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ILLINOIS

UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN

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2015Lk~"

U NIVERS ITY
OF ILLINOIS

578
! B75tTREATISE

ON THE

MICROSCOP E,

FORMING THE ARTICLE UNDER THAT HEAD IN THE
SEVENTH EDITION OF THE

ENCYCLOPAEDIA BRITAIN NIC A.

BY

v/

SIR DAVID BREWSTER, K.G.H. LL.D. F.R.S.

VICE-PRESIDENT OF THE ROYAL SOCIETY OF EDINBURGH,
AND CORRESPONDING MEMBER OF THE
INSTITUTE OF FRANCE.

EDINBURGH:

ADAM AND CHARLES BLACK, NORTH BRIDGE,

BOOKSELLERS TO HER MAJESTY FOR SCOTLAND.

MLCCCXXXVII. MEDINBURGH i
Printed by Thomas Allan & Co*
265 High Street.-W
S,1 STtr

TO

HENRY FOX TALBOT, Esq.

F. R. S., See. &c.

My Dear Sm,

Having been requested to draw up a short
and popular Treatise on the Microscope, for the Encyclopaedia
Britannica, I have endeavoured to give an account of the most
important modern improvements upon that valuable instrument,
and of the most interesting observations which have been re-
cently made with it.

I could have wished to have enriched it with some account of
the very curious discoveries which you have made with the
Polarising Microscope, and which I had the advantage of see-
ing when enjoying your hospitality at Lacock Abbey; but as
these required to be illustrated with finely coloured drawings, I
trust that you will speedily communicate them to the public in a
^ .^separate form.

Y4

Qs

V

it

In placing your name at the head of this little volume, I ex-
press very imperfectly the-admiration which I feel for your
scientific acquirements, and for the zeal with which you devote
your fortune and talents to the noblest purposes to which they
can be applied.

I am,

My Dear Sir,

■'< \

Ever most faithfully yours,

D. BREW STEEL

Allerly, Nov. 16, 1837.

rCONTENTS.

Page

INTRODUCTION.........................................................................1

CHAPTER I.

On Single Microscopes..........................................................................................3

Description ofMrPritchard's Single Microscope........................6

Description of Mr Ross' Single Microscope........................................8

Single Microscopes made of Precious Stones......................................13

Diamond Lenses...................................................................................................14

Sapphire Lenses.............................................................................................22

Garnet and Spinelle Ruby Lenses................................................................24

Single Fluid Microscopes......................................................25

Catadioptric Lenses.....................................................27

The Grooved Sphere........................................................................................28

Microscopes made of the Crystalline Lenses of small Fishes	31

Magnifying Power of Single Microscopes.......................ib.

Spherical Aberration of Lenses of various media............................34

CHAPTER II.

Description of Microscopic Doublets and Triplets	36

Wollaston's Periscopic Doublet......................................................................ib*

Periscopic Sphere....................................................................38

Achromatic Periscopic Spheres...........................................41vi

CONTENTS.

Page

Sir John HerschePs Doublet of no Aberration................................43

----Periscopic Doublet..................................................45

----Piano-Convex Doublet.....................................47

Wollaston's Doublet..................................................................................................48

Pritchard's Triplet......................................................................................................52

Single Achromatic Microscope........................................................................53

Single Reflecting Microscopes..........................................................................54

CHAPTER III.

On Compound Microscopes.......................................................56

Compound Refracting Microscope.........................................57

Dr Goring's improvement on it.........................................62

Mr Coddington's improvement......................................................................63

Compound Achromatic Microscope..............................................................66

M. Selligues' Achromatic Microscope......................................................67

Mr Lister's improvement on it.................................................................69

Compound Achromatic Microscopes with Solid and Fluid

Lenses...........................................................................................73

Pritchard's Compound Achromatic Microsope..................................77

Newton's Compound Reflecting Microscope......................................79

Mr Potter's improvement on it......................................................80

Amici's Reflecting Instrument.............................................82

Dr Goring's ditto...........................................................................84

Dr Smith's ditto............................................................................................................87

Sir David Brewster's ditto....................................................................................90

CHAPTER IV.

On Polarising Microscopes.....................................................95

Single Polarising Microscope.........................................96CONTENTS.	VII

Page

Compound Polarising Microscope................................. 99

Mr Nicol's Polarising Prism....................................... 100

Sir David Brewster's Polarising Rhomb........................ 301

CHAPTER Y.

On Solar and Oxyhydrogen Microscopes................. 105

Common Solar Microscope.......................................... 106

Benjamin Martin's.................................................... 107

Achromatic Solar Microscope.................................... .. 110

Sir David Brewster's ditto ditto......................... ......... 112

Dr Goring's Solar Camera Microscope........................... 115

Oxyhydrogen Microscope............................................ 120

CHAPTER VI.

Description of Micrometers for Microscopes......... 124

Wollaston's Micrometer.............................................721

CHAPTER VII.

On the Illumination of Microscopic Objects.......... 135

"Wollaston's Method.................................................. 141

i Sir David Brewster's method...................................... 145

CHAPTER VIII.

On the Monochromatic Illumination of Microsco-
pic Objects..................................................... 154

Monochromatic Lamps.............................................. 155

Dr Goring's Objections to Monochromatic Illumination
considered......................................................... 160VIII

CONTENTS.

Page

CHAPTER IX.

Ok the Preparation of the Object and the Eye for

Microscopical Observations...................*.......... 163

Preparation of the Object........................................... ib.

Preparation of the Eye.............................................. 165

CHAPTER X.

On Test or Proof Objects for trying the Perfor-
mance of Microscopes......................................... 169

Dr Goring introduces Test^Objects............................... 170

Mr Pritchard's test of them.............................T........... 171

On the Structure of the Lenses of Test-Objects.............. 176

CHAPTER XI.

On Microscopic Objects............................................ 182

Dr Ehrenberg's Siliceous Infusoria............................... 183

Fibres and Teeth in the Crystalline Lenses of Fishes...... 185

List of Microscopic Objects........................................ 187

List of Prepared Objects sent to the British Museum by
Dr E hrenberg.................................................... 192TREATISE

OX THE

MICROSCOPE.

Microscope, from	a small objectt and ffxorew, to

see or examine, is the name of a well-known optical in-
strument for examining and magnifying minute objects,
or the minute parts of large ones. Dr Goring has, in his
various ingenious works on the microscope, used the word
engiscope, from syyvg, near, and cxcwrsw, to see ; but the old
and venerable term is so associated with the history of op-
tical discovery, and is so expressive of the application of
the instrument, that we cannot consent to the proposed
change.

Single microscopes in the form of glass globes contain-
ing water, were used by the ancients. Hemispheres of

A2

TREATISE ON THE MICROSCOPE.

glass, and afterwards lenses, were subsequently used, so
that no person has pretended to claim the invention of
the single microscope* The compound microscope, con-
sisting of two lenses placed at a distance, so that the one
next the eye magnifies the enlarged image of any object
placed in front of the other, was invented by Zacharias
2ansz, or his father Hans Zansz, spectacle-makers at Mid-
dleburg in Holland, about the year 1590. One of their
microscopes, which they presented to Prince Maurice,
was in the year 1617 in the possession of Cornelius Drebell
of Alkmaar, who then resided in London as mathematician
to King James VI.

f There is probably no branch of practical science which
has undergone such essential and rapid improvements
as that which relates to the microscope. It has become
quite- a new iju^umentmmodem times, and it promises
to be the meansof disclosing the structure and laws of mat-
ter, and of making as important discoveries in thse infi-
nitely minute world, as the'tetesebpe'*lf&s done in that
which is infinitely distant# ^SINGLE MICROSCOPES.

s

CHAPTER I.

ON SINGLE MICROSCOPES.

A single microscope is one in which only one convex
teas A tfir it usedfbr magnifyingobjects. The object m n to
be examinedisplaeecT before thelens A	focus;

and the rays which emerge from the lens after refraction
by the eye C D are parallel, and therefore a distinct and
enlarged image M N of the
object n is formed oii the	Fig. 1.

jretina. The simplest form
of the single microscope is
when the lens is fitted into
a rim of brass furnished with
a handle, and the object being
held iix the left hand and the
lent life "dght, if omy be examined with great cor-
recinessi If the convex lens is very miiute, and has
a short focal length, such as from the 10th to the 100th
of an inch, it cannot be conveniently used in the hand,4

TREATISE ON THE MICROSCOPE.

and must therefore be placed in a firm microscope stand,
having a shelf for holding the object, a screw or a rack
and pinion for placing the object in the focus of the lens,
and a lens or mirror, or both, for throwing light upon the
object. In this form, however complex be its structure, it
is still called a single microscope.

A single microscope, in order to have all the perfection
which art can give, must consist of a substance perfectly
homogeneous, like a fluid without double refraction, or any
variation of density. Its figure ought to be that of a plano-
convex lens, whose convex surface is part of a hyperbo-
loid, in order to correct completely the spherical aberration.
Its surface should be perfectly smooth and highly polish-
ed, so as not to disturb the perfection of vision; and the
substance of which it is made should have the lowest dis-
persive power. As it is a great object to obtain high mag-
nifying powers with as little convexity as possible, and a
large aperture, substances with high refractive and low
dispersive ones are the most suitable for single lenses,
such as diamond, or garnet9 which have no double refrac-
tion when well crystallized ; or such as ruby, sapphire, to-
paz, &c. which have double refraction. As fluor spar has
the lowest dispersive power, it might be used with great
advantage when high powers are not wanted, and when
the diminution of colour is an object.	$

Of all the substances we have named, fluids have pro*SINGLE MICROSCOPES*

&

perties best suited for single microscopes. They possess
perfect homogeneity 5 their surfaces, when made into lenses,
are perfectly smooth; and it is possible to mould minute
drops of them into a form approaching to that of the hy-
perboloid. Their defect, however, consists in their not
having a high refractive power, in their want of durability,
and the difficulty of forming sufficiently minute lenses for
producing high magnifying powers. These defects, how-
ever, especially the last, may be overcome by patience and
experience; and in proof of this we may state, that we
have succeeded in forming fluid lenses that were fully
equal to the best sapphire lenses that have been executed.

In the present state of this branch of science, it would
be unprofitable to detail the methods of producing micro-
scopic globules of glass, given by Dr Hooke, Father di
Torre of Naples, Mr Butterfield, or Mr Sivright; be-
cause when they are made after their methods, and in the
most perfect manner which these methods will permit, they
are of no value compared with lenses of glass when ground
and polished to the same focal length.1

We shall therefore proceed to describe a single micro-
scope when fitted up in the best form for observation.

1 These methods may be found by the following references :—■
Hooke's Micrographia; Di Torre', Phil. Trans. 1765, p. 246, 1766,
p. 67 ; Butterfield, Phil. Trans. 1678 ; Sivright, Edin. Phil. Jour-
nal, 1829, vol. i. p. 81.6

TREATISE ON THE MICROSCOPE.

Description of a Single Microscope.

The most essential part of this instrument is the lens or
lenses, upon which the value of the microscope depends.
The lenses are generally made of plate-glass, and should
have focal distances varying from the ^th to the ^th
of an inch. In order that the spherical aberration of these
lenses may be the smallest possible, the radii of their two
surfaces should beias 1to &; $ie surface whose radius is
as 1, or the most convex side, must be turned towards the
eye. The lenses, thus made, are then set in the centre or
the lower surface of concave brass caps, a section of one
of which is shown in Plate I. fig. 1.

The best mode of fitting up the microscope is that con-
trived by Mr Pritchard, which is represented in fig. 2, on
a scale about one third of its real size. It is shown in an
inclined position; but it may be used either in a vertical
or a horizontal one, according to the convenience of the
observer. The body of the instrument rests on a pillar b,
supported by tl^ree leg»> shown at s, and is connected with
it by the clip f being fixed by the pinching screw f
Within the tube c there slides a tube h, connected by a
screw which passes through it to the triangular tube or
bar iy carrying the arm ij, into which is placed the brass
cap J which carries the lens. This lens is adjusted to theSINGLE MICROSCOPES.

7

distinct vision of objects placed on the stage /, by sliding
the tube h up or down, and a perfect adjustment is ob-
tained by turning the milled head k. The stage I, which
carries the objects, is fitted into the triangular box r at
the extremity of the stem, by means of two pins, and can
be removed at pleasure. The spring slider-holder, for
holding the sliders in which the objects are placed, is fixed
by a; bayonet-joint into the stage; and it may be used to
hold stops or diaphragms for limiting the field of view.
The tube above / represents an illuminator fixed to the
slides-holder. Upon the tube c, two sockets rf, 0 slide
with sufficient spring-and fruition to keep' them in their
place. The socket d carries the reflector d, and the socket
e carries the condensing lens, which is not inserted in the
figure.

A section of the stem rck is shown in fig. 3, in order to
exhibit the mechanism by which the adjustment is ef-
fected. Into the box r, screwed into the top of the stem,
is fitted the triangular tube ii\ which carries the arm ij.
In the lower end i of this triangular tube is a small block
with a fine screw working in it, the stem of which turns
along with the milled head &, to which it is fixed. The
upper end of a spiral spring, shown in the figure, bears
against the block f at the bottom of the triahgtto tube,
while its lower end acts against a stop fixed within the
sliding tube h. The method of managing, illuminating,8

TREATISE ON THE MICROSCOPE.

and examining opaque objects with this microscope is
the same as that used in the achromatic compound mU
croscope, in the drawing of which it will be more dis-
tinctly seen.

The preceding instrument of Mr Pritchard's is intended
for general purposes; but as the dissection of botanical
and other objects is now a leading object with naturalists,
we shall add an account of another microscope, construct-
ed by Mr A, Ross, with much still, for Mr W. Valentine
of Nottingham, the parts of which are given in consider-
able detail.

A perspective view of this microscope is shown in
Plate II. fig. 4>. It is supported on a closing tripod, aaa,
whose feet can be folded together, and are made of hard
bell-metal, prevented from springing by edge bars, as seen
on the left-hand foot. A firm pillar, which rises from the
tripod, carries the stage a?, which is fixed on brackets, to
give a steady support to the hands of the operator. A capi-
tal, e9 fixed to the top of the tube by three screws, has in
its axis a triangular hole, into which is fitted the triangu-
lar tube f> the lower end of which passes through another
similar triangular tube in the piece gg> fixed to the tube.
This triangular tube is made to slide up and down by a
fixed screw, i, which is wrought by a large milled head, o,
which is most judiciously placed at the base of the pillar.
At the top and bottom of the triangular tube, at g, andsingle microscopes* T	9

near r, are fitted two pieces, with triangular holes through
them for receiving the triangular bell-metal bar ss9 which
moves up and down in them. This bar carries the arm
10 with the lenses. It is moved up and down, so as to ad-
just the lenses to distinct vision of the objects on the fixed
stage, by the rack and pinion t, when a quick adjustment
is required; but when a slow and nicer adjustment is want-
ed, it is effected by the milled head o. A slit, uv, is made
in the shaft of the pillar, to allow the neck of the small
milled head t to move up and down; for when the screw
is in action by the large milled head 0, the triangular tube
and the bar move together. The triangular bar is per-
forated at both ends, the upper perforation for receiving a
conical pin, and the lower for admitting the adjusting screw
to preserve the length of the bar. The piece w is removed
from the side of the pillar, to show the bearings of the pinion
ty which are attached to the triangular tube. The bar moves
1^ inch, and the tube 1J, so that #€ can command an
elevation of 3 inches. At the ingenious suggestion of Mr
Solly, the screw i, moved by the milled head 0, has fifty
threads in an inch, and the milled head is graduated into
100 parts, for the purpose of measuring the thickness of
any vessel or other object in the direction of the a^is of
vision. For this purpose the upper surface of the body is
brought into distinct vision; the division at which the in-
dex or pin of the tripod stands is then observed; and the

a 210

TREATISE ON THE MICROSCOPE.

under surface being like manner brought into focus by
turning the iniHed head d, the division is again observed.
The number of divisions, which are each SOOOths of an
inch, between these two numbers, will indicate, according
to Mr Valentine, the spacethrough which thelens has pass-
ed, which is the diameter of a vessel*1

In this microscope, different parts of an object may be
brought into the field, either by moving the stage or the
lens, a very important requisite in amicroscope^sed for the
purposes ofdiscovery. With this view, the large stage x
is formed of three plates, the lowest of which is fixed to
the pillar by the ring 1; and,.to make it bear the weight of
the hands, it rests upon the strong brackets 2, 2. The
under side of this plate is shown in fig. 6; (the biiddle
plate, fig. 7, contains two pair of dovetail slits, B, & and
4, 4, the widest orifice of each being on opposite sides of
the plate. The dovetail pieces in 4, 4 screw into the up-*

1 This is not the case, as the refraction of tlie light issuing froni
the lower side of the vessel dr object ier not considered. The right
mode is, after living observed the upper surface of an object lying
upon glass, remove the object, and observe the divisions when the
surface of the glass is seen distinctly : the difference will be the
true thickness. Mr Samuel Yarley is said to have constructed
an instrument on this principle for measuring the thickness of foci
of lenses; but unless he removed his lens after observing the
first surface, his results must have been all erroneous.SINGLE MICRQSC0PE5.

11

per side of the upper plate, fig. 8, Plate III., the points of the
screws being shown at 4 in that figure, while the dovetail
pieced in 3, 3 are secured to the upper side of the under plate
by the screws 3, 3, fig. 7. The plates are thus moved dia-
gonally, and at right angles to one another, by the adjust-
ing screws 7 and 8, fig. 8. Oneof-the screws, with its ball
and milled head, is shown in fig. 9. In the adjusting screw
7, the ball is placed in spring couplings, and fastened to the
under side of the upper plate. These screws are judi-
ciously placed, one on each side of the pillar, that the hand
may reach them easily and not intercept the light. By
turning first one screw, and then the other,or both at
once, any part of the object may be brought into the field.

The arm for holding the lenses is shown at 10, in fig. I
and fig. 10. A conical pin projects from underneath, and
fits into a hole made down the triangular bar, as shown at 9,
fig. 8. The lens will therefore have & circular movement
in a horizontal plane, and it may be placed at any point
in this plane by the action of the rack and pinion at 10.
Hence the most complete adjustment can be obtained
without any motion of the stage.

The elevated stage for holding the objects is shown at
II in fig. 4 and 8. A t6be,12, fig. U, screws into the up-
per plate, and upon this fits the tube 11, carrymg the fin-
ger spring, shown in fig. 4>. Objects of different thickness
are thus kept down upon the plates by the pins sliding in12

TREATISE ON THE MICROSCOPE.

the small pipes. An elevated stage is shown in fig. 12, for
viewing the sides of objects without disturbing them. A
condensing lens, fig. 13, slides into the sockets 5 or 6; and
fig. 14# shows the pincers, to be applied in the same manner.

The large reflector above a, fig. 8, may be removed, and
any other illuminating apparatus substituted. That of Dr
Wollaston is shown in fig. 15, at 19. The handles 18, 18
serve to move the lens up and down in the tube.

The mode of attaching the body of a compound mi-
croscope is shown in fig. 16* For this purpose the arm
10, fig. 4, is withdrawn, and the conical pin 20 is made to
fit in the same hole in the triangular bar.

Mr Valentine informs us, that with this instrument he
can dissect under a lens ^th of an inch focus.

Having recently had occasion to examine one of Mr
Ross's microscopes, with achromatic object-glasses, we
were surprised at the beauty and distinctness with which it
exhibited the most difficult test-objects. We have never
indeed seen any instrument superior to it*

As the stand and apparatus now described maybe used
along with all single microscopes, and also with what are
called doublets and triplets, we shall now proceed to give
an account of the various improvements which the single
microscope has undergone.SINGLE microscopes.

13

Single Microscopes made of Precious Stones.

The low refractive power of glass rendered it neces-
sary, when high powers were wanted, to use lenses with
very short foci, and consequently with very deep curves
and very small diameters, so as to admit only a narrow
pencil of light into the eye.

Sir David Brewster was the first person who pointed
out the value of using other materials for the construction
of lenses; and he remarked, that no essential improvement
could be expected in the single microscope, unless from
the discovery of some transparent substance, which, like
the diamond, combines a high refractive with a low dis-
persive power. Having experienced the greatest difficulty
in getting a small diamond cut into a prism in London,
he did not conceive it practicable to grind and polish a
diamond lens,1 and therefore did not put his opinion to the

1 Mr Pritchard informs us (see Edinburgh Journal of Science,
No. 1, new series, p. 149, July 1829), that Messrs Rundell and
Bridge of Ludgate Hill had, at the time when Mr Pritchard began
his experiments, many Dutch diamond-cutters at work; and that
the foreman, Mr Levi, with all his men, assured him, that it was
impossible to work diamonds into spherical lenses. The same opi-
nion, he adds, was also expressed by several others, who were
considered of standard authority in such matters. When Mrn

TREATISE ON THE MICROSCOPE.

test of experiment. He got two lenses, however, execut-
ed by Mr Peter Hill, an ingenious optician in Edinburgh,
the one made, of ruby, and the other of garnet; arid these
lenses he found to be greatly superior to any lenses that
he had previously used.

Dr Goring, whose zeal and success in the improvement
of microscopes has not been surpassed, directed the atten-
tion of Mr Pritchard in 1824> to?the passages in Sir David
Brewster's Treatise oa iieiri^<»opfekml fe	re-

specting the value oftheprecious stones for single micro-
scopes ; and having immediately seen their full force, it
was agreed that they should undertake to grind a diamond
into a magnifier.

Diamond Lenses.

The history of this attempit is so interesting, that we
must give it in Mr Pritchard's own words:—" For this
purpose," says he, " Dr Goring forwarded me a small bril-
liant diamond to begin upon ; and it was proposed to give

Pritchard had, contrary to the expectation of many, succeeded in
finishing his first lens, it was examined by Mr Levi, who ex-
pressed great astonishment at it, and added, that he was not ac-
quainted with any means by which that figure could have been
effected.SINGLE MICROSCOPES. ~	15

it the curves that in glass would produce a lens of a twen-
tieth of an inch focus, with the proportion of the radii of
their surfaces as two to five. This stone I ground with
the proper curves, and polished the flatter side, contrary
to the expectations of many whose judgment in these
matters was thought of much weight, who predicted that
the crystalline structure of the diamond would not permit
it to receive a spherical figure. When thus far advanced,
fate decreed that I should lose the stone, and my only con-
solation was, to discover afterwards, that, had it been com-
pleted, its thicjk^ess and enormous refractive p<wer would
probably have caused the focus to fail within the subgtance
of the stone,

" Having, however, in this experiment proved the possi-
bility of working lenses of adamant, I set about another,
and selected a i'ose-cut diamond, in order to form it into a
plano-convex lens, and thereby save a moiety of the labour.

" In the progress of working this stone, the heat generat-
ed by friction, in the course of the abrasion of the diamond,
was perpetually melting the cement (shell-lac) by which
the flat side was affixed to the tool, and compelled me to
seek some means by which it might be prevented. After
several trials, I found, that when a portion of finely powder-
ed pumice-stone was mixed with the shell-lac, the cement
was much stronger, and less liable to melt, than any other
similar substance.16

TREATISE ON THE MICROSCOPE.

" On the first of December 18241 had the pleasure of first
looking through a diamond microscope, and it was doubt-
less the first time this precious gem had been employed in
making manifest the hidden secrets of nature* A few days
after, I had polished it sufficiently to put it into the hands
of Dr Goring, who tried its performance on various objects,
both as a single microscope and as the objective of a com-
pound* He states in a letter addressed to me, dated 3d
January 1825, ' that it has shown the most difficult trans-
parent objects I have submitted to itand again, * I can
clearly perceive the amazing superiority it will possess
when completely finished.' I must, however, inform my
readers, that we discovered in this state various flaws in
the stone, in consequence of which we abandoned all
thought of completing it. In this condition the project
remained for about a year, when I determined to resume
my attempts; and having worked several stones into lenses,
I at last succeeded in obtaining a perfect one. In the
course of these labours, a new though not unexpected de-
fect appeared in several lenses, which would have subvert-
ed the whole scheme, had not the first diamond lens been
free from it.

" These lenses, instead of giving a single image like the
first, gave a double or triple one. This rendered them
utterly useless as magnifiers, and made the defects of soft
and hard parts in the same stone, and the small cavities inSINGLE MICROSCOPES.

17

others, of comparatively trifling consequence. The images
exhibited in such lenses overlapped each other, but were
never entirely separated, though the quantity of overlap*
ping varied in different specimens.

" It was now evident that these defects arose from po-
larisation, though this stone is described as 6 refracting
single.' I subsequently learned from Dr Brewster, after
I had overcome these obstacles, that this property of
the diamond had been observed by him, and an account
of it given in the Edinburgh Philosophical Transactions.1
On referring to his paper, it appears Dr Brewster found
that some stones ' polarised in particular parts, while other
portions of the same stone were quite free from any trace of
polarity/ and thus perfectly adapted to our purpose, as had
previously been demonstrated in the first diamond lens.

" Notwithstanding these difficulties, and the consequent
expense and labour they entailed on me before sufficient-
ly experienced in working upon this refractory material
with certainty, I have now the satisfaction of being able,
by inspection a priori, to decide whether a diamond is fit
for a magnifier or not; and have now executed two plano-
convex magnifiers of adamant, whose structure is quite

1 Edinburgh Phil. Trans. voL viii. p. 157, 1817- See also
Geological Transactions, new series, vol. iii. p. 455; and London
and Edinburgh Philosophical Magazine, vol vii. p. 245.18

TREATISE ON THE MICROSCOPE*

perfect for microscopic purposes. One of these i$ about
the twentieth of an inch focus, and is now in the posses-
sion of his Grace the Duke of Buckingham; the other, in
my hands, is the thirtieth of an inch focus, and has conse-
quently amplification enough for most practical purposes."1
Although it is quite certain that many if not most dia-
monds possess a doubly refracting and polarising structure,
owing to their having been irregularly indurated when in.
a soft state, yet the separationofthe images, arising from
this structure, is riot sufficient to account for the overlap-
ping of the images observed by Mr Pritchard. In order
to have this matter investigated, Mr Pritchard sent a bad
diamond lens, with two or three images, to Sir David
Brewster, who was for a long time perplexed with the dif-
ficulties which it presented to him. It occurred to him,
however, to examine if the stone possessed a homogeneous
structure, as he had observed in amber and gums, which
are indurated in a similar manner, a variation in the re-
fractive density capable of accounting for the imperfec-
tions of the diamond* In order to' io' this/beadraitted a
narrow beam of light into a dark toom, and examined by
this light the flat surface of the plano-convex lens of dia-
mond with a hand microscope. After getting the dia-
mond at the most favourable position, he was surprised

1 Microscopic Cabinet, p. 107-111.SINGLE MICROSCOPES*

to see its whole surface covered with thousands of minute
bands, some reflecting more and some less light, thus
proving beyond a doubt that this diamond consisted of an
infinity of layers of different reflective, and consequently
refractive powers, from which arose all the irregularities
of performance which it exhibited as a microscope. In
this case the veins, as we call them, lay parallel, or nearly
so, to the axis of the lens, so as to produce the worst pos-
sible effect; and had Mr Pritchard known previously that
his diamond possessed such a structure, and made the
axis of the km perpen^ic^lir to theplane of these veins
.or laming, its performance as a microscope wduld scarcely
have been affected by them. These observations will, we
trust, induce opticians to use the diamond more frequent-
ly than they were disposed to do when they believed that
its imperfections arose from its doubly refracting structure.

As the expense of the diamond, and the labour of work-
ing it, are very great, about fifty or sixty hours being neces-
sary to complete a diamond lens with double convexity, it
is of the greatest consequence to ascertain beforehand if
the substance of the diamond is homogeneous, that is, free
from difference of density or double refraction, and if it does
not contain any small cavities. The best way is to Exa-
mine the stone, by cutting two flat faces upon it, unless it
is a lashe or table diamond, which always has two flat
faces 5 but this labour may often be avoided by exam-20

TREATISE ON THE MICROSCOPE.

ining it when plunged or held in a glass trough containing
oil of cassia, the fluid which approaches nearest to it in
refractive power. This will diminish all the refractions
at the irregular surface of the diamond, and make any in-
ternal imperfections as easily seen as if its substance was
plate-glass.

By comparing the indices of refraction of diamond and
glass, it may be easily shown that the same magnifying
power may be obtained with a diamond lens having its
curvature with a radius of 8, as with a glass lens, the radius
of whose curvature is 3; and as the spherical aberration
increases with the depth of curvature or the thickness of
the lens, a lens of diamond will bear a much larger aper-
ture than one of glass before indistinctness of vision is
produced. Mr Pritchard has given a very useful ocular
representation of the relative value of a diamond and a
glass lens. In the annexed figure, G is the section of
a semi-lens of glass,

and D the section of	Fig. 2*

one of diamond, so
placed that their prin-
cipal focus F shall be
at the same point. In
the diamond semi-lens the marginal rays will intersect the
axis at d, and in the glass semi-lens at g; the longitudinal ab-
erration being JF in the diamond\ and gF in the glass lens.

1			
\a			SINGLE MICROSCOPES.

21

In order to obtain a numerical increase of these aberra-
tions, Mr Pritcharjd computed them from the formula, and
found that of the diamond lens to be f-ths of its own thick-
ness, that of the glass lens being £ths of its thickness; and
by taking the thickness of the diamond lens to be 255, while
that of the glass is 758, he obtained fths of 255 = 108,
and £ths of 758 z= 884, and hence it follows that the actual
aberration of a diamond lens is only about one ninth of the
aberration of a glass lens of the same power and aperture.

If we suppose the diamond lens to be ground on the
same tool with the glass lens, so as to have the same cur-
vature, the same thickness, and t^e same diameter, the
longitudinal aberration of the diamond will be to that of the
glass lens as 43 is to 117, or nearly one third of it; and if
we suppose the focal length of both to be^\yth of an inch,
the magnifying power of the diamond lens will be 2133,
while that of the glass one will be only 800. In order that a
lens of glass may have the same magnifying power as that
of the diamond above mentioned, its focal distance would
require to be only the 200th part of an inch.

The durability of the diamond lens is also another va-
luable property, which allows it to be burnished into a disc
of metal, and taken out and cleaned without any danger of
being scratched. In treating of microscopic doublets and
achromatic microscopes, we shall have occasion to recur
again to the diamond lens.22

TREATISE ON THE MICROSCOPE.

Sapphire Lenses.

The ruby and the sapphire are the same substance,
differing only in colour. Mr Pritchard has, with his usual
success, executed many lenses of sapphire, which, though
inferior to those of diamond, are vastly superior to the
best executed lenses of glass. When a double convex
lens of sapphire and one of plate-g&ss are ground to the
same focus, so as to have the	Fig. 3.

same aperture and magnify-
ing power, their relative cur-
vatures are as 5 to 3, and their
thicknesses as shown in the
annexed figure, where A is
the section of a semi-lens of
sapphire, whose focus is at F,

and B a section of a semi-lens of glass, having its focus at
the same point. This figure points out in the clearest
manner another advantage of using the precious stonfcs in
place of glass. In sm&ll lenses of glass, the thickness of
the glass is such that there is no room between its ante-
rior surface and the object for the admission of instru*
fnents for dissection, and not even for the thinnest plate
of glass, so that it is impossible to use glass lenses of small
foci in viewing objects placed in glass sliders. If the pre-

SINGLE MICROSCOPES.

23

ceding figure represents lenses with a focus of ^th of an
inch, the distance of the glass lens B from F will be little
more than ^th of an inch, which is less than the thin-
nest glass.

In using the sapphire and ruby, or any precious stone
which refracts doubly, such as zircon, topaz, &c. for lenses,
we are exposed to a very serious defect, arising from the
duplication of minute lines, in consequence of the double
refraction of these crystals. In order to remedy this de-
fect, the optician endeavours to cut the stone so that the axis
of the lens may coincide with the axis of double refraction.
This, however, is a difficult tafek ;	if it w&re ac-

complished, we should not get rid entirely of the duplication
of the images, as in all double convex lenses, as well as in
plane convex lenses with the plane side turned to the object,
the rays cannot pass through the lens in parallel directions,
and therefore must suffer double refraction, however small.
It may be reduced, however, to the smallest possible
amount, and even to nothing, for pencils of rays diverging
from a point in the axis, by making the lens plano-convex,
and turning the plane side to the eye, as in the annexed
figure, where rays issuing
from F, and entering the eye	Fig. 4.

A

parallel at E, mustpass through ~~~ ~

the lens AB in parallel direc- jb-

tions, suffering all their refrac-£4>	TREATISE ON THE MICROSCOPE.

tion at the first or curved surface of the lens. By adopt-
ing this form and position of the lens, we, however, lose
more than we gain; for the lens is placed in the position
which gives a maximum spherical aberration. When the
magnifying power is not very high, the residual double re-
fraction is not injurious; and in proof of this we may state,
that we have in our possession a double convex lens of
sapphire, executed by Mr Pritchard, which exhibits mi-
nute objects with the greatest beauty and precision. The
only way, therefore, is to employ precious stones, such as
the diamond, the garnet, and the spinelle ruby, which have
no double refraction when their crystallization has been
regularly effected.

Garnet and Spinelle Ruby Lenses.

The garnet is superior in its structure to the spinelle
ruby, and the best and purest which we have seen is that
which is brought from Greenland, and has a slight tinge
of purple. We have used lenses made of this substance
by Mr Hill, Mr Adie, Mr BJackie, and Mr Veitch, all of
which exhibit minute objects with admirable accuracy and
precision ; and we can state with confidence, that we have
never experienced the slightest inconvenience from the
colour of the garnet, which diminishes with its thickness,
and consequently disappears almost wholly in very minute
lenses.SINGLE MICROSCOPES.

25

Single Fluid Microscopes.

Mr Stephen-Gray long ago proposed to construct single
microscopes with drops of water, which he lifted up with
a pin, and deposited in a small hole made in a piece of
brass. The drop retained a sort of imperfect sphericity,
and showed objects with some distinctness; but it is ob-
vious that the very weight of the drop destroyed its sphe-
rical form, even if it had not been disturbed by minute
irregularities on the circumference of the aperture in which
it was placed.

Sir David Brewster long ago constructed fluid lenses in
a different manner, so as to avoid the irregularities above
mentioned. He placed minute drops of very pure turpen-
tine varnish, and other viscid fluids, on plates of thin and
parallel glass. By this means he formed plano-convex lenses
of any focal length ; and by dropping the varnish on both
Sides, he formed double convex lenses, with their convexi-
ties in any required proportion. By freeing the glass
carefully from all grease with a solution of soda, the mar-
gin of these lenses was beautifully circular ; and the only
effect of gravity, which diminishes with the viscidity of
the fluid and with the smallness of the drop, is to elongate
the lower lens and flatten the upper one. These lenses

B26

TREATISE ON THE MICROSCOPE#

were found to answer well as the object-glasses of com-
pound microscopes.

After experiencing the extreme difficulty of obtaining
precious stones free of double refraction or difference of
density, and from little cavities and imperfections, as well
as the difficulty of giving their surfaces a perfect polish
and a correct figure, Sir David Brewster has, we under-
stand, recently made an extended series of experiments
on the formation of minute fluid lenses, which should equal
in power and distinctness those made of precious atones.
The primary difficulty which was encountered in this at-
tempt was that of depositing a sufficiently minute drop of
fluid upon a surface of glass. This arose from two causes,
from the difficulty of taking up on the slenderest fibre a
minute globule of a fluid of any moderate tenacity, and the
still greater difficulty of overcoming its adhesion to the fibre,
and laying a portion of it on glass. The first of these difficul-
ties he overcame by a suitable mixture of two fluids, and the
second by a mechanical process. Having thus succeeded
in obtaining lenses too small to be recognised distinctly by
the eye, he next endeavoured to make their figure approxi-
mate to the hyperbolic form when the lenses were not of
the smallest size, and the results which he obtained were
far beyond his expectation. Some of these lenses pre-
served their perfection for more than a year, and if pro-
tected from dust might have been kept much longer. IfSINGLE MICROSCOPES.

27

fluids could be obtained of a high refractive power, and
not of a volatile nature, microscopes of extreme perfection
might thus be readily constructed.

A single lens, by which light is both refracted and re-
flected, seems at first sight to be something paradoxical.
Such a lens, however, which was proposed and used by Sir
David Brewster, is shown in the annexed figure, where
ABC is a hemispherical plano-convex lens, which, if we
Ose it in the common way* will have a certain ffiagm*
fying power 5 but if we	Fig. 5.

magnifying power. Bi-	____

sect the semicircle BAC in A, and join AB, AC. If we
now place an object at mn, and look into the lens B A at F,
we shall see by reflection from the surface BC the object
mn, with the same distinctness and under the same angle
as if we had placed the two lenses Aa *Bd, A a Cd, with
their plane sides AB, AC together. Since the light is inci-
dent on the reflecting surface BDC at an angle of 45°,

Catadioptric Lenses.

ble convex lens of the —
same radius, and has —
consequently twice the

use it as shown in the
figure, it acts as a dou-28	TREATISE ON THE MICROSCOPE.

not a ray of it will be lost, as it suffers total reflection.
The lens thus used has less spherical aberration than when
used as a whole, ABC, and there is no error from imperfect
centring. This lens may be used as the object-glass of
a compound microscope 5 and it will be seen in another
section that it possesses other advantages than those which
have been mentioned.

The Grooved Sphere.

This lens derives its name from its having a deep
groove cut round it in the plane of a great circle per-
pendicular to the axis of vision. Sir David Brewster
was led to its construction by the doublet of Dr Wol-
laston, which will be described in another section. It
consists of a spherical lens or sphere, with a deep con-
cave groove cut round it, so as to cut off the marginal
pencils, and thus give a wider field and a more perfect
image. It is represented in the annexed figure, where
ABCD is a sphere of glass, having Fig. % ;
the unshaded parts below AC and
above BD cut away, in order to pre-
vent rays that fall very obliquely from
reaching the eye. The central thick-
ness of the lens may be made so small
as to render the spherical and even the chromatic aber-SINGLE MICROSCOPES.

29

ration almost insensible. As all the pencils pass through
the centre, every part of the image will be equally distinct;
a property possessed by no other lens whatever.

Mr Coddington,1 who entertains a high opinion of the
value of this lens, observes: " Besides all this, another ad-
vantage appears in practice to attend this construction,
which I did not anticipate, and for which I cannot now at
all account. I have stated, that when a pencil of rays is
admitted into the eye, which, having passed without de-
viation through a lens, is bent by the eye, the vision is never
free from the coloured fringes produced by eccentrical
dispersion. Now with the sphere I certainly do not
perceive this defect, and I therefore conceive that if it
were possible to make the spherical glass on a very mi^
nute scale, it would be the most perfect simple microscope,
except, perhaps, Dr Woll as ton's doublet,2 than which I
dan hardly imagine any thing more excellent, as far as its
use extends, its only defects being the very small field of
view, and the impracticability of applying it, except to
transparent objects seen by transmitted light. Now the
sphere has this advantage, that whereas it makes a very
good simple microscope, it is more peculiarly fitted for the

1 Phil. Trans. 1830, part i. p. 69-84.

* This exception was needless, as the doublet is not a simple
microscope, having two lenses placed at a distance.30	treatise on the microscope.

object-glass of a compound instrument,, since it gives a
perfectly distinct image of any required extent, and that,
when combined with a proper eye-piece, it may without
difficulty be employed for opaque objects," We do not
rightly apprehend the exact import of these observations*
Mr Coddington distinctly asserts that the grooved sphere
is the most perfect simple microscope, or the most perfect
microscope with one Jens; and yet he says in the next pa-
ragraph that It is only " a very good simple microscope,*
being " more peculiarly fitted for a compound instrument.*
With regard to the difficulty of making it on a small
scale, it is by no means great; for if we can grind and
polish its two surfaces, we may readily excavate it round
its margin. We have now before us a grooved sphere
of garnet ^th of an inch radius, executed by Mr Biackie:
The focus is almost close to the lens, which in many kinds
of observation is a great advantage, and its performance is
remarkably fine.

It will be seen in our article on Optics, that when the
refractive index of a sphere exceeds 2*000, its focus falls
within the sphere* Hence a grooved sphere made of dia-
mond is useless. When made with garnet, it is invalu-
| able, and its focus is just thrown so near its surface, that
| the objects may be laid upon its surface, or pressed against
! it by a concave surface of the same radius.SINGLE MICROSCOPES.

81

Crystalline Lenses of Small Fishes.

The crystalline lens of minnows and small fishes may
be taken out of the eye in a state of such perfection, that,
when used as single microscopes, they give a very perfect
image of minute objects. In such lenses, which have an
increasing density towards their centre, the spherical
aberration is almost wholly corrected. Great care, how-
ever, must be taken to make the axis of the lens the axis
of vision, to prevent its form from being injured by pres-
sure dgainst the aperture which holds it. The be& way
is to make a ring at the end of a piece of wire, having its
diameter a little greater than that of the lens. A ring of
viscid fluid is then made to line the ring of wire, and the
lens is suspended in the ring of fluid, some of the fluid en-
croaching upon its anterior or posterior surface.

Magnifying Power of Single Microscopes.

Having thus described the various kinds of single mi-
croscopes, we shall now consider the subject of their mag-
nifying power. When an eye in the prime of life, and
neither long nor short sighted, views the minutest object
which it can recognise, it will generally place it at the dis-
tance of about five inches. When the same object is
viewed through a single microscope, the distance at which32

TREATISE ON THE MICROSCOPE.

it is seen is equal to the focal length of the lens; and as
the apparent magnitude of objects is inversely as the dis-
tances at which they are seen, we have only to divide the
distance, five inches, by the focal length of the lens, in order
to know its magnifying power, or its apparent magnitude
when seen through the lens.

The following table shows the magnifying power of
lenses of all focal lengths, from 5 inches up to the 100th
of an inch, and is applicable to all lenses, of whatever sub^
stance they are made.

Focal Length.

Magnifying
Power.

Focal Length.

Inches.
5

2
Jf

P

i

f
i

I

TT

TV
tV

T4«
TT

1

2|

2j

H

4

5

n
10
15
20
25
80
35
40
45
50
55
60
65
70
75

Inches.

t

TT

1%

t

5

40

h)
5T

6J

I

Tnr

6single microscopes.

33

We have already mentioned the" advantages which the
precious stones have over glass ones, in having a much less
spherical aberration. In order that a glass lens may have
the least spherical aberration, its radii of curvature must
be as one to six, the flattest side being turned to the ob-
ject ; but this is not the case with bodies of a different re-
fractive power. Mr Coddington, in his valuable Treatise on
the Reflection and Refraction of Light,1 has computed the
ratio of the curves when the aberration 'is a minimum for
various indices of refraction from 1*5 up to 2*0, and the
amount of the aberration itself in parts of the thickness
of the lens. This table is very important in a practical
point of view, as will be seen from the observations which
follow it.

1 Page 111, Cambridge, 1829.

b 2S4	TREATISE ON THE MICROSCOPE*

Table showing the Spherical Aberration of Lenses of Glass
and the Precious Stones, and the proper proportion of their
Radii when the Aberration is a Minimum.

Substances.

Fluor spar................

Cryolite..................

Glass plate...............

Oil of almonds...........

Castor oil............

Honey.....................

Flint glass................

Quartz.....................

Topaz.....................

Oil of cassia..............

Glass, lead 1, flint 2....
Sulphuret of carbon

Sapphire...................

Ruby......................

Spinelle ruby............

Garnet....................

Glass, lead 2, flint 1^..

Sulphate of lead........

Glass, lead 2-L flint 1.

Zircon.....................

CalorneL..................

Sulphur...................

Phosphorus...............

Glass, lead 3, flint 1...,

Index of
Refrac-
tion.

1-4

1-5

1-6

1*7

1*8

1-9

20

Ratio of
the Radii
of a Lens
of Mini-
mum Aber-
ration.

1 to 3<

1 to 6

1 to U

1 to —93

1 to—12

1 to — 7

1 to — 5

Longitu-
dinal Aber-
ration, the
thickness
being 10.

10-96

10-71

933

6-66

3-57

1*66

0-62

It appears from the preceding table, that when the re-
fractive index is between 14 and 1-6 and a little more, but
less than 1*7, the second surface of the double convexSINGLE MICROSCOPES.

35

lens must be convex, in order to have the least spherical
aberration; but that when the index is above 1*6 or a
little more, the second surface must be concave, in order
to make the aberration a minimum, and this concavity,
in the case of zircon and sulphur, is so much as — 5, so
that in diamond it must be nearly — 3; so that we must
sacrifice a great deal of magnifying power in order to ob-
tain this advantage in diamond; but the sacrifice will be
well bestowed, for such a lens will be almost wholly free
of spherical aberration. It appears from the table, that
when the index of refraction is a little less than 1*7, a
plano-convex lens, with its plane side turned to the object*
is the best form for a single microscope.

Notwithstanding the difficulty of the task, we would
earnestly direct the attention of artists to the subject of
grinding plano-convex lenses of a hyperbolic form for
single microscopes. The smallness of a tens must increase
the difficulty; but in this case the effect may be accidental-
ly obtained, as it is often done in giving a parabolic and
an elliptical form to specula. Mr Potter has given, and
put to the test of experiment, a method of obtaining any
curve derived from revolution, by giving a particular form
to the grinding and polishing tools.1

1 Edinburgh Journal of Science, new series, No. 12.36

TREATISE ON THE MICROSCOPE.

CHAPTER II.

DESCRIPTION OF MICROSCOPIC DOUBLETS AND TRIPLET?

Under this chapter we propose to describe all combina-
tions of lenses in which two or more are placed in con-
tact, or at such a distance that no image is formed be-
tween them.

Wollastoris Periscopic Doublet.

The earliest proposal of a doublet lens was that which
Dr Wollaston described in 1812,1 under the name of a peri-
scopic microscope. The following is his own description of
it: " The great desideratum," says he, " in employing high
magnifiers, is sufficiency of light; and it is accordingly ex-
pedient to make the aperture of the little lens as large as
is consistent with distinct vision. But if the object to be
viewed is of such magnitude as to appear under an angle

1 Phil. Trans. 1812, p; 375.MICROSCOPIC DOUBLETS AND TRIPLETS.	S7

of several degrees on each side of the centre, the requisite
distinctness cannot be given to the whole surface by a
common lens, in consequence of the confusion occasioned
by oblique incidence of the lateral rays,. excepting by
means of a very small aperture, and proportionable dimi-
nution of light. In order to remedy this inconvenience,
I conceived that the perforated metal which limits the
aperture of the lens might be placed with advantage in its
centre; and accordingly I procured two plano-convex
lenses ground to the same radius, and applying their plane
surface on opposite sides of the same aperture, in a thin
piece of metal (as is represented by a section, fig. 7), I
produced the desired effect, hav-	,

ingvirtually a double convex lens,	Fig. 7.

so contrived that the passage of
oblique pencils was at right angles
with its surface, as well as the
central pencil. With a lens so
constructed, the perforation that
appeared to give the most perfect distinctness was about one-
fifth part of the focal length in diameter; and when such an
aperture is well centred, the visible field is at least as much as
20° in diameter. It i& true, that a portion of light is lost by
doubling the number of surfaces; but this is more than
compensated by the greater aperture which, under these
circumstances, is compatible with distinct vision."38

TREATISE ON THE MICROSCOPE.

Periscopic Sphere.

In the preceding passage Dr Wollaston describes only
a double convex periscopic microscope* consisting of two
plano-convex lenses; but he does not take the case where
these two lenses are hemispheres, and, consequently,
where the doubly convex one which they form is a
sphere. When the lenses are not hemispheres, the pencils
are not centrical, and the rays from different parts of the
object do not each suffer the same species of refraction as
they do when passing through a sphere, so that every part
of the field is not equally distinct It is obvious also that
Dr Wollaston did not think of filling up the central aper-
ture with a fluid of the same refractive power as the leris,
in order to remove the loss of light from the double num-
ber of surfaces, which he mentions as a defect in his mi-
croscope.

The construction of a periscopic sphere proposed by
Sir David Brewster combines these two properties. The
lenses, whether they are hemispherical or not, must be so
placed that their convex surfaces form part of the same
sphere, as shown in the annexed figures.MICROSCOPIC DOUBLETS AND TRIPLETS.

39

Fig. 8*	Fig. 9.

Fig. 10.	Fig. 11.

In fig. 8, the plano-convex lenses AB, CD may be ce-
mented to a piece of plate glass mn, of such a thickness in
one direction as to complete the sphere, and of such a
diameter in the direction mn as to form a suitable contrac-
tion of the aperture.

In fig. 9, the space between the lenses may be filled up?
as in the figure, by two pieces of plate-glass A;wwB, CmnD,
between which is a plate of brass, or of thin black paper,
containing a suitable aperture in the centre of the lenses.
Here there are six surfaces within the sphere*but, by uniting
them with a proper cement, the whole becomes a single
sphere, in which there is no perceptible loss of light.

In fig* 10, lenses of unequal size and thickness are com-40	TREATISE ON THE MICROSCOPE.

bined, either with a plate of glass, or a fluid between them
of the same refractive power as the glass, and the aper-
tures may be placed on the plane surfaces AB, CD.,

In fig. 11, two convex lenses may be combined into a
sphere, the intermediate portion being filled up with a fluid
of the same refractive power. This may be readily done
by cementing each lens on the end of a brass or glass tube
ab, and introducing the fluid by an aperture.

In uniting these lenses, it is obviously necessary that
the convex outer surfaces should be exactly of the same
curvature, and of the same kind of glass. The best way
of effecting this is to grind a
thick lens ABC, either great-	Fig. 12.

er or less than a hemisphere,

and then to bisect it at AD,

and out of the portions ADB,

ADC to form two plano-con-

vex or two double convex B	D	c ?

lenses AB, CD. The outer surfaces of these two lenses
will belong accurately to the same sphere. The perfect
similarity of the inner surfaces is of less consequence, as
all refraction by them is removed by the cement.

By these different steps Sir David Brewster was led to
the idea of the grooved sphere, or the lens already de-
scribed under the head of single microscopes, in which the
aperture is contracted by excavating the sphere all round
in one of its great circles.MICROSCOPIC DOUBLETS AND TRIPLETS.	41

Periscopic Achromatic Spheres.

If, in the construction shown in fig. 13, in place of mak-
ing the fluid op of the same refractive and dispersive power
as the two lenses, we make it such as
to correct the colour of these lenses, Fig. 13.

o

we shall obtain an achromatic sphere,

as proposed by Sir David Brewster 5
and the compound lens may be still far-
ther improved as a microscope, by at-
taching, either by cement or #not, to
the back of the first lens AB a'convex speculum of silver
or steel for illuminating the object, the contracted aperture
being the hole in the centre of the speculum. The con-
cave fluid lens is shown at op ; and such a curvature must
be given to the convex reflector mn, that it may reflect pa-
rallel or diverging rays upon the object.1

Mr Coddington,2 in his excellent treatise on optics, thus
treats of the preceding contrivance.

" An achromatic sphere may be constructed by interpos-

1	Edinburgh Philosophical Journal, voL iii. 5 and Mr Pritch-
ard's Treatise on Optical Instruments, in the Library of Use-
ful Knowledge, p. 41, § 66.

2	Treatise on the Reflection and Refraction of Light, p. 256.42

TREATISE ON THE MICROSCOPE.

ing between two crossed lenses (double convex ones), in
opposite positions, a concave lens of a medium more highly
dispersive than that of the lenses, adjusting the curvatures
so that the outer surfaces of the crossed lenses shall be
portions of the same sphere, and that the interior surfaces
shall exactly fit into each other.

" In such a system, we may consider the effect to be the
same as that of an entire sphere of the same medium as
the two lenses diminished by that of a concave lens hav-
ing for its refractive ratio that which exists between the
two media in question.

" Thus, if [Jj1 be the ordinary index of refraction for the
convex lenses, for the concave one, <r: 1 the ratio of
the dispersive powers, r the radius of the sphere, s9 <r those
of the internal surfaces, we must have

(g-,)e+7)'2^

whence it follows, that - + -=2

* * 1*2—t*i r

For example, let = 1,5; == 1,65; * = x 1,25.

■ 1 1 1 ,5 1 1

Then, 4- ~~ zzl -?■ *—^ sz -—~

s ■ o* 4 j,5 r 1, 2r

If the first internal surface be plane, or 5= oo, we have

in general <r = ~ —-r ; or, in this particular exam-

4 —— 1

pie, <s = l,2r, or <s : r == 6 : 5."MICROSCOPIC DOUBLETS AND TRIPLETS,	43

Doublet of no Aberration.

So long ^go as 1668, a doublet was described in the Phi-
losophical Transactions, in which
a large and fiat field was obtained.	Fig. 14-

Th is subject, however, was ne-
ver investigated with care till Sir
John Herschel took up the sub-
jectinl821. The doublet, made
of glass, which Sir John proposes
for obtaining perfect distinctness in microscopical observa-
tions is shown in the annexed figures, Fig. 15.
14,15, the convex sides being turn-
ed to the eye when the doublet is .

used as a microscope, and to the sun
when it is used as a burning-glass.

The following are the radii and focal length of these lenses.

Fig. 14. Fig. 15.

Focal length of the first lens........+ 10*000 + 10*000

Radius of its first surface............ + 5*833 + 5*833

Radius of its second surface......— 35*000 — 35*000

Focal length of the second lens......+ 17*829 + 5*497

Radius of its first surface*.........*.+ 3*688 + 2-954

Radius of its second surface........ + 6*291 + 8*128

Focal length of the combined lenses + 6*407 + 3*47444	TREATISE ON THE MICROSCOPE.

" Whether we ought or ought not," says Sir John, " to
aim at the rigorous destruction of rays parallel to the axis,
the use to which the lens is to be applied must decide.
In a burning-glass it is of the highest importance. A
slight consideration will suffice to show, that the differ-
ence of temperatures produced in the foci of a double con-
vex lens of equal radii, and one of the same focal length,
but of the best form, must be very considerable. Jn order
to try whether even the latter might not be improved by
the shortening of the focus, and the superior concentra-
tion of the exterior rays, by applying a correcting lens of
one of the forms above calculated, in spite of the loss of
heat in passing through a second glass, I procured two
lenses to be figured to the radii assigned in the first co-
lumn of the foregoing table. They were about three inches
in aperture, and when combined as above described, the
aberration was almost totally destroyed, and probably
would have been so completely had the index of refrac-
tion proper to the glass been employed instead of that
adopted in our calculation for brevity. Their combined
effect as a burning-lens appeared to me decidedly supe-
rior to that of the first lens used alone, and there is there-
fore good reason to presume, that the effect of the other
construction, which, with the same loss of heat, affords a
much greater contraction of the focus, would be still bet
ter 5 and I regret not having tried it in preference. 'MICROSCOPIC DOUBLETS AND TRIPLETS.	45

" In eye-glasses and magnifiers, if we would examine
a minute object with much attention, as a small insect, or
(when applied to astronomical purposes) if we would scru-
tinize the appearance of a planet, a lunar mountain, the
nucleus of a comet, or a close double star, where extent
of field is of less consequence than perfect distinctness in
the central point, too much pains cannot be taken in de-
stroying the central aberration.''1

Mr Pritchard informs us, that he has executed some of
these doublets for the object-glasses of compound micro-
Scopes, " for which," he says, " they answer remarkably
well, but their angle of aperture is small compared with
combinations of double achromatics."2	T

HerscheVs Periscopic Doublet.

We owe also to Sir John Herschel the construction of
a periscopic doublet with a very large field of moderate
distinctness. " In spectacles," says he, " reading-glasses,
magnifiers of moderate power, and eye-glasses for certain
astronomical purposes, the correction of the aberration in
the centre of the field may be sacrificed with little incon-
venience. By far the best periscopic combination I am

1	Phil. Trans. 1821, p. 246-248.

2	Microscopic Cabinet, p. 163.46

TREATISE ON THE MICROSCOPE.

acquainted with consists of a double convex lens of the best
form, but placed in its worst position (radii as 6 to >) for
the lens next the eye, and a plane concave whose focal
length is to that of the other as 2*6 to 1, or as 13 to 5,
placed in contact with its flatter surface, and having its
concavity towards the object, as
in the annexed figure, for the	Fig. 16.

farthest; yet for destroying the
aberration of rays parallel to
the axis nothing can be worse.

In fact our formula gives for the
aberration in this construction 22*02, or about 22 times what
the best single lens of equal power would give; yet, on
accidentally combining two such lenses in this manner, I
was immediately struck with the remarkable extent of ob-
lique vision, with the absence of fatigue on reading some
lines with a power much beyond that of the natural eye,
mid with die freedom from colour at the edges of the field,
arising from the opposition of the prismatic refractions of
the two solids* an advantage which a single meniscus does
not possess*' The focal length of the compound lens
which Sir John tried was 1*84 of an inch. The field of
tolerably distinct vision extended fully 40° from the axis,
and the letters of a book might be read, and the forms of
objects distinguished, with management, as far as the 75th
degree. In using such a combination the lenses should
be very thin, and the eye applied as close as possible.MICROSCOPIC DOUBLETS AND TRIPLETS.	47

Piano* Convex Doublet.

A doublet of much simpler construction, and with its sphe-
rical aberration greatly diminished, has also been proposed by
Sir John Herschel. It is represented in the annexed figure,
and consists of two con-
vex lenses of equal focal	Fig. 17.

lengths, the convex sides
being placed in contact,

and the eye and object
opposite the plane sides.

In this case the aberration
will be only 0*6028. But if we make the focal length of
the first to that of the second as 1 to 2*3, the aberration
will be reduced to 0*2481. In order to have an idea of the
value of such a doublet, we shall give the series of aberra-
tions for single lenses, as investigated by Sir John Herschel.

Aberration.

Plano-convex, plane side first.*........................ 4*2

Plano-convex, convex side first........................10*81

Double convex.....................................................1*567

Best form of a single lens................*........... ... 1*00

Doublet of two equal plano-convex lenses........... 0*6028

Doublet of two plano-convex lenses with their

focal lengths as 1 to 2*3.............................. 0*2481

Doublet of garnet, with suitable focal lengths...... 0*0848

TREATISE ON THE MICROSCOPE.

As these calculations are made for glass, the advantage
of such a doublet made of garnet or sapphire must be much
greater. In a garnet the minimum aberration takes place
when the form of the lens differs very little from that of
plano-convex; and as it is then to that of glass as 35 to
107, the aberration will be only about 0*08, or next to im-
perceptible.

The consideration of the Huygenian eye-piece for astro-
nomical telescopes suggested to Dr Wollaston the proba-
bility that a similar combination should have a similar
advantage, of correcting both achromatic and spherical
aberrations, if employed in an opposite direction as a micro-
scope.1 With this view, he took two plano-convex lenses,
the ratio of whose focal lengths was as 3 to 1, and he
placed them as in the annexed figure, so that the distance of
their plain surfaces was from 1 to

of the focal length of the small-	Fig. 18.

est. The plane sides of the lenses ,------,

are towards the object. The ad- '---1

vantage of the first lens having its

Mr Coddington, has shown that such a correction is impossible.
(Treatise onjhe Eye and Optical Instruments, p. 55.)

Wollastoris Doublet.MICROSCOPIC DOUBLETS AND TRIPLETS. , 49

plane side next the ofcject is, as Dr Woliaston states, that if
it should touch a fluid, the view is not only nortmpaired,
but ^improved, whereas a double convex lens would require
to be taken out and cleaned. The following excellent ob-
servations on this doublet are made by Pritchard.

" Having mounted some plano-convex lenses of the rela-
tive foci named by Dr Woliaston, in such a manner that
the distances might be varied at pleasure, I was surprised
to find, that after the doublet was adjusted by trialy so as
to obtain the maximum of distinctness, the distance be-
tween the lenses did not aecord either with the rule given
by Huygens, or that of Dr Woliaston. Supposing that I had
not got the combination intended by Dr Woliasttfn, I pro-
cured several doublets made by different artists, and to my
astonishment found they agreed with my own, am! there-
fore presumed the doctor was mistaken in the distance by
the thickness of thelenses and the minuteness of the space
between them. The distance which appeared to me essen-
tial to obtain the best effect, is the difference of the focal
length ef the hoo lemes, making a proper allowance for their
thickness. The proportion of the foci of the two lenses
may be varied ad libitum. All that is requisite in this re-
spect is, that the difference must be greater than the thick-
ness of the anterior lens, while it may be observed (in high
powers), that the greater the difference between their twd-

focal lengths, the more space will be left in front; and as

c50

TREATISE ON THE MICROSCOPE,

this is of great practical importance, they should never be
less than as 1 to 3. I have made some very good ones,
differing as much as 1 to 6, Another advantage resulting
from attending to this point is, that we do not lose so
much magnifying power in such combinations as when,
the difference between the lenses is less.

" The delicacy and beauty with which these doublets ex-
hibit the structure of tissues will justify my entering into
some minute details respecting them. The following is
necessary to insure their goodness.

■ " First, The convex surface of each lens must be truly
spherical. If this is not obtained, it will be in vain to pro-
cure a good doublet, however beautifully the lenses may
be polished, or accurately adjusted. From this circum-
stance, I have found globules perform very well, providing
they are free from air-bubbles, which, however, is rarely
the gase. It should be observed, that a slight scratch on
their surface is trifling compared to air-bubbles; for the
latter not only stop"the light, but, by the reflection around
the edges of each bubble, produce considerable fog or glare.
Second, The distance between the lenses is the next point
of importance; its adjustment is best accomplished by
trial, mounting the lenses in such a manner that their dis-
tance can be varied at pleasure, and capable of being
turned round, so as to adjust the centring. When this
is obtained, they should be fixed so that their distance andMICROSCOPIC DOUBLETS AND TRIPLETS.	51

position cannot be altered. This it is necessary to regard,
as I have sometimes spent whole days in re-adjusting $
doublet that had been separated to examine the lenses
singly. Third, The stop or diaphragm, for limiting the
aperture in these combinations, should be placed immedi-
ately behind the anterior lens. From the difference of the
situation of this stop in the various doublets I have exa-
mined, it will appear that their makers did not know that
the field of view depended upon the plane of the stop. I
have found, that when the stop is situated close behind the
anterior lens, no other is required, and the field is enlarged
without sensibly augmenting the aberration. On this ac-
count, the lenses of the finest doublet, when used singly
with the same aperture as combined, has so much aberra-
tion and distortion that distinct vision cannot be obtained,
even with the most rigid adjustment of the focus. From
the difficulty of procuring a flat surface, somo makers have**
worked the anterior surface of the lens next the object
concave : these lenses do not possess any advantage in
point of performance, not even to compensate for the loss
of power from the negative side.''

Mr Pritchard remarks, that when the lens next the ob-
ject is a jewel, the performance of the doublet is improved;
but that he has not observed any advantage when both
lenses are gems. This must be a mistake ; for lenses of any
gem, that are superior to glass ones when acting singly,52

TREATISE ON THE MICROSCOPE*

must, if suitably combined, be superior also when united. In
proof of this, we have a garnet doublet before us, executed
by Mr Blackie, the performance of which i» quite remark-
able. The lenses are made of Elie garnets, and their con-
vex sides are placed towards each other. The radius of
the smallest lens near the object is -^th (one seventieth) of
an inch, [and that of the other ^th (one twentieth) of an
inch. Its magnifying power is very high, exceeding great-
ly that of the semi-jewel doublet made by Mr Pritchard,
with a sapphire lens ^thi (one sixtieth) of an inch focus,
combined with a glass lens x^th (one tenth) of an inch focus.

In the experiments to which we have already referred
on fluid lenses, Sir David Brewster has constructed doub-
lets, one of the lenses of which is made of glass or a pre-
cious stone, and the other a fluid one; and he has been
enabled to increase the power of any single lens by the
^'addition of a fluid one, till the object touches the anterior
surface of the object-lens. He has also made fluid spheres
contracted in the middle, and fluid doublets in which the
centring is effected by an infallible process.

Pritchards Triplet.

Upon the same principle as the doublets, Mr Pritchard
has constructed triplets, the third or posterior lens having
a longer focal length than the two others. This combina-MICROSCOPIC DOUBLETS AND TRIPLETS*	53

tion requires much more precision in the adjustment, and
more attention in the centring. Mr Pritchard remarks,
that, "when perfected, they amply repay the pains be-
stowed upon them, in the accuracy with which they ex-
hibit the most difficult lined objects, though it is to be re-
gretted that neither these nor the doublets of deep power
will show pleasantly cylindrical bodies of large diameter,
such as a large mouse or bat's hair." Having long made
use of one of Mr Pritchard's triplets, we can amply confirm
the account which he has given of the excellence of this
combination. MrBlackiehas executed for us a triplet, the
centre lens of which is garnet, the posterior one of quartz,
and the anterior one of flat glass* It is a very powerful
combination, and performs admirably.

Sir David Brewster has made triplets in which two of
the lenses are fluids and the third a solid, and some in
which they are all fluids.

Single Achromatic Microscope.

In many of the doublets which we have already describ-
ed, the chromatic aberration is partially, and sometimes
greatly corrected, but still not to such a degree as to en-
title them to the name of achromatic. Ttie great improve-
ment which has taken place in the art of grinding and
polishing small lenses, has enabled the optician to execute54

TREATISE ON THE MICRO'SCOPK.

double and triple achromatic lenses, having a diameter so
small as from £th to T^th of an inch. Mr Pritchard has
made them of the latter size with an angle of aperture
of 65°. These lenses may be advantageously used where
very high powers are not required, or when they cannot
be applied, though it is usual to employ them as the ob-
ject-glasses of compound microscopes, as we shall afters-
wards see.

Sir David Brewster has executed achromatic lenses, both
double and triple, by combining a fluid concave lens with
one or two convex lenses of the precious stones or glass*
When the lenses are double, the fluid lens is of course a
xneniscus in which the concavity predominates, as it is
impossible to form a fluid lens doubly concave.

Single Reflecting Microscopes.

Single reflecting microscopes are not much in use.
They consist of a concave metallic speculum of a short focal
length, so that any minute body placed in its focus will
be seen magnified. The form of the speculum should of
course be parabolic. Such a microscope is principally use-
ful for looking at one's own eye, or any part of it not far
from the pupil. In these cases no image is formed, as the
rays enter the eye parallel,

t The following ingenious contrivance for a fluid reflect*MICROSCOPIC DOUBLETS AND TRIPLETS.	55

ing microscope we owe to Mr Stephen Gray.1 Having
taken a small globule of quicksilver, and dis-
solved it in a menstruum of 10 parts of water Fig. 19.
and 1 of nitric acid (aquafortis), he dipped the <^3
end of a stick in this solution, and rubbed with
it the inner circle of the ring A, so as to give
it a mercurial tincture. This ring is made of
brass, and is about the 30th of an inch thick,

having its mean diameter not exceeding §^hs
of an inch. When the inner surface of the
Ting wetted with the solution has been wiped
dry and laid upon a table, pour a drop of
quicksilver within it, and when this drop is gently pressed
with the ball of the finger, ifc will adhere to the ring, arid
when cleansed with a hare's foot will form a convex spe-
culum. If the ring and speculum are now taken up and
carried horizontally, and laid on the margin of the hollow
cylinder B, the mercury will become a concave reflecting
speculum, in consequence of its upper surface sinking
down by gravity. The cylinder B rests upon a pillar with
q. screw on its outside, and supported by the base D. A
stage, ECFG, may be moved up and down, so as to place
the object, which is fixed at G, in the focus of the concave
speculum.

1 Phil. Trans. 1697> No. 228, p. 539.56

TREATISE ON THE MICROSCOPE.

CHAPTER III*

ON COMPOUND MICROSCOPES,

A compound microscope is an instrument in which $
distinct and enlarged image of an object is formed by an
object-glass or a speculum, and this enlarged image again
magnified by one or more eye-glasses.

There is every reason to believe, that tie earliest com-
pound microscopes* [which were used by Zansz and 6a*
lileo, consisted of a convex lens for art object-glass, and a
concave one for an eye-glass,, like the telescope which was
at that time in use.

Fontana i» 1646 used two convex lenses; Dr Hooke
three, and Eustachio Divini four; the two next the eye
being plano-convex, and placed in contact, with their con-
vex sides towards each other, to give a high power and a
large and flat field. In 1691 Philip Bonnani1 used a com-

1 Ofoervationes circa mventia, qua in rebus non viventibus reperiuntm►COMPOUND MICROSCOPES.

57

pound microscope with three lenses, and added to it an il-
luminating apparatus with two lenses.

The reflecting compound microscope was first suggested
by Sir Isaac Newton, and its construction varied and made
more complex by Dr Barker and Dr Smith of Cambridge.
The simple contrivance of Sir Isaac has in modern times
been greatly improved by Amici, Potter, Tulley, Cuthbert,
and Dr Goring, whose labours in improving the micro-
scope have been indefatigable, and in the highest degree
successful. An account of these improvements will be
given in this chapter.

The principle of the compound microscope with two
lenses will be understood from the annexed figure, where
MN is a minute object placed in the focus of the object-
glass AB, or rather a little farther from it than its princi-
pal focus. An image of this ob-
ject will be formed at mn, at some	Fig. 20.

distance behind AB, the distance e

n A is to AM, their distances from the lens AB. If we now
view this magnified image mn through an eye-glass EF, so

Compound Refracting Microscope.

nA increasing as AM diminish-
es. The size of the image mn will
be to that of the object MN as

c 258

TREATISE ON THE MICROSCOPE.

placed that mn is in its principal focus, we shall again
magnify it in the inverse proportion of En to the distance
at which the eye sees minute objects most distinctly, which
is about five inches. The object MN is thus doubly mag-
nified, so that if wA is six times AM, and the lens EF has
& focal length of half an inch, the magnifying power will
be 6 X 5-7-^ —60. While the lenses are the same, the
magnifying power may be increased to any extent by in*
creasing the distance between the lenses EF and AB; but
the object becomes indistinct as the magnifying power in-
creases, so that it is not advisable to make the distance
nA more than five, six, or seven inches; or calling/ the
focal distance of the eye-glass, D = AM, d = An, and A
the distance at which we see objects distinctly, then this

■ $0'

magnifying power M will be M = -jj X -j.

- In this arrangement of lenses the field of view is small,
and therefore we cannot see the whole of many objects at
one view. In order

to remedy this, a large	Fig. 21.

lens, called the ampli-	»

as shown in fig. 21,

where nm is the image formed by AB alone; but in con-

fying glass, is placed
between the image
and the object-glass,COMPOUND MICROSCOPES?*

59

sequence of the interposition of GH, it is contracted into
>/*, and this contracted image is magnified by the eye-
glass EF. In order to widen the field still farther, and make
it flat, two plano-convex lenses have been priced at E, F,
having their convex sides in contact.. In order to find the
magnifying power when the lens.GIJ: is used, we must
multiply the magnifying power, as obtained by the preced-
ing formula, by the quantity <p representing the focal

length of the lens GH and L = --- — d —f b being

the distance between the first and second glasses, ^nd d
the distance between the first and third glasses

Dr Goring prefers the following method of finding the
magnifying power of these microscopes. Measure the
aperture of the object-glass and call it a, make AM = f
and having measured with a micrometer scale the diame-
ter of the usual pencil of rays before they enter the eye,
call it d; then a :/= d : F, F being the focal length of a
single lens having the same power as the compound micro-
scope. But the magnifying power m of a lens F is
Hence the magnifying power M of the compound micro-
scope will be M =

In accurate investigations with the microscope, the com-
mon compound refracting instrument above described is of60	TREATISE On THE MICROSCOPE.

little use, and has been superseded by better combinations.
As it is, however, very serviceable for common purposes?
we shall describe one of them of the most convenient and
modern construction. This instrument is shown in Hate
IV. fig, 19, where the body of the microscope and all
Its parts are seen. They all slide on a triangular bar
S, the sliding pieces having springs on one side* The
three lenses within the body of the microscope are shown
at EF, GH, AB, as in the last figure. The object MN is
placed on a slider attached to the stage PQ. The reflec-
tor for illuminating the field is at R, being moved by a
Hooke's universal joint at XX. A condensing lens is
placed at C, for increasing the light reflected from the mir-
ror R. The body of the instrument is adjusted to distinct
vision of the object MN, by a rack and pinion working a
triangular bar at right angles to the bar S. It is often
thought preferable to move the stage towards the micro-
scope, rather than the microscope towards the stage; but
as the motion of the stage often disturbs animalculse, and
may derange the position of inanimate objects, the motion
of the body of the microscope has many advantages. When
the stage, however, is moveable, it is made to consist of
three platesv The upper plate, which carries the slider-
holder, is secured with a screw across the middle plate,
which middle plate is carried across the lower stage-plate
by a similar motion, but in a direction at right angles toCOMPOUND MICROSCOPES.

61

the upper plate. By these movements the object can be
placed in any position with regard to the axis of the fixed
microscope, first by turning the one screw and then the
other.

Some attention is requisite in adjusting the apertures of
the object-glasses employed. The smaller they are, the les&
will be the spherical and chromatic aberration of the ob-
ject-glass, but the less will be the light. When a plano-
convex lens about half an inch focus is used, its plane side
should be towards the object, and its aperture limited to
jJjth of an inch.

A compound microscope is sometimes so constructed
that it can be used as a single microscope. This is done
by screwing the lower end of the bodjr round the object-
glass into a projecting arm at the top of the stand. When
the body is unscrewed and removed, a single lens or a
doublet or triplet may be screwed into the same place,
and the moveable stage, with the slider and object, brought
near the single lens, just as if it had been the object-glas»
of a compound microscope.62

TREATISE ON THE MICROSCOPE.

Dr Goring1s Improvement on the Object-Glass of the Com-

When the compound microscope does not require to
have a high power, a

trived the combinauon

shown in the annexed figure, where A is a plano-con-
vex lens, with its flat side next the object, having its
focal distance about one half or two thirds that of the piano
or double convex lens B* A stop D is placed in the posr
terior focus of the object-glass A. Mr Pritchard remarks,
that when the focal" length of A " is not less than half
an inch, this combination has been employed with consi*
derable advantage, both as regards distinctness and aper*
ture."

pound Microscope.

compound object-glass
of two lenses m
advantageously er
ed. Dr Goring1 h£

Fig. 22.

1 Quarterly Journal, vol. xvii. p. 202.COMPOUND MICROSCOPES.

63

Mr Coddingtoris Improvement on the Eye- Glasses of the
Compound Microscope,

The improvement suggested by Mr Coddington on the
eye-pieces of compound microscopes is shown in Plate
IV. fig/20. The object of the contrivance is to fulfil
the condition proposed by Huygens in his excellent tele-
scopic eye-piece, namely, to have the refraction of tliq
pencils divided between the two lenses, and to produce
the greatest possible flattening of the field. Mr Codding-
ton found that the most
proper form of the lenses	Fig. 23.

was that shown in the
annexed figure, where ?*-
the two eye-glasses con-
sist of a meniscus A next the eye, and a double equi-
convex lens B, while the field-glass is composed of two
meniscuses C, D. Mr Coddington, however, informs
us, that he " found no sensible error arise from the sub-
stitution of plano-convex lenses for the meniscus-glasses,
which are difficult and expensive to form.,, He remarks
also, that theory indicates H a farther flattening of the
field to be made by separating the eye-glasses a little,
which requires the distance of the first eye-glass from
the field-glass to.be diminished by about half as much
but he did not perceive any improvement arising from64

TREATISE ON THE MICROSCOPE.

this alteration in practice, and therefore he does not re-
commend the change. The object-glass which Mr Cod-
dington uses in this microscope is shown at o, and is the
grooved sphere proposed by Sir David Brewster.

In his treatise on the eye and optical instruments, Mr
Coddington has proposed a different combination for the
eye-glasses and the field or amplifying-glass. Supposing
the distance between the object-glass and field-glass to be
1 inch, and the focal length of the field and eye-glasses 1
inch each, and the distance between the field-glass and near4- *
est eye-glass 1 inch, he finds the distance of the two eye-
glasses to be £th of an inch. He finds also, " that all in-
distinctness arising from the oblique refractions will be
corrected when the field-glass is convexo-convex, nearly
convexo-plane, the first eye-glass convexo-convex1 (ra-
dii as 3 ; 1), and the second eye-glass a meniscus2 (radii
as 1:5)."

Taking another case, he supposes the distance of the
object-glass from the field-glass to be 2 inches, and the
eye-glasses to be in contact, as in fig. 23, then " it ap-
pears that for the achromatism we must have the distance
between the field-glass and second eye-glass 1 inch."
Then the field-glass must be convexo-plane nearly; the

*	The flattest side next the eye.

*	The most convex side next the eye*COMPOUND MICROSCOPES,

65

first eye-glass equi-convex, and the second eye-glass a me-
niscus with the radii as 1:5.

Dr Goring, who examined one of the instruments con-
structed on these principles, states, that both the chromatie
and spherical aberration of the objective part was wholly
untouched, and that the eye-piece, consisting of four glasses,
was achromatic. He adds also, that nothing can surpass
the beauty of the field of this microscope for extent of
flatness. Now we think that Dr Goring has mistaken Mr
Coddington, who never pretended to correct the spherical
and chromatic aberration of the object-glass.1 He consi-
ders the chromatic and spherical aberration of the grooved
sphere, which is the object-glass he uses, as reduced to very
small quantities, by leaving only a small channel in its axis
for the passage of the rays. Whatever the residual aber-
ration may be, Mr Coddington is not answerable for it, as
his object was merely to make the other part of the mi-
croscope good, which, according to Dr Goring, he has suo
ceeded in doing.

1 Treatise the Eye and Optical Instruments, p. 58, 59, § 329.66

TREATISE ON THE MICROSCOPE.

Compound Achromatic Microscope.

Although the achromatic microscope has only recently
come into use as an effective and superior instrument, yet
it can scarcely be considered as a new instrument. Every
person knew that an achromatic object-glass was most de-
sirable in a compound microscope. So early as 1776 Euler
proposed to employ them in compound microscopes ; but
so late as 1821 M. Biot considered their introduction as
out of the question, from the impracticability of achroma-
tizing lenses as small as those which the microscope re-
quires.

In 1823 M. Selligues and Dr Goring were both occu-
pied with the subject, the former having employed MM.
Chevalier, two excellent opticians in Paris, and the latter
Mr Tulley, to execute small achromatic object-glasses.
It is to M. Selligues, however, in so far as we can
learn, that we are indebted for the new and happy idea
of making the object-glass consist of four achromatic
compound lenses, each consisting of two lenses. This
idea is the actual source of the superiority of the achro-
matic microscopes; and in proof of this we may state, that
Professor Amici, who had early been following out the
old idea of a single achromatic object-glass, abandoned his
attempts in 1815, but afterwards successfully resumed
them by adopting M. Selligues' plan of the superpositionCOMPOUND MICROSCOPES.

67

of several object-glasses. In M. Selligues' instrument the
focal length of each object-glass was 18 lines, its diameter
6 lines, and its thickness at the centre 6 lines. In that
of Amici the focal length of each was about 6 lines; and
MM. Chevalier have executed them having only 2 lines in
focal length. More recently, however, Mr Pritchard has
surpassed all these artists, by making them of one sixteenth
of an inch in focal length.

From this brief historical detail, we shall proceed to
give a more minute account of the lenses executed by
these different individuals, for which we are indebted to
Mr Jackson Lister, whose able memoir on this subject is,
as we shall see, one of the most valuable contributions to
the science of the microscope that has for a long time ap-
peared.	*;

The achromatic object-glasses of M. Selligues' micro-
scope made by MM. Chevalier, consisted of a plano-concave
lens of flint glass, and a double convex one of crown or
plate glass, with their inner curves cemented together by a
mixture of mastic and turpentine, to remove the reflection
of the interior surfaces, and prevent the introduction of
dampness. Four of these lenses, of from 1^ to 1§- of an
inch in focal length, were made to screw before each other,
so as to be used either all together, or any of them indivi-
dually, in the usual manner, like the object-glasses of a
compound microscope, The aberration of colour was-thus68

TREATISE ON THE MICROSCOPE.

corrected in a considerable degree, but the glasses were
fixed in their places, with their convex sides towards the
object, which is their worst position; and in consequence
of this, the spherical aberration was enormous, and was dis-
tinctly seen, even with the small aperture to which it was
necessary to reduce them.

Notwithstanding this defect, the grand idea of the com-
bination was acquired; and M. Chevalier having observed
the mistake committed by M. Selligues, made them of less
focal length, and more achromatic; and turning the concave
lens to the object, he produced in 1825 an instrument far
above that of M. Selligues. His deepest glasses were four
tenths of an inch in focal length ; and in his first micro-
scope two such compound lenses were combined - for his
highest power.

The date of Fraunhofer's achromatic microscopes is
not known. Many years ago the writer of this article or-
dered an achromatic object-glass from Fraunhofer for a
large microscope, fbr the purpose of making a particular
class of observations; but at that time he seems not to
have made any compound lenses to be cotnbined after the
manner of Selligues. Mr Robert Brown1 has a series
of five such object-glasses, recently obtained from Utz-

1 Phil. Trans. 1830, p. 188.' COMPOUND MICROSCOPES.

69

Schneider, whose focal lengths are from 1*8 to 0*43 of an
inch.

When Professor Amici visited London in 1827, he
brought with him some compound object-glasses, which
performed very well; and Mr Lister has since learned from
him, that he has executed a combination of 2#7 lines in
focal length, and 2*7 lines in aperture, which greatly ex-
cels the former.

Among the most successful improvers of the achromatic
microscope we must rank Mr Jackson Lister, who has dis-
covered some curious and valuable properties of these
lenses that have escaped the notice of the most skilful
analysts. Mr Lister has investigated the subject entirely
as a matter of observation, and therefore his results are
more likely to have a higher practical value.

Mr Lister takes as the basis of a microscopic object-
glass two conditions, 1. that the flint glass shall be plano-
concave ; and, 2. that it shall be joined by some cement to
the convex lens. The first condition obviates the risk of
error in centring the two curves, and the second dimi*
Dishes by nearly a half the loss of light from reflection,
which is very great at the numerous surfaces of a combi-
nation of compound object-glasses.

Now Mr Lister has found that in every such compound
lens which he has tried, whether the flint glass was Swiss
or English, with a double convex of plate glass, which has70

TREATISE ON THE MICROSCOPE.

been rendered achromatic by the form given to the outer
curve of plate glass, the ratio between the refractive and
dispersive powers has been such that its figure has been
correct for rays issuing from some point in its axis not far
from the principal focus on its plane side; and these rays
either tend to a conjugate focus within the tube of the
microscope, or emerge nearly parallel.

If AB represents such an object-glass, let us suppose
that it is free from spherical and achromatic aberration
for a ray FDEG radiating from F, then the angle of emer-
gence GEH will be about three times as great as that
of incidence FDI. If the radiant point is now made
to approach the lens, the angles of incidence Fig. 24*.
and emergence will approach to equality,

and the spherical aberration produced by the
two will bear a less proportion to the oppos-
ing error of the single correcting curve ABC,

and hence in this case the rays will be over-
corrected for such a focus.

- As F continues to approach the lens, the
angle of incidence continuing to increase, it
will exceed that of emergence, which has *

been in the mean time diminishing, so that
the spherical aberration produced by the two
outer surfaces will recover their original pro-
portion. When F has reached this point F'' (at which theCOMPOUND MICROSCOPES.

71

angle of incidence does not exceed that of emergence so
much as it had at first come short of it), the rays will again
be free from spherical aberration. If F" still comes nearer
the lens, or is carried beyond F in the opposite direction,
the angle of incidence in the former case, or of emergence
in the latter, becomes disproportionately effective, and in
either case the aberration exceeds the correction, or the
rays are under-corrected. Hence Mr Lister gives the fol-
lowing rule.

That in general an achromatic object-glass, of which
the inner surfaces are in contact, or nearly so, will have on
one side of it two foci in its axis, for the rays proceeding
from which the spherical aberration will be truly corrected at
a moderate aperture ; that for the space between these two
points, its spherical aberration will be over-corrected, and
beyond them either way, under-corrected'*

Mr Lister found also, " that when the longer aplanatic
focus is used, the marginal rays of a pencil not coincident
with the axis of the glass are distorted, so that a coma is
thrown outwards, while the contrary effect of a coma di-
rected towards the centre of the field is produced by the
rays from the shorter focus." These interesting results
obviously furnish the means of destroying both aberra-*
tions in a large focal pencil, and of thus surmounting what
has been hitherto the chief obstacle to the perfection of
the microscope. And when it is considered that the72

TREATISE ON THE MICROSCOPE.

curves of its diminutive object-glasses have required to be
at least as exactly proportioned as those of a large tele-
scope, to give the image of a bright point equally sharp
and colourless* and that any change made to correct one
aberration was liable to disturb the other, some idea may
be formed of what the amount of that obstacle would have
been. It will, however, be evident, that if any object-
glass is but made achromatic, with its lenses truly worked
and cemented, so that their axes coincide, it may with
certainty be connected with another possessing the same
requisites, and of suitable focus, so that the combination
shall be free from spherical error also in the centre of its
field.

For this the rays have only to be received by the front
glass B, from its shorter aplanatic focus F, and transmitted
in the direction of the larger correct pencil FA of the
other glass A. It is desirable that the latter pencil should
neither converge to a very short focus, nor be more than
very slightly, if at all> divergent; and a little attention at
first to the kfnd of glass used will keep it within this range,
the denser flint being suited to the glasses of shorter focus
and larger angle of aperture. If the two glasses which
in the diagram are drawn as at some distance apart, are
brought nearer together (if the place of A, for instance, is
carried to the dotted figure), the rays transmitted by B
in the direction of the larger aplanatic pencil of A, willCOMPOUND MICROSCOPES.

73

plainly be derived from some point (Z) more distant than
F", and lying between the aplanatic foci of B; Fig. 25.
therefore (according to what has been stated) this
glass, and consequently the'combination, will
then be spherically over-corrected. If, on the	\

other hand, the distance between A and B is in-
creased, the opposite effects are of course pro-
duced.

In combining several glasses together, it is
often convenient to transmit an under-corrected
pencil from the front glass, arid to counteract its
error by over-correction in the middle one.

Slight errors in colour may, in the same manner, be de-
stroyed by opposite ones; and, on the principles described,
we not only acquire fine correction for the central ray, but,
by the opposite effects at the two foci in the transverse
pencil, all coma'can be destroyed, and the whole field ren-
dered beautifully flat and distinct.1

Compound Achromatic Microscopes with Solid and Fluid
Lenses.

In 1812, a very simple method was employed by Sir
David Brewster, for tnaking both single arid eoriipoiMd

1 Phil. Trans. 1830, p. 199.

•d74

TREATISE ON THE MICROSCOPE.

achromatic microscopes. Almost all objects are seen to
the greatest advantage when immersed in a fluid, even the
finest test objects, such as the scales of the Podura. Hav-
ing placed the object on a piece of glass, he put above it
a drop of an oil having a greater dispersive power than
the single lens, or than the concave lens which formed
the object-glass of the microscope. The lens was then
made to touch the fluid, so that the surface of the fluid
was as it were formed into a concave lens. Now if the
radius of the outward surface of this lens was such as to
correct the dispersion, we have here a perfect achromatic
microscope, both simple and compound. The best way is
to over-correct the colour of the plate-glass lens by the
fluid, and then to reduce the dispersion of the fluid by
mixing it with one of a less dispersive power, This will
be understood from the annex-
ed diagram, where AB is an
unequally convex lens, the flat-?

test side of which is plunged
in the fluid mn9 placed in a
watch-glass CD., The object is
placed at mn, and the disper-
sion of the concave surface of the fluid compensates that
which is produced by the lens. All errors of centring
are here removed, and also the loss of light at the touch-
ing surfaces of solid lenses. If AB is a single microscope,

Fig. 26.

COMPOUND MICROSCOPES.

75

the object mn will be placed in its principal focus, and
the emergent parallel rays will enter the eye; but if it is
the object-glass of a compound microscope, an image will
be formed a few inches behind AB, by withdrawing AB a
little from mn, or placing the object a little without its prin-
cipal focus. We have already had occasion to describe
an achromatic grooved sphere, but in the process of ach-
romatizing it, the sphere loses in a very small degree its
valuable property of refracting in the very same manner
all the pencils that enter the eye. This property, how-
ever, may be preserved in the bird's-eye sphere by the
achromatic method which we have now described. Let
AB be the 'grooved sphere, and CD the watch-glass con-
taining the fluid; it is obvious that every ray which passes
through the centre of the sphere will enter and quit it
perpendicularly, without suffering any refraction. The
same mode of achromatizing the sphere AB may be adopt-
ed with a solid concentric concave
lens ABCD of flint-glass or other
substance, or the sphere may be pla-
ced between two such concentric
lenses. The greater the dispersion
of the flint-glass, the nearer must the
outer surface CD approach to AB.

By these means the grooved sphere

Fig. 27.

76

TREATISE ON THE MICROSCOPE.





may be rendered perfect, both as a single microscope and
as the * object-glass of a compound one.

The principle above described may be applied to a sys-
tem of object-glasses like those of Selligues' microscope.
Let A, B, E be three convex
lenses,^so placed at the end
of the tube of a compound
tnicroscope, that the high-
ly dispersive fluid in the
watch-glass CD will enter
between the glasses A, B, 0,

and E. The concave lenses
of fluid will over-correct the
three lenses A, B, and E; but if a very deep curvature
on the outside of A is not sufficient to compensate this
over-correction, it may be effected by a suitable lens at
F. If the three lenses are made of the precious stones,
with a high refractive power and a low dispersive one, the
concave fluid lenses will not over-correct them.

If we use muriatic acid in the form of butter of anti-
mony, and containing a due quantity of metallic particles,
for the fluid, and crown-glass for the lenses, the secondary
colours will be completely corrected, and an instrument of
the most superior kind produced. If a permanent and
portable aplanatic object-glass is preferred, the butter of
antimony may be placed between a meniscus and a piano-compound microscopes.

77

convex lens of crown glass, as in the annexed

Fifir* 29.

figure, where o is the object, CD a menis-
cus of crown-glass, AB a plano-convex lens,

and mn a concave lens of the fluid. This
construction of the object-glasses of com- aL
pound microscopes is much more easily ap-
plicable in the case of the microscope than
in that of the telescope. In the latter case,

the colour of the fluid, the changes which it undergoes
by time, and the difficulty of retaining it, are objections
of considerable amount; but in the case of the micro-
scope, the colour of the fluid disappears owing to its small
thickness, and it may be retained by capillary attraction
alone, and renewed as often as we choose.

Description of Mr Pritehard's Compound Achromatic Mi-
croscope.

This instrument is represented in Plates V. VI. and VII.
fig. 21, 22, 23, as fitted up by Mr Pritchard. All its parts
are so distinctly shown in the figures that they require
no description, especially as the uses of most of the parts
have been described in a former chapter. Fig. 21 is a
perspective view of the instrument m its most convenient
position for examining transparent objects by reflected
light. The stops and condensing illuminator, which are78

TREATISE ON THE MICROSCOPE.

seen under the stage, should be removed when particular
objects are viewed. When test-objects are to be viewed
by direct light, the instrument can be turned round. Fig.
22 shows the position of the instrument for dissecting.

' The rest for supporting the hands is shown at a, and the
large moveable stage at b. Fig. 23 shows the proper po-
sition of the instrument for viewing opaque objects by the
concave reflector c. In front of c, the object is placed upon
a black or white ground, according to its nature; and
the light of the candle, collected and thrown upon the mir-
ror c by the condensing lens, is again reflected by the
mirror upon the object. Fig. 24 is an eye-piece; and fig. 25
represents an apparatus for holding a bottle to show aquatic
plants and animals.

In Mr Pritchard's instrument the following are the di-
mensions and powers of the lenses for a complete micro-
scope.

Sidereal Focal	» , « Magnifying Powers in Dia-

Length in Parts A £ °	meters by a Standard of

of an Inch.	Aperture.	5 Inches.

I 16°	60 to 100

^ 21	100 to 360

i 42	240 to 500

i 55	500 to 1100

-iV 65	900 to 3000
Of these object-glasses, that whose focal length is £th of
an inch appears to be the most perfect and useful.COMPOUND MICROSCOPES.

79

Compound Reflecting Microscopes.

Sir Isaac Newton seems to have been the first person
who described a reflecting microscope. He communicated
his plan to Oldenburg in 1679, as shown in the annexed
diagram, where AB

is a concave specu-	Fig. 30.

lum, O the object,

A.	n

F the place where
an image of it is

3	"

formed, and CD an

eye-glass for magnifying it. In another letter to Oldenburg,
dated 11th July of the same year, he refers to another im
provement on microscopes, which is to " illuminate the object
in a darkened room, with the light of any convenient colour,
not too much compounded; for by that means the micro-
scope will, with distinctness, have a deeper charge and
larger aperture, especially if its construction be such as I
may hereafter describe." We are not aware that this idea
was ever further developed by its author.1



1 Brewster's Life of Sir Isaac Newton, p. 31U80

TREATISE ON THE MICROSCOPE.

Mr Potter's Improvement upon it.

Mr Potter1 has recently described " a new construction

of Sir Isaac Newton's microscope," principally with the
view of removing the difficulty of illuminating the object.
His first construction was for opaque objects; and in order

to illuminate them, he cut a large circular aperture abc in
the tube, between the object and the speculum ; but the
light which fell on the sides of the tube occasioned a good
deal of indistinctness in the field of view. This defect,
however, was completely removed by lining all the lower
parts of the tube with black velvet. Mr Potter found it
advantageous to concentrate the light on those objects
that required it, by a large lens at d. For transparent
objects he applied a lens, as shown at e. Its convergent
beam is reflected on the object placed at the end of the
wire a, fixed to a handle h, by means of a small diagonal

Fig. 31.

* Edinburgh Journal of Science, Jan. 1832, No. 11, p. 61.compound microscopes.

81

mirror in the axis of the tube, and inclined to this axis
45°. By this means a very strong light may be thrown
through and past the object. By means of moveable caps
to cover the opening abc, and the lens e, all interference
of foreign light is prevented; and without altering the po-
sition of the object, both methods of illumination may be
successively adopted. Mr Potter attaches his objects to
thin brass pins a stuck into wooden handles h7 and these
pins pass through a slit cut into a small piece of cork
attached to the sliding piece g, which at the same time
carries the lens e and the plane mirror, the whole of which
are moved by the small arm connected to the crank, as at
i. The adjustment of the object to the focus of the mir-
ror is effected by turning a nut attached to the pivot on
which the crank is fixed.

In the microscope used by Mr Potter, he employs a spe-
culum one inch in diameter, with a focal length of 1^ inch;
and he generally employs a distance of from 12 to 14#
inches between the object and the image.

This size of the speculum allows him to place an insect
or other object of ^th of an inch square in the tube, with-
out any perceptible bad effect resulting from it.

When Mr Potter had adjusted the illuminators in the
manner which we shall afterwards have occasion to de-
scribe, he " saw quite easily what are called the diagonal
lines on the scale from the wing of the white cabbage

d 282

TREATISE ON THE MICROSCOPE.

butterfly, which has been proposed as a difficult test object
by Dr Goring; and it is such a one as those who have only
seen the stronger longitudinal striae or scales from the
wings of moths and butterflies have little idea of." Mr
Potter was also able to resolve a delicate blue tissue in the
web of a spider called the clubiona atrox, into its compo-
nent fibres.

The great size of speculum used by Mr Potter arises
from his being able to give all his specula a true ellipsoidal
figure, so as to remove all spherical aberration.1 We have in
our possession two of Mr Potter s instruments, one of them
with a spherical and the other with an ellipsoidal mirror.
The quantity of light and the defining power of the latter
are unusual in such instruments.

Amicus Reflecting Microscope.

This instrument is shown in section in the annexed
figure, where a is a small ellipsoidal speculum about 1
inch in diameter, and 2-^ths in focal length. The object
is placed on a stage m», below the tube of the microscope,
and the rays which issue from it fall upon a small specu-
lum b inclined 45° to the axis of the ellipsoidal speculum,

1 The process by which he does this is fully described in the
Edinburgh Journal of Science, No. 12, p. 228, new series.COMPOUND MICROSCOPES*

83

in the same manner as if the object had been placed in the
tube as far to the right hand of the small mirror as it is
below it. An image of this object is of course formed in
the other focus of the ellipsoidal speculum, and may be
viewed by a single or double eye-piece, as in other com-
pound microscopes. Professor Amici, however, uses a
negative eye-piece, consisting of two piano-con vex. len^s
A, B.

Fig. 32:

The new and peculiar part of this instrument is the use
of the small speculum, which allows the object to be placed
without the tube, and illuminated with the utmost facility.
Dr Goring, to whom science is indebted for the perfection
of the reflecting microscope, remarks, that " the instru-
ment was turned out of Professor Amici's hands in a rough
and ineffective state, owing to the concave metal being of
too long a focus and too small an angle of aperture, and the
diagonal one (or small mirror b9 which was half an inch in
diameter) of too large a diameter, which caused it to in-
tercept too large a quantity of light from the other, leav-
ing only a narrow rim of reflexion to enter the reting,
which occasioned a disagreeable nebulosity in the middle84

TREATISE ON THE MICROSCOPE.

of the fi^Id of view, unless the eye-glass was of great
depth."1 '

Dr Goring9s Improved Reflecting Microscope.

Mr Cuthbert, an ingenious London optician, constructed
one of Amici's instruments, the speculum having 1| inch
of aperture, and a focal length of 3 inches, and the body of
the microscope being about 1 foot long. Dr Goring and
he having tried it on the test objects which the doctor
had newly introduced, found its performance quite unsa-
tisfactory. Dr Goring, therefore, recommended that the
speculum should be only half an inch in focal length, and
the body 4 or 5 inches long. Mr Cuthbert accordingly
finished a pair of metals six tenths of an inch in focal length,
and only three tenths in diameter. The excellent perform-
ance of this instrument induced Dr Goring and Mr Prit-
chard to turn their attention to its improvement; and, as
Mr Cuthbert2 has been able to execute perfectly ellipsoidal
metals, having an aperture equal to their sidereal focal

1	Goring and Pritchard's Micrographia, p. 23.

2	The process by which Mr Cuthbert is able to accomplish this

difficult task is similar to that by which he gives truly hyperbolic

figures to the mirrors of small Gregorian telescopes, with three

inches of aperture and five inches of focal length.COMPOUND MICROSCOPES.

85

length, or 54°, and of so small a diameter as three tenths
of an inch, they have been able to produce an instrument
of a very perfect kind.

This microscope is represented in Plate VIII. fig. 26,
where the instrument is seen to rest on a tubular pillar,
its body being held by a split socket. The pillar is screw-
ed to a solid cruciform stand, to one of the legs of which
an adjusting screw is applied, to produce steadiness. The
body moves round a cradle joint at the top of the pillar,
and may be firmly fixed at any degree of inclination. The
body of the microscope is shown at a, the eye-tube at d,
and the eye-piece, which is a Huygenian one, at e. The
focal lengths of the interior glasses of the eye-pieces, of
which there are usually three, are three fourths, three
eighths, and three sixteenths of an inch. The tube con-
taining the specula is shown at be. The triangular bar
which carries the illuminating reflector, the stage, and the
apparatus for adjustment, is shown at f, and is soldered to
the neck of the body. The mirror k is plane on one side,
and has a plaster of Paris surface on the other. The stage
I is a combination of rack and screw work, wrought by two
concentric milled heads at m. The smallest of these
moves the object in the direction of the body, and the
other in an opposite direction. The stage can be lifted out
of the triangular socket g, which carries the adjusting screw
i for obtaining distinct vision, and the clamping screw h.86

TREATISE ON THE MICROSCOPE.

When the body and stand are used for a compound ach-
romatic microscope, a tube, shown in Plate IX. fig. 27,
and containing the compound object-glasses below it, in-
creasing in diameter from the object, is screwed into
the body at b} in place of the tube be. A rectangular
prism, shown in dotted lines, reflects the pencils that
pass through the object-glasses along the axis of the tube
be to the eye-piece e.

The following sets of metals are made for the reflect-
ing microscope.	;

A i n Distance between
No. ^ Solar Focus. A ®	Object and side of

Aperture.	the Tube.

	1...................2 inches. 13J°.................J inch..

	2..................1 ..................j

a..................A	.................A.

4..................h	3 6£...................5V

5..................fo	41 J.........almost 0

6..............55

The metals No. 1, 2, and 3 are those most useful for
examining opaque objects. No. 3 is excellent al|gibr all
kinds of transparent objects. No. 5 can scarcely be used
for opaque objects, as it leaves almost no space between
the tube and the object for allowing the latter to be illu-
minated. No. 6 cannot be used at all for opaque objects,
but is especially intended for the most difficult class o»f
transparent test objects.COMPOUND MICROSCOPES.

87

Dr Smith's Reflecting Microscope.

Having constructed one of Sir Isaac Newton s micro-
scopes in 1738, Dr Smith of Cambridge observed that the
colours of objects were much more beautiful and natural
than in refracting microscopes. He found that objects
were very distinct and sufficiently light when the micro-
scope had the following dimensions :—

Focal length of the speculum........................inches.

Diameter of ditto............. *.......................1

Focal length of the plano-convex eye-glass....... 2£

Ratio of the distance of the object from the fo- '

cus of the speculum to the focal distance of

the speculum.......................................1 to 19. '

Finding that, in order to obtain a high magnifying
power, the speculum required to be very concave and
small, he contrived another microscope with two reflecting
spherical surfaces of any size, but so related to each other
that the second reflexion should correct the aberration of
the first.

Dr Smith's microscope is shown in fig. 33, where AA is
a concave spherical speculum, having its polished convex
surface inwards. The rays from an object a placed in the
slider mn will be reflected from the concave speculum
AA upon the convex CC, and will have a distinct and
magnified image of it formed before the convex eye-glass88

TREATISE ON THE MICROSCOPE.

Fig. 33.

E, by which it will be magnified still more. This instru-
ment, in short, is nothing more than the Gassegrainian te-
lescope converted into a microscope, with this difference
only, that in the telescope distinct vision is obtained by
moving the convex mirror, whereas in the microscope
it is obtained by a motion of the eye-glass. Dr Smith
constructed one of these microscopes, which he found to
perform " nearly as well, in all respects, as the very best
refracting microscopesand the writer of this article has
one of them now before him, which performs wonderfully
well, though both the specula have their polish consider-
ably injured. It shows the lines on some of the test ob-
jects with very considerable sharpness.

The following are the dimensions, &c. of Dr Smith's
reflecting mirror, as given by himself:—

Focal length of both specula.............................1*0000

Distance of the centres of both specula...............1*6558COMPOUND MICROSCOPES.

89

Distance of the image from the centre of the con-
cave speculum............................................1#1337

Focal length of the eye-glass............................0*1407

Distance of the eye behind the eye-glass.............0*1479

Diameter of the eye-hole.................................0 0190

Distance of the object from the centre of the con-
vex speculum............................................0*0626

Length of the concave speculum...................15° 49'

Arch of the convex speculum....................... 4° 50* 49"

Distance of the stop s from the object.................0*4545

Diameter of the stop s....................................0*038

Diameter of the hole in the concave speculum......0*143

Diameter of the hole in the convex speculum........0*049

Magnifying power, the focal length, See. of the eye

being 8 inches.......................................300 times.

The dimensions of the instrument in our possession is
very different:

Diameter of the concave speculum...............2*17 inches.

Focal length..........................................2*17

Diameter of the hole in it...........................0*376

Diameter of the convex speculum................1*03

Diameter of hole in it...............................0*10

Diameter of stop.....................................0*13

Distance of stop from hole in convex speculum..0*67

Distance of specula..................................3*80

Focal length of doubly convex eye-glass........0*1790

TREATISE ON THE MICROSCOPE.

Sir David Brewster's Reflecting Microscope.

Notwithstanding the excellence of Professor Amicus
miscroscope, as constructed with Dr Goring's improve-
ments and with Mr Cuthbert's specula, we are quite
convinced that it does not owe these advantages to the
peculiarity in its construction which constitutes it a dif-
ferent instrument from Newton's. This peculiarity is
in our opinion a disadvantage, and we consider the in-
strument as recommended solely by its possessing an ellip-
soidal speculum, with a large angle of aperture. The only
advantage which can be ascribed to Amici's instrument is
a more convenient mode of illumination, thougli not much
more so than Mr Potter's \ but this advantage, whatever
be its amount, is purchased at great sacrifices. 1. The
whole instrument is an awkward-looking piece of mecha-
nism, with its triangular bar and all its appendages dang-
ling at one end of it. 2. It cannot be used in the vertical
position, which we consider a very great defect. 3. By
the use of the small reflecting speculum, more than one
half of the whole light is lost. 4. With small concave spe-
cula, such as those °f an inch in diameter, opaque ob-
jects cannot be illuminated, as stated by Dr Goring.

The construction which has been proposed by Sir David
Brewster to remedy most of these defects is shown in. theCOMPOUND MICROSCOPES.

91

Fig. 34.

annexed figure, where ABC is the
body of the instrument, which screws
at its lower end C into the horizon-
tal projecting arm DE of the stand,

either of the achromatic microscope
or the single microscope, so that we
get rid of all trouble about the ob-
jects placed at mn> and their mode of
illumination, as every thing concern-
ing them is the same as in other mi-
croscopes. This, we must say, is a
great advantage; for neither natu-
ralists nor amateurs are disposed to
purchase and use two sets of the
extended apparatus necessary for
holding, moving, and illuminating	35 ®

microscopic objects. At the lower end C of the body,
exactly where the object-glasses of the compound and
achromatic object-lenses are placed, is a small tube abed,
at the lower end of which is placed the concave speculum
cd3 perforated with a very small hole at its centre, and
with its concave surface upwards. Above it is the plane
speculum s, fixed by a slender arm to the side bd of
the small tube, and having its diameter a little greater
than the perforation in the speculum cd. This little
tube abed screws into the arm DE, as if it were a micro-92

TREATISE ON THE MICROSCOPE.

scopic doublet or single lens; and the body ABC may
either screw upon the outside of this tube, or, what is bet-
ter, upon a stronger piece of tube forming part of the arm
DE. A concave illuminating reflector kh, for opaque ob-
jects, may screw on the back of the speculum cd, or that
speculum may be made thick, and ground and polished on
both sides, so that while one side magnifies the objects,
the other illuminates them.

It is obvious, that rays proceeding from an object at mn
will be reflected from the plane speculum e, upon the con-
cave speculum cd, exactly as if the objects were placed at
r, as far above e as mn is below it, and an image of it would
be formed in the other focus of the ellipsoid, r being the
one focus, if the rays were not intercepted by the eye-piece
AB, by which the image is farther magnified. By this
mode of construction, the whole of the reflecting micro-
scope, in place of having a separate stand and separate ap-
paratus costing a large sum of money, is comprehended
in the little tube abed, and may be considered as a reflect-
ing object-speculum, forming part of a general microscope,
furnished with single lenses, doublets, and compound ach-
romatics.

By the means now described are removed all the de-
fects which we enumerated as belonging to Amici's com-
bination, except the third, which is one of such import-
ance that it is of consequence to consider how far it isCOMPOUND MICROSCOPES.

93

capable of being remedied. Sir David Brewster has
proposed to get rid of this loss of light by placing the ob-
ject wm, as in Amicus instrument, outside of the tube, but in-
clined to its axis, and refracting its rays upon the specu-
lum cd, by means of an achromatic prism e, in a manner ana-
logous to his method of producing a similar effect in the
Newtonian telescope.1 The faces of this prism are equally

inclined to the axis of the microscope and the axis of the
pencil issuing from the point of the object under examina-
tion. As the prisms of plate and flint glass which compose
e are cemented by a substance of nearly the same refractive
power, there will be no farther loss of light than what is
reflected at the two surfaces. A socket may be placed at
D, for holding an illuminating lens, or the little appara-
tus for opaque objects, shown in Plate IX. fig. 27. But
in order to avoid the encumbrance and expense of separate
stands and apparatus for this, as well as Amici's form of the
instrument, we would propose that a strong piece of tube
should be inserted in the opening, above mn, to screw into

Fig. 35.

* Treatise on Optics, Lardner's Cabinet Cyclopaedia, p. 354.94>	TREATISE	MICROSCOPE.

the upper side of the projecting arm, as shown in the pre-
ceding figure; or a solid screw attached to the upper side
of the tube, a little to the right hand of C, and above the
opening, might screw into the lower end of the projecting
arm DF. In these cases the object at mn will be placed
on the ordinary stage, and illuminated in the common man-
ner ; but it will be necessary to have a counterpoise at D,
to balance the weight of the body ABC.

Those who are acquainted with the principle of the
Cassegrainian telescope, and of Dr Smith's compound mi-
croscope, will readily see that the reflecting microscope,
with the perforated speculum, may be converted into a
more compound reflector, analogous to Dr Smith's, by
making the little speculum e, fig. 35, convex, the figures of
d and e being made hyperboloids.POLARISING MICROSCOPES.

95

CHAPTER IV.

ON POLARISING MICROSCOPES,

The use of polarising microscopes is to observe struc-
tures and phenomena which are invisible with the common
microscope.

These microscopes were first used by Sir David Brews-
ter, upwards of twenty years ago, in his experiments on
the structure of apophyllite, amethyst, and analcime, and
other mineral bodies; and also in his examination of
various animal and vegetable organizations. In using the
single microscope for these purposes, he cemented plates of
agate and tourmaline, with Canada balsam? to the plane sidp
of a plano.-convex lens,1 and thus analysed the polarised
light, by means of which the peculiar structure was render*
ed visible. In other cases he preferred for the analyser an
achromatised prism of calcareous spar, in which one of the

1 Edinburgh Transactions, vol. ix. jp. 141, npte.96

TREATISE ON THE MICROSCOPE.

images only was visible,1 or one in which he had extin-
guished one of the images by a particular process,2 which
he has described.

When considerable magnifying power was necessary,
or when the structure was to be drawn by an artist, he
used the compound microscope, in which the light was
polarised and analysed by various means, accommodated
to the nature of the structure to be examined.

Single Polarising Microscope.

The simplest and most useful polarising microscope is
a hand one, such as AB, con-
taining a convex lens mn. It'	Fig. 36.

is to be held in the right hand,

as in the figure, and a plate
of light reddish-brown tour-
maline, abed, fixed above the
lens, either temporarily by a
little bit of soft wax, or ce-
mented to it by Canada balsam. The last method has
the advantage of preventing the loss of light by reflection
from the first surface of the tourmaline, and removing any

1 Edinburgh Transactions, vol. viii. p. 371.
8 Ibid. vol. ix. p. 141, note.

aPOLARISING MICROSCOPES.

97

imperfection of polish that it may have. It would be ad-
visable, indeed, to construct the microscope with two plano-
convex lenses, and to place the tourmaline between them,
and joining it to both by Canada balsam, so that there
would be no loss of light or imperfection of vision pro-
duced by the surfaces of the tourmaline.

The position of the plate abed should be such, that
when it is held as in the figure, thepolarised light, which
is to illuminate the object, should be unable to pass through
it. This polarised light may be obtained either from light
reflected, at an angle of 56°, from a plate of black glass,
or from a bundle of plates of crown or flint glass, or by
transmission through a bundle of such plates, or from one
of the images of a rhomb of calcareous spar.

When this light is obtained, the observer holds the mi-
croscope AB in his right hand, and examines through it
the object in his left hand, turning AB slightly round, so
as to bring it into the position when it refuses to trans-
mit any of the polarised light which passes through the ob-
ject, and towards which, of course, the observer's eye is
directed. When this is done, the peculiar structure of
the object will depolarise or alter the polarisation of part
of the incident light; and this light, being no longer po-
larised, will pass through the plate of tourmaline to the
eye, and exhibit, on a dark ground, and in luminous and
often beautifully coloured lines, the structure of the body.

E98	TREATISE ON THE MICROSCOPE.

If the body is transparent, and not flat, it may be ad-
vantageously placed in a little glass trough, containing
water or^pil, or a fluid of the same refracting power as
the body, so that the polarised light may be made to pass
through it in $11 directions, and exhibit its entire struc-
ture.1

When the shape and surface of the body present no
difficulties, the best method is to stick it by * transparent
cement, or simply to place it upm a plat^ of tourmaline
held in the left hand. The observer thus carries the po-
lariser and the object in his left hand, and in his right the
magnifier and the analyser.

When a second plate of tourmaline is not at hand, the ob-
ject may be placed upon a rhomb of calcareous spar, above
one of the images which that rhomb forms of a circular
aperture on its lower or farther side, the light of the other
image being stopped out by a piece of wafer.

When'a small lens is needed, and strong light can be
commanded, the magnifier and the analyser may be united
in one by making the magnifying lens of tourmaline.

1 It was in this way, by cementing fragments of crystals of
analcime to a piece of wood, and holding the mineral in a small
trough of almond oil, that Sir David Brewster detected the ex-
traordinary structure of that substance.POLARISING MICROSCOPES.

99

Compound Polarising Microscope.

The simplest form of the compound polarising micro-
scope is to make the eye-glass into an analyser, in any of
the ways described for a single lens, the proper position
of the plate of tourmaline being readily found by the mo-
tion of unscrewing the eye-glass. The polariser is also a
plate of tourmaline, laid on the slider-holder, and having
the object laid upon its upper surface. If the polariser is
laid down in any accidental position, the proper position
of the analyser will be found by a slight unscrewing of
the eye-glass. The best method is to place the small po-
larising piece of tourmaline (which need not be larger
than an object which fills the field of the microscope)
between two pieces of glass, with Canada balsam inter-
spersed. In this way a compound microscope may be
converted into a polarising one, fit for any researches, at
the expense of a few shillings.

When tourmaline cannot be obtained, the light may be
polarised by one or more plates of glass, placed on the il-
luminating mirror so that their surface may be inclined
34° to the axis of the microscope, and the analyser may
be a chip of black, blue, or any other kind of glass, having
the reflection from its second surface removed by grinding,
or by a few drops of black wax. If this chip is placed cn100	TREATISE ON THE MICROSCOPE.

the brass ring above the eye-glass, so as to turn with that
ring, and so that its surface is inclined 34>° to the axis of
the microscope, the observer, by looking into a little re-
flector, will see the object under examination when the
plane of this analysing plate is at right angles to the plane
of the polarising plates,

When the compound microscope is fitted up in the com-
pletest manner, and for the express purpose of examining
structures by polarised light, which we believe was first
done by Henry Fox Talbet, Esq., a NicolV prism is fixed
between the illuminating mirror and the slider-holder, to
polarise the light, and another similar prism is placed above
the eye-glass. The last prism, howrever, is very inconve-
nient, as it contracts unpleasantly the field of view; and it
is therefore necessary to substitute for it a plate of tour-
maline, as already described.

The expense of constructing a Nicol's prism, the diffi-
culty of making the one next the eye perfectly colourless,
and the risk of a change taking place in the cement which

1 This ingenious prism, consisting of two pieces of calcareous
spar cemented together, so as to transmit only one image, derives
its name from its inventor, William Nicol, Esq. of Edinburgh, and
is of great use in all experiments on the polarisation of light, par-
ticularly where the colour of tourmaline would interfere with the
phenomena to be observed.POLARISING MICROSCOPES.

101

unites the two parts of it, render it desirable to have a
simple, a cheap, and a durable substitute for it. The po-
lariser which has been employed by Sir David Brewster
in his experiments on elliptical polarisation, and on the
action of crystallised surfaces upon light, where tourma-
line could not be used owing to its colour^ was a single
rhomb of calcareous spar, with its natural surfaces having
thin plates of colourless glass cemented to them by Canada
balsam, which removes any imperfection of surface, and at
the same time protects the surfaces from any accidental in-
jury, or from the deterioration of the polish, which arises
from frequently cleaning them. This
rhomb ABCD had a circular aperture Fig. S7,

a,	placed upon its lower surface, and of
such a diameter, that it just separated
the two images b c, seen from above.

This rhomb may be placed either be- c^

neath the slider-holder, or upon it, and,

by sticking a piece of wafer upon any one of the images

b,	c, and leaving the other exposed, and placed exactly be-
neath the aperture of the object-glass, we have the most
perfect polariser that can be constructed. The object to
be examined may, if necessary, be laid above the circle b.

By this construction of the polariser, we obtain another
advantage; we may so adjust the size and distance of the
pencils b, c, that both of them may be included in the field

s^o

102	TREATISE ON THE MICROSCOPE.

of view, and, by placing one of the objects to be examined
above 5, and another of the same above c, we may observe
them at the same instant under their opposite colours, if
the depolarised light is coloured, which it generally is.

These rhombs may be made even out of rhombs crossed
with veins, which multiply the images, because the mul-
tiplied images are at too great a distance from the princi-
pal ones to be visible. This is a peculiar advantage, as it
is often very difficult to get good pieces of spar free from
this composite structure.

This method of constructing a polarising rhomb enables
us to take advantage of the two lateral images, which ac-
company the two principal images in crystals crossed by
one vein. These lateral images,

shown ^t m, n, are distant from	Fig. 38,

one another, and from the prin- n* b & rv
cipal images b, c; and as each of • O O •
them consists of light wholly polarised in one plane, we
have only to bring pne of them under the aperture of the
object-glass to have an admirable polariser, without being
at the trouble of stopping out any of the other pencils.

The images m9 n, are much less bright than the principal
ones 6, c ; but this is really of no consequence, as we can
obtain any degree of light we choose in the microscope,
either by the condensation of artificial, or the use of solar
light.POLARISING MICROSCOPES.

103

When the vein by which these lateral images are form-
ed is above a certain thickness, their light is white; but
they are most frequently coloured; and the observer who
understands the cause of these colours may make this
coloured pencil of great service in microscopical observa-
tions. If he uses a rhomb which gives to m a green of the
second order, it will contain none of the extreme violet and
blue rays, and none of the extreme red; so that it affords a
more homogeneous pencil than if it were white light, and
thus improves the performance of a microscope that is not
achromatic*

He may in like manner use tints which give the red ex-
tremity or the blue extremity of the spectrum, or, even
when the tint is divisible by the prism into periodical bands,
he may absorb the least luminous of these bands, and create
a homogeneous pencil of polarised light of inestimable value,
in particular researches, and with particular microscopes.

But, independent of these advantages, the method of
using a lateral pencil has the great advantage of not
requiring any thickness in the rhomb. A Nicol's prism, and
a rhomb in which the two principal images b9 c, are used,
must be about an inch thick in order to be efficacious; but
the distances m9n, or mf b, are the same at all thicknesses, so
that we can use rhombs for this purpose which are quite
useless for any other.

It is scarcely necessary to add, that similar rhombs in104

TREATISE ON THE MICROSCOPE.

which either the principal images b, c or the lateral ones
m, n are used, may be employed for the analyser. For this
purpose, a thin plate in which m or n is white, is peculiarly
applicable, as it enables us to see at once the whole field
of the microscope.SOLAR AND OXYHYDROGEN MICROSCOPES. 105

CHAPTER V.

ON SOLAR AND OXYHYDROGEN MICROSCOPES.

The solar microscope is a well-known popular instru-
ment, for exhibiting on a white screen in a dark chamber,
magnified images of minute objects, illuminated by the
condensed light of the sun. As the sun cannot often be
commanded in our climafce, this instrument may be consi-
dered as having fallen into disuse; but the discovery of
the lime-ball light by Mr Drummond amply supplies the
place of the great luminary, in so far as the microscope is
concerned. The instrument has accordingly been revived
under the name of the oxyhydrogen microscope, and is
now a favourite public exhibition.

The solar microscope was proposed by Dr Lieberkhun
in 1738; and early in 1739, when he paid a visit to Lon-
don, he exhibited an instrument of his own construction to
several members of the Royal Society, and to Mr Cuff, Mr
Adams, and other London opticians.

This microscope is nothing more than a convex lens, in

e2*106	TREATISE ON THE MICROSCOPE.

front of which, a little farther from it than its principal
focus, is placed a microscopic object, the rays of the sun
being reflected in a horizontal line, and condensed by a
lens. This will be understood from the annexed figure,

Fig. 39.

where CD is the convex lens, E the object placed before
it, and AB the illuminating condenser. An enlarged image
of E will be formed to the right hand of CD, on a wall or
screen, and the size of the enlarged image will be to that
of the object as the distance of CD from the screen or
wall is to CE, the distance of the object from the lens,
Dr Lieberkhun's solar microscope had no mirror for re-
flecting the sun's rays into the tube, so that it could only
be used a few hours, when the tube could be convenient-*
ly pointed to the sun. The improvement of adding a mir-
ror was made by Mr Cuff, who constructed the instrument
in a very superior manner.1 Dr Lieberkhun subsequently
fitted up the solar microscope to show opaque objects ; but

1 See Baker on the Microscope, vol. i. p. 22.SOLAR AND OXYHYDROGEN MICROSCOPES. 107

the method which he employed is not known. Since the
time of Mr Cuff, the solar microscope has undergone many
improvements. Mr Benjamin Martin added greatly to the
value of this instrument, by fitting it tip both for opaque
and transparent objects, in the manner shown in Plate
IX. figs. 28 and 29. In fig. 28 it is shown as fitted up
for opaque objects. The body ABCDEF has the part
ABCD of a conical, and the part CDEF of a tubular form.
A large convex lens, corresponding with AB in fig. 39,
is placed at AB, at the end of the conical tube ABCD,
which screws into the square plate QR, which is fastened
to a window-shutter opposite a hole of at least the size of
the lens AB, by means of the screws e, d. Upon the square
plate QR there is a moveable circular plate abc. To this
circular plate is attached the silvered glass mirror NOP,
placed in a brass frame, which moves round a joint PP,
and which may be placed in any position with regard to
the sun, so as to reflect his rays into the tube ABCD by
means of rack-work and pinions at Q and R. The pinion
Q, moves the circular plate abc (to which the mirror NOP
is fixed) in a plane perpendicular to the horizon, while
the nut R gives it a motion in an opposite plane. The
light introduced by this mirror falls upon the lens AB,
which throws it in a condensed state upon any object in
the tube. But before it reaches the opaque object, it is
received by a mirror M, placed in the box HILX, which108

TREATISE ON THE MICROSCOPE.

reflects the condensed light back upon the face of the ob-
ject E, fig. 39, next to the lens CD, fig. 39. This mirror
is adjusted to a proper angle by the screw S.

Above the body ABEF is seen the part /VK which
carries the sliders or objects, and the object-glass or lens
CD, fig. 39. The tube K slides within the tube V, and
V again slides into the box HILX. These tubes carry
each a magnifying lens. The inner tube K is sometimes
taken out of the other V, seen within the box, and used alone.
The sliders and objects are introduced into a slit or open-
ing at H. The brass plate to the left of H is fixed to a
tube A, by means of a spiral wire within the tube, which
presses the plate against the side of the box HILX, so
that the sliders, when placed in the opening, are pressed
against the side of the box.

In using this microscope, the sun's rays are first made
to pass along the tube ABCD, by the nuts Q and R. The
box for opaque objects, HILX, is then slid by its tube G
into the tube EF. The slider con taining the object, hav-
ing its face to be examined tume4 to the right hand, is
then pushed into the opening at H, till the object is in the
centre of the tubes V, K. The condensed light falling on
the mirror M is then thrown back on the face of the ob-
ject in the slider, and the door M shut. Upon a white
paper screen or cloth, from four to eight feet square, and
placed at the distance of from six to ten feet from theSOLAR AND OXYHYDROGEN MICROSCOPES. 109

window, the observer, in the room made thoroughly dark,
will see on the screen a magnified representation of the
object, which may be rendered distinct at different distances
of the screen, by pulling out or pushing in the tubes V, K
containing the convex lenses. As the sun is constantly
moving, its rays must be kept in the axis of the tubes by
now and then turning the nuts Q and R.

When the microscope is to be used for transparent ob-
jects, the box HILX, with its tube G, and other appen-
dages, is removed, and the apparatus shown in fig. 29 sub-
stituted for it. This is done by sliding the tube Y of fig.
29 into the tube EF of fig. 28. A slider containing the mag-
nifying lens is then slipped through the opening at ft* and
a second condenser may or may not be inserted in the
opening at h. The slider with the object is then placed
in the opening m9 and when its magnified picture falls
upon the screen, it is adjusted to distinctness by turning
the milled nut O.

The picture formed by a solar microscope being in Dr
Robison's opinion " generally so indistinct that it is fit
only for amusing ladies," he proposed to use as an object-
glass the achromatic eye-piece of four lenses, constructed
by Mr Ramsden for telescopes. Having made the experi-
ment, he found the image " perfectly sharp," and recom-
mended this application " to the artists, as a valuable ar-
ticle of their trade."110

TREATISE ON THE MICROSCOPE.

i A much simpler method, however, of correcting the
defects of the microscope, is to use compound achro-
matic lenses, which were first suggested by Mr Benjamin
Martin*

Another mode of improving the instrument was pro-
posed in 1812 by Sir David Brewster,1 who has de-
scribed a new solar microscope, which can be rendered
achromatic. The method of doing this is shown in the
diagram, fig. 39, where AB is the condensing lens, and
CD the object-glass, cemented firmly into one end of a
tube mCDn, which has a tubular opening at E, while the
other end of the tube has a circular piece of parallel glass
cemented upon it. The tube mCDn is then filled with
water, or any other fluid; and the object, when placed
upon a slider, or held in a pair of forceps, is introduced
at the opening E into the fluid. The mechanism for pro-
ducing these effects is easily conceived. By the instru-
ment thus constructed, imperfectly opaque and corrugated
objects, rendered transparent, and extended by the fluid
medium, may be examined in this microscope, though in-
capable of being used in any other. Objects may be even
dissected in the aqueous tube. Nay, objects preserved

* Treatise on New Philosophical Instruments, p. 410.SOLAR AND OXYHYDROGEN MICROSCOPES. Ill

* in spirits might be exhibited by immersing the bottle, if it
is small, in the trough or tube mCDw.1

But the most important purpose effected by this form of
the instrument is, that it can be rendered perfectly achro*
matic by using a fluid of higher dispersive power than the
glass lens CD, and making the interior curvature of the
side CD, which touches the fluid, of that degree of con-
vexity which will convert the fluid into a concave lens ca*
pable of correcting the colour of CD. The lens CD may
be made most advantageously of fluor spar, which, from
its low dispersive power, might form an achromatic com-
bination with water.

, Although, in so far as we know, metallic specula have
never been regularly fitted up as a reflecting solar micro**
scope for use, yet every person familiar with, and in the
habit of using, specula and lenses, must have made the ex-
periment of forming magnified images both in solar and ar-
tificial light, with small concave specula. The perfection
of these images cannot be doubted; and it has often ap?
peared to us surprising that the optician did not avail him-
self of such a combination for a solar microscope. Neither
the Newtonian nor the Amician form of the instrument of-

1 See Treatise on New Philosophical Instruments, p. 401, for
an account of the advantages of examining objects immersed in
fluids.112	TREATISE ON THE MICROSCOPE.

fers facilities for this purpose. Sir David Brewster has there* *
fore proposed to employ his forhi of the reflecting micro*
scope for a solar and oxyhydrogen instrument. Its facili-
ties for this purpose are very great, and there can be little
doubt that it will be practically successful, and,will be as
superior to other solar microscopes as the best reflect-
ing compound microscope is to other compound micro-
scopes. Dr Goring made an experiment with the Ami*
cian microscope; but he obviously considers it as not like-
ly to succeed, remarking, that " after all that could be
done, a refractor would be sure to beat it hollow; there-
fore I shall take my leave of the subject, as I cannot con-
scientiously recommend such an instrument."1 It is no
wonder that this experiment failed, because Dr Goring
seems to have used the whole of the Amman microscope,

Fig. 40.

1 Dr Goring states, that a friend of his had constructed a solar
microscope with metals on the Amician principle, and without a
body or eye-glass, which exhibited a variety of test objects in a
highly satisfactory manner.SOLAR AND OXYHYDROGEN MICROSCOPES. 113

eye-glasses and all, as the magnifier in the solar micros
scope, and therefore it could not be considered as a reflect-
ing solar microscope, being in fact as much a refracting one.
The construction to which we have above referred is shown
in the annexed figure, where AB is the illuminating lens,
throwing the condensed rays of the sun upon a transparent
object mn. The rays from this object falling upon the
small speculum e, are reflected to the deep concave specu-
lum cd9 so placed that the image is formed at MN on a
screen at some distance behind it, distinct vision being ob-
tained either by moving the object or the speculum.

For opaque objects this form of the instrument is pecu-
liarly adapted. The parallel rays of the sun falling uponthfc
deep speculum kh, are condensed by it and thrown On thfe
inner face of the object mny of which a magnified image
is formed, as before, at MN. A greater condensation of
light may be obtained by using the lens AB, so that the
speculum kh shall receive its convergent beam before the
rays reach their focus and complete their convergency.

In this construction we have the disadvantage of two
reflections, belonging also to the Amician form ; but this
may be considered as compensated by the image being
without the tube, and more under our command. Though
this is true in the compound microscope, yet the advan*
tage of having the object outside the tube is of less con-
sequence in a solar microscope. To avoid therefore two114

TREATISE ON THE MICROSCOPE.

reflections, and two mirrors with their relative adjustments,
Sir David Brewster has proposed to construct the reflect-

Fig. 41.

ing solar microscope in the manner shown in the annexed
figure, where CD is the perforated concave speculum, mn
the object in one of its foci, and MN the magnified image
in its other focus. The object mn} placed on a slider pass-
ing through an opening in front of the speculum, is illu-
minated as an opaque object by the lens AB, whose re-
fracted rays are farther condensed by a lens placed in the
aperture of the speculum. This form of the solar micro-
scope is therefore singularly adapted for opaque objects;
and as the whole of the effect of the instrument is pro-
duced by a single reflection from a single surface, it is the
simplest optical instrument in existence. In order .to
throw light upon mn as a transparent object, the rays must
pass through it in an opposite direction from the side MN,
and this may be done by the very same method given by
Mr Potter, and represented in fig. 31.

The simplicity and practical value of this instrument will

MSOLAR AND OXYHYDROGEN MICROSCOPES.

115

be immediately recognised by comparing it with the com-
plex opaque box, which in all solar microscopes is a ne-
cessary appendage for opaque objects. See Plate IX. fig.
28 and 29.

Dr Goring's Solar Camera Microscope.

Dr Goring, whose indefatigable genius has improved al-
most all our popular instruments, has described in the Mi-
crographia a very complete solar microscope, which has
the property of exhibiting the image on a horizontal curved
surface, placed in a darkened camera, at which two or more
persons can look at the same time. It is in reality a new-
instrument, but can also be used like the common solar mi-
croscope in a darkened room.1

This instrument, with all its parts, is shown in Plate
X. figs. 30, 31, 32, and 33; fig. 30 being a geometrical
elevation of the instrument, one tenth of the real size, the

1 Br Goring calls this instrument a Solar Engiscope^ while he
gives the name of Solar Microscope to the same instrument when
used in a dark room in the common way. The introduction of
the image into a camera becomes thus the reason for changing a
microscope into an engiscope, The word engiscope, however appro-
priate it may be as a companion to the word telescope, is quite in-
applicable to any kind of solar microscope.116	TREATISE ON THE MICROSCOPE.

various parts being represented as if formed of transparent
matter. A strong framework A of wood rests upon four
legs, having, a large hole in it, into which the instrument
is fixed with two screws F, F. The frame is large enough
to protect the observer from the solar rays. A long plane
mirror B is fixed to an arm C, which moves round a pin
fixed to the side of the mirror frame, and also round a
joint attached to a strong round wire E, which slides back-
wards and forwards in the tube D, having a spring within,
and a pinching nut to fix it in its place. The inclination
of the mirror is varied by pulling out or pushing in the
mirror, which has also another motion produced by the
action of the milled head G on a rack and pinion. A com-
mon illuminating lens, five inches in diameter and one foot
in focal length, is placed at H. Dr Goring recommends
an achromatic lens1 (which would be a very expensive ap-
pendage), though he says that he has never used one* The
main body pf. the microscope is conical, having a bayonet
catch at L to receive the rest of the instrument, viz. the
tube carrying the stage and rackwork. This tube 11 moves
within the conical one by means of the milled head M and
rack and pinion N. The end of this tube is closed, and an
ordinary slider-holder O is fixed to it. On the inner side

1 See Edinburgh Journal of Science, No. 9, new series, p. 85 ;
and our chapter on the Illumination of Microscopic Objects.SOLAR AND OXYHYDROGEN MICROSCOPES. 117

of the stage, near N, is fixed a condensing lens, about one
and a half inch in diameter and two inches in focal
length, which, by means of a sliding wire passed through
a hole in the stage, can be moved from one side of the tube
to the other, and also made to approach to or recede from
the stage. A second tube PPP, slit open at the sides, is
screwed into the tube in which the stage moves; and into
this tube the optical part q is made to slide, the object-
glass being placed at K. Dr Goring here remarks, that
" the focus may of course be roughly adjusted, by sliding
the body backwards and forwards in its containing tube,
before it is attached to the camera, fig. 31; but when this
has been done, it must of course remain immoveable* I
look upon it," he continues, " as a principle in the solar
microscope, that the magnifier or object-glass should not be
moved, but always remain at a fixed distance from the illu-
minator." Perhaps we do not distinctly understand the
import of this passage ; but we apprehend that the magni-
fier or object-glass may be, nay, must be, moved in any
way that is necessary to produce distinct vision upon the
screen, whatever be its distance ; and that the essential
condition is, that the distance of the illuminator and the
object shall be invariable, the object being, if possible, accu-
rately situated in the focus, unless where a slight deviation
is necessary to prevent its destruction by the concentrated
heat of the solar rays.118

TREATISE ON THE MICROSCOPE*

The end of the tube Q is now pushed into another piece
of tube at R, fig. 31, which communicates witha conical
tube of brass, " having a rectangular prism, with its reflect-
ing side silvered,1 or a plane metal adjusted at its head S,
so as to throw down the image to the bottom of the box
or camera, where it is to be received on paper (at T), or
on a surface of plaster of Paris duly curved to suit its
shape." The camera WWXX is constructed with win-
dows V, V, to permit two persons to view the picture on
the table T. Two pieces of wood WW carved out to
fill the slope of the upper part of the face, are placed
as in the figure (one of them is shown separately in a plan
at fig. 33). Dr Goring adds, that " he has found it ne~
cessary to exclude the breath from entering the camera,
as it dims the eye-glass of the engiscope, and thus spoils
the imagebut he does not mention whether this is the

1 Dr Goring is surely mistaken in saying that the side of the
prism should be silvered ; for as total reflection commences at 41°
49' for glass, and takes place at all greater angles of incidence, the
light incident at 45° will be totally reflected. . But even if the
least oblique part of the conical beam should penetrate the re-
flecting surface (which it cannot do), part of the picture would
have the light of silvered reflection, and the other part the double
light of total reflection, which would never answer. We would
prefer a plane metallic speculum to the prism, even if sufficiently
homogeneous not to affect the accuracy of the picture.SOLAR AND OXYHYDROGEN MICROSC0FES. 119

object of the pieces of carved wood, or whether they are
used to keep extraneous light from the eye,1 which, in
so far as the figure indicates, does not appear to be the
case.

The sides U, U of the camera may be removed at
pleasure, to allow the observer to draw the picture on the
table, the light being excluded by some black drapery,
while the hand passes through a suitable opening in it.
Dr Goring recommends that the whole of the exterior (in-
terior ?) of the conical brass tube and camera should be
well blacked, or lined with black silk velvet.2

In applying this instrument to opaque objects, the
opaque box, shown in fig. 32, is applied to the conical tube
in fig. 26 by means of the bayonet catch at L. A plane
mirror R, adjusted by the screw S, throws the light of the
illuminator to the object O placed in the conjugate focus
of the eye-glass K, by means of the milled nut M and
screw T, which causes the stage and the object to approach

1	In using this, and all other optical instruments where perfect
vision is either agreeable or essential, we,would recommend the
use of the Greenland snow spectacles, cut to suit the individual
from a plaster of Paris cast of the eyes, nose, and brow.

2	Mr Potter found black velvet to be superior to any other
blacking for the interior of his reflecting microscope. Edin. Journ.,
of Science, No. 11. p. 62, new series.120

TREATISE ON THE MICROSCOPE.

to or recede from the lens K.1 The stage is formed by a
piece of cork covered with black velvet. PP is the tube
into which the body q of the microscope is inserted, as in
fig. 26.

This instrument may be converted into a common so-
lar microscope by unscrewing and removing the tube PP,
and placing a simple object-glass in an appropriate mount-
ing at M. The whole apparatus is then removed from the
frame A, and screwed to a window shutter in the usual
way.

On the Oxyhydrcgen Microscope.

The great popularity of the public exhibition made with
this instrument*4 has turned the attention of opticians and
amateurs to its improvement. Mr Pritchard has written
a long and interesting chapter of nearly fifty pages on the
subject of solar and oxyhydrogen gas microscopes, in the
Micrographia^ already referred to, and has given a most po-
pular and minute account of all the details of the instru-
ment. These details, to which we must refer our readers,

1 The illumination is here far too oblique. The mirror should

be nearer P, and the screw MT should be made to move the ob-

ject-glass K, in order that the focus of the illuminator may always

fall on the object O.solar and oxyhydrogen microscopes. £21

do not belong to an article like the present; and we shall
content ourselves with explaining what an oxyhydrogen
microscope is, and how the optical apparatus of a solar
microscope may be readily converted into that of an oxy-
hydrogen one, and vice versa.

An oxyhydrogen gas microscope differs from a solar one
chiefly in this, that a brilliant light obtained by igniting a
ball of lime the size of a pea (hence called the pea or lime
light, or more appropriately the Drummond light, from its
inventor Mr Drummond) with oxyhydrogen gas, is substi-
tuted in place of the solar rays. This enables us to en-
joy the amusement of the solar microscope apparatus in all
weathers and at all hours of the day,

As the lime-ball light, however, is at our elbow, it sends
forth diverging rays ; whereas the rays of the sun are pa-
rallel. A very beautiful principle, already referred to in
our article Micrometer, enables us to give the simplest
direction for this purpose. Let AB be the illuminating
lens of the common solar microscope, throwing the parallel
rays e f of the sun upon the object mn, and let the whole
instrument be in perfect adjustment; then, without mov-
ing or changing any part of it, we may convert it into an
oxyhydrogen microscope, where the light diverges from
the lime-ball L, simply by placing in front of AB another
lens CD, whose focal length is equal to the distance of the
lime-ball light L from the lens AB* The oxyhydrogen122

TREATISE ON THE MICROSCOPE.

Fig. 42.





microscope will then have its objects at mn illuminated in
precisely the same way as they were by the sun's rays.
The two lenses CD, AB, should be in contact, the space
being left to show the parallel rays ef. Now, as L is the
focus of the lens CD, the converging rays ef will be paral-
lel, and consequently will be refracted by AB, exactly as
if they had been the rays of the sun.

If the instrument had been made originally as an 0x3^
hydrogen microscope, with a large and deep lens at AB,
■which would be required to refract rays diverging from L
to then we might convert the instrument into a solar
microscope, by simply placing a concave lens in front of
AB, whose focal distance is equal to the distance of L from
AB. This concave l&is will give such a divergency to the
parallel rays of the sun that they will have their focus at
mn.

Our readers will find the most ample details respecting
the gas apparatus, and the method of managing and usingSOLAR AND OXYHYDROGEN MICROSCOPES. 123

the instrument, in Mr Pritchard's Essay in the Microgra-
phia, already cited.

Notwithstanding the precautions to prevent an explo-
sion of the oxygen and hydrogen employed in this appara-
tus, we would recommend to Mr Pritchard, and those who
may construct such instruments, to use a common lamp,
supplied with oxygen gas, such as Sir David Brewster some
years ago recommended as a safe substitute for the lime-
ball light, when it was proposed to use the latter for light-
houses. This oxygen lamp, equally safe and brilliant, has
been tried with the most perfect success at the Trinity House,
and will, we are confident, be soon in universal use, not
only in light-houses, but wherever strong lights are re-
quired.m

TREATISE ON THE MICROSCOPE.

CHAPTER VI.

DESCRIPTION OF MICROMETERS FOR MICROSCOPES.

All &&micrometers above deoei'ibcd may be adapted to
compound microscopes, where the eye-glass has a consi-
derable focal length. A good micrometer, however, for sin-
gle microscopes, which can be used with facility, and at
the same time give accurate results, is still a desideratum.
When the single lens is so minute, or when the first lens
of a microscopic doublet or triplet almost touches the sur-
face of the object, it is an extremely difficult matter to in-
troduce any scale, or any minute body of known dimen-
sions, with which the object may be compared. In some
cases, when the object to be measured is minute, the seed
of the lycoperdon bovista or puff-ball might be introduced,
its diameter being about the	Part an inch 5 and

when the object is less minute, the seed of lycopodium
may be used, its diameter being -gl^th of an inch. We
may advantageously adopt, in some cases, the method ofMICROMETERS FOR MICROSCOPES.

125

Dr Jurin,1 who introduced into the field small pieces
of silver or brass wire, whose diameter he had previously
ascertained by coiling the wire round a cylinder, and ob-
serving how many breadths of the wire were contained in
a given number of inches.

This method of introducing a substance of known di-
mensions may be carried much farther. We may use all
the variety of hairs and wool which have a known diame-
ter ; and for this purpose Dr Young's table of substances
measured by the eriometer will be of great use. The fol-
lowing are a few of them:

Diameter in Parts
of an Inch.

Lycoperdon bovista, seed of...............8500th of an inch.

Smut of barley...............................4600th ditto.

Silk, fibre of (average).....................2500th ditto.

Human blood, particles of (Bauer)......2500th2 ditto.

Mole's fur.....................................1875th ditto.

Goat's wool....................................1575th ditto.

Saxon wool....................................1320th ditto.

Farina of Laurestinus.......................1100th ditto.

Seed of lycopodium,........................ 940th ditto.

The distance of the fibres of the crystalline lens of fishes

1	Physico-Mathematieal Dissertations, p. 45.

2	We consider this the best measure.126	TREATISE ON THE MICROSCOPE.

may also be advantageously used, and also the distance of
the teeth which unite the fibres. For this purpose, the
lens must be well dried, and perfectly hard, so that with a
sharp knife we can detach minute portions of any of the
laminae. The thinnest should be used; and as the fibres
always taper to the pole, and the teeth become smaller in
proportion as the fibres diminish, we must determine the
distance of the fibres, and also those of the teeth, at both
ends of the laminae, by the method described by Sir David
Brewster in the Philosophical Transactions for 1833, p.
324?. The larger lined scales of moths and butterflies may
also be used, especially as we can measure the distance of the
lines by the coloured spectra which these lines produce.1
The minute subdivisions in the shells of the infusoria dis-
covered by Dr Ehrenberg, and shown in Plate XIII. fig.
24 and 25, "are admirably adapted for micrometrical pur-
poses. These operations will require much dexterity on
the part of the observer, and they are recommended only
to those who cannot succeed in their measurements by
other methods.

An excellent method of measuring microscopic objects,

1 The late Mr Pond has observed, that the pale, slender, double-
headed scales of the Pontia or Pieris brassica, which taper to a
point, and terminate in a brush-like appendage, are of an invari-
able length, about one-eightieth of an inch.MICROMETERS FOR MICROSCOPES.	127

is to project the image of the object against a divided
scale, at a given distance from the eye. The scale must
be seen either by the same eye which is looking into the
microscope, or by the other eye. In the first case, the
rays from the microscope will enter one side of the pupil,
and the rays from the divided scale the other side; the aper-
ture through which we look at the scale, and the aperture of
the microscope, being at a distance less than the diameter
of the pupil. When the right eye looks at the divided
scale, the left, which looks into the microscope, will see
the object projected against the scale, although it has no
vision of the scale itself. This second method may be
carried into effect in two ways. The seale may form no part
of the instrument, and may be viewed by the naked eye ;
or it may form part of the instrument, like a binocular
telescope, the left eye looking into one tube, viz. the mi-
croscope, while the right eye looks into another tube, in
which a divided scale is magnified by an eye-lens.

Dr Wollaston has constructed and used a very ingenious
micrometer on the first of the principles above mention-
ed, viz. when the object and the scale are viewed by the
same eye; but its use is limited to microscopes with small
lenses. When the lenses are larger, we have adopted
another method, namely, to perforate the lens with a small
hole in or near the centre, or, if it is thought better, near
the margin of the lens. A slit extending from the margin
of the lens may often be executed more easily.128	TREATISE ON THE MICROSCOPE*

The following is Dr Wollaston's own description of this
instrument:

" This instrument," says Dr Wollaston, " is furnished
with a single lens of about th of an inch focal length.
The aperture of each lens is necessarily small, so thafc
when it is mounted on a plate of brass, a small perforation
can be made by the side of it in the brass, as near to its
centre as ^jth of an inch.

"When a lens thus mounted is placed before the eye
for the purpose of examining any small object," the pupil
is of sufficient magnitude for seeing distant objects at the
same time through the adjacent perforation, so that the
apparent dimensions of the magnified image might be com-
pared with a scale of inches, feet, and yards, according to
the distance at which it might be convenient to place it.

"A scale of smaller dimensions, attached to the instru-
ment, will, however, be found preferable, on account of the
steadiness with which the comparison may be made; and
it may be seen with sufficient distinctness by the naked eye,
without any effort of nice adaptation, by reason of the
smallness of the hole through which it is viewed,

" The construction that I have chosen for the scale is
represented in Plate XI. fig. 34 It is composed of small
wires about ^th of an inch in diameter, placed side by
side, so as to form a scale of equal parts, which may be
with ease counted by means of a certain regular variation
of the lengths of the wires^MICROMETERS FOR MICROSCOPES.	129

u The external appearance of the whole instrument is
that of a common telescope consisting of three tubes. The
scale occupies the place of the object-glass, and the little
lens is situated at the smaller end, with a pair of plain
glasses sliding before it, between which the subject of ex-
amination is to be included. This part of the apparatus
is shown separately in fig. 37. It has a projection, with a
perforation, through which a pin is inserted to connect it
with a screw, represented at b, fig. 36. This screw gives
lateral motion to the object, so as to make it correspond
with any particular part of the scale. The lens has also a
small motion of adjustment, by means of the cap c, fig. 36,
which renders the view of the magnified object distinct.
b " Before the instrument is completed, it is necessary to
determine with precision the indications of the scale,
which must be different, according to the distance to
which the tube is drawn out. In my instrument, one
division of the scale corresponds to joioo^ an *nch
when it is at the distance of 16-6 inches from the lens;
and since the apparent magnitude in small angles varies
in the simple inverse ratio of this distance, each division
of the same scale will correspond to	at distance

of 8^ inches; and the intermediate fractions ^qq, ToW
&c. are found by intervals of 1 66 inch, marked on the
outside of the tube. The basis on which these indications
were founded in this instrument was a wire, carefully as-130

TREATISE ON THE MICROSCOPE.

certained to be of an inch in diameter, the magnified
image of which occupied fifty divisions of the scale when
it was at the distance of 16*6 inches; and hence one di-

vision = 50 x 200 = mm- Since any error in the

original estimate of this wire must pervade all subsequent
measures derived from it, the substance employed was
pure gold drawn till fifty-two inches in length weighed
exactly five grains. If we assume the specific gravity of
gold to be 19*36, a cylindrical inch will weigh 3837 grains;
and we may hence infer the diameter of such a wire to be
g^th of an inch, more nearly than can be ascertained by
any other method.

" For the sake of rendering the scale more accurate, a
similar method was, in fact, pursued with several gold
wires of different sizes, weighed with equal care; and the
subdivisions of the exterior scale were made to correspond
with the average of their indications.

" Inmaking use of this micrometer for taking the mea-
sure of any object, it would be sufficient, at any one acci-
dental position of the tube, to note the number on the
outside as denominator, and to observe the number of di-
visions and decimal parts which the subject of examina-
tion occupies on the interior scale as numerator of a frac-
tion, expressing its dimensions in proportional parts of an
inch j but it is preferable to obtain an integer as numera-MICROMETERS FOR MICROSCOPES.	131

tor, by sliding the tube inward or outward, till the image
of the wire is seen to correspond with some exact number
of divisions, not only for the sake of greater simplicity in
the arithmetical computations, but because we can by the
eye judge more correctly of actual coincidence than of the
comparative magnitudes of adjacent intervals. The small-
est quantity which the graduations of this instrument pro-
fess to measure, is less than the eye can really appreciate
in sliding the tube inward or outward. If, for instance,
the object measured be really ^gVo> it may appear xonoo>
or -g^oQ, in which case the tloubt amounts to ^th part of
the whole quantity. But the difference is here exceed-
ingly small in comparison to the - extreme* division of other
instruments, where the nominal effect of its power is the
same.

" A micrometer with a divided eye-glass may profess to
measure as far as 7^0 o^1 an inch; but the next division
is fo§o(T or Jooo' and though the eye may be able to
distinguish that the truth lies between the two, it receives
no assistance within one-half part of the larger measure."1

The micrometer microscopes used for reading off the
divisions on the graduated limb of astronomical instru-
ments differ in no respect from the eye-pieces of tele-
scopes fitted up with micrometers.

1 Phil. Trans, 1813, p. 119.132

TREATISE ON THE MICROSCOPE

Notwithstanding the value of the methods described
above, the want of a simple micrometer for microscopes
of high power is felt by every person who has been prac-
tically occupied with this class of researches; and we can-
not give a better proof of this than by adducing in support
of our opinion the different measures that have been given
by able and ingenious observers, of the size of the particles
of the human blood.

	„,; 1-6060th part of an inch*	
Dr Wollaston....................	l-5000th	ditto.
	, ,l-4076th	ditto.
Captain Kater...................	,l-4000th	ditto.
Dr Ehrenberg1..................	l~3600th	ditto.
Messrs Hodgkin and Lister...	..1-3000th	ditto*
Sir David Brewster............	l-2556th	ditto.'
	,1-1940th	ditto-

1 In measuring the size of the fossil infusorias recently dis-
covered by himself, Dr Ehrenberg assumes a globule of human
blood to be the three-hundred tli of a line in diameter, or the
three-thousand eight-hundredth of an inch, but of what inch is
not mentioned. He does not state whether this measure is taken
by himself or not. He reckons the thickness of a human hair
as the forty-eighth of a line at its mean thickness, or the five-hun-
dred and sixty-seventh of an inch.

* This result was confirmed by Leeuwenhoeck, who used the same
wire, which was sent to him by Dr Jurin. Phil. Trans. No. 377*

1 In measuring the size of the fossil infusorias recently dis-
covered by himself, Dr Ehrenberg assumes a globule of human
blood to be the three-hundred tli of a line in diameter, or the
three-thousand eight-hundredth of an inch, but of what inch is
not mentioned. He does not state whether this measure is taken
by himself or not. He reckons the thickness of a human hair
as the forty-eighth of a line at its mean thickness, or the five-hun-
dred and sixty-seventh of an inch.

* This result was confirmed by Leeuwenhoeck, who used the same
wire, which was sent to him by Dr Jurin. Phil. Trans. No. 377*MICROMETERS FOR MICROSCOPES.	13S

Mr Bauer's best observation.....l-2500th part of an inch.

next best.............l-2000th ditto.

worst observation...l-1000th ditto.

The three measures of 1000, 2000, and 2500, have
been recently given by Mr Bauer himself, as the different
steps which he made towards what he conceives the best
measure, viz, l-2500th, which he obtained repeatedly
with an improved achromatic microscope. As Dr Young
obtained his measure eriometrically, namely, by measuring
the diameter of the first red ring produced by looking
through the blood at a luminous object, we cannot con-
ceive it possible that he could have committed such a mis-
take as to make the diameter of that ring nearly thrice as
great as it should be, according to Mr Bauer's results, or
more than thrice as great as the concurring measures ob-
tained by Jurin and Leeuwenhoeck. The only explanation
we can give is, that the particles of the blood must have
an organized structure, or consist of portions separated by
lines which have the magnitude assigned by Dr Young.
In order to submit this explanation to the test of experi-
ment, Sir David Brewster examined the particles of blood
a few minutes after it was drawn, when dried by natural
evaporation on a plate of glass. Each particle he found
to consist of a dark rim, within which is a bright circle,
then a darkish central spot, which spot in some globules
may be resolved into a dark ring, a bright ring within this*134

TREATISE ON THE MICROSCOPE.

and then a small central black spot. Here, then, is the
cause of Dr Young's mistake. The red ring of light which
he measured in the eriometer was not that which was due
to the globule as a whole, but to the parts of the globule.
Being anxious to obtain more complete evidence of this
fact, we placed lycopodium powder beside the globule of
blood, and fQund that the diameter of the globule was to
that of the lycopodium seed as 5 to 18. We then com*
pared the diameter of the red ring produced by the seed
with the diameter of the red ring produced by divisions on
steel, in which there were 1250 to the inch, as executed
for us by the late Sir John Barton, and found the di-
ameter of the seed to be the 697th of an inch. We com-
pared it also with the ring produced by divisions of which
there were 625 to the inch, and found its diameter the
717th part of an inch. The mean of these two is the
710th1 part of an inch, which, increased in the ratio of 5
to 18, gives the 2556th part of an inch as the measure of
the diameter of the globules of blood, agreeing almost ex-
actly with the recent measure of Mr Bauer.

1 Dr Young makes this the 940th of an inch, but he has cer-
tainly committed a mistake in his observation.ILLUMINATION OF MICROSCOPIC OBJECTS. 135

CHAPTER VII.

ON THE ILLUMINATION OF MICROSCOPIC OBJECTS.

The methods of illuminating microscopic objects that
have been long in use have been described in the preced-
ing chapters. They consist in throwing light upon the ob-
ject, either by means of a mirror or a lens, or both combin-
ed ; but the nature of the light employed, the magnitude
of the pencil, its condition with regard to parallelism, di-
vergency, or convergency, and the diameter of the pencil
employed, of the direction in which it falls upon the object,
have never been discussed as matters of science, and upon
which the performance of the finest instrument essentially
depends.

In so far as we know, the most important of these to-
pics was pressed upon the notice of the scientific reader
by Sir David Brewster, in the year 1820 ; and in order
that the progress of improvement in this essential branch of
the art of making discoveries with the microscope may be
understood, we shall quote his observations on the subject.136

TREATISE ON THE MICROSCOPE.

" The art of illuminating microscopic objects is not of
less importance than that of preparing them for observa-
tion. No general rules can be given for adjusting the in-
tensity of the illumination to the nature and character of
the object to be examined; and it is only by a little prac-
tice that this art can be acquired. In general, however,
it will be found that very transparent objects require a less
degree of light than those that are less so; and that ob-
jects which reflect white light, or which throw it off from
a number of lucid points, require a less degree of illumi-
nation than those whose surfaces have a feeble reflective
force.

" Most opticians have remarked, that microscopic objects
are commonly seen better in candle-light than in day-light;
a fact which is particularly apparent when very high mag-
nifying powers are employed; and we have often found
that very minute objects, which could scarcely be seen at
all in day-light, appeared with tolerable distinctness in
candle-light. So far as we know, the cause of this has
not been investigated ; and as it leads to general views re-
specting the illumination of microscopic objects, we shall
consider it with some attention.

" Let LL, fig. 43, be a single microscope placed before
the eye at E, and let /be a microscopic object placed in
its anterior focus, and illuminated by two candles at A and
B. As the rays Afa and Bfb cross at f the focus of pa-ILLUMINATION OF MICROSCOPIC OBJECTS. 137

Fig. 43.

rallel rays, and as the two shadows of the microscopic
object will be formed at a and b, as it were, by rays di-
verging from f, the images of these two shadows formed
upon the retina will coincide and make only one image,
so that the object / will appear perfectly distinct. If the
object, however, is placed either within or without the
focus f its shadows being formed, as it were, by rays di-
verging from a point either within or without the prin«
cipal focus/ will not coincide on the retina, but appear to
form two images, either overlapping each other, or com-
pletely separated. If, instead of two candles, A, B, we have
four, five, or six, we shall have four, five, or six overlapping
or separated images. Now, as it is impossible to place the
different parts of a microscopic object exactly in the focus
/, and as every lens has different foci for the differently
coloured rays, and even for homogeneous light, in conse-
quence of its spherical aberration, it necessarily follows,
that when microscopic objects are illuminated by light pro-
ceeding from several points, the image upon the retina
must consist of a number of images not accurately coinci'
dent 5 and hence it becomes of the greatest importance that138

TREATISE ON THE MICROSCOPE.

the object be illuminated only from one pointy and not from
a large surface of light, such as the sky, which is equivalent
to an infinite number of radiant points. ■

" The following rules may therefore be laid down respect-
ing the illumination of microscopic objects, and the method
of viewing them.

" 1. The eye should be protected from all extraneous
light, and should not receive any of the light which pro-
ceeds from the illuminating centre, excepting that portion of
it which is transmitted through or reflected from the object.

" 2. Delicate microscopical observations should not be
made when the fluid which lubricates the cornea of the ob-
server's eye happens to be in a viscid state, which is fre-
quently the case. See Brande's Journal, vol. ii. p. 127.

" 3. The figure of the cornea will be least injured by the
lubricating fluid, either by collecting over any part of the
cornea, or moving over it, when the observer is lying on
his back, or standing vertically. When he is looking down-
wards, as into the compound vertical microscope, the fluid
has a tendency to flow towards the pupil, and injure the
distinctness of the vision.

" 4. If the microscopic object is longitudinal, like a fine
hair, or consists of longitudinal stripes, the direction of the
lines or stripes should be towards the observer's body, in
order that their form may be least injured by the descent
of the lubricating fluid over the cornea.ILLUMINATION OF MICROSCOPIC OBJECTS. 139

" 5. The field of view should be [contracted, so as to ex*
elude every part of the object, excepting that which is un-
der immediate examination.

" 6. The light which is employed for the purpose of illu-
minating the object should have as small a diameter as pos-
sible. In the day time it should be a single hole in the
window-shutter of a darkened room, and at night it should
be an aperture placed before an argand lamp.

" 7. In all cases, and particularly when very high powers
are requisite, the natural diameter of the light employed
should be diminished, and its intensity increased by opti-
cal contrivances.	;

" 8. When a strong light can be obtained, and indeed in
almost every case, homogeneous light should be thrown
upon the object. This may be done either by decomposing
the light with a prism, or by transmitting it through a
coloured glass, which has the property of admitting only
homogeneous rays."

In the same article Sir David Brewster has described " a
new method of illuminating objects in the solar and the lu-
cernal microscopes." " The great defects," says he," which
still attach to the solar and lucernal microscopes^ arise
from the imperfect method of illuminating the objects. The
method suggested by iEpinus, and employed almost univer-
sally by opticians, of reflecting the light concentrated by a
lens upon the objects, by means of a plane mirror, is good140	TREATISE ON THE MICROSCOPE*

enough so far as it goes ; but in consequence of the light
arriving from one direction only, the surface of the illume
nated object is covered with deep shadows, and the inten-
sity of illumination is by no means sufficient when the power
of the instrument is considerable. We propose, therefore,
that in the solar microscope the sun's light should be re-
flected by a very large mirror through four apertures, A, B,
C, D (surrounding the tube T), each of which is furnished
with an illuminating lens. The four
cones, if condensed, are then re-	Fig 44.

ceived, before they reach their
focus, each by an inclined mirror,

which reflects them upon the ob-
ject ; the distance of the lens from
the mirror, added to the distance of
the mirror from the object, being
always less than the focal length of
the illuminating lens. In the lu-
cernal microscope it would be desirable to place an argand
lamp opposite each of the apertures A, B, C, D. By these
means the light would fallupon the surface ofthe objectinfour
different directions; a high degree of illumination would be
obtained for very dark objects; andby shutting up one or more
of the four lenses, or parts of them, we shall be enabled to find
the particular direction of the light which is best suited for
developing the structure which it is the object of the observerILLUMINATION OF MICROSCOPIC OBJECTS. 141

to discover " Although the focus of the illuminating rays
should always fall upon the object, for the reasons already
assigned, yet in the preceding method, applied to the solar
microscope, a deviation from this rule becomes necessary,
for two reasons: 1st, Because, if the focus of the illuminat-
ing lens fall exactly upon the object, it might burn it, or
destroy it by corrugation ; and, 2dly> In the ordinary illumi-
nating lenses, the diameter of the focal spot, or image of the
sun, is not sufficient to cover the whole object, or to give a
sufficient luminous field around it. For these reasons it is
recommended in the preceding extract to place the object
a little way within the focus of the illuminator, that is, be-
tween the illuminator and its focus. But if the object is
such that it cannot be injured by the solar heat, or if the
illuminator is sufficiently large to give a focal spot capable
of filling the field of the microscope, then the object should
be placed in the solar focus of the illuminator.

After a lapse of nearly ten years, the subject of micro-
scopic illumination was discussed by Dr Wollaston, in his
paper on the microscopic doublet, published in the Phil.
Trans, for 1829. This eminent philosopher, whose inge-
nuity never failed in executing in the best manner what-
ever he attempted, was then on his death-bed; and this,
among other papers, was published without that complete
N revision which its author would otherwise have given it.

" The state of my health," says Dr Wollaston, " in-142

TREATISE ON THE MICROSCOPE.

Fig. 45.

duces me to commit to writing rather more hastily than I
have been accustomed to do, some observations on micro-
scopes ; and I trust that, in laying them before the Royal
Society, they will meet with that indulgence which has
been extended to all my former com-
munications.

" In the illumination of microsco-
pic objects, whatever light is collect-
ed and brought to the eye beyond
that which is fully commanded by
the object-glasses, tends rather to im-
pede than to assist distinct vision.

" My endeavour has been to col-
lect as much of the admitted light as
can be dpne by simple means, to a fo-
cus in the same plane as the object
to be examined. For this purpose I
have used with success a plane mirror
to direct thelight, and a plano-con-
vex lens to collect it ; the plane side
of the lens being towards the object
to be illuminated."

These two principles of illumina-
tion, the first of which is the same as
the first and fifth of the rules already
given, though not so fully developed,ILLUMINATION OF MICROSCOFIC OBJECTS. 143

and the second founded upon a mistaken principle, have
been carried into effect by Dr Wollaston in the following
manner:

" T, U, B, E represents a tube about six inches long,
and of such a diameter as to preclude any reflection of
false light from its sides; and the better to insure this,
the inside of the tube should be blackened. At the top of
the tube, or within it at a small distance from the top, is
placed either a plano-convex lens ET, or one properly
curved, so as to have the least aberration, about |ths of
inch focus, having its plane side next the object to be
viewed ; and at the bottom is a circular perforation A,
of about j§ths of an inch diameter, for limiting the light
reflected from the plane mirrof R, and which is to be
brought to a focus at a, giving a neat image of the per-
foration A, at the distance of about <^ths an ^rom
the lens ET, and in the same plane as the object which is
to be examined. The length of the tube, and the distance
of the convex lens from the perforation, may be somewhat
varied. The length here given, six inches, being that
which it was thought would be most convenient for the
height of the eye above the table, the diameter of the
image of the perforation A lpciust not, excepting with lower
powers than are here meant to be considered, exceed one
twentieth of an inch.

** The intensity of illumination will depend upon the144	TREATISE ON THE MICROSCOPE*

diameter of the illuminating lens and the proportion of
the image to the perforation, and may be regulated ac-
cording to the wish of the observer. # * *

" The lens ET, or the perforation A, should have an
adjustment by which the distance between them may be
varied, and the image of the perforation be thus brought
up to the same plane as the object to be examined. * *
" For the perfect performance of this microscope, it is
necessary that the axis of the lenses, and the centre of the
perforation A, should be on the same right line. This may
be known by the image of the perforation being illumi-
nated throughout its whole extent, and having its whole
circumference equally well defined. For illumination at
night, a common bull's-eye lanthorn may be used with great
. advantage,	*	. * .	*

i( Supposing the plano-convex lens to be placed at its
proper distance from the stage, the image of the perfora-
tion may be readily brought into the same plane with the
object, by fixing temporarily a small wire across the per-
foration with a bit of wax, viewing any object placed upon
a piece of glass upon the stage of the microscope, and
varying the distance of the perforation from the lens by
screwing its tube until the image of the wire is seen dis-
tinctly at the same time with the object upon the piece ofglass'9
In the preceding passages we have extracted every one
of Dr Wollastons observations in reference to his methodILLUMINATION OF MICROSCOPIC OBJECTS.

of illuminating microscopic objects, so that the read# will
be enabled thoroughly to understand it.

This method of illumination was highly commended by
optical writers. Dr Goring1 considered it as most effec-
tive, and enumerates it among the inventions which found-
ed a new era in the history of the microscope ; and he
elsewhere states, that " there is no modification of day-
light illumination superior to that invented by Dr WoUas-
ton."2

The marked difference between the methods of illumi-
nation proposed by Dr WoUaston and Sir David Brewster,
induced the latter to publish, in 1831, a paper " Oh the
Principle of Illumination of Microscopic Objects^ In
this paper the mistake committed by Dr WoUaston is clear-
ly pointed out. The rays which Dr WoUaston throws
upon the object, in place of being rays actually converged
to a focus, as they ought to be, are rays which diverge from
a focus situated between the object and the lens. He makes
the focal point of the circular margin of the perforation fall
upon the object, without considering that the rays which
pass through that perforation do not diverge from it, and
therefore cannot be collected in the conjugate focus cor-

1	Microscopic Illustrations, Exord. p. 1, Lond. 1830.

2	Microscopic Cabinet, p. 181, Lond. 1832.

3	Edinburgh Journal Qf Science, new series, No. 11, p. 83.

c146	TREATISE ON THE MICROSCOPE.

responding to the perforation. In Dr Wollaston's diagram
(Phil. Trans. 1829, plate ii. fig. 1), the rays which are
incident on the mirror R are actually drawn as parallel
rays; and it is quite clear that he meant them to be paral-
lel rays issuing from the bull's-eye Janthorn, which he re-
commends. But if we suppose that a common flame is
used, the error is just of the same nature* It is a distinct
image of the flame that should be thrown upon the object;
and hence the perforation A should be placed close to the
flame,—the source of light and the illuminated object form-
ing the conjugate foci of the lens. After explaining this
principle, Sir D. Brewster adds in the same paper :—" I
have no hesitation in saying, that the apparatus for illumi-
nation requires to be as perfect as the apparatus for vision ;
and on this account I would recommend that the illuminating
lens should be perfectly free of chromatic and spherical aberra-
tion, and that the greatest care be taken to exclude all extraneous
lightt both from the object and from the eye of the observer

At the meeting of the British Association at York in
1831, the preceding methods were communicated to Mr
Potter, who was then engaged in inquiries with the re-
flecting microscope, and who had used only the common
method of illuminating his objects. The effect which he
obtained by it is thus described.1 " I am indebted to

1 Edinburgh Journal of Science, new series, No. 11, p. G4.ILLUMINATION OF MICROSCOPIC OBJECTS. 147

Dr Brewster for information on the necessity of having
the focus of the illuminating lens for transparent objects
to fall exactly upon the object, when great nicety of visiori
is required. Having adjusted my microscope carefully on
this point (see our figure, p. 80, where the object is seen in
the focus of the illuminating rays), I saw quite easily what
are called the diagonal lines on the scale from the wing of
the white-cabbage butterfly, which has been proposed as
a difficult test object by Dr Goring; and it is such a one
as those who have only seen the stronger longitudinal
striae on scales from the wings of moths and butterflies have
little idea of" By the same means Mr Potter's instru-
ment " showed him easily not only the strise on the scales
of the wing of the small house*moth, but also the diagonal
lines." Mr Potter afterwards applied his microscope, and
the new method of illumination, to "a much more difficult
object than those just referred to/' This object is the
broad bluish band first notijced in the web of the spider,
the Clubiona alrox? " There can be no doubt," says Mr
Potter, " that this blue band consists of lines produced by
the spider, and woven into the delicate tissue. To demon-
strate these fibres, however, is a work for an expert micro-
scopist, provided with a first-rate instrument. So critical

s It is found in the crevices of old walls, and may be recognised
by its irregular fleecy-looking web.148

TREATISE ON THE MICROSCOPE.

a defining power is required, at the same time with a large
quantity of light, that I doubt much whether any compound
refracting microscope, even the best achromatic, will ever
show the construction of this web on a transparent object.
When viewed in this manner through good common com-
pound microscopes, the blue band can scarcely be per-
ceived at all with a moderately high power. It is better seen
as an opaque object by the light of the sun, and it was on
this method that I discovered it, when highly illuminated
and highly magnified, to be covered very regularly and
closely with white spots. This was sufficient information
that it was of an uniform texture ; but as there is always
in such a light a strong display of irradiations and prisma-
tic colours, it was impossible to trace the fibres. I had dis-
covered something of the texture with small globules of
glass, used after the manner prescribed by Leeuwenhoeck ;
but with very high powers the distinct field of view is so
small, that 1 dared hardly to pronounce decidedly upon
the general structure ; and it was only after adjusting the
illuminating, lens of my microscope very carefully, that I saw
with it the complete structure of a regularly woven net"1

1 Mr Pritchard received from Mr Potter a specimen of this web;
but though he detected the Hue bands, yet, as the specimen was net
a recent one, he was unable to perceive " the complete structure
of a regularly woven net." {List of2000 Microscopic Objects, p. 6, 7.)ILLUMINATION OF MICROSCOPIC OBJECTS* 149

After this strong testimony to the practical utility of Sir
David Brewster's method of illumination, and the unques-
tionable optical principles on which it is founded, we were
surprised to observe that Dr Goring and Mr Pritchard
should, in the Microscopic Cabinet, published in 1832,
still recommend and use a method so decidedly erroneous
in theory, and founded on no optical principles whatever.
Dr Goring has described what he calls an improved illu-
minator, which is just Dr Wollaston's, with a stop in the
focus of the lens.

As the progress of discovery with the microscope must
depend upon the scientific illumination of the objects un-
der examination, we shall proceed to describe, in detail,
the method of illumination used by Sir David Brewster.
Let mn be the plane surface
on which the object rests
accurately perpendicular to
the axis of the lens, lenses,

or mirrors, which consti-

A

tute the microscope. Let
PQRST be a tube from
one and a half to two
inches long, and wholly
lined with black velvet, it
This tube has an opening at ST, and must be so attached
by an universal joint, or any analogous contrivance, to the

Fig. 46.150	TREATISE ON THE MICROSCOPE.

slider-holder, that the axis FL of the tube can be inclined at
any angle to the surface mn irom 90°, its general position,
to 60° or less, as circumstances may require. It should
also have a circular motion about its axis, in order that the
inclination may be made in any azimuth. A doublet
AB, CD, of no aberration, and having a focal length of
from half an inch to an inch, is then placed in the tube,
with a rack and pinion, or any other adjustment, to bring
its focus for parallel rays F, or its conjugate focus for di-
verging rays, accurately to a point in the plane mn, and
upon the object lying in that plane, for examination. A
short way below it is placed a metallic speculum (not a
silvered glass one), which receives parallel or diverging
rays, entering the tube at ST, and reflects them upon the
doublet ABCD. This speculum should be of pure virgin
silver, notwithstanding its liability to tarnish, and should
be wrought with the same care as the plane speculum of
a Newtonian telescope ; or it might be a rectangular prism
of good homogeneous glass, acting by total reflection.
This part of the ijluminator forms part of the microscope.1"
The other part of the illuminator, which is detached, is-
no less essential. It consists of the flame S, which should

1 if the axis of the microscope is placed horizontally, or even
with some obliquity, we may dispense with this speculum alto-
gether, and direct the tube at once to the illuminating flame.ILLUMINATION OF MICROSCOPIC OBJECTS. 151

be as bright and small as will give the necessary quan-
tity of light after condensation. As close to it as possi-
ble is placed a stand for holding a screen, with different
circular apertures, and a variable rectilineal aperture. If a
stronger light is required than can be obtained from the
plane S, its light must be condensed into a parallel beam
SL, by another doublet of no aberration, A'B'C'D', the
flame S being in its anterior focus.

The illuminator, as now described, is adapted to homo-
geneous light, either as obtained from a monochromatic
lamp, or by means of coloured glasses, or from the pris-
matic spectrum; but if we employ common light, the
doublets ABCD, A'B'C'D' must be achromatic. We have
mentioned above a variable rectilineal aperture. This is
a most essential accompaniment for giving perfection to
the vision of lined objects. The aperture should be made
to form every possible angle with a vertical line, and
should be opened and shut by means of a screw, till as
much light is introduced as is necessary to obtain a per-
fect view of the object. The image of the slit, which is.
close to the flame, jnust be thrown upon mn9 so as to be
parallel with the lines of the object. When the objects
are circular, circular apertures are preferable to any other.

We have already stated that no light should reach the
eye, either from the field of the microscope, or any other
source. For this reason it would be desirable to have cir-152

TREATISE ON THE MICROSCOPE.

cularand rectilineal apertures of different sizes, to be placed
immediately beneath tfm, so as to allow no part of the field
to be seen, excepting that which is occupied by the object
or part of it under examination.

The above apparatus being provided, let us suppose that
the observer is called to examine some structure very dif-
ficult to be resolved, such as the blue band of the Clubiona
atrox, or the structure and nature of the lines on test ob-
jects. We omit at present the consideration of the prepa-
ration of the object and the eye of the observer, and also the
nature of thejight which he is to use, as these will be se-
parately considered; and confine ourselves to the use of
the illuminator. The object is first placed on a piece of
thin colourless parallel glass, or film of topaz or sulphate
of lime, near its middle, and the microscope is directed to
it, so that it can be seen distinctly in the ordinary way.
Put the illuminator in its place, and set the proper aper-
ture close to the small plane. Adjust the doublet ABCD
by its screw or pinion till a distinct image of the aperture
GH is seen in the field; and, by means of the apertures
below mn, any strong or unnecessary light may be still
more completely excluded. If the structure is not ren-
dered sufficiently distinct by this process, it will be proper
to try the effects of oblique illumination, by inclining the
axis FL of the illuminator to the plate mn9 and observing
carefully the effects which it produces in different azi*ILLUMINATION OF MICROSCOPIC OBJECTS. 15$

muths. If all these means are insufficient, we must have
recourse to new auxiliaries,—to monochromatic light if
the microscope is not achromatic, or to monochromatic il-
lumination if it is achromatic ; and we must prepare both
the eye and the object, the one for exhibiting and the
other for viewing to the best advantage the structure which
we are anxious to develope. These important topics we
shall treat in their order, and with as much brevity as
possible.154k

TREATISE ON THE MICROSCOPE.

CHAPTER VIII.

ON THE MONOCHROMATIC ILLUMINATION OF MICROSCOPIC
OBJECTS.

If a simple and easily applied system of monochromatic
illumination, that is, of illuminating objects with homoge-
neous light, which a prism, and consequently a lens, is not
capable of dispersing or refracting in different directions,
could be contrived, we should never again hear of compound
achromatic microscopes. We believe it will be admitted,
that in Sir John HerscheFs doublet of no aberration, the
spherical aberration is more completely corrected than in
any double or even triple achromatic object-glass. Hence
it follows, that in homogeneous light such a doublet would
be a better microscope than the compound lens. But in
the best system of achromatic compensation that can be
executed, the secondary spectrum still remains without a
remedy ; and hence the doublet of no aberration, in which
there can be no secondary colour in homogeneous light,
must be a superior instrument to the compound achroma-ILLUMINATION OF MICROSCOPIC OBJECTS. 155

tic lens. Now, in telescopes it is impossible, except in
viewing the sun's disc,1 to work with homogeneous light;
but in microscopes, where the quantity of light is in our
power, it is perfectly practicable to make that quantity so
great that all the yellow or red rays which it contains may
give sufficient light for microscopical observations. This
insulation of homogeneous light may be effected in two
vrays ; 1st, by a monochromatic lamp, as proposed and con-
structed by Sir David Brewster ; 2dly, by the absorption
of coloured media; and, 3dly> by the prism.

The monochromatic lamp is shown in the following fi-
gure, where AB is a lamp having its globe A filled with:
diluted alcohol, which descends gradually through th^
tube C, into a thin platina or metallic cup, in which it
burns. A strong heat is kept up by a spirit lamp L en-
closed in a dark lanthorn, and when the diluted alcohol is
inflamed, it will burn with a fierce and powerful yellow flamet
If the flame should not be perfectly yellow, or rather of a
nankeen colour, owing to an excess of alcohol, a small pro-
portion of salt thrown into the cup D will have the same
effect as a farther dilution of the alcohol. Sometimes
a little blue light will be found mixed with the yellow,

1 A solar telescope should never be an achromatic one, but should
consist of a compound lens of no aberration, all the colours of the
spectrum but one being absorbed by the darkening glass.156

treatise on the microscope.

but this may be easily

absorbed by a piece of	Fig. 47.

scope through which a

ently powerful for all

microscopic observations.1

A stronger flame may be produced by using a gas
lamp, or, what is still better, a portable gas one contain-
ing compressed gas, This gas, when rushing out in a
full stream, explodes when burned ivith atmospheric air,
emitting much heaV anc* a faint bluish and reddish light.
As the force of the issuing gas is sufficient to blow out
the flame, a contrivance for sustaining it becomes neces-
sary. The method which we contrived for this purpose is

1 Edinburgh Transactions, vol. ix. p. 435.ILLUMINATION OF MICROSCOPIC OBJECTS.

157

Fig. 48.

shown in the annexed figure,

where PQ is the main body
of the lamp, MN the principal
burner and A the screw, which
opens the main cock. A small
gas tube abc> communicating
with the main burner, termi-
nates above the burner, and
has a short tube de moveable
up and down within it, but so
as to be gas light. This tube
de, closed at d, communi-
cates with the hollow 'Hhgjp,

in which four apertures are

perforated so as to throw their jets of gas to the apex of
a cone whose base isfy. When the gas is made to issue
from the burner M, it rushes also into the tube abcdg> and
issues in four small flames at the apertures in the ringfg;
and the height of these flames is regulated by the stop-
cock at b. The explosive mixture of air and gas which
rushes up through the ring is sustained in combustion by
these small flames, through which it passes. A broad col-
lar, made of coarse cotton-wick, and thoroughly soaked in
a saturated solution of common salt, is fixed on a ring h ;
and when the bluish flame of the explosive mixture rises
.above h, it will be converted by the salted collar into a158	TREATISE ON THE MICROSCOPE.

strong mass of homogeneous yellow light. A hollow cy-
linder of sponge, with numerous projecting tufts, may be
substituted for the cotton collar, or a collar of asbestos
cloth might be used, and supplied from a capillary foun-
tain containing a saturated solution of salt.1

When the few blue rays which sometimes mingle them-
selves with this yellow light are absorbed, every part of
the light will be found to have a definite refrangibility,
greater than any other artificial light that can be produ-
ced. The minutest objects, and the smallest type, will
appear perfectly distinct in this light when seen or read
through the largest possible angle of the greatest dispersive
prism, an irrefragable proof of the perfect homogeneity of
the light.

2. The second method of producing homogeneous light,
and by far the simplest and most easily applicable to mi-
croscopes, is that of absorption; and the best rays to leave
unabsorbed or insulated are the red. It requires some
experience and scientific knowledge of the action of differ-
ent absorbing media to select those which will leave the
narrowest andbrightestband of the red space in the spectrum.
We have now under our microscope (a grooved sphere of
garnet executed by Mr Blackie2) two scales of a moth ly-

1	Edinburgh Journal of Science, new series, No. 1, p. 108.

2	This sphere, which we have already mentioned, is made of theILLUMINATION OF MICROSCOPIC OBJECTS. 159

ing in sulphuric acid, and covering each other. With
solar light the spaces between the lines glitter with all the
hues of the rainbow; but when a thickish plate of red mi*
ca is combined with another plate of red glass, and placed
beneath the object, all these colours instantly disappear;
and a perfection of vision is obtained, which can be dis-
turbed only by the very small portion of spherical aberra-
tion which must exist in the sphere, and which an increas-
ed depth of the groove would render almost insensible.
Blue glasses, and green and yellow, as well as coloured
fluids, may be successfully :u&e(Linjoarrowing the range of
refrangibility of the red space.

3. The third method, or that of prismatic refraction, is
perhaps the surest and the best method of obtaining ho-
mogeneous light with the smallest extent of refrangibility.
A certain effect may be produced by small prisms ; but in
order to have a perfect apparatus, the microscope should
form part of the apparatus for examining the lines of the
solar spectrum; that is, it should screw into the eye-
piece of the telescope, in front of the object-glass of which
is placed a fine large prism, for forming the spectrum within

urest garnet, and is executed in the most admirable manner ; and
though we cannot say that we see any defect in it, yet if the
groove were still deeper, both its spherical and achromatic aberra-
tion would be diminished.160	TREATISE ON THE MICROSCOPE.

the telescope. By this method, which we have put to the
test of experiment, microscopical observations can be carried
on with mi accuracy and satisfaction which nothing can ex-
ceed. We enjoy the luxury of perfectly monochromatic
vision, greater than which the most perfect achromatic com-
pensation cannot give; and while we have the spherical aber-
ration corrected, we have no secondary colours, and none
of the imperfections of vision which must arise in trans-
mitting light through six or eight leases of plate and flint
glass.

Although we hope that the scientific reader will admit
that the preceding views are demonstrably correct, yet
Br Goring has pronounced a most unfavourable opinion of
the system of monochromatic illumination.1 We have al-
ready endeavoured to convert him from this heresy, and
hoped that we had succeeded ;• but in the Micrographiaf
just published, he has devoted a whole chapter to the re-
production and Support of his former views.3 We shall
therefore again examine his objections in their order, as
they obstruct the progress of improvement among those
who justly admire Dr Goring's ingenuity and knowledge
in every thing which relates to the microscope.

1. Dr Goring's first objection to monochromatic il-

1 Edinburgh Journal of Science new series, No. 9, p. 52.

2 Ibid. p. 143.	8 Page 73.ILLUMINATION OF MICROSCOPIC OBJECTS. 161

lamination is, that it is too weak, and must be about one
seventh of the whole beam of light. This we are not dis-
posed to dispute; but Dr Goring is too well acquainted
with the resources of optical science, to forget that this
monochromatic seventh of a beam of light may be made
seven times more intense than the whole beam. The ob-
jection, however, does not apply to the solar spectrum,
for one seventh of the sun's light is too intense for any
eye to bear.

2. The second objection of our author is, that the co-
lours of the spectrum, when separated by the prism, are
actually separated into different colours when they are re-
fracted ift oblique pencils bfk	If tMs Obser-
vation is correct, then we must denounce the prism that
produced such a spectrum as utterly useless. Dr Goring,
however, conceives his observation and his prism to be
good, and endeavours to explain the result by referring to
Sir David Brewster's analysis of the spectrum, in which it
is shown that white light exists at every point of it; but
this white light, which has been rendered visible by ab-
sorption, cannot be decomposed by refraction of any kind,
as it consists of red, yellow, and blue rays, of the same
refrangibility. Such white light is the light that is wanted
for the microscope ; and there can be little doubt that ab-
sorptive media will yet be discovered to effect its insula-
tion in sufficient quantity for praofcic^l purposes.162

TREATISE ON THE MICROSCOPE.

3. Another objection to monochromatic light is, that it
will not show the real colours of microscopic bodies.
This is true ; but the object of the microscope is not to find
out colours, but structures. A common glass lens, with
common light, will let the observer have all that he wants
of the colours of objects; and when he has learned this, he
will then gladly avail himself of coloured light for more
important purposes. We can truly say, that though we
have wrought with the microscope for thirty years, we do
not at present recollect a single case where we required
to know any thing of the precise colours of minute bodies.
Notwithstanding this discussion, Dr Goring concludes his
chapter with the following observation, in which we en-
tirely concur. " A monochromatic light, therefore, being
once obtained in a sufficient state of intensity for practical
purposes, bids fair to conduct us to the highest perfection
of which aplanatic object-glasses and magnifiers are sus-
ceptible/' It may be proper to add, that the best system
of compound achromatic object-glasses now in use would
be freed of all their secondary colours, by using monochro-
matic light; and they may be also greatly improved, by
employing suitable coloured media to absorb what are
called the outstanding rays in an achromatic combination.PREPARATION OF THE OBJECT AND THE EYE. 163

CHAPTER IX.

ON THE PREPARATION OF THE OBJECT AND THE EYE FOR
MICROSCOPICAL OBSERVATIONS.

! In using lenses of short focus, either singly or in doub^
lets and triplets, the object is so near the lens, and its thick-*
ness, even when very small, forms such a considerable part
of its distance from the nearest refracting surface, that any
bend or want of flatness in the object completely inters
feres with the distinct vision of its parts. When the ob-
ject will bear pressure, the best way is to lay above it a
thin transparent film, with perfectly parallel and polished
surfaces, such as a splinter of New Holland topaz, a
thin plate of sulphate of lime, or a film of mica. If these
plates are a little less thick than the distance between the*
lens and the object, a little bees' wax should be interposed
between the brass setting of the lens and the plate, so
that the lens in the act of adjustment would press the wax
against the plate, and the plate against the object, till dis-
tinct vision is obtained. The object should be placed up-
on a deeply curved ooncavejsurface*164

TREATISE ON THE MICROSCOPE.

The proper flattening of the object, when it is tender,
may be effected by pressing a thick balsam above it, and
allowing the lens to dip into the balsam. If the surface of
the lens is flat, its magnifying power will suffer no dimi-
nution.

When grooved spheres are used, such as the garnet
one above mentioned, the object requires to be very near
its surface. It should therefore be placed in a concave
lens of glass, of a little greater radius than the sphere,
and pressed into the concave form by interposing a nar-
row strip of a thin film of mica, and, if necessary, pieces
of wax or India-rubber, as before; the pieces filling up the
space between the lens and the mica, so as not to inter-
fere with the part of the object under examination.

If a diminution of the magnifying power can be permit-
ted, a fluid maybe placed between the concave object-plates
and the grooved sphere, and the chromatic aberration greatly
reduced. By the interposition of a fluid a grooved dia-
lfload inhere may be used; for though its focus for pa-
rallel rays falls within the sphere when the refractions are
made from air and into air, yet when the first refraction
is made from a suitable fluid, the focus will fall without
the sphere.

When the object is put into the best possible condition
for observation, and the illuminator appKed in the best
possible manner, the observer will have every advantagePREPARATION of the object and the eye. 165

ib his researches 5 but still the structure which he seeks
to develope may escape his eager research. Under these
circumstances, he must perform his part of the observa-
tion in the best possible manner.

Unless particular arrangements are made by the ob-
server for his own comfort, there is no bodily fatigue to
be compared with that of the use of the microscope. The
eye, the mind, and the whole frame, are on the stretch.
The observer must therefore first try if his own eye is in
a right state for observation. The fluid which lubricates
the cornea, which must be considered as a lens, and as
part of the microscope, is sometimes in such a viscid state,
that when the eyelids roll over the cornea by that beauti-
ful provision of nature by which it is* kept smooth and
clean, the lubricating fluid, which is pushed into a ridge
between the eyelids, does not quickly recover a convex sur-
face. This state of the cornea is incompatible with deli-
cate microscopic observations, and its existence may be as-
certained by viewing the expanded image of a luminous
point held close to the eye, and, after shutting the eyelids
and again opening them slowly, observing if the luminous
disc recovers its uniform luminosity quickly or slowly. If
the luminous line produced by the fluid accumulated be-
tween the eyelids continues to be visible, and the gene-
ral surface mottled and spotted, the lubricating secre-
tion should be excited by exposing the eye to the va-186	TREATISE ON THE MICROSCOPE*

pour of hartshorn, raised by pouring a few drops on the
surface of boiling water. The secretion will now flow co-
piously, the cornea will be swept clean by the purer and
less viscid fluid, and the vision of the observer greatly im-
proved. But this moveable fluid surface of the cornea
generates another imperfection of vision, which has al^
ready been referred to. This fluid, when undisturbed by
the eyelids, descends by the influence of gravity in verti-
cal lines, and the minute ridges thus formed obliterate and
render indistinct all horizontal lines seen by the eye, but
have a tendency rather to improve the vision of vertical
lines. In proof of this, we may state the unquestionable
fact, that if we take a striped pattern of any fabric, and
bend part of it into a horizontal direction, while the rest
remains vertical, or vice ver$a% the vertical part will always
appear the most distinct. Hence, in viewing lined objects,
when the position of the observer's head is either vertical
or oblique, the lines of the lined object should be always
placed parallel to the direction of the descending fluid. If
the axis of the lenses is vertical, and the eye looks down-
wards, the lubricating fluid will collect irregularly at the
apex of the cornea, and injure vision. If the axis of the
lenses is horizontal, and the observer's head in its natural
position, the fluid will descend in vertical lines ; but if the
observer lies on his back and looks into the microscope up-
wards, a position, we admit, not favourable for research,*

PREPARATION OF THE OBJECT AND THE EYE. 167

the fluid will flow equally in all directions from the apex
to the margin of the cornea, and leave a clear centre well fit-
ted for distinct vision. We may here notice the beautiful
contrivance, not mentioned by natural theologians, that the
effect of vertical descent of the lubricating fluid is counter-
acted by the eyelids opening horizontally, and consequently
effacing the tendency of the fluid to form vertical currents.
Had the eyelids opened vertically, the vertical ridges would
have been increased, and vision greatly impaired.

When every thing has been done to fit the cornea (the
only part of the eye over which we have any direct com*
mand) for accurate vision, the general state of the health,
or any casual irregularity of diet, will be sometimes found
to affect the state of the organ, and unfit it for nice obser^
vation. To remedy this defect of vision, we must refer the
patient to the simplest prescriptions of his physician.

When these precautions have been taken, the observer
must protect his eye from all extraneous light; and the
most effectual way of doing this is to use the snow spec-
tacles of the Greenlanders, which are cut out of wood, so
as to exclude all light whatever, except what enters through
a circular aperture the size of the pupil, and directly in
front of it.1 A cast should be taken in plaster of Paris,

-J In the snow spectacles a long narrow slit is used, to enable the
wearer to look on each side of him. An interesting account of the168

TREATISE ON THE MICROSCOPE.

from the part of the face to which they are to be applied,
to enable the artist to cut them of a proper shape; and
when finished they should be lined with black velvet.

The last requisite for accurate microscopical observa-
tion is steadiness in the microscope, and a steady and com-
fortable position for the observer. The first is easily at-
tained. The second may be accomplished by the observer
resting his doubled arms upon a stool or frame nearly the
height of the eye-glass of the microscope, but unconnected
with it, while his chin rests either upon his arms or upon
his breast.

When all these means and precautions fail in unravel-
ling a mysterious structure, we have often derived advan-
tage, in the case of lined objects, by looking through cy-
lindrical lenses or good prisms; the length of the cylinder,
or the refracting edge of the prism, being at right angles
to the lines. Narrow slits may also be used with advantage
next the eye; but in all these cases, while we improve and
give a finer definition to the lines and the spaces between
them, we deteriorate vision for other parts of the struc-
ture.

great value of these spectacles, by the celebrated Professor Blumen-
bach, will be found in the Edinburgh Phil. Journal, vol, viii. p. 261.test or proof objects.

169

CHAPTER X.

on test or proof objects for trying the perform-
ance of microscopes.

This class of objects, and their application to the mi-
croscope, we owe to Dr Goring; and to their introduc-
tion we must ascribe much of the rapid improvement which
this instrument has undergone. The finest test-objects
are the scales of butterflies and moths, which were sug-
gested to Dr Goring by the following passage in Leeuwen-t
hoeck. "If we examine the wings of this creature (the
silk-worm moth) by the microscope, we shall find them
covered with an incredible number of feathers (scales), of
such various forms, that if an hundred or more of them
were to be seen lying together, each would appear of a
different shape. To show more clearly this wonderful ob-
ject, I caused eight feathers to be delineated, for I do not
remember that I ever saw them of so curious a make in
any flying insect.

" Although the microscope, by which these feathers
7	h170

TREATISE ON THE MICROSCOPE.

were drawn, represented objects very distinctly, the limner
could not through it see the ribs or streaks in each feather,
until I pointed them out to him ; therefore, I put into bis
hands a microscope wbich magnified objects almost as
much as that by which the silk worm's thread was drawn,
desiring him to give the figure of that feather which through
it he could see the most distinct."1

From this passage, Dr Goring naturally inferred, that
there were some peculiar properties in the lines on the fea-
thers and scales of insects, which rendered them more diffi-
cult to be discovered than other microscopic objects ; and
hence he discovered their properties as test or proof objects
for trying the penetrating powers of microscopes. Mr Prit-
chard regards the penetrating power of a microscope as de-
pendent on its angle of aperture, and its defining power as
in the inverse ratio of the quantity of chromatic and spheri-
cal aberration. yVhen the angle of aperture was less than
a certain quantity, he found that the lined structure of the
scales could not be rendered visible, however perfect the
instrument was. The following is his list of test-objects,
arranged in relation to penetrating and defining power.
Several of these are represented in Plates XII. and XIII.

1 Select Works, p. 63.TEST OR PROOF OBJECTS.

171

I.—Penetrating Power.

Sect. I. Easy. 1. Petrobius maritimus, scales of, fig. 1*

2. Lepisma saccharina, scales of*

Sect. II. Standard. 1. MorphoMenelaus, feathers of, fig. 2.

2.	Alacita'pentadactyla, feathers of the.

3.	Alacita hexadactyla, feathers from

the body of it.

4.	Lycaenae Argus, feathers of the.

5.	Tenea vestianella, or clothes moth,

feathers from under ends of the
wing, fig. 3.

Sect. III. Difficult. 1. Pieris or Pontia brassica, or cab-
bage butterfly, feathers from
the, fig. 4, 4a, 4c.

2. Podura plumbea, scales from the,
fig. 5, 6, 7, 8, 9, 10, 11.

II.—Defining Power.

1.	Hair of the common mouse, fig.

12-15.

2.	Hair of the bat genus, fig. 16,17.

3.	Leaf of the moss Hypnum, spe-

cies unknown,

4.	Spotted .scales of the Lyccence

Argus, fig. 24.172	TREATISE ON THE MICROSCOPE.

Besides these test-objects, which are copied from Mr
Pritchard's drawings in the Microscopic Cabinet, we have
added from the same source the following interesting ob«
jects:

1.	A hair from the larva of the common Dermestes, fig,
18.

2.	A white hair from a young cat, fig. 19.

3.	The hair of a Siberian fox, fig. 20.

4.	The hair of a common caterpillar, fig. 21.

5.	A scale from the under side of the wing of the blue
butterfly, the Lycaenae Argus, fig. 22.

In some of the preceding figures of the lined proof-ob-
jects the reader will perceive diagonal or oblique lines, as
in fig. 9, 10, different from the longitudinal ones seen in
fig. 1, 2, 3, 4, &c. These lines are reckoned more diffi-
cult to be seen than the longitudinal ones; and Dr Goring
has mentioned those on the scales of the white-cabbage
butterfly as a difficult test-object. Mr Potter, who has
seen these diagonal lines, mentions also his having ob-
served them on the scales of the wing of the small house-
moth. As the nature of these oblique lines is far from
being understood, and as their real existence is in our opi-
nion questionable, we shall lay before our readers Mr
Pritchard's observations on those observed in the scales
of the Podura plumbea and Pieris brassica.

" In the foregoing account," says he, " of the differentTEST OR PROOF OBJECTS.

ITS

scales from the wings and bodies of insects, the design
has been to give their appearances under microscopes or
engiscopes, without in the least determining their actual
structure. When it is considered that these lines are less
than the twenty thousandth of an inch distant, it must
be allowed there is some difficulty in.accurately determin-
ing their construction. The Morpho Menelaus and Lepis-
ma saccharina are of sufficient size to distinctly perceive
they are composed of two delicate tissues with longitudi-
nal cords (probably tabular) disposed between them ; but
in the two delicate ones, the subject of these remarks, we
perceive other systems of lines disposed obliquely; and as
they are extremely delicate, it becomes a question whether
they actually exist, or whether they are appearances pro-
duced under certain- modifications of the illumination. As
there is only one set shown at a time, and I have never
been able to see them in the decided way of the longitu-
dinal lines, I have been induced to consider them as ap-
pearances only, and not real lines. To determine this
point, it became necessary to ascertain the cause that
would produce such an effect; and it immediately occurred
to me that these oblique lines were occasioned by the dis-
position and pressure of the superambient scales, in the
same manner as the watering or wavy appearance com*
municated to corded silks and moreens by the pressure of174

TREATISE ON THE MICROSCOPE.

two pieces passed between rollers.1 In examining the
scales of the Pieris brassica under a deep power and
large angle of aperture, I found them broad in some parts
and almost invisible in others; and the same appearance
presented itself in the curved lines on the scale of the
Podura plumbea, some idea of which may be obtained by
examining figs. 9 and 10 of Plate XII.

^ The motive that has induced me to offer the above
remark is, that it may lead to a complete investigation of
the subject. What is here given is merely the crude idea
that presented itself in the course of their examination as
proof-objects." 2

Dr Goring has published, in the Journal of the Royal In-
stitution, vol. xxii. and also in the Micrographia? many in-
teresting observations on lined objects, of which it is ne-
cessary to give some account. In order to explain the
effects of aperture on lined objects, he has represented in
the circles shown in fig. 23, Plate XIII. the different
appearances of a portion of the scale of the Marpho me-
nelausy shown in fig. 1, Plate XII. produced by increasing

1	I have since examined the Petrolius maritimus as an opaque
object, which confirms this view of the nature of the oblique lines
on its scales.

2	Microscopic Cabinet, p. 160, 161.	3 Hall, 159, &c.TEST OR PROOF OBJECTS.

175

the aperture. He used a triple achromatic object-glass
nine tenths of an inch focus, and half an inch in aperture,
with a negative eye-piece of one fourth of an inch.

No. 1. shows the appearance of the scale when the
aperture was one tenth of an inch, not a vestige of lines
being visible.

No. 2. Aperture three twentieths 5 Dr Goring fancied he
saw indications of lines or scratches.

No. 3. Aperture one fifth; traces of irregular scratches
seen.	,

No. 4. Aperture three tenths; nascent lines recognised
by a practised eye, like an aggregation of dots, but inter-
rupted and Ijroken. .

No. 5, Aperture four tenths 5 the lines resolved, but not
fairly. They are very faint, and seem ragged, as if still
composed of dots and points.

No. 6. Aperture five tenths; the full aperture of the lens.
The lines appear in their true character, as if drawn by a
pen with some blue pigment on light violet-coloured paper.

No. 7. Same aperture. When the object is turned one
fourth round, the cross striae become perceptible.

Our limits will not permit us to give Dr Goring's excel-
lent observations on the lines of the Pontia brassica, as
seen also with apertures of different sizes in a reflecting
microscope. With a well-figured metal, three tenths of
an inch focus, and an angle of 55±° aperture, the lines and*76

TREATISE ON THE MICBOSCOPE.

cross striae he found never to be resolved into dots and
points, but to appear in what he supposes to be their proper
character. " The two sets of diagonal lines," he remarks,
" will be shown with a force and effect which will leave no
doubt of their existence in the mind of a candid obser-
ver ; the various lines, the longitudinal, the cross striae,
and the two sets of diagonals, being all observable succes-
sively by a slight change of illumination, though we can
scarcely see two of the systems well at the same instant."1
Dr Goring elsewhere observes,2 that the reflecting mi-
croscope invariably shows the diagonal lines onthe brassica
as distinct as the eye sees the ruled lines on a copy-book;
that, in some " petscales," one of the systems of oblique lines
may be seen by looking into the instrument directly^ and
t^e other by looking into it obUqudyr without any altera*
Hon in the illumination;3 and that if one instrument shows
the lines dotty, broken* interrupted* or ragged, while
another shows them clearly made out as veritable lines ®r
stripes drawn with a pea and ink, the latter is the best,4

Notwithstanding these repeated decisions of Dr Goring,
he seems, in an earlier part of his volume,5 to have had
some misgivings on the subject. In speaking of the lined
tests, he says," There is an inexplicable mystery about them,

1 Micrographia, p. 163.

* Ihid. p. 102* note.

* Micrographia^ p. 130 and 144.

4 Ibid. p. 104. 5 Ibid. p. 44.TEST OR PROOf OBJECTS.	1*77

for if their lines are in reality produced on the same princi-
ple as those of micrometers, why are they not as easily seen f
No penetrating power or large angular aperture is requi-
site to bring out the lines on a micrometer, though divid-
ed nearly as finely as ordinary tests, to the extent perhaps
of 10,000 in an inch." These observations are just and phi-
losophical, and we would add only a single observation in
support of them, that Dr Wollaston made platina wires the
18,000th of an inch in diameter, and saw them distinctly;
and we venture to say, that in no instrument whatever
would such lines appear either dotted or ragged

Such was the state of this subject when these lined ob-
jects were examined by Sir David Brewster, both in re-
ference to their action upon light when observed by the
naked eye, and when placed under the microscope as test-
objects. Having been occupied for several years in a se-
ries of analogous observations on the lines which apparent-
ly separate the component fibres of the crystalline lenses
of animals, he was familiar with the class of optical illu-
sions which interfere with the accurate development of
such structures.

Upon exposing the finest lined objects to a bright light,
and excluding as much as possible all other extraneous
rays, he saw distinctly the fringes of colour produced by
interference; and on measuring the angular distances of the
first red fringe from the light, he found that the distance

h 2178

treatise on the microscope.

of the lines, or rather the diameter of one black line and
half the bright space between the lines, varied from the
10,000th to the 22,000th of an inch. Hence, if we take
the black lines and their intervals to be equal, the dia-
meter of each will var}r from about the 13,000th to the
29,000th of an inch.

Although these apparent lines give colours by inter-
ference, exactly like the analogous lines in the laminae
of the crystalline lens, yet neither of them are real lines,
as decided upon by Dr Goring. With small apertures the
lines in the crystalline lens appear dotty, interrupted, uneven,
and ragged, and exhibit, in short, all the general pheno-
mena of the lines on proof-objects; but with a good mi-
croscope and a large aperture, we discover the true se-
cret of all these appearances. They are not lines, but a
succession of teeth arranged in lines; and from the great
number of lines forming the sides of the teeth, they appear
dark. Each fibre, in short, has teeth on each side of it,1
and the teeth of one fibre lock into the spaces between
the teeth of the adjacent fibres. When we trace these fibres
towards the pole to which they converge, they become
smaller and smaller, the teeth diminishing in the same
proportion, so that they become as difficult, and finally
more difficult, to resolve, than the lines in the proof-objects.

1 See Phil. Trans. 1830.TEST OR PROOF OBJECTS.	179

After a laborious examination of the lined tests, and
the use of every optical resource which he could* com-
mand, Sir David Brewster has found that the mysterious
lines on these test-objects are only apparent lines, being
composed of a succession of interlocking teeth, by which
the fibres to which they are attached form that delicate
film which composes the scale of a moth. We now see
the source of all the perplexities which have beset this
class of observations. We understand why such lines are
not seen so distinctly as the real lines on micrometers,
and the dots and the raggedness are all explained. In
the lenses of quadrupeds the teeth of the fibres are not
round like those of fishes, but are often sharp i^j^ted
and extremely short, like a jagged line, or & ling with
points projecting from it. In like manner, the separation
of the teeth is much more distinct in some of the lined
objects than in others. See fig. 24, in which we have
given a rude representation of the lines.

With regard to the diagonal or oblique lines, which have
been such a source of perplexity to microscopical observ-
ers, we have little hesitation in pronouncing those which
we have seen to be optical illusions, from the accidental
aligmment of the sides of the teeth in different grooves,
when similarly illuminated by oblique rays. When the
scales are immersed in diluted sulphuric acid> we have
never seen the diagonal lines. When the sulphuric acid180	TREATISE ON THE MICROSCOPE.

is too strong, the scales curl up, and often in this state ex-
hibit the lines very beautifully. We have observed dia-
gonal lines singularly developed in the laminae of the crys-
talline, and clearly arising from the interference of the rays
acted upon by the lines on one side of the lamina, with the
rays acted upon by the lines on the other side, and there-
fore we have been the more confirmed ki o«r opinion. As
we have not had the advantage, however, of using any of
the fine reflecting microscopes with which Dr Goring ob-
served the oblique lines so distinctly brought out, it is still
with considerable diffidence that we place our conclusions
in opposition to so direct and distinct an observation, made
by such skilful and experienced observers as he and Mr
Pritchard.1

With the view of arriving at a just decision respecting
the nature of the lines, Sir David Brewster endeavoured to

1 Mr Pritchard informs us that the diagonal lines or cross striae
are most easily seen in the scales from the wing of the Euplcea
limniacae, and in the blue scales from the Papilio Paris, where they
" are strongly and easily developed under a power of from 100 to
200 times." In speaking of the ordinary lines, Mr Pritchard re-
marks, that these tines or " markings," with his best instruments,
" appear detached like short hairs or spines covering the delicate
tissue of the scale. This latter appearance is correct, the promi-
nent portions of the lines which have escaped the pressure of those
of the surrounding scales being in a plane above the other por*TEST <>& PROOF OBJECTS*

181

ascertain the disposition of the colouring matter on the
scales. Owing to the great brightness of the lines on the
black scales, especially near their root, he was at first dis-
posed to infer that, at least in these scales, the colouring
matter was arranged along the black lines, the particles
being more readily detained in their places by the edges
of thg teeth. He has found, however, that in other scales
the colouring matter lies also along the bright lines ; and
it is only when this colouring matter is removed, or its ef-
fect masqued, by removing the refraction at its surface by
immersion in a fluid, that the lines of proof-objects are de-
veloped with perfect distinctness.

Sir David Brewster has made an attempt to count the
number of scales and teeth in the wing of a brown moth, or
in one superficial inch, the area of the two surfaces of each
wing. He supposes, of course, all the scales to be the
same in size and structure, and he finds that there are

Scales..........................................158,400

Teeth...................................19,800,000,000

or nineteen thousand eight hundred million.

tions of the lines." This opinion of the structure of the lines, pub-
lished in 1835, is not repeated in the Micrographia, published in
1837, where Dr Goring decides that they are real lines. See List
of 2000 Microscopic Objects, p. 10. Lond. 1835.182

TREATISE ON THE MICROSCOPE.

CHAPTER XI.

ON MICROSCOPIC OBJECTS.

In the preceding chapter we have already described
some of the most interesting objects for microscopical ob-
servation. Every department of nature is full of. objects*
from the examination of which the most important disco-
veries may be expected ; but though the zealous observer
can never be at any loss for subjects of research, it is de-
sirable to know what has been done by our predecessors,
and what trains of inquiry, are. most likely to prove of ge-
neral interest. There are subjects of microscopic inquiry
which are closely connected with the most interesting parts
of physiology ; and even geology itself, conversant with the
grandest subjects of research, has recently been illustrated
by the aid of the microscope.

Dr Ehrenberg of Berlin, to whom we are indebted for so
many important discoveries respecting the organization ofon microscopic objects.

183

infusoria] animalcules,1 has lately made the most remarkable
discovery of infusorial organic remains. These remains
are the siliceous shells of animalcules belonging to the di-
vision Bacillaria, and form strata of Tripoli, or poli-schiefer
(polishing-slate), at Franzenbad, in Bohemia.2 M. Ehren-
berg has still more recently discovered them in the semi-
opal found along with the polishing-slate in the tertiary
strata of Bilin, in the chalk flints, and even in the semi-
opal or noble opal, of the porphyritic rocks.3 The size of
a single individual of these animals is about ^gth of a
line, or ^j^-th of an inch. In the polishing-slate from
Bilin, in which there appear to be no vacuities, a cubic lint
contains, in round numbers, 23 millions of these animals,
and a cubic inch contains 41,000 millions of them !

The weight of a cubic inch of the polishing-slate is 270
grains. There are, therefore, 187 millions of these ani-
mals in a single grain, or the siliceous coat of one of these
animals weighs the 187 millionth part of a grain !

In Plate XIII. figs. 24? and 25, we have given representa-
tions of these singular microscopic objects, as seen by
Ehrenberg. The siliceous shells found in the Franzenbad

1	Organization Systematise dev Infusions Thierchen, 3 vols, folio,
Berlin, 1830-1834.

2	PoggendorfPs Annalender Physik, 1836, No. V. p. 225.

3	Ibid. No. VI. p. 464.184?	TREATISE ON THE MICROSCOPE.

poli-schiefer are much more distinct than those found in
the Bilin strata. Among the former we discovered a mi-
croscopic shell of a quite different kind, whose length was
equal to half that of the shells shown in fig. 24, and in
which there were lines or grooves exactly resembling those
in the scales of moths.

Another example of the value of microscopical observa-
tions may be drawn from the discovery of the teeth of the
fibres which compose the crystalline lenses of almost all
animals. The crystalline lens is composed of innumerable
fibres of nearly the same length, each of which tapers from
its middle to its two extremities, where it comes to the
sharpest point. The sides of each of these fibres are fur-
nished with teeth like those of a watch-wheel, and the
teeth of the one lock into those of the adjacent ones,
as shown in fig. 28, Plate XIV. When the power is
small, or the microscope not good, or the laminae too
thick and not nicely detached, each row of interlocking
teeth appears as a dark line, sometimes as sharp as a black
line drawn upon paper with a pen. Sometimes the lines
appear rough and ragged, and as the fibres become jess
and less in approaching the poles, the black lines are as
difficult to resolve into teeth as the lines on test-objects
already described. The following measures, taken by Sir
David Brewster, will show what a wonderful, structure in
the eye has been thus disclosed to us by the microscope*ON MICROSCOPIC OBJECTS.

185

The calculations refer to the lens of a cod, four tenths of
an inch in diameter.

Number of fibres in each lamina or spherical coat....2*500

or the lens of a cod contains five millions of fibres, and
sixty-two thousand five hundred millions of teeth; and if
we reckon the curved end of the tooth as one surface, each
tooth will have six surfaces,1 which come into contact with
the corresponding surfaces of the adjacent tooth, so that
the number of touching surfaces willbe threekundred and
seventy-Jive thousand millions ;2 " and yet this little sphere
x)f tender jelly is as transparent as a drop of the purest
water, and allows a beam of light to pass across these al-
most innumerable joints without obstructing or reflecting
a single ray J*

There is another class of objects of extreme interest,
which Mr Pritchard has omitted to notice, and the de-
velopment of which called forth all the resources of opti-
cal knowledge and practical experience with the micro-
scope. These objects are the microscopic cavities in mine-

Number of teeth in each fibre............

Number of teeth in each spherical coat.
Number of fibres in the whole lens......

Number of teeth in the lens..............

............12,500

......31,250,000

........5,000,000

62,500,000,000

1 This includes the concave surface between two adjacent teeth.

2 Philosophical Transactions, 1833, p. 329,186	TREATISE ON THE MICROSCOPE.

rals, containing two fluids unknown to the chemist, groups
of crystals, floating balls, and exhibiting actual chemical
operations going on in these minute laboratories when ex-
posed to changes of temperature. These various pheno-
mena have been described and represented in drawings, in
two papers by Sir David Brewster, published in the Trans-
actions of the Royal Society of Edinburgh. In some of the
precious stones, particularly in diamond, garnet, &c. these
cavities are perfect spheres ; but, owing to the great refrac-
tive power of the gem, they appear completely black and
opaque, though the microscope descries a small spot of light
in their centre, which is the pencil of light which they re-
fract. These spherical cavities, and this central spot, are
the finest objects for examining the aberration of lenses
and specula, and are infinitely preferable to the reflected
patches of light from small spherules of quicksilver. Dr
Goring has observed spherical cavities or air-bubbles in
fluids, and, with his usual ingenuity, recognised their uti-
lity for indicating the effects of aberration. Those which
we have used in the gems are, however, permanent instru-
ments of much greater utility, not only from our being
able to use the same bright spot with all instruments and
on all occasions, but from the dark ring round the bright
spot being incomparably greater in the gems than in fluids.1

1 The ratio between the diameter of the dark sphere and of theon microscopic objects.	ib?

Representations of some of the cavities in fluids are
given in Plate XIV. fig. 29, 30, 31. Fig. 29 shows the
cavities containing the two new fluids, which will not mix,
though in the same cavity. The little circle is the bubble
either of gas or of vacuity. The fluid round it is a highly
evaporable fluid, and the fluid in the angles and ends of
long cavities is a thick and unevaporable fluid, which in-
durates when exposed to the air, Figs. 29 and 30 are beau-
tifully formed cavities in topaz.

Our limits will not permit us to pursue this subject far-
ther, and we shall conclude the article with a very brief
selection of microscopic objects from Mr Pritchard's ad-
mirable little pamphlet, entitled a List of 2000 Microscopic
dbjects.

1. Insects,—Eggs, wings, tongues, antennae, and scales
of.

Eyes of, Agrion, 12,000 eyes; Bombyx mer,
6236 eyes ; Phalaena cossus, 11,300; Sca-
rabaeus, 3180; Hawk-moth, 20,000; Li-
bellula, 12,544; Melalontha, 8820; Mor-
della, 25,088 ; Papilio, 17,000.

2. Hairs of Animals. Hair of an infant, Ornithoryn-
chus, mouse, bat, bee, Acilius canaliculars, Melecta

small luminous spot gives a measure of the refractive power of the
solid or fluid.188

treatise on the microscope.

punctatus, Siberian fox, spider, wing of Tipalis, stag-
beetle, white cat, dormouse, dermestes, caterpillar,
badger, ant-eater, civet cat.

3.	Scales op Insects. Podura plumbea, Pontia brassica,

Pierisbrassica, Parnassus Apollo, Atlas moth, diamond-
beetle, Euplcea limniacse, house-moth, Lepisma sac-
charina, 10-plumed moth, 20-plumed moth, Morpho
Menelaus, Papilio Apollo, Papilio Paris, Urania lei-
lus, privet moth. .

4.	Circulation in Plants, or Cyclosis. Nitella hyalica,

Nitella translucens, Chara vulgaris, Caulinia frigalis,
Hydrocharis or frog-bit in the stipulae of the leaves
and the ends of the roots, Tradescantia virginica or
spiderwort in the filaments around the stamina, Sene-
cio vulgaris or groundsel in the hairs surrounding the
stalks and flowers.

5.	Circulation in Animals. In the arachnoida or spi-

der tribe at the joints of the legs, Peria viridis and
Semblis bilineata on the antennae and wings when
they have just emerged from the chrysalis, larva of
the Ephemera, larva of Hydrophilus, small Dysticus,
Agrion puella, Libellula, round Lynceus, fresh-water
shrimp, water-hog (Oniscus), Ligia, water-flea
(Daphnia pulex). (See Pritchard's Microscopic Illus-
trations, and Microscopic Cabinet.)

6.	Circvlation jn Zoophytes. Mr Lister has discover-on microscopic objects.

189

ed a circulation resembling that in plants in some of
the polypiferous zoophytes, as the Tubularia indivisa,
Sertularise, Campanulariae, Plumulariae, &c.

7.	Crystals. For an account of various interesting mi-

croscopic phenomena observed by H. F. Talbot, Esq.
of Lacock Abbey, we must refer the reader to a se-
ries of interesting papers in the recent numbers of
the London and Edinburgh Philosophical Magazine,
and to others which will be found in the Philosophical
Transactions.

The oxalate of chromium and potash dissolved in water
and rapidly crystallized is a fine object. In polarised
light the most splendid object is the Faro Apophyllite
when the prisms are complete, as represented by Sir
D. Brewster in a coloured drawing in the Edinburgh
Transactions, vol. ix. p. 317, plate xxi. fig. 1.

Size.

8.	Animalcules. Monas Termo, 18,000th of an inch.

Monas atomus, 4000th of an inch.

Monas volvox, 3456th to 1728th of an
inch.

Volvox globator, found in stagnant
water, 30th of an inch.

Vibrio bipunctatus, 200th of an inch.

Vibrio spirillum, like a screw, 2000th
to 1000th of an inch.190

TREATISE ON THE MICROSCOPE.

Size.

Vibrio glutinis.1

Kolpoda cucullus, 28th of an inch.
Cercaria podura.

Cercaria viridis.

Cercaria hirta.

Leucophrys fluida, 400th of an inch.
Trichoda vulgaris, 1200th to 240th of

an inch.

Trichoda longicauda.

Vorticella polymorph a.

Vorticella convallaria.

Vorticella senta, 100th of an inch.
Vorticella rotatoria.2
The reader will find beautiful drawings and full descrip-
tions of these and many other animalcules in Mr Prit-

1	Figured by Dr <*oring in the Microscopic Cabinet.

2	The Rotifer vulgaris. See Microscopic Cabinet, chap. vi. p.
58-69; and Pritchard's Nat. Hist, of Animalcules, p. 167. The
reader will find a very interesting discussion of the apparent ro-
tatory movements of the wheels of this extraordinary animalcule,
by Mr Faraday, in the Journal of the Royal Institution, vol. i.
New Series, p. 220} October 1830, May 1831. See also Baker,
Of Microscopes, vol. ii. p. 266-295; Adams on the Microscope, p.
548 ; and Leeuwenhoeck in the Phil. Trans. No, 283, 295, 337.ON MICROSCOPIC OBJECTS.

191

chard's interesting work entitled The Natural History of
Animalcules, London, 1834; in the Microscopical Illus-
trations of Mr Pritchard and Dr Goring; and in the Mi-
croscopic Cabinet by the same authors he will find every-
thing that he desires respecting microscopic objects.

Dr Ehrenberg, whose discoveries we have already re-
ferred to, has lately presented to the British Museum a
series of dried microscopic objects, consisting chiefly of in-
fusorial animalcules, globules of the blood of the sheep and
frog, &c. along with a brief notice of his method of pre-
paring them. " Dr Ehrenberg," says Mr Children, who has
most judiciously published1 an account of this valuable
donation, "preserves these most minute and perishable of
known organic forms by means of rapid desiccation on little
plates of mica, in which manner he informs us that he
has succeeded in making a very satisfactory dried collec-
tion, not only of nearly 300 species of Infusoria, but also
of othei* kinds of microscopic objects. He mounts them
between double plates of mica, fixed in the cells of slides,
in the usual manner of preparing the scales of butterflies
and Podurce, and other transparent microscopic objects ;
and thus he says, '1 have not only preserved the form and
colour of the shielded cuirasses, Rotatoria and Bacillarice,
but also the softest and most delicate of the polygastric

1 Lond. and Edin. Phil. Mag. August 1836, vol. ix. p. 90.m

TREATISE ON THE MICROSCOPE.

Infusoria, even those of the genus Monas ; as well as the
tissue of plants; the Spermatozoa and Cercarice; the differ-
ent sorts of globules of blood, with their nuclei; globules
of lymph, chyle, and milk; and the nervous tubes, &c. of
a great number of animals, and of man.''

A power of about 300 (linear) is sufficient for viewing
these objects; " but a lower power does not show them
satisfactorily, however well*they may be illuminated."

We subjoin a list of the subjects presented to the mu-
seum by Dr Ehrenberg.

Slide No. 1.	4. Carchesium polypinum/

1.	Monas viridis.	5. Epistylis nutans.

2.	Poly soma uvella, and Mo- 6. Euplotes Charon.

lias termo.

3.	Spirillum undula, and Vi-	No. 3.

brio bacillus.	1. Stentor niger.

4.	Euglenia acus, Eu. viridis,	2. Paramecium aurelia.

Eu. pyrum.	3. Glaucoma scintillans.

5.	Coleps hirsutus.	4. Stentor polymorphus.

6.	Volvox globator.	5. Stentor caeruleus.

6. Idem, compressed to show
No. 2.	testiculi.

1.	Paramecium caudatum.

2.	Glaucoma scintillans.	No. 4.

3.	Trichoda carnium.	1. Nassula ornata.MICROSCOPIC OBJECTS.

193

2.	Nassula elegans.	6. Anuraea aculeata.

3.	Nassula aurea.

4.	Idem, crushed to show the	No. 6.

teeth.	1. Globules of blood of the

5.	Chilodon uncinatus.	sheep (ovis aries).

6* Chlamydomonaspulvis-	2. Ditto of the frog (rana

cuius.	temporaria).

3. Grains of the retina of the

No. 5.	eye of the same.

1.	Hydatina senta.	4. Spermatozoa vespertilio-

2.	Idem, crushed to show	nis murini.

the teeth.	5. Arhnanthes longipes.

3.	Polyarthra trigla.	6. Meridion vernale ;

4.	Brachionus pala, with its	Fragilaria rhabdosoma:

eggs.	Navicula acus;

5- Brachionus rubens, ditto.	Na. Amphisbaena.

FINIS.

EDINBURGH:

Printed by Thomas Allan & Co,
265 High Street.ERRATUM.

Page 124, line 1, for all the micrometers above described, read all micrometers.PLA TE I.

A Single Microscope.rjjA.TE 17.

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