the focus a little, up and down alter¬
nately, taking great care to move the milled head equally
each way. Both ways the outline will open, or enlarge into
a nebulous coma, from the focal image ; and it will probably
be seen that the enlargement is greater (for the same motion)
one way than the other (Wenham). If it is greatest when
the objective is drawn back, open the lenses; if greatest
when the objective is focussed down below the true point, close
the correction (collar towards higher numbers). Do this till
the expansion appears about equal both ways ; the correc¬
tion will now probably be nearly correct, and with some
observers may be quite so.

Next examine the image itself, and see if the small
markings appear in the same plane as the grosser outline. If
they appear distinctly above the gross image, the correction
is not right yet. Work a little more till both seem to focus
nearly, at all events, in the same plane. The correction will
now be very near, possibly quite so.

MICROSCOPE MANIPULATION

109

But finally, the observer must learn to judge of the
sharpness and density of the image itself. Getting as large
and good a cone as he can, or as the lens will bear, and test¬
ing with the highest power eye-piece he has which it will
stand, let him now study on the diatom or Podura scale the
sharpest marking. On the diatom he should get at will,
with a very slight change of focus, either an array of sharp
little white discs, or in their place black dots, and no others ;
working the collar till both are as sharp as possible. On the
Podura, the white exclamation mark on the top and within
each black one, should also be as sharp and distinct as
possible. Bacteria should appear cleanly drawn and thin.

When this is attained on such representative slides, the
student will have learnt sufficient of collar-correction for his
present purposes; anything more thkt he needs will gradually
come of itself.

CHAPTER VI

MICROSCOPICAL DRAWING, MEASUREMENT, AND PHOTOGRAPHY

The genuine student will occasionally require to draw what
he sees, and may also desire to measure either the size of
the objects or the magnification of his images. Both
operations are simple and easy.

62. Drawing. — This is performed by some modification
or other of the camera lucida. The simplest, cheapest, and
easiest for a beginner to use is that known as Dr. Beale’s
neutral-tint reflector (fig. 58). The usual cap of the eye¬
piece is removed, and replaced by that in the figure, outside
which is a simple mount which supports a piece of trans¬
parent neutral-tinted glass at an angle of 45°. The micro¬
scope is adjusted with its tube horizontal, and, if the stand

no

THE MICROSCOPE

Fig. 58

be small, should be supported on a book or box, or the
stand suggested in § 47, so that its centre is ten inches
from the table, on which the paper is laid. The image is

then reflected upwards to the eye looking
straight downwards, and which also per¬
ceives the paper and pencil through the
tinted glass. This apparatus costs from
5s. to 7s. 6d., the latter sum procuring
several glasses of different shades.

Various more costly prism arrange¬
ments are also in use, and Abbe’s camera lucida has a re¬
flector which enables the microscope to be used at any angle.
Such arrangements will be found figured in first-class cata¬
logues, and need not be particularly described here. Some
are rather difficult to use ; but it is singular that one observer
will find most easy, the very one which another finds most
difficult. Only one practical remark need be made, viz. that
success depends much upon the relative brightness of the
image as seen on the paper, and of the paper and pencil them¬
selves. Hence a separate lamp to illuminate the paper is
often desirable, and upon increasing or diminishing the light
of either, comfort and success may often depend.

63. Measurement of Size. — The real sizes of minute ob¬
jects are easiest measured by the aid of a stage micrometer ,
costing about 5s. This consists of a line ruled on a glass slide,
and divided by cross lines into small spaces, usually the
and iVoir °f an inch, or and y-^-g- of a millimetre. The
object being conveniently adjusted, its leading dimensions
are sketched on the paper (a mark or two usually sufficing).
Then it is replaced by the micrometer ; and the image of the
small spaces being made to coincide, gives at once the
dimensions of the details of the object.

Another method is to employ an eye-piece micrometer,
containing a glass scale in the field of the eye-lens, and
which is consequently seen in the same plane as the image.

ME A S U RE ME NTS

hi

The stage micrometer is then imaged (of course this should
be done with the same power and tube-length employed on
the object) and the value discovered of say one or more youths
of an inch, on the larger divisions of the eye-piece micro¬
meter. Then the dimensions of the object
are known by comparison. Cheap eye¬
piece micrometers on circles of glass are
sold (fig. 58 a), which can be dropped into
ordinary Huygenian eye-pieces. An eye¬
piece with micrometer fitted is much more

. Fig. 58 a

expensive.

64. Measurement of Magnifying Power. — This also is
simply effected with the aid of a stage micrometer and the
camera lucida. The paper must be placed at the proper
distance of 10 inches (§ 1) from the neutral-tint glass.
Then the image of the micrometer is focussed and dimen¬
sions sketched on the paper. The most convenient division
so sketched, has then simply to be measured to give the
magnification. Thus, let us suppose the division known ta
be Tyoth of an inch measures when drawn 34 inches, it is
obvious that the magnification is exactly 350 diameters.

65. Photography. — The highest class of photo-micro¬
graphy demands costly apparatus, the very finest lenses,
and the highest manipulative skill, and to enter into it is
out of the question in these pages ; we can only give a
very few hints towards success in earlier and simpler
attempts, which may, perhaps, lead on towards more ambi¬
tious work. In that case, elaborate treatises on this special
subject are accessible to the student. The delicacy of
such work as highly magnified photographs of the more
difficult bacilli may be gathered from the fact, that it is
hopeless to attempt it except on the solid ground, in situa¬
tions practically free from the vibration always present in
the crowded parts of large cities.

Micrography having additional difficulties of its own, the

1 12

THE MICROSCOPE

operator should first of all have sufficient ordinary camera
practice to master the ordinary photographic details. He
should learn to judge by the behaviour of his plates
whether the exposure has been correct, too long, or too
short ; and how to ‘ humour ’ development according to
these circumstances. It is also advisable that he should
become familiar with the use of the quicker plates, and
especially of some of the best brands of isochromatic plates,
which latter give far better results with many insect
subjects, or with others of difficult colours.

The microscope for early work need not be expensive ;
but must be steady, capable of being balanced with the tube
in a horizontal position, and should have a focussing and
centering substage. Some amount of skill in proper illu¬
mination and display of the objects to be photographed
must also be acquired ; for if the visual image be not good,
it is impossible for the photograph to be so. These points
are almost self-obvious ; but observation proves that dozens
of plates and much time are absolutely wasted in efforts
that cannot yield good results, for want of attending to them.

When thus ready for actual microscope work, there are
two general methods of operation ; but in either case, with
most magnifications it will be needful to provide a thick and
solid base-board four to six feet long, which is all the
better if provided with strips on the edges of the top surface,
to form a trough or slide for the apparatus to move in,
easily but in line. Along one edge of this, in two or more
eyes, should revolve easily a brass rod or tube, with a
pulley fixed on it, round which and the milled head of the
fine adjustment passes a cord or tape. Usually it will be
a tape ; but when a microscope is intended for photography,
a groove is often turned in the milled head, and a cord used
in the groove. The purpose of this is, that when the image
which has to be focussed is too far off from the microscope
for the operator to reach the fine adjustment with his hand,

P HO TO -M ICR 0 GRA PH Y

1 13

it may be worked by turning the rod, which has a milled
head on the end. A very solid table should be used, on the
most solid floor that can be found ; but Dr. Charters-White
says that on such a table he has found that vibration is
largely prevented by supporting the base-board on two piles
of unbound magazines.

The first method of working was recommended some
years ago by the gentleman just quoted, and consists
essentially in placing the microscope and lamp themselves
in a box open only at one side, which also can be closed
entirely by an efficient black curtain ; and with an aperture
in one end opposite the end of the microscope body. Over
the lamp must of course be also a chimney, which must be
light-tight during exposure. The aperture through which
the microscope protrudes is easily made light-tight by fixing
outside a short tube, round which and the microscope tube a
loose tube of black velvet or cloth is kept tight by india-
rubber bands. Outside the box is provided any kind of
moveable stand to carry the focussing screen and plates,
which can be adjusted at any distance from the microscope
required, according to the size of image.

Large objects with low powers may be focussed on an
ordinary ground glass ; but for minute detail this will not
be sufficient. In some cases it answers well to have white
paper pasted on the front side of a glass plate, and look at
the image, projected as on a white screen. One advantage
of this method is that no focussing rod is needed, as the fine
adjustment can usually be reached, while getting a critical
view of the image on the white paper. But the usual plan
is to use a glass plate, on the face of which (corresponding
to the film in position) are ruled some fine lines, and to
carefully focus these and the image together with a ‘ focus¬
sing-glass.’

The procedure with this sort of apparatus is pretty
obvious. The lamp, substage condenser, and microscope are

H

THE MICROSCOPE

114

carefully centred, and the illumination carefully adjusted in
the usual way, till the very best image possible is attained,
and the screen is so adjusted as to get the image the required
size and sharply focussed. Often it will be best to project
it direct from the objective alone, at several feet distant. In
other cases an eyepiece may be employed, at a shorter dis¬
tance. The best results of all, for high powers, are obtained
with ‘projection’ eyepieces, but these are unsuited for low
powers, unless apochromatic.

Thus far the side of the box is left open to give light in
the room : now the curtain must be closed and the whole left
for ten or fifteen minutes. All will then probably appear out
of focus, owing to the expansion of the metal by the heat
inside the box. This must be readjusted : and not until the
image appears to remain stable should exposure be attempted.
Then the plate is placed in position, and a loose cap over
the microscope end ; the shutter removed (all being now in
total darkness), and finally the loose cap removed and the
exposure given, by watch. Correct exposures must be learnt
by experience ; but a pretty good average exposure to start
trials with, at say 100 diameters, with ordinary lantern
plates, may be 20 to 30 seconds.

Many good ordinary objectives may be made to give fine
definition, equal to that of most apochromatics, by interpos¬
ing a sheet of Chance’s ‘ signal-green ’ coloured glass. The
time of exposure is, however, increased considerably. Other
photographers have obtained splendid results from various
coloured screens and isochromatic plates. These last give
the best results with the more difficult insect subjects.

With low powers, the ground-glass method of illumina¬
tion will often be found of great use. With high powers it
absorbs too much light, and reliance must be placed on
carefully-adjusted central cones, as large as the lenses and
the object will bear. In all cases the photographer should
be careful to stand or sit still during exposure, as walking

PHOTO-MICROGRAPHY

1 15

about the room would probably spoil the image by the
vibration so caused.

The other method of work is to place the lamp and
microscope free on the base-board, and an ordinary camera
with its dark slides at the other end, removing its lens, and
making a light-tight connection between its tube and micro¬
scope, in the way already described. With the usual short
cameras, an eyepiece will be necessary to get a sufficient
magnification ; but additional bellows, up to several feet in
length, can be obtained for a small sum. To use these the
focussing-rod will of course be necessary. Fig. 59 shows
such an arrangement in conjunction with the simple stand

9

Fig- 59. — Microscope and Camera

illustrated in fig. 31, with an ordinary lamp, and bull’s-eye to
supply parallel rays. Mr. E. M. Nelson has devised and
described a simple arrangement of light-tight boxes open at
both ends, blackened inside, and fitting into one another,
with blackened internal flanges. With these a camera of
any length can be built up, according to the number of boxes
used end to end, which could be made by any amateur car¬
penter.

This latter method of work (i.e. with a camera) is gene¬
rally preferred by practical men. It allows the room itself
to be well lighted throughout, and gives facility for using a
lime-light jet, or any other powerful method of illumination.

h 2

1 1 6

THE MICROSCOPE

And the apparatus becomes far less heated than when itself
enclosed in a dark box, though it will be found that even the
proximity of the lamp will affect the focussing somewhat,
until the whole has been some time in position.

The majority of good modern objectives are fairly well
corrected for photography. Nearly all are corrected and
sharp in focus, if a signal-green glass is used.

In photographic work, it is of the utmost importance to
prevent all ‘flare ’ from bright reflected light. Some otherwise
good objectives reflect a great deal of light from bright brass
round the edges of their lenses ; all lens mounts should be
blackened. The larger in diameter the tube of the micro¬
scope is, the better ; and this should be carefully blackened
inside with black velvet ; no mere varnish is ‘ dead ’ enough
to prevent all reflection from the sides of the tube. Or
another plan as good or better, is to cut several discs of card
of a size which will jam or stick anywhere in the tube ; cut
apertures in the centre, of rather different sizes, the largest
slightly smaller than the field-lens of the eyepiece ; and
make them dead-black with suitable varnish or Indian ink.
The smallest is pushed two-thirds down the tube, and
two others spaced so that the last and largest is about an
inch from the bottom of the eyepiece. These side-stops
catch all the surplus side-rays. The bellows or camera-
boxes should be protected in the same way, square-shaped
stops being easily placed at intervals along the camera.

Many plates produced are the veriest rubbish (unless of
some rare or unique object, of which any photograph may
be better than nothing). It should be understood that in
this branch of photography there is as much room for taste
and manipulative skill as any other. Of some objects the
sharpest definition and the sharpest contrast in black and
white may be desirable ; but in others good half-tones,
harmony of shades, and general effect are as important as in
other subjects. Exquisite slides can be produced of suitable

PHO TO-MICROGRAPH Y

H7

objects by the ‘dark-ground method’ of illumination. Then
the thickness of the microscopic object has to be considered.
If very appreciable, only certain portions can be ‘ focussed ’
at one time by a high-power objective ; and if tolerably high
magnification is required, it must be obtained by using a
power low enough to get ‘ depth of focus,’ and making up
the magnification by either an eyepiece, or greater distance
of the plate.

These difficulties and refinements should not be en¬
countered all at once, but the first attempts made upon easy
and flat objects, of which a flat image can be at once obtained.
Sections of the stems of plants, and the wings of some
insects, are very good objects to commence with. If serious
and systematic work is then determined upon, the student
will do well to obtain one of the detailed manuals concerning
this branch of microscopy.

CHAPTER YII

MANIPULATION, PREPARATION, MOUNTING, AND SELECTION OF

OBJECTS

Whole treatises have been written upon this division alone :
we must be content with those elements suitable for a
beginner.

66. Natural Objects. — Very many objects are well
worth examination in their simple, natural state. Small
flowers whole ; their parts ; insects and parts of these — all
such will well repay observation as found. For this purpose
an indispensable appliance is the stage forceps, fig. 60.
The stem of the forceps fits into a hole in one corner of the
stage, or the limb of the microscope, as most convenient ;

THE MICROSCOPE

1 1 8

and the slide, combined with the universal motion, enables
a small object to be placed in any position, angle, or direction
desired.

Most objects examined in this way are illuminated as
opaque objects, by the bull’s-eye alone ; but the parabolic
reflector (§ 57) or the Lieberkubn can also be used. Some
may be transparent, such as the wings of many insects ; and
for these the concave mirror or condenser may be used as
usual.

67. Live Objects. — One way of examining live insects has
already been described on page 42, but only serves for the
head or mouth organs. Another method is more generally

useful. A flea or other
insect may be stupefied
by the vapour of alcohol,
ether, or chloroform, and
while unconscious at¬
tached by a particle of
gum on its side to a very
small disc of paper
gummed on the centre
of a glass slip. When
it recovers, the move¬
ments of any part can be well seen, by aid of the parabolic
reflector or a Lieberkuhn.

Another very usual method is to employ the simple
apparatus called the live box (fig. 61). On a slip of brass is
fixed a short wide tube, in one end of which is mounted a

HANDLING NATURAL OBJECTS

119

circular plate of glass. Another short tube sliding a little
stiffly, either in or outside the other, has mounted in one end
a thinner plate of glass ; thus with care the two glasses can
be adjusted at any distance desired, so as slightly to compress
an insect or small animal between them. This apparatus is
made either with the glasses at the bottom of the tube, or at
the top, for convenience, and costs 5s. to 8s. Mr. Rousselet’s
form is best, in which the glasses are at the bottom of a
much wider cell, so that an objective can focus anything up
to the very margin of the glasses. More perfect contrivances
are made at higher prices, with which the pressure can be
more precisely adjusted.

68. Pond and Marine Life. — Living objects in water are
handled in various ways. Infusoria, and many Rotifers, are

Fm. 62. — Troughs

best examined by simply putting a drop of the water contain¬
ing them on a glass slip, and a loose cover-glass over, which
is held on by capillary attraction. If so large as to be
crushed by this, a filament of wet cotton, or of some confer-
void growth, will keep off the pressure, and yet confine the
movements of the animals ; or it is very usual to employ
the live box, enclosing now a drop of the water between the
two glasses.

But the majority of creatures will require glass troughs,
the smaller of which, resembling fig. 62, can be obtained from
Is. each upwards, and consist of cells cemented on ordinary
3x1 glass slips. Thinner cells, and with thinner front glasses
than can be generally purchased, will, however, often be
required; and I have been in the habit of making mine.
Provide half an ounce of cover-glass, about 1 J x f inch, for

120

THE MICROSCOPE

the fronts of the troughs. For the thickness, or wall, half of
an ordinary vulcanised-rubber ring of suitable size will do ;
or for very thin troughs, a wall cut out of thin card. Either
may be cemented to the slip and the front quite securely, by
the ordinary balsam and benzol.

Another good way is to use glass slips with concave cells
ground out of the centre ; these provide a certain depth of
liquid, over which a cover-glass can be placed. Such cells
are sold in great variety, and also with subsidiary cells and
grooves, so arranged at their sides that a current of aerated
water may be maintained if necessary. These devices are
known as ‘ life-slides,’ and after a little experience the
manner of use will suggest itself. A great variety are figured
in Beck’s larger catalogue.

For larger animals, Polyzoa, &c., large troughs are sold,
of all sorts of shapes and sizes. When a trough is rather
deep, and an animal has to be kept near the front, this is
done by wedging up a slip of glass behind it. Marine life
attached to weeds requires rather a large trough. What is
called ‘ Botterill’s ’ trough consists of two glass slips, with
half a vulcanised-rubber ring pinched between them, or two
layers if required, by means of two outer plates of vulcanite
and set screws. Personally, I have found made troughs most
convenient.

Vegetation, with or without life attached, is best arranged
in a trough with a slender pair of forceps or needles.
Separate animals may be withdrawn from the water they
are kept in by a ‘ dropping tube.’ The tip of the forefinger
is held tightly over the top end of an empty tube of suitable
size, and the lower end being brought over the animal, when
the finger is lifted the water rushes up and carries the creature
with it. Replacing the finger, the whole is lifted out, and
removing the finger again will deliver the animal in the trough
or on a slide. But by far the most convenient implements
are the ‘ fillers ’ sold with stylograph or fountain pens ; if

PREPARATION OF OBJECTS

1 2 1

a few are at hand with different bores at the point, small
animals can be captured and again expelled with the greatest
facility.

69. Dissecting. — The minute details of many objects have
to be carefully separated in order to be examined. The
tools needed for these manipulations consist of fine pointed
scissors, forceps (fig. 63), needles for teasing-out fine struc-

Fig. G3.— Forceps

Fig. 64.— Needles

Fig. 05. — Knives

tures (fig. 64), and dissecting knives (fig. 65). Forceps are
required both straight and curved, and are usually of metal ;
but for wet work I prefer them with ivory tips. Fig. 64
shows three ways of mounting needles in mechanical handles.
But it does just as well to provide some cedar handles, as
used for camel-hair pencils, to bind one end round with
thread, and having made a hole in the end with the point of
a needle, to push the butt-end firmly in with a pair of pliers.

122

THE MICROSCOPE

There should be several straight needles of various thickness,
one or two curved, and one or two bent quite to a right
angle and into hooks. Knives need no description. A few
camel-hair pencils, with perhaps one or two sables for their
greater stiffness, are also required.

In dissecting under the simple microscope, the hands
will require some support, which can be obtained from blocks
or books at the sides ; but the instrument shown on page 40
is provided with hand-rests, as are those of other makers.
A portion of vegetable tissue, previously macerated in water,
may be simply placed in a few drops of water upon the
glass stage. When more fluid is required, watch-glasses may
be used, cemented for steadiness into apertures cut in a plate
of cork. Or for Is. apiece can be obtained thick plates
of glass with deep concave basins ground out and polished,
which are very convenient. For some purposes it is needful
to use square flat-bottomed glass troughs. It is not always
necessary to work under the microscope ; with light thrown
on the object from the bull’s-eye condenser, a great deal
can often be done by the naked eye, or with rather short-
focus spectacles (fig. 66).

Many objects, such as an insect, have to be fastened
down to be dissected. A plate of cork loaded with lead is
often convenient. But it is sometimes better to run into
the bottom of the trough some waxy material, into which
pins may be thrust. A mixture of paraffin and stearine will
be tolerably transparent. Where transparency is not needed,
beeswax softened with tallow may be used, and if desired
can be easily coloured dark with lampblack. Sometimes an
object is best fixed by melting a portion of the ground
material with a hot wire, and slightly embedding the object
in the melted surface ; in other cases, or on cork, pins must
be used.

Generally the liquid in the trough will be water. Some¬
times diluted alcohol is better, especially in dissecting the

MOUNTING SLIDES

123

nervous system of insects, as the alcohol hardens the
tissues. Some tissues dissect best in glycerine and water.

Subjects from the plant- world have to be prepared for
dissection by long soaking in water to soften them ; often
that is not enough, and solution of caustic soda of greater
or less strength is employed ; but this acts very powerfully,
and the progress must be carefully watched. Insects to be

Fig. 66

dissected should always be kept in fluid till wanted;
diluted methylated spirit being the most generally suitable.

70. Mounting Slides. — It will best serve the purposes of
this work to select from the countless methods of mounting
microscopic objects, three only, most suitable for the be¬
ginner, and most widely applicable.

Microscopic objects are almost universally mounted now
upon glass slips 3x1 inches. Wooden slips warp, and
are discarded even for opaque objects. The paper covers
once used have also quite gone out, and the microscopist

124

THE MICROSCOPE

should employ only good flatted glass slips with ground
edges. Over the objects are placed thin ‘ cover-glass/
which can be had either in largish oblong pieces for large
objects, or in circles and squares of various sizes.

Before use, all glasses must be cleaned. For very high-
class and minute work, strong solutions are often employed
to do this ; but for the class of objects here in view, the
glass will be sufficiently cleaned with a little soap or soda,
rinsing in clean water, and drying with glass-cloth, finally
polishing with clean linen or a chamois leather. To polish
the cover-glasses, after a little experience the finger and
thumb may be used with the linen or leather, moving the
cover by its edges between them, with finger and thumb of
the other hand. Many use two flat pieces of wood with
a chamois leather strained over the face of each ; the
cover-glass is placed on one, the other applied, and the two
twisted together. The cover should be reversed, in each
case, as it usually sticks to one of the leathers, and is only
rubbed by the other.

71. Cement and Cells. — A very great variety of cements
and varnishes, and materials for cell-making, are used by

various microsco-
pists ; but for the
processes here de¬
scribed a few will
suffice. In regard
to the walls of cells,

FlG. 67.— Cell-making Instrument (Sliadbolt)

very shallow or thin
cells are built up with varnish alone. For this process,
and for sealing in some cells, some kind of hern-table
(fig. 67) is absolutely necessary. That shown is the sim¬
plest, and is known as Shadbolt’s, the glass slip being
simply held down by two strong springs, a, b, while the table
with the slip on it is turned round by the finger on the milled
ring, c. Other forms are chiefly distinguished from this by

MOUNTING SLIDES

125

some arrangement or other for automatically centering the
slip, so that it shall turn round the centre, and for holding it
more tightly. There is, however, no difficulty in centering
even with the Shadbolt form. It will be understood in an
instant how, if the slip be turned round with the left fore¬
finger, while a camel-hair or sable pencil with some varnish
be held touching the slide, a circular ring of the varnish will
be laid on. For very minute objects, such as the scales of
some insects, one thin ring of varnish lifts the cover-glass
sufficiently high ; if more be required, this coat is allowed
to dry, and a second laid on the top of the first ; and even a
third or more.

Deeper cells require a solid wall of some kind. Rings of
glass, tin, celluloid, and other materials are sold for the pur¬
pose. Rings may also be used stamped out with a punch
from paper or card, and soaked in melted solid paraffin.
Without this precaution damp would enter the cell if dry ;
or fluid contents might evaporate.

Of cements and varnishes it will suffice to commence
with a bottle of ‘ gold-size,’ another of asphalt and caout¬
chouc in naphtha, called ‘ black varnish,’ and another of
somewhat similar varnish known as ‘ black Japan,’ or, what
is now preferred by many mounters, the ‘ Club Black Enamel,’
sold for bicycles by the Silico-Enamel Company.

72. Mounting ‘ Dry’ Objects. — By ‘dry’ preparations are
meant such as are only protected from dust by a cover-glass,
and not immersed in any fluid medium. Nearly all opaque
objects are so mounted, and many transparent ones may be
treated the same way, such as scales from insect wings,
diatoms, leaves, and petals of plants.

Many of the latter class require only very thin or shallow
cells. These are made by ‘ ringing ’ a slip on the turn-table
with one or more coats of gold size or Club enamel. If more
than one be needed, the first one must dry before the second
is added ; but the final one should only be half dry, or sticky,

126

THE MICROSCOPE

when the cover is applied. Thin flat objects like a piece of
wing need no cement, but will have ‘ spring ’ enough to keep
them in place. Other objects may require a morsel of gum
to fix them to the slip, and in all such cases the gum, and
the object also if it has contained any moisture, must be
most thoroughly dry before covering in, else the moisture
will gradually deposit on the cover in small globules in¬
visible to the eye, but too evident when magnified.

Gum cannot be used in this way with very minute
objects, such as insect scales or the pollen of flowers. For
such, the bottom of the cell may be brushed over with very
dilute gum, which is allowed to dry into an invisible trans¬
parent film. The objects being placed on the surface, very
gentle breathing on the slide, or the steam from some hot
water, will moisten the gum again, and make the objects
adhere. Or they may be affixed to the underside of the
cover in the same way. The gum must be thoroughly dried
again before closing the cell. Gum arabic for slide-mount¬
ing should have six drops of glycerine per ounce added, to
prevent cracking when dry, and a few drops of some essen¬
tial oil to avoid mildew ; the oil of cloves used in balsam¬
mounting will answer very well.

The best way to attach scales to the dried gum surface
of a slip or cover-glass, is to rub a bit of the wing on the
surface. Sufficient will adhere, and can then be fixed as
already described. Or, with a hand magnifier or dissecting
microscope, a bristle set in a handle may be employed.

When all is thoroughly dried , the cover-glass is pressed
down on the ‘ tacky ’ surface of the varnish ring. The cover
should not reach to the outer edge of the ring of varnish.
It should if possible be pressed into contact all round.
When the ring appears dry, the edge of the cover is ringed
round with one or more coats of black varnish or enamel, or
with a coat or two of gold-size first, and black varnish over
it when dried. The gold-size and varnish should extend a

MOUNTING SLIDES

127

little on the slip, when the varnish cell is very thin, and
to the edge of the ring when it is thicker, or when solid
cell-walls of any material are used. Such solid cells of
paraffined paper or card, tin, glass, or other material, should
be cemented on and thoroughly dried before the object is
inserted.

Dry opaque objects usually require a cell, and most of
them look better on a black ground. This may be obtained
with transparent slides by using a ‘ dark well,’ as mentioned
on p. 104, but opaque cells are often used. A disk of asphalt
varnish may be put on by the turn-table, and the cell built
up on that. Or on the black varnish may be gummed a
disk of dead black paper, and the cell built up over that.
And for some objects it is better to bed a cover-glass on a
disk of black varnish, before the latter is dry, and gum the
object, or objects (such as a group of
Foraminifera) upon the glass ; the cell
and real covering glass being built up
over the whole. The cover-glass is
put on and sealed as in the preceding
paragraph.

73. Mounting in Benzol Balsam. —

This is the most durable and generally
useful mode of mounting, doing excel¬
lent work with entomological subjects,
botanical sections, and anatomical
sections.

The solution is made by baking
Canada balsam on a water-bath, or in a
cool oven, Till it is as dry as pitch, and FlG- 68.— Capped Bottle
then dissolving it in benzol to about the consistency of thick
cream. Usually this requires about equal weight of the dry
balsam and the fluid. The dry balsam can be purchased, or
the solution ready for use. The latter is to be kept in the
wide-mouthed capped bottle, shown in fig. 68, with a glass

I2&

THE MICROSCOPE

rod drawn to a blunt point, by which the thick fluid can
be dropped upon a slide.

Objects are chiefly mounted in balsam without cells, but
simply pressed between the slip and cover-glass till dry. If
they are too thick for this, as with some hard insects or
insect organs, it is usual to employ slips with a concavity
ground out of one side. Pressure is maintained either by a
small weight, such as a large printer’s type, or by one of
various patterns of spring clips or compressors, which can
be seen at any dealer’s shop. Where strong pressure can
be borne, common American wooden clothes-pegs answer
very well ; but much less force is generally advisable.

Insects and parts of insects are best kept till mounting
in a bottle of methylated spirit. When wanted, an insect
is to be soaked in one or two waters for some hours to
remove this, then placed in a solution of one part caustic
potash to ten parts of water, to soften and dissolve the fatty
parts. This may take a few hours, or it may need days, and
must be watched ; if overdone, the insect is bleached too
much to be of use. When the object is thought soft enough,
it is well soaked in several waters to remove the potash, and
then treated under water with two camel-hair pencils ; one
in the left hand, holding down the insect by the thorax ; the
other with a rolling motion, squeezing out the contents of
the body, which are carefully washed away. At this stage,
if not convenient to proceed at once, the insect may be kept
in acetic acid, which, in that case, must be washed out again
with water ; but otherwise it may at once be placed on a
glass slip and arranged, a slip of paper or thin visiting card
laid on each side as a wall or protection, and another glass
slip being laid over all ; the slips pinching the object are tied
tightly together, and put into methylated spirit to get rid of
the water ; in fact, they should be left in it for hours, and
taken out into a saucer of fresh, strong spirit to remove the
last traces — really valuable specimens are worth strong

MOUNTING SLIDES

129

alcohol. From this the object is removed into spirit of
turpentine, and allowed to soak for a variable time. The
turpentine gradually makes the brown, hard, chitinous parts
more transparent, and some parts may be the better for days
of the process, while other organs only need half an hour.
It is still better to place an insect from the spirit first into a
small saucer of clove oil to remove the alcohol, and from
thence into the turpentine ; but this is not absolutely neces¬
sary. In all cases the turpentine soak is the final stage
before transference to the slide.

The mounting may be done in two ways. One way is to
place sufficient balsam solution on the slip with the glass rod,
take out the object with a tiny spatula, called a section-
lifter, and draining away as much turpentine as possible,
work it off the lifter with a needle-point into the balsam if
possible, or, if needful, another drop of balsam may be placed
on top of the object. The cover-glass is then taken with
forceps and gently lowered on the balsam from one edge,
which will prevent air-bubbles. The cover is then pressed
down gently, any extra balsam exuded being removed by
blotting-paper, a number of small slips of which are cut and
kept ready. When all but enough is taken away the slide is
put in a clip and laid away in a warm place to harden ; or
may be gently warmed for some hours on a brass plate,
heated by a lamp, but not enough to cause bubbles.1

The other way, strongly advised by Mr. Martin Cole, is
to put the balsam on a cleaned cover-glass, and get the object
well into it ; then cover it from dust,2 and put away for
twelve hours. Warm a clean slip, apply a fresh drop of
balsam solution to the centre of the partly dried balsam with
the object in it, and lay the cover and object carefully down
on the warmed slip, avoiding bubbles. Keep the slide

1 Some mounters prefer to lieat the slide over a lamp just enough to cause
the benzol to boil very gently, the bubbles escaping at the sides. This is all
right if the bubbles do not get entangled in the object.

2 A good way is to invert common tumblers over slides.

I

130

THE MICROSCOPE

warmed a few minutes, and press down rather more strongly.
The advantage of this plan is that the balsam is more dried,
and the object is nearer the cover-glass.

Smaller portions of insects, such as legs, antennae, &c.,
will need scarcely any soaking in potash, or none, but merely
enough in turpentine to make them sufficiently transparent.

Portions like wings need only be dried, then soaked in
spirit and finally in turpentine, to avoid air-bubbles.

Sections of stems or vegetable structure may be mounted
in the same way ; their preparation is briefly described
further on.

Balsam slides do not need ringing. When dry, all that
is exuded may first be cut round the cover and nearly
removed with the tip of a penknife, not going quite close.
The slip can then be quite cleaned off with a bit of linen
soaked with a few drops of sulphuric ether (keeping this well
away from any flame). It can be left so, or the slide very
slightly warmed again, to gently melt the edges and make
them smooth. Should it be preferred, however, a ring of
black varnish or enamel can easily be added on the turn¬
table.

74. Glycerine Jelly Mounts. — This method also is most

simple for the beginner, and one of the most generally useful
to the end. It particularly suits small transparent insects,
and vegetable subjects, especially mosses and algae, pre¬
serving the colour well. It can be procured in shilling
bottles at any microscope optician’s. It is a jelly when cold,
but liquefies with a gentle heat.

Objects to be mounted in this have to be finally soaked in
water, or water with a little glycerine mixed in it. Insect
subjects therefore only need to be freed from the potash
solution ; or vegetable tissues to be teased out or dissected
in water. The chief thing is to get rid of air-bubbles. It
helps this if tiresome subjects are soaked in water that has
been well boiled, and finally warmed in it. Insects and

MOUNTING SLIDES

*3*

vegetable subjects which have been ‘ treated ’ as before
described, are generally already well soaked with fluid.

The jelly is well kept in a capped glass bottle like fig. 68,
and must be liquefied by placing in a cup or basin of hot
water. Then, a drop or two being placed on the slide, the
object upon that, more jelly over that, and finally the cover-
glass gently squeezed down, the whole should be kept warm
till all has well settled down, when the mount may be allowed
to cool. Superfluous jelly is cut away with a penknife, and
the traces left cleaned with a wet camel-hair pencil.

The slides may be finished and sealed by a couple of
rings of gold-size, dried successively, and followed by one
of black varnish or enamel. But unless the objects (and
consequently the layer of jelly) are thick, they will last for
years without any sealing at all. My friend, Mr. Henry
Scherren, and the Rev. T. R. R. Stebbing, one of the
greatest living authorities upon Amphipods, from whom he
learnt the method, are in the habit of mounting their smaller
specimens in glycerine jelly with no other preservation what¬
ever ; and some of these preparations have reached a quite
respectable antiquity.

75. Protection against Oil. — Objects which will ever be
examined under oil-immersion lenses must however be
protected, whatever the medium they are mounted in. This
is done by putting on, outside of anything and everything else,
two successive rings of Hollis’s liquid glue, or its equivalent
in a solution of shellac in alcohol. This is not acted upon
by cedar oil. It is generally best to put on two coats or
rings of gold-size before the glue.

76. Section-cutting. — We shall only here describe briefly
the preparation of vegetable sections, as of stems, leaves, &c.,
which are the most accessible to a novice, and form some of
the most pleasing objects. Two implements will be required ;
viz. a razor, or hollow-ground knife, or plane-iron for cutting,
sold for the purpose (though an ordinary razor may often be

132

THE MICROSCOPE

found which will answer) ; and some sort of a microtome, the
simplest form of which is shown in fig. 69. The substance
to be sectioned is either embedded or wedged in an inner
tube, which slides in an outer one ; and is forced up a little
for each section by the screw, the knife being kept down to
the flat flange surface on the top. Better instruments are
sold, with a clamp to fix to the table, a glass flange, &c.

With more complicated apparatus for pro¬
fessional section-cutting this little work has
nothing to do.

Wood and stems will need preparation
before sectioning. Stems or roots should
be soaked some days in alcohol to dissolve
but the resins, and then for days in' water
to soften, and dissolve out the gum. Dry
or hard wood may need long soaking in
water to soften, before the alcohol ; or even
long boiling may be needed. Leaves and buds, as a rule, only
need some soaking in diluted spirit, or may be cut without
any soaking at all.

To fix into the microtome, apiece of stem maybe wedged
in with bits of pith or soft deal; but in most cases it is better
to ‘ embed ’ objects. Mr. Cole and others recommend for em¬
bedding, four or five parts solid paraffin and one part lard,
melted together ; but this often slips in the microtome tube,
and it is better to use a mixture of stearine and naphthalin,
using the less stearine the warmer the weather. This is found
to hold tightly. The piece of material should be dried on the
surface, and then dipped in a gum solution made as follows :

White Gum Arabic
Glycerine

Alcohol (Methylated)
Water

1 dram
5 drops

15 drops

2 ounces

After removal, lay the piece on blotting paper till the
surface is again dry. It is then placed in the microtome and

STAINING

the naphthalin composition (melted) poured in, the object
being held in place till cold.

In cutting, a thick slice or two will first be detached to
level down, well wetting the surface, and the razor, with
dilute spirit — say ten parts water and one part methylated
alcohol. The knife or razor should be dipped into or flooded
with the methylated spirit before every cut, and the sections
floated off the razor into a saucer of water. This will dis¬
solve off the glycerine and gum, and the naphthalin will float
to the surface of the water. Whether embedded or not, all
sections are cut with a knife well wetted with dilute methy¬
lated spirit, and the razor will need sharpening according
to the nature of the material.

77. Staining the Sections. — Most botanical sections re¬
quire bleaching before further preparation. This can be done
in a weak solution of chloride of lime — the clear part of a
quarter of an ounce in a pint of water. Some prefer chlori¬
nated soda — the clear part of one ounce of chloride of lime
and two ounces washing soda in a pint of water, well shaken
after fully dissolving, and left to stand. The bleaching must
be watched. When the sections are taken out they should
be placed, for an hour or two, in a solution of five drams
sulphite of soda in a pint of water to get rid of the chlorine,
and then washed and soaked in several clean waters. They
are now ready for staining, or may be kept in dilute spirit
indefinitely. Two methods of staining will be sufficient.

78. Plain Log-wood Staining. — Dissolve half a dram of
haematoxylin in 3^ fluid ounces absolute alcohol, and half a
dram ammonia alum in 3| ounces distilled water, to which
add 3 ounces Price’s glycerine and 3 drams glacial acetic
acid. Mix the two together and shake, and let it stand in a
stoppered bottle in a cool place for some weeks before using.
For use add 30 drops of the matured solution to an ounce
of distilled water, and immerse the section for a quarter
to half an hour, or in some cases more. Then wash well in

134

THE MICROSCOPE

changes of water. Remove the water by a quarter of an hour
each in two baths of methylated spirit ; clear off the spirit
by ten minutes in clove oil, or oil cajeput. Finally mount
in benzol balsam. The last operations are the same as in
mounting insect slides ; but for vegetable sections the clear¬
ing oil is necessary.

Eosin is also much used as a single red stain.

79. Double Staining in Red and Green. — This is the

most beautiful method of treating stems and many other
sections. In no way can the amateur collect such a number
and variety of beautiful slides, with little expense and only
moderate experience, as by systematic sectioning of the
stems of trees and plants. There are many recipes : the
following is by Mr. A. C. Cole : —

For the carmine stain, dissolve 10 grains borax in 1 ounce
distilled water and add half an ounce of glycerine and half
an ounce alcohol. Mix also 20 minims liq. ammoniae fort,
with 30 minims distilled water, and dissolve in this, in a test
tube gently warmed, 10 grains pure carmine. When cooled
mix the two, and filter into a stoppered bottle.

For the green stain, make a saturated solution of iodine
green (also called methyl green) in alcohol.

Stain the washed section in the carmine solution from
ten to thirty minutes, then soak in methylated spirit till
colour ceases to come out. Wash in clean spirit, to which
has been added sufficient of the green solution to make it a
bright colour. They may stay in this as long as convenient :
then wash in clean spirit, clear this with clove oil, and
mount as before.

These stains or their equivalents may be bought ready
prepared. Acid aniline green and others are also used.
Sometimes it is desired to stain a trial section quickly, and
Mr. F. Ritchie states that this can be done in one operation
by placing a teaspoonful of ammonia-carmine solution
in a watch-glass, and stirring into it a drop of the acid

STORAGE AND SELECTION

135

aniline cjreen solution sold everywhere for double-staining,
taken up on a section-lifter. Dip the section for a few
seconds, then remove, and without washing draw off excess
of stain with blotting paper. Float and dabble quickly in
alcohol, and draw off excess of that ; then float in clove oil
and mount in the balsam.

Want of space forbids further details, and either of these
will produce beautiful results, which cannot be really
surpassed. The microscopist who needs further informa¬
tion may obtain it in Mr. Martin Cole’s instructions for
sectioning, staining, and mounting in ‘ Modern Microscopy,’
or in Squire’s ‘ Methods and Formulas.’

80. Charred Sections. — A method of treating vegetable
sections mentioned years ago by Dr. Carpenter, is good and
easy, and has been too much neglected. It consists in simply
charring a section, held between two glasses over a lamp,
till it has become charred a dark brown ; then mounting
plain in benzol balsam.

81. Handling Sections. — What were called section-lifters
were formerly much used to transfer delicate sections from
the final fluid to the glass slip. They consist simply of
narrow slips of thin German silver, with one end bent at a
convenient angle. The modern, and in most cases better,
practice is to take a clean cover in a pair of forceps, and to
introduce this under the section in the last fluid used
before mounting. Lifters are, however, often necessary in
moving from one medium to another in the processes of
washing and clearing, and for balsam mounts.

82. Storage and Selection of Slides. — The most econo¬
mical, safe, and convenient plan of keeping slides is in the
pine boxes of trays, as shown in fig. 70. These are sold by
scores, of two sizes, holding respectively three dozen and
six dozen slides, at about 2s. and 3s. 6d. each. They
keep the slides flat and safe at a very small cost, and can be
added to just as required. They have the further advan-

THE MICROSCOPE

136

tages, that a selection wanted for any occasion can be
looked out and put together; and that, as a collection
increases, it can be conveniently sorted out and classified.
Double trays holding one dozen are also convenient for a
few test slides, or to carry a small assortment to a meeting.

The microscopist is by no means dissuaded from buying
slides. Such exquisite work is now produced by skilled
mounters, that much beauty and interest will be missed by
the man who neglects to avail himself of it. But he is
strongly advised against idly ordering a ‘ collection.’ It is
better to pick up a slide every now and then which excites

Fi(x. 70. — Pine case

admiration or arouses interest. Many subjects are adver¬
tised from time to time. Bought in this occasional way, a
little expenditure gives much more enjoyment, is much less
felt, and keeps up the interest, which every purchase will
renew as it is made. Slides will also be only purchased when
a really good one offers ; and a much higher average of
quality will be secured. The few hundreds of slides which I
possess, each chosen with a view to illustrating some
occasional lecture, upon the screen, with the lantern
microscope, have all been selected in this way, better

STORAGE AND SELECTION

1 37

specimens of some objects being substituted whenever they
offered. Thus the very collection becomes a source of
interest and gratification, whilst the cost is spread over
many years.

CHAPTER VIII

POND AND MARINE LIFE

The space remaining in this little volume for some rough
outline of what the microscope has to reveal, is quite in¬
sufficient for any strict orderly classification. We must
divide this part of our subject into a few large and very
roughly comprehensive groups, and attempt only a brief
survey of each, with some practical hints from a general
point of view. Thus the chapter following this, though
entitled ‘ The Insect World,’ is made to include spiders,
which are only insects in common parlance, and not in the
scientific sense at all.

We allot purposely more proportionate space to this
chapter, brief as it must be, than to the remainder, for special
reasons. It is rather desirable, after the novelty of the instru¬
ment, and the amusement and interest of such slides as were
borrowed or purchased with it have worn off, that the budding
microscopist should commence real work for himself with
this branch of study. It is in itself one of the most interesting
branches — for there is always a living interest about life and
motion. It carries the student into country air and life, and
affords him a dash of that ‘ sporting ’ interest which has
such an attraction for the average man. It gives him some¬
thing actually to do, out of doors as well as indoors. It
teaches him from the first to look for objects of interest and
study, and to look below the obvious surface of things for
them. And it often leads to pleasant acquaintance and

138

THE MICROSCOPE

companionship, especially if a good society of working
microscopists be within reach.1

83. Collection of Pond Life. — A few glass tubes with corks
will be required, an inch wide by three or four inches long
being a good size. Half-a-dozen will bring home enough
for some little time, and larger bottles or jars are only
required when it is meant to stock an aquarium. And there
must be some kind of ‘ collecting-stick,’ with appliances
fitting into the end. It is to be feared that a well-known
and fascinating book 2 upon this subject, is responsible for
a great deal of disappointment and useless work. I myself
certainly gathered from it an impression that the interesting
subjects described were, as a rule, obtained by dipping into
ponds a small wide-mouthed bottle at the end of a stick.
Such is of use occasionally, chiefly to introduce (upside-
down and empty) into some special spot, in order that when
inverted it may collect a specimen of what there may be just
there and nowhere else ; but a novice no better equipped and
instructed than that, would have long and tedious work for
very little return. The really useful implements are shown
in figs. 71, 72, and 73.

The chief of them all is the net , shown in fig. 71. This
is funnel-shaped, and attached at the wide end to a ring 6
inches diameter, while the narrow bottom end is clasped by
a rubber ring round a flanged glass tube an inch diameter
and three or four inches long. The net is made of rather
fine or mull muslin. As this net is swept through the water,
either on the surface or elsewhere, the water passes through it,
but the rotifers, crustaceans, small larvae, or whatever else

1 The London student cannot do better than join the Quekett Micro¬
scopical Club, whose frequent organised Saturday excursions each season
will find him companions, and initiate him into practical pond-hunting under
the guidance of skilled old hands. Once at least every season there is also a
marine excursion. The subscription is only ten shillings per annum, which
includes the Journal. There are many local societies with similar pursuits
in different parts of the country, which can be found on inquiry.

2 Marvels of Pond Life.

POND AND MARINE LIFE

139

was in the water, are retained ; then finally when the ring
is turned into a horizontal position and the whole lifted out
of the pond, most of the water drains away, and nearly all
the contents are condensed into the flanged tube. This
need not be removed from the net ; but by bringing up the
mouth above the ring, the contents can be poured out into
a bottle or another tube. Where a mere bottle would bring
up hardly anything, a few sweeps of the net (supposing of
course that material is really present) will produce a tube
crowded with specimens.

Fig. 71. — Ring, Net, and Tube Fig. 72.— Hooked Knife Fig. 73. — Drag

Of less importance, but useful occasionally, is the hooked
knife in fig. 72, by which portions of plants may be both
severed, and drawn to land. In such cases the rushes or
other plants should be cut as low as possible ; when polyzoa
may often be found that would be searched for in vain near
the surface.

A proper collecting-stick, fitted, costs from 12s. to 15s.
and is made telescope fashion, in three lengths, resembling a
walking-stick when shut up, and stretching almost three
times the length when extended. It usually includes a

140

THE MICROSCOPE

small ring for a bottle, which is however seldom of use. A
good stick is well worth its cost ; but when every shilling
must be studied, a little ingenuity can fit a ring of wire to
the end of any ordinary stick or pole, to carry the net. And
it is the net and tube which is the essential implement.

Another useful apparatus Is the drag hook (fig. 73).
This also can be made by anybody, casting two or three
ounces of melted lead round the hooks, fashioned out of
wire. There is no use in having the points too sharp, and
some danger. Three hooks equi-distant are best. This will
answer most purposes of the knife, as it can be thrown into
a mass of weeds, and will drag out a portion as well as the
latter. It will also drag a plant up by the roots. And it is
also very useful in dragging up sticks or branches or dead
plants lying on the bottom, to which may be found
attached many forms of life that cannot be found near the
surface.

A few hints must suffice for what may be found, and
how and where it should be looked for.1 Infusoria, free-
swimming Rotifers, Desmids, Yolvox, Water-fleas and other
crustaceans, and miscellaneous small fry, will be obtained
by sweeping with the net, examining the result of every cast
with a pocket-magnifier, in one of the tubes, unless it is
seen at once what has been obtained. A pond cannot be
found that will not yield something ; but it may not be
what is wanted. It will soon be found also, that the kind
of pond affects what is found. A different run of Rotifers
will come from a pond in a farm-yard polluted by manure-
water, and from a clear one ; and what may be called
‘ clay-muddy ’ water will also have character of its own.
Water-fleas and crustaceans like sun and open ponds :
many rotifers and infusorians the same : others are found in

1 For fuller hints and details respecting the contents of this chapter, the
reader should consult Ponds and Bock Pools, by my friend Mr. Henry
Scherren, F.Z.S., to be obtained of the same publishers as this book, and to
which I am indebted for several of the illustrations.

POND AND MARINE LIFE

14 1

deep shade, or under pond-weed. Yolvox and Desmids also
like open water and sun ; and most of the algae are found in
the same circumstances.

The stationary and tube-dwelling rotifers must be sought
differently, on the pond -plants. The common weed
Anacharis is one of the greatest favourites of these. Por¬
tions may be dragged to land by the hook or knife ; but
often it ‘ pays ’ better to reach spray after spray from the
bank, and examine by the lens in one of the tubes, throwing
back each bit if it does not bear anything interesting. It
requires a little attention and practice to recognise Stephano-
ceros or a Floscule, even with a good glass ; but Hydra,
Melicerta, and Stentors are easily seen. The favourite
localities of Vorticella are the rootlets of duckweed, and
the general appearance of a group of Vorticella or Gar-
chesium is that of a tiny lump of bluish-white mould or
scum. Any such morsel of apparent ‘ mould,’ upon almost
any water-plant, will repay examination, unless the mi-
croscopist already knows what it is.

Polyzoa are more rare, and must be sought for more
persistently. As a rule they are found more or less below
the surface of the water, preferring shade. Rootlets of trees
on the banks of ponds and streams, a foot to three or four
feet deep, or submerged stems of plants, or submerged
sticks or branches, are the usual places. These have been
so ‘ hunted ’ by collectors and Clubs (at least in the neigh¬
bourhood of London, and doubtless elsewhere) that they are
more difficult to find than formerly. Trips in the country
should be utilised for them as opportunity offers. The first
time I ever found a really fine lot of Alcyonella was upon a
tricycle excursion, a few miles from Herne Bay ; on the
rootlets of pollard willows, two feet beneath the surface of
a perfectly clear stream, were large masses. Let the col¬
lector rigidly obey the golden rule, and whenever he finds
a prize, take no more than enough for his needs, leaving the

142

THE MICROSCOPE

rest for others, or to multiply in future. I have known a
so-called ‘ microscopist ’ nearly strip a pond in Epping
Forest of the beautiful polyzoon just named ( Alcyonella ),
carrying home a number of large bottles, the great bulk of
which were of no possible use to him, but would speedily
become a mass of decomposition. There is some reason to
fear that rare Polyzoa may he hunted to extinction in a
populated island like ours, as much as giraffes and elephants
in Africa ; and such destructiveness cannot be too much
condemned.

Many infusoria and rotifers are best collected with the
small bottle amongst the stems of rushes or other water-
plants. Others are found chiefly amongst the moss or
confervae that fringe walls or boards at the sides of canals.
Diatoms and desmids are found in the green and brown
films which cover the mud at the bottom in many places.
Amoeba, Actinophrys, and other lowly forms of life are to be
sought for amongst the flocculent mud or debris which falls
to the bottom of the tubes, or of the micro-aquarium.

To what was said on handling objects in § 68, it only
need be added here, that it is often well to look over a good
spray of some water-plant in a comparatively large trough,
with a low power such as 14 inch or 2 inches. A great
variety of objects will often be found, which can then be
separated. For detailed examination it is generally best to
put into a trough (as thin as possible, and filled with clean
water) a single leaf or small hit, with the specimen attached,
that the view may not be obstructed. Such small bits of
plant with animals attached, are easily handled with a
slender pair of forceps.

84. Water Fleas and Entomostraca.— Taking first in our
broad view what is likely to come first to hand, the most
evident result from casts of the net will probably be a catch
of these, which belong to the larger group of Crustaceans,
containing crabs and lobsters. The word Entomostraca de-

POND AND MARINE LIFE

M3

notes an 1 insect in a shell/ and will be seen to be very
appropriate, though it is a curious example of an entirely
‘ popular ’ meaning rendered into Greek. The proper water-
fleas or Daphnice well deserve their name, from their size,
general look, and peculiar jerky motion in the water. There

Fig. 74. — Female Fig. 75. — Male

are seven or eight species, which differ in size and shape,
but have a strong general resemblance ; and they are very
common in open ponds which are not very constantly dis¬
turbed. So numerous are they sometimes, that clouds of
them may be seen in the water in the summer; either

144

THE MICROSCOPE

whitish clouds like oatmeal, or in other cases of a quite
reddish colour. One dip of the net into such a cloud will
catch hundreds, sometimes of two or three species ; but in
almost any pond not much fouled, the net will soon collect
some. They are interesting objects, because of their trans¬
parency, which allows the digestion, the heart, and blood
circulation to be easily seen ; and they are easily examined
by pushing into a trough just narrow enough to squeeze the
enclosing shell slightly together, so as to check their motion ;
or by very slightly squeezing in the live-box.

Fig. 74 represents the female, which is much the
largest, and fig. 75 the male, of Daphnia psittaca, and fig. 76
is a rough diagrammatic section across the
body. The shell is not hinged, but joined in
'b one as shown in fig. 76, though it opens and
closes to a considerable extent by elasticity at
the back. Its outer wall is hard and reticu¬
lated, the inner soft, and in the space between
globules of blood (c, fig. 76) can be seen in motion. In most
varieties the shell ends in a pointed spine, and the edges are
often serrated or bordered with teeth.

The head alone of the body is outside the shell. It ends
in a beak, which is, however, not a mouth, and its most
conspicuous appendages are a pair of superior antennae sa
(see figs. 74, 75) which are near the beak, and very small in
the female, and a much, larger pair ia of inferior antennae,
which spring near the neck. These last divide into branches,
which are furnished with filaments thickly studded with
finest hairs. With these large antennae the creature
moves or swims, so that they must be considered a pair of
fore limbs.

There is but one eye o, though its growth has been
traced from two spots, one on each side. It has twenty or
more lenses on a black spherical surface, and is shown
more clearly at o, fig. 77.

POND AND MARINE LIFE

145

Farther back come the real mouth m and the mandibles
md, shown more clearly also in the front view of the head
of D. vetula, fig. 77. They work from the sides of the head
as in insects, and the serrated ends grind together. The
D. vetula here figured, opens the sides of the shell more
than usual, and can often be got on its back so as to show
this front view ; it is pretty common, and may be known by
its nearly round profile and the absence of any spine at the
end, as shown in the side view of it,

fig. 78. Following the mandibles come
a pair of serrated maxillae or jaws,
much smaller.

Fig. 77

Fig. 78. — D. vetula

The next appendages are interesting, and consist of five
pairs of ‘ legs,’ numbered 1, 2, 3, 4, 5, in figs. 74, 75. There
is no doubt that they answer to legs in other crustaceans ;
but the wonderful variety of result, and adaptation of means
to ends, of which the Creator’s ‘ natural laws ’ are capable, is
shown in the fact, that whereas in these creatures the
antennae have become the locomotive organs, the function
of the legs is to assist in feeding and respiration ! The first
pair may be excepted perhaps from these functions, offering
(as seen in fig. 79) the most conspicuous marks of sexual
distinction, next to the presence or absence of eggs ; for
while the first pair in the female have branches or appen¬
dages, those of the male are terminated by a hook and

K

146

THE MICROSCOPE

single curved filament, which projects beyond the shell as at
/, fig. 75. The next three pairs are furnished with peculiar
comb-like structures (cb, fig. 74) (more especially developed
in two of the pairs) and shown magnified to a much greater
degree in fig. 80, termed the branchial plates. These are in
constant motion, and serve to create a current of water up
between the sides of the shell. This both conveys infusoria
and other particles of food to the mandibles, and air to the
gills or breathing organs. At one time it was thought they

Fig. 79. — Forelegs
/. Female to. Male

were gills ; but the real respiratory structures appear to be
certain pouches also attached to the legs, and seen most
clearly in fig. 75, in whose walls the blood can with care be
seen to circulate rapidly. The hooked ‘ tail ’ of the animal,
curled round to the front, and often protruded considerably
from the shell, is constantly passed up and down between
the pairs of legs as if to cleanse them.

The stomach and intestine st are in one, and the con¬
tents can easily be seen in motion, until the refuse matter
is ejected at the anus a . The heart h is a structure with a

POND AND MARINE LIFE

147

kind of cleft in the middle, which opens and closes in beat¬
ing, and the whole will be seen to pulsate vigorously.

Passing over many other points of detail, we can only
refer in conclusion to the wonderful provision made for the
propagation of the species. The body of the animal is only
united to the shell about as far down as the heart, and
beyond that is quite detached. The eggs are first formed in
ovaries at the sides of the stomach-intestine ; but are soon
transferred into the space between the back of the animal b
and the shell, as shown at e in the diagrams, figs. 76, 81. Here,
in the ordinary course, they gradually mature until ready
for ‘ hatching ’ about the fifth or sixth day, when the young
emerge into the water. Their progress can be easily seen
in the microscope. This is the ordinary mode
of propagation ; but it is believed that all such
eggs, and probably all the adults, perish in a cold
winter, and this has to be provided against. Like
larger crustaceans, the Daphnia has at intervals
to ‘ moult ’ its shell for a larger one. At certain
intervals, a thickening of the shell takes place
previous to this, at the back, and an extra chamber (e, figs. 74
and 81) begins to form outside the ordinary hatching cavity.
At first this opens out of the latter, as in fig. 81 ; but the cavity
gradually closes entirely in, with from one to three eggs e
within it — generally two, forming a sort of saddle (fig. 74, e )
which is hence called the ephippium. The eggs so en¬
closed, also acquire much more thick and horny shells
than the others, and do not hatch at the same time. Next
moult sees the ephippium thrown off ; to float about pro¬
bably until the spring, when the eggs thus doubly-protected
hatch out in the rays of the sun.

Another of the Entomostraca, known as Cyclops (fig. 82)
is more common even than the Daphnicc , being often found
in drinking water. Scarcely any piece of water will fail to
yield specimens of this, but it does not abound in such

Fig. 81

148

THE MICROSCOPE

multitudes as the preceding. In this creature may be noted
the single eye (whence its name) and in most specimens

(the great majority being females) the
curious pair of bags of eggs dragged about
by the parent till ready to hatch ; also the
curious resemblance in form to the larger
Crustaceans. Except to the systematic
student, however, there is not so much
interesting detail as in the Daphniae.
There is another small crustacean pretty
common, the Cypris genus, which may
be known from its oval bi-valve shells.

85. Infusoria. — Next to Entomostraca, and such larvae
and other larger creatures as are easily weeded out, the
most plentiful product of collection will probably be a

number of really microsco¬

pical animalcules, or Infu¬
soria ; so called because
first observed in infusions
of vegetable matter. In¬
deed, as good a way as any
of procuring many forms,
is to place a little hay in a
tumbler of water and leave
it awhile ; or many of the
monacl forms breed most
freely (from the infinitesi¬
mal spores which are con¬
stantly floating about in the air) in water containing animal
matter, such as bits of meat, or of the head of a codfish.
Taking our pond-water, however, and putting a drop on a
slide, (only a cover-glass placed over the drop being needed
for these small creatures) the field of the microscope will be
seen more or less occupied by tiny bodies in motion. Some
may be Rotifers, to be mentioned presently : but smaller

Fig. 83. — Infusoria

POND AND MARINE LIFE

149

than these may appear as in fig. 83. The small dots we
must leave for advanced microscopists. One of the larger
creatures is Paramecium, and it is easily made out that the
body contains near one side two contractile vesicles ; that
there is also a nucleus, and one or more vacuoles or spaces
for food ; that there is an opening on one side, evidently a
mouth ; and that the whole body is covered with fine hairs,
known as cilia, in constant motion, by which the creature
swims, and by which food particles are swept into the mouth.
These particles enter the vacuoles, which after awhile ap¬
proach the surface and reject the refuse.

Nearly as common is the Swan animalcule (fig. 84),
named for obvious reasons. Very different from the other,

and twisting about in the most
ludicrous way, the same general
features may still be observed.

|| So also they may in the Barrel

Pig. 84. — Swan Animalcule

Pig. 85. — Barrel Animalcule
A. Adult B. Dividing

animalcule ( Coleps hirtus ) shown in fig. 85, which in
general appearance is more different still, being sculptured
into rectangular areas on the outside. This is also very
common, and uncommonly voracious. We have purposely
here selected three of both the most plentiful, and apparently
most diverse.

The Ciliated Infusoria are of strange significance in
the vast scheme of animal life, especially as regards the

150

THE MICROSCOPE

cilia from which their family name is taken. These are
evidently sense organs and motor organs, and especially
cause motion in surrounding particles. On the one hand, it is
deeply interesting to trace back their possible beginning in
still lowlier forms ; and on the other to see (as we shall in
Chap. XI) how ciliated cells have been preserved in the
highest animals.

These creatures are also of unique interest as being,
according to the majority of biologists, those in which * the
life of the single cell finds its highest expression .’ 1 Very
much materialistic argument has been based by a certain
school of biologists upon ‘ the single cell,’ and upon the
utter ‘ absence of structure ’ in the protoplasm (§ 86) of
which it is said to be composed. Any one of these ‘ single
cells ’ is simply crowded with * structure ’ and with marvels.
It has a skin, outside which the cilia protrude ; and this
skin consists of distinct and different layers, one of which
is strangely striated, much like muscle. And while the
mode of multiplication for a while is that of gradual con¬
striction in the middle (as seen in the Coleps , fig. 85) and
ultimate transverse division into two ; at intervals two
individuals conjugate, to form as the result, and often in
definite parts, ova2 or spores. The higher power we put
upon it, the more the ‘ single cell ’ teems with details
clearly differentiated, which only the imperfection of our
lenses hid from us till a few years ago.

86. Lower and Higher Forms of Single-celled Animals.
Leaving this question for the moment, let us take a look at
still lower forms of our ‘ single cell.’ Lower than the ciliated,
are the flagellated Infusoria, which only possess one, two,
or three whip-like filaments, by the motion of which they
swim. These flagella are so inconceivably fine, that in many
cases only the finest objectives and most skilled manipulation

1 Dr. Dallinger, summing up biological opinion,

2 Balbiani and Gruber,

POND AND MARINE LIFE

T5i

will reveal them ; and we must simply state, that here
again more perfect lenses, following i twenty years' almost
constant observation , have only lately revealed most
marvellous and complex structure in what was formerly
called ‘ structureless.’ Especially in that smaller ‘ nucleus ’
which was once held to be simple and homogeneous, it is
now demonstrated that wonderful changes of structure
invariably precede and originate the more obvious changes
in the outer portion of the animal. Only with the most
modern lenses were these facts
possible to be observed.1

We may go to lower forms
still, and look at the Amoeba,
the Materialist’s special fa¬
vourite. It is apparently a
formless lump of jelly, vary¬
ing from xouth to ^th of an
inch in diameter, and plentiful
everywhere, but troublesome
for the beginner to find. It is
generally found amongst the
flocculent sediment at the
bottom of tubes, taken from ponds of water near decayed
vegetation ; and is seen as a little shapeless spot of trans¬
parent matter moving slowly across the field of the micro¬
scope, pushing out projections now here and now there,
and contracting somewhere else. Some are clear, others
orange-coloured. In the indefinite extensions of this lowly
form of life, whose normal state may be taken as at a in
fig. 86, while extensions are shown at p in b and c, we have
probably the first approach to flagella and cilia. There is
here no definite vacuole or stomach, but these pseudopodia ,
as they are called, reach out to and engulf and flow round
some particle of food, as at p in d of fig. 86. Even here,
1 Dr. Dallinger’s Presidential Address to the K.M.S. 1886,

Fig. 86. — Amoeba

152

THE MICROSCOPE

however, is seen the nucleus, and the cavity which alter¬
nately expands and contracts, called the contractile vesicle,
and a distinctly firmer skin or ectoderm, and a number of
very fine granules which can be seen flowing about.

There are other low forms of life, which Professor Haeckel
especially insisted upon as consisting of ‘ an entirely homo¬
geneous and structureless substance, a living particle of
albumen/ or again, as that eminent Materialist expresses it,
‘ the whole body of the Monera, however strange this may
sound, represents nothing more than a single, thoroughly
homogeneous particle of albumen in a firmly adhesive con¬
dition. . . Our sharpest discrimination can detect no trace
of an internal structure.’ We were taught that in a mere
‘ homogeneous ’ particle of matter, the result of chemical
combination, resided as a ‘ function ’ all the essential charac¬
teristics of life. This was, and is still in some quarters,
stated as the now ascertained result of modern scientific
knowledge.1

And all this is now known to be simply the result of
ignorance and imperfect observation. It is the microscope,
in its highest forms, which has made the truth known. As
Dr. Dallinger records, (Quekett Club, Feb. 20, 1891) ‘ it was
gradually shown that a distinct structure was discoverable
in some cells, and subsequently it was shown that nearly all
cells and all forms of protoplasm, show a microscopic network
of fine fibres. In short, it became plain that the reputed
structurelessness of the cell was due to the inefficiency of
the lenses used, and was dissipated when competent optical
aids were employed.’ Down to the very bottom of the
ladder a most delicate and complicated ‘ structure ’ is found,
whose further exploration still baffles our utmost efforts at

1 See various passages in The Story of the Creation, by Edward Clodd. And
Mr. Herbert Spencer, from whose Principles of Biology much of it is taken,
writes himself that ‘the lowest living beings are not, properly speaking,
organisms at all, for they have no distinction of parts, no traces of organi¬
sation,’

POND AND MARINE LIFE

153

decipherment ; but the more power the microscope can bring
to bear, the farther into the background recedes that inscru¬
table mystery of Life, which the sheer ignorance of men
had rashly claimed to have unveiled.

Let us now take the upward direction. In the so-called
Sun-Animalcules (Actinophrys and Actinospherium) which
can with a little search be always found before long, we
have a nearly spherical central morsel of a cell, with many
thin filaments projecting ra¬
dially in all directions — not
nearly so thickly as the cilia
we began with, and longer,
but obviously pointing in that
direction. It is to be specially
noted that many other small
animals planned on this type
have the power of forming
curious shells round them¬
selves, of either calcareous or
silicious matter, which are
pierced with tiny holes
through which the pseudo¬
podia protrude. These creatures are then the Foraminifera,
or the Polycystina and other Badiolaria mentioned in
Chapter XII.

Passing by the forms of ciliated Infusoria from which we
started, we shall arrive at higher types of the supposed ‘ one-
celled * animals. One of these is a plentiful and lovely
microscopic object. By examining in tubes a few little
portions of duckweed from almost any pond, there will
generally be soon found what looks like a little white mould.
Any stalk of this or other vegetation with such appearance,
put into a trough and examined, will probably appear as the
Vorticella, or Bell- Animalcule (fig. 87), the cell assuming a
vase-shaped form. The rim is not quite circular, but of a

154

THE MICROSCOPE

kind of spiral shape, with a sort of disc inside ; and it is
round this rim cilia are developed. Their motion takes place
in such successive order as to produce the idea of circular
motion ; and it does produce an actual vortex in the water,
by which particles of food are swept into the cup or mouth,
to enter vacuoles and be digested as in the other Ciliata.
The contractile vesicle, and other parts of the so-called
‘ cell ’ will also be observed. The bells are as a rule attached
to long stalks, which either stretch out to great proportionate
length, or every now and then contract into a short spiral :
this motion takes place independently in each stalk, so that
the whole group is in constant motion in a beautiful manner.
The most striking way of exhibiting Vorticella is upon the
dark ground, with the stopped condenser, paraboloid, or
spot-lens (§ 56).

Reproduction takes place by division, both across, and
down the middle, as in other Infusoria. But as in their
case, at intervals there has to be a conjugation or pairing of
two individuals, resulting in the production of swarms of
spores. The cells are also capable of a quite free, swimming
existence, apart from the stalks. These stalks reveal a
striated character, with a strange resemblance to that of
contractile muscle in the higher animals.

The stalks of a colony of Vorticella usually spring at
the base from a little gelatinous mass, thus showing some
approach towards a colony or assemblage of many animals.
This kind of development finds fuller expression in actual
tree-like combinations, known as Epistylis, Carchcsium, and
Zoothamnium, in which the stems actually do branch off
from larger stems, with differences which need not be
described here.1

The largest animals of this type are the Stcntors or
Trumpet-Animalcules, of which the finest variety is known
as Stentor polymorphic (fig. 88). This is as much as ^jth
1 They are explained in Ponds and Bock Pools,

POND AND MARINE LIFE

155

of an inch long, and generally green in colour : in one
splendid variety, here figured (it is not very common, but in
no other respect departs from the type), the green is studded
with beautiful scarlet spots. The disc and rim by the cilia
much resemble that of the preceding; and like Vorticella, a
group is generally rooted in one little mass of mucus : but
every foot is capable of self-attachment, and individuals are
constantly going off as free-swimmers ; much more frequently
than the preceding.

Reproduction takes
place in the same
ways. There are
other kinds in the
family ; and the
Black Stentor is
much shorter, and
blunter at the foot
and always a free-
swimmer.

Yet another
colony form is
found in various
kinds of Ojrfiry-
dium. The animals
are vase - shaped
and ciliated on the
rim like Vorticella, but secrete a kind of jelly outside, by
which a number of them are joined together, the ciliated
ends protruding. The stalks meet in the centre. One
variety is not uncommon in ‘ nice ’ ponds, in the shape of
apparent balls of rather greenish jelly, in size from that of
a small pea to that of a marble. This type will strongly
remind us of similar gpoups of Rotifers, later on (§ 88).

We must look at one more type, generally considered the
highest of the Infusoria, inasmuch as the young creature in

Fig-. 88. — Stentor polymorplius x40

156

THE MICROSCOPE

some genera starts as a Ciliated Infusorian, but afterwards
assumes another type. The class is called Tentaculifera ,
or the Tentacled Infusorians. The example before us is
Acineta (fig. 89). 1 It is common enough on filaments of
confervoid plants more or less loaded with decayed matter,
but being only -j^th of an inch in height, has to be searched
for rather carefully : thus it differs also, in being a fixed
creature. It cannot be mistaken, being known by its stiff
stalk and the triangular shape of the cell ; from the outer
corners of which can be protruded or drawn in bunches of

minute tentacles, with knobs at their
ends. These knobs are suckers, and if
one of them touches some small animal¬
cule or other particle of food, others
immediately come to assist. But they
do not, like the cilia in other forms, sweep
down the entire object into the mass
below, where it might pass into a vacuole
to be digested : they suclz out the fluid
portion, after which the rest is rejected.
As the young are free swimmers, moving
fig. so. -Acmeta freely with the aid of cilia, the growth

out of this larval form into another, is a step upward in
organisation ; and it is remarkable that in the somewhat
rare Dcndrosoma, many individuals in the adult form unite
by their roots in a branched colony, showing also approach
towards the Hydrozoa presently mentioned.

87. Volvox. — Before passing on to the more highly-
organised Rotifers, let us look at one of the most beautiful
of all the forms of pond-life, Volvox globator. Its character
is still debated. At first it was considered an animal colony :
by the middle of the century it was generally classed as an
assemblage of lowly algce or water-plants ; now some

1 That here figured is a salt-water variety; but the fresli-water type
closely resembles it in all but being much larger,

POND AND MARINE LIFE

157

of the best modem biologists again class it as an animal ;
but it is also possible that the simple zooids of which
it is composed may stand on the boundary line between
the two worlds.

It is most common in clear pieces of water. It is a little
capricious ; and when none could be found in a likely large
pond, I have found plenty in a small tributary pool sur¬
rounded by rushes, close by. But the invaluable ring and
net will often find it in satisfactory quantities, when mere
dips with bottle alone result in disappointment. A well-filled
tube appears thickly studded even to the naked eye, with
bright light-green specks float¬
ing about. To see them well,
put them in one of the thin-
fronted troughs which has just
space enough to let them move
freely, but in one layer only,
and use from an inch to half¬
inch power.

Fig. 90 shows the object in
black and white, but this gives
no idea of its beauty. The
whole is brilliant orange-green
tracery or lacework, like an open-knitted green silk ball
kept stretched to a sphere by some invisible agency. This
is in constant majestic 1 rolling motion across the field of
view. Inside the large sphere are several smaller spheres,
seen to be precisely similar except in size, and which,
if the large one is carefully ruptured by pressure, roll about
independently, and would gradually increase in size. So
much for the whole. More critical scrutiny reveals that
each knot in the netted silk is really connected with the
next ones by threads of transparent material (protoplasm),

1 Small as the spheres are, when once seen it will he felt that there is no
incongruity in the term.

Fig. 90. — Volvox x 75

i$8

THE MICROSCOPE

but that each is, besides, furnished with two loose or free
flagella such as were mentioned in § 86. These are easily
seen round the edge of the great sphere, and by their motion
the sphere is rolled along.

If by more pressure the Yolvox is more completely broken
up, it can be seen that each of these little zooids with its
pair of flagella , is capable of swimming about alone. We
have in Euglena, a single-celled green organism furnished
with only a single flagellum , and which is so thick in some
ponds as to give the water itself, apparently, quite a green

Fig. 91. — Rotifers

colour. This too has been considered both vegetable and
animal, and there are not so many reasons for placing it in
the latter class as in the case of Volvox. In any case the
latter is to be considered a colony , composed of many single-
celled individuals.

88. Rotifers. — In the Vorticella, Stentors, &c., we found
advance from unsystematic cilia, to defined circles of them,
producing vortices in the water, which swept food into the
cavity serving as a mouth. In nearly all pond-water, be¬
sides such mere Infusorians, will be found a greater or
less number of larger creatures, in which this feature is

POND AND MARINE LIFE 159

the leading character, and which are hence called Rotifers.
But it is seen almost at the first glance, that they are
of much higher organisation. They are of all sizes and
shapes, but two special types stand out easily — the long
and narrow, and the broader and pitcher-shaped, as shown
in fig. 91.

The commonest type of the former is one in the above
figure found everywhere, both
in salt water and fresh, and
known as Rotifer vulgaris, a
more magnified view of which
is given in fig. 92. This was the
first to be observed, and has
named the whole tribe. It
shows at once that Rotifers
are highly -organised creatures.

There are evident signs of
segments ; from which the class
is generally held to have some
affinity with the worms. The
foot with which most are
furnished, obviously has toes,
and is able to take distinct
grasp of fixed points. There is a distinct stomach and in¬
testine ; and there are a pair of powerful actual jaws, which
work with a machine-like regularity to grind up the food,
swept in by the vortex set up by the two sets of cilia. On
most genera there are spots which it is thought fulfil at
least some of the functions of eyes.

Fig. 93 shows a species of Brachionus, one of the short
and broad type, very common in most ponds. The stem is
very flexible, and this Rotifer and many others twist about
on the foot in the most grotesque way ; or the tail can be
quite drawn into the sheath. There are, between the two
ciliated discs, tuffs of setae which do not move while extended.

i6o

THE MICROSCOPE

Amongst the many tribes, there are many more modifications
of this kind, which we cannot describe in detail.

A characteristic class of these animals is known as the
tube-dwelling Rotifers, from the fact that the creature
surrounds itself with a case or sheath, either of gelatinous
matter secreted, or of pellets built up round it. Fig. 94
shows one of the Floscules. The tube is of transparent
jelly, and the crown is divided into lobes which differ in

Fig. 94. — Floscules

Fig. 95. — Stephanoceros

number with the species. From each of these lobes projects
a radiating bundle of transparent straight filaments. Below
these and the edge of the crown are the cilia. To exhibit
the full beauty of a Floscule, will tax the utmost skill of a
microscopist. It requires dark-ground illumination ; and
the more perfectly the optical conditions are adjusted, and
the finer the lens, the farther and farther out the long
filaments are seen to extend, far beyond what was imagined
at first, and of almost inconceivable tenuity.

POND AND MARINE LIFE

i6i

Allied to the Floscules is Siephanoceros, which also
secretes a transparent tube. It however is furnished at the
top with five curved processes, armed with cilia all the way
up, and which spring outwards and then curve inwards
again, like the petals of a flower. This is a great favourite,
and is also displayed to best advantage on a dark ground.
It is generally found on the weed Anacharis or the filaments
of confervas, and is very easily seen in a glass tube with the
pocket magnifier. Floscules on the other hand are hard to
find. I have found both Stephanoceros and the Melicerta
next mentioned, breed very freely in the micro-aquarium,
and have had long filaments of Spirogyra studded
thickly with Stephanoceros from end to end.

The interesting brick-building Rotifer, Meli¬
certa ringens, is easily seen with the naked eye ;
and few large masses of Anacharis can be searched
long without finding it. When at work it pro¬
trudes four circular lobes with cilia, which look
like revolving cog-wheels ; but it is very shy, and
the least shake to the table makes it drop back
into the tube. It looks like a hit of brown
pack-thread ^th of an inch long (when fully
built up), sticking out perpendicularly from some small
leaf or stem, and with apparently the very tiniest morsel
of something like thistledown at the top. Under the
microscope, the brown tube is seen to be made up of little
pellets or bricks : and as close observation is continued, each
little pellet can be seen being ground up and formed in the
little mill nearly in the centre of the four ‘ wheels.’ When
complete the pellet is seized, and the upper portion of the
animal bends over and deposits the new brick on the top of
the wall it is building. There are some allied varieties ; but
this is both the commonest and most interesting.

We can only mention in the last place the clustering or
colonial Rotifers, which are combined in masses bound

L

Fig. 96.
Melicerta

]62

THE MICROSCOPE

together by a centre of gelatine, like the Infusoria of similar
type (page 154). All are plainly allied, though the shapes
of the ciliated crowns differ a little, and also the precise
mode of attachment. The most beautiful is perhaps
Conochilus volvox (fig. 97), in which the individuals are
gathered together at the feet by a central lump of jelly,
into a spherical ball, much in the fashion of a child’s
cowslip-ball. The whole group revolves through the water
like Yolvox (to which of course it is in no way allied), but
every separate Rotifer is quite capable of separate free-
swimming existence. The drawing is from a beautiful
slide mounted by Mr. Rousselet. Both Megalotrocha and
Lacinularia are usually attached to some plant or twig, and

Fig. 97.— Conochilus Fig. 98.— Megalotrocha Fig. 99.— Lacinularia

are found in running water, whilst Conochilus is found in
ponds. In Lacinularia there is a somewhat large central
lump of jelly, (fig. 99) whilst in Megalotrocha the feet only
are attached and centred at almost a point (fig. 98). The
latter may also be known by having four opaque white spots
set round the neck, or space under the ciliated corona.

It is interesting to see in these different forms of clustered
Rotifers, an evident instance of adaptation to somewhat
different conditions. The two last mentioned are capable of
revolving in a free condition ; and a small new colony often
passes some time in that way. It is obvious that in still
water this plan of existence offers advantages, both in
respiration and finding food ; whereas in running water,

POND AND MARINE LIFE

163

which of itself brings food and air, the balance of advantage
must be in a fixed position.

These clustered Eotifers are seen to the best effect on
the dark ground with a low power, needing as they do much
‘ depth of focus ’ to see the entire group fairly. They are
also beautiful objects treated as opaque, by rays from either
the bull’s-eye, parabolic reflector, or Lieberkilhn.

89. Hydrozoa. — We have in this class of animals quite
another type, of which the fresh-water Hydra is the general
pattern. There are three varieties of Hydra :
the common, a browner form, and the green,
or Hydra viridis, with much shorter and
more clumsy tentacles, looking rather like
a small budding flower. One or the other
can be found in almost any pond where
there are plants or duckweed, to which it
fixes itself by a sucker at the foot. Fig. 100
shows the Common Hydra ; but it differs a
great deal in appearance. After a full meal
the tentacles are drawn in and blunt : when
food has been scarce, the arms may stretch
out as thin threads over an inch long.

From time to time little buds form as at b,
which grow into branching Hydras. There
are also eggs at certain seasons.

In one respect the Hydroids are very low and simple
animals. There is no stomach or intestine, only a mere
bag. On the inside of this are simple ciliated amoeboid
cells ; and when the food has been dragged down into the
cavity by the tentacles, particles of it pass into the vacuoles
of these to be digested, just as in the lowest forms of life
(§§ 85, 86). The stomach is a mere sort of colony of amoebae.
Hence it is that the Hydra is able to reproduce itself from
pieces, as everybody knows ; but it is not the fact that the
outer skin will do for a stomach if turned inside out, as well

FIG. 100. — Hydra
A. Natural size
B. Enlarged

THE MICROSCOPE

164

as the inside ; it is a skin, not furnished with amoeba cells,
and has to be turned back again if the creature is to live.

But this simple stomach colony is served by a complicated
apparatus. Under the microscope the tentacles are seen to
he studded with suckers, as shown in b, fig. 100 ; and if one
of these touches a small Rotifer or Infusorian, the latter
appears to be paralysed, and in a moment or two is dead.
Even Entomostraca are usually killed in a few seconds ;
but I learnt in a curious way that a strong one, owing to
the resisting power of its shell, may sometimes escape. A
gentleman told a friend of mine that he was giving a lecture
on Pond Life to an audience of soldiers, illustrated by the

Lantern Micro¬
scope, and a living
Hydra was upon
the screen, when it
grasped a water-
flea. The latter,
instead of being
paralysed as usual,
made the most
violent efforts to

Fig-. 101. — Thread cell of Hydra viridis

escape, turning over and over, amid excited murmurs from the
soldiers, who looked on with intense interest, the power of
the Hydra having just previously been explained to them.
At last the Daphnia hurled itself free ; when suddenly every
man in the room sprang to his feet and joined in a deafening
cheer— the queerest experience, the lecturer said, he ever
had !

This deadly effect is produced by ‘thread-cells,’ with
which the tentacles are thickly studded, and the nature of
which is shown in fig. 101. Each consists of a little sac or
bag filled almost to bursting with a poisonous fluid ; and
coiled up in the fluid is a long thin filament ; the filament
as well as the fluid is therefore in a state of tension. Con-

POND AND MARINE LIFE

I6s

tact or pressure ruptures the sac, when both fluid and
filament are forcibly ejected, the end of the filament entering
the victim, and piercing it ready for the poison. The small
Hydroids are not powerful enough to incommode large
animals; but Medusae or jelly-fishes sting in precisely the
same way, and it is well known that their sting amounts to
agony. They belong to the same great class as the Hydra ;
and it is remarkable that many of the Hydrozoa, particularly
marine forms, throw off young which arc medusae, and swim
about in the same way. The thread-cells of the Hydra can
generally be made to eject for examination by adding a little
magenta or iodine to the water in the trough.

There is another single polyp in the tanks of several
Botanic Gardens, which has a beautiful medusa form also :
it probably came on some plant from South America. In
Cordylophora, which Mr. Scherren says is becoming com¬
moner in England, we have a multiple or colonial form,
many hydra-like heads branching away from one stem.
Marine forms are numerous (§ 92).

90. Polyzoa. — These are amongst the most beautiful pond
subjects. They bear much the same relation to multiple
Hydrozoa, as a colony of Botifers to one of Infusoria : that
is, the individuals making up the colony are highly orga¬
nised, with a stomach and other organs. In mere outward
appearance, some marine Polyzoa greatly resemble some
Hydroids ; but the internal organisation places them upon
a higher plane.

Only six fresh-water kinds are often found. Alcyonella
was once called the fresh-water sponge, and forms in spongy
masses round sticks or roots rather deep in the water, in
both clear ponds and streams. A small slice cut out will
display the beautiful flower-like zooids, with their circular
crowns of ciliated tentacles. There are several forms which
branch like a tree, Plumatella being one of the most beau¬
tiful. Two others have each zooid enclosed in a membranous

i66

THE MICROSCOPE

and gelatinous mass. Fig. 102 shows one or two individuals
of a colony of Lophopus crystallinus, with their crowns of
waving tentacles.1 The most remarkable however in some
respects of all the Polyzoa, either in fresh water or salt, is the
Cristatclla mucedo, which when fully developed has an
extraordinary resemblance to a thick hairy caterpillar. A
single polypide can start a colony, which at first is a small

Fig. 102. — Lopliopus Fig. 103. — Cristatella mucedo ( Allman ) x 5

convex above. The polypides are found arranged in three
rows round the back, leaving a space up the middle in which
the statoblasts or eggs are developed. And the under side
assumes a regular fluted structure, which travels over the
pond-weed or twig like the foot of a slug ! This is very
wonderful and even mysterious ; for in some respects the
undoubtedly independent individuals appear to be combined

1 The drawing is from Allman : but is far too stiff and rigid. Thei’e is a
flexible curling beauty about the crowns, as they are withdrawn or protruded,
of which this drawing gives no idea.

POND AND MARINE LIFE

167

into a sort of single animal , which moves as a whole by-
common consent, and possesses functional parts (in the foot)
which cannot be said to belong to any of the single polypides.
This problem cannot be said to be understood. The whole
is like a bed of living flowers, and its beauty can be imagined.
Fully developed colonies are rare, and chiefly found in autumn
in ponds well lit by the sun : the other Polyzoa are mostly
found deep, in more or less shady pools. The condenser or
spot-lens best brings out their exquisite beauty.

91. Miscellaneous Pond Life.— The Desmids, Algae, and
other lowly forms of vegetation are left for Chapter X. Space
does not permit us to mention in detail the many objects
furnished by larval forms of Insects which inhabit the water.
The larvae of various
gnats and other small
insects are interesting
objects. One larva
known as Corethra

7 • • .i.j. Fig. 1U4. — Water Bear

plumicornis, or that ot

the plumed gnat, is as absolutely clear and transparent as
glass, and under a low power all the internal organs can be
examined. Another beautiful object is the larva of the
Mayfly, which is furnished on both sides with many sets of
gills, by which oxygen is conveyed to the air-tubes.

Many small worms are also well worth examination. Stand¬
ing out of the mud will often be seen clusters of red threads
waving about : these are the small worms called Tubifcx.
The whole digestive and circulatory apparatus is evident.
Shown on the screen by a good lantern microscope, the
creature looks like an immense python, gaudily coloured,
and ten or more feet long.

There are also interesting varieties of water-mites and
water-lice. The most curious of the former is perhaps the
very small creature, not larger than some Rotifers, known
as the Water Bear. Except that it possesses four legs

THE MICROSCOPE

1 68

on each side, it does look ridiculously like a bear, as it
crawls about and pokes its bear-like snout into the debris
on which it feeds.

92. Marine Life. — A harvest of beauty is to be reaped
by the seaside, but the ground already gone over will
enable us to be very brief. The net will be as useful as
ever in obtaining small crustaceans, in greater numbers and
variety than fresh water affords. It will also obtain plenti¬
ful variety of salt-water Rotifers and Infusoria ; and on

many parts of the coast, skimming of the surface will
obtain various marine diatoms. There are also many beau¬
tiful forms of minute vegetation, more fully dealt with in
the handbook already mentioned.

Sponges are a study in themselves. There are one or
two fresh-water forms of interest, but the majority are
marine. It hardly need be said that sponges are now re¬
garded as colonial animals. Most of them are plentifully
furnished with a kind of skeleton of calcareous, or more
generally silicious Spicules, which of themselves make

POND AND MARINE LIFE

169

interesting slides. They may be all joined into one
skeleton, as in the beautiful Euplectella or Venus’ Flower
Basket; or they may unite as a looser but connected
lattice-work ; or (more generally) be detached. The forms
vary greatly, from the simplest needles, to types of which
a few only are given in fig. 105. Somewhat similar spicules
are found amongst Echinoderms, and the ‘ anchors ’ of
Synapta and ‘ wheels ’
of Chirodota are well-
known microscopic ob¬
jects.

Pig. 11)6. — Sertularia pumila (: natural size ) Fig. 107. — S. pumila ( magnified )

But the greatest variety, and the easiest of the more
beautiful subjects to obtain, will be found in the marine
Hydrozoa and Polyzoa, which are far more plentiful than
the fresh-water forms. Some must be sought in rock-
pools, in rather dark cavities, or under the sea-weed :
some will be got by the drag-hook (fig. 73) from depths
too great to reach otherwise : some are only obtained by
dredging from a boat. But a great deal may be gathered
from shallow pools, especially the little pools in sand
formed by the swish of the tide round points of rock ; and
many beautiful forms can be obtained by merely turning

170

THE MICROSCOPE

over the sea-wash, or loose sea-weed thrown up by the
tide, whilst still wet and fresh. Let us take only an
example or two of what the tyro can be certain to get
almost directly.

It is impossible to search long amongst sea-wash with¬
out finding a bit of sea-weed with some tiny growth upon
it like fig. 106. Place a little bit in a trough, under an
inch or two-thirds, and the beautiful Sea-oak Coralline
( Sertularia pumila, fig. 107) appears, each polypite appear¬
ing like a tiny flower, one on each side of each joint of
the branching stem. Quite as common in most places is
Corync fruticosa , shown of the natural size (a) and mag¬
nified ( b ) in fig. 108.
The tentacles here
are seen to be of
another type, with
globular heads and
shorter, and not so
free in motion as the
preceding. These
club-shaped heads
are veritable armou-

FiG. 108. — Coryne fruticosa

ries of the

stinging

‘ thread-cells ’ noticed in the case of the Hydra, and common
to the tribe : also there may generally be found some gono-
phores (c) or reproductive scions. Great interest will be found
in observing these differences, and comparing the polypites,
their mode of arrangement on the stem, their grouping
&c., which cannot here be indicated in detail. The last of
the Hydroids we will mention is the pretty Lucernaria
(fig. 109). This is usually found on sea-weed or Zostera
about (or rather outside of) low-water, and ds shown here
about the natural size : under the microscope each of the
eight arms is seen to have a beautiful tuft of tentacles,
which paralyze prey in the usual manner, while between

POND AND MARINE LIFE

171

Fig. 109. — Lucernaria

each pair of arms is a smaller organ which serves as a
sort of foot or anchor when the creature moves about, as
it is constantly doing. This beau¬
tiful Hydroid is not so common as
the foregoing.

At least one of the marine Poly-
zoa is as plentiful as the common
Hydroids. This is the pretty Nit
Coralline. It abides in rather deep
water, outside low-water mark as a
rule, on sea-weeds there found, and
especially on the pretty delicate pink
weed itself known as the common
Coralline. But I have often found
it in sea-wash after a storm. The
figure shows the appearance to
unaided vision, and when magnified ; the spectacle under
the microscope being of wonderful beauty. The distinction
in character of the Poly-
zoa will be at once seen,
in that amidst a consi¬
derable general resem¬
blance, we have obviously
a greater importance and
complexity of the indi¬
vidual animal.

Here however we
must conclude this chap¬
ter, in the hope that some
readers may find sufficient
interest in Pond and
Marine life to follow up
the study with more of
method and detail.

Fig. 110. — Nit Coralline
A. Natural size

B. Magnified

172

THE MICROSCOPE

CHAPTER IX

THE INSECT WORLD

Next to pond and marine life the Insect World probably
opens to the microscopist the most generally attractive,
extensive, and easily accessible array of subjects. The ease
and facility with which these may be prepared for preserva¬
tion, has been indicated in Chapter VII.

93. The Study of Insects. — We would specially urge the
student to make general observation of the head and mouth
organs of the living insect, in the manner indicated in §§ 94, 97.
They are so utterly different from all ideas he may have
gained from animals, that in no other way can an intelligent
conception of their arrangement, and extraordinary modifica¬
tions, be attained ; and every one who has thus examined a
few of the leading types under a low power — from 1 inch to
2 inches — will have acquired an additional range of ideas,
and an amount of realistic knowledge, which will impart to
his further studies in detail the highest interest.

Our next advice is to carry out microscopic work, as oppor¬
tunity offers, upon some definite system. This will wonder¬
fully add to the interest from the very first ; and as the
system is carried on, will maintain it to the very last. It
may be done in several ways, of which two only need be
mentioned as hints for a beginning. We may set out to
study one insect all through, or at least as regards its more
obvious parts and organs. Or we may, on the other hand,
set out to study similar organs as modified in different insects
— as for instance wings, legs and feet, mouth organs, &c.
It is obvious that the two plans may be happily combined ;
and that by pursuing either the microscopist will gradually
gather a really valuable collection, instead of a mere hetero-

THE INSECT WORLD

1 73

geneous collection of slides. Let us take an example of each
method.

94. A House Fly . — The common house fly— with refer¬
ence also to the Blow Fly as larger, but otherwise nearly
identical in structure — will serve well for the study of an
insect, and we can indeed not indicate a tenth of what it
offers for study ; if anyone desires an idea of what there is
to study in an insect, let him refer in some library 1 to Mr.
Lowne’s grand monograph on the Blow Fly. We must take a
more general view of the crea¬
ture, shown as by a strong
pocket magnifier in fig. 111.

From this we see its place
among Insecta, since its many
sedgments are plainly * cut
into ’ three divisions of head,
thorax, and abdomen. We
notice its apparently single
pair of wings, as distinguished
from two pairs in many in¬
sects : thus classing it among
the Diptera. We can perceive
in general its large compound
eyes ; and with a little care can see the spots on the sides of
its thorax and abdomen, which are the spiracles, or openings
of the air-tubes. Its three pairs of legs are plain enough,
and mark off true insects from spiders and mites.

A very little dissection, also under a low power,2 will
show how totally the plan of an insect differs from that of a
vertebrate animal. The latter has an internal skeleton : the
insect has an outside one of horny substance called chitine,

1 The book costs, I believe, three guineas.

2 Insects can be killed for dissection in a wide-mouthed stoppered bottle
in which is placed either : (1) Some fresh chloroform on wool. (2) Or some
fresh pounded laurel leaves, covered by a piece of perforated card or zinc. A
cyanide bottle is more powerful, but somewhat dangerous to have about.

Pig. 111. — House Fly

174

THE MICROSCOPE

to which its muscles are attached. The great nervous chord
runs in animals along the back : in insects it is along the
front. The animal’s circulation is from a central heart
through arteries and veins : in insects there are several
chambers or hearts, near the hack, which drive the blood
forward, not through veins, hut through the spaces between
various organs. And instead of localised lungs or gills,

Fig. 112. — Foot of Fly

the whole body is traversed by a net-work of air-tubes, with
ever so many open entrances along the sides, through which
the blood is refreshed with oxygen.

Let us now examine some of the parts and organs under
higher power. And first for the legs and feet. The leg
consists of many joints (see § 95) which are studded with
pretty long and coarse hairs. At the foot the most notice-

THE INSECT WORLD

175

able features are, first, the double claw which is general in
nearly all insects ; and secondly, the two pads or pulvilli,
which are by no means so. It is by their means that the
fly can walk upon glass ; but how this is effected can hardly
be said to be certain even at the time this is written. For
years we used to read that the pads were suckers holding on
by a vacuum ; but it was conclusively shown that this at
least was a mistake. Since then it has been generally
believed that the small hairs with which the pads are
covered, and which appear to have small disks or knobs at
the tips, exude a sticky fluid, by which the fly sticks on.
Though such hairs would let go as the pad was ‘ rolled ’ off
the surface, it would take a little force to do this ; and so in
winter, when the flies become feeble, they may often be
found stuck by their feet to the window. It is also a fact
that they do leave a little sticky matter behind them. In
1895, however, Mr. Merlin, using lenses of higher class and
wider angle than had been usually employed upon this well-
known object, claimed to have discovered that the knobs or
disks resolved themselves into very fine curved hairs. His
observations have been confirmed by Mr. Nelson and others,
and it is possible that in many cases these curved hairs may
act as very minute hooklets, in the same way as the cater¬
pillar prolegs described on p. 181. But it is hard to see how
adhesion to glass can be obtained in such a manner ; and
my own observation is that while many hairs, especially
round the edges of the pad, can be resolved as described,
there are others which cannot be so resolved ; and that in the
feet of some flies — such as Asilus for instance — whose parts
are coarser and easier observed, the knobs or disks are
unmistakeable. The question cannot be regarded as settled
at the time this is written ; but the case is mentioned as a
good illustration of how much work yet remains to be done,
with modern improved apparatus, upon even objects sup¬
posed to be well-known to every one.

176

THE MICROSCOPE

The wing consists of a double membrane, exquisitely
thin and transparent. A moderate power will show that it

is thickly studded all
over with small hairs.
The veining is of course
conspicuous, and maybe
compared with that of
other dipterous flies.
This is really the fore¬
wing only. At the base
of the posterior edge is
a curious double semi¬
circle called the winglet
or alulet : when the wing
closes one of the semi¬
circles slides in over the other, and the fore-wing over
both. This however is not the rudiment of the posterior
wing, nor is its function clearly known : the real hinder wing
is represented by the little rod with a knob at the tip, also
shown. The pair of these are called Jialteres or balancers.

They have some such function : but there are
some minute structures and nerves at the
thickened base, which will tax the highest
powers of the microscope, and which have
been thought to signify sense-organs of some
sort : here again is further work yet waiting
to be done.

In the body, we must content ourselves
with the respiratory system. A bit of the
skin with an air-opening or spiracle is easily
dissected out and mounted : it is well to
take one from both thorax and abdomen. Fig. 114 shows
one from the thorax, which is a pretty object, and shows
how the opening is guarded by a sort of sieve-like arrange¬
ment from the entrance of any coarse particles. Fig. 115

Spiracle

Fig. 113. — Wing of Blow Fly

THE INSECT WORLD

1 77

shows a portion of the branched net-work of tubes which
communicates with these openings. These tubes have spiral
threads of chitine to strengthen them, precisely as wire is
coiled inside or outside of rubber tubing for the same purpose,
as shown on a larger scale in fig. 116. The structure of
these air-tubes is more conspicuously seen in those of larger
insects ; but a portion of those of our fly can be teased out
and mounted without any difficulty.

Turning now to the head, the proboscis (fig. 117) is per¬
haps the most familiar object in every collection. The

Fig. 115. — Tracheal Tubes

Fig. 116. — Tubes more magnified

figure shows the organ squeezed out flat, which displays
well the haired maxillary palpi, mp, the powerful muscles
and tendons, the fleshy lips l with hairs round the margin,
the very minute delicate hairs which stud the whole mem¬
brane, and whose tips are tests for the very best lenses, and
especially the pseudo-tracheae or sucking-tubes, tr. These
are guarded by chitinous rings as in the tracheae, but not of
the same structure. There is no continuous spiral ; but each
ring is detached, and does not go all round, the ends being
alternately slightly thickened, and of horseshoe form, as
shown at b in fig. 118. It seems probable that fluid can be
sucked up through the entire Assure caused by the opening

M

I7S

THE MICROSCOPE

between the ends of the rings, which opens on the face of
the proboscis ; and it is also possible that the hard ridges
may have a scraping action : but here again certainty has
hardly been obtained.

There are also minute sharp piercing instruments, which
are however hidden by this mode of mounting, and are in
this hy smaller than in many other Diptera. The proboscis

Fig. 117.— Proboscis of Fly

should also be examined from a side-view, preferably without
pressure, for the general relation of parts, and for these
piercing organs, which are far more developed in some other
flies (§ 97).

Fig. 119 is the antenna. This has six joints, much less
than in many insects ; but some of them are too small to be
discerned except under the microscope. They are numbered

THE INSECT WORLD

179

in successive order in the figure. The third joint is peculiar,
and will tax the highest powers. Under these the surface
is seen to be studded with minute sacs covered with a fine
membrane. A somewhat similar structure is far more highly
developed in the flattened antennae of the Cockchafer. It
is plain that such delicate organs must minister to some
delicate sense ; but whether that sense is hearing, smell,
feeling, or some extra sense inconceivable to us, is by no
means certain— here again is work still to be done.

The eye is amongst the most remarkable features. In
the case of many insects, the great hulk of the head consists
of two immense compound eyes, as may well be seen in the

Fig. 118.— Sucking Tubes Fig. 119.— Antenna

A. Tube B. Rings magnified

head of a Tipula (daddy long-legs). Each mass is seen to
consist of thousands of facets, every one of these being a
separate lens, focussing the image of the tiny bit of the
world immediately in its axis, at the foot of a crystalline cone
behind the lens. All rays but a small axial portion are
stopped by a diaphragm of black pigment. The general
plan of a fly’s eye is shown in fig. 120, the nervous rods
whose ends take the place of a retina being expanded at
their tips near the base of the cones. The precise arrange¬
ment differs in various insects. In most flies the lenses
have outer convex surfaces ; some being plano-convex, and
others double-convex. In the eye of the Water-Boatman the

iSo

THE MICROSCOPE

outer surface is plane, ancl the inner convex. In the eye of
the Cockchafer the lens consists of two parts, an outer lens
and an inner one, in contact. The lengths and shapes of
the cones also differ.

To cut good and thin sections of these compound eyes
will tax an experienced section-cutter ; but it is easy to wash
away the rest of the eye from the cornea of lenses, with a
camel-hair brush, and mount this cornea flat, snipping the
edges a little, that the spherical surface may lie down. The
set of lenses of a large insect, such as a water-beetle or

dragon-fly, will exhibit
the focal power of the
lenses, by showing
multiple images of the
source of light. The
best plan I find to be
to place in the substage
(without a condenser,
and racked as far back
as it will go, close to
the mirror) some
bright coloured object.
Then focus first the
cornea itself. On now racking bach the objective carefully,
this will go out of focus, while there will appear many small
images of the luminous object. This is a pretty experiment ;
but it does not convey a true idea of insect vision, or of any¬
thing at all beyond the fact that these tiny lenses are real
lenses. It has been shown by actual experiment, that when
an insect’s compound eye is used to take an actual photograph
in its natural spherical condition (which has been done by
Professor Exner) there are not many small images, but that
one large image is truly pictured.

Besides these compound eyes, many insects have several
single eyes on the top of the head, between them. Three is

Fig. 120. — Eye of au Insect

THE INSECT WORLD

1 8 1

the most usual number, and these are set in a triangular
form. In our fly they are very small ; in wasps and drone-
flies they are conspicuous. The relation of these single eyes
to the others, and to the total vision of the insect, is at
present a great mystery.

Here we must leave our fly, with much about it not even
alluded to. There are the digestive system, the muscles,
the nerves, and many other details which will all repay
study ; but we must pass to another point of view. Let us
now suppose that the student desires to make comparisons,
and study resemblances and differences. As we began
about the fly with its feet, let us take for example of this
method, a few specimens from the legs and feet of other
insects.

95. Legs and Feet of Insects. — First take a couple of
larval forms, one quite general, the other much the reverse.
Fig. 121 represents the feet of the Goat-moth caterpillar, a
general type of many other caterpillars. The top figure
is the end of one of the * pro-legs,’ which
are mere membranous projections from the
skin, a pair springing from each of four
segments, and another pair from the last
segment of all. The end or foot is fur¬
nished with a row of recurved hooks all
round ; and it is the hold of these hooklets
upon the skin, which causes the ‘ creepy ’
feeling when a caterpillar crawls on the
hand. All these * legs ’ totally disappear

° J 1 r Fig. 121

with the larval stage ; and the three pairs
of real legs are represented by the short- jointed organ,
with a single claw, shown in the lower part of the
figure.

Not legs in the proper sense, but certainly locomotive
organs of a peculiar kind, are shown in fig. 122, the very
minute larva (magnified) of Tanyjpus maculatus, a small two-

1 82

THE MICROSCOPE

winged fly, found in the flocculent vegetable matter at the
bottom of ponds. It is very transparent, and a beautiful
object ; but we have only to do with the peculiar organs
shown, projecting from a segment behind the head, and at

the ‘ tail.’ At first nothing may
be visible ; but gradually a little
projection comes out, then this
forks into two, and finally from
the end of each starts a mass of
little hooklets, as shown in the
upper of the more magnified
figures. In the other fork these
are shown drawn in, as the
whole can be at will by the
muscle seen passing down the
centre of the branch. The
pair at the tail are very similar,
as shown in the other enlarged
figure.

Going now to adult insects, a normal example will be the
leg of a cockroach, which shows the five parts. First is the

coxa , a, next a small joint called the
trochanter , b, next the thigh or

Fig. 122

e 0

Fig. 123. — Cockroach leg

femur, c, then the tibia, cl, and
finally the tarsus, e, here and nor¬
mally with six joints and ending in
a double claw, which ends the limb in the great majority of
cases. Leaving the host of beetles and other insects which
have legs closely adherent to this type, we will look at some
curious developments and modifications.

Perhaps the most striking are amongst the water-beetles.
Fig. 124 is the foot of the fore-leg of the male of the great
water-beetle, or Dyticus marginalis. Here the first three
joints of the tarsus are expanded into a broad surface, which

THE INSECT WORLD

183

is fringed all round with curved hairs. From the surface

itself spring a great number
disks at their extremities ;
one very large indeed, and
others) of intermediate size,
general structure, as show:

Fig. 124. — Tarsus of Dyticus

of short hairs with cup-shaped
the greater number small, but
another (in some varieties two
All have practically the same
1 in the more magnified view
given in fig. 125. Ridges of
chitine go from centre to cir¬
cumference of the cups, and
the idea of their being ‘ suckers ’
is almost irresistible ; but it
has been all but proved that
they are not, hut simply ap¬
paratus for conveying a viscid
adhesive fluid, and in fact a fur¬
ther development of the hairs

Fig. 125. — Hairs with Disks

mentioned in describing the foot of the fly, page 175. The
stem of the large ‘ sucker ’ is easily overlooked unless looked
for and examined sideways. The other two joints are
terminated by a double claw, in this case unusually large.
The hind legs of the same insect are also worth mounting,
for their immense fringe of long and thick hairs, which act
as a propelling paddle when expanded.

Our next example is the expanding paddle of the Gyrinus
or Whirling beetle. The front pair of legs are of the ordinary
type, but the hinder pairs are peculiar (fig. 126). The
trochanter tr, femur fan, and tibia (tib) are flat plates of

1 84

THE MICROSCOPE

triangular shape, jointed at outer angles, from which the apex
of each next plate springs. But the tarsus {tar) is jointed on

at the inner angle
of the outer end
of the tibia, and
each of its four
joints is ex¬
panded on one
side into a flat
paddle - blade.
These are shown
nearly closed in
the upper figure,
and expanded and
more enlarged in
the lower, where

Fig. 126.— Paddle of Gyrimis the joints of the

tarsus are num¬
bered. The paddles are moreover fringed with flat hairs
which further increase the surface. The fourth joint bears
a small fifth one with its usual double hook.

Fig. 127 is the foreleg of the Water Scorpion. Here the
femur / becomes by far the largest joint, containing muscles
of comparatively tremendous power. These
muscles close the tibia like the blade of a knife
upon the femur, which is grooved like the handle
of the knife to receive it. The claw alone

here represents the
tarsus tar, and is not
jointed. This powerful
instrument is employed
by the insect to seize
and crush its prey. FlG- 128
Fig. 128 shows the tibia of the foreleg of the Great Green
Grasshopper, which is remarkable for the pair of orifices,

tar

Fig. 127

THE INSECT WORLD

185

oval in shape, near the upper end. These orifices are in
direct communication with the main tracheal tubes which pass
down the limb. Such a remarkable feature must have some
special function ; but
this has not yet been
discovered. It is possi¬
ble that other instances
of the same kind might
be observed if search
were made for them.

Many interesting
variations may be found
in the legs and feet
of flies, Lepidoptera,
and other orders ; but
we must only further
notice the hind-legs of
the worker-bee. Here
both the tibia and the
first joint of the tarsus
are broadened into wide
plates, but the two sides
of these plates are
differently furnished.

On one side they are
rather hollowed out and
comparatively bare ;
and in these hollows are
carried home the pollen-
bread and the resinous propolis used in finishing and
cementing cells. On the other side are a thick coating of
hairs, those on the tarsus taking the form of a brush, which
appears to be used in collecting pollen. Neither of these
special developments are found on the hind-legs of the
drones or of the queens.

3 1 '!

Outside
Fig. 129.-

Inside

-Leg of Worker Bee

THE MICROSCOPE

1 86

So much for examples of quite different systems — but
still systems — for the microscopical study of the insect world :
we must content ourselves further with a few miscellaneous
subjects of interest amongst insects and spiders.

96. Wings of Insects. — These offer very interesting and
easy subjects for the ‘ comparative ’ method, all the more
instructive because upon them the great insect orders were
originally, and are still mainly founded. The Orthoptera
were the ‘ straight’ wings, the Neuroptera the ‘ nerved’ wings,
the Trichoptera the * hairy ’ wings, the Coleoptera the ‘ cased ’
or ‘ sheathed ’ wings, the Diptera the ‘ two ’ wings, the

Fig. 130.— Scales from Convolvulus Hawk-moth ( x 20)

Hymenoptera the ‘ married ’ wings, the Lepidoptera the
‘ scaled ’ wings.

In all the orders there have been found most interesting
variations, especially in such wings as are very minute.
Some are best mounted dry : for instance the wing of the
Hairy Midge ( Psychocla ) is apt to lose the hairs with which
it is loaded if immersed in fluid — they come off so easily.
And the smaller membranous wings are so transparent that
some of them are really better as dry mounts. But the
majority mount in benzol-balsam quite well. The thick
horny cases of many beetle wings need to be made trans¬
parent by long soaking, in the manner already described
(Chap. VII). The elytra or wing-cases of iridescent beetles
make beautiful objects mounted opaque for the Lieberkuhn

THE INSECT WORLD 187

or reflector, and so do bits of wing from the iridescent and
bright-coloured butterflies and moths.

Whole wings of the latter mount well in balsam. It is
very easy to mount the whole or a piece of a wing to show
the arrangement of the scales. Some wings will show the
scales so sparse that they stand clearly apart, and in the

In flight

Separated

Fig. 131.— Wings of Bee ( x 7)

clear-winged moths the scales are rare, and more or less
interspersed with hairs.

A scale is in fact only a peculiar development of a hair,
and the particular development varies amazingly. Scales
rubbed off from different genera, and even species, and
mounted dry, will make an interesting set of slides. Also
from the Convolvulus Hawk-motli (and many others) sets of

1 88

THE MICROSCOPE

scales may be so selected and mounted as to illustrate
clearly the gradual development of hairs into scales. Fig. 130
is drawn from a set of scales I had so mounted, all of which
were taken from the same individual insect, and show the
transition clearly. Again, it will be found that the mem¬
branous wings of all insects are covered with small hairs ;
and if the wing of a Gnat be carefully mounted, it will show
that on many portions, especially the veins and the edges,
these hairs have become scales, in no essential respect differ¬
ing from butterfly scales.

Fig. 131 shows the Hymenopterous or ‘ married ’ type of
wings. The two are also shown separated, to manifest on the
hinder wing the row of little hooks standing up, and on the
posterior edge of the fore-wing, the turned-down edge into
which the hooks catch. Thus, when in position, the two
are joined together. It will be found very interesting to
trace this character running not only through the Bees and
Wasps, but through such totally different-looking groups of
insects as the Ichneumons, Saw-flies, Gall-flies, and Ants,
even of small size.

97. Mouth Organs. — Similarly interesting and important
are the mouth organs, equally important in classifying with
the wings, and sometimes more so. For a primitive and
normal type take the Cockroach (fig. 132). The main portions
of the apparatus are here shown separated. With care the
mouth-parts of insects can be so separated, and laid down
upon the slide, arranging them in benzol-balsam, and leav¬
ing the preparation to nearly dry in the balsam, protected
from dust, before laying down the cover-glass. Such a
preparation is known as the trophi of an insect, and a set of
trophi, of various species throughout the orders, is a most
valuable and instructive set of slides, such as cannot be
bought for money anywhere.1

1 A couple of dozen I possess, kindly mounted for me by the late Mr.
Tatem, of Reading, are among the most valued slides in my collection.

THE INSECT WORLD

189

Looking now at the cockroach’s mouth, the whole ap¬
paratus is a complete upset of all our vertebrate notions,
though in many respects similar to that of Crustacea. There
is a sort of roof or upper lip a called the labrum. Next
come the first and principal pair of jaws b , the mandibles,
but which work from side to
side, as do all the organs, and
not like ours. These are hard
and strong and toothed. Below
these are a second pair less
strong, but toothed also, c, called
the maxillce, and each of which
has a palp or feeler, but which
is more really a sort of hand
to help in manipulating things.

Beneath all is the lower lip or
labium , e, but which under
higher power is made out to
be essentially a third, but still
less powerful pair of maxillae, each of which also has a palp.
In the cavity of the mouth is a lingua d, which is something
like a real tongue, receiving the duct of the salivary glands.

Leaving this ancient type, it will be most interesting to
see how in the main it is preserved in all the Beetles or
Coleoptera, with chiefly the more or less complete disap¬
pearance of jaw-like character from the parts of the labium ,
or third pair. Next to be modified are the second pair, the
maxillae. It will be found of great interest to follow up, with the
help of some really intelligent work on entomology, the trans¬
formations of these organs and their palpi, with those of the
labium, into the ‘ tongue ’ of bees and wasps, the ‘proboscis’
of moths and butterflies, and so on ; the mandibles either
remaining as biting jaws — as in the bee— or being reduced
to rudiments, as in butterflies. We take two examples from
a different order again, the Diptera, in which these organs

THE MICROSCOPE

190

Fig. 133.— Mouth Organs of Drone Fly ( x 25)

THE INSECT WORLD

191

are so modified as to assume more or less of the character
of weapons. Fig. 133 shows those of the Drone-fly, taken
first as resembling in many respects those of the common
fly already mentioned (fig. 117). The soft parts of the
proboscis, with their pseudo-tracheal tubes, are almost
precisely similar, as will be seen, and represent the anterior
part of the labium. The labrum, mandibles, and maxillae
are in the Diptera converted into lancet-shaped organs
more or less developed, which both pierce the skin of
animals, and form tubes by which the juices may be sucked

Fig. 134. — Trophi of Tabanus

up. In the Drone-fly they are much larger than in the
House-fly, and easily made visible. The figure shows
clearly the maxillary palpi, next which come a couple of
lancets, and finally two instruments resembling by com¬
parison straight two-edged swords, and a very peculiar one
with prongs or teeth at the end, the purpose of which
appears to be to enlarge and irritate a wound and so
increase the flow of fluid, which is probably sucked through
a tube up the centre.

The further development of this type of mouth is well
seen in the organs of a Tabanus or Breeze-fly, fig. 134. These

192

THE MICROSCOPE

are here shown laid out separately, with the antennae added,
as a specimen of the mounted ‘ trophi ’ already recom¬
mended. The general strong resemblance in the several
organs is easily seen at a glance, and also that the line of
development is carried still further, the fleshy ligula or
proboscis being diminished, while the offensive organs are
increased in power, till they appear like half a dozen
different instruments from a cutler’s shop.

In Bees the mandibles remain, and both pairs of palpi, but
other organs are modified into the tubular proboscis, so well
known. In Lepidoptera the mandibles are mere rudiments,
and the proboscis is immensely long, and kept when not in
use curled up into a spiral. It is really in two halves, the
tube being made up of two half-tubes or gutters with their
open diameters in contact.

98. Antennae. — Many of these organs are of exceeding
beauty, especially the feathered or plumed ones of many
Lepidoptera and of the Gnat family. They differ to an
altogether extraordinary extent in different orders, and even
different genera. They appear in very many orders to have
most to do with the mysterious power the males of many
insects possess, of finding the females ; so that a crowd of
some moths will gather round an unpaired female, even
when concealed in a pill-box about one’s person. At all
events, in a large number of cases, as the common Cock¬
chafer — the antennae are immensely more developed in the
male ; and when this is the case it is well to mount both
sexes upon one slide.

Eyes have been sufficiently dealt with on p. 180. Such
exquisite sections can be obtained professionally mounted,
that it is a pity not to avail oneself of results which can
hardly be rivalled by any amateur.

99. Ovipositors. — The ovipositors of female insects offer
immense variety in the different orders. In Crickets and
Locusts they are very large indeed. The most interesting

THE INSECT WORLD

193

are perhaps those of the Hymenoptera. In Ichneumons
they are mostly long and pointed. In some genera they
become boring organs, as in the great Sir ex gig as. In
Bees and Wasps they are connected with poison reser¬
voirs and become stings, which are accordingly confined
to the female sex entirely, the males being helpless. More
or less serration at the tips belongs to nearly all. In Bees
and Wasps the teeth are barbs ; but the most interesting

Fig. 135.— Saws of Saw-fly ( x 40)

development of this toothed feature is in the Saw-flies, of
which there are many genera. The saws of a very
common fly are shown in fig. 135. In every case the saws
are in pairs, and each ‘ blade ’ or cutting-saw works
backward and forward in a grooved sheath which em¬
braces its back edge. The two saws work side by side
with alternate motions, and thus cut a groove in the leaf,
or rind, in which the eggs are to be inserted. The exact

194

THE MICROSCOPE

pattern of teeth differs somewhat ; but a very curious fact
is that most of them, as can be seen from the figure,
almost exactly resemble a pattern which has been adopted
in some surgical saws, and some saws made in America for
various purposes.

100. Eggs. — Ovipositors naturally remind us of the eggs
of insects. Many of these are very beautiful objects.
Fig. 136 shows some eggs of butterflies magnified, while the
upper right-hand figure illustrates the natural size, and the
manner in which they are generally deposited. This how-

Fig. 136.— Butterfly Eggs

ever differs a great deal ; some being attached to a kind of
stalk, and others laid in coils or spirals round small twigs.
They are best mounted ‘ dry ’ upon a black ground.

Still more striking are the eggs laid amongst the feathers
of birds by the numerous parasitic insects which infest them
— each genus of birds, as a rule, having its own parasite, as
each genus of animals has its own flea. Fig. 137 reproduces
by permission a beautiful drawing by Dr. Dallinger of
three of these eggs. That numbered 1 is the egg of a para¬
site of the Peacock; No. 2 that of a parasite on the
Hornbill ; and No. 3 is from the Australian Mallee-bird.
But analogous forms are to be found in the plumage of
our home birds, game, and even of domestic poultry.

THE INSECT WORLD 195

101. Spiders. — Various points in these wonderful crea¬
tures are worthy of microscopical demonstration. Their eyes
are single instead of compound, and usually eight in number,
arranged as an upper and lower row of four each. By skil¬
ful section-cutters, thin sections of these are prepared, show¬
ing the lens, and the rods of the retina. The head and

Fig. 137.— Eggs of Bird Parasites

jaws are easily mounted, and convey a vivid idea of the
terrible offensive ‘power of these creatures in proportion to
their size. Imagine a cat with a pair of claw- jaws the size
of those of a large crab, and with poison in the hollow
fangs : that is the scale on which a spider is armed. But
perhaps most general interest attaches to the organs asso¬
ciated with the web and its formation.

n 2

196

THE MICROSCOPE

Fig. 138 shows the cloaca and spinnerets of the Garden
or Geometrical Spider. The Spinnerets at a first glance
appear four, but are soon seen to be six in number. They
are not single teats or tubes, but contain in some species
up to a thousand separate tubules. The microscope itself
will make apparent, that the lower pair contains tubes
much larger than the others. But it has required atten¬
tive observation to discover, that these large tubes emit a
more liquid and viscid secretion than the others, and that
the spider has the power of ejecting filaments from the
hundreds of pores at once, either so that they remain

PiCi. 138. — Spinnerets

Pig. 139. — Foot of Garden Spider

separate threads, or so as to unite almost immediately
into one stronger thread. She spreads them when making
the holding end, or anchorage of a line, before it unites
into one ; also she spreads them when winding up a vic¬
tim in a shroud of silk, turning it over and over with her
legs whilst emitting the tangle of silky filaments. In ordi¬
nary web-spinning she converges them, so as to join into
one cable almost at once. Statements about her ‘ hand¬
spinning ’ are fables.

Yet the foot has to be in relation to the lines, upon which
the insect is constantly running with the greatest rapidity.
It is terminated by claws armed with many comb-like teeth.
The great claws may be two, but in many garden sorts are
three, with smaller claws which can oppose them as well—

THE INSECT WORLD

197

the latter are shown separate in the figure. However quick
the pace, the line falls between some pair of the teeth in the
comb, and the foothold is sure and steady.

The radii of the web are spun with ordinary silk (which
can in fact be gathered and spun as silk, were there enough
to be worth while) which is not at all adhesive. By these
the spider runs from the centre to her prey. She spins these
first, after her marginal foundations ; and next spins a con¬
tinuous Archimedian spiral thread from centre to circum¬
ference, of the same non-adhesive thread. She then goes
back from the circumference to the centre with another
spiral ; but this time the large spinning tubes are also
brought into play, and the hard thread as it is spun is

B _ _ _ _

Fig. 140. — Spider Threads A. Adhesive B. Unadhesive

covered with viscid non-drying fluid , which appears chiefly
as globules strung upon the thread. Larger globules at
pretty regular intervals, have generally very tiny ones inter¬
spersed all along the intervals, as in fig. 140. It is these
globules which detain the flies.* 1 They adhere to the spider
also, if her foot touches a globule, contrary to the common
belief. She constantly has to tear away rungs from her
ladders to get free of such mishaps, and as constantly
repairs them : and in some cases at least, unless a fresh web
is made altogether, which is often done, the adhesive lines
have to be renewed next day, the fluid probably drying.

1 It is often stated that in spinning the viscid set the spider cuts off and
eats the unadhesive spiral, which is used as a bridge only. Without affirm¬
ing that this never occurs, I am absolutely certain that it was not done in
cases which were watched. It may be that species differ in that point.

198

THE MICROSCOPE

Much has been written, in ignorance of the true state of
the case, about the wonderful skill of the spider, and her
dexterity, in forming such a number of these viscid globules
so rapidly. If it be remembered that there are hardly ever
less than 100,000, and often a quarter of a million, in a web
spun in less than an hour, it will be seen that such dexterity
is impossible. The truth is far more really wonderful, than
any such mere marvel would be (if physically possible).
Years ago the late Mr. B. Beck observed that these threads
when first spun were quite smooth, but that first undulations
and then the globules rapidly appeared. But only quite lately
has Professor C. Vernon Boys, F.B.S., shown clearly that if
a fine quartz fibre be stroked with a straw dipped in castor-
oil, the fluid in a few moments forms undulations and then
drops, in just the same way ; nay, these drops of castor-oil
are equally effective in catching flies. This, then, is what
happens. The tough thread, as fast as it is spun, is coated
with the fluid ; and the web-spider’s mode of subsistence,
and her complicated spinning organs, are bound up with,
and work in association with, those profound molecular laws
which cause a jet of water to break into drops, or hold
together a soap-bubble !

102. Life Histories. — Only one more line of work can be
indicated here. There are many cases, especially where
very minute insects are concerned, in which the use of
organs, or the complete life-history, is yet unknown. Three
illustrations must suffice.

More than thirty years ago (1863) Sir John Lubbock
discovered that a minute hymenopterous insect (called by
him Polynema natans) descended into water, both swimming
and crawling therein. Since then the insect is only known
to have been observed three times until quite lately (May 4,
1895), when it was rediscovered near London by Mr. W.
Burton and Mr. F. Enock, and kept under observation. The
first result was the discovery of the unknown fact, that the

THE INSECT WORLD

199

specimen observed never left the water during several days !
Possibly its history may be completed by these observers ;
but when it is added that some allied species only measure
-73-th inch long, and that there are other insects quite as
small (the Trichopterigia among beetles being only ^th to
^th inch) it will be seen what lines of inquiry amongst the
more minute insects await the microscopist.

There is another class of histories, in those insects which
present the phenomena of alternate generation. Thus, the
Aphides will go on for generations producing only females,
which bud off as if were, asexually ; while late in the autumn
true males and females appear, which lay eggs in some
sheltered place, thereby preserving the species through the
winter till next spring. But M. Lichtenstein discovered that
some Aphides presented more complex phenomena, several
different forms appearing in succession before the final true
males and females appeared. It has not been determined
yet how far this complicated system of generations is common
to the family ; but it is pretty certain that there are many
Aphides whose full history is unknown.

Our last illustration is from the Gall Flies, and that
member of the Cynips family which produces oak-apples will
do as well as any. Everybody knows the oak-apple, which
is a large gall formed by irritation of the tissues in the twigs
of oak-trees, by maggots hatched from eggs laid therein by
flies of the Hymenopterous order. On the gall the maggot
feeds, changes to a pupa, and hatches into a hymenopterous
fly. All this seems like the ordinary development of an in¬
sect. But it is only half the story. The flies thus developed
do not deposit eggs on the twigs again, but on the roots,
where each gall is only as large as a pea ; many galls being
however often fused into one mass. The galls are also this
time reddish, and not golden-brown as the oak-apples were.
These galls are formed in autumn, and the larvae hatch in
winter, and are pretty similar to the others. But the perfect

200

THE MICROSCOPE

insects are so widely different, that till quite recently they
were classed in different genera. They are all without wings,
and all females only, which lay their eggs in embryo buds
on the twigs ; these eggs forming oak-apples in turn. These
facts were discovered by Dr. Adler; and since then many other
double forms of galls and insects have been discovered, in a
few of which the second form develops both sexes, though
of different forms to the first. In nearly all these cases the
two forms were formerly described as distinct genera, and
the eggs are laid and galls formed in different situations,
and are of totally different appearance.

There is little doubt that patience and research will
discover other cases of these double forms, and perhaps
associate into one descent other insects now considered dis¬
tinct. But we have said enough to indicate more than one
line of work calculated to afford the greatest interest, in
the wonderful Insect World.

CHAPTER X

THE VEGETABLE WORLD

Under this heading, again, we can only briefly indicate some
directions in which the microscopist may find interesting
and profitable occupation.

103. Cells. — Thin slices of the cuticle of leaves, or of
seeds, and sections in various directions of any tissues, or
hairs from any plant, removed and placed on a slide under
a cover-glass with a drop of water, will soon give a general
idea of the numerous cells of which any highly-organised
plant is composed, and of their great variety in form and
character. The tissues of the whole plant are at once seen,

THE VEGETABLE WORLD

201

far more easily than in the case of animals, to be built up of
many single cells.

104. Single-celled Plants. — If the student possesses
already a little skill, and a really excellent ^th which will
bear a high-power ocular, a small particle of yeast will enable
him to study cell-growth and multiplication in a very simple
form : but this plant is too minute and transparent for
beginners.

Fig. 141. — Closterium lunula

A. Perfect cell B. One end enlarged to show cyclosis D. Cell just divided

Bacteria are not treated here for the same reason. The
serious student will have recourse to one of the many
special treatises, and will require a good J and an immer¬
sion. The ordinary ones can be well seen with a good ^ or
^ and compensating ocular, and slides can always be pro¬
cured at 2s. or 3s. each.

Desmids grow in quantities in most ponds, especially those
in which the water looks green ; that colour being usually given
by swarms of these plants, on which many aquatic creatures
feed. Some may be got in the net : others require to be

202

THE MICROSCOPE

scraped off the bottom, or from the stems of submerged
plants. They are of great variety in form, some of which
are very beautiful : we can take but one example, from the
group of Closteria (fig. 141). This is a very common
Desmid, and shows clearly the main phenomena of the family.

First of all, it is easily seen in this Desmid that the living
cell is full of living protoplasm, whose constant motion in
streams presents the phenomena of cyclosis (§ 114). The
stream of particles is most clearly seen along the edges, but
with care can be made out anywhere under the enclosing
membrane. Every now and then it carries along with it one
of the larger particles (marked b in a) into the chamber at
the end. These larger particles are not found in the central
dividing space (a) between the two halves of the Desmid ;
which leads to the Inference that the cell in this state (a) is
already partially divided into two. At b the end is shown
much more enlarged. It is then seen that the inner mem¬
brane or endochrome is furnished with cilia (b, a) which are
in motion. The protoplasm outside the endochrome shows
currents in opposite directions, as denoted by the arrows : in
the end chamber are also very strong currents shown by the
arrows converging towards b, and these currents appear
thrown back or reflected as by the arrows pointing to c.
The whole gives a very vivid idea of the active life going on
in the cell.

At d is shown the simplest form of propagation, by cell-
division. The endochrome retreats more and more from the
central space a in diagram a, while there is outer constriction
at the same time, till the two separate entirely as at a a in
diagram d, the exterior becoming rounded as at b. The
exterior ends b then elongate, while each half begins to
constrict in the middle ; and in about six hours each half
has become symmetrical or nearly so, like the parent.

This and other Desmids present also at intervals
the phenomenon of conjugation ; two individuals uniting,

THE VEGETABLE WORLD

203

and producing a zygospore which forms a fresh series. Some
good fortune as well as patience will be required to secure
specimens in the stage of conjugation. This simple con¬
jugation of two cells apparently precisely alike (though it
cannot be proved that it is really indifferent what cells enter
into union ; there may be differences, of the nature of sex,
which the microscope cannot discover) is evidently the
earliest steps towards the truly sexual mode of propagation.

105. Algae. — Almost every pond open to the sun will
furnish a variety of these plants, mostly in the shape of
long green filaments, which only need to be placed upon a
slip with a drop of water and a cover-glass. In these
filamentous forms we have a number of similar cells joined
together end to end, the simplest form of a complex structure.

Fig. 142. — Spirogyra conjugating (Sachs)

Many of the plants are of great beauty when seen on the
stage, especially those of the Spirogyra family. The cells
of this group for a while divide in the middle, each half
lengthening so as to extend the filament ; and during this
period the endochrome and chlorophyll bodies usually re¬
main a compact mass. But as the cells mature, the endo¬
chrome contracts, and shapes itself into a beautiful spiral
green band inside the outer tube, from which the family
takes its name.

The Spirogyra furnishes another example of conjugation
between two cells similar to all appearance : but there is a
distinct further advance in the process. Two adjacent
filaments whose cells are mature, throw out from each cell
lateral protuberances (fig. 142, a). These grow till they

204

THE MICROSCOPE

meet ( b ), and meantime the spiral contents of each cell
shrink and gather into an ellipsoidal mass. Then the cell
wall opens between the two projecting parts, and one of the
two ellipsoidal masses passes gradually through to coalesce
with the other. Close observation, however, shows that
the spiral band is preserved, though forced together into a

compact body ; and moreover after
coalescence the two bands are so
arranged as to form one band. The
passage of one mass into the other
is shown nearly complete at a in
fig. 143. fig. 143, and quite so at b. The

Conjugation complete (Sachs) , > , i p j

‘ zygospore thus formed remains
apparently quiescent for some time, often months ; and then
begins to elongate and divide to form a new filament.

It is to be remembered that, while the two cells show no
observable difference, the contents of one are entirely
absorbed by the other : also that all the cells in one filament,
are usually absorbed by the cells in the other. These facts
appear to indicate that there is a difference in the two cells,
though it eludes present observation ; and that we have here
a further advance towards difference and union of sexes.

106. Alternation of Generations. — We further have in the
above an early stage of the phenomena called Alternation of
Generations. Just as in the Aphides (p. 199) we had gene¬
rations budding from imperfect females only, to be succeeded
at intervals by true union of sexes ; so here in the vegetable
world, we have multiplication for a period by division and
growth of cells only, but followed after a while by union
of two individuals. These so far appear alike ; and yet we
have indications that already they are in some way different.
But in some form or other, Alternation of Generations
between some form of cell-multiplication, and the formation
of new individuals by conjugation of two, prevails through
the entire vegetable world. The details of Fern propagation

THE VEGETABLE WORLD

205

are very remarkable, but must be studied in botanical works :
in this class of plants alternation is regular. Potatoes have
been propagated almost entirely for generations by * eyes ’
or tuber-buds. In mosses, it is the asexual product usually
observed. The Cham family form at intervals perfect fruits ;
but there is one species — C. crinita — in which only female
organs have been found for many generations in most
Continental localities, though a very few male organs have
now and then been observed in Transylvania. There are
many various phenomena of this kind, which are not fully
understood, and will furnish subjects of profoundly interest¬
ing microscopic research to the botanical student.

107. A Life History . — As just one illustration, let us take
the history of the common mildew or rust in wheat, which has
only lately been established by the re¬
searches of De Bary, and which may be
compared with that of the Gall Fly in
§ 102. As in that case, investigation has
shown that what were once considered
fungi of different genera, are but stages
in the history of one. There are found
in spring on the leaves of the common
Barberry orange-coloured swollen spots,
which under a powerful hand-magnifier present the appear¬
ance of clusters of small cups, as in fig. 144. On making
vertical sections, the swollen parts are found to be full of
the thread-like mycelium of a Fungus. There appear
however to be two forms of fruits, which themselves were
once considered two fungi, but are now known to be
but two forms of one, as shown in fig. 145. Here x is the
natural thickness of the leaf, which is so swollen by the
thread-like mycelium. From the mass protrude on one side
the ‘ spermogonia * sp, urn-like receptacles lined with my¬
celium, whose threads protrude through the epidermis. From
these receptacles are detached minute bodies called spermatia .

206

THE MICROSCOPE

The other form of fruit is much larger, and is the old
JEcidium of botanists ; a name still used, but only as a
stage in the history of the mildew Puccinia graminis.

Fig. 145. — Section of Barberry Leaf (Sachs)

u

These also spring from the mass of mycelium, and at first
are nearly globular bodies per beneath the cuticle, lined
with a layer of cells called the peridium. These finally

break through to form the open seci-
I dium cups cie, and which give off
chains of larger aecidium spores. As
^ will be seen, a section showing these
stages of development makes a very
beautiful microscopic object.

These spores (whose union in
some way with the spermatid is pro¬
bable, but has not even yet been
proved) cannot germinate upon the
Barberry. But on the stem or leaf
of wheat or grass, they throw out
germinating filaments which enter
the pores or stomata (§ 113) and form
another network of mycelium there.
This mycelium produces another form of spore or fruit known
as ztredo-spores or fruits, and shown at u in fig. 146, which

Fig. 146.

Uredo-spores (Be Bary)

THE VEGETABLE WORLD

20 7

are borne at the ends of filaments, and are known as the
‘ mildew ’ on the wheat. These uredo-spores germinate on
the same grass or wheat, and the mycelium from them forms
similar spores, this going on for several generations. At
last the older uredo-plants
begin to produce amongst the
others, at first single speci¬
mens te (fig. 146) of another
and larger two-celled kind of
spore, known as teleuto spores.

Then the uredo-spores cease,
and the uredo-plant produces
only groups of teleutospores,
as in fig. 147, which break
through the cuticle of the wheat-plants. These teleuto¬
spores remain on the haulms through the winter, and in
the spring they germinate and throw out filaments, at the
ends of which yet another kind of sporidia are produced.
These germinate and produce mycelium only when they find
a lodgment on the surface of the leaves of the Barberry,
with which event the cycle commences over again.

Fig. 148. — Moulds

108. Fungi, Mosses, &c. — All the lower kinds of plants
present beautiful objects for the microscope, but cannot be

Fig. 147. — Teleutospores ( De Bary)

2o8

THE MICROSCOPE

particularised. Almost any kind of 1 mould ’ becomes, when
magnified with a low power, a miniature forest of great
beauty. Higher power reveals the cell arrangement. In
fig. 148, No. 1 is a mould growing on the stem of a plant;
No. 2 Aspergillus glaucus , the cheese-mould : No. 3 a mould
found on many decayed vegetable substances.

Many of the Mosses, Lichens, and Liver- worts are also
interesting objects. We may just mention the peristome
of many Mosses. The product of fertilisation in these

plants is a theca or urn, which is at first
closed by a lid. When the fruit is mature
this lid falls off, and the mouth of the
urn is then found surrounded by a toothed
or fringed structure — the peristome. Fig.
149 shows a very beautiful peristome in
which the fringe is double, the inner one
Double Peristome being joined by cross-bars so as to make a

kind of trellis-work. There are many
beautiful varieties of double as well as single peristomes to
be found in the Mosses, and a representative series would
make a beautiful and interesting collection of objects.

109. Development of Sex. — We can hardly help seeing,
even from the few facts mentioned in § 105, how very deep
down and fundamental is the phenomenon of sex in the great
realm of organised Nature. Even in the kingdom of plants,
only in the very lowest types are two conjugating individuals
alike, or can one take the place of the other ; if indeed it is
possible even in them, which we have seen reasons for
doubting. The higher we get in organisation, the more
obviously different in appearance, as well as work, do the
two beings become, and the more utterly impossible for one
to fulfil the functions of the other.

This development may be traced very low down. There
is a filamentous Alga called Sphceroplea which is composed,
like Spirogyra , of many single cells joined end to end, and

THE VEGETABLE WORLD

209

which multiplies by lengthening and division for a time.
But when mature, instead of two (apparently) similar cells
conjugating as in figs. 142, 143, some cells develop into spo-
ranges, while others are filled with small active bodies with
flagella, called antherozoids. A small aperture forms in the
side of each kind of cell, by which the antherozoids escape
from their own mother-cell, and enter the other cells, which
are thus fertilised.

As we go higher the organs become more complex. There
is a small fucoid plant common in marshes, called Vaucheria,
which at one stage expels from the ends of its club-shaped
filaments large spores, remarkable for being completely

Fig. 150. — Vaucheria
Before Fertilisation

Fig. 151. — Vaucheria
After Fertilisation

ciliated all round their exterior : these are considered non-
sexual zoospores. When the mature stage has arrived, the
filament develops two different organs very close together,
short and broad but rather curled, as in figs. 150, 151. The
sporange a is an enclosed vessel filled with cells ; the adjacent
antlieridium b (fig. 150) is filled with active antherozoids.
The sporange bursts open first, and the antheridium soon after,
when the antherozoids enter the sporange and fertilisation
takes place, the result being a kind of fruit-spore as in
fig. 151, containing several brown bodies. This large spore
it is which sprouts into the next generation, after an inter¬
val, usually of three or four months.

In the Mosses and Ferns the analogous organs are further

o

210

THE MICROSCOPE

developed, into pairs called antheridia and archegonia, many
of which form beautiful objects for the microscope, especially
when cut into very thin sections.

110. Flowers and Fertilisation. — It is in the true flower¬
bearing plants, however, that we find reproductive organs
most perfectly developed. In these the antheridia become
anthers , carried on the ends of stamens, and forming pollen-
grains instead of spermatozoids. These pollen-grains are of
very various forms, a few being shown in fig. 152. They are

easily collected, and many make
beautiful slides mounted simply in
benzol-balsam. Sections across the
anthers also make fine objects if
skilfully made, and will show the
pollen-grains within.

The female portion of the flower
comprises the receptacle called the
ovary, containing from one to many
bodies called ovules , each of which
(if fertilised) becomes a seed. From
the top of the ovary extends a
column termed the style , composed
of soft tissue, the stigma or ex¬
tremity of which exudes when mature a viscid and sticky
secretion, easily seen on any fair-sized stigma with a pocket
magnifier. The unfertilised stigma presents no opening
whatever.

If pollen-grains are placed in water, they often burst and
disperse numerous particles ; and it was formerly thought
that these were a kind of antherozoids, and were dispersed
upon the stigma, to work their way to the ovules. Not until
1822 did Amici discover that this was not so, but that the
pollen-grain, on adherence to the viscid fluid, began to push
out and grow , first as a short protuberance ; then a short
tube ; until this tube gradually grew all the way down

Fi(t. 152. — Pollen-grains
1. Primrose 2. Auricula

3. Anemone 4. Iris

5. Clematis 0. Tulip

7. Rose 8. Buttercup

9. Hollyhock 10. Passion

Flower

THE VEGETABLE WORLD

21 1

between the cells of the stigma. This is represented in
fig. 153, showing a section of the stigma of a Thorn-apple.
The stigma with its viscid surface is shown studded over
with pollen-grains, which have grown downwards as shown
by the dark shading. To exhibit this in section will tax the
skill of the microscopist, especially
in section-cutting. The stigma of
the CEnothera or Evening Primrose
is one of the easiest to manage ;
and the best way is for the opera¬
tor to artificially apply pollen to
mature stigmas, and then to section
one morning and evening every
day, till he gets what he requires.

A single tube can rarely appear in
the section for any great length ;
but the most interesting portion is
that near the original pollen-grain.

This growth of the pollen-tube
is a most wonderful phenomenon.

Its length is often hundreds of
times the diameter of the pollen-
grain, and may be as much as two
or three inches in some plants.

The question suggests itself as to
how it occurs. The answer is not
doubtful. The viscid sugary secre¬
tion first nourishes the pollen into
growth, but the tube quickly grows
beyond this source of nourish¬
ment, opening for itself a kind of canal down the style.
The cells lining this canal become however also charged with
sugary matter , which continue to nourish the pollen-tube,
and further stimulate its growth. This secretion of
nutritive matter is plainly produced by the irritation caused

0 2

Fig. 153.— Pol len-tubes growing

212

THE MICROSCOPE

by the pollen-tube, as it pushes its way farther and farther
down, the phenomena being analogous to the secretion
caused by Gall Insects, described in § 102. The lining of
extra-nutritive conducting-tissue around or touching the
pollen-tubes, can only be perceived with good lenses, in
thin sections cut across the fertilised stigma.

At last the tube or tubes reach the bottom part of the
style, pass into the cavity of the ovary, and there enter the

minute orifice or micropyle of each indivi¬
dual ovule (if more than one) as seen in
fig. 154, which represents the style and
single ovule of buckwheat. Thus the ger¬
minal vesicle in the embryo-sac is fertilised,
and certain further changes commence,
which change the ovule into a seed or fruit,
embodying for the next generation all the
work of the past, all the hope of the future.

It cannot be here detailed how every¬
thing in Nature is made to bear upon this
end, in the greatest variety of ways. It
is easy to write volumes upon the ‘ Loves
of the Plants,’ and such have been written.
The scents and colours of flowers are all
to serve this end ; the visits of insects, with
nectar to tempt them, and with colour and odour to attract
them, being relied upon to bring the pollen of one flower to
the stigma of another. In not a few cases one insect alone is
able to do this. In other cases the wind performs the
hymeneal office, and appendages to catch the wind are pro¬
vided. In many cases the pollen fertilises the stigma of its
own flower. Anthers and stigma combined in one flower are
very common, but it is also common to find the sexes on
altogether separate flowers, as in the willow and yew. But
all such marvellous instances of adaptation belong to
Botany rather than Microscopy.

Fig. 154
Style and Ovule
(Buckwheat)
o. Ovule

v. Germinal vesicle
s. Embryo sac

THE VEGETABLE WORLD

213

Sections of flowers, and ovaries, and buds, both trans¬
verse and longitudinal, are very beautiful objects. Being
soft, they are prepared with no special difficulty.

111. Stems. — A few words must suffice for the structures
found in plants. Of the preparation of sections of Stems, in
general, and their staining or double staining, sufficient for
the purposes of this book has been said in §§77-79. Shoots
or branches from J to J inch diameter are most suitable ; but
this must depend upon the rate of growth. If possible,
three sections should be prepared of each — one transverse,
one longitudinal in a radial direction from the centre of the
shoot, and a third also longitudinal, but cut at a tangent to
that centre. In regard to exogens, or those plants which
add rings every year, it is a good plan to prepare three
transverse sections, of one, two, and three years’ growth.
The common pine shows the annual rings clearly.

Endogens, on the contrary, exhibit independent bundles
of fibro-vascular tissue over the area of the stem. Many of
them, well cut and stained, are of exquisite beauty. Boots
also offer many interesting sections.

112. Vessels. — The stems and other main parts of
plants mostly consist of longitudinal vessels, which are
formed essentially by the elongation of cells and thickening
of the cell walls. We cannot go into detail; but fig. 155
shows one very characteristic type, in which the harder
thickening material is arranged in spiral form, just as the
chitine in the tracheal tube of an insect (fig. 116) shown for
comparison in the same figure. Spiral vessels are easily,
after a little maceration, teased out of a piece of the stem of
the common rhubarb, and of many other plants, especially
from the larger veins of soft leaves.

If the rings are separate, and more or less complete, we
have annular vessels, or the thickened walls may be dotted
over with thinner spots, which give punctated vessels. In
some plants, Ferns especially, the walls of many vessels are

214

THE MICROSCOPE

more or less polygonal, and the rings very close, and con¬
nected by very short longitudinal fibres : these forms are
known as scalariform vessels.

In woody fibre the cell-
walls are much thickened and
hardened. The most interest¬
ing form of woody tissue is
found in deal and other coni¬
ferous woods. Here each long
cell is studded with thin spots
surrounded by concave depres-

Fig. 155. — Spiral vessels (Trachea of an
insect shown in centre for comparison)

Fig. 156. — Pitted vessels

sions, on the flat faces which are turned towards the
medullary rays, or radial planes from the centre of the
stem. The flat face of the cell with the pits is shown at a,
fig. 156, and the lenticular cavity formed by two pits (which
always face each other) is shown in section at b. A very
thin slice from a lucifer match will show these pits ; but to
get the two positions perfectly, sections must be cut both
radially and tangentially as regards the stem.

THE VEGETABLE WORLD

215

113. Leaves. — These offer many interesting objects.
Separating and staining a piece of the under cuticle, or
surface layer of cells, this will in most cases show under a
quite low power the Stomata or apertures, leading into
cavities in the cellular tissue, by which plants ‘ breathe ’
(fig. 157, a). With a higher power it will be seen (c, which
shows the little piece marked b in a, further magnified) how
each aperture is bounded by two kidney-shaped ‘ guard-
cells.’ The cuticle will also exhibit cellular structure plainly.

Fig. 157. — Stomata of Lilac Leaf

Vertical sections of leaves should also be prepared, and
are very interesting. Under the cuticular layer, is usually
one of deeper and narrower cells, often termed ‘ palisade ’
cells (fig. 158). Beneath this the cellular arrangement
varies much, but as a rule the upper layers are dense, whilst
amongst the lower layers 'are many cavities, into which the
stomata open. A great deal depends on chance ; but out of
a few sections one or two should exhibit some of the stomata
in section, as in the figure. Some of the thicker leathery
leaves, such as laurel, or of the India-rubber plant ( Ficus
elastica) make fine vertical sections with the least trouble, j-j

2l6

THE MICROSCOPE

Portions of the fronds of Perns may be mounted either
dry, or transparent. Many petals of flowers, which are
forms of leaves, are lovely objects. A favourite one is a
petal of geranium, stripped of its cuticle, then dried, im¬
mersed in turpentine for two hours, and mounted in balsam.
The petal from a flower of the common Chickweed is another
beautiful object, and shows in the ‘veins’ spiral vessels in
their natural position.

The cuticles of many leaves are studded with hairs of
various shapes. Many cells of leaves and other parts con¬
tain crystals , either singly or in masses. In others there

are grains of starch ,
which phenomenon
is best shown in a
thin section of
potato. Hard tis-

Fig. 158.— Vertical Section of Lilac Leaf x 100 sues SUch as the

‘ stones ’ of fruits ; thin cuticles such as enclose the edible part
of a walnut ; pith ; rind ; all these and other structures present
various forms of tissue built up of cells, for further details of
which reference must be made to works on Structural Botany
or the Physiology of Plants.

114. Cyclosis. — We must conclude this chapter by refer¬
ence to one phenomenon of living cells, which more than any
others impresses the observer with the idea of vital activity.
This is the circulation in each cell of the living protoplasm,
in a current round the outer walls, which can be best traced
by the current carrying with it green particles of chlorophyll.
The probability is that all cells containing fluid protoplasm
present more or less of this motion (which must not be
confounded with the circulation of sap through the whole
plant : it is simply a current round the interior of each cell).
But only thin cells with transparent walls are suitable for
observing the phenomena.

The hair of a Nettle, or still better of the Tradescantia

THE VEGETABLE WORLD

217

Virginia, detached and put on a slip with a drop of warm
water, shows this phenomenon well. So do the terminal
shoots of the aquatic plant Chara, or the thinner ones of the
allied Nitella. A leaf of Anacharis treated in the same way
shows this protoplasmic movement (termed ‘ cyclosis ’) well,
and with no trouble, in the cells up
the midrib, or round the edge of the
leaf. But it is best shown, though
with rather more trouble, in a piece
of leaf of Vallisneria. A thin slice
should be shaved (with a lancet is
easiest) from the surface of the leaf,
so as to come as near as possible
to a single layer of the cells, and put
on a slip with warm water. Such
a thin section is nearly transparent,
and the cells are on a larger scale
and more regular in form. A fair
quarter-inch with a moderate eye¬
piece, or a high-class half-inch or
four-tenths with a high-power ocular, will exhibit the motion
of the chlorophyll granules well, as in fig. 159, which is
drawn under a half-inch apochromatic.

Generally the detachment of the piece of plant, or of the
hair or leaf, as may be used, appears at first to paralyse the
motion, as if the cells were really killed. This is however
only temporary, and if the leaves or small slices are placed
for an hour or two in a small tube full of warm water, or
carried in a pocket next the body so as to be gently warmed,
the cells are revived, and the motion recommences.

2l8

THE MICROSCOPE

CHAPTER XI

VERTEBRATE PHYSIOLOGY

That study of the most minute structure of the human or
other body which is termed Histology, does not belong to
the purposes of this work. Medical or biological students
will work out such studies by the aid of the treatises of
Schafer or Klein, and such material as is provided in class.
So far as the human body is concerned, material for personal
section-work or dissection will rarely come in the ordinary
reader’s way ; and it must suffice to indicate in this chapter
a few typical organs or structures of which the microscopist
will probably desire to possess a few purchased slides, as
being of popular interest.1

115. Blood. — The examination of a drop of blood is an
exception, being so easily made. Winding a few threads
round a finger below the nail, a prick near the side of the
nail with a fine needle is hardly felt, and is followed by the
desired drop, which can be transferred to a slip. If the
cover be placed on the drop so as to form a thick layer,
the red corpuscles show a great tendency to gather into rolls
adhering by their flat sides like a pile of shillings (fig. 160, a).
But if the cover-glass is laid on the dry slip and pushed
along till the blood is drawn under by capillarity, a thin
layer is secured, when the individual corpuscles stand out

1 For those interested in Histology as such, an exception may be made to
the recommendation on p. 136 not to purchase ‘ sets ’ of slides. From any of
the metropolitan microscope houses a set of six dozen slides, mounted in
balsam, in a pine cabinet like fig. 70, and representing (according to a refer¬
ence-list) nearly all the important organs and tissues of the human body, pan
be obtained for two guineas. They are known in the trade as ‘ Jones ’ sets,’ and
are largely used by medical students and teachers. The sections are admirably
prepared-

VERTEBRA TE PHYSIOLOGY

219

separately, c. The little disks, averaging 3^0 inch ’m
diameter, have rounded edges, and are rather hollowed on
the sides. This concave lenticular form results in a differ¬
ence of focus showing either
light or dark centres.

of fishes, reptiles, and birds are
larger, oval in form, and with a
very distinct nucleus. Those
of some reptiles are very large
in proportion, and slides are
procurable of any dealer show¬
ing the nuclei stained green,
with the rest of the corpuscle
stained red.

Here and there appear among the round disks, rather
larger corpuscles d d, not so smooth at the edge, and at times
with distinct protuberances. They rarely average more than
1 in 250 to 400 of the red ones. If the slip can be kept
warmed to blood heat, they are found to change their form
and place as in fig. 161. In short, except that they are
much smaller than the ordinary amoeba of fig. 86, they

The corpuscles of the blood

Fig. 160. — Blood Corpuscles
A. Boll of red corpuscles
BB. Red corpuscles in profile
C. Red corpuscle flat
DD. White corpuscles

Fig. 161— Amceboid Changes in White Corpuscles

appear to behave exactly the same in every respect. They
are, in fact, distinct living bodies of the same kind, taking
food also in exactly the same way. Especially have they
been observed to feed upon such bacteria as infest the body ;
and when irritation or inflammation is set up anywhere, by
the attack of these disease-producing germs, the white

220

THE MICROSCOPE

corpuscles or leucocytes rush to the spot from distant points,
and appear to attempt to devour them. The point is still
debated ; but a large school of biologists and medical men
believe that resistance to such attacks of disease-germs, or
recovery from the attacks, depends upon whether the white
corpuscles of the blood can succeed in devouring the germs
before fatal mischief is done.

116. Circulation of the Blood. — The display of this is
always interesting. It is usually shown in the transparent
web of the foot of a frog ; but the tail of a tadpole, or tail or

fin of a minnow, are also used as convenient. We will take
the frog. The orthodox manner of treating him was to have
a ‘ frog-plate ’ of brass aaa (fig. 162) with holes g g g round
one end, and an aperture across which the foot is to be
stretched. The frog is put head first into a bag b b, whose
neck can be drawn round the one leg allowed to protrude
by the strings d d , and by tapes c c sewn to the bag, it can
be tied securely round the plate, or what is better, to studs
projecting from it. A few pins h with slots i cut in them
are provided to fit the holes g g. To the principal toes fff
threads are tied : then these threads are stretched out,
stretching the web with them, and the threads being turned

VERTEBRATE PHYSIOLOGY

221

two or three times round the pegs and then pulled down
through the slit i in each, the whole is fastened. The bag
should be of linen or cotton, not waterproof, and be wetted,
or a little wet moss put in with the animal ; and the web of
the foot also needs wetting occasionally. If the bag or the
tapes be tied too tightly, the circulation will sometimes
stop, and it should be loosened a little. Mere fright some¬
times seems to paralyse the animal ; but there is very
seldom any difficulty.

But most demonstrators prefer Dr. Carpenter’s plan of
using a plate of cork, with a hole half an inch diameter in
the centre. The threads attached to the toes have little
loops made in their ends, or are tied round common pins ;
and these pins are stuck into the cork so as to stretch the
web, as with the plate. An inch power is amply sufficient.

117. Capillary System. — The circulation thus seen is
near the smaller extremities of the veins and arteries, where
the two pass into each
other through the ca¬
pillary tubes. With a
little adjustment the
capillary connections
themselves can be
brought into view, the
diameter of these tiny
tubes being in many
cases only sufficient
for the free passage of
one corpuscle at a time. To exhibit these capillary blood¬
vessels, sections are prepared of tissues in which the blood¬
vessels have been injected with carmine, so as to show up
in bright crimson. Fig. 163 represents a small portion
magnified of the capillary system in the lung.

118. Ciliated Cells. — The amoeboid white blood-corpuscles
are not the only case in which we find grouped, in the higher

222

THE MICROSCOPE

animals, individual cells reproducing the lower forms of
life. It is true that the cellular construction of a vertebrate’s
body is not (to a very low power) so conspicuous as in the
case of plants, in which it cannot be overlooked ; but the
use of moderately high powers speedily reveals that here
also we have a complex life, embracing in it the individual
life of innumerable single cells. We cannot enumerate all
the various kinds ; but one of the most interesting (from
this point of view) is the ciliated cell.

The ciliated animalculae were briefly described in § 85,
and the part the minute cilia played in the motion of the

animal, or (by causing cur¬
rents in the surrounding fluid)
procuring food or assisting
respiration, was alluded to.
Taking now an example far
above an Infusorian, yet far
below any vertebrate animal
in structure, if we examine
a section through the gills of
a mussel, with a high power,
we shall see (fig. 164) that the
exterior of the gill-leaves or plates is composed of longish
cells in close contact, each with a nucleus, and with their
outer ends furnished with cilia c. These are in incessant
motion, and by this motion the fresh water is swept into
the gills, and the breathed water expelled. This office is
what we should probably expect. The motions of the
cilia average some ten or twelve per second ; and that
the individual cell-life is apart from the higher or organic
life, is proved by the fact that the cilia are seen to move,
hours after a piece of gill has been detached from the animal.1

1 Dr. Carpenter states that the cilia have been seen in motion on the trachea
of a criminal seven days after decapitation. And detached ciliated cells will
swim freely about in warm water like an Infusorian.

Fig. 164. — Gills of Mussel c. Cilia

VERTEBRATE PHYSIOLOGY

223

But it is wonderfully interesting to know that we find
similar ciliated cells in the highest Vertebrates, including
ourselves, and that they perform most important functions.
In this case they line interior parts of the body, which arises
from the mode of embryonic development. If the germ of
a hatching egg be kept under examination, the first trace of
the future ‘ plan ’ of the animal is an open groove down the
centre. This groove gradually grows deeper, and at last
closes in, into a tube. Thus the originally outside layer of
cells, becomes an inside lining. The internal organs are
developed from the cavity thus formed. It is only on some
of these surfaces, which are kept moist by thus becoming
internal surfaces, that the cilia survive. Fig. 165 illustrates
a few forms, A being from
the lining membrane of the
nose, b from the trachea of
a’cat, and c from the mouth
of a frog. They belong to
the delicate lining known
as epithelium. Every cell
is seen to be distinct, and to
have its own nucleus ; but
the cells differ in shape.

When oblong and regular
as in a, they are known as
‘ columnar ’ epithelium, and
when in addition ciliated, as here, ‘ ciliated ’ epithelium. Only
a most skilled section-cutter can furnish sections showing
these cells ; but if the reader has a good J or jr, he can
probably procure detached ciliated cells by scraping the
inside of a frog’s cheek with a spoon,1 and placing a drop
of the mucus upon a slip with a cover.

The great interest of these cells lies in their position and

1 No more pain is caused than in scraping one’s own. This will also pro¬
duce epithelial cells; but in man these are not ciliated.

224

THE MICROSCOPE

the work they do. The main positions they occupy are
along the spinal cord or marrow, a narrow tube in which is
lined with cilia ; the cavities of the brain ; the windpipe,
trachea, and bronchi, and nasal tubes— i.e. the respiratory
passages ; and the ear passages. How these originally out¬
side appendages came to be on inside places, we have seen.

Their office is a cleansing and freshening office. In the
case of the respiratory passages, it has been ascertained that
the cilia keep the fluids lining those passages in continuous
motion towards the mouth, thus getting rid of dust, or effete
and diseased matter. It is when the offending matter gets
too much for this action, or when the action of the cilia has
not sufficient vigour, that we get distressed, and have to
cough the matter up. The lining of the nervous cavities
probably has to be cleansed in the same way.

119. Muscle. — Of the more obvious structures of the ver¬
tebrate body, the first to occur to mind is muscle — the

substance of ‘ lean ’
flesh. This is of two
great classes, sharply
distinguished. In all
cases where the motion
produced by the muscle
is voluntary , the fibres

Fig. 166. — Voluntary or Striped Muscular Fibre . . -. , . ..

are striped or striated
as in fig. 166. The coarseness, and some characteristics of
the striation, may differ in different classes of animals, as in
mammals, reptiles, fish, and insects ; but striation is common
to all. On the other hand, where the motion is not voluntary,
as in thejntestines, the fibres are plain or unstriped.

Muscle can be prepared easily. A bit of either raw or
boiled flesh may be teased out with two fine needles into the
finest possible fibrils, under an inch power of the single or
dissecting microscope. Raw flesh is sometimes teased out
more easily if first macerated in a weak solution of chromic

VERTEBRATE PHYSIOLOGY

225

acid. The flesh of the pig or the frog yields good fibres, as
do lobsters and crabs. Some insects furnish muscle with
coarser striation, the Dyticus or great Water-beetle being a
favourite. The striations are thought to show more plainly,
if the muscle is mounted in glycerine or glycerine jelly : but
on the whole balsam is to be preferred.

120. Bone.— Sections of hone are interesting. Fig. 167
is a transverse section of human hone, showing first a series
of very small tubes (averaging 3^ inch diameter) known as

Fig. 167. — Human Collar Bone x 100
Transverse Section

Fig. 168. — Fore-arm of a Turtle x 75
Transverse Section

the Haversian canals, which are in connection with the
great central cavity (when there is one, and like it are filled
with marrow), and supplied with blood-vessels. Bound these
are concentric lamellae of the organic structure, infiltrated
with earthy matter. Between these lamellae are spaces
called lacunae, shown by the black spider-like spots, from
which radiate in all directions a multitude of very fine
canals — caniculi. The lacunae are generally about one-third
deeper than they appear broad, and the proportion between
their apparent length and breadth (the section being sup-

p

226

THE MICROSCOPE

posed to be cut across the Haversian canal) differs in the
great classes of animals, so that a bit of bone will usually
tell of itself whether it belonged to mammal, bird, or reptile.
Thus, in the reptile bone shown in fig. 1G8, the lacunae are
longer but narrower than in the human bone : also the
bone being a flatter one, they tend more to straight parallel
rows. By these lacunae and their caniculi, which are filled
with protoplasmic matter, communication is kept up
throughout the hone with the Haversian canals and larger
hollows, and vital action maintained. It is remarkable that
there should be a general correspondence in the*size of the
lacunae, and those of the blood-corpuscles of the animal to
whom the bone belongs : but the caniculi are far too small
for blood-circulation.

121. Skin and Hair. — Passing now to the external sur¬
face of the body, the shin presents many points of interest.
Transverse sections or layers will display the ridges and
furrows as in fig. 169, and at intervals the mouths of the

pores by which perspiration
is exuded. These pores were
counted

Wilson, and found (on the
palm of the hand) to number
rather over 3,500 in a square
inch. On an average the tiny
tubes of which they are the
mouths, penetrate to a depth
of about a quarter of an inch,
making a total length of
drainage-tubes in that small
area, of about 73 feet ! It is easy to see the serious results
which must follow if such an amount of drainage is not kept
free and open ; and what work must be thrown upon other
excretory organs (such as nasal passages or kidneys) beyond
their proper share.

by Dr. Erasmus

VER TEBRA TE PH YSIOL OGY

22 7

Fig. 170 is a vertical section of skin. At a is the epi¬
dermis, or scurf skin, raised in ridges, the outer part of
which ■ consists of dead horny matter which is constantly
peeling off, as can be seen very prominently in a section of
the skin on the sole of the foot. The inner portion or
Malphigian layer receives projections or papilla cl of the
true skin beneath, which are supplied with capillary blood¬
vessels and nerve endings (see
fig. 173). Deeper down in
the true skin are the perspi¬
ratory glands, c, each gland a
coiled up narrow tube, which
finally passes out to the sur-

Fig. 171. — Scalp with Hairs

face, assuming through the epidermis a cork-screw spiral
form. The glands lie amongst masses of fat-cells c.

The skin is also furnished with sebaceous or oil-secreting
glands, not very different in structure from the preceding.
They are most conspicuous round the roots of hairs.

Fig. 171 is a section of the skin of the scalp, showing
two hairs and hair-follicles. The sebaceous glands and
muscles by which each hair can be moved, are clearly seen.
A double-stained section of the scalp is a very beautiful
object, as are double-stained preparations cut so as to give
transverse sections through the hairs and hair-follicles.

Fig. 170. — Vertical Section of Skin

228

THE MICROSCOPE

Many hairs themselves are very interesting, and the
most marked varieties are always to be purchased. The

whiskers of lions and tigers give
very large transverse sections.
Fig. 172 shows the very peculiar
arrangement of the medullary and
cortical portions of the hair of the

fig. 172.— Hair of Peccary peccary. The hair of the sable
Transverse Section . , i p , i

is remarkable tor the very
rounded cells occupying the centre. A most beautiful ob¬
ject is a good double-stained and ‘injected’ section of the
lip of a cat or rabbit, showing the sensory hairs, and their
follicles and appendages.

122. Nerve Endings. — This leads naturally to the organs
of sensation , through which we are impressed by the external
world. We will only mention here the sensations of touch,
of taste, and of sight.

The skin itself is an organ of feeling, and fig. 173 is a
much more magnified view of the papillce seen in fig. 170,

Fig. 173. — Tapillfe of the Skin, showing capillary vessels and touch-bodies

with the epidermis stripped away. Every one contains a
loop of capillary blood-vessels, shown by the darkest shading.
Many of them contain also the ends of minute nerves, shown

VERTEBRATE PHYSIOLOGY

229

in white, and which are twisted into oval bulbs. These are
generally considered and termed ‘ touch-bodies.’ It is by
no means easy to find a section of skin which exhibits one
or more of these touch-bodies really well, as they elude the
section-knife owing to their softness and minute size.

The more delicate sensations, as we should expect, require
more complicated mechanism. The terminal taste organs
are chiefly found in a certain region of the tongue covered

with large papillae, called circumvallate papillae, between
which are pits. In the hare and rabbit these pits become
more or less parallel leaves and grooves, more easily cut
across, and the tongues of these animals are therefore

Fig. 175. — Taste-organs x 450. A. Flask-shaped bulbs 13. Taste-cell with outer cells

usually sectioned to show the taste organs, which resemble
minute necked flasks, whose mouths open into the sides of
the pits or grooves (fig. 174). The flask-like bulbs are more
clearly shown in fig. 175 under a higher power, and it will
be seen that the outer part of the bulbs consist of oblong

23°

THE MICROSCOPE

cells, each containing a nucleus, and being tapered towards '
each end. Inside these are the true taste-cells (see right-
hand cell in b) in which the central portion containing the
oval nucleus is drawn out into an exceedingly fine process

at each end. The outer end projects
a little through the mouth of the flask
like a tiny hair, the other is usually
branched, and connected with the
nerve-fibres below ; but the precise
mode of connection has even yet
hardly been made out.

Our last example is from the nerve-
endings connected with vision. How
an ‘ image ’ is thrown upon the retina
at the back of the eye-ball, has broadly
been shown in fig. 9 ; and it has been
indicated in § 22 that the rays which
form this image are inconceivably
minute and rapid vibrations in the ether,
propagated along the course of the
‘ ray.’ Organs to receive such delicate
impressions must themselves be of
almost inconceivable delicacy ; and
fig. 176 is a section of the retina only,
from the eye of a frog. Here j are
fibres from the optic nerve, and i large
ganglion cells connected with them :

Pig. 176. — Section of Frog's

Retina

a. Layer of pigment cells

b, c, d. Rods and cones

e, g. Nuclear cells

f, h. Molecular layers

i. Ganglion cells j. Nerves

these ganglion cells and the smaller

nuclear cells presently mentioned ap¬
pear to act in a manner as relays or
fresh centres of nervous energy. At In and / are what are
called molecular layers, and alternating with them at eg are
two layers of nuclear cells, of which it may be remarked
that while in the frog the layer at g is much thicker than
at e, in the human retina the reverse is the case. The outer

VER TEBRA TE PH YSIOL OGY

231

nuclear cells are connected by nerve-fibrils with a layer of
extremely fine rods and cones, thicker at the base c d than
the outer ends b. These inconceivably fine rods and cones
(which are free or unconnected, whilst up to their bases the
whole structure is joined by connective tissue) are received
universally as the seat of vision. The remarkable thing is
that in Vertebrates (owing to that outside-inside embryonic
development indicated in § 118) they are at the bach of the
retina, and the rays have first to pass through all the pre¬
ceding layers, which are transparent. The analogy is pretty

Fig. 177. — Pigment Cells of Retina

r

evident between these rods and cones and the cilia of certain
epithelial cells (§ 118).

At the further end a of the layer of rods and cones is a
sort of pavement of pigment cells, hexagonal in shape, as at
a in fig. 177. The side next the rods throws out a number
of pigmented fibrils ( b ), which pass down amongst the outer
ends of the rods and cones as at c. Light causes these
fibrils to stretch down farther ; darkness to recede. They
appear to have an important office. The outer ends of the
rods and cones contain a colouring matter termed the visual
purple, which bleaches on exposure to the light. But the
colour returns when the rods and cones are in contact with
the pigment cells, even in the dead retina, and probably also
in the living one.

All these details of the intricate and beautiful mechanism
are known ; but how it acts is a profound mystery still.

2 22

THE MICROSCOPE

Some think the fine rods and cones ‘ feel ’ the vibrations.
Others rather believe that the bleaching of the purple, and
its renewal by the pigment cells, are of the nature of a
photographic process, and that this is vision. But no one
knows. No one knows the action of the complicated layers
of nuclear cells ; no one knows the nature of the defect in
the organ which makes some people colour-blind. Defects in
the eye as an optical instrument, we can trace out to the last
detail, and remedy with our prisms and lenses : the nature of
the vital mechanism, no one has yet been able to understand.

The organs of smell and of hearing are equally wonderful
in their minuteness of structure ; but reference must be
made for them to works upon anatomy or histology. Many
other organs supply beautiful objects, especially when (as
is often done) the blood-vessels are injected so as to show
arteries red, and veins blue, the tissues also being stained.
Sections of liver and kidney so treated are not only of great
beauty in themselves, but will give a realistic sense of the
exceeding delicacy of the mechanism of these important
secreting organs.

The above will suffice most microscopists for a general
collection : some others can only be just mentioned. The
various glands and secreting structures of the stomach and
intestines are both very interesting and very beautiful,
affording some of the clearest and most exquisite examples
of the cells of columnar epithelium. A fine illustration of
the arrangement of tendons at a joint, is furnished by a
longitudinal section of a ‘ decalcified ’ joint in the tail of a
mouse. A developing tooth, or the entire jaw of a kitten or
some other small animal, is a very beautiful and interesting
slide. So is a section through the entire eye of a bird, or of
a newt ; or the entire head of a young newt or frog. This
brief list is entirely confined to objects which are almost
always to be obtained, most beautifully prepared by the
professional mounter.

MISCELLANEOUS MICROSCOPIC OBJECTS

-JJ>

CHAPTER XII

MISCELLANEOUS MICROSCOPIC OBJECTS

In this chapter we collect a few notes on various subjects of
microscopic study, or classes of objects, all of which are
attractive, and some of which secure almost altogether the
interest of specialists. We must be content with indications.

123. Diatoms.— One well-known authority has for some
time never spent less than 100Z. a year upon these beautiful
objects, which have been termed ‘ Nature’s jewels.’ Many
microscopists have studied little else for years ; and there is
no doubt that the attempt to make visible the finer of
their markings, ranging in some cases to 100,000 per inch
and upwards, has worked marvels in the improvement of
microscope lenses. Hence it will be understood that any¬
one intending to take up this study seriously, must possess
first-class objectives, both as regards aperture and definition.
But the mere beauty and symmetry of scores of the larger
forms can be seen with ordinarily good half and quarter-inch
powers.

Diatoms are generally considered unicellar plants, growing
in a silicious two-valved case,1 and are found in all sorts of

1 Though it be heresy to say so, I am extremely doubtful of this. Many
years since I was most unmercifully ‘ chaffed ’ by a small body of microscopists
for presumed ignorance in the expressed belief that both Volvox, and at least
some of the Diatoms, might probably be ultimately relegated to the animal
kingdom. Some of the best recent biologists have already adopted that view
as to Volvox : diatoms are not yet so regarded. But the character of the
motion in some forms, and of the silicious skeleton in most (it is not clear to
me that all so-called Diatoms are on the same plane of existence) are of great
weight from analogy ; while I think that in some cases fine pseudopodia can be
observed in the living form. And some of the reasons which were once power¬
ful in deciding for vegetable character — one for instance was the presence of
chlorophyll — are admittedly now of no weight at all. I have however no title
to speak with authority, and merely — for what I think adequate reason — put
my doubts upon record.

234

THE MICROSCOPE

places. In fig. 178 are a group of fossil marine forms, some
of which are still found on the surface of the sea. No. 1 is
a Surirella, No. 2 a Pinnularia, No. 3 Gomphonema, No. 4
one of the genus Navicula, No. 5 a cross section or broken
piece of Melosira.

Diatoms are also found attached to sea-weeds, as green
or orange or reddish scum on the mud at the bottoms of
ponds and brackish river-banks near the sea, and in all sorts

of places. The
beautiful Arach-
noidiscus (fig. 179)
is found in places
as diverse as upon
fronds of sea¬
weed, and stale

jam

f

But the

Fig. 178. — Fossil Diatoms

most ample sup¬
plies of many
forms are fossil,
in what are
called diatom a-
ceous earths, or
in guano. The
tooth-powder sold

as ‘ Zozodont ’ will furnish for sixpence a large quantity,
being a diatomaceous earth, but it varies a great deal; some
year or two ago there were many fine forms in it, but the

last sample I obtained contained only very common forms,
resembling those in fig. 178.

The most beautiful forms are the discoid or equi-angular
ones. Fig. 179 reproduces the drawing of the late Mr. Richard
Beck of the beautiful Arachnoidiscus, showing both of the

two sides of the valve (which are different in all diatoms)
and the way in which the two valves are joined into the
‘ double atom,’ in this case resembling a pill-box, the whole

Fig. 179, — Araclinoidiscus Japonicus x 450

236

THE MICROSCOPE

being seen edgeways. There are several varieties of Arach-
noidiscus, with no really essential difference : one called
A. ornata is perhaps the most beautiful of all. Fig. 180 is
another circular Diatom called Heliopelta. There are
varieties of this, with a different number of sections in the
disk ; but all are alike in the fact that the sections are not
flat, but undulate ; hence the whole valve cannot be in sharp
focus at once. This is common to many discoid diatoms ;

while others re¬
semble a convex
shield. In such
the more minute
markings can

only be focussed
in a certain por¬
tion at a time.

Fig. 181 is a
larger represen¬
tation of another
Nctvicula (so

named from re¬
semblance to the
deck or plan of a
boat). There are
many varieties of
N. lyra , differing
much in coarseness of marking, and one or two are now sepa¬
rated under other names, such as N. metadata. Some are
much flatter towards the edges than others ; and of every dia¬
tom selected, endeavour should be made to secure specimens
which lie flat and focus as evenly as possible, either all over,
or all round the edge, except when it is desired to show the
edge or side view.

Of equi-angled diatoms, Triceratium fcivus is three-
cornered, and similar marking but much coarser is found in

Fig. 180. — Heliopelta x 200
{From photo by Mr. A. A. Cornell.)

MISCELLANEOUS MICROSCOPIC OBJECTS 237

T. septangulatum, with seven corners. A much larger three-
cornered diatom, still coarser in marking, and with rather
bulging sides, is Trinacrium. In all these and many others,
inside the larger marking alone apparent under moderate
powers, are a number of much smaller markings of various
kinds, some of which demand the very
highest powers and skilful illumination
to make them visible.

Thus, Coscinodiscus aster omphalus
is a discoid diatom, which when x 100
appears the size of half a crown (for
an average specimen : the size of many
diatoms differs greatly) and the portion
in focus (for it is not flat) appears
studded with dots or spherules (accord¬
ing to focus and excellence of the
objective) of which about 40 in line
go to an inch ; these spots therefore
average about 4,000 per linear inch,
and are the larger markings. But
when x 2000 each ‘ large ’ circle is
surrounded by a ring of smaller round
spots, and within it are a number of
much smaller spots. And a most
minute difference in focussing, with
the same illumination, changes the
whole system from white to black, so that one image is like
the photographic negative compared with a positive. This
class of work, and many of the finer diatoms, require
immersion lenses and high-class condensers.

This difference in appearance from a mere shade of
focussing, will show the exceeding difficulty of determining
diatom structure. It appears as if the very fine dots were
on, or apertures through, a very thin layer below or above a
thicker one with larger apertures. And the general opinion

Pig. 181. — Naviculalyra x600
{Photo by Mr. C. L. Curties.)

THE MICROSCOPE

333

is that the valves of diatoms are composed of either two or
three layers. It may differ in various genera, and many
good authorities believe that P. angulatum consists of aper¬
tures in a thicker layer, probably of silex, which is between
two inconceivably thin ones, possibly of membrane, or a thin
ectoderm. That the small spots are either perforations, or
hollows, has been proved by the fact that Mr. C. H. Gill has
succeeded in filling them with pigment. The further fact that
when by proper arrangements we have got rid of the many
spurious ‘ diffraction-images ’ caused by a small cone, which
in times past have greatly obscured the subject, there remain
with a wide aplanatic cone and good objective two opposite
appearances and two only— a white spot or a black dot —
also seems conclusive that each spot is either an actual
aperture, or a section like a very small lens.

Under low powers many diatoms appear very beautiful,
but instead of the marking, to be covered with glowing
iridescent colour. This is iridescent colour, caused by in¬
terferences of the light passing through their regular marking
as through a diffraction grating. A higher power, if of
sufficient aperture, resolves the flat colour into dots or
spherules.

Living Diatoms. — The motion of some of these is very
interesting. The best for demonstration are Bacillaria
paradoxa, of which a number of the rod-like forms adhere
and slide along each other in a curious way : and Pleurosigma
quadratics, resembling P. angulatum, but much shorter and
broader. These move over the slide freely in a drop of
water, in a very conspicuous way. Both are to be found
pretty easily in the Thames estuary.

Test Diatoms. — For the purposes of this book, these have
been sufficiently treated of in § 37.

124. Foraminifera.— These minute calcareous shells are
found of many forms, which exhibit a wonderful variety.
Many beds of chalk scores and even hundreds of feet thick

MISCELLANEOUS MICROSCOPIC OBJECTS 239

are formed of them (§ 127), but hundreds of species are to
be found in their detached form. ‘ Spread ’ slides containing
many species from the ocean-bed in various localities are
always to be obtained, one of the most attractive being the
‘ Globigerina ooze ’ brought up in the Challenger expedition
from the bottom of the Atlantic. The Mediterranean also
furnishes many beautiful forms, and so does the estuary of
the Dee.

There are two sources of supply ready to the hand of
every microscopist. The first is in new sponges. Everyone
knows the sand that soaks out of a newly-purchased sponge.
If this be carefully collected and examined, in a large pro¬
portion of cases (not of course always) this sand will be
found to consist of Eoraminifera. These may either he
mounted on a black ground in a dry cell, as opaque objects,
or spread upon a slip and mounted in balsam.

The other source is the sea-shore. With two or three
little bags or bottles, and a spoon, the collector may walk
along high-water mark, and low-water mark, and at each
level scrape up a little of the cleanest and whitest-looking
sand left in the ripples caused by retiring wavelets, in fairly
calm weather. Stormy weather mixes things up too much,
and the loose sand among the weed at high-water mark also
fails for the same cause to yield other than small results.
Between high and low water, a little of the sharp whiter
sand should also be scraped up where tiny little pools are
scooped by the water round the corners of rocks or stones.
Of course the Steinheil loup or other pocket lens should be
used from time to time, to see if there is anything to be got
at each spot attempted. Any chance of dredging off shore
in the ooze or mud or sand, is also not to be despised ; and
the blue clay-mud at the mouths of many estuaries will also
yield specimens.

The produce of these proceedings has next to be washed
and cleaned. First the salt and mud are washed away in

240

THE MICROSCOPE

several waters. Next the mass is to be passed through two
sieves — the top one of perforated zinc or coarse 'gauze with
meshes or holes about £ inch, which will take out weed and
other large masses ; the lower one of wire or silk gauze
120 to 150 meshes per inch. If dirty, boil in solution of
potash, then again in water to dissolve the potash out.
Finally dry entirely in the oven, and when cool throw into
water and stir : the shells being full of air will mostly float
on the surface and can be separated, while the solid sand
will sink to the bottom. Further sorting out, into sizes or
otherwise, can be carried out as desired.

Some of the Foraminifera exhibit well by dark-ground
illumination : but as a rule they are best treated as either
opaque or transparent objects.

125. Radiolaria. — The most beautiful of these minute
forms of life, however, are the silicious or flinty shells of
the group of animals named by Ehrenberg Polycystina , but
which are since merged into the larger group known as
Badiolaria. They are what it is the fashion to call ‘ single-
celled ’ animals ‘ without organs ’ ; nevertheless their lead¬
ing characteristic is that a central portion is enclosed in a
capsule of membrane, within which reproductive growth
takes place, while the extra-capsular portion extends pseudo¬
podia, and by these procures food. According as the mem¬
branous capsule is pierced by many fine pores, or by one or
more larger apertures for communication, the Radiolaria are
grouped into two great divisions of Porulosa or Osculosa.
Further details of classification cannot be entered into here.1

The flinty skeletons of these animals, of which the term
Polycystina is still applied to those in which the skeleton is
outside the capsule, are as transparent as those of diatoms,
and they consequently make splendid objects for dark-ground
illumination. Fig. 182 represents a few shells thus shown,

1 The classical work on the subject is Haeckel’s magnificent Report on this
department of the Challenger expedition.

MISCELLANEOUS MICROSCOPIC OBJECTS 241

reproduced from a fine drawing by Dr. Dallinger. All these
forms are very common in a thick bed of silicious sand¬
stone found in Barbadoes, which almost entirely consists of
such minute silicious shells. The present living forms
appear to live chiefly on the surface of the sea, from which
their shells sink after death, forming an * ooze ’ at the
bottom of the ocean. From this ooze the Challenger

Fig. 182. — Polycystina x 100

expedition has made known thousands of varieties. Such an
ooze, if uplifted as dry land by geological change, would
doubtless result in a silicious sandstone much resembling
that of Barbadoes.

The Frontispiece shows a number of additional forms,
all fossil, reproduced from a most beautiful set of drawings
made from nature by the late Mr. Mungo Ponton. Many
of the specimens figured are of exceeding rarity ; and I have

Q

2z)2

THE MICROSCOPE

made selection either for this reason ; or for unusual beauty ;
(omitting purposely those shown in the drawing by Dr.
Dallinger), or lastly, to show by simple ocular demonstration
what amazing complexity and minuteness of real structure
of some kind must lie hidden in what it pleases biologists like
Professor Haeckel to term a ‘ simple cell,’ or * a speck of
primeval slime.’ Gradual development is evident enough :
for instance, gradual evolution seems clearly written on the
forms along the bottom of the page. But what must there
be in the ‘ single cell ’ which inhabits, and has formed, such
a lovely -dwelling as the central one of that bottom row ; and
if his a 'priori theory of Life leads a man to say there is no
‘ organisation ’ there, surely it must be said, So much the
worse for the theory ! It is certainly not the microscope’s
fault ; it is that of the anti-teleologic, and therefore unphilo-
sophic biologist at the eye-end of the instrument : the
microscope itself has done its very best to show him his folly !

126. Echinus Spines. — Thin sections across these are
very beautiful and perfectly symmetrical objects, which
amongst the various genera and species afford a marvellous
variety. In a lecture upon ‘ Star Fishes ’ by the late Dr. W. B.
Carpenter, which I once had the pleasure of demonstrating
for him with the projecting microscope, if my memory
serves me aright he had nearly twenty specimens of these
sections, besides his other preparations. A few generally find
a place in every collection, and exhibit well either by trans¬
parent or dark-ground illumination.

127. Mineral Sections.— These are a distinct, large, and
interesting class of objects, which furnish a study in them¬
selves. Many of them require polarised light (§ 40, § 58)
for their full understanding, and those who pursue this
study seriously will need the aid of a properly fitted petro¬
logical microscope. But very much interesting detail does
not require this. A very, moderate power will suffice to
show that most varieties of chalk are almost entirely com-

MISCELLANEOUS MICROSCOPIC OBJECTS 243

posed of the compressed remains of Foraminifera (fig. 183),
and that these and much larger organisms — largely consisting
of Encrinites — compose the hulk of many carboniferous
limestones (fig. 184). Other organic rocks, fossil Corals
and Madrepores, Coal sections, and fossil Woods supply
many beautiful objects. Inorganic minerals also frequently
display very beautiful phenomena of colour and iridescence,

but as a rule the polariscope is needed to bring out their
beauty.

128. Polarising Objects. — Any ordinary section of granite,
for instance, is a beautiful object when seen by polarised
light (§ 185). By ordinary light its main constituents of
mica, quartz, and felspar can be fairly made out ; but when
the polarising prisms are brought into action, their different
qualities cause the most beautiful ‘ play ’ of colours. Some
of them will give two colours only (always ‘ complementary ’
pairs of colours) when the analyser or polariser is adjusted
in its two main positions : but other constituents have the
property of exhibiting every colour in succession as one of
the prisms is rotated. This last phenomenon depends upon
the precise direction in which the section cuts a particular
constituent part of the mineral ; and occasionally some

q 2

244

THE MICROSCOPE

minerals — such as amethyst, some agates, chalcedony, and
some opals, exhibit bands of these succession colours of
enchanting beauty. Any dealer in this class of slides can
select an assortment of any extent specially adapted for
colour display.

Another class of colour phenomena is shown by sections
of many crystals, when polarised light is used in the form

Fig. 185. — Section of Granite by Polarised Light x 15

of a wide cone sent through them by the condenser, the
cone crossing in the crystal and being re-collected by a very
similar condenser system as an objective. These figures
are not images, however, but interference fringes produced
by the crystal ; and their explanation must be left to treatises
upon Petrology or Physical Optics.

Crystal Films of various salts in solution, allowed to
crystallise on a glass slip, and mounted when dry, are most
beautiful objects, and present most gorgeous colours in
polarised light. An immense variety are sold at Is. each,
but the student can purchase the salts and prepare for
himself if he prefers. It costs nearly as much, however, by

MISCELLANEOUS MICROSCOPIC OBJECTS 245

the time he has selected a good one out of his various trials.
A drop of saturated hot solution spread upon a warmed slip,
will enable the process of crystallisation to be watched ; but
the easiest way to exhibit this pretty phenomenon is to
purchase a few slides of the fatty acids , which are prepared
for the purpose. These melt at a gentle heat over a lamp,
taking care not to crack the glass or melt the ring of varnish ;
beautiful crystals, in colours, are seen to shoot out as the
slide cools. The same slide will serve scores of times, or
until some accident happens to it.

If a film of crystals does not happen to show colours
alone, the selenite (§ 40) can be used with it : but the con¬
trasts are better when the colours appear alone. This how¬
ever depends upon the thickness required of any particular
salt. Most crystals will mount in benzol-balsam ; but some
gradually dissolve in this, and have to be mounted in copal
varnish or castor-oil. The beautiful Hippuric acid crystals
require the last medium to preserve. The acid is dissolved
in absolute alcohol gently warmed, and if a drop is dipped
on to a clean warmed slide, it will show the most extra¬
ordinary variety of effects, by alternately cooling and warm¬
ing, breathing on the slide, &c. Eno’s fruit salt, tartaric
acid, borax, and carbonate of soda, are common domestic
articles that crystallise effectively from aqueous solutions ;
and will give different effects if hastened over a lamp.

The favourite and beautiful crystallisation of salicine is
best prepared by flooding a slip with a saturated solution in
warm water containing a little alcohol. This is then held
in forceps over a lamp, keeping level, at a moderate heat till
the fluid has evaporated. The heat is then increased, when
the film melts in its own ‘ water of crystallisation,’ and
gradually the beautiful little rosettes start out, the effect
being hastened, and varied adlib., by alternate breathing and
cooling and heating in turn. Some salts, such as sulphate
of copper, and sulphate of copper and magnesia mixed, melt

246

THE MICROSCOPE

and crystallise in the same way ; and heat will modify the
crystallisation patterns of tartaric acid and many other
crystals.

Organic sections show impressively the power of polarised
light in revealing structure. Their variety is countless ; and
many which exhibit sufficiently well in the ordinary way,
come out with added beauty and detail when this additional
power of analysis is brought into play. Sections of bone
and teeth generally polarise, but without adding much : but
sections of horn, hoof, and cartilage are exquisite objects, and
develop the details with startling effect. Every variety of
fish scale makes an effective polarising object, and is easily
cleaned and mounted, either in jelly or balsam. Many
hairs polarise strongly ; and a bit of horsehair crinoline, or
alpaca, cleaned of stiffening or other contamination, and
mounted in balsam, makes a very pretty object.

Another class of objects, and one that lends itself to, and
opens up another line of, systematic study, may be found in
what are commonly called the 1 palates ’ of various molluscs,
but are more properly known as the ‘ odontophores.’ The

organ consists essentially of a

cartilaginous strap, bearing a
series of rows of hard detached
teeth (fig. 186). A portion of
the strap at the back is gene¬
rally tubular, and needs to be
split up in the preparation.
The odontophore is usually
curled round at the front of the mouth so as to return back
again, and powerful muscles at the two back ends work the
front part where it is curled round backwards and forwards,
so as to produce a to-and-fro rasping action, which rasps
away the substance to which it is applied, even .the shells of
other molluscs, which are often thus cut through.

The teeth and their arrangement differ greatly in different

Fig. 186. — Five rows of teeth on Odonto-
phore of Common Garden Snail

MISCELLANEOUS MICROSCOPIC OBJECTS 247

molluscs, and a typical set is interesting. After dissecting
out the organ it is simmered in hot solution of caustic
potash, then well washed from soft integuments and dirty
particles. Then it may be treated as other anatomical
sections, mounting preferably in jelly for ordinary examina¬
tion, but in balsam for polarised light.

Of course many vegetable structures also make beautiful
objects. Amongst the most characteristic are the starches ,
every grain of which by polarised light exhibits a black
cross, which rotates as the analyser or polariser is rotated.

But the space which can be allotted to enumeration of
objects is exhausted. It will be readily understood that
almost every division we have mentioned, is a world in itself,
if followed up with the aid of any systematic treatise dealing
with it : step by step the microscope will go hand in hand
with the treatise, and give to it life, and a reality never
appreciated before.

CHAPTER XIII

MICKOSCOPICAL EXHIBITIONS

Anyone known to possess a fairly good microscope, is likely
to be asked, sooner or later, to use his resources in the
instruction and entertainment of others. Such a request is
reasonable, and to comply with it may be a duty ; but it
very often proves to be anything but a pleasure. I have
often observed much pains and trouble bestowed, only to be
treated by those who had requested it with such con¬
temptuous indifference as to justify positive refusal on any
future similar occasion. A microscopist should guard
against treatment of this kind, even for the sake of the

248

THE MICROSCOPE

conversazione or other occasion on which his assistance
may be desired. If it is desired, all parties should unite in
the endeavour to make it of value. Proper accommodation
and facilities should he stipulated for, and some aid towards
securing the interest of the company. Granted these
essentials, however, either one, or still better several micro¬
scopes, are capable of yielding much interest.

129. Precautions. — It is better, as a rule, not to use high
powers on such occasions, unless the company is to some
extent a skilled one ; and for the same reasons exceedingly
difficult objects should be avoided. At a regular micro¬
scopical soiree, it is different : there, an exhibitor may find it
worth while to spend an hour or more in adjusting some
special object, which may attract interest for the entire
evening amongst the many experts present. Even then,
constant vigilance is necessary to prevent serious damage to
valuable apparatus by rash handling from those not expert ;
hut how much more in a mere general company. To meet
this danger, several opticians actually supply ‘ dummy
milled-heads to go over those working the rack motion, so
that the ignorant spectator in turning them may not be able
to rack the objective down upon the slide. Such a fact
proves how real the danger has been found to be. Moderate
powers will be found sufficient, especially with good modern
lenses and deeper eyepieces, to show well all that can be
shown with profit in a general company.

130. System. — The chief necessity for giving real and
living interest to a popular microscopic exhibition, is that it
should take place upon some system, and that the idea of
the exhibition should be generally known to the company.
A regular microscopical conversazione is not here considered.
At such it is usual to simply have a card describing each
exhibit, which may remain for the whole evening, the com¬
pany finding their interest in inspecting the exhibits at a
hundred or more instruments in succession. We are here

MICROSCOPICAL EXHIBITIONS

249

considering either a single instrument, or a few, up to say
half a dozen ; and we desire to give corresponding interest
to these. Experience proves that this is not done by the
common plan of just changing a slide now and then, and in
an order the company knows nothing about, except from
what they may happen to see upon haphazard inspection.

But now let us systematise the matter. If there be but
one instrument, let it be understood that some subject will
he illustrated during the evening, and that the slides illus¬
trating it will be changed at set definite times, say from
every five to every fifteen minutes. Subjects from the in¬
sect world, for instance, are very popular and suitable for
such occasions. Going back to our chapter on that subject,
either of the two methods there illustrated, or any other,
might be adopted. At the stated fixed interval, another
slide in the series will be substituted for the last ; and the
series may be either accompanied by one brief oral descrip¬
tion of its main features, given loud enough for the room ;
or those features can be set out on a card, prepared for each
slide beforehand. Everyone will know that at a certain time
another item in the series will be ready for inspection, and
the whole will have a connected bearing, and really add to
intelligent knowledge.

With several instruments, the same plan may be pur¬
sued, in either of two ways. The whole team can be
brought to bear upon one subject if preferred; and in that
case the oral description, given audibly once for all, may be
retained. Going again for example to the insect world, the
various instruments may display a series of similar organs
in various orders, or even genera ; or upon the other plan,
the prominent organs of one insect might be distributed
amongst the whole. These might be then changed for
another insect ; and so on.

Or each several microscope may follow up during the
evening its own individual succession of objects. In this

250

THE MICROSCOPE

case dependence must chiefly be placed upon cards, which
should make clear both the nature of the series as a series,
and the points of the objects from time to time brought
forward to illustrate it.

Other departments, such as pond life, plant life, the
anatomy and physiology of the human body, health and
disease &c., lend themselves equally well to such systematic
methods of exhibition and brief comment.

But though such is undoubtedly the most satisfactory
method of microscopic exhibition, it may often happen that
time, or knowledge, or opportunity, or material, may be in¬
sufficient to carry it out. In that case the microscopist must
trust to the attractiveness of such detached subjects as he can
exhibit ; and it may be useful to indicate some such, which
rarely fail to arouse more or less eager interest. When
several instruments are in use, some of these may be worth
exhibition during the whole time. Where this is not the
case, change should still be made at known intervals , so that
spectators may not be in ignorance whether or not they will
find something fresh upon another peep.

131. Exhibition Slides. — A little expense is sometimes
willingly borne, and a special class of very showy slides are
prepared and well known as ‘ exhibition objects.’ Such are
symmetrical groups of diatoms, polycystina, spines and
anchors, &c. Some ‘ spread ’ slides of the same are of
equal beauty. Moderate powers, or dark field, such as will
bring out iridescent colours in place of minute marking, will
attract more admiration of ‘ Nature’s jewels ’ than the fine
detail of the diatomist as seen under very high power.
Eggs of insects are treated in the same way, and some will
excite much marvel. Groups representing sprays or
bouquets of flowers are also prepared of iridescent butterfly
scales, to be shown with the Lieberkuhn. Such slides cost
from 65. to as much as 3 1. Some of the groups of eggs, &c.,
show best in the same way.

MICROSCOPICAL EXHIBITIONS

251

132. Live and Natural Objects. — Mites of various kinds,
as from sugar, cheese, &c., afford a great deal of mere
amusement. The heads of living insects, secured as de¬
scribed on page 42, are generally very attractive, as will be
the head and jaws of a spider. If a flea can be procured and
starved for a day or two to get transparent, and gently
pressed (just to keep it still) in the live box, the beating of
the heart or dorsal vessel will be very clearly seen.

An assortment of the smaller flowers , under low powers
ranging from 4 inches to 1 inch, and illuminated by the
bull’s-eye, will often excite the liveliest admiration.

Circulation and cyclosis are always popular (§§ 114, 116).
The antenna of the large water-louse ( Asellus ) shows
blood-movement well. A good inch objective will suffice
for any of these.

Well-chosen specimens of Pond Life always attract
attention. Rotifers, freed from dirty specks by several
changes into clean water, and shown on dark ground, are
very beautiful : their own instinct keeps them always in the
focussed light of the condenser.1

133. Plant Structure. — In this department double-stained
sections of stems ; transverse sections of ovaries, buds, and
flowers ; various forms of pollen ; and cuticles and sections
of leaves showing stomata , are the most popular subjects.

134. Miscellaneous. — Sections of chalk and foraminiferal
limestone, of coal showing organisms, and fossil wood are
interesting. With the polariscope, sections of various horny

k 1 It may be of occasional service to many to state that Rotifers, Polyzoa,
Volvox, and any other fresh- water microscopic life, can generally be obtained
for Is. per tube from Mr. John Hood, 50 Dallfield Walk, Dundee; or Mr. T.
Bolton, Balsall Heath Road, Birmingham. Marine Polyzoa, Hydrozoa, &c.,
can be obtained alive (and often preserved, or mounted as slides) of Mr. J.
Hornell, Biological Laboratory, Jersey (average Is. &d. per tube) or of the
Director, Marine Biological Association, Citadel, Plymouth (which issues a price
list). A very large and comprehensive catalogue of slides (the only one of the
kind) in all branches of microscopy, published by Messrs. Watson & Sons,
Holborn, will often be found of service.

252

THE MICROSCOPE

tissues, and minerals, and series of crystallisations, are
favourites : many crystallised salts are of marvellous beauty.
The cornea of an insect’s eye arranged to show multiple
images of some brilliant object, such as a star in coloured
glass, generally excites admiration. More need not be
added for the class of exhibition here in view, as experience
will readily suggest other subjects.

135. Lantern Exhibitions. — A few sentences must suffice
under this head. Insect subjects are peculiarly suitable for
projection, and stand out clearly on the screen. In this
case something in the way of a lecture will of course be
planned. There might be a general review of the Insect
World, giving types of organs in various insects; or a single
class of organs may be taken, as the wings, or the mouth
organs ; or such a subject as Household Pests, or Household
Insects, or Ponds and Ditches — such are actual subjects
which I have myself demonstrated, either in personal lectures
or for friends. Minerals demonstrate admirably by polarised
light. Botanical subjects exhibit easily and well, and good
double-stained sections are very beautiful. Sections of organs
and parts of the human body exhibit well upon the screen up
to about 2,000 diameters with the oxy-hydrogen light, which
is sufficient for nearly anything a popular lecturer would
desire. In illustrating for a medical friend a popular lecture
upon ‘ The Skin,’ I was able to show every point he desired,
embracing in all about thirty slides. These brief hints will
suffice to indicate what is within the reach of a really good
projection instrument and competent demonstrator; and it
only need be added, that the Lieberkiihn and spot-lens are,
for some objects to be shown upon a screen, very valuable
and effective adjuncts.

THE END

INDEX

-•o*

Abbe, Prof., and the microscope,
28, 30, 34, 35, 36

— condenser, 74
Actinophrys, 153
Actinospherium, 153
Alcyonella, 165
Algie, 203

Alternation of generations, 204
Amoeba, 151
Angle, 26, 28
Antennae of insects, 192
Aperture, 26, 28

— measure of, 30

— numerical, 30

— and resolution, 31
Aphides, life history of, 199
Apochromatic objectives, 35
Arachnoidiscus, 234

BARREL-animalcule, 149
Bees, 185

Bell-animalcules, 153
Benzol, mounting in, 127
Binocular microscopes, 63
Blood, 218

— circulation of, 220
Bone, 225
Brachionus, 159
Breeze-fly, mouth of, 191
Bull’s-eye condensers, 86
Butterfly eggs, 194

Camera lucida, 109
Capillary system, the, 221
Care of apparatus, 83
Cells, 124

— ciliated, 221

— life of, 150

— vegetable, 200

Cement, 124

‘ Challenger ’ expedition, objects
from, 239, 241
Chirodota, 169
Chitine, 173

Chromatic aberration, 23
Ciliated cells, 221
Cleaning instruments, mode of, 83
Closteria, 202
Cockroach, the, 182

— mouth of, 188
Coleps, 149

Collar correction, 107
Collecting stick, 139
Compound microscope, 21
Condensers, 74

— use of, 92

Cones, illuminating, 95

— ground glass, 97
Conjugation, 202
Conochilus, 162
Corallines, 170, 171
Cordylophora, 165
Corethra plumicornis, 167
Corpuscles of blood, 218
Coryne fruticosa, 170
Coscinodiscus, 237
Cover-glass, effect of, 25
Cristatella, 166

Critical angle, the, 15

— image, a, 105
Crystallisations, 244
Cyclops, 147
Cyclosis, 202, 216
Cynips, life history of, 199

Daelinger, Dr., on protoplasm,
152

Daphniffi, 142

254

THE MICROSCOPE

Dark-ground illumination, 100
Dendrosoma, 150
Desmids, 201
Diatoms, living, 238

— mounting of, 233

— testing by, 71, 106
Diffraction spectra, 33
Dissecting, mode of, 121

— microscopes, 39
Drag hook, the, 140
Drawing, 109
Drone-fly, mouth of, 190
Dropping tube, 120

Dry objects, mounting, 125

Eciiinoderms, 109
Echinus spines, 242
Eggs of insects, 194
Entomostraca, 142
Euglena, 158
Euplectella, 109
Eye of frog, 230

— parts of, 230
Eye-pieces, 72

— shades, 82
Eyes, care of the, 81

Ferns, 210

Fertilisation of flowers, 210
Floscules, 100

Flowers and the simple micro¬
scope, 41

Foraminifera, 238
Forceps, 117, 121
Frog, eye of, 230
Fungi, 207

Gall flies, history of, 199
Glass, mode of cleaning, 124
Glycerine jelly mounts, 130
Gnat, larva of, 107
Goat-moth caterpillar, legs of, 181
Gonophores, 170
Grasshopper, great green, 185
Ground-glass illumination, 89, 97
Gum, use of, 120
Gyrinus, 183

Hair, 226

Hairy midge, the, 180
Haversian canals, 225

Hearing, organs of, 232
Ileliopelta, 236

High powers, precautions with, 90
Histological microscopes, 54
House fly, the, 173 ; legs of, 174 ;
wings of, 170 ; spiracles of, 176 ;
head of, 177 ; antenme, 178 ;
eye of, 179
Hydra, 103
Hydrozoa, 103

Illuminating apparatus, 74
Illumination, dark-ground, 100

— oblique, 98

— opaque, 103

Images, 11 ; size of, 13 ; and lenses,
18; critical, 105
Immersion, 28

— homogeneous, 29

Infusoria, 148 ; ciliated, 149 ;

flagellated, 150; tentacled, 156
Insects shown by simple micro¬
scope, 42 ; study of, 172 ; eggs
of, 174 ; legs and feet of, 181 ;
wings of, 186 ; mouths of, 188 ;
antenme, 192 ; ovipositors, 192
Inversion of images, 13

Knife, the hooked, 139
Knives, 121

Lacinularia, 102
Lamps, 84

Lantern exhibitions, 252

— microscopes, 80
Leaves of plants, 215
Legs of insects, 181
Lens, action of a, 16

Lenses, and images, 18; aberration
of, 22; single and compound, 37
Life histories, 198
Light, mode of using, 84
Live box, 118

— objects, 118
Log-wood staining, 133
Lophopus, 166
Lucernaria, 170

Magnification, 11
Manipulation, 81, 87
Mayfly, larva of, 107

INDEX

255

Measurement, 110
Megalotrocha, 102
Melicerta, 161
Micrometers, 110, 111
Microscope, the, simple, 20 ; com¬
pound, 21; power of, 22; aberra¬
tion of lenses, 22 ; chromatic
aberration, 23 ; objectives, 24 ;
effect of cover-glass, 25 ; angle
and aperture, 26, 28, 31 ; im¬
mersion objectives, 27 ; homo¬
geneous immersion, 29 ; measure
of aperture, 30 ; numerical
aperture, 30 ; resolution, 31 ;
diffraction spectra, 33 ; apo-
chromatic objectives, 35 ; com¬
pensating eye-pieces, 36; dis¬
secting, 39 ; use of simple, 40 ;
flowers under, 41; insects under,
42 ; modern compound, 44 ;
choice of, 45 ; parts of, 47 ; foot,
47; stage, 48; tube, 49; adjust¬
ments, 50; mirror, 51; sub-stage,
51; stands, 52; ‘ Star,’ 54; his¬
tological, 54 ; ‘ Edinburgh,’ 59 ;
‘ Economic,’ 62 ; ‘Paragon,’ 62;
binocular, 63 ; objectives, 65 ;
apochromatic objectives, 67 ;
qualities of objectives, 68 ; test¬
ing objectives, 70 ; eye-pieces,
74 ; illuminating apparatus, 74 ;
polarising apparatus, 77 ; lantern,
80 ; manipulation of, 81, 87 ;
care of the apparatus, 83 ;
cleaning, 83 ; light for, 84 ;
bull’s-eye condensers, 86 ; pre¬
cautions with high power, 90 ;
searching slides, 90 ; nose-piece,
91; use of condensers, 92 ; illu¬
minating cones, 95 ; ground
glass cones, 97; oblique illumina¬
tion, 98 ; dark-ground illumina¬
tion, 100; opaque illumination,
103 ; polarised light, 104 ; various
methods, 105 ; a critical image,
105; collar correction, 107; with
camera, 114; use of, in exhibi¬
tions, 247

Microscopical exhibitions, precau¬
tions for, 248 ; system for, 248 ;
slides for, 251 ; live and natural

objects for, 251 ; plant structure,
251 ; lantern, 252
Microtome, the, 132
Mildew, wheat, 205
Mineral sections, 242
Mosses, 207

Mounting slides, 123; dry slides,
125 ; in benzol, 127
Mouths of insects, 188
Muscle, 224

Natural objects, 117
Navicula, 236
Needles, 121
Nerve endings, 228
Net, the, 138
Nit coralline, 171
Nose-pieces, 91

Oak-apple, cause of, 199
Objectives, 24,65; immersion, 27;
apochromatic, 35, 67 ; qualities
of, 68 ; testing, 70
Odontophores, 246
Oil, protection against, 131
Ophrydium, 155
Organic sections, 246
Ovipositors of insects, 192

Palates, 246
Parabolic reflector, 103
Paraboloid, 101
Paramecium, 149
Parasites, eggs of, 194
Peristomes of mosses, 208
Photography, 111
Plants, leaves of, 215

— stems of, 213

— vessels of, 213
Plumatella, 165
Pocket lens, uses of, 43
Podura test, 70
Polarised light, 104

— objects, 242
Polycystina, 240
Polynema natans, life of, 198
Polyzoa, 141, 165

Pond life, collection of, 138
Power of a microscope, 22
Prism action of a, 16

256

THE MICROSCOPE

Protoplasm, 150
Psychoda, 186

Radiolaria, 240
Reflection, total, 15
Reflector, parabolic, 103
Refraction, 13

— law of, 14
Resolution, 31
Rotifers, 141, 158

— clustering, 161

— tube-dwelling, 160
Rust in wheat, 205

Salicine crystals, 245
Saw-fly, 193
Sea-oak coralline, 170
Searching slides, 90
Section-cutting, 131
Sections, staining, 133

— charred, 135

— handling, 135
Sertularia, 170

Sex, development of, 208
Sirex gigas, ovipositors of, 193
Skin, 226

Slides, exhibition, 250

— mounting, 123

— storage and selection of, 135
Smell, organs of, 232
Sphrcroplea, development of, 208
Spiders, 195

— webs of, 197
Spirogyra, 203
Sponges, 168
Spot lens, 100
Stage forceps, 117
Staining sections, 133

- in red and green, 134

Stands, microscope, 52
Starches, 247

Steinheil lenses, 39

Stems of plants, 213
Stentors, 154
Stephanoceros, 161
Stick, the collecting, 139
Stops for condenser, 100, 102
Storage of slides, 135
Sun-animalcules, 153
Swan-animalcule, 149
Synapta, 169

Tabanus, 191
Tanypus, larva of, 182
Taste-cells, 230
Teleutospores, 207
Tentaculifera, 156
Tests for objectives, 70
Thread cells, 164
Touch-bodies, 229
Triceratium, 236
Troughs, 119

Trumpet-animalcules, 154
Tubifex, 167

Turn-table, Shadbolt’s, 124

Vallisneria, 217
Vaucheria, development of, 209
Venus’ flower basket, 169
Vessels of plants, 213
Vision, organs of, 230
Volvox, 156
Vorticella, 153

Water bear, 167

— beetles, 182

— fleas, 142

— scorpion, 184
Webs of spiders, 197
Wheat rust, 205
Whirling beetles, 183
Wings of insects, 186
Work, mode of, 87

ritlNTED BY

srornswoooE and co., new- street square

LONDON