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A TREATISE

ON THE

Cflttstocfimt, fjnrjer Sfet artir (fopaMIifi

OF

SMITH, BECK, AND BECK'S

ACHROMATIC MICROSCOPES.

BY

RICHARD BECK.

LONDON:

PRINTED EOR SMITH, BECK, AND BECK, 31, CORNHILL :

PUBLISHED BY

JOHN VAN VOORST, PATERNOSTER ROW.

MDCCCLXV.

.

PRINTED BY TAYLOR AND FRANCIS,
RED LION COURT, FLEET STREET.

INTRODUCTION.

The purpose of this work is to give, by a clear and con-
cise description, combined with superior illustrations, the
most complete directions for the use of Smith, Beck, and
Beck's Achromatic Microscopes and the accessory appa-
ratus.

The publication has been much delayed by various
causes which it is unnecessary to explain at length. It
may, however, be mentioned that a considerable amount
of matter is now included in the work, which it could not
have contained at a much earlier issue ; such, for instance,
as a description of Wenham's Binocular Body, the third-
class "Popular Microscope," and several new pieces of
apparatus.

By giving more time than was at first intended to
the preparation of this treatise, the Author has been able
to allude more particularly to some of the peculiar fea-
tures connected with the illumination of objects under
the Microscope. Great care has also been bestowed upon
the delineation of test-objects; for, preceded as this
work has been by many others upon the same subject,
no one has given satisfactory evidence, by illustration, of

IV INTRODUCTION.

that superior performance which belongs especially to
the English Microscopes.

This treatise is strictly confined to the subjects already
mentioned; for although the improvement of the Micro-
scope is intimately connected with much that has been
done by its aid, it is impossible in this work to make any
satisfactory reference to that wide range of observation
which is so continually extending with the increase of the
power of the instrument.

In concluding this introduction, the firm of Smith,
Beck, and Beck have gratefully to acknowledge the assist-
ance they have received, whilst improving the Microscope,
from the suggestions or from the contrivances of many
amateurs, and especially from those of the late Mr. George
Jackson and Mr. Wenham. But it is not with a simple
mention only that any author on the Achromatic Micro-
scope is justified in passing over the name of Mr. Lister,
all the manufacturers of the improved instrument in
England being indebted to him for that theoretical and
practical information at the outset, which has enabled
them to advance it to its present state of perfection.

Erom Mr. Lister's designs and suggestions various
improvements were made in the arrangement and the
appendages of the Microscope; but his attention was
especially directed to the object-glasses, for the construc-
tion of which he not only determined the principle, but
also recommended those combinations of lenses which,
either unaltered or with modifications, are adhered to at
the present day by the best makers.

INTRODUCTION. V

From consideration for those who were engaged in the
manufacture, he abstained from taking credit for these;
and various misstatements since published having been
allowed to pass without notice, his valuable services have
lately been alluded to in a manner most superficial and
erroneous.

Upper Holloway,

March 1865.

CONTENTS.

Page

First- and Second-class Achromatic Microscopes 1-8

Microscope-Stands ; The Stage ; The Mirror ; The Sub-
stage; Revolving and Folding Bases; Eyepieces; Ob-
ject-glasses; Universal Screw ; -^-th Object-glasses.

Directions for use of Microscope ; Transmitted Illumination . 8-23

The Mirror ; The Diaphragm ; The Achromatic Condenser ;
Tests for Object-glasses ; Adjustment for High Powers ;
The Podura-scale ; Methods of measuring Aperture ;
"Lined Objects" as Tests; Nobert's Lines; Oblique
Illumination.

Illumination from above 23-34

Side Condensing-Lenses ; Side Silver Reflector; Lieber-
kuhns ; Forceps ; Opaque Disk-revolver ; Splinter of
Lucifer Match ; Podura-scale ; Tarsus of Spider ; Feather
of Pigeon ; Arachnoidiscus Japonicus.

Dark-field Illumination 34-36

Erecting-Glass for Low Power and Dissection 36, 37

Polarized Light, as applied to the Microscope 37-48

Mcol's Prisms ; The Selenite Plate ; Darker's Retarding-
plates of Selenite ; Darker's Selenite Stage ; Tourma-
lines ; Polarizers for large objects ; Experiments with
Double-image Prisms ; Crystals to show Rings.

Wenham's Binocular Body for Achromatic Microscope 48-55

Sundry Apparatus 55-70

Live-boxes and Trough ; Screw Live-box ; Lever Com-
pressor ; Wenham's Compressor ; Reversible Compress-

viii CONTENTS.

Page
ors ; Frog Plate ; Camera Lucida ; Micrometers ; Indi-
cator ; Double and Quadruple Nosepieces ; Leeson's
Goniometer; Maltwood's Finder; Microscope Lamps
and Table.

Cases toe Fikst- and Second-class Microscopes 70-72

The Third-class Microscopes 73-92

The Popular Microscope ; Series of Object-glasses ; Descrip-
tion of Stand ; Diaphragm ; Side Condenser ; Forceps ;
Glass Plate ; Pliers ; Case ; Binocular Body ; Mechanical
Stage ; Achromatic Condenser ; Lieberkuhns ; Dark
Well ; Parabolic Reflector ; Polarizing Apparatus ; Ca-
mera Lucida ; Micrometer ; Live-box ; Trough ; The
Educational Microscope ; Diaphragm ; Forceps ; Tray
for extra Apparatus.

The Fourth-class Microscope. The Universal 93-101

Object-glasses ; Eyepieces ; Forceps, Pliers, and Glass
Plate ; Extra Apparatus and Box ; Mechanical Stage ;
Combined Body ; Binocular Body.

Single Microscopes and Magnifiers 102-113

Darwin's Dissecting Microscope and Apparatus ; Improved
ditto, with Binocular arrangement : Patent Achromatic
Binocular Magnifiers and Stand ; Hand-magnifier and
Stand for ditto ; Coddington Lenses.

Instruments used in preparing Objects 113-119

Knives ; Points ; Hooks ; Needle-holder ; Scissors ; For-
ceps ; Quekett's Forceps ; Wood-cutting Machine.

Instruments and Materials used in mounting Objects 119-130

Glass and other Slips ; Cutting and Writing Diamonds ;
Thin Glass ; The Disk-cutter ; Canada Balsam ; Brass
Table and Lamp; Page's Forceps; Deane's Medium;
Farrants' Medium ; Glass and other Cells ; Gold Size
and Asphalt ; Cell-machine ; Glass Cells ; Labels ; Small
Glass Bottles ; Case for Instruments and Materials.

Cabinets and Microscopic Objects 130-134

A TREATISE

ON

ACHROMATIC MICROSCOPES.

Description of Construction.

A Compound Achromatic Microscope consists essentially
of two parts, an object-glass and an eyepiece — so called be-
cause they are respectively near the object and the eye when
the instrument is in use. The object-glass screws, and the
eyepiece slides, into opposite ends of a tube termed the
" body," and upon the union of the two the magnifying power
depends. The microscope-stand is an arrangement for carry-
ing the body ; and is combined with a stage for holding or
giving traverse to an object, and a mirror or some other
provision for illumination.

Microscope-Stands.

Three microscope-stands are shown, one-third their size,
in Plates II., III. & IV.; they differ from each other in con-
struction, but the following explanations will apply to all.

* b

2 CONSTKTTCTION OF STANDS.

The joint at (A) allows the body (B) to be placed in a
vertical, horizontal, or any intermediate position ; and for the
adjustment of the focus of the object-glass, a quick motion
is obtained by turning either of the large milled heads (C),
a smaller one (D) giving a slow motion.

The Stage.

The stage has a ledge (F), upon which the object is most
frequently merely placed, but if necessary it can be clamped
by carefully bringing down the spring-piece (G); the ledge
will slide up or down, and the object may be pushed side-
ways*: these are the only provisions for moving the object
in the plain stage (Plate IV. fig. 2) ; but in the other " stages
with actions " this arrangement forms the coarse adjustment,
— finer movements in directions at right angles to each other
being effected by the milled heads (H, I) ; and that part also
to which the ledge is attached will rotate.

The Mirror.

The mirror (K) is flat on one side and concave on the
other; it swings in a rotating semicircle (L), which will slide
up and down the stem (M), or can be turned on either side.

The Substage.

As the mirror alone is insufficient for many kinds of
illumination, some provision has to be made for holding vari-
ous pieces of apparatus between the object and the mirror.
In the first-class instruments a cylindrical fitting or " short

* In the plain stage the spring-piece (G) is almost invariably used, to supply
some slight resistance, and thereby to steady the side movement of the object,
which is directly dependent upon the fingers.

OBJECT-GLASSES. 3

body" (T) is mounted perfectly true with the body, and
can be moved up or down by rack and pinion connected with
the milled heads (U, Plates II. & III.). In the second-class
instruments a short piece of tube (Plate IV. fig. 2, R), equally
true in its position as in the former case, fits by a bayonet-
catch into the bottom plate of the stage ; but it has no rack
movement. The way in which the several pieces of appa-
ratus fit into these substage receiving-pieces will be explained
in each particular case.

Revolving and Folding Bases,

In the two first-class instruments there is a revolving fitting
on the base (N), by which means the microscope can be
turned round without being lifted from the table. Each of
these stands can also be made exceedingly portable for their
size by the application of a folding base and a removeable
stage ; the exact dimensions of their cases under such circum-
stances are specified in our priced catalogues.

The Eyepieces.

There are generally three eyepieces, distinguished by the
numbers 1, 2, or 3 (see Plate V.) : but it is not unusual, in
the examination of many object-glasses by test-objects, to
employ eyepieces of still higher power, and we then supply
a No. 4 or a No. 5 in addition ; the latter magnifies about
twice as much as the No. 3, and the former is intermediate.

The Object-glasses.

The object-glasses (see Plate V.) are numerous, and vary
in many particulars ; the list of them on page 5 may there-
fore require a few explanations.

b 2

4 OBJECT-GLASSES.

The " focal length" is in each instance that of a single
lens magnifying the same as the object-glass.

The numbers given under " linear magnifying power " (or,
as they are sometimes termed, " diameters ") must be squared,
to give the superficial measurement or real increase of size.
The magnifying power is increased with the same object-
glass by changing the eyepiece from a lower to that of a
higher power, the extreme range being from No. 1 to No. 5 ;
and also when both the object-glass and the eyepiece are
the same, by pulling out the draw-tube of the body. This
tube, shown at full length in Plate II. fig. 3, serves several
purposes, which will be alluded to further on ; it may, how-
ever, be mentioned here, that the graduations of inches and
tenths on this tube should generally be kept on the left-hand
side.

The " aperture " is the measurement in degrees of the cone
of light admitted by each object-glass.

The erecting-glass (Plate V. fig. 4) has a special notice
at p. 36. Many particulars connected with the use of the
object-glasses are supplied in other parts of this treatise;
and we will only add here, in connexion with the list, that
we have made most of these object-glasses for many years.
The construction of some of them was for a long time peculiar
to ourselves ; and although both this and their nomenclature
have been copied by others, we are still persuaded that the
attention we give to them in every respect obtains a large
amount of favour with microscopists.

Another list of object-glasses, which we term our Edu-
cational Series, is given further on, in the description of our
third-class instruments.

OBJECT-GLASSES.

0

List of Achromatic Object-glasses.

Focal length.

3 inches
2 inches

1| inch

|- inch

Y4^ inch

r4^ inch

5 inch

i inch

^ inch

£ inch

J^ inch

Draw-tube closed ....
Ditto if drawn out, add

for each inch

Draw-tube closed ....
Ditto if drawn out, add

for each inch

Draw-tube closed ....
Ditto if drawn out, add

for each inch

Draw-tube closed ....
Ditto if drawn out, add

for each inch . . .
Draw-tube closed .
Ditto if drawn out, add

for each inch

Draw-tube closed ....
Ditto if drawn out, add

for each inch

Draw-tube closed ....
Ditto if drawn out, add

for each inch . . .
Draw- tube closed .
Ditto if drawn out, add

for each inch . . .
Draw-tube closed .
Ditto if drawn out, add

for each inch . . .
Draw-tube closed .
Ditto if drawn out, add

for each inch . . .
Draw-tube closed .
Ditto if drawn out, add

for each inch . . .

Linear magnifying power nearly,
with eyepieces

Degrees
of angle
of aper-
ture,

No. 1.

No. 2.

No. 3.

No. 4.

No. 5.

about

12

20

40

48

74

r 12

2

4

6

7

10

20

38

70

85

130

r 18

4

6

8

12

15

30

56

100

120

190

I 23

5

7

12

15

22

70

120

220

270

410

I 35

8

14

25

27

48

120

210

370

460

710

I 55

14

24

34

46

70

'

146

255

460

560

890

I 90

18

32

48

60

80

200

340

590

720

1120

r 75

24

42

63

85

120

225

400

700

860

1450

I 85

18

35

60

80

130

225

400

700

860

1450

1 100

18

35

60

80

130

500

870

1500

1850

2800

I 120

60

100

180

190

370

900

1570

2750

3450

4950

1 140

80

150

300

350

900

The list of magnifying powers, as given above, is only approximate ; but if
the exact power of any object-glass be required, it may be easily obtained in
the way described at page 63.

6 ONE-TWENTIETH OBJECT-GLASS.

The " Universal Screw"

All the object-glasses in both lists are made with what is
called the "universal screw" — a standard size which the
principal microscope-makers of England have adopted for
the attachment of the object-glass to the instrument. When
this uniformity of screw was proposed by the Microscopical
Society of London, we immediately approved the suggestion,
and also established a series of gauges connected with Whit-
worth's standard sizes, which have been found to answer
most satisfactorily*.

The -^oth Object-glass.

The most important addition we have made of late to our
object-glasses is the -2^th. This high power is constructed for
the examination of those objects which require the greatest
amount of amplification, but not that extreme angle of
aperture which involves the employment of the very thinnest
glass and the most careful preparation of the object. The
■^th will adjust through any covering-glass not more than
•005 in. thick ; and, when in focus, there is sufficient space
between the front lens and the object to admit of its use in
the examination of ordinary preparations : under these con-
ditions it can be employed with the same facility as an object-
glass of only half the power.

In Plate XXIY. are illustrations of two objects, selected
from the animal and vegetable kingdoms ; these are shown
as they appear under a -^th, with the No. 1 eyepiece, the
linear magnifying power being about 900 linear. Fig. 1 is
the under side of the head and thorax of the Demodex folli-

* Quarterly Journal of Microscopical Science, July 1859, " Eemarks on
the Universal Screw." By Bichard Beck. (Read May 26th, 1859.)

ONE-TWENTIETH OBJECT-GLASS. 7

culorum, a minute parasite infesting the sebaceous and hair-
follicles of the human skin. They are easily obtained by
pressing out the contents of the follicles on the sides or bridge
of the nose : if this matter be gently stirred up with a small
camel' s-hair pencil in a little olive oil, the parasites become
disengaged, and should be removed to a small quantity of
fresh oil, with a piece of thin glass placed over ; and under
this pressure they will retain their life and correct appearance
for one or two days. The legs are very short, apparently
composed of three joints, and the last one is terminated by
a single claw : each leg is moved in a very deliberate manner,
and describes a semicircular course at its extremity ; during
the backward stroke the claw is retracted, but it is jerked
out again rapidly at the commencement of the forward
movement.

There are two organs (A, B) at the side of the head, which
are perhaps palpi ; they are constantly moved up or down
during life, and are apparently provided with two flaps as a
means of clasping. The parts occupying the central portion
of the head may represent the mandibles and a labium ; but
this parasite is altogether a remarkable instance of the dif-
ficulty of determining the exact organization of a transparent
object. The magnifying power is abundantly sufficient
for the purpose ; but, owing to the refraction of the light
through the denser parts, and the upper and the under sur-
faces being equally apparent in the thinner parts, it has only
been after an examination of many specimens for several
days that we have been able to determine the structure so
far as it is here shown, whilst we know there is still much
left in perfect obscurity.

Figs. 2 and 3 are respectively the front and profile
views of the stinging-hairs from the stem of the common

8 MANAGEMENT OF THE LIGHT.

nettle (Urtica dioica). It appears at first sight somewhat
surprising that these hairs, which are known to enter the
skin with so slight a touch, should have a blunt bulbous ex-
tremity ; but this is the very provision for the peculiar effect
they produce. When any slight pressure is brought upon
the extremity of the hair, the bulbous part breaks off, leaving
an exceedingly sharp-cutting point (see fig. 4), admirably
adapted for entering the skin, at the same time making an
aperture at the extremity of the hair, from which the contents
of the cell escape and enter the puncture. When the hairs
are young and unbroken, a beautiful circulation is visible.
Objects of this class can be examined under the ^th with
as much facility as under a -^th ; the conditions being such
as to cause many high powers of large aperture to be per-
fectly useless.

Directions for Use of Microscope. — Management of the

Light.

For general purposes the body of the microscope is inclined
as shown in the Plates, but this position is varied according
to circumstances.

The light should, if possible, be on the left of the observer :
the best is that from a white cloud on a bright day ; but a
most satisfactory effect can be obtained from a wax or
Palmer's candle if protected by a glass, a good oil-lamp, or
an argand gas-burner, provided they are not more than 10 or
12 inches from the microscope: but with all the artificial
means of illumination there should be some arrangement for
raising or lowering the light, and for holding a shade as a
protection to the eyes (see Description of Lamps).

In the examination of an object choose the object-glass
that appears best suited for it, remembering that in investi-

THE DIAFHKAGM. 9

gation it is best to begin with the lower powers for a general
view, and afterwards to ascend to the higher, which give
greater detail of minute parts.

The right management of the light is indispensable for
obtaining beauty of picture and fine definition, and is only
to be acquired by practice; for the illumination must be
varied with different objects, and often even with the same
to exhibit every feature.

Every microscopic object may be said to be either trans-
parent or opaque : this is not strictly correct, as will be seen
hereafter, but the distinction is made here for the sake of
division.

The Mirror.

The illumination of a transparent object is most frequently
produced hf* reflexion from the mirror (K) below, which
should generally have its centre coincident with the axis of
the body. The flat side is sometimes preferable when there
is abundance of light by day; but artificial light almost
always requires the concave mirror to condense the light to
a focus upon the object.

The Diaphragm.

With the i^jths and lower powers the light is generally
in excess, and has to be diminished by one of the smaller
openings of the diaphragm (P), which is attached to the
under part of the stage : its perforated plate will revolve, and
each hole when central is stopped by a weak spring. If it be
necessary to remove the diaphragm altogether, it will slide
off at its fitting (Plate II. fig. 2, R); and the plain circular
ring (S) which is then left can also be detached from its
bayonet-fitting to the stage, by turning it in the direction of
the arrow.

10 THE ACHROMATIC CONDENSER

With the higher powers there is seldom too much light ;
but the diaphragm must often be removed altogether ; and
the mirror requires the most careful adjustment by its dif-
ferent movements, especially that by which it can be moved
up or down on the stem (M), to ensure its exact focus being
thrown upon the object. It must be borne in mind that by
daylight the rays are parallel, and then the focus of a con-
cave mirror will be much shorter than by an artificial light,
which cannot be used advantageously at more than 10 or 12
inches from the microscope.

The Achromatic Condenser.

When the nicest illumination by the mirror fails to exhibit
the structure of an object, or the best definition of an object-
glass, an achromatic condenser must be employed : this is a
combination of lenses by which the light is concentrated to
a minute spot upon the object, without the colour and other
defects which would be produced by a single lens alone. Of
the two mirrors, the flat one should be invariably used ; but
as this gives a somewhat imperfect reflexion which interferes
with the very best definition, a right-angle prism (Plate VI.
fig. 4), which fits on the mirror-stem of the best instrument,
is sometimes employed instead of the mirror; or the lamp
may be placed at the end of the microscope, in a direct line
with the body.

In the first-class instruments (Stands Nos. 1 & 2) the achro-
matic condenser is mounted, as shown in Plate VI. fig. 2,
and fits by its tube (a) into the top of the cylindrical fitting
(Plates II. & III., T) under the stage; in the second-class
Stand, it slides by its fitting (Plate VI. fig. 1, b) into the
short tube under the stage (Plate IV. fig. 2, R). When
the achromatic condenser is in use, it must be central with

THE ACHEOMATIC CONDENSER. 11

the body of the microscope : this may be tested by screwing
a low power on the instrument, and if the top of the con-
densing-lens should not appear in the centre of the field of
view, the necessary movements can be made by turning the
small milled heads (<?, c) ; and these, to be convenient for use
with the right hand, should be in the position shown in the
drawing. The change of object-glasses involved in this plan
of centering may be obviated in the first-class microscopes
by sliding into the lower end of the cylindrical fitting (Plates
II. & III., T), the diaphragm (Plate VI. fig, 3), when its
smallest opening should appear as a bright spot in the middle
of the field of view (see fig. 7). The achromatic condenser
may, however, be so eccentric as to require a larger opening
in the diaphragm to be used first. Or if the spot of light does
not appear defined on the edge (see fig. 5), the cylindrical
fitting must be moved up or down by either of the milled
heads (Plates II. and III., U). The small milled heads (<?, c)
are used under the same circumstances as before mentioned,
and full illumination is obtained by turning on the largest
opening of the diaphragm.

The achromatic condenser in the second-class Stands will
slide up and down in the circular ring (Plate IV. fig. 2, R) ;
and if slightly rotated at the same time, the movement can be
made very exactly. In the first-class Stands the cylinder (Plates
II. & III., T) will rack up or down by turning either of the
milled heads (U). This rackwork, if connected with the
achromatic condenser, serves for the adjustment of its focus,
which should generally be thrown upon the object. When
such is the case, the image of the flame by artificial light, or
of the window-bars or objects in the landscape by day, will
be seen in the microscope at the same time as the object.
Moving the mirror or right-angle prism will not alter the

12 TESTS FOE OBJECT-GLASSES.

adjustments, and any particular cloud, or part of the flame,
can be selected for the illumination. Another essential point
is the aperture of the condensing-lenses ; this is reduced
from 65 to 40 degrees by removing the front lens (Plate VI.
figs. 1 & 2, d). But in the most complete arrangement a
perforated plate (Hg. 6) is inserted at the back of the lenses,
as shown in fig. 2 ; in this position it can be rotated on its
axis (e), and the openings near its circumference can be
brought in succession behind the lenses, each aperture of the
plate being stopped by a slight spring when central. Five
of the openings vary the aperture from 90 to 45 degrees with
the front lens (d), and without it from 50 to 25, whilst the
other three apertures cut off the rays of light in the centre
and admit only those at the circumference.

Some additional instructions for the use of the achromatic
condenser will be found in those parts of this treatise where
the higher powers, and test-objects, are considered.

Tests for Object-glasses.

The principal points on which an object-glass has to be
examined are, the spherical and chromatic aberrations, the
aperture, flatness of field, and workmanship. When the
term "achromatic"* is applied to an object-glass, it is in-
ferred that all the errors consequent upon the refraction of
light through its lenses are reduced to a minimum; with
such a result the object-glass is called " corrected." And in
connexion with the spherical and chromatic aberrations, the
term "under-" or "over-corrected" is applied as the right
point is exceeded or otherwise, — a single lens being regarded
as " under-corrected."

* Achromatic really signifies the absence of colour only.

ADJUSTMENT FOE HIGH POWERS. 13

Spherical aberration, which is the extension of the focus
of a lens, can be entirely corrected, so that an object shall
appear sharp and denned in one decided plane ; whilst, with
a proper object, the appearance a little within, or as much
beyond, the point of distinct vision will be the same.

It is impossible, so far as is yet known, entirely to correct
chromatic aberration; but this aberration or separation of
light into various colours, with which every one is so familiar
in connexion with prisms and single lenses, can, by a proper
combination of lenses, be so far counteracted as to leave only
a pale green, resulting from the union of blue and yellow,
which is inoffensive and quite immaterial.

Adjustment for High Powers.

In the higher powers, both of these aberrations are con-
siderably affected by variations of the thickness in the covers
of objects; for if an achromatic object-glass be corrected for
an object which is completely uncovered, a piece of thin
glass or any fluid, placed between the object and the object-
glass, will alter that correction and produce indistinctness.
It is not perceptible in the lower powers, but in the higher
powers the error caused by the thinnest covering-medium
becomes injurious.

To provide against this, the ^0-, \, |-, and •£$ object-glasses

have a moveable collar divided into ten divi- r— 1

sions (c) (see accompanying figure), by turn- ^ —f

ing which the distance is varied between the r7T". =^
front lens and those behind it. ®

When the object is uncovered, 0 on the

a
Uncov

screw-collar should stand opposite the small
screw (<z), and the line on the small piece
of brass (b) which is let into the tube (d) should coincide

14 THE PODURA-SCALE.

with the line (e) engraved " uncovered" : this is as far as the
collar will go in that direction.

For covered objects the collar must be turned from the
uncovered point, so that the numbers 1, 2, 3, &c. come in
succession opposite the small screw. It is best in practice
to have the object-glass on the microscope, move the collar
a little at a time, focus for each alteration, and carefully
watch the appearance of the object until the best definition
is obtained. This mode of adjustment can be easily and very
correctly made if the object be well known to the observer, or
if its structure be simple and distinctly marked : the latter is
peculiarly the case with the Podura-scale, and as it is easily
obtained we consider it one of the very best tests.

The Podura-Scale.

The Frontispiece shows the correct appearance of a small
but coarse specimen under different object-glasses with the
No. 3 eyepiece ; but Plate VII. more directly refers to the
matter now under consideration. Each of the first six figures
in this Plate represents xo^o^n °f an mcn square, con-
sequently the magnifying power is about 1300 linear, ob-
tained by the ^th object-glass and the third eyepiece. The
illumination employed is that of a small
lamp, close to the microscope, with its light
reflected by the right-angle prism so as to pass
through the achromatic condenser, which, besides
being carefully centred and focused, is found to
produce the best effect in this case with the
smallest aperture and without the front lens.
The light will be reflected most brilliantly when
it is placed a little behind the right-angle prism,
as shown in the annexed diagram ; a being the light, b the

THE PODUKA-SCALE. 15

right-angle prism, and c the direction of the light passing to
the achromatic condenser. Good daylight will answer quite
as well ; and nearly the same results can be obtained with the
concave mirror alone.

The drawings referred to have been very carefully made
by aid of the camera lucida (the use of this piece of appa-
ratus is described at p. 61) ; and whilst some size and pro-
minence have been given to the features, great care has been
taken not to exaggerate any one of the appearances.

Fig. 1 gives a representation of the markings when there
are no errors in the object-glass. Its adjustment is known
to be correct by the object presenting the same perfectly in-
distinct appearance (fig. 2) when thrown a very little either
within or beyond the focus, and also, when this is done,
by each marking dividing equally, somewhat as shown in
fig. 3.

If the adjustment be incorrect, the appearances within
and beyond the focus are never similar, but on the one side
there will be strong lines (fig. 4), and on the other side a still
greater indistinctness than that shown at fig. 2 (see fig. 5) ;
whilst the best focus under such circumstances shows the
markings in a general fog (fig. 6), without either the sharp
black sides or the white central spot shown in fig. 1.

The exactness with which an error in adjustment can be
detected is one of the best qualities of a test ; and with this
particular structure, using a ^th or |-th object-glass, a vari-
ation of Toootli of an inch in the distance of the lenses, or
about half a division on the screw-collar, can be easily de-
tected by a practised eye.

Any chromatic aberration, as excess of colour, will be im-
mediately seen in the light spaces between the markings of
this object.

16 ADJUSTMENT FOE HIGH POWERS.

The spherical aberration should be entirely corrected so far
as the adjustment by the screw-collar will work ; this varies
in each object-glass: thus the -nyths has a range of about
1\ turns, equivalent to 25 divisions, and will correct for glass
about *025 thick ; the ^th has 2 turns for glass measuring *02 ;
and the one turn of the -^th will only adjust for a thick-
ness of #012. Whilst on this subject, we would caution some
persons against concluding that an object-glass with a move-
able collar must necessarily possess an adjustment; we have
seen many instances in which such an arrangement made no
alteration in the correction, and other cases in which the
alteration that was made exactly reversed the proper condi-
tions. With such an object as the Podura-scale, it is not at
all difficult to establish a rule for the adjustment which shall
be entirely independent of the appearance presented by the
object; for if, after such an adjustment as has just been
made, we measure the thickness of the covering-glass, first
by focusing its upper surface, which is almost sure to
have dust or spots upon it, and then noticing how many
divisions of the slow-motion milled head of the microscope
it takes to bring the object in focus, a comparison of these
divisions with those recorded in the former observation will
supply the standard. As, for example, the Podura-scale
shown in Plate VII. is covered with a piece of glass, and
when the appearance is correct, the collar of the \\h object-
glass is moved five divisions ; and it takes six divisions on
the slow-motion milled head to focus from the top to the
under surface of the thin covering-glass ; so that if, with the
same object-glass, the cover of another object measure 9
divisions of the slow-motion milled head, 7 '9 divisions must
be taken on the collar of the object-glass.

If the body of the microscope be lengthened by pulling

APERTURE OF OB JECT-GL ASSES. 17

out the draw-tube, some further correction is required for
the -roths : thus, for 1 inch drawn out, add two divisions on
the collar, for 3 inches four, and for 4 inches five divisions.
The object-glasses of higher power are not so much altered
by lengthening the body ; but with the ^th and ^th, one
division may be advantageously added for the first 3 inches
drawn out.

In the lower powers an object should not differ in appear-
ance, or require any alteration of focus, if moved from the
centre to the edge of the field of view. This " flatness of
field " is not of so much importance in the higher powers,
and it is seldom obtained in them, especially when the aper-
ture is large ; but its absence the Podura-scale will show to a
remarkable degree.

Aperture, as before stated, is the measurement in degrees
of the cone of light admitted by the object-glass; and the
definition is dependent upon it, provided the object-glass is
at the same time well corrected. In the lower powers it is
difficult under such conditions to exceed a certain point,
whereas in the higher powers it may be said there is no limit
within about 170°; but it must be remembered that the use-
fulness of an object-glass is destroyed when the range of
adjustment, or the distance between the front of the object-
glass and the object, is sacrificed to an increase of the angle
of aperture. With the Podura-scale a larger aperture in the
lower powers is evidenced by the mere fact of seeing the
markings, and in the higher powers with the deeper eye-
pieces it gives a remarkable blackness and brilliancy to the
picture ; but, as a real test of aperture, the Podura-scale is
not equal to many other objects, more especially to those
siliceous valves of the Diatomaceae which show lines under
oblique illumination. After all, however, the only certain

c

18 METHODS OF MEASUKING APERTUKE.

mode of ascertaining the aperture is by some mechanical
means.

Methods of measuring Aperture.

Of these the simplest is that known as Mr. Wenham's, in
which, by merely holding an object-glass in front of a strong
light, the angle of the cone of light issuing from the front
lens can be measured by means of a divided semicircular
card (see Plate VI. fig. 14); but care must be taken to keep
the focus of the object-glass in the centre, and also in the
plane of the semicircle. This plan, however, at the best,
is only intended to give a general estimate within a few
degrees.

The aperture may be much more exactly measured by the
instrument shown in Plate VI. fig. 12. This consists of a
semicircular piece of wood about 8 inches in radius, divided
into degrees on its edge (a, a), with a narrow slip of wood (f)
carrying two V-pieces (b, b) turning on the centre (c). To
use this, the body of the microscope (or some similar tube),
with a shallow eyepiece, should be placed on the V-pieces,
and there should be a small bright light in a line with the
axis of the body, at a distance of about 6 feet. The object-
glass to be measured is screwed on, and the point of its focus
brought exactly over the centre (c); the side (d) of the nar-
row slip is then kept flush with the side (e) of the semicir-
cular board, whilst the whole apparatus is moved to the right
on an imaginary centre, until half of the field of view is cut
off; this is the zero-point, and if the semicircular board be
then kept firm and the slip (/) only be moved to the left
until the field of view is again bisected, the number of
degrees indicated at the point (d) will be the aperture of the
object-glass.

NOBEET'S LINES. 19

" Lined Objects " as Tests.

" Lined objects," as such, are invariably examined by
oblique illumination in a direction at right angles, or nearly
so, to the lines to be observed.

Plate VII. figs. 7, 9, 10, & 12 will illustrate this by show-
ing the various appearances of the structure of Pleurosigma
quadratum under the different directions of illumination as
indicated by the arrows. That which constitutes the test in
this class of objects is the mere fact of being able to sepa-
rate lines, — a power in great measure depending upon the
aperture of the object-glass.

In Navicida rhomboides (fig. 13) the lines which appear in
the direction there shown are as nearly as can be counted
75,000 to the inch (the lines and the interspaces being equal),
and an object-glass of less than 120° will not separate them.
The specimens, however, of the same species of the Diato-
maceee differ so considerably in the distances of their lines
that none of them are equal in value as a test to the object
known as Nobert's lines (Plate VIII.).

Nobertfs Lines.

This is a piece of glass upon which are ruled, with the
most remarkable regularity, and in a way entirely peculiar to
M. Nobert, of Barth, twenty bands of minute lines; these
are again divided into four sets, but from the first to the last
the distance between the lines in each band is on a gradually
diminishing scale of from about 13,000 to 70,000 to the inch;
the particulars are given exactly on the Plate, and the
number of lines in each band varies from about eight to
thirty-five. The following observation may be taken as an
instance of their power to test the aperture of an object-
ed

20 OBLIQUE ILLUMINATION.

glass: — an ^th of 120° will show them all; but when cut
down to 110° it will not separate the twentieth band, at 100°
the seventeenth is the limit, at 80° the fourteenth, and at
60° the tenth.

Oblique Illumination.

The simplest way to obtain the necessary illumination for
this class of observations is with the concave mirror, by turn-
ing it considerably on one side and making use of the
lengthening arm (Plates II. III. & IV., W.) ; when this
method is pushed to an extreme, the focus of the mirror
must be accurately thrown upon the object, and the lamp
should not be nearer than 12 inches. If the object require
a still greater obliquity of illumination, it may be obtained
by an Amici prism (Plate VI. fig. 11) : the way in which the
light passes through this is shown by the three lines (a, a!, a"),
and it is necessary that the prism should be so mounted
that its focus may be thrown very obliquely and yet accu-
rately upon the object (c). When in use, the mounting of
the prism fits by its tube (h) into the top of the cylindrical
fitting of the first-class instruments ; in all the other micro-
scopes it requires a separate stand.

The prism itself can be rotated by the small milled head
(d) in the semicircle (e) ; and this is attached to an arm (/"),
sliding in a dovetail box (</), which also turns on a centre
at g ; so that by these movements, combined with the rack-
work of the cylindrical fitting, every adjustment can be
made.

With the mirror or the Amici prism the object has almost
always to be rotated either more or less, so that the lines
may be at right angles to the direction of the illumination :
this may be obviated by using Nachet's prism (fig. 10), but

OBLIQUE ILLUMINATION. 21

then the obliquity of the illumination is limited ; the direc-
tion the rays take in passing through this prism is shown in
fig. 9, and it will there be observed that the emergent
pencil (fig. 9, a) comes to a focus in the axis of the mounting,
shown by the line b, c ; therefore when the prism is rotated,
the centering of the illumination remains the same, whilst the
direction is altered. The whole piece of apparatus slides
into the cylindrical fitting under the stage, the rackwork of
which, in the first-class instruments, will give the requisite
adjustment up or down, whilst in the second class this move-
ment is made with the fingers, and the light must in both
cases be reflected from the mirror below.

The prism, when in its place, can be rotated by means of
the milled edge (fig. 10, a); it can also be taken off at a fitting
at b, if the inner surface require cleaning ; the other surfaces
are all exposed.

When the object is transparent, and the illumination is
transmitted obliquely and on one side only, it is very seldom
that reliance can be placed upon the appearance as a true
indication of the structure. There is a striking example of
this in the change the markings of Pleurosigma formosum
assume (Plate IX. fig. 1, A, B, C, D) when only slight variations
are made in the direction of the light ; and we may here state
that the most correct way of viewing this peculiar structure
of the Diatomacese is undoubtedly by illuminating with the
largest aperture of the achromatic condenser very carefully
centered, whilst the appearance is considerably improved, as
shown in fig. 1,D, by using one of the stops (Plate VI. fig. 6,
a, b, c) in the diaphragm of the condenser. Under this kind
of illumination, Plate VII. fig. 11 shows the appearance of
the markings on Pleurosigma quadratum; and the upper
part of the same figure also illustrates how entirely the pre-

22 INJUKED CONDITION OF OBJECT-GLASSES.

sence of moisture will obliterate the markings, as is very
frequently the case with specimens that have been mounted
for some time.

It is quite possible for an object-glass, although correct in
all the points hitherto mentioned, yet to be deficient in its
performance from errors in workmanship; these are very
various, and the causes are often as difficult to detect ; but
the one most evident of all is when the surfaces of the lenses,
the lenses themselves, or the brass cells in which they are
mounted, are not true with each other, and the object-glass is
said to be badly " centered." The Podura-scale (Plate VII.)
will immediately show this fault, even if the error be small,
by one side of each marking being darker than the other ;
and the scale should be examined in a horizontal as well as
a perpendicular position, for it is quite possible that an
object-glass will show the markings well in one direction and
not at all in the other : the scale itself, not being flat, may
produce a similar appearance to that arising from bad cen-
tering ; and so also will the illumination to a certain extent,
if it be not central.

While on this subject, we would add one word of caution
as to the appearance of an object-glass : a small chip out of
the edge of a lens, a spratch, or even a surface not thoroughly
polished, does not necessarily affect the performance ; whereas
if an object-glass be not clean (it may have been made for
some years, and may require wiping at the hands of the
maker, or the front lens may have been touched only by the
finger), its definition will be considerably impaired, and by
many it might be most unjustly condemned. We cannot
also too strongly urge upon microscopists the care required
in the use of the object-glasses, as only a slight blow or a
fall may in a moment make the best glass utterly worthless.

SIDE CONDENSINGh-LENSES. 23

The subject of test-objects has received a large share of
attention from English observers, not only from the earliest
times of the microscope, but more especially during the
recent improvements and up to the present day ; this diligent
investigation, although somewhat fruitless in itself, has been
of no little aid in perfecting the instrument for those who
have used it as a means of research, and we strongly recom-
mend every microscopist to test his own instrument : if the
result be satisfactory, it will enable him as a scientific
observer to be accurate and confident in his deduction ; or if
he be only an amateur, he will pursue his recreation with
pleasure and comfort.

Illumination from above.

Opaque objects require, of necessity, that the light be thrown
upon them from above ; but this kind of illumination is fre-
quently employed in the examination of many specimens
which are more or less transparent, and when under such
circumstances the term " opaque illumination" is used, it must
be remembered that, although the appearance is exactly the
reverse of that produced by light transmitted from below,
the real condition of the object remains unchanged.

Side Condensing-Lenses.

The "bull's-eye" condenser (Plate X. fig. 7) and the
smaller condensing-lenses (figs. 9 & 10) are the simplest
means of obtaining illumination from above, by merely con-
centrating the light to a focus and in an oblique direction
upon the object; the best results, however, can only be
obtained by some little care in the adjustments. When the
bull's-eye is used alone for this purpose, its flat side should
be next the object; and the various means by which it can

24 SIDE SILVER EEELECTOE.

be brought into position are a ball-and-socket joint at
fig. 7, A, a sliding horizontal tube at B, and a movement
up or down on the perpendicular rod (C).

The smaller condensing-lens (fig. 10) is shown as mounted
for use with the first-class stands ; it fits by a square (a) into
either of the two holes in the limb of the best instruments
(Plates II. & III,), the rod (B) slides through a pipe (C), which
has a ball-and-socket movement at D, and the lens swings
in the semicircle (E).

Fig. 9 shows the same lens mounted on a separate stand,
with a ball-and-socket at (D), a draw-tube (F), and a sliding
rod (B) carrying the lens at its extremity; this being the
arrangement for the second-class instruments.

When any of the condensers are in use, the lamp should
not be more than 6 or 7 inches from the object, except when
two of them are combined to increase the intensity of the
illumination, and then the lamp may be moved 3 or 4 inches
further off, with the flat side of the bull's-eye nearly close to
it, so that the light is concentrated to about the diameter of
the smaller condenser, and from it to the object. This kind
of illumination must necessarily be one-sided ; for some pur-
poses it may be advantageously so, in many instances it is
abundantly sufficient, and for large objects it is the only
method available.

Side Silver Reflector.

The light, if required, may be thrown more perpendicularly
by the side silver reflector (Plate X. fig. 1) ; this, when in
use, is generally attached by its square bar (A) to one of the
holes on the right-hand side of the limb of the best instru-
ments; it is most advantageously used when reflecting the
light which has in the first instance been concentrated upon

LIEBEKKUHNS. 25

it by one of the condensers. The focus of the reflector, and
therefore the right distance from the object, is a little more
than an inch, and the particular position in which it may be
wanted is obtained by the two ball-and-socket movements at
D and E and by the sliding rod (B) ; the small screw (G),
when tightened, will prevent the rod from turning round,
which it has a tendency to do from the weight of the re-
flector being on one side. •

For the second-class instruments the side silver reflector,
with the movements just described, is mounted on abase and
stem similar to that shown by fig. 9.

Lieberkuhns,

But the illumination which is the best for opaque objects,
because it is not only the most brilliant, but also permits any
alteration in the direction of the light, is that afforded by
the Lieberkuhns; these are highly polished silver cups,
sliding upon the fronts of the object-glasses, and reflecting
to a focus the light thrown upon them by the mirror. They
are supplied to the J-inch and all the lower powers (see
Plate X. fig. 11); but several things are essential to their
proper use. The diaphragm under the stage must be in-
variably removed, and neither the object nor the substance
upon which it is mounted should stop any more light from the
mirror than is really necessary ; for although a diameter of
6 or 7 tenths of an inch may make no obstruction with the
1^-inch Lieberkuhn, about 2 tenths is the largest size that
the J-inch will admit of without impairing the illumination.

If the object be too small or transparent, one of the dark
wells (fig. 12, A, B, C) may be used as a background ; of these
the largest is, of course, intended for the lowest power ; and
they slide into the arm of the holder (fig. 12, D), which fits by

26 EOECEPS FOE HOLDING OBJECTS.

its tube (E) into the cylindrical fitting under the stage. A
dark well, when in use, should be brought as close behind the
object as possible; and it must also be central, or nearly so,
with the body of the microscope.

The direction of the illumination, when a Lieberkuhn is
used, is regulated by moving the mirror, the flat side of
which is generally used by daylight, and the concave is the
best for artificial light ; but the most intense illumination is
obtained by throwing parallel rays from the lamp upon the
mirror by a condensing-lens, and the flat or concave side is
in this case immaterial.

The proper illumination of an object from above requires
quite as much care as the most accurate treatment of trans-
mitted light. Very slight variations in the illumination will
entirely alter appearances ; and the adjustments of the higher
powers require especial attention, as this mode of examining
objects is often the most severe test of the performance of
an object-glass.

But a feature which is peculiar to this mode of illumina-
tion is the change which almost always has to be made in
the position of the object itself.

Forceps for holding Objects.

If the specimen be mounted permanently on a slip of glass,
it is only capable of rotation by turning the top plate of the
stage of the microscope on its fitting ; and this may be quite
sufficient when the object is flat, and an exact analysis of
its structure is unnecessary; but if otherwise, the forceps
(Plate X. fig. 6) are generally used; these fit by their pin (A)
into a small socket (Plates II. III. & IV., Y) on the clamping-
piece of the stage, and the object is either held between the
points of the forceps at B, or it may be attached by a pin

OPAQUE DISK-EEVOLVEE. 27

or some other means to the cork end (C), the various move-
ments of the forceps affording in either case considerable
range for varying the position of the object.

The three-pronged forceps (fig. 5) are for holding large or
irregular-shaped objects, the specimen being placed between
the three points A, B, and C, which are made to approach each
other or spread out by sliding the piece (D) to which they
are attached either up or down on the stem (E) ; the other
movements being the same as those of the ordinary forceps
(fig. 6) already described.

It will be found by trial that many of the movements of
these forceps are limited, and, except for objects that require
but little alteration of position, there is considerable difficulty
in turning an object about quickly and with certainty under
moderately high powers.

To obviate these difficulties, we have contrived a piece of
apparatus termed an " opaque disk-revolver" ; its construction
is as follows : —

Opaque Disk-revolver.

A brass plate (Plate X. fig. 2, A), which is adapted for
clamping in the ordinary way upon the stage of the micro-
scope, has a hole through it sufficiently large for a low-power
Lieberkuhn. On the right side, at B, is attached a short
upright stem, which turns at its base; and at its top is
another fitting, in which the arm (C) can be revolved by a
milling at D. But the most important movement of all is
the rotation of the small socket (E) at the extremity of the
arm (C). This is accomplished by means of a fine chain
which communicates with the milled head (F). The object
is gummed upon a small disk (G) which fits into the socket (E),
and with these arrangements, as many as five sides of a cube

28 OPAQUE DISK-EEVOLVEE.

may be examined with perfect ease, and without disturbing
the position of the object on the disk.

This fact is illustrated in Plate IX. figs. 3, 4, 5, 6, & 7,
which represent the appearances of the cast skin from the
head of the silkworm in five different positions, the whole of
these being easily brought in succession under examination
with the microscope by means of the disk-revolver just de-
scribed, without moving the object from the disk or holder
to which it was in this case gummed, on the side opposite to
that shown in fig. 7.

To preserve those objects that are permanent, the box
(Plate X. fig. 4) may be used. It will contain twenty-four
disks, and, the holes in which they fit being numbered, the
objects may be easily registered ; the top of the box on the
outside at A will also hold a paper label, and a wire fixed in
the centre of the outside case of the box (B) serves as a guide
when it is screwed or unscrewed to prevent the objects being
rubbed off. The space between the inner and outer cases is
also so arranged that, if a disk should become loose, there is
no room for it to get quite free and shake about to the injury
of the other specimens.

Another apparatus for keeping the disks is shown in fig. 8,
and consists of a brass plate capable of holding two dozen
objects ; each hole is numbered, and the centre is occupied
by a piece of paper as a label, which is held in its place by
a brass mat and milled head. The plates may be placed one
above the other, as they are kept the right distance apart by
four pins at the corners ; and the usual plan is to pack three
plates, together with the disk-revolver, in a small mahogany
box by themselves.

Each disk has a small groove on the edge to facilitate its
being put on or taken off by the pliers (fig. 3), which are

OBJECTS ILLUMINATED FEOM ABOVE. 29

specially made for the purpose, as the object is not unfre-
quently injured when the fingers only are used.

Having now described the various means of illuminating
objects (whether opaque or otherwise) from above, we would
draw attention to some of the results as shown in the dif-
ferent illustrations.

Objects illuminated from above. — Splinter of Lucifer Match.

Plate XT. fig. 1 represents a part near the termination of
an oblique fracture of a lucifer match, and shows the struc-
ture of coniferous wood. We give this illustration more for
its general beauty and superiority when compared with that
of a section viewed as transparent, than as any remarkable
test. It exhibits, however, very strikingly the necessity of
illuminating in more than one direction ; for if the light be
thrown in a line with the medullary rays (<?), they are almost
invisible. To exhibit this object, Lieberkuhn illumination is
much the best, and with it there is no difficulty in showing,
by one adjustment of the light, the longitudinal woody tissue,
the transverse medullary rays, and the beautiful coniferous
glands.

Podura-Scale.

The advantage of a one-sided illumination from above, and
the extent to which it may be carried, are shown by an exa-
mination of the Podura-scale (a test already alluded to as the
best for transmitted light) under a ^th with the 3rd eyepiece.
It is necessary that the object be uncovered ; for, with the ob-
lique illumination that is required under such a power, a thin
glass cover is the most perfect reflector ; but, without this,
quite sufficient light can be obtained by means of the usual
large and small condensing-lenses combined; and we have

30 TAESUS OF SPIDEE.

the following results with this particular object, as shown in
Plate XI. fig. 2, the arrow at the left-hand side indicating
the direction of the light. When the markings are at right
angles to the direction of the light (A), they are illuminated
on the sides furthest off; when they lie in the same direction
as the light, with their narrow ends pointing to it (B), the
broad ends appear like brilliant spots ; but when this direc-
tion is reversed (C), the light from the points is so slight
that the scales appear to have lost their markings altogether*.
Now if the object were an opaque substance, this result would
have been a convincing proof that the markings were depres-
sions ; but as we know it to be transparent, it follows that
these particular appearances can only be produced by eleva-
tions. The continuity of the markings, which is a character-
istic feature of the same object when the light is transmitted
from below, is not so evident by this method, most probably
for the reason that the quantity of light is much diminished
when only reflected from the object ; there may, however, be
other causes.

Tarsus of Spider.

After the two illustrations already given, the tarsus or last
joint of the leg oiTegenaria atrica, a large spider quite com-
mon in outhouses and buildings (Plate XII. fig. 1)^ may be
taken as a good medium object; for it is exceedingly beautiful,
and a really good test. Under a power of about 50 linear,
and with a Lieberkuhn, three or four different kinds of hairs
may be detected ; whilst nearly all of these are again covered
with the most minute corrugations or very short hairs, as
shown in the surrounding figures, which are more highly
magnified. This structure is more especially minute, and to

* The markings are shown too distinctly in fig. C.

PEATHEE OE PIGEON. 31

see it clearly and well defined requires the best performance
of an object-glass and the nicest management of the light.
The object itself wants no preparation whatever, but care
must be taken that the hairs are not broken off by careless
handling ; and although cast skins will often furnish suffi-
ciently good specimens, these are generally so far injured or
dirty as to make it preferable to secure a living specimen for
the purpose.

Feather of Pigeon.

The illustrations on the right-hand side of Plate XII.
show the structure of the barbs of a pigeon's feather,
examined under illumination from above. The figures
above and below give the upper and under surfaces of the
vane of the feather, and the intermediate one is a section in
the direction of the letters a, b, fig. 2.

According to the late Professor Quekett*, there are "at
least six elements entering into the composition of a single
feather: viz., primarily, the quill (I) (see accompanying figure),
the shaft (K), and the vane (L, L) ; and, secondarily,
the barbs (M), with their barbules, and the barbu-
lettes."

Each barb consists of a central rib and two dis-
similar sides (see Plate XII. figs. 2 and 4) ; on the
under side, and in the part shown by the left-hand
portion at c (fig. 3), there are a series of isolated
hooks or barbules, which fasten on to the opposite \
portion of the adjoining barb, which will be the
same as the structure at d, consisting of a number
of continuous ribs with recurved edges (shown here in
section), and not isolated hooks similar to the left-hand

* Trans. Micr. Soc. 1849, vol. ii. p. 25.

32 AEACHNOIDISCUS JAPONICUS.

portion (at <?), as they have been described by the author
already mentioned.

The processes or barbulettes (e) on the top of one side of
each barb are in some birds much more fully developed than
in the Pigeon, and they are especially so in the feathers of
the Owl, as described in the paper already alluded to ; but
instead of holding the opinion given there, that such struc-
ture is for the purpose of rendering the flight of the bird
noiseless, it appears to us far more probable that these pro-
cesses repel any moisture and prevent it soaking into the
plumage — a provision which the Owl would more especially
require as a night-flying bird, and constantly exposed to
heavy dews.

Arachnoidiscus Jwponicus.

Plate XIII. illustrates the results that can be obtained by
Lieberkuhn illumination with ^th-in. object-glass, and repre-
sents the Arachnoidiscus Japonicus in three positions. This
diatom, when complete, consists of two valves connected by
a transparent annular membrane, shown in the edge view at
the bottom of the plate.

The inner and outer surfaces of each valve vary consider-
ably in their appearance. For whilst externally only minute
annular markings, somewhat varied at the centre, are to be
detected over one general plane, as in the lower figure on
the left; the structure internally, as shown by the large
figure at the top, is much more striking and complicated,
the material here being evidently distributed so as to furnish
great strength to the valve, and yet to leave a considerable
space and depth unoccupied. The following diagram (fig. 1)
represents the section of a valve in which the curved upper
line is the elevated portion of the inner surface ; it starts

ABACHNOIDLSCTLS JAPONICUS.

33

from the circumference as an annular plate (a, b), from
which it proceeds in numerous radii or spokes, alternately

h k

^Wkr^j I

dipping down to the lower surface, or continuing to and
through a smaller central annulus (c, d), which descends by
a beautiful curve to the lowest surface of the valve; the
radii in some specimens proceed even beyond this point,
as at e,f, and the appearances (g) near the circumference
are most probably caused by other very short perpendicular
radii beneath the plate (a, b). The lower surface internally
differs but little from that externally, and is shown in fig. 3
highly magnified : A, h, h represent the radii connected by
somewhat rectangular lattice-work, in which the silex in the
lines (i, i) is often of such considerable thickness as to be-
come a distinct feature.

These particulars of structure can be easily and quickly
verified by a binocular microscope, excepting perhaps the
appearance shown in fig. 3, a clear view of which is de-
pendent upon the part being quite clean and unconfused by
very thick or very irregular ribs ; but, with such condition,
the lattice-work structure is easily detected internally, exter-
nally, or on the edge of the valve.

In endeavouring to determine the structure of any of the
siliceous valves of the Diatomaceae, it must be more particu-
larly remembered that they consist of a transparent substance,

D

34 " DAEK-MELD ILLUMINATION."

and consequently it is only the refraction or the reflexion of
the light at the various edges, or irregularities of the different
parts, that makes the object visible at all; it is owing to this
circumstance that the dark centre of the valve has often been
taken for an aperture, whereas the silex is present in this
and many other parts that appear black and structureless.

"Dark-field Illumination" — Wenharrfs Parabola.

An illumination which gives an appearance to objects very
similar to that produced by light coming from above is the
" dark-field illumination." This method of examining ob-
jects only came into general use when perfected by Mr. Wen-
ham's invention of the parabolic reflector ; but the principle
itself had long been known and practised, by throwing very
oblique light with the mirror.

In every kind of " dark-field illumination " the light comes
upon the object from below, but at such an oblique angle as
never to enter the object-glass direct; when, however, a suit-
able object intervenes, it disperses the light and appears in
the microscope brilliantly illuminated, the field of view still
remaining dark.

The action of the parabolic reflector may be easily under-
stood by reference to Plate VI. fig. 8, which represents it in
section, and shows that the rays of light, r, r\ r", entering
perpendicularly at its surface (#), are reflected by its parabolic
surface (b) to a focus at f, and then diverge without entering
the angle cde, which may be supposed to represent the pen-
cil of light admitted by an object-glass. To prevent any light
passing through direct from the mirror, a stop (</), attached to
a wire (A), is fitted in the centre, and when raised in the sliding
fitting (#), it will cut off the rays which are reflected at the
least angle, such as r, r'. This adjustment is necessary in
cases where the object-glass has a large angle of aperture,

WENHAM'S PAKABOLIC KEFLECTOE. 35

the efficient use of the parabolic reflector being dependent
upon a dark field of view.

In the first-class instruments the parabolic reflector (Plate VI.
fig. 15) slides by its tube into the cylindrical fitting under
the stage; and the adjustment of its focus (which is attained
when its apex almost touches the object) is made by rack and
pinion connected with the milled heads (Plates II. & III., U).
In the other instruments it (Plate VI. fig. 13) fits into the
tube (Plate IV. fig. 2, It) under the stage ; and by giving it
a spiral motion when in this position, that is, carefully push-
ing it up or down at the same time that it is turned round
by the milled edge (Plate VI. fig. 13, a), the focus may be
adjusted with every nicety.

As it is always necessary that the rays of light should be
parallel when they enter the parabolic reflector, some care is
required in the illumination. The flat mirror must always
be used, and daylight has only to be reflected directly from
it ; but the rays from any artificial source have to be made
parallel by the aid of a condensing-lens. Either the bull's-eye
(Plate X. fig. 7) or small condenser on stand (fig. 9) answers
well ; but in using them for this purpose, whilst their dis-
tance from the mirror is immaterial, some five or six inches
being best, they will only refract the rays parallel when they
are placed the exact distance of their respective foci from the
lamp or other source of light.

Objects for which any kind of "dark-field illumination" is
available must be more or less transparent, and many which
it is almost impossible to illuminate at all from above will be
brilliantly shown by the parabolic reflector; the specimens
will also exhibit their natural colours, which are generally
quite obliterated when the light is transmitted through them
in the ordinary way.

The objects we have chosen for the illustration of this sub-

D 2

36

THE EBECTING-GLASS.

ject are some of the Polycystina from the fossil Barbadoes
earth (Plate XIV.), a deposit which is now universally known
amongst microscopists. The drawings just alluded to, give
somewhat the effect produced by illumination from above ;
this has only been done to give some idea of the shape of the
specimens, which would otherwise appear too flat.

The Erecting-glass for reducing the magnifying power,

and for Dissection.

This piece of apparatus (Plate V. fig. 4) screws into the
stop at the lower end of the draw-tube, and is used in com-
bination with the frds-inch object-glass. There are two
results obtained by it : first, the object, which, is always in-
verted by the ordinary compound microscope, is shown in
its natural position, which helps considerably in dissection or
when the object requires any manipulation; and, secondly, it
reduces the magnifying power, but not to any one definite
amount, for a very considerable range is obtained by merely
sliding the draw- tube in or out, as may be seen by the follow-
ing Table : —

Linear Magnifying Power of Erecting-glass, when used with
the \rds-inch Object-glass and No. 1 Eyepiece.

Inches and tenths.

Mag. power.

Draw -tube pulled out

•5

•7

•9

1-15

5

10
15

20

)? 5?

1-55

30

1-95

40

?J >>

2-35

50

2-8

60

3-2

70

3-6

.80

5? ??
>5 ?

4-
4-45

90
100

POLARIZED LIGHT. 37

The New Halfpenny.

To illustrate these facts we are at least successful in select-
ing a most familiar object, for it can only be the very richest
who are not thoroughly acquainted with the new halfpenny
coin of the realm ; and yet, whilst this perfect knowledge of
the object saves us from giving any very minute description,
we venture to draw attention to one or two particulars. The
coin is an exact inch in diameter (see Plate XV. figs. 2 & 3),
but when magnified 5 times (about the lowest power the
erecting-glass gives) it appears as seen in fig. 1, and may ex-
plain to many, in a very familiar way, what linear magnifying
power really is. With the power raised to 25, the object be-
comes so much beyond our scope of illustration, that we have
to confine ourselves to Britannia's foot alone ; and we must
claim the discovery of a new locality for this ubiquitous mem-
ber, if henceforth, through our instrumentality, it should
find place and rest in quiet microscopic cabinets.

For once we must decline a more minute microscopic
examination of our object, and any further criticism we leave
to those who are gifted with some knowledge of art-design
or execution. On our own part we are content to point
out with pride on this coin the Eddystone Lighthouse, de-
signed and built by Smeaton, whom, originally a mathe-
matical-instrument maker, we may honestly claim as a fellow-
craftsman.

Polarized Light, as applied to the Microscope.

The use of polarized light in the microscope always pro-
duces, in suitable objects, the most beautiful effects, and it
frequently assists in the accurate determination of structure
when no other method is of any avail.

38 POLAEIZED LIGHT.

NicoVs Prisms.

To apply this kind of illumination to the microscope, two
Nicol's prisms are generally used ; and it is necessary that
one, the " polarizer," should be under the object, whilst the
other, the " analyzer," should be somewhere above ; both
prisms being mounted so that they can be turned round when
in their proper positions.

In all the microscopes alluded to in these pages, the pola-
rizer (Plate XVI. fig. 2) will slide in the fitting (fig. 1) which
generally receives the diaphragm, under the stage ; but in the
first-class instruments it will also fit in the cylindrical fitting
(as shown by fig. 22, A) : it must, however, be remembered
that, except when used with the achromatic condenser, as
described hereafter, it is important to bring the polarizer as
near as possible to the object.

The best application of the " analyzer " (fig. 4) is over the
No. 1 eyepiece, in the place of the eyepiece-cap: another
position for it is immediately above the object-glass, and this
may be attained by either of two adapters; the one (fig. 19)
screws into the stop at the lower end of the draw-tube, the
other (fig. 20) forms an intermediate piece between the nose-
piece of the microscope and the object-glass. In the former
the analyzer cannot be rotated without turning round the
draw-tube; in the latter the outside of the adapter has
openings on opposite sides, so that an internal tube (fig. 20, B)
which carries the prism may be easily reached by the tops of
the fingers : but before the analyzer can be applied by either
of these adapters, it is necessary to reduce its mountings (as
shown in fig. 15), by removing the ordinary eyepiece-fitting
(fi.g. 4, C) and the screw-cap (D).

These two positions of the analyzer may require some ex-

POLAEIZED LIGHT. 39

planation. When the prism is placed over the eyepiece, it
necessarily, from its particular shape (although made espe-
cially for the purpose), removes the eye some considerable
distance from the top lens, and with all the eyepieces, except
that of the lowest power, it cuts off considerable portions of
the field of view : this is not the case when the analyzer is
placed immediately above the object-glass, but then any de-
fects in the workmanship of the prism (and there are sure
to be some) are magnified by the eyepiece. The choice of
position is therefore entirely dependent upon the character
of the object ; if the best definition be required, the analyzer
must be used above the eyepiece, whilst the second position is
often quite satisfactory to an observer who looks more at the
general display than at the minute details.

The effect produced by the prisms alone may be observed
by leaving one of them stationary whilst the other is turned
round, and it will then be seen that twice in each revolution
the light will be entirely stopped : this is, of course, sup-
posing that an object-glass and an eyepiece are on the micro-
scope, and that, without the prisms, there is an ordinary full
illumination. If now a polarizing object, such, for instance,
as that shown in Plate XVII. fig. 1 (crystals of sulphate of
copper and magnesia), be placed under the microscope when
the prisms stop the light, the object will present the appear-
ance shown by the drawing, and the dark and light parts
will change their relative positions when either prism is
revolved: with this object, and under these conditions, no
colour whatever is shown; but this deficiency may be sup-
plied by a plate of selenite specially prepared for the pur-
pose; for if this be put in the place of the object, the field
of view will appear coloured, instead of being only black and
white as with the prisms alone.

40 DAEKEE'S SELENITE PLATES.

The Selenite Plate.

The particular colour given by a selenite plate is de-
pendent upon its thickness ; but each piece, during one half-
revolution of either prism, will always show two distinct or,
as they are termed, complementary tints, and the plates, as
generally supplied, produce either red and green or blue and
yellow. The colours of the selenite may be almost entirely
neutralized in certain positions of the Nicol's prisms ; it is
therefore necessary, for varying the tint or for obtaining the
greatest intensity of colour, not only to revolve either prism
separately, but also to change the relative positions of both
of the prisms to the selenite plate ; or, what amounts to the
same thing, where practicable, to rotate the selenite itself.

Either selenite plate (Plate XVI. figs. 17 or 18, the former
mounted in brass, and the latter between two pieces of glass)
has to be placed under the object, and does not admit of
rotation ; but we make provision in the first-class instruments
for this movement in two ways.

In the simpler form, a circular plate of selenite (fig. 6)
drops into the brass cell (fig. 8), and is held in its place by
the ring (fig. 7), which fits tightly over it ; the whole fitting
(fig. 8) will either slide by its larger diameter (E) into the
cylindrical fitting (fig. 22), or by its smaller diameter (F) it
can be attached to the brass-work of the polarizer; but the
milled ring (G) by which the selenite can be rotated must
either project above the cylindrical fitting, or it must, as when
attached to the prism, come opposite to the side opening
where the finger can reach it.

Darker s Ret ar ding-Plates of Selenite.
This other arrangement is specially contrived as the most

DARKEY'S SELENITE PLATES. 41

convenient way of using Darker's series of re tar ding-plates
of selenite; these consist of three plates (Plate XVI.
figs. 9, 10, & 11), each of them engraved on the brass rim
" P|A," together with a number, J, f , or f , which repre-
sents a certain power of retarding a wave of polarized light.
When the plates are superposed with their marks P|A in
the same positions, the power they represent is the sum of
their numbers ; but when placed at right angles (see figs.
12 & 13), they oppose each other, and then their differences
will give the power : thus if \ and f are placed over each
other with their marks P|A in the same direction, they will
represent f ; but if either of them be turned round one
quarter of a revolution, they will only give f , and conse-
quently the three plates, with their various alternations, will
give thirteen different colours, together with their com-
plementary tints, as shown in the following list : —

Prisms at right angles. Complementary tint.

i by itself very light lavender straw-colour.

J — -J = -^ darker ditto light yellow.

J by itself deep blue light maize.

J -|- 1 = -J very light blue orange.

J — -J — -J= -J lake emerald green.

■£■ — x= "I" deep blue bright yellow.

f + 4 — f = 4 k^ green Jigkt Purple.

•J — -J = -j light plum-colour pea-green.

■x by itself blue-green salmon.

^- -|- -i == lp green -yellow mauve.

f + f — J = -41- pink light green.

$r-\-^-=.^c- light pink deep green.

Q -\-^ -\- ^ = —$- very light red stone -green.

The brass-work mounting, by which the alternations just

given may be made under the microscope, is shown in Hg. 16 ;

42 OBJECTS UNDEE POLAEIZED LIGHT.

the selenites are fitted into three cells, which can be rotated
in the three separate arms (J, K, L) ; these arms adapt by a
short dovetail (M) to the cylindrical fitting (fig. 22) under the
stage ; when in that position, they may be turned either in or
out by means of the three nibs (N, O, P), and the finger can
reach the milled rings through the opening (fig. 22, H) for
the purpose of rotation. By this method any change of the
selenites can be made with quickness and facility, and with-
out interference with the position. of the object, or any other
arrangement.

Darker s Selenite Stage.

Darker 's selenite stage (Plate XVI. fi.g. 14) is also a very
complete piece of apparatus ; the three plates (figs. 9, 10, 11),
already described, drop into a ring (fig. 14, R) which can be
rotated in the plate (S) by turning the small milled head (T),
and an arrow upon this may be used for registering any par-
ticular amount of rotation, sixteen turns making one revolu-
tion : the only objection to this arrangement is that the object
which rests on the ledge (U) must be removed at every change
of selenite ; but in many instances this is of no consequence.

The tints produced by the selenites are again much varied
by the object: thus in Plate XVII. fig. 2, the same object as
in fig. 1 is represented, but with an infinite variety of colour
imparted by the interposition of a plate of selenite, which
by itself, and in that particular position, gives only the blue
ground, as shown round the crystal.

Then, again, there are many instances in which the use
of selenite only detracts from the brilliancy and the variety
of the colours of such objects as are represented by the
other figures, whose appearance is entirely due to the use of
the prisms only. Fig. 3 is an oblique section of rhinoceros

TOUEMALINES. 43

horn ; fig. 4 is a section of dried tendon of the ostrich ; figs.
5 & 6 are crystallized salicine (an alkaloid from the bark of
the willow), these last two drawings showing the change pro-
duced by one-quarter of a revolution of one of the prisms.

All polarized light considerably diminishes the intensity of
any illumination ; and although this loss may not interfere
with the appearance of an object under the lower object-
glasses, yet, when the magnifying power is increased, some
colours are quite lost, and all are much less brilliant. This
deficiency of illumination may be obviated by employing the
achromatic condenser to concentrate the polarized light ; and
the arrangement will then be to have the polarizer at the
lower and the achromatic condenser at the upper end of the
cylindrical fitting (Plate XVI. fig. 22), — the selenite, if used,
being placed between these two, or immediately under the
object ; and the smaller apertures of the condenser, without
the front lens, are generally quite sufficient.

Tourmalines.

Besides Nicol's prisms, there are many other means of
polarizing light ; of these the most remarkable are tourma-
lines (Plate XVI. Hg. 5). Very thin slices of some of these
crystals are sufficient for the purpose, and consequently as
analyzers they have a great advantage over Nicol's prism in
giving a full field when used above any of the eyepieces.
Their only fault is that they are never free from some colour
in themselves, and therefore they make material changes in
the true appearance of an object. They are, moreover,
when of a sufficiently light tint and still retaining their
polarizing power, very scarce ; so that the small pieces even,
which are quite large enough for the microscope, can only
be purchased at high prices.

44 EXPERIMENTS WITH DOUBLE-IMAGE PKISMS.

Polarizers for large objects.

Before the extra large polarizer (Plate XVI. fig. 3) was in-
troduced, the illumination of the whole of an object under a
low power was often found to be imperfect, and was remedied
by the use of some other polarizer : either a piece of black
glass (fig. 21) was placed over the mirror and then inclined
at the proper angle, or a bundle of thin glass plates (fig. 23),
properly mounted, was slid into the cylindrical fitting under
the stage, so that the light would be polarized by either
passing through or by reflexion from it ; but these methods
are not much employed at the present time.

Experiments with Double-image Prisms.

The microscope may also be made to exhibit various
results produced by polarized light, as the subjoined extract
from a paper by Mr. Legg* may explain: —

" The following experiments, if carefully performed, will
illustrate the most striking phenomena of double refraction,
and form a useful introduction to the practical application
of this principle.

" The apparatus necessary is —

" A Nicol's prism (Plate XVI. figs. 2 or 3) to be adapted
under the stage.

" A selenite plate, figs. 17 or 18.

" Two double-refracting prisms, adapting to each other
and to the eyepieces, figs. 24 and 26.

" A film of selenite adapted to the double-refracting
prisms, fig. 25 ; and

* " On the Application of Polarized Light in Microscopic Investigations."
By M. S. Legg. Read before the Microscopical Society of London, Dec. 9,
1846.

EXPERIMENTS WITH DOUBLE-IMAGE PRISMS. 45

a A plate of brass, 3 inches by 1, perforated with a series
of holes, fig. 27.

" Exp. 1. — Place the piece of brass so as for the smallest
hole to be in the centre of the stage of the instrument,
employing a low-power object-glass, and adjust the focus
as for an ordinary microscopic object; place one double-
refracting prism in the place of the cap of the eyepiece, and
there will appear two distinct images (Plate XVIII. fig. 1, a);
then by revolving the prism the images will describe a circle,
the circumference of which cuts the centre of the field of
view ; the one is called the ordinary, and the other the extra-
ordinary ray. By passing the slide along so as for the larger
orifices to appear in the field, the images will not be com-
pletely separated, but will overlap, as in fig. 1,5.

"Exp. 2. — Place the polarizer (Plate XVI. figs. 2 or 3)
into its place under the stage, still retaining the double-
image prism over the eyepiece ; then by examining the
object there will appear in some positions two, but in others
only one image ; and it will be observed that at 90° from the
latter position this ray will be cut off, and that which was
first observed will become visible; at 180°, or one-half of the
circle, an alternate change will take place; at 270° another
change; and at 360°, or the completion of the circle, the
original appearance (see Plate XVII I. fig. 3).

" Before proceeding to the next experiment, it will be as
well to observe the position of the Nicol's prism used as a
polarizer, which should be adjusted with its acute angles
parallel with the sides of the stage (see fig. 7), in order to
secure the greatest brilliancy in the experiment : the proper
relative position of the selenite may be determined by no-
ticing the natural fractures or flaws in the film, which will
be observed to run parallel to one another: these flaws

46 EXPERIMENTS WITH DOtXBLE-IMAGE PRISMS.

should be adjusted to about 45° from the sides of the stage, to
obtain the greatest amount of depolarization (see fig. 8).

" Exp. 3. — If we now take the plate of selenite thus pre-
pared, and place it under the piece of brass on the stage, we
shall see, instead of the alternate white and black images,
two coloured images, as in fig. 2, 0, and fig. 4, composed of
the constituents of white light, which will alternately change
(by revolving the double-image prism over the eyepiece) at
every quarter of the circle; then by passing the plate of
brass along, so as to bring the larger orifices in succession
into the field, the images will overlap, and where they over-
lap, white light will be produced (see fig. 2, d). If by accident
the prism should be placed at 45° from the position just in-
dicated (see fig. 9), no particular colour will be observed, and
it will then illustrate the phenomenon of the neutral axis of
selenite ; because when placed in that relative position, no
depolarization takes place.

" The phenomena of polarized light may be further illus-
trated by the addition of the second double refractor (fig. 26,
Plate XVI.) and the film of selenite (fig. 25) between the
double refractors.

" Exp. 4. — By placing the apparatus as described in the
first experiment (that is, removing the Nicol's prism and
plate of selenite, but retaining the brass plate), we shall ob-
serve the two images as shown in fig. 3, Plate XVIII. ; then
by placing the second double refractor over the first, so as for
all the faces of the one to be parallel to all the faces of the
other, as if they formed but one piece, the eye will perceive
two distinct images, but at twice the original distance from
each other (see fig. 5, e). If we now turn the prism nearest
the eye from left to right, two faint images will appear ; con-
tinuing the turn, at 45° the four images will be all equally

CRYSTALS TO SHOW RINGS. 47

luminous (fig. 5, f) ; and when the prism has turned round
90°, there will be only two images of equal brightness (fig. 5, </) ;
continuing the turn, two other faint images will appear;
further on the four images will be equal ; still further they
will be unequal; and at 180° of revolution they will all
coalesce into one bright image (fig. 5, h).

" Exjp. 5. — The above results will be rendered more inter-
esting by interposing between the double refractors the film
of selenite. Instead of the two white images, as in the pre-
ceding experiment, we shall see three, of which the two
outer ones will be one colour (say green), and the middle,
its complementary colour or red (fig. 6, i); by turning the
prism nearest the eye, the middle image will gradually
divide, until the completion of a quarter revolution, when
four images will appear of equal brilliancy, two of each
colour (fig. 6, Jc): revolve the prism until the completion of
the half-circle, and the three images will reappear, but with
different properties, the outer images being red and the
middle green (fig. 6, I) ; at another quarter revolution, four
images, but with opposite colours, will be observed (fig. 6, m),
and at the completion of the revolution the original appear-
ance (see fig. 6, i).

" In this experiment the relative positions of the double-
refracting prisms and the selenite must be carefully observed,
as, if the neutral axis of the selenite be parallel or perpen-
dicular to the plane of polarization, no depolarization takes
place, and no colours will be produced, the results then
appearing as if the selenite were not interposed."

Crystals to show Rings.

The systems of coloured rings, produced by crystals cut
perpendicularly to their axes, can also be beautifully shown

48 WENHAM'S BINOCULAE MICROSCOPE.

in the microscope with polarized light; they are best seen
by placing the polarizer (Plate XVI. figs. 2 or 3) under the
stage, and employing a low-power object-glass; it is usual
to place the crystal over a No. 2 eyepiece, made without a
stop ; and either a short prism or a tourmaline may be used
as an analyzer over the crystal. Plate XVIII. fig. 10 shows
the appearance of calc-spar under such circumstances.

The following is a list of the most interesting specimens,
each of which is generally mounted in a brass fitting, as
shown in Plate XVI. fig. 28.

List of Crystals.

Quartz, right-handed.

Nitre.

Quartz, left-handed.

Topaz.

Calc-spar.

Sugar.

Borax.

Arragonite

Wenhams Binocular Microscope.

Wenham's binocular microscope is now so well known,
that it may appear hardly necessary to occupy a portion of
these pages with any explanation or defence of its principle ;
but binocular vision itself, as well as this particular applica-
tion of it, still meets with such strong opposition from many
microscopists, that we venture to give with some minuteness
a few facts connected with the subject, and, more especially,
because we consider Wenham's binocular body to be the
most valuable addition to the compound microscope since
the perfecting of the object-glasses by Mr. Lister.

That binocular vision is advantageous, if not necessary, is
quite sufficiently proved by the fact of our having two
eyes, and it is also known that the stereoscopic effect con-
veyed to the mind increases as the object approaches
the eyes; consequently, when the simple design of the

WENHAM'S BINOCULAE MICEOSCOPE. 49

microscope is to enable the eyes to see an object, or the
image of an object, at a shorter distance than the natural
limit, under such circumstances stereoscopic vision must be
of the greatest importance.

In Mr. Wenham's contrivance, the binocular body is merely
an addition; when adapted to an instrument, it does not
interfere in any way with the use of the single body only, in
its perfect condition. This is a most important feature ; for
it also affords the opportunity of an immediate comparison
between the appearances seen by single or double vision ;
and thus not only can the accuracy of the binocular body be
easily tested, but the observer may obtain in an instant a
double analysis of the structure or condition of an object.

We believe most of the opponents of the binocular prin-
ciple to be those observers who have accustomed themselves
to the examination of objects as transparent ; and we venture
to draw especial attention to this particular condition, under
which so many judge of the value of the Binocular Micro-
scope. This method of examination, peculiar to the micro-
scope, and one to which the naked eye is quite unaccus-
tomed, conveys to the mind a most imperfect idea of the
form or vertical distance of the various parts of an object,
whilst in very many instances the appearances bear no re-
semblance even in shape to the true structure: this may
be readily conjectured from the various shapes the markings
on Pleurosigma formoswn assume when illuminated in dif-
ferent directions (see Plate IX. fig. 1) ; but most conclusive
evidence is furnished by a careful examination of the scales
of Lepisma saccharina.

This insect may be found in most old houses, frequent-
ing damp, warm cupboards, or as an associate of black
beetles and cockroaches, and its scales have long been

50 WENHAM'S BINOCULAR MICROSCOPE.

known to microscopists. The insect, which is very active,
may be caught without injury in a clean pill-box, with a
few pin-holes in the lid ; and a drop of chloroform over these
holes will soon make the inmate insensible, when it may be
turned out upon a piece of clean paper.

The best way to remove the scales is to press gently one
of the ordinary 3x1 glass slides upon the part from which
the scales are required, and they readily adhere to the glass,
appearing to the naked eye like a smear of dust ; under the
microscope they present considerable variety, not only of
size and shape, but also in the character of their markings.
The scales that are most abundant resemble the one shown
in Plate XIX. fig. 3, and upon it the more prominent mark-
ings appear as a series of double lines which run parallel
and at considerable intervals from end to end of the scale,
whilst other lines, generally much fainter, radiate from the
quill and take the same direction as the outline of the scale
when near the fixed or quill end ; but there is in addition
an interrupted appearance at the sides of the scale, which is
very different from the mere union or " cross-hatching " of
the two sets of lines.

In the first place, the scales themselves are truly trans-
parent objects; for water instantly and almost entirely obli-
terates their markings, but they reappear unaltered as the
moisture leaves them; therefore the fact of their being
visible at all is due to the refraction of light by superficial
irregularities.

The following experiment establishes the fact still more
conclusively, and also determines the structure of each side
of the scale, a matter which it is impossible to do from the
appearance of the markings in their unaltered state.

Having removed some scales from the insect to a glass

WENHAM'S BINOCULAE MICEOSCQPE. 51

slide as already described, cover them with a piece of thin
glass, which may be prevented from moving by a little paste
at each corner. The subjoined drawing may be taken as an
exaggerated section of the various parts.

Ar

Z1B

A, B, is the glass slide with scales, c, c, closely adherent
to it, and D the thin glass cover secured to A, B, by a very
little paste at the corners : in this state the whole should be
placed under the microscope, — a high power, such as a ^th
and No. 3 eyepiece with achromatic condenser illumination,
answering well. If under these circumstances a small drop
of water, E, be placed at the edge of the thin glass, it will run
under by capillary attraction ; but when it reaches one of the
scales, it will run first between it and the glass slip A, B,
because the attraction there will be greater, and consequently
the markings on that side of the scale which is in contact
with the glass slip will be obliterated, whilst those on the
other side will, for some time at least, remain unaltered;
when such is the case, the strongly marked vertical lines
disappear and the radiating ones become continuous (see
Plate XIX. fig. 4, left-hand lower portion). To try the same
experiment with the other, or inner, surface of the scales, it
is only requisite to transfer them by pressing the first piece
of glass, by which they are taken from the insect, upon
another piece of glass, to which a few scales will adhere ; and
then the same process of introducing fluid, as already de-
scribed, may be repeated with them, when the radiating
lines will disappear and the vertical ones will become con-
tinuous (see fig. 4, right-hand side of scale). These results
show, therefore, that the interrupted appearance is produced
by two sets of uninterrupted lines on different surfaces.

e 2

52 CONSTRUCTION OE BINOCULAR MICROSCOPE.

It may be interesting to many to try these experiments
for themselves, whilst others may be satisfied with the
appearance which some scales are almost sure to present in
every slide that is mounted. Fig. 4, -already alluded to, is
a camera-lucida drawing of a scale which happens to have
the opposite surfaces obliterated on either side : this is
seldom the case, for it is generally only the outer surface
of the scale that is in such a condition.

Fig. 1 shows a small scale in a dry and natural state, the
direction of the markings upon which is shown by the faint
lines : at the upper part (a) the interrupted appearance is not
much unlike that seen at the sides of the larger scales ; but
lower down, at b, where the lines are of equal strength and
cross nearly at right angles, they are entirely lost in a series
of dots ; and exactly the same appearance is shown in fig. 2
to be produced by two scales at the part (c) where they over-
lie each other, although by themselves they only show the
parallel vertical lines.

These kinds of contradictory results produced by the re-
fraction of light, combined with an illumination to which
the naked eye is unaccustomed, make the binocular micro-
scope of little avail in the examination of true transparent
objects; but with those specimens that are opaque, or nearly
so, its assistance is most remarkable, and we do not know a
single object of this class in which its use is not essential.

Construction of Binocular Microscope.

The only plan for a binocular microscope, as yet known
to be practicable, is the equal division of the rays after they
have passed through the object-glass, so that each eye may
be furnished with an appropriate one-sided view of the ob-
ject. The methods at first contrived to effect this not only

CONSTRUCTION OF BINOCULAR MICEOSCOPE. 53

materially injured the definition of the object-glasses, but
also required expensive alterations in their adaptation, or
more frequently still a separate stand ; whereas the following
arrangement, contrived by Mr. Wenham, forms no obstacle to
the ordinary use of the instrument, and the definition even
of the highest powers with the binocular body is scarcely im-
paired. It consists of a small prism mounted in a brass box
(Plate XX. fig. 3), which slides into an opening immediately
above the object-glass (fig. 1, I), and reflects one half of the
rays, which form an image of the object, into an additional
tube (B) attached at an inclination to the ordinary body (C).
One half of the rays take the usual course, with their con-
ditions unaltered; and the remainder, although reflected
twice, show no loss of light or definition worthy of notice, if
the prism be well made.

As the eyes of different persons are not the same distance
apart, the first and most important point to observe in using
the " binocular " is, that each eye has a full and clear view
of the object; this is easily tried by closing each eye alter-
nately without moving the head, when it may be found that
some adjustment is necessary by racking out the draw-tubes
(D, E) by the milled head (K), which will increase, the
distance of the centres; or when moved in the contrary
direction, they will suit those eyes that are nearer together.

If the prism be drawn back until stopped by its catch
(fig. 3, F), the field of view in the inclined body is darkened,
and the whole aperture of the object-glass passes into the
main body as usual, neither the prism nor the additional
body interfering in any way with the ordinary use of the
microscope. By pressing back the spring-catch (fig. 3, F) of
the prism, it can be withdrawn altogether for the purpose of
being wiped ; this should be done frequently and very care-

y

54 CONSTRUCTION OF BINOCULAE MICROSCOPE.

fully, on all four surfaces, with a perfectly clean cambric or
silk handkerchief or a piece of wash-leather; but no hard
substance must be used.

This construction of binocular body can be applied to any
of the microscope-stands already described and drawn with
the single body only. The first few plates of this work were
engraved before Mr. Wenham's contrivance, or the illustra-
tions would have included a binocular body in each instance,
as it is now but seldom that we send out instruments with
only the single body.

The most suitable kind of illumination for the binocular
microscope has been already alluded to, and includes all
those methods in which the light is thrown upon the upper
surface; but for those objects that are semitransparent, as
sections of bone or teeth, Diatomacese, living aquatic ani-
malcules, &c, the dark-field illumination, by means of the
parabolic reflector, will give a very good result. For the
illumination of perfectly transparent objects, it is best to
diffuse the light by placing various substances under the ob-
ject, such as tissue-paper, ground glass, very thin porcelain,
or a thin film of bees-wax run between two pieces of thin
glass.

When employing the polarizing apparatus with the " bin-
ocular," the analyzer is fitted into an adapter (fig. 2), which
applies immediately between the object-glass and the prism,
with the means of revolving the prism by turning its tube
round at opposite slots in the sides of the adapter, and the
effects are exceedingly beautiful.

Although the binocular body makes the various features
of an object so distinct and so easy to distinguish, yet the
representation of them in drawings is extremely difficult, be-
cause the stereoscopic appearance can only be given by the

LIVE-BOXES AND THOUGH. 55

management of the shadows : this has been attempted in the
following illustrations : — cast skin of silkworm, Plate IX. ;
splinter of lucifer-match, Plate XI. ; tarsus of spider and
structure of feather, Plate XII. ; Arachnoidiscus Japonicus,
Plate XIII. ; and Polycystina from Barbadoes earth,
Plate XIV. To see these illustrations with the best result,
they should be held so that the light falls on them from the
top right-hand corner of the plate; only one eye should
be used ; and if the margin round the drawing be cut off by
looking through the hand when nearly closed, the effect is
considerably improved.

Sundry Apparatus. — Live-boxes and Trough.

To confine small living objects for examination under the
microscope, different pieces of apparatus are used, according
to the size or the nature of the object. Those that are dry and
active may be secured in a live-box (Plate XXI. figs. 1 & 4).
This consists of two parts : the lower piece (fig. 4, a) is a brass
plate, with a short piece of tube (b) in the centre, carrying a
circular piece of thick glass (c) at the top ; over this fits the
upper piece or cap (d), carrying a thin glass. The object is
held by the pressure of the two glasses, and the amount of
separation may be varied at pleasure by sliding the cap
more or less on. The thin glass is in a cell, and may be
changed or replaced by unscrewing the top milled ring (e).

A smaller live-box is shown in fig. 1.

These pieces of apparatus answer well for
many objects in fluid; but if employed in
this way, the quantity of water, or whatever
it may be, should, if possible, be limited, as
shown by the accompanying figure, in which
the small central ring represents the appearance of a drop

56 LIVE-BOXES AND TROUGH.

of water when held between the glasses of a live-box.
The object, if small enough, may be confined thus, either
with or without pressure, whereas, if the fluid be in excess,
the objects will move about over too large a space, or fre-
quently they reach the edge of the live-box and disappear
altogether.

When fluid objects are very small and cannot be injured
by moderate pressure, a very simple and efficient plan is to
use the plate-slip (fig. 5, a) ; the fluid is placed upon this, and
then covered with a thin piece of glass (5), which is pre-
vented sliding off by the ledge (d).

The glass trough (figs. 2 & 3) is intended for larger ob-
jects in water ; it must be used with its thinner plate of glass
(fig. 3, a) in front. The modes of confining the objects, and
keeping them near the front surface, must vary according to
circumstances. For many it is a good plan to place the piece
of glass (b) diagonally in the trough, its lower edge being
kept in its place by a strip (c) at the bottom ; then if the
object introduced be heavier than water, it will sink till
stopped by the sloping plate (b). Sometimes the whalebone-
spring (d) may be applied behind this plate to advantage,
with the wedge (e) in front to regulate the depth.

The glass tubes (fig. 6) are employed to catch any animal-
cule or other object that may be swimming about in a vessel
of water.

If a finger be pressed close upon the small opening at the
rounded end (a), the other end of the tube may be dipped
in the water to any extent without the water entering the
tube ; but immediately the finger is lifted off, the water
rushes up to the same level as that in the vessel, and
together with it any animalcules which were near the lower
and of the tube; the finger is placed again upon the small

LEVER COMPRESSOR. 57

opening at the top of the tube, which can then be removed,
together with its contents, from the vessel.

Compressors.

In many cases, objects require careful and yet considerable
pressure whilst under the microscope, and there are many
contrivances for the purpose.

Screw Live-box.

In the screw live-box (Plate XXI. figs. 9 & 10) the pres-
sure is produced by two small spiral springs (a, a) acting against
the upper plate (#), on the top surface of which a piece of thin
glass is held by two springs (c, c) ; the lower plate holds a
thick circular glass, and outside it is a milled ring (d), on
which the upper plate is constantly pressed ; the distance be-
tween the two glasses is regulated by the milled ring (d),
which screws up or down, and two pins (e, e) prevent the
plate turning round.

Lever Compressor.

The lever compressorium (figs. 13 & 14) has also a thick
glass on the bottom plate ; but the thin glass is cemented in
a ring at the extremity of an arm or lever (a), which is moved
up or down by turning the small screw (b). In this instrument
the amount of pressure is the force imparted by the screw
(5), and it is very considerable ; it is equally distributed over
the glasses by the ring (c) being swung in the centres (d, d),
and by the semicircle (e) having a revolving fitting at (f);
the lever (a) can be turned on one side, so as to expose both
glasses when they want cleaning, or when the object is to
be changed.

58 REVERSIBLE COMPRESSORS.

Wenhams Compressor.

Wenham's compressor (figs. 7 & 8) was designed especially
for use with the parabolic reflector or achromatic condenser.
The bottom plate {a) is very thin, and a circular thin glass
is cemented in the middle ; the top plate (b) is a thin brass
arm, also carrying a circular thin glass cemented to it;
the pressure is imparted by a spring or set which is given
to the arm (#), and the two pieces of glass will approach or
separate according to the direction in which the small milled
head (c) is turned; the arm will also turn aside, as in the
lever compressor. The simple construction of Wenham's
compressor makes it a very easy and convenient instrument
to use ; it has, however, one or two disadvantages — the two
pieces of glass do not move in parallel directions, and the
amount of pressure is rather limited.

Reversible Compressors.

The next two compressors are our own contrivance, and
both of them individually combine in one piece of apparatus
the excellences of those already described, together with the
following new qualities: viz., a parallel and equal compres-
sion, with an immediate action up or down; the means of
turning the compressor over, so as to see both sides of an
object without disturbing it; the use of thin glass only
either at top or bottom ; and lastly, such restrictions in the
thickness of the brass-work as permit the use of the achro-
matic condenser, or the parabolic reflector, on either side of
the object.

Mr. Slack suggested the basis of the construction in each
case ; he proposed that two oblong pieces of thin glass (fi.g. 15)
should each be secured to a separate brass plate by the heads

KEVERSIBLE COMPRESSORS. 59

of two screws ; spaces were then to be made for the heads of
these screws in each opposing plate, and the two pieces of
glass could be brought close together. This simple plan has
great advantages : the pieces of glass are easily cut ; for no
great nicety of size is required, and they can be changed, or
replaced, without difficulty and with little loss of time.

The "parallel plate compressor" (figs. 11 & 12) consists
of two brass plates, furnished with thin glass, as already
described, and connected by four small rods (a) : this is in fact
the commonest parallel-rule motion, and very nearly a ver-
tical one ; for when the plates are near together, the position
of the connecting rods is such as to cause no perceptible end-
movement. The brass plates, when about i^th of an inch
from each other, are kept apart by two springs, which supply
the separating power, the compressing force being given by
a small screw (b) with a conical end, attached to the one
plate, working into a corresponding recess in the other
plate; the first hold the point of the screw gets is at the
bottom of the recess, so that the plates are drawn together
as the screw advances. When the inner surfaces of the thin
glasses want cleaning, or when the object or the glasses
have to be changed, the screw (b) must be brought quite
back, and the plates will then separate to the extent of a
full inch.

In the "cell compressor" (figs. 18, 17, & 18), the thin
glasses are secured to two circular plates of brass; the one
fits the ring (fig. 18), and the other the counter ring (fig. 16)
of a brass cell (fig. 17, a) ; when these are screwed together,
the plates approach each other, or when unscrewed, they sepa-
rate, and some slight intermediate brass springs counteract
any loss of time that there may be in the screws or in the
fittings of the circular plates. Two upright pins in a flat

60 FEOG PLATE.

brass plate (fig. 17, b) pass through both the circular brass
plates to prevent them turning round whilst the cell is
screwed or unscrewed, and a small screw (c), working by a
slight taper against a push-piece (d), holds the cell itself
firmly in a sunken ring of the brass plate (b\ into which it
drops. For cleaning the glasses, &c., the circular plates
may be entirely separated by unscrewing the cell. The chief
novelty in both of these compressors is the opportunity they
afford of examining both sides of an object whilst under
pressure. The "parallel plate compressor" maybe turned
over altogether, and used from either side ; in the " cell com-
pressor," after the push-piece is loosened by slightly un-
screwing the small milled head (c), the cell will lift out of
the sunken ring and off the pins, to be placed on again with
whichever side is wished uppermost.

Frog Plate.

The frog plate (fig. 19) supplies the means for viewing
the circulation of the blood in the web of the frog's foot,
and consists of a long thin brass plate (a, a), with a square
aperture (b) at one end covered with a piece of glass. A
small bag is supplied, in which the whole of the frog is put,
except one hind leg; and to prevent this being drawn in, the
neck of the bag should be pulled tight ; the frog, thus con-
fined, is then secured to the unglazed end of the brass plate
by ribbon wound over it and through the holes at the sides
of the plate, the ends being held by a spring underneath.
The foot is next drawn out over the glass (#), and the web
extended by fastening to the extremity of the toes, pieces of
fine silk which can be wound round the pins (c, c). The
glazed end of the plate must be clamped firmly to the stage
of the microscope, and the frog as well as its foot should be

CAMERA LUCID A. 61

constantly moistened with cold water; if also a piece of
thin glass with water underneath be placed over the part of
the web that is under examination, the effect is much im-
proved.

Camera Lucida.

Wollaston's camera lucida, when applied to the achro-
matic microscope, is of the greatest use and importance ; by
its aid the most intricate structure of objects can be drawn
with perfect accuracy and to an exact scale.

This piece of apparatus (Plate XXII. figs. 6, 7, & 8) slides
on in the place of the cap of either eyepiece, with its flat
side (fig. 7, a) uppermost, as shown also in fig. 8, which is a
drawing to scale of the camera lucida when in use. The
body of the microscope must be in a horizontal position, and
a piece of paper laid upon the table should be exactly ten
inches from the flat side of the prism. Then if the eye be-
placed so that its pupil is divided as it were by the edge of
the prism (see accompanying figure, in which p
a represents the pupil of the eye and b the top 1
surface of the prism), the object will appear
upon the paper, and can be traced on it by a pencil, the
point of which will also be seen. Only one eye is to be
used; and the light must be regulated so that no more
than is really necessary is upon the object, whilst a full
light is thrown upon the paper. Perfect steadiness of the
eye is also necessary, and the eyebrow may be gently rested
upon the prism-fitting as shown in fig. 8. Any one who
cannot see to read distinctly at ten inches should use a lens
in the position shown in figs. 6 & 8 ; it must be convex for
long sight* and concave for short sight : in general the former
only is supplied, but it is easily changed when necessary.

It must not be supposed that any carefully finished draw-

62 CAMEEA LUCID A.

ing can be made with the camera lucida; this is seldom
possible; and when the object to be sketched is irregular on
the surface, or larger than the field of view, it often requires
no little care and patience to retain the relative position of
the parts correctly ; but considerable facility is soon acquired,
and when observers who are quite unable to draw from the
eye alone succeed in producing a faithful representation of an
object, the result is most gratifying and frequently of some
value, more especially if the specimen be not permanent.
It is well known amongst observers that generally no minute
comparison of objects can be made without drawings, and it
is almost impossible to obtain such without the aid of the
camera lucida ; at the present time there are many illustra-
tions of objects which are completely puzzling and compara-
tively worthless because they are not drawn to scale, but
merely represent the appearances presented to the eye of
the artist.

If there be substituted in the place of the object a piece
of glass ruled into lOOths and lOOOths of an inch, termed a
stage micrometer (fig. 3 or 4), its divisions can be marked on
the same or another piece of paper, and by comparing them
with the drawing of the object, the most accurate measure-
ment can be made. The whole of Nobert's lines (Plate
VIII.) and the markings on Navicula rhomboides (Plate VII.
fig. 13) were measured in this way.

In Plate XXII. fig. 11 is a simple sketch to illustrate the
usefulness and capabilities of the camera lucida ; it represents
the stellate tissue as occurring in the transverse section of a
rush (Juncus conglomeratus), very commonly used in London
for tying up bunches of watercresses. The left-hand portion
of the sketch is the mere outline exactly as drawn by the aid
of the camera lucida, the right-hand portion being touched

THE MICEOMETEES. 63

up more or less to show how it may be finished if necessary,
and at the bottom are traced IGOths and lOOOths of an inch
to serve as a scale for measurement.

The magnifying power can be easily ascertained by com-
paring the magnified stage-micrometer lines traced by aid of
the camera lucida with a rule divided into inches and tenths :
thus, supposing y^ o^n °f an inch, when marked on the paper,
to measure 1 inch and -nyths, the magnifying power would be
130; but in such calculations particular care must be taken
that the distance from the edge of the camera lucida to the
paper is exactly 10 inches, as this is the standard distance
of distinct vision with the naked eye.

The Micrometers.

The following is a description by the late Mr. George
Jackson, who was the inventor of this method of using
micrometers with the compound microscope : —

" In ordinary observation, when no drawing is made, a
micrometer in the eyepiece is the readiest method of mea-
suring. But here it is obvious that we shall be comparing
an enlarged image of the object with the divisions themselves,
and not with their enlarged image. It is therefore necessary
to ascertain the value of these divisions with each of the
object-glasses of the microscope; or, in other words, to deter-
mine the relation which the micrometer in the eyepiece bears
to the image of the one on the stage, subject to the same
magnifying power that is to be applied to the object to be
measured.

" Insert the proper micrometer (Plate XXII. fig. 2) in the
eyepiece (see fig. 1), and adjust the upper lens {a) by its
screw, so as to see the lines clearly when the light comes
through the body of the instrument. Place a micrometer

64 THE MICROMETERS.

(fig. 3 or 4), divided into lOOths and lOOOths of an inch, on
the stage, adjust the focus, and make the two sets of lines
parallel by turning the eyepiece; then notice how many
divisions in the latter correspond to one in the former.
Suppose that nineteen and a half cover one-hundredth of an
inch (fig. 5, a), by carefully adjusting the length of the draw-
tube, the coincidence may be rendered exact at 20 divisions
(fig. 5, b). Then, as twenty divisions cover one-hundredth
of an inch, 2000 (20x100) will cover an inch, and each
division will be equal to 1-2 000th of an inch.

" In doing this with each object-glass, the draw- tube of the
microscope should be so adjusted as to give a number that
can be reduced to a decimal fraction by multiplying or di-
viding by a single figure ; but if it be preferred to express
the dimension by a simple vulgar fraction, we have merely to
divide the number of divisions in an inch by the observed
number. Thus, if the length of an object be eight divisions
of the above scale (8-2000ths), it will be l-250th of an
inch, or decimally *004.

"A memorandum should be made, in a tabular form, of
the value of the eyepiece micrometer with each object-glass,
together with the length of the tube drawn out, so that
the instrument may be readjusted for measuring without
repeating the above operation : thus —

" Value of each division of Eyepiece Micrometer with the following

Object-glasses.

Amount of tube drawn out, Fraction of Decimal of

Object-glass. in tenths of an in(jh ftn inch> an inch>

I 7 ^Vtt '0005

" When moving the lines to their coincidence, the action
of the stage will be found rather too quick ; but the most
correct adjustment can be made by the small screw (fig. 2, a)
attached to the eyepiece micrometer."

LEESON'S GONIOMETER. 65

Indicator.

Quekett's indicator (Plate XXII. figs. 9 & 10) is a fine
pointer, fitted in the interior of the eyepiece, and capahle of
being turned in or out of the field of view by means of the
small quadrant (fig. 9, a). It answers admirably for the very
useful purpose of indicating some particular part of an ob-
ject; and any of the eyepieces can be furnished with it.

* Double Nosepiece.

Brooke's double nosepiece facilitates the change from one
object-glass to another, and avoids the loss of time incurred
by screwing and unscrewing in the ordinary way. This
piece of apparatus (Plate XXII. fig. 14) is attached to the
microscope by the screw-piece (a); two object-glasses are
screwed to the extremities (#, #), and, by merely rotating
the arm (c, c) on the centre (d), either object-glass may be
brought into the position for use : a pin (e) in each instance
forms a stop to ensure correct centering.

Quadruple Nosepiece.

We have extended the same principle as that of the
foregoing to a quadruple nosepiece (Plate XXII. fig. 13);
this is applied to the microscope by a screw-piece, as in the
double one ; but the object-glasses are changed in position by
drawing the plate (fig. 13, a), to which they are screwed, a
little forward, to release it from a pin, when it may be turned
round, and a slight spring, confined in the top (#), will press
the plate home again, when the next object-glass will be
only central with the body of the microscope.

Leeson's Goniometer.

This instrument (Plate XXII. fig. 12) is admirably adapted
for measuring the angles of microscopic crystals. It con-

F

66 MALTWOOD'S EINDEE.

sists of a circular divided plate, above which a Biot's double-
refracting prism is mounted so as to admit of rotation by
means of the arm (A), which also serves as an index-hand.
The whole piece of apparatus fits on the eyepiece, to the
flange of which it is secured by a pin, which drops into a
corresponding hole.

When a crystal, or any angle of a crystal, is viewed
through the prism of the goniometer, there will appear two
images, which may be made to occupy various relative posi-
tions by revolving the prism, as shown in the annexed wood-
cuts, figs. 1, 2, & 3.

Fig. 1. Fig. 2. Fig. 3.

y*~

if y

Let at, y, z be the angle to be measured : hold the arm (Plate
XXII. fig. 12, A) at zero, and revolve the prism by the milled
ring (B, B) until the lines forming one side of the angle to be
measured coincide in both images, as, for instance, the lines
#, y, x\ y' (fig. 2), then move the arm over the graduated
circle until the two lines forming the other side of the angle
y, z, y\ z! are made to coincide (fig. 3) ; the amount of rotation
thus obtained is the measure of the angle, or its complement,
according to the direction in which the arm is moved. In-
stead of starting from zero, it is of course sufficient to take
the difference of the readings in the two positions.

Maltwood's Finder.

A " finder," as applied to the microscope, is the means of
registering the position of any particular object in a slide,
so that it may be referred to at a future time, and by any
microscopist who possesses the finder.

MALTWOOD'S FINDEE.

67

The subject of the best form was very fully discussed in
the pages of the ' Quarterly Journal of Microscopical
Science;' amongst the various schemes we selected the fol-
lowing, which was proposed by Mr. Maltwood, and it has
now become a universal standard of reference.

It consists of a glass slide, 3 X 1 J inches, with a scale (a)

MALTWOOD'S
FINDER

4r^-m STOP

"Wigy PHOTOGRAPHED

BY

SMITH. BECK & BECK

^S?ti?9B3iS3S5SBBiJl|3Si5SS53!»ttitf!iW

LONDON.

True size,
occupying one square inch, and consisting of 2500 squares,
each of which is separately numbered with a longitude and
latitude.

The scale is in each instance at an exact distance from the
bottom and left-hand end of the glass slide, which, when in
use, should rest upon the ledge of the stage of the microscope,
and be pushed against a stop at the end ; this stop, which is
best as a simple pin, should be about one inch and a half
from the centre of the stage, and at a point from the ledge
indicated by an arrow upon the finder at b.

The object-slide must be placed under the microscope
with the same care as the finder ; and when the particular
object is in the field of view, remove the object-slide, put
the finder in its place, and read the numbers of the square
that comes into view; this maybe recorded upon the object-
slide ; and to refer to the same object at a future time, the
process has only to be reversed, by first finding the particular
square of the finder, and then by placing the object-slide in
its place.

f2

a

b

24

12

6

d

6 8 MICEOSCOPE-LAMPS.

There is an easy way of recording the numbers by which
each square may be subdivided into 5 ; thus : — Supposing the
following figure to represent one of the squares ;
if the object should be out of the centre of
the square, put two lines in addition to the
figures, to indicate the particular corner in

which it occurs; thus, if it is <z, write -,«, if at 8, -.J, if at
c, I-, q, or if at d, -* «u whilst if near the centre, the figures

-j ^ may be recorded without any lines.

We have taken great care to preserve an exact uniformity
in the size of the scale and in its position on the slide, so
that the registrations may avail any microscopist who pos-
sesses one of these finders; we should at the same time
mention that the ledges on the stages of microscopes are
generally made at an angle, and when this angle varies it
may make a slight difference in the readings from the finder ;
and, principally on this account, we have adhered to Mr. Malt-
wood's original size of fiftieths of an inch, rather than any
smaller size, for the squares in the scale.

Microscope-Lamps.

One of the greatest advantages possessed by the micro-
scope is its entire fitness for use at night ; but to experience
the full enjoyment of it at this time, the illumination must
be suitably arranged and sufficiently bright ; any of the four
lamps shown in Plate XXIII. are suited for the purpose.
Fig. 1 is known as the " Cambridge reading-lamp," and burns
oil. Figs. 2 & 3 are for Belmontine, Leucoline, or some
other of those kinds of oil, which are admirably adapted for

MICEOSCOPE-T AELE. 6 9

the purpose, if they are only properly selected and prepared ;
we are now in treaty for some specially made for use with
the microscope, which we expect to be most brilliant, per-
fectly safe, and free from any unpleasant smell. Fig. 4 is
a gas-lamp with a flexible tube for a connexion. All these
lamps are provided with the means of raising and lowering
the light upon the stems (A), and also with movable shades
(B) for protecting the eyes from any annoying glare. The
intensity of the light may also be increased or diminished by
appliances common to other lamps, and therefore not re-
quiring explanation here. In any case when a wick is used,
it should be carefully dried before being put into the lamp ;
and if there be any hardening or incrustation at the top of
the wick after it has been used, it should be cut or rubbed
off previous to being lighted. But above all give the glass
chimney a wipe each time before it is put on ; there is no
need to clean it all the way up, but any smear or fog below
the top of the flame forms a serious obstruction to good illu-
mination.

Microscope-Table.

However absorbing and sometimes exclusive microscopical
observations may be, there are many occasions upon which
the cooperation of other observers is required, and not unfre-
quently the microscope is enlisted in the sociable entertain-
ments of an evening. Under either of these circumstances a
frequent change of seat, or even, if the instrument be placed
perpendicularly, the constant standing, detracts much from
any pleasure, and makes a quiet study unnecessarily irksome
and disturbing.

The table (Plate XXIII. fig. 5) is contrived to obviate
these annoyances; it will comfortably accommodate three

70 CASES FOE MICROSCOPES.

people sitting, and by revolving the top, the microscope may
be passed round without any one rising : this principle can
be, and frequently is, extended to tables of much larger
size, but the arrangement here shown is the one more gene-
rally approved.

Firmness of construction is most essential in these tables,
and the revolving fitting especially has to be made with
great care. The manufacture throughout is of the best de-
scription, consequently their cost contrasts rather unfavour-
ably with that of much of the furniture of the present day ;
but tables of inferior construction have only proved most
annoying, or entirely useless, when required for microscopical
purposes.

Another convenient and simple method of passing the
microscope to any number of persons seated at an ordinary
table without disturbing the adjustments of the instrument,
is shown in Plate XXVII. fig. 2. It consists of a neatly
framed wooden stand, covered with leather on the top, which
is sufficiently large to receive the microscope and lamp ; and
the under side is provided with four carefully made brass
casters, so that the stand and all that is upon it may be
pushed about smoothly in any direction that may be required.

Cases for Microscopes. — First-class.

The microscope-stands, the apparatus, and the object-
glasses are almost invariably packed in some kind of case ;
it is a subject we have paid considerable attention to, because
upon it the good preservation of a microscope, whether at
home or when travelling, very much depends.

Plate XXV. shows a best Spanish mahogany case, con-
taining a large best binocular microscope complete ; the tri-
pod of the stand is fitted on a board (A) at the bottom of

CASES FOR MICROSCOPES. 71

the case, which confines the instrument to a certain posi-
tion, and much facilitates the sliding of it in or out ; at the
top, the eyepiece-caps slide in two grooves, and a remov-
able block (D) in the left-hand one fills up a vacant space
between the front of the eyepiece and the door. Two boxes
(E & F), containing the apparatus, slide in at the sides of
the instrument, and are secure in their places when the case
is closed. The bull's-eye condenser packs on some blocks
(G & H) at the back, and the draw-tube slides into two
holes on the left at I; with these two exceptions, the
whole of the apparatus and object-glasses fit into the various
holes or recesses cut in the boards placed in the insides
of the two boxes as shown in Plate XXVI. ; various blocks,
faced with velvet, are fastened to the lids of the boxes, so
that when closed and secured by the hooks, the whole of the
apparatus will remain undisturbed, even when turned upside-
down or shaken. When there is a smaller quantity of appa-
ratus than that shown in Plate XXV., only one box is
used. We also make a cheaper form of outside case in
Honduras mahogany.

Our large best binocular microscope, complete in every
respect, may be made exceedingly portable by using a fold-
ing tripod stand and a movable stage; with these altera-
tions the whole will pack in a case, the outside measure of
which is only 19 inches long, 9^ inches broad, and 5 inches
deep.

Second-class Cases.

The second-class instruments are packed with the same
care as those of the first class, but generally in flat cases,
such as the one represented in Plate XXVII. The micro-
scope is shown with but little apparatus ; there is, however,

72 BELL-GLASS COVERING.

abundant room in the case for any additions ; we also specify
in our catalogue for upright cases of this class.

Another plan which may be adopted for keeping any
microscope at home is a simple bell-glass placed over a
base, to which the stand of the instrument is fitted ; but this
method is almost invariably additional to the ordinary
arrangement previously described.

73

THE THIRD-CLASS MICROSCOPES.

These instruments are intended to meet a want that has
been much felt by students and others, who have been
desirous of possessing an instrument equal to their require-
ments, whether in physiology, chemistry, or the minute
forms of animal or vegetable life, but have not been able
or not disposed to incur the cost of such as are larger or
more elaborate.

Of this class we give descriptions of two different forms of
Stand, distinguished from each other by the titles of the
Popular and the Educational Microscopes.

THE POPULAE MICEOSCOPE.

This entirely new form of Stand has been contrived because
the construction of the Educational Instrument, described
at p. 89, was not suited to the addition of Wenham's Bino-
cular Body. Whilst, therefore, the optical arrangement of
the Popular Microscope remains exactly the same as that of
the Educational, the stand is greatly improved, without any
additional cost.

The object-glasses and eyepieces of the Popular Micro-
scope are so similar in appearance to the best ones figured
in Plate V., that any additional illustrations would only
appear superfluous ; but the magnifying powers and other

74

THE POPULAE OBJECT-QLASSES.

particulars more especially relating to the object-glasses of
this microscope are given in the following list.

The Popular Series of Achromatic Object-glasses.

The object-glasses in the following list are intended for
all kinds of general use in which any very large aperture is
not necessary.

There are many persons who would not, with ordinary
objects, and using only the first and second eyepieces, see
any difference between these powers and the very best, the
corrections being perfect, and the workmanship really good
so far as the aperture goes.

With severe test-objects, and employing the third or
higher eyepieces, the difference is very perceptible ; but
many who work with the microscope never require this
high-class performance ; and the price of these object-glasses
is very considerably below that of the best.

Focal
length.

2 inches
li inch
1 inch
-1 inch
^ inch
■J- inch

Linear magnifying

power, nearly, with

eyepieces

Degrees of
angle of
aperture.

No. 1.

No. 2.

Draw-tubes

closed

24

40

10

ditto

29

48

15

ditto

55

90

22

ditto

120

200

40

ditto

210

350

75

ditto

420

700

85

All these object-glasses have the Universal Screw (see
p. 6).

With the ^-inch and higher powers the condition of the
chromatic and spherical aberrations may be considerably
altered by the use of different thicknesses of thin glass, or

THE "POPULAE MICROSCOPE." 75

other media, in covering the object.. As a provision to com-
pensate for the errors arising from such causes, the front
tube of each of these object-glasses is made to slide for a
very short distance ; and when pulled out as far as it will go,
the object-glass is adjusted for an uncovered object, and the
tube must be pushed back more or less for an object that is
covered, the extreme adjustment being for glass *01 thick.

Description of the Stand.

The construction of the Popular Microscope (see figs. 11,
12, and 13) is as follows : —

The body, A (in these illustrations shown as binocular), is
carried by a strong arm, B ; and this is attached to a square
bar, C, which may be moved up or down by a rackwork
and pinion in the lower part of the stand, where the stage,
D, and the mirror, E, are attached.

The base, F, is triangular ; and it is connected with the
parts of the instrument already described by a broad stay,
G, which moves on centres at the top and bottom, so as to
allow the end of the tube, H, to fit by its projecting pin into
various holes along the medial line of the base. With this
arrangement, if the body of the microscope be required in a
more or less inclined position (see fig. 11), four holes are pro-
vided near the extremity of the base for the pin of the tube
to fit into. A hole near the stout pin, L, is used when a ver-
tical position is wanted (see fig. 12) ; while to obtain the
horizontal position, shown in fig. 13, the pin of the tube is
placed in a hole in the stud, K, the inner surface of the stay,
G, resting at the same time on the top of the stout pin, L.
This form of construction is entirely new, and has the fol-
lowing advantages :— it is strong, firm, and yet light ; the
instrument cannot alter from any particular inclination it is

76

THE "POPULAR MICEOSCOPE.'

put into, which is not. unfrequently the case when the
ordinary joint works loose ; and in every position the heavier
part of the stand is brought over the centre of the base to
ensure an equality of balance.

Fie;. 11.

^ size.

Directions for Use.
To adjust the focus of the object-glass, turn the milled

THE "POPULAE MICEOSCOPE."

77

head, O, for a quick movement, or the milled head, P, for a
slow one.

Pig. 12.

^ size.

The stage, D, is circular ; and upon it fits a plate, T ; this
again carries the object-holder, U, which is provided with a
ledge, V, and a light spring, W ; it is held on the plate T by
a spring underneath, so that it can be moved about easily
and smoothly by one or by both hands. The small spring,

78 THE "POPTILAB MICKOSCOPE."

W, is fastened to the object-holder by a milled head, which
will unscrew; so that the position of the spring may be
altered to give more or less pressure upon the edge of the
object, or it may be removed altogether, if necessary.

When a stage with only a flat surface is required, the
object-holder, U, may be removed by unscrewing, from the
under side of the plate, T, two small milled heads which con-
nect a circular spring with the object-holder; or, by re-
moving the plain stage altogether, an extra simple flat plate
may be substituted.

Beneath the stage there is a cylindrical fitting for the
reception of a diaphragm, or for any additional apparatus
that may be required in that position.

The mirror, E, besides swinging in a rotating semicircle,
will slide up or down the tube, H, or it will turn on either
side for oblique illumination.

The light should in general be on the left of the observer ;
the best is that from a white cloud on a bright day ; but a
satisfactory effect can be obtained from a wax or Palmer's
candle, if protected by a glass, a Cambridge or Moderator
oil-lamp, a small Paraffin or Belmontine lamp, or an Argand
gas-burner, provided they are not more than 10 or 12 inches
from the instrument.

The management of the illumination demands particular
attention; that of a transparent object is produced by re-
flexion from the mirror below, which should most frequently
have its centre coincident with the axis of the body, and
should be at such a distance that the light reflected from it
may nearly converge to a focus at the object: this distance
will be about 2\ inches when daylight is used ; but the rays
from a lamp or candle 10 or 12 inches from the mirror are
so divergent, that the focus for them will be lengthened to

THE "POPULAB MICROSCOPE.'

79

about 3 inches ; and the mirror may have to be slid up or

down accordingly.

Fig. 13.

Fig. 14.

Accurate adjustment of this focus is often required with
the J-inch object-glass; and some details of objects, such as
delicate striae, are best seen with this glass when a
strong light is thrown on them obliquely by turning
the mirror on one side of the axis. With the
1-inch object-glass the light is generally in excess,
and has to be lessened by fitting the Diaphragm
(fig. 14) under the stage ; this
admits only so much light as
passes through one or other of
the two apertures in a small
revolving disk, by which con-
trivance, together with sliding
the diaphragm up more or less
under the stage, every neces-
sary variation can be made.
The diaphragm is generally
left in its place when the in-

Fig. 15

size.

80 THE "POPULAE MICKOSCOPE."

strument is packed away ; or, if there be a vacant space, it
may be put in the Tray (fig. 20).

To illuminate opaque objects the light is thrown upon
them from above ; a Small Condensing Lens (fig. 15) mounted
upon a separate stand, and capable of being turned in any
direction, answers well for this purpose : its focus, for a lamp
or candle 4 inches from it, is about 3 inches ; for daylight,
2 inches. A large object can be placed upon the stage at once ;
but small ones are generally either laid on a piece of glass or
held in the Forceps (fig. 16): these are of a superior construction
to those we supply with our Educational Microscope ; they fit
upon the pin at the top of the small milled head, which
fastens the spring on the stage, and by the ball-and-socket
movement at a, and the sliding wire b, every requisite move-
ment can be obtained. In illuminating objects from above,
all light that could enter the object-glass from below should
be excluded ; and the Diaphragm (fig. 14) will do this very
effectually when placed under the stage, with the blank space
of its revolving disk turned over the aperture.

A Glass Plate with a ledge, and some pieces of Thin Glass
(fig. 17) are applicable for many purposes, but are specially
intended for objects in fluid. Thus, a drop is Pig. 17.

placed upon the plate and covered by a piece
of thin glass ; or, the object being put upon
the plate, and the thin glass over it, the
fluid is applied near one side, and runs under by capillary
attraction.

A pair of Brass Pliers (fig. 18) completes the necessary
apparatus furnished with the microscope, and pi lg
is kept, with the forceps and glass plate, in
the space on the left-hand side of the Tray,
fig. 19.

THE "POPIILAK MICROSCOPE." 81

The Case is made of mahogany, French-polished, with brass
hooks, a good lock, and a strong handle. To pack away the
stand, it must first be put in the horizontal position (iig. 13);
but no part has to be taken to pieces. At the right-hand
end of the case, provision is made for all the apparatus. In
the first place, & Removable Tray (fig. 19), lying at the bottom

Fig. 19.

-Jrd size.

of the case, is divided into two spaces : the one is for keeping
the forceps, pliers, glass plate, &c. ; and the other is racked
for 18 objects. Above this fits another Tray (fig. 20), which is

Pig. 20.

^rd size.

contrived to receive not only the object-glasses and apparatus
already alluded to, and here shown in their places, but also
the whole of the apparatus hereafter described, a is the
diaphragm; c, the 2nd eyepiece; and n and o, the object-
glasses, for which some brass cells are fitted in the board ;

G

82 THE "POPULAK MICROSCOPE."

and when the object-glasses are put away, the engraved brass
caps should always be screwed on, to prevent any dust
settling on the inner glasses, which cannot be easily wiped.

The other spaces in the tray are arranged as follows : — in
the holes, a, b, d, g, the achromatic condenser, the parabolic
reflector, the dark-well holder, or the large polarizing prism
will fit ; the small analyzing prism will pack in c ; the camera
lucida in Jc; p and r are spaces for additional object-glasses;
I and m will receive the 1-in. and ^-in. Lieberkuhns ; an extra
eyepiece can be packed at/; and h and i are finger- and thumb-
holes for taking hold of the tray.

The whole or any part of the extra apparatus for which
spaces are left in the Tray, and which is described in pages

83 to 88, may be added to the instrument at any time, with-
out its being sent back to the makers.

Glass of any kind requires occasional cleaning ; a piece of
soft wash-leather is the best for the purpose.

The fronts of the Object-glasses may be carefully wiped ; but
if anything more be required, it must be done by the makers.

When cleaning the Eyepieces, which should be done fre-
quently, the cells containing the glasses must be unscrewed
and replaced one at a time, so that they may not be mixed.

Any dirt upon the Eyepieces may be detected by turning
them round whilst looking through the instrument ; but if
the Object-glasses are not clean, or are injured, it will for the
most part only be seen by the object appearing misty.

Wenhams Binocular Body.

Thus far in this description the Popular Microscope has
been considered as having a single body only ; the addition,
therefore, of the Binocular Body, as shown in the illustra-
tions, requires a few explanations and directions for use,

ADDITIONAL APPARATUS.

83

which will be found in pp. 48 to 55, where there is a full de-
scription of the contrivance, in every way applicable to this
instrument, excepting a slight difference in the fitting of the
prism, which in the Popular Microscope can only be removed
after a smaller milled head (fig. 12, S) has been unscrewed.

Mechanical Stage.
When the movement of the object requires greater nicety
than a direct action from the hand can give, the plain stage
may be taken off and replaced by a Stage with Mechanical
Movements (fig. 21). By this arrangement the plate (a) with
a fitting, sliding up or down, will
receive the object, which can also
be moved sideways, these two
movements forming a quick ad-
justment; the slower movements
in rectangular directions being
given by turning the milled heads
b and c, which, for convenience in
use, are placed on the same spin-
dle. For rotation of the object, the whole stage may be
turned upon the bottom stage-plate, which is central with
the body ; and consequently the part of the object that is
under examination will always remain in the field of view
during the rotation.

Additional Appaeatus.

When the light from the concave mirror proves insufficient
for any object requiring an intense transmitted light, the
Achromatic Condenser (fig. 22) may be employed with ad-
vantage : this slides, by its tube, into the fitting under the
stage of the instrument, in which it has to be moved up or
down until the focus of its lenses falls upon the object, the

G 2

kd size.

84

THE "POPTJLAE MICEOSCOPE."

light having been previously reflected in the proper direction
by the flat mirror.

Pig. 22.

Pig. 23.

Pig. 24.

^rd size.

The Illumination of Ojpaque Objects, already described,
must be more or less one-sided; and in most cases it is
desirable that it should be so. An illumination on any or
every side is, however, easily obtained, provided the object is
not too large, by means of the LieberJcuhn (fig. 23). This is
a silvered cup, which slides upon the front of the object-
glass, and light thrown upwards by the mirror will be re-
flected by it down upon the Fig. 25.
object ; it will then be found
that, by slightly varying the
inclination of the mirror,
every necessary alteration in
the direction of the illumina-
tion can be obtained. The
LieberJcuhn here shown is in-
tended for the 1-inch object-
glass.

It is in most cases neces-
sary, when using the Lieber-
kuhn,to slide a Baric Well(fi.g.

24) under the stage, to pre- Section, true size.

vent any light entering the object-glass direct from the mirror.

Dark-Field Illumination is, to appearance, a means of
seeing a transparent object as an opaque one. The principle,
however, is that all the light shall be thrown under the

ADDITIONAL APPAKATUS. 85

object, but so obliquely that it cannot enter the object-glass
unless interrupted by the object: this is best accomplished
by Wenharris Parabolic Reflector (fig. 26). It may be easily
understood by reference to fig. 25, which represents F- 26.
it in section ABC, and shows that the rays of
light, r rV, entering perpendicularly at its surface
C, and then reflected by its parabolic surface A B
to a focus at F, can form no part of the largest jrd size.
pencil of light admitted by the object-glasses and represented
by GFH; but an object placed at F will interrupt the
rays and be strongly illuminated. A stop at S prevents any
light from passing through direct from the mirror.

In this microscope, the Parabolic Be/lector fits under the
stage by the tube A (fig. 26), and the adjustment of its focus
upon the object (which is when its apex almost touches it)
is made by giving it a spiral motion when fitted in — that is,
carefully pushing it up or down at the same time that it is
turned round by the milled edge, B B. As the rays of light
must be parallel when they enter it, a Flat Mirror, which in
this case should be added to the instrument, is generally
used; daylight will then require only direct reflexion, but
the rays from an artificial source will have to be made
parallel by putting the condenser (fig. 15) between the light
and the mirror, about If inch from the former and 4^ inches
from the latter. Nearly the whole surface of the mirror
should be equally illuminated, which may be tested by tern*
porarily placing upon it a card or piece of white paper,
Parallel rays can also be obtained from the Concave Mirror,
if the light is put about 2^ inches from it.

Polarized Light, invaluable to some microscopists, and to
others a beautiful appliance by which many objects other-
wise almost invisible are shown in every imaginable colour,

8Q

THE "POPULAE MICEOSCOPE.'

^rd size.

can here only be treated of by describing the way in which
it is applied to this microscope by the following apparatus : —

A Nicol's prism as a Polarizer (A, fig.
27) fits, and can be turned round,
under the stage; another prism, B,
slides in the place of the cap of either
eyepiece, and also revolves ; or by un-
screwing its outer tube, <?, and its cap,
d, it screws, as e, in the place of the
back-stop, f9 of either object-glass, and then the
object-glass together with the prism are attached to
the nosepiece of the microscope by the adapter (H), which
has a revolving fitting at k. When the prism B is over the
No. 2 eyepiece, the field of view is considerably cut off ; and
although it is not so when the prism is screwed above the
object-glass, yet the definition is then somewhat impaired:
its position therefore must be regulated by the character of
the object. When only alternate black and white images
are given by the prisms alone, a plate of selenite, G, will
produce coloured ones.

To draw an object, the Camera Lucida (fig. 28) is used.
It slides on in the place of the cap of either eyepiece, with
its flat side uppermost, as shown. The body of the micro-
scope must be in a horizontal position, and the whole instru-
ment has to be raised until the edge of the prism is exactly
10 inches from a piece of paper placed upon the table. If
the side of the case be used for this purpose, the proper
distance is exceeded by f of an inch ; but the paper may
easily be raised this amount by some pad. The light must
be so regulated that no more than is really necessary is upon
the object, whilst a full light should be thrown upon the
paper. Only one eye is to be used ; and if one half of the

ADDITIONAL APPAKATUS.

87

pupil be directed over the edge of the prism, the object will
appear upon the paper, and can be traced on it by a pencil,
the point of which will also be seen. Should any blueness
be visible in the field, the prism is pushed too far on, and

should be drawn back till the colour disappears.

Fig. 28.

jrd size.
Substituting in the place of the object a piece of glass
ruled into l-100ths and l-1000ths of an inch, termed a
Micrometer (fig. 29), its divisions can be marked on the same
or another piece of paper, and, by comparing them with the
sketch, the object can be most accurately measured. These

88 THE "POPULAR MICKOSCOPE."

divisions also, if compared with a rule divided into inches
and tenths, will give the magnifying power : thus, supposing
1-1 00th of an inch when marked on the paper measured
1 inch and -^ths, the magnifying power would be 130.

The Live-box (fig. 30) hardly needs description : the object
is confined between the glass, $, of the lower part, B, and
that of the cap, C ; the distance between them can be varied
by sliding the latter more or less on. As the thin glass is
only dropped into a slight recess in the top of the cap, and is
held there by the heads of the two screws, it can be easily
taken out for wiping, or be replaced by another when broken.

The Glass Trough (fig. 31), for larger objects in water,
must be used with its thinner plate of glass, #, in front.
The modes of confining such objects, and keeping them near
Fig. 29. Fig. 30. Pig. 31.

?rd size.

3J

the front surface, must vary according to the occasion. For
many it is a good plan to place a piece of glass, e, diagonally
in the trough, its lower edge being kept in its place by a
strip (d) at the bottom; then if the object introduced is
heavier than water, it will sink till stopped by the sloping
plate. Sometimes a very slight spring (f) may be applied
behind this plate to advantage, with a wedge (g) in front to
regulate the depth.

Arrangements are made for all those parts which may
require cleaning. Thus, the Parabolic Reflector unscrews
from the tube ; the Nicol's Prisms will push out of their
fittings ; and the Camera-lucida Prism can be taken out by
turning aside the plate that covers it.

THE EDUCATIONAL MICKOSCOPE.

89

THE EDUCATIONAL MICROSCOPE.

This is the first third-class instrument that we constructed ;
and although we consider the stand to be superseded by the
Popular Microscope, last described, there are many persons
who, not requiring a binocular body, are of opinion that the
Educational Microscope possesses many advantages in its
form and arrangement ; we therefore give the following full
description of the instrument.

Fig. 32.

-J-th size.

When the microscope is taken out of the case, as shown
in fig. 32, first turn up the mirror G to O, and draw out the
body, A; the stand, turning on the centre P, can then be
placed in a perpendicular, horizontal, or any intermediate
position. Next restore the mirror to its former place, slide
the body, A, into the upper end of the tube B, and the in-
strument is ready for use. (See fig. 33.)

To adjust the focus of the Object-glass : — A quick motion
is obtained by sliding the body, A, in the tube B ; and a slow
motion by turning the milled head, C. The Stage, D, has
two springs, E, the pins of which may be inserted in any of
the four holes, F, and by their pressure, which can be varied
by pushing them more or less down, they will hold the ob-

90

THE EDUCATIONAL MICROSCOPE."."

ject under them, or allow it to be moved about with the
greatest nicety.

The Mirror, G, besides swinging in the rotating semicircle,

Fig. 33.

^rd size.

I, is on the tube H, which may be drawn down, or turned on
either side.

There are some cases in which a supplementary stage

THE EDUCATIONAL MICROSCOPE.

91

(fig. 33 a) may be found useful, as It entirely prevents the
springs of the stage, EE (fig, 33), interfering with an object
or its covering when under the microscope ; such a stage can
be used under all circumstances, except with the dark field
and Lieberkuhn illuminations.

When the instrument is put away, the object-glasses are
kept in the boxes, K. The eyepiece No. 1 is left in the
body, and No. 2 fits in the space at L. The two vacant
spaces in the board at M and N are for an additional object-
glass, and a Lieberkuhn to be used with the 1-inch, if these
should be required.

The diaphragm which fits under the stage is shown in
fig. 34; this admits only so much light as will F- 34
pass through its aperture (A), but by sliding it up
more or less in the fitting, every necessary varia-
tion can be made, or the diaphragm may be used
to exclude all light from below, in which case the
small shutter, B, is turned over the aperture,
phragm packs in the board at O (fig. 33).

A small condensing lens, R, attached to . one of the up-
rights of the stand answers well for the illumination of
opaque objects : its focus for a lamp or candle four inches
from it is about three inches ; for daylight, If inch.

Fig. 35 shows the forceps which are supplied with the
Educational Microscope, ri 35

and the different ways in
which they will hold small
opaque objects.

The remaining apparatus, together with the object-glasses
and eyepieces suitable for application to this stand, are the
same as those already described under the Popular Micro-
scope (pp. 83 to 88) ; but there is some difference in the way

The dia-

92

THE EDUCATIONAL MICEOSCOPE.

in which the extra apparatus is packed, as shown by the
accompanying figure, which represents a small mahogany
board provided with the various packings, and fitting into a
groove near the top of the case.

Fig. 36.

93

FOURTH CLASS.

THE UNIVEKSAL MICROSCOPE.

This instrument is the result of an endeavour to make a very
low-priced Compound Achromatic Microscope by reducing
its construction to the simplest possible form, still retaining
all that a really useful instrument requires, together with
such an arrangement as would admit of considerable additions
being made without returning the stand to the makers.
These features, together with other details, are fully explained
in the following description : —

The foundation of the stand is a large circular base
(fig. 37, A), and near its circumference, on the left-hand side,
is a strong pillar (B) ; at its top is the axis upon which the
remainder of the instrument turns, and with so equal a balance
as never to require more than a slight screwing down of the
small milled head, C, to secure any particular position.

On the same centre as the axis is a large milled head, D,
by turning which a quick motion is given to the body, E ; and
depending from the smaller part of the same milled head is
a lever, F ; this of itself hangs free, but when held at the
lower end, and pressed sideways, either nearer or further from
the pillar, it obtains a gripe upon the milled head, which
can then be turned so slowly as to constitute a very good
slow motion.

The quick-motion milled head and the slow-motion lever
are always in the same position, and do not alter with any
inclination of the body; they are also so low down, that in
using them the hands are very little raised from the table ;

94

THE UNIVERSAL MICROSCOPE.

THE UNIVERSAL MICROSCOPE.

Fiff. 37.

This illustration is drawn to a scale of half size.

THE UNIVERSAL MICKOSCOPE. 95

this latter advantage also applies to the stage (H), which is
screwed on at the lower end of the limb (G) at less than
four inches from the bottom of the stand.

On the top of the stage is a double spring (I) branching
right and left over a brass plate (K) ; on this there is a ledge
(L) for the object to rest upon, and in continuation on the
right-hand side the plate is bent over so that it may be
firmly grasped by the fore and middle fingers underneath,
and by the thumb above. With this arrangement the object
can be moved freely in every direction, and will retain its
position after the hand is withdrawn. If necessary, the
short spring (M) may be used when the object has to be
held firmly on the plate. The pin (N) is for holding the
forceps. Beneath the stage is a cylindrical fitting (P) for
all the apparatus required in that position. .

The diaphragm (fig. 38) is, however, made
a fixture to the mirror-stem ; but it will turn
away entirely on the left side, when necessary ;
it is provided with one small aperture (X)
for the lower powers, and this can be closed by
a small shutter (Y).

A concave mirror (fig. 37, S) swings in a i size-
rotary semicircle (T) which is attached to an outside sliding
tube, the inner tube being screwed beneath the stage; on
the opposite side a condenser (U) is fixed for the illumina-
tion of opaque objects; it is provided with ball-and-socket
joints, which afford any necessary movement, and also the
means of turning it out of the way when not in use. Its
focus for a lamp or candle five inches from it is about
1\ inches ; for daylight, If inch.

There are five object-glasses with which the Universal
Microscope may be furnished, viz. 2-inch, 1-inch, ^-inch,

96

THE UNIVERSAL MICROSCOPE.

J-inch, and £-inch (fig. 39) ; they differ from the powers of all
other instruments in having, a smaller-size screw for their
attachment to the body, and the settings of their lenses made
as short as possible, these variations having been made to
suit them to the " Combined " and " Binocular " bodies, de-
scribed further on.

Fig. 39.

2-inch. 1-inch

£-inch.

i-inch.

i-inch.

size.

The eyepieces (fig. 40) are of a construction introduced by
Kellner, and they give a flat and, for their size, a large field
of view. Their chief fault will, we believe, prove a general
advantage : any dust or moisture upon the field-lens is so
annoyingly apparent from its being in the focus of the eye-
lens, that those who use this form will be compelled to wipe
the lenses frequently ; and not only this, but they will soon
learn the necessity for the constant examination and the
occasional cleaning of every surface of glass that they have

about their microscopes.

Fig. 40.

No. 1.

No. 3.

* size.

The apertures of the object-glasses of the Universal Micro-
scope and their linear magnifying powers when combined
with the eyepieces are given in the following list, together
with the increase that may be obtained by the addition of a
lengthening tube to the body — an arrangement which, under
many circumstances, is of great advantage.

THE UNIVERSAL MICEOSCOPE.

97

List of Achromatic Object-glasses for the Universal

Microscope,

Focal length.

Linear magnifying powers, nearly,
with eyepieces

No. 1.

No. 2.

No. 3.

2 inches,

20

30

65

with lengthening tube

40

55

100

1 inch,

55

75

155

with lengthening tube

95

125

250

j inch,

with ordinary body

115

150

325

with lengthening tube

190

250

500

I inch,

with ordinary body

175

225

480

with lengthening tube

275

350

650

g inch,

with ordinary body

315

425

865

with lengthening tube

470

630

1250

The higher powers, viz. the \ inch, \ inch, and \ inch, have
no adjustments for variations in the thin glass or other
media interposed between their front lenses and the object;
but they are corrected for a piece of glass *008 thick, and
these object-glasses will define the best when the object is
covered with glass of such measurement.

The forceps (fig. 41, O), a small
pair of pliers (V), and a glass plate
with a ledge (W) are also, generally
supplied with this microscope.

The case is made of mahogany, and
of an upright form ; the instrument,
when put away, slides into it so that
the pillar occupies the left-hand corner
next the door; the stand is blocked
to prevent any inj ury in carriage. O ne
object-glass and one eyepiece are intended to remain on
the microscope ; a small board on the door receives the re-
maining apparatus, with room for any additional object-
glasses or eyepieces that may be required. Provision is also

H

size.

98

THE UNIVEKSAL MICEOSCOPE.

made in the case for a small box (fig. 42), to contain the extra
apparatus alluded to hereafter.

This microscope can be kept in perfect working order by
a little attention to one or two parts. If the body work
loose in its dovetail fitting, it can be corrected by unscrew-
ing the screw (fig. 37, a) at the upper part of the limb half
a turn, and by screwing up the one (b) at the lower part of
the limb to the same extent ; or if this should not be suffi-
cient, it may be repeated until the body will not rock in its
fitting. This operation is really that of pushing up by the
lower screw, a dovetailed wedge; but, to retain this in its
proper place, it is always necessary that the upper screw
should also be screwed down firmly upon it.

The chain connected with the quick and slow movements
may also work loose ; if so, it can be tightened by carefully
turning round from left to right the smaller screw-head (c)
at the upper end of the limb.

The extra apparatus, which may be added to the instru-
ment at any time without its being sent back to the makers,
is common to the third as well as this class of microscopes,
and is described in pages 83 to 88 inclusive ; but the case for
containing it is shown in the accompanying figure.

Eig. 42.

THE COMBINED BODY.

99

Stage with Actions.
The double spring (fig. 37, I) which holds the stage-

plate

(K)can be removed by unscrew-
ing; after this is done, the open-
ing (P) will receive what is
commonly called a Stage with
actions (fig. 43) ; this is so con-
trived to fit on as to admit of its
being turned round as a whole,
and consequently always central
with the body; or, in other
words, during the rotation, the
object will not move out
of the field of view; the
amount of movement at
right angles is half an inch
each way, and is effected
by means of the milled
heads (I, K) ; the object
can be moved to and fro
on the ledge (L), which
slides up and down, and a
small spring can be turned
to clamp the object, when
necessary.

The Combined Body.
This is a contrivance for
the attachment of three ob-
ject-glasses and three eye-
pieces to the instrument at
once (see fig. 44).

Fig. 43.

size.

H 2

100

THE UNIVEESAL MICEOSCOPE.

Only one of each of these can be central with the axis
of the body at the same time ; but any of the others can
be brought into the central position by pressing in the
rounded head of the pin (B), when either of the disks (C)
can be turned in the required direction, and will be stopped
again by the pin (B) springing out.

This arrangement may be appreciated by those who
are deterred from making a casual use of the microscope,
either from the trouble of putting an instrument up,
or from the delays which the necessary changes involve,
whilst it will considerably assist in the investigation of
objects which are undergoing a change either in their
position or their structure, and when a great range of

power is required with the least
possible delay. As a luxury, it
will apply to every use of the
microscope.

The change from the square body
(fig. 37, E) to this one is made by
unscrewing a large screw which is
at the back of the dovetail piece
fitting into the limb, and the body
will then push off downwards.
This process is, of course, reversed
when a body has to be put on ; but
then some care must be taken that
the screw in the lower end of the
body fits the slot it slides into at the
lower end of the dovetail piece, to
effect which the screw may require
either a little screwing up or un-
screwing before the body is put on.

Eig. 45.

I size.

THE BINOCULAE BODY. 101

WenharrCs Binocular Body for the Universal Microscope.

This Binocular Body (fig. 45), as applied to the Universal
Microscope, possesses the following advantages : — The object-
glasses are mounted on a rotating disk, as already described
under the combined body ; an adjustment for different dis-
tances between the eyes is made by turning the milled head
(F), which will move the draw-tubes (E, E) up or down. The
reflecting prism is placed close behind the back lens of each
object-glass, and with this arrangement the field of view is
not cut off when the objects are viewed as transparent, with
the highest power.

The change to the binocular body can also be made with
the same facility as has been already described under the
" combined body.''

Full directions for the use of Wenham's Binocular Body
have been already given in pages 48 to 55, inclusive, of this*
treatise, and apply generally to this particular form of it.

102

DAEWIN'S SINGLE MICROSCOPE.

SINGLE MICROSCOPES.

Darwin s Instrument.

An instrument and apparatus exactly similar to those
shown in figs. 46 and 47 were made to the express design
of Mr. Darwin, and it is with such arrangements that he
has carried on many of his valuable investigations.

•i-rd size.

The base (fig. 46, A) is a stout block of oak or mahogany,
near one edge of which is screwed an upright stem (B) ;
into this is fitted a triangular bar (C), having a slight cross-
arm attached at the top, to carry the magnifiers. This arm
is provided with a fitting for rotation at C, or it can be
moved backwards or forwards by turning either of the small

DAEWIN'S SINGLE MICEOSCOPE.

103

milled heads (H) ; and by using these movements, separately
or combined, the magnifier may be moved over any part of
an object which is placed upon the stage (I). The focus of
the lenses is adjusted by turning either of the milled heads
(L, L) ; and a large concave mirror, with a smaller flat one at
its back, provided with all the necessary movements, supplies
the means for transmitting light through an object.

Fig. 47.

-Jrd size.

Various pieces of apparatus, which may be required in
dissection, fit into the stage of this instrument, which is pre-
pared for the purpose with a deep recess. Into this will fit
either of the two large saucers with glass bottoms (T, U) ;
they differ in depth, but are both intended for the dissection
of large objects in fluid; and in those cases where trans-
mitted light is not required, the specimen may be pinned to
a piece of cork (O), which is loaded with lead to keep it
firmly at the bottom of the saucer.

For small objects the size of the stage may be reduced by

104 DAKWIN'S SINGLE MICEOSCOPE.

dropping into its recess the brass plate (R), which has a cen-
tral aperture of only one inch and a half ; and the following
pieces of apparatus are provided of this size : — two flat and
two concave glasses, a piece of boxwood covered with cork,
and a holder (P) to receive the ordinary 3x1 glass plates.

Beneath the stage of the instrument is a dovetail fitting
for the brass plate (M), which is provided with a small aper-
ture, and serves as a diaphragm; or when the opening is
closed by the small shutter (N), the brass plate forms a dark
background for the stage.

The lenses are mounted so as to drop easily into the ex-
tremity (fig. 46, E) of the arm, and are generally either
single lenses or Coddingtons ; but Mr. Darwin recommends
in addition a large doublet (fig. 47, S), the two lenses of
which are mounted, so as to be used separately or in com-
bination, and in this way to furnish three low powers. The
small socket of the bent arm to which the doublet is
attached fits the arm at the end (fig. 46, D) which is oppo-
site to that part where the ordinary lenses drop in.

This instrument, which is generally termed Darwin's Dis-
secting Microscope, together with all the apparatus and some
dissecting instruments, is carefully packed in a small maho-
gany case with a lock and handle.

IMPEOVED SINGLE MICEOSCOPE.

105

AN IMPEOVED SINGLE MICROSCOPE,
with or without a Binocular Arrangement to the Magnifiers.

Fig. 48.

-i-rd size.

The base (fig. 48, A) of this instrument is a square block
of mahogany, with the upper edge and corners rounded off,
and with four strong brass pillars (B) at the angles, which
support, at a height of four inches from the table, a brass
plate (C) six and a half inches square; above this are two
circular brass plates, the first (D) of the same diameter as
the square plate, with a revolving fitting at the centre, and
the means of tightening it down underneath by a large
milled head (E) ; the second and top plate of all (F) is much
smaller, and is held down by springs, which allow it to be
moved pleasantly over the lower plate to the extent of three-
quarters of an inch in any direction; this is equivalent to
a space at the centre of one inch and a half diameter, and of
this size the opening (G) of the top stage is made for the
reception of various glasses, troughs, or holders for dissection.

106

IMPBOVED SINGLE MICROSCOPE.

The magnifiers drop into the extremity of an arm (H)
which comes in a diagonal direction from the left-hand back
pillar; here it is fixed to a triangular bar (I) which can be
moved up or down by a milled head (K) connected with
a rack and pinion for the adjustment of the focus : the arm
can be turned aside to the left, but in the other direction it
stops when central with the revolving fitting of the first cir-
cular plate already alluded to.

Pig. 49.

-|rd size.

When light is required from below for transparent objects,
a concave mirror (L), provided with a semicircle and other
necessary fittings, can be turned up from a recess in the
mahogany base, and a side condensing-lens (M), with ball-
and-socket and other movements, supplies all necessary illu-
mination from above.

In nearly all dissecting microscopes the magnifier is moved
over the object, and consequently away from either con-
denser- or mirror-illumination. In this instrument the

IMPEOVED SINGLE MICEOSCOPE. 107

magnifying power is stationary; and not only does the
illumination continue to be the same whilst the object is
moved about in every direction, but any particular part that
is being examined will remain in view whilst it is rotated by
turning the larger circular brass plate (D).

The following are the advantages of this construction.
The size or, as it might be termed, spread of the microscope
gives great firmness, and also plenty " of room on the top
plates for the hands to rest upon during dissection.

The stand is comparatively low ; for in a single microscope
it is necessary to allow some considerable open space beneath
the lowest part of the stage, to admit an illumination from
below when the light in front is at a moderate elevation ;
but the unavoidable height is taken full advantage of in this
instrument by giving a range of three inches and six-tenths
to the rack of the triangular bar, and thus an object of con-
siderable thickness may be placed under the magnifiers.

The Binocular Addition.

The binocular arrangement for this microscope is based
upon the fact that, with single microscopes, when the object is
in focus the rays emerge parallel from the magnifier ; there is,
therefore, on this account nothing to prevent the use of both
eyes, one being close to the lens and the other some distance
off: the size of the object will be the same in each instance,
and the union of both images perfect.

This result is effected by the arrangement shown in figs.
48 and 49, half the aperture of the magnifier being allowed
to proceed direct to one eye, whilst the other half is reflected
by two l prisms (O, P) to the other eye. The two fields of view
are unequal, and the space included in a small circle only is
stereoscopic; but this result is no worse than the ordinary

108 IMPEOVED SINGLE MICEOSCOPE.

disadvantage of smallness of field common to all binocular
single microscopes of moderate power ; whereas the advan-
tages which arise from employing reflecting prisms for one
eye only are — much saving of light, only half the cost, and
great facility in turning the prisms out of the way when
one eye only is used.

The mounting required by this binocular arrangement is
as follows : — To a semicylindrical tube (S) are attached two
L prisms ; one (o) is a fixture, but the other will slide in or
out to suit the distances of different eyes ; the two together
are mounted upon a second arm (R) which fits immediately
above the first arm, carrying the magnifier, and, when both
eyes are used, the right-hand prism (o) should exactly cover
one half of the magnifying lens.

The arm (R) is stopped when it reaches this position ; but
it can be turned out of the way to any extent in the other
direction, and the fittings of the prisms allow of their easy
removal when they require cleaning.

The apparatus supplied with this form of single micro-
scope is very similar to that already described as belonging to
Darwin's instrument. The brass saucers with glass bottoms
are smaller ; but a gutta-percha tray, to which any object
may be pinned, is added for those specimens requiring dis-
section when in fluid ; and the piece of cork, loaded with
lead, is of an oblong form, and capable of being moved about
upon the top stage-plate.

The case for the complete instrument is made of maho-
gany, and nearly all the spare room inside is made use of —
a good-sized box for the dissecting instruments and apparatus
being fitted between the four pillars of the instrument, and
a narrow strip of wood for the magnifiers occupying the top
right-hand corner.

PATENT BINOCULAE MAGNIEIEKS. 109

Hand Magnifiers, with and without Stands.

No exact line of distinction can be drawn between this
class of instruments and 4he single microscopes already-
described, because the various forms of hand magnifiers are
frequently mounted upon some kind of stand ; the stage for
the object, if it require one, being extemporized in any way
that may prove most convenient for the occasion. As
examples of such arrangement, we give descriptions of the
following instruments : —

The Patent Achromatic Binocular Magnifiers.

The great advantage secured by this arrangement of lenses
is the employment of both eyes when using a magnifying
power which, with an ordinary lens, would confine the ob-
server to the use of one eye only.

Under these binocular magnifiers the object retains its
natural or, as it is sometimes termed, stereoscopic appearance ;
the light, upon which definition mainly depends, is doubled ;
the magnifying power is apparently much increased ; and the
binocular lens will, as a rule, give a better result than a
single one of double the magnifying power, without bringing
any strain whatever upon the eyes.

The three sizes figured in Plate XXVIII., the foci of
which are 7, 5, and 3 inches respectively, have also another
advantage, in addition to those already enumerated, in being
achromatic.

Directions for Use.

The lenses are plano-convex, and when in use the flat
sides should always be placed next the eyes ; and the mount-
ing is so contrived that this may be done when the mag-
nifiers are held in either hand.

110 PATENT BINOCULAE MAaMFIEKS.

As the distances of the eyes in different persons vary con-
siderably, these lenses have a sliding fitting at A, by which
the proper amount of separation may be obtained : the cor-
rect distance is easily determined by experiment ; but, as a
general rule, if a flat object appear convex, the lenses are
too near together; or when too far apart, the same surface
will appear concave, — the right distance being somewhere
between.

It will be found by many, and especially those who have
been accustomed to make constant use of a single lens, that
these binocular magnifiers require at first some rather careful
adjustment in holding them; for, besides the alteration of
distance already described, the eyes must be directed ver-
tically, upon the flat surfaces of the lenses ; and each eye
should see the object clearly defined. The former may be
tested by slightly altering the position of the lenses, and
watching whether the definition improve or otherwise ; the
latter by shutting each eye alternately, without moving the
head or the magnifier.

Stand for Binocular Magnifiers.

For really careful work, and when neither hand is at liberty
to hold the magnifier, some Stand is necessary, and one spe-
cially contrived for the purpose is shown in the Plate. The
lenses, after the ordinary handles (b) are unscrewed, can be
attached by means of the milled head (c) to the semicircular
arm (d) fitting by a pin into the tube (e), which slides upon
an inner rod (f), and this again rotates on a fitting at (g).

By this plan of mounting, the lenses can be placed in any
necessary position, or any requisite change may be effected
with ease and certainty. The adjustment for focus takes
place on the vertical rod (h), a slow movement being given

HAND MAGNIFIER. Ill

by the milled head (l) ; or if any great or quick alteration
be necessary, the milled head is thrown out of gear by
pressing in the lever (k), and the whole of the arm and its
fittings can be moved rapidly up or down to any position on
the rod (h), the mere act of releasing the lever fixing the
arm again immediately in any place.

The Binocular principle as thus applied to single magni-
fiers is of course limited in power, and no lenses of shorter
focus than 3 inches (the highest power here given) can be
used in this way with advantage ; but within their range it
is quite impossible to enumerate all the purposes for which
these lenses are peculiarly suitable.

Hand Magnifier.

The most generally useful form of hand magnifier, and one
which may be carried in the pocket, is shown in fig. 50 ; it is
provided with three lenses ($, #, #), the curvatures of which
are so arranged as to admit of the powers -p- 5q

being used separately or in combination,
in this way furnishing seven different mag-
nifying powers.

The lenses are mounted in tortoiseshell
frames together with a stop (d), which
may be used with advantage to cut off
some of the circumferential rays of the
high powers. The smallest lens (c) is » slze*

plano-convex, and, whether used separately or in combination,
should always be held with its flat side towards the object.
To use this or any other kind of hand magnifier with the
greatest advantage, the lens should be kept as near to the eye
as possible, the object being gradually brought into the
focus.

112

CODDINGTTON LENSES.

Fig. 51.

Stand for Hand Magnifier.

This form of hand magnifier may also be attached to a very
simple form of stand (fig. 51), which will convert it at once
into a very useful and inex-
pensive dissecting instru-
ment. To accomplish this,
the tortoiseshell frame of
the magnifiers is provided
with a square metal socket,
to fit a small arm with a
sliding fitting upon a short
stem, which is screwed at
its lower end into a small
circular base. The ordinary
forceps of the compound
microscope are also some-
times fitted to the rod of the
same stand, and the various
provisions for the movement
of the object, or for the adjustment of focus, are so evident
as to make any further description superfluous.

Coddington Lenses.

Coddington lenses (fig. 52) are also employed as hand mag-
nifiers; and although their

^ size.

foci are short, any one with
a steady hand can use
them easily. The mount-
ing shown here is of an
improved form, and meets
the following require-
ments. When closed (see

Kg. 52.

^ size.

DISSECTING INSTKUMENTS.

113

a), the surfaces of the lens are completely protected from any
injury they might receive whilst kept in the pocket. If the
Coddington be uncovered for use (see b), the convex surfaces
project beyond the mounting, and cut off no light in the ex-
amination of an opaque object : the opening or closing is
managed by moving the small milled head down or up the
central stem ; and this may be done with one hand, if the
mounting be attached to a braid hung round the neck, or
fastened to a waistcoat button.

The Coddington lenses vary in focus, and are mounted in
German silver, aluminium-bronze, silver, or gold ; one of the
last kind is represented at c.

INSTRUMENTS USED IN DISSECTION.

Knives.

Fig. 53.

114 DISSECTING INSTKTTMENTS.

The instruments which may be required in the process of
dissection are very various. Only a few are shown in figs. 53,
54, and these require but little further explanation than that
which is so evidently conveyed by the illustrations. The knife
lying across in fig. 53 is known as Valentine's: the purpose
for which it is contrived is that of making thin sections of soft
substances by means of two blades, the distance between
them, and, of course, the thickness of the section, being regu-
lated by means of two screws (a, b); the former passes
through both blades, the latter through one only, and pushes
against the other, by which arrangement the parallelism of
the blades can be maintained when they are more or less
separated.

At the left-hand end of fig. 53 is shown a needle-holder,
in which the needles may be stored at the end (d), or, when
required for use, can be held firmly at the other end by
pushing up the ring (c). The next illustration is that of
a hook ; and then follow curved and straight points which
merge into knives, commencing with blades of lancet-shape
and passing on to others of very various forms, any choice of
which must be entirely dependent upon the service for which
they are required.

It frequently occurs, when many of the subjects for
dissection are very minute, that any definite cutting is
impossible ; and various devices have to be adopted for the
separation of their various parts,. Sometimes when these are
sufficiently hard, they may be quickly and satisfactorily
detached by two needle-points, after the entire specimen
has been completely crushed ; this process must, however,
generally be done in some fluid and between two pieces of
glass, so that the results of more or less pressure may be
watched under the microscope.

DISSECTING INSTEUMENTS.

115

Scissors and Forceps.

The illustrations of the scissors and forceps in fig. 54 re-
quire even less comment than those of the knives. A form
of spring scissors is shown at the top ; and of the other three
pairs, one has straight and the others have either curved or
bent blades. Of the forceps, the pair nearest the scissors is

Fig. 54.

supplied with cutting points ; that at the left-hand end has
crossed points which close by their own spring, and are sepa-
rated by pressure lower down, exactly the reverse of ordinary
forceps, two pairs of which are shown in the other illus-
trations, only differing from each other in the shape of their
points.

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v.:v

WOOD-CUTTING M ICHIN1

n ;

snvn (^), ami i\\c brass plug is prevented (iivuui;: ivuml b\

i slot in us side, which fits i pin La the iron hole,

Che substance tivm which sections ire required is placed
in the central aperture of one

of the brass plugs, The fitting
of the materia] must be quite
tight, and it is best fco push
it up from tin* lower end of
the hole until it projects
about f^thofan bach from tin*
upper end, The brass plug,
when thus furnished, is placed
in thr machine, and pushed
down until the projecting end

Fig m

■■

!

rr y

\ gift

of the substance to be cut is about i<^<vi with the iron sur-
face («, a) of the machine,

in making i section (see fig, 57), tin* machine is held by
the fingers of the lefl hand, whilst the cutting instrument, in
the form of a large chisel, is grasped in the right band, with
the end of its handle pressed against tin* right shoulder, and
the level of its edge resting on tin- case-hardened surface of
the machine,

The chisel cannot be held too firmly . and whilst the right
hand keeps it in position, it should be pushed forward bj the
shoulder, the thumb of (in- lefi hand giving it at the same
time n side motion ; so that commencing to make the section
with tin* right-hand corner of the chisel, and completing the
cut with the other oorner, the cutting edge is carried steadilj
ami in an oblique direotipn across the material for the pur*
pose of making wy thin sections, this is the most advan-
tageous waj in which anj cutting edge can be employed,

Afin each out with the chisel, the brass plug (6) contain-

118

WOOD-CUTTING MACHINE.

ing the material is pushed up by the screw (<?), its milled
edge (g) being easily reached by a finger and thumb through
the two openings in the sides of the iron pillar. The screw
has fifty threads in an inch, and there are twenty divisions
on its bevelled edge ; therefore by using any side of the two

Fig. 57.

•A-th size.

openings just mentioned as an index, an exact upward move-
ment of one thousandth of an inch or more is easily made,
and can be repeated with certainty.

In making thin sections by any method, the chief difficulty
is in the preparation of a very fine, sharp edge to the cutting
instrument ; and we have adopted the particular form already

INSTEUMENTS USED IN MOUNTING. 119

described, not only because we know that it is used in the
trade with great success, but also because a careful carpenter
or cabinet-maker can easily sharpen the chisel, if any person
should find a difficulty in the matter.

Nearly all substances require some preparation before they
can be properly sliced : woods and horns have to be soaked
or sometimes even boiled in water before they become suffi-
ciently soft ; hairs are generally glued together in a bundle,
or they are drawn quite tightly into a piece of cork or soft
wood ; whilst other substances will not offer sufficient resist-
ance to the edge of the knife until they are hardened. Under
such a variety of circumstances no very definite rules can be
established; and, after all that might be done, there is no
instruction equal to that which may be obtained by being
thrown upon one's own ingenuity.

INSTRUMENTS USED IN MOUNTING OBJECTS.

Glass and other Slips.

The specimens or preparations intended to be kept as per-
manent objects are now almost universally placed on slips
of glass, wood, or tin, measuring three inches by one inch.
The first material is by far the most frequently employed,
and these slips are generally of plate or crown glass with the
edges ground ; but rough edges as left from the diamond-cut
are no disadvantage when the slides are covered with paper.

Cutting Diamond.

The glass slips are now so easily obtained at moderate prices
that it is seldom any person finds it advantageous to cut them
up for himself; but, if otherwise, a glazier's diamond (fig. 58)
is the best instrument for the purpose, and after a little prac-

120

THE DISK-CUTTEK.

tice in holding the diamond at the proper angle, there is no
difficulty in cutting the glass so as to leave rig. 58. Fig. 59.
a tolerably smooth edge.

Thin Glass.

It is an almost invariable practice to
cover the object with very thin glass ; this
material comes from the manufacturers in
small, angular-shaped sheets, which require
careful cutting by a diamond (fig. 59) to
bring them into suitable sizes and shapes,
the most frequent forms being circles or
squares of not less than three nor more
than nine tenths of an inch, with all the in-
termediate gradations in tenths ; there are,
however, frequent cases in which oblong
or oval pieces are required, and there is no
difficulty in obtaining them.

^ size.

The Disk-cutter.

An instrument for cutting small circles of
thin glass is shown in fig. 60, and is used in the
following manner : — The central inner stem (a)
is held in an upright position upon the piece
of thin glass by the forefinger of the left hand,
so that there is a gentle pressure at the small
base (e), which is padded with cork, and round
this the diamond (d) is rotated by turning the
tube (f) with the thumb and finger of the right
hand, from which also must be given at the
same time the necessary pressure for the cut of

j size.
Eig. 60.

a

MOUNTING-MEDIA. 121

the diamond. This operation may be repeated over a large
piece of glass, after which, a slight bending of the glass back-
wards and forwards will develope the various annular cracks ;
and when these are fairly through the thickness of the glass,
the disks or circles are released by carefully breaking away
in pieces the thin glass surrounding them.

By means of the transverse sliding arm (b) the diamond is
readily adjusted for cutting circles of any required diameter;
and the small milled head (c) is used to fix the arm securely
at any part of the range.

The same diamond may also be used in cutting the thin
glass into squares, for which purpose some small wooden rules
only are required.

The Mounting of Objects.

There are various ways of preserving microscopic objects:
many may be mounted, as it is termed, " dry," which implies
that the specimen is merely covered with a piece of glass as
a protection from injury or dust ; or when the specimen will
not bear any pressure, it is usual to employ a ring of card or
paper to carry the thin glass. The dry plan is usually
adopted with sections of bones or teeth, hairs, feathers,
scales of insects, sections of recent woods, fructification of
ferns, some of the Diatomacese, zoophytes, small shells, and
many other objects.

Mounting -Media.

The purpose for which preparations are mounted in a
medium of some kind, is either to preserve the specimen or
to make it more transparent. But in many cases both of
these results- may be required; for instance, many of the
Diatomacese, spicules and gemmules of sponges and Gorgo-
nias, sections of Echinus-spines, some of the Foraminifera, &c.,

122

CANADA BALSAM.

cannot be well examined by transmitted light until they are
rendered more transparent by some medium ; whilst other
preparations require, in addition, the preserving property of
the material, such as minute insect-dissections, Acari and
parasites, the finest injected preparations, many crystals and
polarizing objects.

It is therefore impossible . to establish any definite rules ;
but it will be found that the best course of proceeding may
easily be determined by experiment.

Canada Balsam.

Of the various media, Canada balsam is perhaps more
frequently employed than any other in mounting objects; it
is generally used in a semifluid state, or, if necessary, it
may be made more fluid by mixing turpentine or chloroform
with it.

The material itself, although generally supplied in a
bottle such as is shown in fig. 62, is best kept for general

Fig. 61. Pig. 62.

use in a bottle with a ground cap (see fig. 61), and the

BEASS TABLE AND LAMP.

123

quantity that is required for use may be taken out by the
glass dipping-rod.

Brass Table and Lamp.

With Canada balsam some amount of heat is almost
always required, either previously to the object being placed
in it, or afterwards, to harden it sufficiently, and prevent
any alteration or shifting of the specimen ; under such cir-
cumstances it is best to place the slide upon a small brass
table, with a spirit-lamp underneath (see fig. 63) ; in general

Pig. 63.

\ size.

only a slight warmth is required, and sufficient heat will be
communicated by the brass top of the table when the flame
of the spirit-lamp is quite small. If, however, under any
circumstances any greater heat be required, the object may
be placed so as to be over the small opening near the end
of the table, with the lamp immediately underneath. It
must nevertheless be borne in mind that air-bubbles are
produced by the boiling or overheating of the balsam, and

124 DEANE'S MEDIUM.

also, from the same cause, the balsam will turn of a brown
colour and become very brittle.

Page's Forceps.

Another method of warming a slide is by merely moving

it backwards and forwards across the flame of a spirit-lamp ;

and if by this plan the glass should become too warm to

hold in the fingers, Page's forceps (fig. 64) maybe employed.

The wood of which the

_ _ _ Pig. 64.

sides of these are made

will not conduct the heat,
so that whilst the slide
is placed at one extremity
and submitted to any amount of temperature, the other
end of the forceps may be held in the hand with impunity.
The sides of the forceps are separated by pressing the pins
(&, b); and to let the slide cool without coming in contact
with any cold surface which might crack it, the forceps,
together with the slide, will stand upon a table in the posi-
tion shown by the drawing.

Some of the conditions connected with the employment of
Canada balsam, such as its high refractive power, the heat
required in its use, and its not amalgamating with an object
unless perfectly dry or saturated with a solvent, entirely
prevent its use in the preservation of many objects; there
are, however, other and valuable substitutes.

Dearie's Medium.

Deane's gelatine medium requires much less heat than
Canada balsam, and is admirably suited to objects in a
moist state. The following directions for its use are

FAEBANTS' MEDIUM. 125

given by Mr. Deane upon the bottles in which he usually
supplies it : —

" Set the bottle in water sufficiently hot to liquify the
medium perfectly, and then with a piece of quill-glass tube,
made warm by being immersed at one end in hot water,
take up a portion of the medium, and drop it on the object
previously arranged in its place on a warm glass-slip, and
then cover with its appropriate thin glass, also made warm,
lest the jelly set before the operation is completed. A
pewter hot-water plate answers well for warming the glass.

"Objects to be mounted in this medium should be quite
moist and free from air-bubbles ; some may be floated into
their places in water, or weak spirit and water, the super-
fluous moisture being drained away by tilting on end, and
then with bibulous paper or a cambric handkerchief. Care
must be taken to prevent the object becoming dry before
the medium is put to it."

Farrants' Medium.

Farrants' medium is composed of gum and glycerine: it
requires no warmth whatever in its use, and is usually kept
in a semifluid state ; but a small quantity of it will become
tough on exposure to the air for about an hour, if it be
made in the right proportions.

This material is admirably suited for many kinds of prepa-
rations, more especially for moist specimens, and for small
objects living in either air or water.

Glass and other Cells.

The three substances already mentioned, Canada balsam,
Deane's medium, and Farrants' medium, form not only the
materials in which the objects are mounted, but also the

126

CELL-MACHINE.

Cell-machine.

The cells of either or
both of the materials just
mentioned are best made
by means of the instru-
ment shown in fig. 66.
The slide is held on the
small circular plate by the
two springs (a, #), and
whilst the right hand, rest-
ing on the wood block,
holds a camel's-hair pencil
containing the cell-mate-

Fig. 66.

Fig. 65.

cements for keeping the covering (thin glass) in its proper
place upon the slide. This cannot be the case with many
mounting materials, of which the following may be men-
tioned as possessing valuable preservative properties : — dilute
spirits of wine, distilled water, Goadby's fluid, Thwaites's
fluid, a solution of chloride of zinc, and castor-oil.

To confine such materials as these, it is almost always
necessary to make a cell of some substance upon
which they cannot act as solvents, and a mixture of
asphalt and gold-size may generally be employed
for this purpose, or either of them may be used
alone ; but the former often proves too brittle, and
the latter takes some time to harden. Asphalt or
gold-size is generally supplied in bottles the same
as that shown in fig. 62 ; but the gold-size is
considered by some persons to be more convenient
for use when kept in a small metal tube shown
in fig. 65.

'J>*

j| size.

^ size.

GLASS CELLS.

127

rial, the circular brass table is spun round by quickly passing
a finger of the left hand over the small milling (c) underneath ;
a thin ring of the material is thus painted on the slide, and
the thickness of the cell is regulated by the number of the
coats, each of which should be allowed some time to dry
before another is added.

After the cell is made and is sufficiently hard, the object,
together with the mounting-material, are placed in it and
covered with a piece of thin glass ; the superfluous mounting-
material is soaked up by blotting-paper or some other means,
and a few additional coats of the cell-material are added at
and over the edge of the covering-glass to permanently seal
the preparation.

Glass Cells.

The method of cell-making just described is only available
with objects of very moderate thickness; but if a clean,
secure, and moderately deep cell be required, there is no
material equal to glass.

Fig. 67.

A few of different sizes are shown in fig. 67 : c andy have
solid bottoms, and e and i are made from the microscopic

128

LABELS.

thin glass ; the others are sections of different kinds of glass
tube, and may be had of any thickness.

All glass cells are generally cemented to the glass slips
with marine glue. This material is usually supplied in small
pieces packed in a bottle similar to fig. 62 ; but, when re-
quired for use, thin slices are placed between the cell and
the glass slips, and whilst exposed to considerable warmth
the cell is pressed down hard, so as to come in as close con-
tact as possible with the glass slip. The superfluous marine
glue is easily removed with a knife when the slide is cold.
Although marine glue is invariably the best cement for
fastening the cell to the glass slip, gold-size and asphalt will
answer perfectly in securing the thin glass cover and sealing
up the cell.

Labels.

When a slide requires covering with paper after the object
has been mounted, the labels (fig. 68) may be used to give
a neat finish; and the gummed square labels (fig. 69), ruled
with six faint lines, give considerable space for writing the
name or any other particulars connected with the prepara-
tion. If the object be not covered with paper, it is usual to
write upon the unoccupied spaces of the glass slips with the
diamond shown in fig. 59.

Pig. 68.

True size.

Pig. 69.

True size.

COLLECTING-BOTTLES.

129

Small Glass Bottles.

For the preservation of some kinds of microscopic objects
Dr. Guy has recommended the use of small flattened glass
bottles (fig. 70), and they may answer well under some cir-
cumstances, whilst in other cases the irregularities and
imperfections of the glass will be found to obstruct most
annoyingly the best definition of an object-glass.

Kff. 70.

Pig. 71.

Fig. 72.

In the dissection or other preparation of microscopic ob-
jects, reagents or other fluids may be occasionally required
in small quantities, and in such cases bottles similar to the
one shown in fig. 71 may be used. The tapering mouth-
piece (which is removable for the introduction of the fluid
into the bottle) only permits a small quantity to escape at a
time, and the ground-glass cap, when fitted over the neck,
excludes dust and prevents evaporation.

The bottles shown in fig. 72 are used in collecting speci-
mens, for which purpose they are exceedingly convenient;
three sizes are represented in the illustration, which so fully
explains their capabilities that any further description would
be superfluous.

K

130

CABINETS FOE OBJECTS.

Case for Instruments and Materials.
Fig. 73.

The materials and instruments required at different times
in the preparation of microscopic objects are so numerous
as frequently to run the chance of being mislaid. Such a cir-
cumstance might be most inconvenient; it is therefore a
common plan to have them all arranged and packed in a
mahogany case : the one shown in fig. 73 is of intermediate
size, and illustrates the fitting of the top tray only ; but this
lifts out, because the space underneath is also fitted up for
the reception of various articles.

Cabinets for Microscopic Objects.

The cabinets or boxes for holding microscopic objects are
very various ; but the best are those in which the specimens
lie flat, as this position not only materially helps to preserve
the preparations which are in fluid, but it greatly assists in
the arrangement of a collection; and an object is much
more easily found when lying flat than when placed on edge.

CABINETS FOE OBJECTS.

131

Pig. 74.

|th size.

One of the best cabinets, with twenty-eight drawers, to hold
1000 objects, is shown in fig. 74 ; each drawer is divided into

k2

132

QUARTO CABINET.

three compartments by two partitions, which prevent the
slides shifting backwards or forwards and becoming mixed
when the drawers are moved in and out. The bottom of
each drawer is made of strong paper, a material much
superior in such a position to wood, which has always a ten-
dency to warp and twist the work. Each drawer has in front
two porcelain labels, which can be written upon in ink or
pencil ; and both are easily rubbed out, so that any record of
the classification is readily made or altered.

Similar cabinets for objects are also made of a smaller size
than that just described ; but the workmanship of all has
necessarily to be of the best kind, and when two of the
same size differ in price it is because of the amount of work
on the outside case, or the quality of the wood.

Quarto Cabinet,

Fig. 75.

^th size.

QUAETO CABINET.

133

The " Quarto Cabinet " for objects, when closed, somewhat
resembles a book composed of five thick leaves, which at a
short distance from the edges are recessed on both sides
(see fig. 75). The two outside surfaces are covered with
leather ; but the inner ones are lined with velvet, and narrow
elastic, stretched rather tightly, is pinned down at intervals,
so as to hold each object securely and separately (see fig.
76). This case will contain 144 specimens; and besides the
choice that this plan admits of keeping the objects flat or on
edge, according to the position in which the case is kept,
the slides require no packing of any kind before they can be
carried about, the elastic holding each object firmly in its
place, and the leaves being fastened together by two double
hooks, which, turning on a centre in the edge of the middle
leaf, pass over two pins on the outside leaves.

Fig. 76.

|th size.

134 MICEOSCOPIC OBJECTS.

All the cases, smaller than those already described for
containing objects, are provided with racks, and the better
ones are made of mahogany to hold six dozen objects; but
the commonest are made of deal or card, covered with cloth,
and the usual sizes are those containing two dozen, one
dozen, or half-a-dozen.

Microscopic Objects.

Specimens for the Microscope, so prepared as to be per-
manent, have now become a very large and important adjunct
to the Microscope. The subject is too extensive to enter upon
in this treatise ; but we may mention that we have been en-
gaged for some time past in making complete lists of micro-
scopic objects : two or three of these are already published,
and it is probable that we may complete the catalogue in the
course of a year or so.

LIST OF ILLUSTRATIONS.

Fkontispiece.

Podura Scale, as shown by five different Object-glasses with No. 3 Eye-
piece.

Plate II.

First-class Microscope. Fig. 1, No. 1 Stand. Fig. 2, Diaphragm of Stage
and fitting. Fig. 3, Draw-tube of Body.

Plate III.
First-class Microscope, No. 2 Stand. Woodcut of Lever Stage.

Plate IY.
Second-class Microscope. Fig. 1, No. 3 Stand. Fig. 2, Plain Stage to ditto.

Plate Y.

Eyepieces, Nos. 1, 2, 3. Fig. 4, Erecting-glass. Eight Object-glasses,
viz. 2-inch, lj-in., f-in., T\-m., |-in., i-in., §-in., ^j-in.

Plate YI.

Achromatic Condenser, first- and second-class, and with and without dia-
phragm. Right-angle Prism; "Wenham's Parabolic Reflector; Na-
chet's Prism ; Amici's Prism ; Instruments for measuring apertures of

Object-glasses.

Plate YII.

The Podura Scale as a test ; Pleurosigma quadratum, as shown by oblique
illumination and by Achromatic Condenser ; Navicula rhomboides, as
illuminated by Amici's Prism.

Plate YIII.

Nobert's Lines, as shown by § Object-glass and No. 3 Eyepiece, with
oblique illumination.

136 LIST OF ILLUSTRATIONS.

Plate IX.

Fig. 1. A, B, C, and D, Pleurosigma formosum, as seen under various illumi-
nations. Fig. 2, Pleurosigma formosum, as seen with a low power
under oblique illumination. Figs. 3, 4, 5, 6, and 7, cast skin from
head of Silkworm, to show use of Disk-revolver.

Plate X.

Apparatus used when objects are illuminated from above: Side Conden-
sing Lenses and Reflector ; Lieberkuhns ; Dark Wells and Holder ;
Forceps ; Disk-revolver.

Plate XI.

Splinter of Lucifer-match, and Podura Scale, illuminated from above.

Plate XII.

Tarsus of Spider (Tegenaria atrica) and Pigeon's feather, illuminated from

above.

Plate XIII.

Arachnoidiscus Japonicus on marine plant, as seen by Lieberkuhn illumi-
nation with 2j-in. object-glass.

Plate XIV.
Polycystina from Barbadoes, as shown by " Dark-field " illumination.

Plate XY.
The new Halfpenny Coin of the Realm, as seen under the Erecting-glass.

Plate XVI.

Polarizing Apparatus, as applied to the Microscope : Mcol's Prisms ; Black
Glass and Thin Glass Polarizers; Selenite Plates; Double-image
Prisms ; Tourmaline ; Crystal fittings, &c.

Plate XVII.

Objects seen under polarized light : Crystals of sulphate of copper and mag-
nesia ; Rhinoceros horn, oblique section ; tendon of Ostrich, section ;
Crystallized salicine.

Plate XVIII.
Illustrations of experiments with Double-image Prisms, &c.

LIST OF ILLUSTRATIONS. 137

Plate XIX.
Structure of the Scales of Lepisma saccharina.

Plate XX.
Wenham's Binocular Body, for Achromatic Microscope.

Plate XXI.

Live Boxes; Glass Troughs; Compressors; Frog Plate; Glass Plate with
ledge ; Glass Tubes.

Plate XXII.

Micrometers ; Camera Lucida ; Quekett's Indicator ; Leeson's Goniometer ;
Double and Quadruple Nosepieces.

Plate XXIII.
Lamps and Revolving Table for the Microscope.

Plate XXIV.

Head and thorax of Demodeoo folliculorum ; and stinging-hairs of Nettle, as
seen under the one -twentieth Object-glass.

Plate XXV.
Mahogany Case for Large Best Microscope.

Plate XXVI.
Apparatus Boxes, as packed for a complete Large Best Microscope.

Plate XXVII.

Best Student's Microscope and Apparatus, in case; movable Stand for
Microscope and Lamp.

Plite XXVIII.
Patent Achromatic Binocular Magnifiers and Stand.

LIST OF WOODCUTS.

FIG. PAGE

1. Front of object-glass to explain adjustment of high powers . . 13

2. Diagram to show the best relative positions of lamp and [_ prism 14

3. Sketch of feather, showing the component parts 31

4. Diagram of form and structure of Arachnoidiscus Japonicus . . 33

5. „ to illustrate experiment with Lepisma-scales .... 51

6. „ of drop of water in live-box 55

7. „ to show position of eye when Camera Lucida is used . 61

8. „ of crystals, to explain use of Leeson's Goniometer . . 66

9. Maltwood's Finder 67

10. Diagram of one square of Maltwood's Finder 68

11. Popular Microscope : inclined position of stand ...... 76

12. „ „ vertical ditto 77

13. „ „ horizontal ditto 79

14. „ „ diaphragm of stage 79

15. „ „ side condenser on stand 79

16. „ „ forceps of stage 79

17. „ „ glass plate with ledge, and thin glass cover . 80

18. „ „ pliers 80

19. „ „ lower removable tray 81

20. „ „ upper ditto, for apparatus 81

21. „ „ mechanical stage 83

22. „ „ achromatic condenser . 84

23. „ „ Lieberkuhn for 1-inch object-glass ... 84

24. „ „ dark well 84

25. „ „ section of parabolic reflector 84

26. „ „ parabolic reflector 85

27. „ „ polarizing apparatus 86

28. „ „ diagram to explain use of Camera Lucida . 87

29. „ „ micrometer ^

LIST OF WOODCUTS. 139

PIG. PAGE

30. Popular Microscope : live box 88

31. „ „ glass trough , 88

32. Educational Microscope, as taken out of its case 89

33.

33a.

34.

35.

36.

ft

when set up for use . 90

supplementary stage . . 90

diaphragm of stage 91

forceps of stage 91

tray of extra apparatus 92

37. Universal Microscope 94

38. „ „ diaphragm of stage 95

39. „ ,. object-glasses -96

40. „ „ eyepieces 96

41. „ „ forceps of stage, pliers, and glass plate . 97

42. „ „ box of extra apparatus 98

43. „ „ mechanical stage 99

44. „ „ combined body 99

45. „ „ binocular body 100

46. Darwin's single Dissecting Microscope 102

47. „ „ saucers and other apparatus 103

48. Improved single Dissecting Microscope 105

49. „ „ as seen from above . . 106

50. Hand magnifiers in tortoiseshell Ill

51. „ „ on stand 112

52. Coddington lenses 112

53. Dissecting instruments ; knives, points, hooks, and needle-holder 113

54. „ scissors and forceps 115

55. Quekett's forceps 116

56. Wood-cutting machine 117

57. „ in use 118

58. Diamond for cutting thick glass 120

59. „ for writing or cutting thin glass 120

60. Disk-cutter for thin glass 120

61. Stoppered bottle for Canada balsam 122

62. Ordinary bottle for mounting materials 122

63. Brass table and lamp for mounting objects 123

64. Page's forceps 124

65. Metal tube containing gold size 126

140 LIST OF WOODCUTS.

FIG. PAGE

66. Cell-machine 126

67. Glass cells 127

68. Labels for covering objects 128

69. Name-labels . 128

70. Flattened glass bottle for preserving objects . 129

71. Small stoppered bottle for reagents 129

72. Small collecting-bottles 129

73. Case for mounting materials and instruments . \ . . . . . 130

74. Eest cabinet for 1000 objects . .... , . . . . . . 131

75. Quarto cabinet for objects, outside .132

76. „ „ inside .......... 133

INDEX.

A. PaSe

Aberration —

Chromatic 12

Spherical 12

Achromatic Condenser : —

First-class 10

Second-class 11

Third-class 83

Achromatic Microscope : —

Construction of 1

First- and Second-class 1

Third-class 73

Fourth-class 93

Binocular 48

Adjustment for high powers. ... 13, 16

Amici Prism 20

Aperture —

of Object-glasses 4, 17

measurement of 18

Apparatus : —

Tray, third-class 92

Box, fourth -class 98

B.

Balsam, Canada 122

Bases of Microscopes, revolving and

folding 3

Binocular Microscope, Wenham's . . 48

Binocular body, fourth-class 101

Binocular Single Microscope .... 107

Binocular Hand Magnifiers ...... 109

Bottles, small glass —

for mounting objects 129

for reagents 129

for collecting objects 129

Bull's-eye Condenser 23

C. ^e

Cabinets for objects 130

Camera Lucida : —

First- and Second-class 61

Third- and Fourth-class 86

Canada balsam 122

Cases for Microscopes : —

First-class 70

Second-class 71

Popular Microscope 81

Cases for mounting-materials .... 130
Cast skin, head of Silkworm .... 28

Cell-machine 126

Cells of glass, for mounting objects 127

Coddington Lenses 112

Collecting-bottles 129

Combined body, of Universal Mi-
croscope 99

Compressor —

Lever 57

Reversible 58

Wenham's 58

Condenser, Achromatic : —

First- and Second-class 10

Third-class 83

Condenser, Bull's-eye 23

Condenser, side 23

Construction of Achromatic Micro-
scope 1

Crystals —

of Sulphate of Copper and

Magnesia 39

to show rings 47

D.
Dark-field illumination 34

142

INDEX.

Page

Darker's selenite plates 40

Dark wells : —

First- and Second-class 25

Third- and Fourth- class 84

Darwin's single Dissecting Micro-
scope 102

Deane's medium 124

Demodex folliculorum 6

Diamond, cutting , 119

Diaphragm of Stage : —

First- and Second-class 9

Third-class 91

Fourth-class 95

Diaphragm of Achromatic Con-
denser 11

Directions for use of Microscope . . 8

Disk-cutter for thin glass 120

Dissecting Microscope : —

Darwin's 102

Improved 105

Dissecting-instruments 113

Draw-tube 4

E.

Educational Microscope 89

Erecting-glass 36

Errors in workmanship 22

Extra apparatus, fourth-class .... 98

Eyepieces , 3

of Universal Microscope .... 96

F.

Farrants' medium 125

Feather of Pigeon 31

Finder, Maltwood's 66

Flatness of field 17

Forceps for stage : —

First- and Second-class 26

Third-class 79

Fourth-class 97

Forceps —

Three-pronged 27

Page's 124

Quekett's 116

Frog plate 60

G.

Glass cells .... 127

Page

Glass, thin 120

Glass, slips 119

Goniometer, Leeson's 65

H.

Halfpenny, the new 37

Hand Magnifiers 109, 111

Binocular 109

Stand for 112

High powers, adjustment of 13

I. J.

Illumination —

oblique 20

from above 23

dark-field 34

Indicator, Quekett's 65

Jackson's Micrometers ,63

Juncas conglomeratus, section of . . 62

K.

Kellner's Eyepiece 96

Knife, Valentine's 114

L.

Labels for covering objects 128

Lamps for Microscope 68

Lamp and table for mounting ob-
jects 123

Leeson's Goniometer 65

Lenses —

Side condensing 23

Coddington 112

Lepisma saccharina, scales of .... 49

Lever compressor 57

Lieberkuhns 25

Light, management of 8

" Lined" objects as tests 19

Lines, Nobert's 19

Live-boxes 55

Live-box, " Screw" 57

Third- and Fourth-class 88

Lucifer-match, splinter of 29

M.

Machine —

for making sections of wood . . 116
for making cells 126

INDEX.

143

Page

Magnifiers, Hand 109

Patent Achromatic Binocular . 109

Magnifying power —

linear 4

to ascertain 63

Maltwood's Finder 66

Management of light 8

Measurement —

of objects by Camera Lucida. . 62

by micrometers 63

Mechanical stage —

of Popular Microscope 83

of Universal Microscope .... 99

Medium —

Deane's 124

Farrants' 125

Micrometers, Jackson's 63

Microscopes, Achromatic : —

First- and Second-class 1

Third-class 73

Popular 73

Educational 89

Universal 93

Microscopes, single : —

Darwin's 102

Improved 105

Microscope Cases : —

First-class 70

Second-class 71

Microscope Lamps 68

Microscope Table 69

Microscopic objects 134

Mirror, use of 2, 9

Mounting of objects 121

N.

Nachet's Prism 20

Navicula rhomboides 19

Nicol's Prism 38

Nobert's Lines 19

Nosepiece —

Double 65

Quadruple 65

O.

Object-glasses 3

Focal length of 4

Aperture of 4

Page

Object-glasses, list of first-class. ... 5

One-twentieth 6

Popular series 74

Universal series 96

Tests for 12

Objects —

Mounting of 121

Labels for 128

Cabinets for 130

Quarto Cabinet for 132

for Microscope . 134

Oblique illumination 20

Opaque objects 23

Opaque Disk-revolver 27

P.

Page's Forceps . . . , 124

Parabolic Reflector, Wen ham's : —

First- and Second-class 34

Third-class 84

Pigeon, feather of 31

Pleurosigma quadratum 19

Pleurosigma formosum 2L

Podura, scale of 14

Polarized light 37

Polarizing apparatus, third- and

fourth-class . 86

Polycystina from Barbadoes 36

Popular Microscope 73

Prism —

Right-angle 10

Amici's 20

Nachet's 20

Q.

Quadruple Nosepiece 65

Quarto cabinet for objects 132

Quekett's Indicator 65

Quekett's Forceps 116

R.

Reflector, side silver 24

Reversible compressor 58

Right-angle prism 10

S.

Scale of Lepisma saccharina 49

Scale of Podura - 14

144

INDEX.

Page

Screw —

Universal 6

Live-box 57

Selenite plates 40

Darker' s 40

Side silver reflector 24

Side condensing-lenses 23

Third-class 79

Silkworm, cast skin of head 28

Single dissecting Microscope : —

Darwin's 102

Improved 105

Binocular 107

Slips, glass 119

Spider, tarsus of 30

Splinter of lucifer-match 29

Stage of Microscope 2

Stinging-hairs of Urtica dioica. ... 8

Substage 2

Sulphate of copper and magnesia,

crystals of 39

T.

Table for Microscope 69

Table and lamp for mounting ob-
jects 123

Tarsus of Spider 30

Page

Tests—

for object-glasses 12, 23

" Lined," objects as 19

Thin glass .....' 129

Thin glass disk-cutter 120

Tourmalines 43

Trough , 56

U. V.

Universal screw 6

Universal Microscope ...... 93

Mechanical stage of 99

Combined body of 99

Binocular body of 101

Urtica dioica, stinging-hairs of . . . . 8

Use of Microscope, directions for. . 8

Valentine's knife 114

W.

Wenham's mode of measuring aper-
ture 18

Wenham's Parabolic Reflector. ... 34

Wenham's Binocular Microscope . . 48

Wenham's Compressor 58

Wood-cutting machine 116

Workmanship, errors in 22

THE END.

Printed by Taylor & Francis, Red Lion Court, Fleet Street.

Description of Frontispiece.

THE PODURA-SCALE.

The drawings of this object already published are very numerous,
and to the same extent various and incorrect. The sources of error
may easily be recognized by a practised observer as being generally
due to a want of care in those adjustments of the illumination, or of
the object-glass, which are fully explained in this treatise under the
directions for the use of the higher-power object-glasses.

The Podura- scale is an admirable test, as it is only under the right
conditions in every respect that it appears well-defined and brilliant.
Nearly all the markings on the scale should appear black, but with
a light median line or spot near the top ; and each marking should be
distinctly separate, except where it is connected longitudinally with
other markings by a faint spreading line which is not perceptible
with the lower powers.

There are at least five or six species of Podura which have scales of
somewhat similar character to the one drawn here, but no good
description of them has yet been published. The insects which have
these particular scales are about j^th of an inch long, and of a dark
lead- colour ; they walk slowly, but can jump a very considerable
distance ; and they may be found under pieces of wood or stone in
outhouses which are slightly damp.

There is some difficulty in obtaining the best strongly-marked
scales. An insect apparently the same as that just described is to be
found almost everywhere, and will furnish scales which only differ
from the right one in the markings being less brilliant and distinct.
It is generally necessary to search carefully in drier parts of the
locality inhabited by the insect ; and under any circumstances it is
advisable to make a careful comparison of specimens before being
confident of having that scale which forms the best test.

The specimen here figured is a small one, but the markings are of
the full size. Some further particulars of this object are given at
pp. 14-17, and there are other drawings of a portion of the scale in
Plate VII. figs. 1-6.

Description of Plate II.
FIRST-CLASS MICROSCOPE.

Fig. 1. Large Best, or No. 1, Stand.

A. Joint for moving the Instrument generally, to inclined or

other positions.

B. The Body, supported by a strong limb from the joint.

C. Milled head, for quick motion of body.

D. Milled head, for fine adjustment of focus.

F. Ledge of Top-stage, upon which the object is placed.

G. Sliding-piece, to clamp the object when necessary.

H. Milled head of Stage, for horizontal movement of object.
I. Milled head of Stage, for vertical movement of object.
K. The Mirror.

L. Semicircle, in which the mirror swings.
M. Triangular bar, upon which the mirror-fitting slides.
N. A revolving fitting, on which the whole of the instrument

above the base will turn.
O. The draw-tube of the body, or when drawn out as in fig. 3.
P. The Diaphragm under the stage, shown also in fig. 2.
Fig. 2. R. The fitting-tube of diaphragm.

S. A short tube to receive the diaphragm, and attached by

bayonet-catch to the bottom plate of stage.
T. A cylindrical fitting, to receive all the kinds of illuminating

apparatus which are required under the stage.
U. Milled head, for movement of cylindrical fitting up or down

by rack and pinion.
W. Lengthening- arm to mirror, chiefly for supplying the means

of very oblique side illumination.
Y. Small socket, in which the forceps fit.
Fig. 3. The draw-tube of body.

(All these drawings are to a scale of one-third the true size.)

PLATE II

G^tt^s (^lap Q^/^^^tzd^

FIG.l.

H°l STATO.

FIG. 2

FIG. 3.

3rd SCALE.

Description of Plate III.

FIRST-CLASS MICROSCOPE.

Small Best, or No. 2, Stand.

A. Joint for moving the Instrument generally, to inclined or

other positions.

B. The Body, supported by a strong limb from the joint.

C. Milled head, for quick motion of body.

D. Milled head, for fine adjustment of focus.

F. Ledge of Top-stage, upon which the object is placed.

G. Sliding-piece, to clamp the object when necessary.

H. Milled head of Stage, for horizontal movement of object.

I. Milled head of Stage, for vertical movement of object.

K. The Mirror.

L. Semicircle, in which the mirror swings.

M. Stem tube, upon which, the mirror-fitting slides.

N. A revolving fitting, on which the whole of the instrument
above the base will turn.

O. The draw-tube of the body, figured when drawn out in
Plate II. fig. 3.

P. The Diaphragm under the stage (for detail, see Plate II.
fig. 2).

T. A cylindrical fitting, to receive all the kinds of illuminating
apparatus which are required under the stage.

U. Milled head, for movement of cylindrical fitting up or down
by rack and pinion.

W. Lengthening- arm to mirror, chiefly for supplying the means
of very oblique side-illumination.

Y. Small socket, in which the forceps fit.

(This drawing is to a scale of one-third the true size.)

A

The accompanying figure shows a
u lever stage " which is sometimes sup-
plied with the First-class Instruments.
It does not differ from the stage already
described, so far as concerns the hold-
ing or rotation of an object; but its
peculiarity consists in the arrangement
for moving an object in all directions
(instead of only opposite rectilinear
ones) by means of a lever, which when
in use is held firmly at its extremity
(A) by the right hand.

PLATE III

*& &€a^ Q^^&zwme/Z

^° 2 STAND

3rdSGALE.

Description of Plate IV.
SECOND-CLASS MICROSCOPE.
The Best Student's, or No. 3, Stand.

Fig. 1. The Stand, with actions to the stage.

A. Joint for moving the instrument generally, to inclined or

other positions.

B. The Body, supported by a limb from the stage.

C. Milled head, for quick motion of body.

D. Milled head, for slow motion of body.

F. Ledge of Top-stage, upon which the object is placed.

G. Sliding-piece, to clamp the object when necessary.

H. Milled head of stage, for horizontal movement of object.
I. Milled edge of stage, for vertical movement of object.
K. The Mirror.

L. Semicircle, in which the mirror swings.
M. Stem tube, upon which the mirror-fitting slides.
O. The Draw-tube of the body, figured when drawn out in

Plate II. fig. 3.
P. The Diaphragm under the stage (for detail, see Plate II.

fig. 2).
W. Lengthening- arm to mirror, chiefly for supplying the means

of very oblique side- illumination.
Y. Small socket, in which the forceps fit.
Fig. 2. Plain stage, for a Best Student's Stand.

F. Ledge, upon which the object is placed.

G. Sliding-piece, with spring at the end, to give a slight resist-

ance when the object is moved sideways by the fingers,

or to serve as a clamp.
P. The Diaphragm under the stage (for further detail, see

Plate II. fig. 2).
R. A short tube, to receive the Diaphragm or any other pieces

of apparatus required under the stage.
Y. Small socket, in which the forceps fit.

(These drawings Rre to a scale of one-third the true size.)

ff//.

PLATE IV.

¥° 3 STAND.

fig-. 1

fig-, z.

iT.aSCALE

Description of Plate V.

Nos. 1, 2, 3. Eyepieces of different magnifying powers, No. 1 being
the lowest ; there are, besides these, two higher powers,
viz. No. 4 and No. 5, but they do not differ in appearance
from No. 3.

Fig. 4. The Erecting-glass. This is generally used in combination
with the §rds object-glass, for the purpose of showing an
object in an erect or natural position, and also for giving
the great range of magnifying power of from 5 to 100
linear (for description, see p. 36).

The eight figures at the bottom of the page represent
different object-glasses : the focal length is engraved on
each, and the Joth is the highest power we make ; but the
2-inch is not the lowest, as we frequently supply a 3-inch.

PLATE V.

N°l.

N°2.

N93.

WWW< V

Vi! .

Ilj^gf^i ffn r ilc'lifillif T|l|iin f»i| n^^^iwm^ ' ''% % I i?fiiBi:iii!if«iiii itiffliii"

"ll lit ] ii , ilf

FULL SIZE

S.Allen. Sc

Description of Plate VI.

Apparatus for illuminating objects from below, and two methods of
measuring the apertures of object-glasses.

Fig. 1. The Achromatic Condenser, plain.

Fig. 2. The Achromatic Condenser with perforated diaphragm, a

front view of which is shown in fig. 6.
Fig. 3. The Diaphragm under the stage, employed also in centering

the achromatic condenser.
Fig. 4. Right-angled Prism, used instead of mirror when a more

perfect reflexion is required.
Fig. 5. A small aperture of the Stage- diaphragm, as it appears in the

field of view when the achromatic condenser is out of

focus and not central.
Fig. 6. The Diaphragm of the Achromatic Condenser, front view

(see side view, fig. 2) .
Fig. 7. A small aperture of the Stage-diaphragm, as it appears in the

field of view when the achromatic condenser is central

and in focus.
Fig. 8. Section of Parabolic Reflector, showing the directions in

which it reflects the light, for dark-field illumination.
Fig. 9. Section of Nachet's Prism, showing the directions in which

it reflects the light, for oblique illumination.
Fig. 10. Nachefs Prism, as mounted for use.

Fig. 11. Amici's Prism, for oblique illumination, as mounted for use.
Fig. 12. The ordinary arrangement for measuring the aperture of

object-glasses.
Fig. 13. The Parabolic Reflector, as mounted for second-class micro-
scopes.
Fig. 14. Wenham's method of measuring apertures by a divided card.
Fig. 15. The Parabolic Reflector, as mounted for first-class micro-
scopes.

~TJ T A rP TP T7~T

i LA 1 J±i vl.

FIG.S.

FJG.l

FIG. 8. d
FULLSIZE

10 o i°

XSCALE.

S. Allen, So

Description of Plate VII.

Fig. 1. Appearance of the Podura-scale when the adjustment of the
object-glass is correct and the markings are in focus.

Fig. 2. Appearance of the Podura-scale a very little within or beyond
the focus, when the adjustment of the object-glass is cor-
rect.

Fig. 3. The way in which the markings individually divide when all
the adjustments are correct, and when the focus as shown
in fig. 1 is altered the least possible amount either way.

Figs. 4 & 5. The two appearances on one and the other side of the
best focus when the adjustment of the object-glass is in-*
correct.

Fig. 6. The appearance of the Podura-scale at the best focus when
the adjustment of the object-glass is incorrect.

Figs. 7 j 9, 10, & 12. The different directions of the lines on Pleuro-
sigma quadratum under oblique illumination, in directions
indicated by the arrows.

Fig. 8. Outline of Pleurosigma quadratum, x 400 linear.

Fig. 11. The markings on Pleurosigma quadratum as shown by the
achromatic condenser with a large illuminating pencil.

Fig. 13. The appearance of lines on Navicula rhomboides when very
obliquely illuminated by the Amici prism.

PLATE VII

Fia.i.

FIG. 2

FIG. 3.

FFG. 4.

PODERA SCALE, AS A TEST.
JZaofa Squares =. 001 of am lru>h, X, 1, 300

I iWA

L.lftl

1 1 1!

FIG. 7.

PLEURO SIGMA QUADRAT UM

FIG.

X 1j
FIG. 10.

FIG. 11 .
X 1,300.

*:#:%:#:•:•..-.•. *,.;:: ^&:-&

•'.''.•'.-'.•'.''.•'.•'.•'.•'.•.'.•I'l'''.'',

FIG. 13 .

NAVTCTJLA KHOMBOIDES, X 1,300

S.iflle

Description of Plate VIII.

NOBERT'S LINES.

There is but little to explain in this Plate, which is a camera-lucida
drawing of the 20 bands of lines constituting the test-object known
as Nobert's lines.

Under the drawing of each band is given the number of lines equi-
valent to one-thousandth of an inch. In the first and last bands the
lines are ruled at the respective rates of 13 and 70 thousand to the
inch, the other bands being intermediate, and the whole forming a
beautifully graduating series.

Only the true lateral distances of the lines are intended to be
given in this Plate. Their length is very considerable ; and the bands
follow consecutively, and not, as in this illustration, in rows one
above the other.

PLATE VIII

■ -

€j .=^Wl

) 15

tl. MVfc-

x -7,

zea&/

If PI

13 =-001

HP 2

15 =-001

WP3

18 =-001

WP4

21= -001

If? 5

23= -001

HP 6

27=-001

If0 7

31 = - 001

K°8

36= -001

lf°9

41 = - 001

N°10

45- -001

N? 11

■001

Df°12

52=- 001

WP13

55= -001

HP 14

57=-00l

If? 15

59=- 001

2f?16

61 = -001
RBeck.Del1

W°17

63—001

HP 18

65=- 001

HP19

67=-

HP 20

70=-001

C A L E

Description of Plate IX.

Fig. 1. A, B, & C represent the different appearances the markings
on Pleurosigma formosum assume under oblique illumina-
tion in different directions. This result appears to be due
to the refraction of light by more than one plane of unequal
transparent structure. At D is shown an entirely different
appearance, the result of central illumination with the achro-
matic condenser.

Fig. 2. Siliceous valve of Pleurosigma formosum, as seen under a low
power with slightly oblique illumination.

Figs. 3, 4, 5, 6, & 7. Drawings of the cast skin from the head of the
Silkworm, as seen from different points of view. These illus-
trations are for the purpose of showing how an object, when
illuminated from above, may be examined in almost every
position by means of the disk-revolver figured in Plate X.
fig. 2. The object in this case was fastened to the small
disk G by some gum, on the side opposite to that shown in
fig. 7; and, without any alteration in this setting, the
whole of the five views given here were obtained.

PLATE IX.

FIG. I

FIG. 3.

A

C

it

jFj

FIG. 5

Mi '"

j7! a, z

A. 51.

S.Allen, Sc

Description of Plate X.

Fig. 1. Side silver reflector, for the illumination of opaque and

other objects from above.
Fig. 2. Opaque disk-revolver, an instrument for facilitating the

change of position of small opaque objects.
Fig. 3. Pliers for taking hold of the small disk, G.
Fig. 4, A and B. Box for packing the small disks which fit into

the opaque disk -revolver, fig. 2, — another method being

shown in fig. 8. \

Fig. 5. Three-pronged forceps for holding large opaque objects;

they fit into a small socket on the top stage-plate.
Fig. 6. The forceps for holding small objects ; the pin at A fits into

a small socket on the top stage-plate.
Fig. 7. Large Bull's-eye Condenser, for concentrating the light

either upon the object or the mirror.
Fig. 8. Brass plate for holding the small disks which fit into the

opaque disk-revolver, fig. 2, — the other method being

shown in fig. 4.
Fig. 9. Side Condensing-lens on stand, for the illumination of

opaque or other objects from above.
Fig. 10. Side Condensing-lens, as adapted for first-class microscopes,

for the illumination of opaque or other objects from

above.
Fig. 11. Lieberkuhns for the four object-glasses specified under the

figures ; each of these silvered dishes fits on the front of

an object-glass, and the light thrown up from the mirror

below is reflected by them down upon the object.
Fig. 12. Three dark wells and a holder, for making a dark back-
ground to an object when the Lieberkuhns (fig. 11) are

used.

PLATE. X

Fiai.

ALE.

S.Allen, Sc.

Description of Plate XI.

Fig. 1 . The general appearance of coniferous wood as seen in a simple
splinter. The ordinary lucifer-matches are mostly made
of deal or pine, and the most beautiful specimens are
easily obtained from them. Small fractured pieces of
woody tissue are much superior to any sections ; and in
the present instance the shape of the longitudinal tubes,
the form and arrangement of the circular disks or glands,
together with the transverse medullary rays, form as a
whole, and when under the binocular microscope, one of
the most beautiful and instructive objects. It is impos-
sible to represent their true appearance in a drawing ; but
we are fairly entitled to ask for a comparison between this
and previous illustrations of the same subject.

Fig. 2. This illustration is intended to serve two purposes : first,
the extent to which illumination from above may be car-
ried, the magnifying power being 1300 linear, obtained
by a ^th and 3rd eyepiece ; and secondly, how such
an illumination may be employed in the determination
of superficial structure, — A, B, and C representing the
change of appearance in the markings of the Podura-scale
when they are placed in different directions to the light.
(This subject is treated of in detail at page 29.)

PLATE XI.

JR. Beck, del.

X 1300.

Description of Plate XII.

Fig. 1. Tarsus of Tegenaria atrica (a spider very common in out-
houses) as seen by Lieberkuhn illumination. It is an inter-
esting and beautiful object, from the variety of hairs with
which it is studded : besides those of the upper and under
sides being very different in general shape, there is a
distinct group round the claws, a few scattered branching
hairs, others again which approach to spines, and lastly,
and perhaps the -most interesting of all, some very deli-
cate hairs which rise more or less at right angles from the
upper surface of the joint, and during life wave about from
a ball-and-socket root when there is the least motion in
the air, and are no doubt intimately connected in some
way with sensation. The larger figures of the hairs show
that they also are all covered with other minute short
hairs, the distinct separation of which forms a very good
test for an object-glass.

Figs. 2 & 4. Part of a Pigeon's feather, showing the arrangement by
which the barbs are attached to each other. The top
and bottom figures are respectively the upper and under
surfaces of the vane of the feather, whilst fig. 3 is a section
in the direction of the letters a, b, and shows the barbules
(c) and the barbulettes (e).

PLATE Xn.

Description of Plate XIII.

THE ARACHNOIDISCUS JAPONICUS.

This illustration is given to show the result that can be obtained
with a £-inch object-glass and Lieberkuhn illumination.

The drawings have been made from specimens most kindly lent us
by Mr. Deane, of Clapham, who was the first to draw the attention of
microscopists to this diatom in its natural position, having found it
in considerable abundance on some seaweed used in Japan for soups.

Our Plate, however, does not represent specimens from this loca-
lity, but from a marine plant of Mauritius, given to Mr. Deane by
Professor Harvey, of Dublin.

The three figures in the Plate, although drawn from valves of dif-
ferent sizes, serve just as well to explain the structure of this diatom.
The bottom right-hand figure shows the complete form, two siliceous
valves being joined by a connecting membrane; but the process of
self- division has commenced, as indicated by the narrow central light
band. The other figure at the bottom gives the appearance of the
external surface of the valve, whilst the more striking structure of
the internal surface is represented in the larger figure above.

PLATE XIII.

Description of Plate XIV.

This Plate is intended to show the effect produced by " dark-field
illumination " on some of the Polycystina found in an extensive fossil
deposit in the island of Barbadoes. Of the true character of these
beautiful microscopic objects, which are closely allied to the Forami-
nifera, there is but little known ; they were discovered by Professor
Ehrenberg, and by him have been most carefully classified. There
is every reason to suppose that most of the specimens in the mounted
slides are more or less mutilated ; this, however, does not detract from
their remarkable beauty as microscopic objects, nor from their value
as illustrating the advantages of a peculiar illumination, for which
purpose they are introduced here.

The following are the names of Figs. 2, 3, and 4. Figs. 1 aud 2
we are unable to identify with any illustrations that have been
published.

Fig. 1.

Fig. 2. Haliomma Humboldtii.
Fig. 3. Astromma Aristotelis.
Fig. 4. Lychnocanium lucerna.
Fig. 5.

PLATE XIV.

A.T.HoRick, De

Description of Plate XV.

These illustrations are to explain the use and capabilities of the
Erecting- glass, which may be employed with great advantage when
objects of a large size have to be examined under low powers of from
5 to 100 linear; and with it the image of the object is erect, not
reversed, which considerably helps in any manipulation or dissection
of the object.

Fig. 1. The new halfpenny coin of the realm, magnified only 5 times
linear, by employing the erecting-glass with the §rds.

Figs. 2 & 3. The two sides of a halfpenny, the full size. These
figures, when compared with fig. 1, convey a good idea of
linear magnifying power.

Fig. 4. The foot only of the figure of Britannia under a magnifying
power increased to 25 diameters, obtained by pulling out
the draw-tube of the body ; and by the same process the
range of power may be extended to 100 diameters, but
this object then becomes quite unsuitable as an illustra-
tion.

PLATE XV

FIG. 1

'IG. k

FIG. 3

AT.Hollick,

X 25

Description of Plate XVI.

Fig. 1. The short tube, fitting by a bayonet-catch to the under

plate of the stage.
Fig. 2. The ordinary polarizer of NicoPs prism.
Fig. 3. The extra-large polarizer of NicoPs prism.
Fig. 4. The analyzer of NicoPs prism.
Fig. 5. Tourmaline mounted as an analyzer.
Fig. 6. The selenite plate.

Fig. 7. The ring to hold the selenite plate in its fitting, fig. 8.
Fig. 8. The fitting for the selenite plate, fig. 6.
Figs. 9, 10, 11. Darkens three retarding- films of selenite.
Figs. 12, 13. Two of Darker' s retarding-films when placed at right

angles.
Fig. 14. Darkens selenite stage.
Fig. 15. The analyzer of NicoPs prism when its cap and its fitting

to the eyepiece are removed to suit it for either of the

adapters, figs. 19 or 20.
Fig. 16. Darker' s three retarding-films, fitted to rotating cells of three

arms, which, when in use, fit on the cylindrical fitting

under the stage, as shown in fig. 22.
Fig. 17. Selenite mounted in plain brass plate.
Fig. 18. Selenite mounted between two slips of glass.
Fig. 19. An adapter which screws into a stop at the bottom of the

draw- tube to receive the analyzer, fig. 15.
Fig. 20. An adapter which screws on the nosepiece of the microscope

to receive the analyzer, fig. 15.
Fig. 21. A polarizer of polished black glass, fitting on the rim of the

mirror.
Fig. 22. The Achromatic Condenser combined with a polarizer, and

Darker's series of selenites, to increase the brilliancy of

the colours under high powers.
Fig. 23. A bundle of thin glass as a polarizer.
Figs. 24, 25, 26. Two double-image prisms and plate of selenite, for

experiments with double refraction.
Fig. 27. A brass plate with three small holes for use with double-
image prisms.
Fig. 28. A section of a crystal as mounted to show rings.

ATE XVI

FIG. I

FIG. Z.

FIG. 3

FIG. 4

S. SCALE.

Description of Plate XVII.

OBJECTS SEEN UNDER POLARIZED LIGHT.

Fig. 1. A crystal of the sulphate of copper and magnesia as it
appears when the Nicol's prisms only are nsed.

Fig. 2. The same object, showing the change produced by the
addition of a selenite plate.

Fig. 3. An oblique section of Rhinoceros-horn as seen under po-
larized light produced by NicoPs prisms only.

Fig. 4. A section of the tendon of the Ostrich under polarized light
produced by NicoPs prisms only.

Figs. 5 & 6. Crystallized salicine (an alkaloid from the bark of the
willow) under polarized light produced by NicoPs prisms
only. The two drawings show the change produced by
one quarter of a revolution of one of the prisms.

PLATE XV !i

FIG. 1

- !-n

id

\ .

Description of Plate XVIII.

ILLUSTRATIONS OF EXPERIMENTS WITH
DOUBLE-IMAGE PRISMS.

Fig. 1. The appearance of three small holes when a double-image
prism is placed over the eyepiece.

Fig. 2. The same as fig. 1, with the addition of a polarizer and an
interposed plate of selenite. Where the larger images
overlap, the complementary tints form white light.

Fig. 3. The change that takes place in the double image, at four
equal points, in one revolution of the polarizer.

Fig. 4. The same experiment as that shown in fig. 3, but with the
interposition of a plate of selenite.

Fig. 5. The appearances of a small hole, without a polarizer, but
with two double-refracting prisms placed over the eye-
piece ; and the changes that take place when one of the
prisms is moved round to four equal points in one revo-
lution.

Fig. 6. The same experiment as fig. 5 repeated, with the addition of
a polarizer and interposed plate of selenite.

Fig. 7. A diagram to show when the NicoPs prism is adjusted with
its acute angles parallel with the sides of the stage.

Fig. 8. A diagram to show the natural flaws or veins in the selenite
plate, and the position in which the greatest amount of
depolarization may be obtained.

Fig. 9. A diagram to show when the NicoPs prism is turned 45 de-
grees from the position shown in fig. 7, and when no
depolarization will take place.

Fig. 10. The black cross and coloured rings produced by a piece of
calc-spar when its surfaces are cut perpendicular to the
axis of the crystal, and when placed under polarized light,
by fitting over the cap of the eyepiece of the microscope
with an analvzer above.

FI& I

PLATE XVEL

fi& 2

FIG 3

fit

. .... . .. .

H :', .... '....':

Description op Plate XIX.

STRUCTURE OF THE SCALES OF LEPISMA
SACCHARINA.

Fig. 1 (a and b). The different appearances, on a small scale, of

Lepisma saccharina when the superficial corrugations

cross at different angles.
Fig. 2 (c) . The appearance resulting from the superposition of two

scales.
Fig. 3. One of the more common scales of Lepisma saccharina

under a magnifying power of 650 linear.
Fig. 4. Parts of a scale in which the appearances have been altered,

by the structure on opposite sides being obliterated by

moisture.

PLATE. XIX.

R.JBe, '!: ,diil

Description of Plate XX.

Fig. 1 . The body and part of the limb of one of the ordinary micro-
scope-stands, with an additional tube, forming Wenham's
Binocular Microscope. This addition does not interfere
with the ordinary use of the single body, and any of the
microscope-stands as yet figured will admit of the altera-
tion.

B. The additional body-tube.

C. The original body-tube.
D&E. Draw- tubes.

I. The part of the nosepiece where the prism-box is inserted.
K. The milled head, connected with a pinion and racks, by which
the draw- tubes D and E may be adjusted for eyes of dif-
ferent distances.
Fig. 2. An adapter to receive the analyzer of the polarizing appa-
ratus, so that polarized light may be employed with the
binocular body.
Fig. 3. The fitting for the small prism, which reflects one-half of the
rays passing through the object-glass into the additional
tube of the binocular body.
F. A small catch for stopping the prism-box when it is drawn
back so as to allow the whole of the rays from the object-
glass to pass into the straight body. This catch must be
pressed back when the prism-box has to be removed.

PLATE XX

• -.-,./'

k#

FIG. 1

I, SCALE

FULL SIZE

Description of Plate XXI.

LIVE-BOXES, COMPRESSORS, &c.

Fig. 1. Small live-box.

Fig. 2. Glass trough, front view-

Fig. 3. Glass trough, end view.

Fig. 4. Large live-box, with the cap lifted from lower part.

Fig. 5. Glass plate with ledge, and thin glass cover.

Fig, 6. Three glass tubes for catching animalcules.

Fig. 7. Wenham's Compressor, front view.

Fig. 8. Wenham's Compressor, side view.

Fig. 9. Screw live-box, front view.

Fig. 10. Screw live-box, side view.

Fig. 11. Parallel plate compressor when the plates are separated.

Fig. 12. Parallel plate compressor when the plates are close toge-
ther.

Fig. 13. Lever compressor, front view.

Fig. 14. Lever compressor, side view.

Fig. 15. The arrangement of the two pieces of thin glass in the two
compressors, figs. 11 & 17.

Fig. 16. Counter cell of the compressor, fig. 17.

Fig. 17. The reversible cell compressor.

Fig. 18. The cell of the compressor, fig. 17.

Fig. 19. The frog plate.

PLATE XXI

FIG. I.

FIG A.

TIG. 2.

d

FIG. 3.

: --'r*'f'\

FIG. S.

■ FIG. 9.

FIG. 13

FIG. 6.

::.'^JM

FIG. 7.

.

FIG. II

FIG. 12.

FIG. 16

FIG. 15.
-■ — €>- -I

P

-o-

FIG. IS

t.SCALE.

Description of Plate XXII.

MICROMETERS, CAMERA LUCIDA, &c.

Fig. 1. No. 2 Eyepiece, with the micrometer in position. The cap
of the eyepiece is here drawn apart, to show how the top
lens may be adjusted at a.

Fig. 2. The Eyepiece Micrometer, front view.

Fig. 3. The Stage Micrometer, as mounted in brass.

Fig. 4. The Stage Micrometer, mounted in card.

Fig. 5. The lines of the eyepiece and stage micrometers, as they
appear in the field of view.

Fig. 6. The Camera Lucida, as seen from above.

Fig. 7. The Camera Lucida, side view.

Fig. 8. A drawing to scale, showing the way to draw an object in
the microscope by means of the Camera Lucida.

Fig. 9. The top view of an eyepiece, showing the small arc at a,
by which Quekettfs Indicator may be turned in or out of
the field of view.

Fig. 10. Quekett's Indicator, as it appears when in the field of view.

Fig. 11. A sketch, made by the Camera Lucida, of the stellate tissue
of a rush ; the right-hand portion is more or less touched
up, but that on the left is exactly as traced by aid of the
camera.

Fig. 12. Leeson's Goniometer.

Fig. 13. A quadruple nosepiece, for the attachment of four object-
glasses to the microscope at once.

Fig. 14. A double nosepiece, for the attachment of two object-glasses
to the microscope at once.

PLATE XXII

Fia.i

^KT~Z

QJwyn

^7

?/

2. SCALE.

I I I I I I. I I I I I

Description of Plate XXIII.

LAMPS AND REVOLVING TABLE FOE, THE
MICROSCOPE.

Fig. 1. Cambridge reading-lamp for burning oil.

Fig. 2, Best Belmontine lamp, with circular wick.

Fig. 3. Cheaper Belmontine lamp, with flat wick.

Fig. 4. Gas-lamp for use with flexible tube.

Fig. 5. An engraving to show the use of the revolving microscope-
table. Only two sitters are represented in the Plate;
but three or four persons may sit round without incon-
venience.

PLATE XXIII

FIG. 2.

FIG. 1.

FIG. 3

FIG. 4

iJh SCALE.

FIG. 5 .

IZ . SCALL.

Description of Plate XXIV.

OBJECTS AS SEEN UNDER THE ^th OBJECT-GLASS
MAGNIFYING 900 LINEAIt.

Fig. 1. The thorax, legs, and parts of head, as seen on the under
side, of Demodeoc folliculorum, a parasite found in the
follicles of the human skin, more especially about the
nose.

A & B. Moveable organs, which are probably palpi ;
but the correct determination of the various parts of this
parasite is very difficult, on account of its transparency,
which confuses the upper and under portions, whilst the
denser parts of its structure refract the light and falsify
the appearances of many of the features.

Figs. 2 & 3. Front and profile views of the stinging-hairs from the
stem of the common nettle ( Urtica dioica) .

Fig. 4. A stinging-hair similar to those in the preceding figures,
but with the bulbous extremity broken off by being
touched : in this way a very sharp point is provided, which
is admirably adapted for puncturing; and an aperture
being made at the same time, the fluid escapes from the
interior of the hair and enters the wound.

PLATE XXIV

FIG. 4.

.

Description of Plate XXV.

THE BEST UPRIGHT CASE FOR THE LARGE BEST

MICROSCOPE.

This Plate shows the way in which the Large Best Binocular Micro-
scope, together with the most complete apparatus, is packed in an
upright case. The extremities of the tripod base are let into a board
at the bottom, to facilitate the sliding of the instrument either in or
out, and also to confine it to one fixed position ; the eyepiece ends are
also secured by two grooves, the central one having a sliding dovetail
block which fills up the intervening space between the eyepiece and
the door.

The Bull's-eye Condenser is fastened by various packings on the
back of the case, and the long draw-tube fits into two blocks on the
left, the remaining apparatus being packed in two boxes which slide
on either side of the Microscope- stand. (The interiors of these boxes
are shown in the next Plate.) By this arrangement, when the door
is closed, the whole instrument is securely packed for any kind of
ordinary travelling.

PLATE XXV.

?&.£imo.cU4

rrcl scale.

Description of Plate XXVI.

THE TWO BOXES OF APPARATUS BELONGING TO A
COMPLETE LARGE BEST MICROSCOPE.

There is but little explanation required for this Plate, which shows
the various ways in which the different object-glasses, eyepieces, and
other apparatus are packed in the two boxes, which in the preceding
Plate are shown as packed on either side of the Microscope- stand.

Each separate piece of apparatus is held in these cases, when they
are closed, by a block on the lid ; so that all are secure from any in-
jury by displacement when the cases are turned over or are shaken.

This Plate may also be of some service to a beginner, who may occa-
sionally find some difficulty in replacing the apparatus; for, unless
each piece is in its proper place, the cases cannot be closed without
some strain or injury.

PLATE XXVI,

krd scale.

Description of Plate XXVII,

Fig. 1 . The flat mahogany case in which the Second-class Microscopes
and apparatus are usually packed. No part of the Stand
has to be taken to pieces when placed in the case, which is
sufficiently large to receive the instrument and the most
complete set of apparatus.

Fig. 2. A neatly framed wooden Stand, covered with leather on the
top, which is sufficiently large to allow a Microscope and
Lamp to be placed upon it. The stand is provided beneath
with carefully made casters ; so that it can be pushed about
in any direction, upon an ordinary table, at which several
persons may be seated ; in this way the microscope may
be passed from one person to another, without any altera-
tion in the adjustment of the instrument, or any inter-
change of seats.

PLATE XXVII.

FIG. 1.

+rd scale.

FIG. 2.

£th scale.

PLATE XXVIII

i

CATALOGUES, ETC.

PUBLISHED BY SMITH, BECK, AND BECK,

31, CORNHILL.

Price 6d. each.
Illustrated description of the Popular Microscope.
Illustrated description of the Educational Microscope,
Illustrated description of the Universal Microscope.

Scientific Catalogue of Microscopic Preparations.

Price 6d. each.
Part 1. Vegetable.

Part 2. Bones, Teeth, Cartilages, Hairs, and Feathers.

Part 3. Entomological.

Price 6d.
General Catalogue of Achromatic Microscopes, etc. — Descrip-
tion of Wenham's Binocular Microscope. — Illustrated de-
scription of the Patent Binocular Magnifiers. — Illustrated
description of the achromatic table and patent mlrror
Stereoscopes.

Price 2s. 6d.
ILLUSTRATED CATALOGUE OE SCIENTIFIC INSTRUMENTS.

Part II.
Meteorological Instruments.

Will be published early in April.

Price 2s. 6d.

ILLUSTRATED CATALOGUE OE SCIENTIFIC INSTRUMENTS.

Part I.

Microscopes, and Apparatus, Instruments used in preparing
Objects, Materials used in Mounting Objects,
Cabinets, Lamps, Tables, etc.

!

COVNTWAY LIBRARY OF MEDICINE

QH

211

B38

c

RARE BOOKS DEPARTMENT

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