Boston

Medical Library

8 The Fenway

Digitized by the Internet Archive

in 2011 with funding from

Open Knowledge Commons and Harvard Medical School

http://www.archive.org/details/micrographiacontOOgori

AueroareUlnta. /'/u/, I

- m z

London. WhittaAer & Cf Jen f83

.'-,/ , , Join CUakcrn

MICROGRAPHIA:

CONTAINING

practical afr&ags

REFLECTING, SOLAR, OXY-HYDROGEN GAS

MICROSCOPES; MICROMETERS;

EYE-PIECES, Sfc. $c.

BY ,

C. R. GORING, M.D.

AND

ANDREW PRITCHARD, Esq. M.R.I.

HON. MEM. SOC. ARTS, EDIN. &C.

AUTHORS OF " MICROSCOPIC ILLUSTRATIONS,'' " MICROSCOPIC CABINET,'

" NATURAL HISTORY OF ANIMALCULES," &C.

LONDON:

WHITTAKER AND CO., AVE-MARIA-LANE.

1837.

-A.

V\

LONDON :

Wilson and Son, Printers,

57 j Skinner Street.

PREFACE.

The importance and utility of the modern improve-
ments in Microscopes being- now generally recognized
throughout Europe by all practical zoologists and
botanists*, the present publication seems imperatively
called for as a necessary sequel to the te Microscopic
Illustrations" and " Microscopic Cabinet," and is in-
tended to supply all the practical information micro-
scopists can require, which is not afforded by those
works.

The first chapter contains a description of all the
reflecting Microscopes that are worthy of note, and
in particular an account of the most improved form
of that of Professor Amici, of Modena (the only one
which has come into use) : and it may fearlessly be
maintained, that Dr. Goring has omitted no argument
which could be adduced in its favour. To this is at-
tached full and perspicuous directions for its manage-
ment, in every possible manner in which it can be
mounted. In my humble opinion, this chapter is an
excellent model for this kind of writing.

* The recent discoveries of the fossil remains of Infusoria, Plants,
&c. will cause the geologist also to call to his assistance the aid of the
Microscope.

IV PREFACE.

The tract on Micrometers, whatever may be its faults,
is the best practical treatise that has yet appeared ;
and it is worthy of remark, that few theoretical or
even practical opticians can determine the foci or
powers of small lenses with any degree of precision : put
a minute lens into their hands, and ask them to tell you
what its solar or visual focus is, and ten to one if they
can give any thing but a rude approximation to the
truth. Now, all difficulties of this kind are obviated ; for
by his tract, the foci, powers, &c. of all sorts of lenses,
simple and compound, as well as those of Engiscopes,
can be determined with accuracy. To the description
of the various constructions of Micrometers is added
that of a new one, for single or compound magnifiers.

The chapter on Monochromatic Illumination con-
tains many curious particulars : that on Solar Engi-
scopes may be said to be written on a new species of
instrument, or one at least not as yet introduced into
use, but which may be expected one day to become
highly popular with the more refined and fastidious
admirers of the wonders displayed by Solar Micro-
scopes.

The chapters* on trying Microscopes and Engi-
scopes against each other, and that on Eye-pieces,
contain much valuable information, and are written
in a very forcible style. The tract (Chapter VII.),
illustrative of the effects of large angles of aperture
on the vision of objects, as contradistinguished from

* The opinions expressed iu each chapter are those of the writers
whose initials are severally subjoined.

PREFACE. V

magnifying power, touches the very nerve of the
question respecting the importance of the modern
improvements of object-glasses, metals, doublets, and
triplets ; the extension of their angular aperture, or
the taking in of a large pencil of light (free from aber-
rations) from every part of the object, being, in fact,
nearly the sum and substance of every thing which
has been effected in ameliorating the optical part of
Microscopes and Engi scopes ; for so far from increas-
ing the magnifying power (as is generally imagined by
persons not acquainted with the subject), the principal
improvement may be said to consist in so augmenting
the penetrating powers of these instruments, that a very
high degree of amplification becomes in a manner unneces-
sary. There is also some useful information on
Objects in this chapter.

My own essay on Solar and Oxy-hydrogen Gas
Microscopes will, I doubt not, receive due attention
from the public, being the first account of the latter
which has appeared ; it will be found to contain a vast
body of very useful practical information respecting
these highly popular, attractive, and amusing instru-
ments, which may be said to have conferred on man-
kind a new source of pleasure.

In Chapter XI. I have briefly described a few con-
trivances for facilitating the observation of Micro-
scopic Objects, and perfecting the mechanical part of
instruments.

Correct Drawings of Microscopic Objects are ex-
ceedingly valuable ; they are evei'y day sought after

VI PREFACE.

with increasing eagerness; and as my friend, Mr.
Bauer, has made more of them than any other person
in England, it appeared to me highly desirable to
present the reader with an account of the method
pursued by him in their production. For this pur-
pose, I have been favoured with a short essay written
by himself, and feel confident it will be perused with
much interest. The other paper in the Appendix, by
the Rev. J. B. Reade, on his new method of Illumi-
nating Microscopic Objects, will be found highly
useful, either for investigation or amusement ; with it,
even the most fastidious may employ a miscroscope
without excitement from light, as in the ordinary
illumination, while it is equally available for the larger
class, or for the scales of the Podura.

ANDREW PRITCHARD.

263, Strand, London.

CONTENTS

OPTICAL SECTION OF THE MICROGRAPHIA.

Chap. I.

Page
History and description of the Araician Reflecting En-

giscope 1

Chap. II.
On Micrometers, and their use in determining the foci
and power of object-glasses, single and compound
magnifiers, &c. 42

Chap. III.
On Monochromatic Illumination 73

Chap. IV.

On Solar Engiscopes, and on the exhibition of Tests by
them 82

Chap. V.

On trying Microscopes and Engiscopes against each
other ; with rules for ascertaining their comparative
merits 99

Chap. VI.
On the Spherical and Chromatic Aberration of Eye-
pieces o 134

CONTENTS.

Chap. VII.

Page
Illustrations of the Alterations produced in the vision of

certain Microscopic Objects, by using instruments

having various angles of aperture, but a fixed power . . 1 59

Chap. VIII.

On the construction and management of Solar and

Oxy-hydrogen Gas Microscopes, &c 168

Chap. IX.

I. — On Cuvier's method of dissecting Microscopic Sub-
jects under Fluid ; with Additions 215

II.— On a new method of obtaining a delicate adjust-
ment to the focus of Microscopes , 217

III. — On an improved mode of supporting a candle or

lamp for Microscopes 218

APPENDIX.

No. 1. — On making Drawings of Microscopic Subjects,

in a letter, by Francis Bauer, Esq. F.R.S. F.L.S. &c. 221

No. 2. — On a new method of illuminating Microscopic-
Objects. By the Rev. J. B. Reade, M.A. of Caius
College, Cambridge, and Honorary Member of the
Halifax Philosophical Society , , 227

MICROGRAPHIA.

CHAPTER I.

HISTORY AND DESCRIPTION OF THE AMICIAN
REFLECTING ENGISCOPE.

While one sect of Microscopists is pragmatically employed
in the production of fancied novelties, and not unfrequently of
doivnright retro gradations in the structure of Microscopes,
another party seems sturdily determined that there shall be
nothing new under the sun in these latter times : we may rack
our brains till they are addled, invent and excogitate all sorts
of wonders, ay, and get rewarded by some learned body for
our pains, and lo ! it is certain to be found out in the end
that we have merely resuscitated some old thing which the
ignorance, or want of recollection, of the public had overlooked,
and that our utmost endeavours have only succeeded in
dressing it out in new habiliments, or engaging it in some new
occupation.

When Professor Amici, of Modena, invented his admirable
reflecting Engiscope, (for it cannot be pretended that we ever
had a reflector really capable of practical application before,)
the spirit of detraction instantly exclaimed that he had merely
reversed a Newtonian telescope made on a small scale, and of
course refused to concede the merit he had so richly earned in
inventing and introducing into use what every candid person
must allow to be a real novelty. Sir Isaac Newton never
recommended any such adaptation of his telescope to the
purpose of acting upon diverging rays, but merely a concave
meta! to form an image, and an eye-glass to view it with, being

B

-*:

4 THE MICROGEAPHIA.

in fact the simplest kind of reflecting microscope which can
possibly be constructed, and the best also, if it could but be
easily applied to the examination of minute objects ; which,
however, is not the case. (Vide fig. 25.) In fact, it is in the
exterior only of the Amician microscope that there is a prima
facie resemblance to a Newtonian telescope, for the form of
the concave metal and the situation of the radiant point are so
totally different, as to constitute a new instrument.

It has always been remarked that the English invent nothing,
but improve upon every thing ; and the Amician reflecting
Engiscope may, in its perfect state, certainly be considered as
a fine sample of English improvement on a foreign invention.
For, though Professor Amici joins to his acquirements as a man
of science an admirable genius for mechanics, and is, in fact,
one of the best workmen the continent can produce, yet the
instrument was turned out of his hands in a very 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 of too large a diameter, which caused it to intercept too
large a quantity of light from the other, leaving only a narrow
rim of reflection to enter the retina, which occasioned a dis.
agreeable nebulosity in the middle of the field of view, unless
the eye-glass was of great depth ; the mounting was, moreover,
very deficient, and greatly disabled and cramped the powers of
the instrument. The original dimensions of Amici's Engiscope
were the following: — Elliptic metal 1 inch aperture, and 2 to
inch focus ; diameter of the diagonal | an inch ; length of tube
1 foot ; and however well such metals may be worked, it will
be found impracticable to render them capable of exhibiting
even ordinary minute objects as they ought to be shewn, much
less tests.

I shall now proceed to describe the instrument in its effective
form, as constructed at my suggestion, with the final improve-
ments in the mechanical part, which my worthy coadjutor, Mr.
Pritchard, and myself, have effected in it.

REFLECTING ENGISCOPE. O

To begin from the foundation : — The stand or pillar of the
Engiscope is not unlike those of small spy-glasses ; it grasps
the body of the instrument by means of a split socket, as they
do, and will readily serve as a stand to a 20-inch glass, if re-
quired : the peculiarities of it I shall now notice. The pillar
is screwed on to a solid cruciform stand : — it is well known to
every cabinet-maker that a claw table stands much more se-
curely on four legs than on three, and is much less liable to be
overturned ; of this any person may easily satisfy himself;
and though a table on three legs is always sure to rest truly on
an uneven surface, one on four may be made to do so likewise
by means of an adjusting screw applied to one of the legs,
made purposely a little too short, and, moreover, will not re-
quire to be made so massive, or to have its feet sprawled out
so far, in order to procui'e a given degree of stability. Now the
Amician Engiscope requires to be raised to a considerable
height, in order to come to the height of the eye without a
scaffolding of boxes, books, or pieces of wood, &c. : nothing
can be more agreeable to use when it is on a true level with
the eye, or more uncomfortable when it is not ; on this account
the cruciform stand seems to be peculiarly suited to it. At the
same time I freely admit, that if the legs are wanted to be made
to fold up together, in order to pack in a small compass, the
tripod is the preferable construction, for the cruciform must
be made of one piece. Mr. P., of course, makes them to suit
the fancy of purchasers : I have little doubt that he would, to
oblige a good customer, mount an instrument on a mop-stick,
or the top of an old brass candle-stick, as Sir I. Newton did the
original of his telescope. It will be observed that there are
two joints in the pillar, like those of a pull-out telescope :
these are composed of tubes, the recipient part of which is
split, and screwed on the outside, and firmly grasps the inter-
nal tube by means of the pinching milled heads, so that the
whole is as strong and solid as if made of a single tube. As
will be seen by reference to the scale, the instrument may be

O THE MICROGRAPHIA.

elevated to the height of 18 inches, if required. All the par-
ticulars I have described are sufficiently well represented in
Plate 1, fig. 1 ; the adjusting screw for the fourth foot is seen
in front. Fig. 17 is a key for tightening the cradle-joint at the
top of the pillar, so that the body may be firmly fixed in any
degree of inclination without swerving when turned round in
the split socket.

The body (a) hasnothing very peculiar about it ; it has the
usual eye-tube (d), into which is screwed the eye-piece (e),
consisting of two glasses on the Huyghenian principle, but in
the lower powers further separated than in the regular con-
struction, to accomodate them to the metals : it is difficult to
procure a field of view of more than 20° with a very shallow
eye-piece, acting with a short body, if a maximum of distinct-
ness is required. Not unfrequently the edges of the field of
view with low powers are rendered dark and shadowy, or alto-
gether invisible, owing to the hole in the side of the tube of
reflectors (fig. 28, c.) not being sufficiently opened out. The
same effect is even, to a certain extent, sometimes produced by
the hole in the stage being too small, or closed up partially by
the traversing plates. The foci of the anterior glasses of the eye-
pieces, of which there are usually three, are 3-4ths, 3-8ths, and
3-16ths of an inch; the powers given by which of course de-
pend upon the depths of the metals they are used with, and
also upon the elongation or contraction of the length of the
body, by means of the eye-tube, h, c, is the tube containing
the metals ; a section of it, fig. 28, shews them of the full
size for one inch acting focus, in situ : b, is the elliptic one ;
a, the plane diagonal, c; the hole in the side of the tube to
admit the rays from the object, d, the radiant point, or object
itself. The rays proceed from the object in the manner marked
in fig. 28, and are reverberated into the opposite focus of the
ellipse, where they form an image between the two eye-glasses,
which is viewed by the eye through the anterior glass.

On the outside of the tube of reflectors, at 6, will be re-

REFLECTING ENGISCOPE.

marked a lappet, or segment of tube, the use of which is to lap
over the hole in the side of the tube, to keep out dust and
damp, &c, when the reflectors are not in action : for the same
reason a cap is made to cover the open end of the tube, when
removed from the body. A circular line is also turned on the
tube, passing exactly through the centre of the hole in the side,
as a rough guide for placing the object in its proper place.

It here becomes necessary to say somewhat concerning the
manner in which the tube of reflectors and the body of the
instrument are attached to each other : this is effected in
the ordinary manner in which the component tubes of a pull-out
spy-glass are connected with each other. Every one must
have observed that the tubes of spy-glasses rather get tighter
than otherwise from age and use, owing to the partial oxidation
of their surfaces ; moreover, if well executed, a tube consisting
of a number of pieces is just as stiff and straight as if formed
only of one. In this way, therefore, are the tubes of reflectors
fastened to the main body ; one of them, of the full size, is seen
at fig. 11, with the spring-joint*" (d), which will, I doubt not,
be instantly recognized by any optical turner-]-. The only pe-
culiar point necessary to be attended to is to form a notch, or
a mark of some kind, on the milling of the sliding tube (e),
which must be made to coincide exactly with another on the
neck of the body, as may be observed in fig. 1, in order that
the metals may be always affixed so that the prolongation
of their axis and of the body may be in the line of the bar.
Too much care cannot be bestowed on this circumstance, for if
the plane of the stage and object presents itself obliquely to the
metals, their performance will be seriously deteriorated.

* One of these, of course, suffices/or the whole of the tubes, and always
remains screwed to the neck of the body.

t I think the joints of flutes, clarionets, &c. might be made with great
advantage on this principle, and would not be at all liable to get out of
order ; a little bee's-wax and suet would render them perfectly wind-
tight, though I suspect if the tubes were carefully drawn, and merely-
wetted when put together for use, the wax would not be required.

8 THE MICROGRAPHIA.

f, fig. 1, is the triangular bar of the Engiscope, soldered to
the neck of the body, which, with all its apparatus, is much
like that of other engiscopes. Its mirror is plane on one side,
and should be a dead white surface on the other. I do not
hold with concave mirrors ; they always produce an indistinct-
ness in vision, along with an increase of light, when used with
transparent objects, and for opaque ones they are not wanted,
as the condensing lens, fig. 7, acts much better, either used by
itself or in conjunction with the plane side of the mirror, as
circumstances may require. The stage (I) is undoubtedly the
best yet contrived for giving traversing motions, being the ce-
lebrated combination of rack and screw work, worked by
the two concentric milled heads at m, invented by Mr.
Tyrrell. The small anterior head gives a motion to the object
in the line of the body, the other in the contrary direction ;
both are under the command of the finger and thumb of
the right hand, and both may be moved together, giving a
diagonal motion. The said stage is removable at pleasure,
from the triangular socket (jg) being made to lift out.

To the triangular socket (g) is appended that which carries
the clamping screw (h), and the adjusting screw fij, by which
the object is brought into the focus of the Engiscope.

Fig. 2 is the tube which carries the aplanatic object-glasses
belonging to the instrument ; the rectangular prism, which
enables them to act with the Amician, may be seen in dotted
lines. Where several object-glasses are screwed together, they
ought to be made of decreasing diameters, as shewn in the
plate ; the anterior ones (which are always of shorter foci than
the first screwed on) being the smallest, so that whenever one
objective is screwed on before another, it shall always be of
less diameter than that to which it is attached, and thus leave
space/or the illumination of opaque objects. When all are made
of the same diameter, in the usual manner, it is impossible to il-
luminate opaque objects shewn by them otherwise than by a cup.

Fig. 3 is a silver cup which is fastened on to the tube of

REFLECTING ENGISCOPE. 9

reflectors by a clasping piece of brass, the perforation in it
being made to coincide with that in the side of the tube. Each
pair of metals capable of acting upon opaque objects has one
adapted to it.

Fig. 4 is an illuminator for opaque objects, about an inch
focus : its stem is passed into the spring tube (n) carried by
the arm attached to the under side of the stage ; it has a motion
upwards and downwards in the tube, and likewise a rotatory
one ; the lens also can be turned round in its socket (a), by
means of the milled head (b). Nothing, in my opinion, can be
a greater blunder than to give these sort of things a greater lati-
tude of motion than is really requisite, by means of cradle-joints
and balls and sockets, which only render them more difficult to
manage, and increase the expense of their construction. By
the by, all sorts of needles and pins, &c. ought to be made of
brass, or, what would be better, of gun-metal ; if made of
steel they soon rust by being handled, and then move very
stiffly and awkwardly in their sockets.

Fig. 5 is another illuminator for opaque objects, intended
to co-operate with that just described, and also to act by itself
along with the object-glasses and the metals of long focus.

It is about 1\ inches focus, and is passed into a spring tube
travelling on a slide, with a pinching screw underneath, by
which it is adjusted at a proper distance from the object, or the
second illuminating mirror. It is represented in a detached
state, but fixes to the under plate of the stage on that side op-
posite the milled heads.

Illuminators and burning lenses present much analogy to
each other, the caustic and illuminating power of each depend-
ing, first, upon their actual diameter, and, secondly, upon their
angular aperture : thus a burning lens, even of 2 feet diameter
aud 4 feet focus, does not exert any great caustic power alone,
because, though it draws a vast body of calorific rays together,
it does not condense them into a sufficiently small space. But,
let a second lens be interposed at ten inches from the focal

10 THE MICROGRAPHIA.

point of the first, and of sufficient diameter to catch the whole
cone of rays proceeding from it at that point, and converge to
a focus perhaps only six inches distant from the small one, and
we shall soon see what an enormous increase of caustic power
will be the consequence. On this principle the two illumina-
tors are combined ; the first is only of an inch diameter, but
might be made larger to increase the effect, for the vision by
the Amician Engiscope is so dark and sombre on opaque ob-
jects as to require all the help from illuminators which can
possibly be given to it. Every one must, I think, have re-
marked the intense illumination given by silver cups acting with
illuminators of large angular aperture, which is, in fact, an
example of the intrinsic brightness produced by a primary lens
having its light condensed by a cup placed at a proper distance
from it ; though a silver cup can never reflect so much light as
a lens of the same focus and aperture can transmit, acting in
lieu of it.

It might perhaps be worth while to construct an illuminator
of two lenses placed at a proper distance from each other with
relation to their foci, (somewhat after the fashion of those opera-
glasses which are worn about the neck,) having no tube, but
mounted on a bar, the largest being made to travel on the bar
by means of a socket, and the small one to fold down ; the bar
must be equipped with a cradle-joint attached to a stem to be
passed into the spring tube of the slide.

Fig. 6 is a raised stage to come up to the level of the tube
of reflectors, since the primary stage itself cannot easily be
made to do so : it has a sunk rim turned to receive a watch or
plane glass, black and white discs, &c, like other microscopes,
it is also convertible into a black box for viewing certain
opaque objects when closed at the bottom by the part (e),
which is covered on the inside with black silk velvet, (a) is
an arm made for convenience to revolve round on a shoulder
at the top of the stage, and even to remove altogether at the
pleasure of the observer ; at the end of the arm is a sprung

REFLECTING ENGISCOPE. 11

tube to receive the needle (d), which is furnished at one end
with a miniature vice to grasp pins, at the other with a small
cylinder to act with silver cups. At c is a small black velvet
ground, which is employed when opaque objects, not attached
to discs, are to be seen by silver cups, in order that their
minutiae may be displayed to the greatest advantage ; the
more sombre and obscure these are, the greater is the necessity
of an intense black ground to contrast and draw them out.
The aforesaid little ground serves the important purpose of
keeping all direct rays from the illuminating lens from getting
into the Engiscope, so long as it remains in the line of the axis
of the metals ; but if the traversing plates of the stage are
called into full action, it must evidently fail in this purpose ;
they must therefore be quiescent, or nearly so, and the observer
will have to move the object about by means of the needle,
which, having no cradle-joint, or ball and socket, I know by
experience he may easily do, giving a small final motion only
by means of the screws. If, however, it is determined that the
traversing plates shall act to their full extent, (which, by the by,
will be sure to cut off some of the light coming from the
illuminator,') then the wire carrying the disc or black ground
must be made to fix to one of the arms of the stage : then it
will probably happen that the traversing tnotions ivill carry the
object away from its influence ; so, upon the whole, 1 think it
is best as I have arranged it. Different sizes of discs will be
required to suit different cups.

Fig. 7 is the illuminator co-operating with the silver cups.

Fig. 8 is a species of raised stage, which may be made
serviceable for many purposes, but is especially contrived for
the reception of test-lined objects, both transparent and opaque ;
d is a piece of brass fitted to the aperture of the stage, and
attached by two catches, corresponding to the nicks in the
superior plate ; 6 is a milling attached to a piece of turned
work, having in its upper portion a piece of sprung tube with
a sunk margin (a) adapted to receive the disc (c), of which a
magnified representation may be seen in fig. 23.

12 THE MICROGRAPHIA.

These circular object-holders may be made of ivory, having
a disc of ^talc-glued to one side, and another kept down by a
ring over it, between which the object is to be included in the
usual way ; the side on which the ring does not appear being
placed uppermost. The scales are to be examined by a mag-
nifier, and two marks (a, a, fig. 23) made with ink to mark the
direction in which the lines on the scales happen to lie most
commodiously for resolution, which will be when the said
marks are at right angles to the lines: then the disc is
placed in its recipient (a, fig. 8,) and by means of the milling
fb) it is turned round till the said marks come into a line with
the body, when it will be in a situation most suited for develop-
ment : of thisll shall speak more at large hereafter.

If the subject is opaque, the disc is made merely of a bit of
ivory hollowed out and blackened, to which the object is
cemented. When placed in the recipient, the two ink marks
should be at right angles to the body, and should cross the
lines to be resolved at right angles also. The illumination
will then come with most effect in the direction of the said
marks, or from the side of the stage on which the illuminator
(fig. 5) is placed, as it ought to do.

Fig. 9 is a piece of apparatus made to fix on to the body
of the Engiscope, in lieu of the metals and object-glasses, to
carry single lenses or doublets, when it becomes an excel-
lent microscope.

Fig. 10 is the plan of a four-pillar circular slider-holder
with double plates, and a good stiff spring. A segment fa, a,)
must be cut from the plates on that side next the bar when in
situ, otherwise they will come in contact with the neck of the
body when the superior plate of the stage is made to traverse
in that direction ; 6 is an object-holder having a bit of sprung
tube in the middle to carry discs, like fig. 8, which are to be
fixed into it, on the same principle. Charles the Xllth. of
Sweden, may it please your worships and reverences, always
spread his bread and butter with his thumb ; many Asiatics
prefer their claws and digits to knives and forks ; and I am

REFLECTING ENGISCOPE. 13

very fond of using my fingers to move an object about, instead
of traversing screws, provided the spring of the slider-holder
does not permit them to move too easily. It will be observed
that a slider-holder, fashioned like that in the figure, is capable
of much latitude of motion in a slider-holder even of moderate
size*.

Fig. 1 1 is a view of a metal of 3-10ths of an inch aperture, and
3-10ths of an inch focus, of its natural size: it will serve also to
represent any other metal of 3-10ths aperture made on the same
plan, to avoid increasing the size of the diagonal metal along with
the angle of aperture, and thereby obliterating the effect of the
best part of the elliptic speculum, e is the lappet to cover
up the part of the tube from dust, &c. which is cut away at f,
in order to admit the approach of a slider-holder to the focus,
which lies within the tube : the diagonal is shewn in dotted
lines. Fig. 12 is the trough slider-holder, an indispensable ap-
pendage to a pair of metals constructed in this way : it is of the
four-pillar model, but its superior plate is bent hollow to adapt
itself to the tube containing the metals ; it is also blackened
and perforated. The slider-holder, which must be very small,
very thin, and likewise blackened, is seen in situ at a. Be-
neath the trough may be seen a pair of plates, between which
is passed the slide of diaphragms (fig. 13), for improving the
vision of certain objects not contained by sliders, having very
small holes in them, which of course supersede its use. The
said diaphragms applied immediately behind objects seen by the
other metals, are of little use, and do not answer so well as
those in the slide placed under the stage. The three figures,
included by the bracket, are of the full size.

Fig 14. is the piece which carries the slide of diaphragms

* T have sometimes thought a very good and simple slider-holder might
be made of a circular magnet, or two semicircular ones slightly separated,
or even of two bars with the help of a steel slider-holder like that at b,
which would always cling to the magnet in any position, and still allow of
being moved about.

14 THE MICROGRAPHIA.

(fig. 15.); it has a groove in it which greatly resembles that
part of the old compound microscopes which used to carry the
slide of object-glasses, and has, like it, an internal spring, which
catches in the notches of the slide, when each hole becomes
concentric with the aperture in the tube of reflectors. This part,
however, of the apparatus is not essential. I myself prefer
a slide of diaphragms to a wheel, because it is evident that a
wheel can only give an eccentric pencil for lined objects in two
directions, as the wheel moves backwards and forwards ;
whereas a slide does in any direction, except in the line of the
body, because the catches which fasten it to the under plate of
the stage are placed in that direction. This faculty of giving
an eccentric pencil, or, what is the same thing, an oblique light,
in almost any direction, will be found of great use in resolving
test objects by daylight, — with artificial light, diaphragms are
of but little service. Where the diaphragms and the body are
both fixtures, of course no oblique illumination can be had,
unless, indeed, a fixed arrangement is made to that effect, and
then of course a direct light cannot be obtained : on this
account, sliding tubes below the stage, affixed to its immoveable
under plate, are inadmissible in the kind of instrument under
consideration.

Fig. 15 is the slide belonging to the groove : it has four holes
in it, the largest half an inch in diameter, as may be seen by
reference to the scale. It would have been better to have
been full three-quarters, and the rest in proportion. The aper-
ture in the under plate of the stage should be a full inch, or
even more, lest it should, under any circumstances, operate on
the visual pencil, or the edges of the field of view.

Fig. 16 is a glass tank, chiefly intended for holding aquatic
plants. The back and sides are composed of thick plate glass,
to which a very thin piece is attached in front, the whole being
cemented and firmly bound together, both at top and bottom,
with a slip of brass. A piece of plate glass is introduced at
pleasure, to press that part of the plant to be examined against

REFLECTING ENCISCOPE. ]5

the front of the glass trough. This tank is fixed to the superior
plate of the stage by a peg to be screwed to it at pleasure,
somewhat after the way in which a salt box is to a wall. This
piece of apparatus is much superior to a phial or flattened tube.

Fig. 17 is a clasp of diaphragms to be applied to the tube of
reflectors itself: it is seen flattened, and not curved, as it should
be, like that belonging to the silver cup (fig. 3), to cling to the
tube. I have already observed that we are unable to apply a
shallow eye-glass, or to obtain a large field of view with the
Amician Engiscope, unless the aperture in the side of the tube
admitting light from the object is opened out considerably more
than will consist with distinctness when deeper eye-glasses are
employed, which take in but a very small portion of an object,
and therefore will do very well wieh a smaller hole, without
having the edges of their field encroached upon thereby.

By means of this contrivance the hole in the tube of reflectors
may be made as large as is necessary for the low powers, and
again contracted for the high ones, without, as it seems to
me, diminishing the quantity of light, when transparent objects
are viewed ; though with opaque ones the case is very different.
d is the fragment of a tube of reflectors ; c, c, the direction of
the circular line passing through the centre of the hole in the
side ; a, a, a longitudinal line passing also through the centre
of the hole ; b, b, a line passing through the centre of the three
holes in the clasp made to coincide with the circular line on the
tube (c, cj. In like manner any of the three lines passing-
through the three holes may be made to correspond with the
longitudinal line on the tube_, and when the lines in both are
adjusted, of course the holes in the clasp of diaphragms must
be concentric with that in the side of the tube. The holes,
when high powers are used, may be as small as those made
by a large pin, or a little larger than the space taken in by the
field.

These diaphragms may also be adjusted by putting on the
lowest eye-piece, and causing the spot of light which will be

16 THE MICROGRAPHfA.

seen in the field to become centrical, moving the diaphragm
about till this is effected. It will be observed that fig. 16 is of
the full size for a metal of 3-lOths of an inch aperture.

All the figures described are one-third of the natural size,
except where the contrary is stated.

There is a variety of apparatus common to this and other
engiscopes, such as fish-pans, aquatic and dry live-boxes, tubes,
slides for crystallizations, diagonal and polarizing eye-tubes,
micrometers, apparatus for viewing the circulation of the blood
in animals, and the cyclosis in plants, &c. &c. which the
reader will find fully described in the " Microscopic Illus-
trations,'' " Microscopic Cabinet," " Treatise on Animalcules."
" List of Microscopic Objects," &c. I have, I think, described
every thing peculiar and indispensable to the instrument under
consideration. I may observe, that though the instrument is
represented in perspective (which will not in strictness consist
with a regular scale), yet many parts of the drawing supposed
to be nearly in the point of sight;, and not foreshortened, are
made to correspond with the scale, and to shew their true di-
mensions according to it. Thus the superior plate of the
stage cannot have its parts measured by the scale, but its late-
ral view may ; the diameter of the mirror may be obtained,
the size of the body, pillar and stand, &c. &c.

I do not know of any improvements which can now be made
in the instrument, unless perchance that the mirror might
be made like that in fig. 1 in the " Microscopic Illustrations,"
but this is not very material.

I will, however, venture to suggest one thing, though I am
afraid it will be thought rather fanciful.

It would not be amiss if the concave metals, and the piece
to which they are soldered, were perforated like those of the
Gregorian telescope, as a lens of the same focus as the acting
one of the concave metal might be set in the hole, which might
act as a finder to the Engiscope itself : indeed, a light small
body, with a single eye-glass having cross hairs in its focus,

REFLECTING ENGISCOPES. 17

so adjusted that their intersection should always take place on
an object placed in the centre of the field of view of the
Engiscope, might also be applied,, and have its focus adjusted
to suit the primary instrument, by means of setting the object-
glass in a tube sliding on that containing the metals. Thus
two persons might view the object under consideration at the
same time, and one manage for the other, at least while the
instrument was in the position shown in fig. 1, in which, how-
ever, all its capabilities may be developed : this at all events
would strongly remind us of the two kings of Brentford smell-
ing at one nosegay.

THE VARIOUS METHODS OF OBSERVING WITH THE AMICIAN

CATADIOPTRIC ENGISCOPE AND HORIZONTAL ACHROMATICS*,

MADE EASY AND SIMPLE AT LAST.

General Observations,

As horizontal achromatics must have their stands, mountings,
and apparatus, constructed very nearly in the same manner as
the Amician reflectors, — the same sauce which does for the

* First introduced into use by Professor Amici, of Modena : they pos-
sess in particular the property of giving an erect image, when used with a
diagonal metal or plate of glass applied to their visual pencils for the pur-
pose of causing their image to be seen on paper placed underneath, to
assist observers who make drawings of microscopic objects, which are
thus represented exactly in their natural positions, while they are confined
to the stage purely by their own gravitation, which circumstances are fre-
quently a great accommodation to microscopists.

c

18

THE MICROGRAPHIA.

goose shall serve for the gander also., by your leave, ladies and
gents., observers with the aforesaid instruments.

I have often wished it was possible to invent some kind of
SLveritable finder for Engiseopes, like those used for telescopes ;
in the present case such a piece of apparatus would be in-
valuable, for it would make the management of the instruments
I am now treating of as simple as that of any others of the re-
fracting kind ; but as I conceive this to be impracticable, it will
always involve somewhat of the difficulty (at least with be-
ginners) which is at first experienced in using a Newtonian
telescope, viz., we must be content to acquire the habit of
pointing the instrument in a direction totally different to that
in which we look, and in like manner the Amician reflector
and horizontal achromatics must be managed exactly as if the
observer had to look in a line formed by the axis of the illumi-
nating mirror passing through the centre of the stage, instead
of at right angles to the same, as will be seen in the sequel ;
but before I proceed to business, I must premise a few remarks
on the optical part of the instrument.

There have been five and six sets of metals made for the
improved Amician Engiscope, as follow :

No.

Solar Focus.

Angle of Aperture.

1

2 Inches.

m°

2

1 Do.

181°

3

To

27J"

4

4
10

36i°

5

3
TO

4H°

6

fo

55°

Various eye-glasses operate along with these objective metals,

REFLECTING ENGISCOPE. 19

giving magnifying powers almost ad libitum, which may again
be considerably increased by drawing out the inner tube into
which they slide : thus every degree of power may be obtained
between that given by a lens of half an inch focus, and one of
only l-60th of an inch. Each eye-piece should have engraved
upon it, by the maker, the power it gives with each of the sets
of metals, the body being contracted to its minimum length.
Each set of metals should also have engraved upon its outer
tube the focus of the concave metal and its angle of aperture,
for there is frequently but little difference in their outward
appearance, save that one seems a little longer than the other :
this circumstance has frequently given interested persons an
opportunity of using a metal with a small angle of aperture
instead of a large one ; and thus the character of the instru-
ment has been injured in the opinion of simple confiding people.
It will be observed that no attempt is made to give the
metals of long focus a large angle of aperture, as might have
been done*. The reason of this is, that it is a point of the
utmost importance to gain space between the object and the
side of the tube which contains the reflectors for the illumina-
tion of opaque objects ; accordingly, No. 1 gives half an inch
for this purpose, No. 2 one-fifth, No. 3 one-tenth, No. 4 one-
twentieth, No. 5 scarcely any space, and No. 6 cannot be used
for opaque objects at all, except by removing the diagonal
metal, and simply placing an object in its focus : it is especially
devoted to the most difficult class of transparent test objects.

* Since diminishing the focus of these metals adds so greatly to the
difficulty of working them, especially if the angle of aperture is considera-
ble, it may be worth while, under existing circumstances, to try what can be
done by giving the metals of long focus a large angle of aperture. The
oblique lines on the scales of the brassica have been seen, though faintly,
with a metal of so long a focus as an inch and a half with an inch of aper-
ture, having a very exact figure. This aperture is certainly not sufficient
to shew them perfectly ; it should be equal to the focus. All the common
lined tests may be seen very well with a one inch focus metal, with half
an inch of aperture.

20 THE MICROGRAPHIA.

Nos. 1, 2, and 3, are those metals most useful for examining
opaque objects : No. 3 is also a capital working metal for all
sorts of transparent objects ; indeed, there are not above four
or five objects known that it will not exhibit.

The microscopes and engiscopes produced by myself and
Mr. Pritchard, are constructed on the same principles, as far
as possible ; they must, therefore, be used and managed nearly
in the same way : consequently the various cautions and direc-
tions given in the " Microscopic Illustrations" for managing
the Operative Aplanalic Engiscope, and in the " Microscopic
Cabinet" for dealing with the Jewel and Doublet Microscopes,
must not be overlooked as altogether irrelevant and inapplicable
to the instruments now under consideration ; for, in fact, the
only peculiarity in the Amician Engiscopes and horizontal
Achromatics is, that their bodies are a fixture, and motions are
given to their stages.

I should strongly advise you never to attempt to wipe or
clean the metals yourself: you may clean the prism (fig. 2) I
have spoken of, if necessary, with a bit of chamois or wash-
leather perfectly free from dust, and whiting, and a little
very strong spirits of wine : the same material also serves for
wiping the metals, if it must be done by yourself, but recollect
it will be a singular piece of good fortune if after having cleaned
them you should be able to return them into their containing
tubes so as to be in perfect adjustment. Many half-taught people
fancy that the metals of the Amician are out of adjustment when
they are in, and in when they are out, because they are not
aware that the plane or diagonal metal, being only of the exact
size wanted, and every atom of it actually operating up to Us
extreme edges, is neoer precisely concentric with the concave.
A section of a cone, at an angle of forty-five degrees, will shew
and verify this fact (vide also fig. 28) : the same circumstance
holds good with the Newtonian telescope, only this instrument
having a very small angle of aperture, and a small diagonal
metal, the edges of which are never in use, permits us to place

REFLECTING ENOISCOPE. 21

the said diagonal in a truly centrical position. The adjustment
of the metals of the Amician with each other may be judged
of according to the rules laid down in the "Microscopic
Cabinet," p. 209.

I once saw a rich fellow, who passed for a man of science,
(by virtue of buying scientific books and instruments, and
giving good dinners to persons who really were entitled to
that appellation) deliberately take out the metals of a Gregorian
telescope and deliver them to his butler to clean, observing,
that he expected a party of friends to examine some double
stars with the instrument in the evening ! Emboldened by his
sublime example, as individuals are frequently placed in situa-
tions where the assistance of an optician cannot be had, I
shall here give the method of taking out and cleaning the
metals : — First, make a mark on the head at the end of the tube
of l'eflectors corresponding to another on the tube itself, — this
is done to enable you to return the concave back precisely into
the place it should occupy after you have cleaned it: — then,
with a strong pair of pliers or hand-vice, twist the metal out by
forcibly grasping the projecting bit of brass at the extremity,
and turning it round ; the "plane metal can then be got ut so
easily that it will hardly be necessary to remove it : then wipe
both metals with the leather moistened with the strongest
alcohol, and afterwards with another piece of dry leather. If
any dust has collected on their surface, which may be expected
to scratch them, remove it with a camel's hair pencil before
you apply the leather, or wipe them very gently at first.
There is frequently a brown tarnish formed on metals of a
certain age, which cannot be removed by this process : in this
case it will be requisite to apply a saturated solution of citric
acid in distilled water to them for an instant, licking it off
with the tongue, and then washing the speculum thoroughly
with alcohol. As the citric sold is often adulterated with sul-
phuric and other' acids, (which would be destructive to the
polish) it will perhaps be safer to use boiled lemon juice.

22

THE MICROGRAPHIA.

If, however, they have lost their polish, and become grey
and oxidated, you may, if you are a bold man, with a mecha-
nical turn, attempt to repolish, (one of the most delicate
operations the imagination can conceive) ; that is to say,
if the metal has become useless, and you therefore cannot
alter it for the worse, otherwise you had better let it alone;
for if you should succeed in repolishing it, it is ten to one but
that the figure will be utterly ruined. Procure then a piece
of very thin chamois or white kid-skin leather, of an equal
thickness throughout ; melt some bees'-wax, and having first
stretched the leather into a concave figure over one of your
fingers, put some of the wax, when nearly cold, into it, and
apply it to the metal, stretching the leather well, and pressing
it hard, so that it may take an exact impression of its figure :
when the wax is quite hard, trim off the edges of the leather
with a pair of scissors, leaving its convex surface of the same
size as the metal to be polished. Then rub this tool over with
some of the finest-washed crocus powder, and rub the loose
particles off with a clean brush or piece of leather. Make your
tool fast to something or another, taking great care not to in-
jure its figure, and lay the metal on it, working it steadily
backwards and forwards across and across, but never circu-
larly, and using the least pressure possible ; have patience,
and the polish will gradually return ; try the figure from time
to time, according to the rules laid clown in the " Microscopic
Cabinet," p. 199, and do not attempt to get a perfect polish ;
be content with one which is sufficient for practical pur-
poses. The figure of the metal will probably become too
spherical with the cross motions : if you find this to be the
case, you may give it a few circular strokes, but be very
cautious, or it will in this way become too flat at the edges,
i. e. parabolical or hyperbolical*, which is a much worse

* The whole secret of figuring metals (an operation effected in the
process of polishing) seems to reside in the knowledge that a movement
across and across in a right line given to the metal, (whether the tool

REFLECTING ENGISCOPE. 23

fault than too spherical. The metals are polished originally
on a pitch stool, which you may use if you like instead of a
leather one ; but unless you have some practice, you will
probably have the best chance of retaining the original figure
of the metal with the leather and wax*.

The plane metal is taken out by unscrewing the adjusting
screw : at its back marks must be made, as before directed,
to enable you to screw it on again in adjustment : and now 1
am sorry to say, the figure of this is still more ticklish than
that of the others ; I am afraid it is scarcely possible for an
ordinary person to do any thing with it. You may, if you
please, make the following experiment : —

Get three pieces of brass planed perfectly flat, place a little
of the finest-washed crocus powder, or putty mixed with water,

which forms it is stationary or revolving in a lathe,) gives a spherical
figure, and that this figure can be converted, first into an ellipsis, then
into a parabola, and finally into a hyperbola, or ultra-hyperbola, by
merely changing the cross motion into a circular one, and cutting it short
at the requisite point. A perfectly plane or flat figure seems to be of still
more difficult attainment than any concave or convex one : it is procured
by working three metallic surfaces successively against each other ; two
only worked together are sure to become concave and convex respectively,
but of the same radius. When metals are of long focus and small angular
aperture (as is the case in telescopes), the great difficulty seems to be in
keeping their figures sufficiently near the sphere, at least in the operation
of polishing: nearly all the reflecting telescopes I ever saw incline to be
too hyperbolic (I mean too flat at the edges) in figure ; but it is quite
possible to hit off the figure of an ellipse (which is that of the metal of an
engiscope), perfectly true, even with a true black polish.

* A critique on the merits and defects of the Amician Engiscope has
just appeared in Napier's " Encyclopaedia Britannica," art. Microscope,
part 84, p. 43, col. C2.—A. P.

24 THE M1CR0GRAPHIA.

between two of them., and work them together till smooth, and
also work these two alternately with the third piece, so that
the three being ground together may always preserve their
flat surface, as they will do ; they will imbibe enough of the
polishing powder, even after they are wiped perfectly clean,
to polish a plane metal indifferently well, which is drawn in
one direction over any one of these charged surfaces, and if
this operation is carefully done, the figure will be pre-
served*.

I dare say every regular optician will laugh heartily at these
directions ; but until their worships and reverences will please
to learn to work these small metals, we must endeavour to
preserve those already in existencef, the best way we can. I

* The plane metals are originally formed in the following manner: —
About half a dozen of the small pieces of brass which carry the screws,
by which the diagonal metals are retained in their places, are soldered to
a plate of a very thin speculum metal, about an inch in diameter, (the
thickness used may be rendered evident by abrading the edge of one with
a file) ; this plate is then figured as a single plane metal, just as those of
Newtonian telescopes are; when polished, it is cut or broke into pieces,
each having the bit of brass at its back, and the edges of such as turn out
good and true are carefully ground down to the required size : the arms
which carry them are carefully cut out of a disc of brass, soldered to a
piece of very thin tube, and then inserted in situ. The adjustment of the
plane metal is performed by a single screw connecting the arm and the
metal together, and here the skill of the workman is shown, and put to
the test, in the nice manner in which he must adapt the two little plane
surfaces to each other, having to perform in this way what is, in the New-
tonian and other telescopes, effected by the joint action of three screws.
Little do observers know of the mass of work there is in a pair of these
delicate metals duly figured and adjusted.

t I conceive that even where an artist possesses the necessary capacity,
and has learnt the principle and method of working metals of very large
aperture, the practice of nearly a lifetime will be requisite to attain the

REFLECTING ENGISCOPE. 25

have merely recommended such means of keeping the specula
in order as I think most likely to succeed with amateurs and
observers in general. No man knows better than I do how
very small an error in a reflecting surface is sufficient to
create confusion in an image, especially in metals worked on
so very small a scale : I dare say a millionth part of an inch
of error or inequality at the edges of their curves, that is to
suy, being too flat at their margins, or too deep in their centres,
by some such quantity, will be quite sufficient to render the
performance of a metal very bad.

I forgot to mention, in its proper place, that a small sable's.
hair pencil warmed at the fire, (which may be easily introduced
i nto the tube of reflectors, without disturbing the metals,) will
suffice for cleaning the specula from any casual particles of
dust, &c. which may adhere to them.

I shall now proceed, on the supposition that every thing
about the instrument is in good order and condition ; that you
have selected the metals you mean to use ; fixed them on in
careful adjustment ; drawn back the slide from the lateral hole ;
screwed on the cruciform stand, and drawn out the slides of
the pillar to the proper height*.

FIRST METHOD OF OBSERVATION, WITH THE BODY HORIZONTAL,
AND THE BAR DEPENDENT AND VERTICAL.

TRANSPARENT OBJECTS.

Fig. 1 represents the Engiscope set up and prepared for
action, in the aforesaid method ; and observe, gentle reader,

tact of producing exquisite figures in such specula at pleasure; even if it
•were otherwise, the production of fresh works of art of the highest merit
would not diminish the value of those of the ancient masters, which,
though they may be equalled, cannot be surpassed.

* It will probably be found convenient in packing the instrument, to
put the cruciform stand into a shallow drawer, at the bottom of the case,
by itself; and to retain as much of the apparatus attached to the bar and
body in situ as may be, in stowing away the rest of the things.

26 THE MICROGRAPHIA.

that here you may set up your staff and proceed no farther,
unless you like, for every species of object may be duly
observed in this way ; neither will it matter whether the bar
is a fixture, and supports the body, as in most of the foreign-
made instruments, or whether a pillar performs this office, for
the mode of procedure will be just the same. Now mind what
I say : this instrument is like the Irishman's crooked gun,
which used to shoot round the corner ; therefore, the first
thing you have to do is to mount your object, just as if you
had to look at the dotted eye (r) in the line of the dotted
arrow (q p\ and as if the hole in the tube of reflectors was a
magnifier : recollect the distance of the focus of the pair of
metals you are using from the side of the tube, and observe
that there is a line drawn round the said tube, passing through
the hole by way of a rough guide for you to place the object
in its proper situation. The second thing you have to do is
to direct the light by means of the mirror (k), also just as if
you had to look at r, in the line p q, instead of round the
corner at e. The third thing you have to do is to adjust
the focus, just as if you had to look at 1c, in the line p q,
instead of round the corner at e, by means of the screw (i),
first screwing up the milled head of the pinching screw (h) ;
till this is done the said screw (i) does not act, though the
stage will move at pleasure up and down the bar.

Notwithstanding the explanation given of the use of the
various apparatus belonging to the instrument, in the pre-
ceding pages, as a da capo is often necessary to produce a
decisive effect on the mind of the reader, I shall observe that
all the nicknacks used for the examination of transparent
objects apply to this instrument very nearly in the same way
as to any other, according to the humour of the observer.
Thus, if your objects are contained in sliders, you put on the
slider-holder, (fig. 10), and turn it one quarter round to fix it
to the stage, and you may either use the double screws (in),
to call into action the traversing plates, or move the slider-

REFLECTING ENGISCOPE. 27

holder about with your fingers, which were made before tra-
versing screws were invented ; and if the object does not yield
too easily to the touch, and your elbows are rested in some
way together with your wrists, you will, I think, find little use
for artificial movements, unless your hand trembles.

If the nature of the object to be examined allows of its
being laid on a disc of glass, then use the raised stage, (fig. 6);
you may remove its arm («), if you find it in the way, and lay
the glass on the rim thereof, and proceed as before : recollect,
whenever you use daylight, not to employ the concave mirror,
(if there is one) but the plane one ; also affix on the slide of
diaphragms (15) to the under plate of the stage : be sure yon
turn it round to fix it, or it will tumble down and break your
mirror ; try the various holes, to ascertain which best regulates
the light, and tempers it to the shade best adapted for exhi-
biting your object well : recollect that oblique light is rarely
required, except, for proof-lined objects ; therefore keep the
diaphragm concentric with the hole in the tube of reflectors,
and then you have direct light. Of course, if you have living
aquatic objects or animalcules to observe, you use an aquatic
live-box ; if crystallizations of salts, a slide proper for them,
&c. &c.

If you use artificial light your lamp or candle must have a
bull's-eye lens or condenser applied to it, to render its rays
parallel ; every thing then goes on as usual, except that, in my
opinion, the diaphragms will do no good with artificial
light, which can always be obtained of the precise degree of
intensity required by removing it to a proper distance from the
instrument.

A lamp fed with oxygen gas introduced into the wick, such
as has been frequently used of late years by exhibitors of objects
with ■pretended oxy-hydrogen microscopes, would be an ad-
mirable addition to an Amician Engiscope, both for the purpose
of obliterating the brownish tint given to objects by all reflectors,

28 THE MICROGRAPHIA.

when their powers are high;, and of rendering its light equal to
that possessed by refractors under ordinary circumstances.

If the tube carrying the object-glasses is used instead of the
metals, the raised stage will not be needed; but the other
arrangements will not be disturbed.

Touching transparent lined objects, I beg you to study the
cautions given relative to their resolution, farther on, particularly
as to the method of illuminating them aright. You will easily
acquire the method of managing your mirror, so as to get ob-
lique light with it in any direction at pleasure, by having a piece
of rubbed glass to lay on your stage, and observing with your
naked eye the direction in which the spectrum of a candle or
window travels over it as you move your mirror about ; only
you must, of course, remove your eye from the instrument for
this pm-pose, which is inconvenient: there are other methods of
ascertaining this very essential point, as you will see farther on.

OPAQUE OBJECTS.

If you desire to examine an opaque body requiring shade as
well as light to reveal the nature of it, and it be thought most
advisable to employ day-light, it may be sufficient, if the day
is bright and sunny, and the power employed low, merely to
place the instrument, duly equipped with its slider-holder and
slide, &c. near a window, in such a position that the body shall
be parallel to it ; and thus let the light fall upon the object,
turning the body, perhaps, a little round in its split socket, so
that the end of the bar shall point towards the window, if the
tube of reflectors shall appear in any degree to shade it : if it is
a subject not contained in a slider, nor placed on a disc or cy-
linder, but simply mounted on a pin, as, for example, an insect
from a common entomological cabinet drawer, the black box
(fig. 6) must be used, with its bottom (e) fixed on, but without

REFLECTING ENGISCOPE. 29

the little supplementary black velvet disc, grasping the pin by
means of the miniature vice (d) on the arm (b).

It" the use of the condensers (figs. 4 and 5) is requisite to
brighten the object, these may be used either separately or
combined. The focus of the largest (fig. 5) is about 24 inches,
but. should be rather more than less than that; accordingly it
must be so adjusted in its slide as to be at 2h inches distant
from the object it is to illumine, but still as much nearer than
that as may be necessary to cause the image of the window to
disappear* ; the small one is about an inch focus, (but rather
more than less) and is in like manner to be adjusted, so as to be
at about an inch from the object.

If, however, the power is high, and the object dark, combine
both the condensers as follows : — fix the smaller half an inch
distant from the object, and the other large one an inch and
three-eighths from the small one ; take care that the axes of both
are in a line with the object and the light, and also that the
planes of both lenses are parallel to each other. This combina-
tion gives a degree of brightness of illumination equal to what
would be obtained from a lens of half an inch focus, and an inch
of aperture, (if, indeed, it could have such an aperture,) barring
the light lost by the two surfaces of the second lens, and
might be rendered twice as great by giving an aperture of an
inch and a half to the illuminator which first collects the rays,
instead of an inch.

When this arrangement is made it will do as well for artificial
light as day-light; all that is requisite will be to place a lamp
or wax-taper, furnished with a bull's-eye lens to render its rays
parallel, on the side of the Engiscope on which the illuminators
are placed, a little above the level of the stage.

If the object is best illuminated by silver cups, select that
which is adapted to the specula you use, and fix it on their

* Tbis gives the maximum of brightness, which always lays within
the focus of an uncorrected lens, and prevents the object from being con-
fused by the traits in the spectrum of the window.

30 THE MlCROGRAPHIA.

containing tube, taking care that both the holes coincide :
you must now, if your object has no black ground attached
to it, use the little black velvet one (a, fig 6), and remove
the bottom of the black box (e), to allow the light of the
illuminator (fig. 7) to reach the cup. The said illuminating lens
is to be fixed on the bar between the stage and the plane
mirror (k), in order to effect which, the mirror must be stripped
off, and then put on again in its place, after the lens has been
slid on the bar. You now reflect either the light of the sky,
or that of your bull's-eye lamp upwards, by means of the plane
mirror.

When you are observing with cups, be very sparing in
using the traversing motions of the stage : employ them only
for the finishing touch, after you have done your best to get
your object in its place, by moving about the needle (d, fig. 6.)
I have given my reasons for this already, in the description of
the black box and its accessory parts.

The object-glasses, of course, have cups adapted to them,
both singly and in combination, by being screwed to their
cells. When used, they require the same apparatus to be em
ployed as those made for the metals. I have not given any
figures of these, because they are so common that every one is
acquainted with them.

The best situation for the illuminating lens will be pretty
close under the stage ; this, however, will depend a good deal
on the length of its focus, — a few experiments will soon teach
you its proper locality.

I suppose I need not say that the slide of diaphragms must
be taken off by turning it one quarter round, whenever opaque
objects are to be shewn by silver cups. I frequently find it
necessary to tell my readers what they are not to do, as well as
what must actually be done.

If the focus of the silver cup you use does not happen to
fit the metals well, owing to its being of too short a focus, you
may make it fit, by drawing out the eye tube, which shortens

REFLECTING ENGISCOPE. 31

1 lie anterior conjugate focus of the metals. The focus of the
Amician usually falls a little beyond that of the cups, when the
body is contracted to its minimum length*.

A great deal may be done with cups having single lenses
inserted in them which they do not fit : by raising or lowering
their pouts or settings by means of rings of thin metal, till the
focus of the lens and of the cup fall on the same point, the
brightness of the object is then very remarkable, even with a
lens of l-60th of an inch focus.

Now if you should not happen to succeed in getting every
thing to go on exactly to your mind at first, do not curse the
instrument by your gods, and throw your wig behind the fire,
and then dance up and down the room like a maniac, as I
once saw a Frenchman do ; but have a little patience ; read my
directions over again, weigh well the import of every sentence,
and try your luck over again. I am sure, that so lung at least
as the instrument is kept in its present position, its management
and that of all its apparatus is so exactly like that of all other
microscopes having upright stems, that I think a man pays
a small compliment to his understanding in quarrelling with it.

The body, instead of being kept truly horizontal, may be in-
clined a little at the pleasure of the observer, without disturbing
any of the arrangements described above.

If at any time during cold weather you should find the
steam from your eye continually condensing upon the eye-glass,
and thereby impeding your observations, warm the whole in-
strument at the fire_, especially the body and eye-glasses, and
you will no longer be annoyed by this inconvenience.

* The method of knowing whether the focus of a silver cup coincides
with that of the lens, object glass, or metal attached to it, is this : having
illuminated the object in the best way you are able, put it in and out of
focus, and see whether the sensible brightness of your field of view is
greater either within or without the focus, than at the focal point ; if it is,
it is a sign that the cup is not properly adjusted; if the light is strongest
vithin the focus, the cup or its focus is not near enough to the object ; if be-
yond thefocus, then it is too near.

32 THE MICROGRAPHIA.

SECOND METHOD OF OBSERVATION, BODY INCLINED, BAR POINT-
ING UPWARDS, FOR VIEWING TRANSPARENT OBJECTS BY
PURE INTERCEPTED DAY-LIGHT.

This and the following modes of observation, though not ab-
solutely necessary to be learnt, are yet highly useful, because
they cause the instrument to put forth its whole mettle, — they
are of course only intended to be studied by graduates of the
upper form, who have got over the asses' bridge we have just
passed.

Turn the body round in its spring socket till the bar assumes
the direction pourtrayed in the miniature skeleton figure marked
(19). The mirror must be taken away altogether : the object
to be viewed must then be mounted between the bars of the
slider-holder as before : the instrument must be placed
pretty near the window in this way of using it, that the bar
may be so directed that the light of the sky shall fall freely on
the object ; if oblique light is wanted, turn the body about in
its socket till the bar points a little on one side, and tilt the
body at the same time till you have attained your object.
The slide of diaphragms (fig. 15) will be almost certain to im-
prove the vision of objects seen in this method of mounting,
and should therefore be employed, whether the light required
is direct or oblique ; if the latter is wanted, it must be so
placed that the aperture in use shall not be in the axis of the
metals, but a little on one side of it, and it will effect this point*.
The aforesaid mode of observation is equivalent to that given
in the "Microscopic Illustrations," p. 66, for the Explanitic
Engiscope. 1 now proceed to describe what may be con-

* Whenever oblique light is obtained it matters not in what manner : on
viewing the visual pencil with a magnifier, it will be found not to present
a perfect round disc of light, but one partially illuminated only, the brightpor-
tion being always near the margin. Vide " Microscopic Cabinet,'' p. 172.

REFLECTING ENGISCOPE. 33

sidered merely a modification of the present way of setting up
the instrument.

THIRD WAY OF MOUNTING, BODY HORIZONTAL. BAR VERTICAL,

AND ABOVE THE BODY.

Turn the body round in its socket till the bar is truly vertical,
and then clamp it tight. The plane mirror must be fixed on
the bar as in fig. 18, to reflect the light downwards. This
method is extremely useful for observing the crystallization of
salts, the bellies and inferior parts of aquatic insects and ani-
malcules, and all sorts of chemical actions. The details areas
follow : —

A slip of glass, in a proper brass case, must be warmed, and
then inserted between the bars of the slider-holder, to receive
the drop of salt to be crystallized, or it may be merely smeared
over its surface, so as to present only a very thin film of the
fluid, which is perhaps the better way : sometimes the surface
of the glass is greasy, and will not take to the salt ; in this case
wash it writh pure soda, and afterwards polish it a little with a
bit of perfectly clean wash-leather impregnated with putty.
It must be obvious that this is the best way of viewing crys-
tallizations, for thus the water can pass off" freely without steam-
ing the metals or object glasses, and the drop being exposed on
a horizontal surface will not spread or run about.

As living aquatic insects constantly endeavour to keep in one
position, viz. with their bellies undermost, in whatever situation
they are placed, this is the best way in which their inferior parts
can be viewed ; it is very probable that many animalcules are
in a similar predicament*. It is no doubt possible to confine

* The infusoria are always found on the surface of the liquids which gene-
rate them, probably because air is necessary , not only to their production, but
to their existence ; their most natural way of exhibition will therefore be
in a naked drop of a liquid taken up with the head of a pin, but such a
drop will always present a spherical surface on its upper side, which will

D

34 THE MICROGRAPHIA.

an aquatic insect closely in a live-box, and then examine its
under parts, but it will be always struggling to regain its na-
tural position, which circumstance will disturb our observations.
Chemical actions, in a small way, will also be best seen in this
mode, as any gaseous matter which may escape from the bodies
in action can pass off freely, and they are, besides, quite under
the control of the experimentalist, who can carry on his ope-
rations without removing them from the instrument ; a thin
watch-glass, having a segment of its middle flattened by being
pressed into an appropriate mould while hot, must be fixed into
a piece of flat brass of a certain thickness, and capable of being
introduced between the bars of the slider-holder, to carry the
substance whose actions are to be examined.

FOURTH METHOD OF OBSERVATION, BODY AND BAR

HORIZONTAL, OR SLSGHTLY INCLINED.

This is the best arrangement for viewing transparent objects
by artificial light, especially tests, as it allows a taper or lamp
to be introduced directly behind the stage, vide fig. 20 : it also
enables us to use the condensing lens, with a taper, for illumi-
nating opaque objects by means of the silver cups fixed to the
tube of reflectors, as already described, without employing the
mirror, which only renders the operation more complicated,
while it occasions a great loss of light. The skeleton figure
(20) is a top view of the instrument, or an apology for a plan
taken from above, the eye being supposed to look down upon
it. The parts will, I think, be easily recognized by merely

operate like a lens, and thus interfere with the action of any instrument
employed to view its superior surface ; it will also evaporate rapidly unless
renewed ; but in the way described, we look through the under flat surface
of the drop, by which means all the aforesaid inconveniences, together with
that resulting from the naked liquid coming in contact with the side of the
tube, when metals of very short foci are used, are easily obviated.

REFLECTING ENGISCOPE. 35

considering that the body has been turned one quarter round in
its socket, carrying the bar along with it.

The light of the taper or lamp* is so very easily managed
at the pleasure of the observer, that I should only make the
simplest thing in the world difficult and complicated by saying
much about it. Of course the nearer it is placed to the stage
the stronger and more intense its light will be, and vice versa:
if direct rays are required, it must be placed in the axis of the
metals directly behind the aperture in the stage ; if oblique, on
one side of it : vide the dotted figure of the taper. Recollect that
the flame must be kept of the exact height of the tube of re-
flectors : the proper kind of candlestick for the purpose is
represented in the " Microscopic Cabinet," p. 245, art. 22.

OPAQUE OBJECTS.

Fig. 21 shows the method of fitting up the instrument in the
same position for examining opaque objects with silver cups,
in which the same apparatus is used as was described in the
first method of observation, except that the plane mirror's ser-
vices are not in request, the taper or lamp casting its rays im-
mediately on the illuminating lens, which refracts them towards
the cup. The lamp, when it has no bull's-eye lens, should be
placed at such a distance from the illuminator as is equal to
about twice its solar focus : if this is an inch and a half, it will
be about 3 inches distant from the said lens ; the brightest part
of the flame must be just in the axis. If the needle turns
round in its socket, and thereby annoys you, (as it will some-
times do when in the position in which it is represented in the
figure,) turn the arm round on the top of the black box till it

* In this case it may be used without a bull's-eye, or other lens, to make its
rays parallel ; indeed it is frequently most effective in its simple state, es-
pecially with proof-lined objects : of this more hereafter.

36: THE MICROGRAPHIA.

becomes vertical, and the needle will then remain quiet in any
position you may require.

OF THE EXHIBITION OF TEST OBJECTS BY THE AMICIAN
REFLECTING ENGISCOPE.

I conceive it to be absolutely necessary that all proof-lined
objects, both transparent and opaque, intended for demonstra-
tion by the Amician reflector, should be mounted in little
circlets of ivory or metal, of which a representation may be
seen at fig. 8 (c), and also on an enlarged scale at fig. 23. This
may cause a little extra trouble and expence. but it will be
found well worth while ; for I have frequently seen the
Amician fail, or perform but indifferently, on tests, not from any
lameness or impotency in its optical part, which is second to
that of no instrument whatever, but merely because (if 1 may
be allowed the expression) it could not get at them properly, so as
to be able to exert its full power. This is owing to the impractica-
bility of turning a slider fairly round, when mounted for inspec-
tion, on account of its proximity to the side of the tube of re-
flectors, which is sure to cause it to come in contact with the
neck of the body ; no such inconvenience, however, will occur
when the object-glasses are employed, because their focus will
be sufficiently distant from the side of the tube to allow the
slider to turn Tound till it comes into its best position for per-
mitting the object to be illuminated in the most effectual manner.
Fig. 8 is a piece of turned work to be applied to the stage,
like the other pieces already described, for carrying the said
circlets, the milling at b being intended to be turned round until
the two ink marks (a a), fig. 23, shall come into a line with the
body if the object is transparent, or at right angles to it if it
is opaque ; the two marks having been previously made in
such a manner that an imaginary line drawn through them shall
pass at right angles across the particular lines to be demon-

REFLECTING ENGISCOPE. 37

strated: all that is necessary then will be, if the test is a diapho-
nous one, to use the instrument in the fourth way of mounting,
vide fig. 20, to keep the system of lines to be resolved perfectly
vertical, and to place the taper in the position indicated by the
clotted figure of it. All this I think a mere child might be able
to do.

If the object is opaque, the instrument will operate best in
the first way of mounting (fig. 1*), and the ink marks being
placed at right angles to the body will be ready to receive the
illumination from the side of the stage in their most favourable
attitude for exhibition.

If the object-glasses are used, the same system must be
acted upon, though, as I have already said, sliders in this case
may be employed : a couple of ink marks are to be made on
their sides, as before recommended, that the observer may
always know at a glance how they are to be placed.

Fig. 22 represents an enlarged view of the metals of an
Amician Engiscope, seen by taking out the eye-piece, after
the instrument has been adjusted in the best manner for
showing a transparent test ; the space occupied by the small
metal being dotted, as well as the image of the taper, in
order to allow of an imaginary representation of the object, (a)
with its penumbra [b), as seen when the eye-piece is on,
that the relative positions of the lines and the flame of the
taper may be compared. I have done this for the purpose
of showing the reader a circumstance I think I have con-
stantly observed : — First, that the long pyramidal flame of a
wax taper acts more favourably on lines than any lamp with
a round flame can ; and, secondly, that the wick of the said
taper must be parallel to the lines to be brought out. If the
illumination is managed in the opposite way, {vide fig. 24,)
where the wick of the taper is (by taking out the eye-piece)

* It will frequently be found advantageous to employ a single eye-glass,
on account of its superior light, in viewing opaque objects, especially if
they are tests, and the power is high.

38 THE MICROGRAPHIA..

seen in a position either above or below the lines, and at
right angles to them, though the lines may certainly be shown
in this manner, yet, in my opinion, they are never resolved
in that satisfactory manner which leaves us nothing to wish
for or desire. If a lamp is employed instead of a taper (which
it should not be, for transparent tests), it is of small moment
how it is made to act.

When metals are employed, having their sides exit away,
and where it is in consequence utterly impossible to turn the
slider round, or to employ circlets, (vide fig. 11 and 12),
showing the slider in its holder, we must be content to illu-
minate any how we can ; and a lamp in this case is most
likely to operate best.

In Art. 10, line 1, p. 180, of the "Microscopic Cabinet,"
I have described a method of knowing when the light of an
engiscope or microscope is oblique, and when it is direct,
together with the mode of ascertaining the direction in which
the obliquity takes place ; viz. by putting the object out of
focus, and observing whether its penumbra is stationary or
not, and if not, in what direction it travels. Moreover, in
Art. 9, I have endeavoured to show how and in what manner
the light must be made to traverse the system of lines to be
resolved, in order to bring them out to the most advantage.
Now by the help of attending to these rules, we may always,
even if we have not the power of shifting the direction in
which a test lies, cause the light to fall upon it in the best
way practicable ; for by using a lamp possessing a motion up
and down on its stem, as well as from side to side, we can,
by a combination of these motions, cause its rays to flash
across our object in any direction we please, and this direc-
tion, as I have already said, may always be ascertained by
putting the object out of focus ; and if we find that its pe-
numbra, or double image, laps over it, and passes backwards
and forwards at right angles to the system of lines to be

REFLECTING ENGJSCOPE. 39

exhibited, it is a sign that the illumination is right, (vide
fig. 22 b) : if it does not, we must shift the lamp from right
to left, or up and down, till our point is attained. Again,
this method may be confirmed by examining the visual 'pencil ;
a line drawn from the centre of which towards the middle of
the illuminated portion of it at the margin also lets us know
what direction the light is taking, as I have already re-
marked. If, instead of artificial light, any modification of day-
light is used, the same precautions must be taken, and the
light made to suit itself to the object, since the object can-
not adapt itself to it. I need scarcely say that illuminating
.mirrors, having a double motion, give oblique light in any
direction at the pleasure of the observer.

After all, it is possible, by giving ourselves a little trouble,
to fit up sliders, in such a manner that some of the favourable
specimens of scales and feathers shall have their lines pa-
rallel to the sides of the slider ; for when this point is gained,
the lines will be in their best possible position for resolution,
and the full power of the metals can be exerted. The pene-
trating and defining power of metals, accurately figured, seems
so much greater than that of any thing made of glass, not-
withstanding the superior light of the latter, that it is in my
mind worth any pains which can be bestowed on fitting up a
slider, to give the specula an opportunity of doing their utmost,
as we know not what discoveries may be missed by pre-
senting an object to them in an unfair and improper manner.

One more caution, and I have done. Whenever the tubes
of reflectors, having the largest angular apertures, are used, it
is of the utmost consequence that the object should be placed
exactly in the centre of the field of view, for the error of the
oblique pencils continually increases with the extension of the
angle of aperture, and causes the vision to be bad at the
edges of the field, however perfect the figures of the metals
may be. In fact, under such circumstances, the artist can

40 THE MICROGRAPHIA.

only work to a point, and there is scarcely more than room
for one scale or feather in the perfect part of the field.

If you are desirous of using simple or compound magnifiers,
the piece (fig. 9) slips into the neck of the body, and will
carry them for you ; the instrument then becomes an excellent
and very effective microscope, and the body of the reflector a
capital handle to twist it about any how you like.

You now may observe in the four modes, precisely as if
you were using the reflectors, or the horizontal objectives, only
you have not the trouble of looking round the corner ;*indeed,
a course of observations with mere magnifiers would be an
excellent preparatory step to using the engiscopic part of the
instrument, for it cannot fail to make the student perfectly
acquainted with all the capabilities of his instrument a priori,
sso that the management of it, when converted into an en-
giscope, will become mere child's play to him.

I have neglected to describe in their proper places, figs. 26,
27, and 29. Fig. 26 is a reflecting engiscope of its full
size, recommended by the late Charles Tulley, of Islington, and
executed by his son William Tulley, whose untimely death
every lover of the microscope must deplore. It is an elliptic
metal, perforated at the back like that of a Gregorian telescope,
in order to allow the rays of a taper, rendered parallel by a
lens, or simple daylight, to fall on an opaque object placed in
its focus. This is the only kind of engiscope or microscope
which will allow an opaque object to be illumined by light
absolutely direct and -parallel to its axis ; and the effect of it is
singular enough upon some objects : thus, a piece of dial
plate enamelled black, upon which by no other instrument
could any scratches or markings be seen, instantly appeared
of a brownish instead of a black colour, and scratched all over.

I expected that this property would render it a very powerful

REFLECTING ENGISCOPE. 41

agent on opaque-lined objects ; this, however, was not the case,
for it scarcely shewed them at all ; indeed, as the reader well
knows, they require oblique, not direct light: doubtless it may
reveal things not to be seen by other instruments, and requires
to be fully tried.

Fig. 27 is a modification of the Amician Engiscope also
origiually executed by the late William Tulley. The plane
diagonal (a) is drawn back and diminished in size, in order
that it may operate as an illuminator for a transparent object
placed at c. This, as to principle, forms the best reflecting
engiscope in existence for transparent objects, and has been
carried to its utmost limit of perfectibility by Mr. Pott ; the
difficulty of applying objects to it is, however, so great, that it
will never, I think, come into use. The same may be said of
25 and 29.

Fig. 29 is a Catadioptric Engiscope, invented by Mr. Wo
Tulley : it consists of an elliptic metal (a), acting with a plane
oval one (b) placed diagonally, and perforated not far from
its centre by a small hole near which the object (d) is placed.
The rays proceed from the object upwards to the elliptic metal,
and are reverberated back from it to the plane one ; from
thence, at right angles, to the eye-piece. This instrument per-
forms, as well as the Amician, either on transparent or opaque
objects, but is not so convenient, neither can it be made on so
small a scale or of so short a focus* ; the object, however, not
being within the tube, is much more easily managed than in any
of the o.thers, always excepting that of Professor Amici.

I described these instruments shortly after they were exe-
cuted, in a paragraph in the Scientific Intelligence of the
Quarterly Journal of the Royal Institution. — C. R. G.

* The plane perforated metal might perhaps be made wilh advantage
of steel, as it requires to be made very thin, to gioe room for placing the object,
(though steel certainly does not reflect so much light as speculum metal) ;
for it is very difficult and hazardous to make the said plane so thin as it
requires to be of the latter substance.

CHAP. II.

ON MICROMETERS,

AND THEIR USE IN DETERMINING THE FOCI AND POWER OF OBJECT-
GLASSES, METALS, SINGLE AND COMPOUND MAGNIFIERS,
&C. SfC.

The celebrated Mr. Coventry and the late Sir J. Barton greatly
distinguished themselves by their delicate Micrometers, di-
vided on metal, ivory, horn, mother-of-pearl, and glass, fre-
quently to the extent of 10,000 divisions to the inch : the
artists of the present day find great difficulty in rivalling their
exquisite workmanship. We have, however, micrometers
very exactly executed to a sufficient degree of fineness for real
use ; and others formed by delicate screws acting on cobwebs,
fine fibres of silk, hair, &c, with which quantities far less than
the 10,000th part of an inch can be measured in the most
easy and comfortable manner, so as to leave little to be de-
sired in this respect.

It here behoves me to take notice of a remarkable property
possessed by grooved surfaces, such as those of micrometers,
first noticed, I believe, by Frauenhofer. When v/e look at
the image of a candle, reflected by a grooved surface in the
line of its divisions or grooves, we see first a white image of
the candle, and on each side of it a number of prismatic
images thereof gradually diminishing in strength and intensity,
as they recede from the primary image : by turning the micro-
meter round, they will revolve till they all come to be at right

ON MICROMETERS. 43

angles to their original position, which was in a line at right
angles to the divisions. These images are generated by what
is called interference ; and it has been duly ascertained
that their distance from each other is always proportioned to
the distance of the divisions of the micrometer, each from each,
casteris paribus : thus, if there are two sets of divisions on the
same micrometer, (say of 500 and 1000 parts to an inch,) the
prismatic images will be separated twice as far by the 1000
divisions to an inch, as they are by the other of 500. When
we compare two sets of divisions in this manner, we must take
care that they are on the same plane, and that the light is
reflected by both at the same angle, otherwise a calculation will
be requisite to obtain the real value of the divisions, as indi-
cated by the distance of the prismatic images from each other.
Now, as the finer the divisions of a micrometer are made, the
more the images are separated, this affords an excellent test
of the goodness of micrometers, which are often very care-
lessly divided, and one easily applied — at least to such as are
executed on opaque substances. The coloured images may be
seen on glass as well as on metal, &c. ; but the double
images produced by the two surfaces create so much con-
fusion that it is not safe to place much reliance on observa-
tions made with them.

Under certain circumstances very accurate measurements of
natural grooved surfaces may be executed by means of inter-
ference, which, indeed, could hardly be done by any other
means. Thus Sir D. Brewster traced and measured in this
way the lines or grooves on the laminae which compose the
crystalline lenses in the eyes of animals, to their extreme point
of convergency at the poles of the lenses, where " they could
not be rendered visible by the best microscope." I cannot too
strongly recommend to the real connoisseur the whole of Sir
B.'s paper, " On the Anatomical and Optical Structure of the
Crystalline Lenses of Animals, particularly that of the Cod."

44 THE MICROGRA.PHIA.

in the Phil. Mag. for March 1836, p. 193, as a model both
of anatomical and optical investigation executed with the
microscope. The whole tribe of lined tests shew iris tints,
especially by transmitted light ; it is therefore probable that
they are all grooved, though there is an inexplicable mystery
about them, for if their lines are in reality produced on the
same principle as those of micrometers, why are they not as-
easily seen ?

No penetrating power, or large angular aperture, is requi-
site to bring out the lines on a micrometer, though divided
nearly as finely as ordinary tests, to the extent, perhaps, of
10,000 divisions in an inch; (the Morpho-Menelaiis has only
12,000 to an inch, and many others even less than 10,000.)
The minuteness of the feathers and scales of insects is a suffi-
cient reason why we cannot observe the effect of the inter-
ference of light upon them, for a certain extent of grooved
surface seems a necessary condition for our perceiving the
prismatic images of the candle, which became more and more
distant from the white one, as the distances between the
grooves are lessened, and the scale becomes finer : if the
scales were about a quarter of an inch square, instead of their
actual minute size, I make no doubt we should be able to
measure the interval between their apparent lines by inter-
ference, as accurately and readily, or perhaps more so, than
by any other means.

Micrometers for engiscopes and telescopes are made much
in the same way, and on the same principles ; thus we have
the divided object-glass micrometer for both, only made on
a small scale to suit the engiscope, and, in the case of this
instrument, generally by a divided concave lens only, placed
before the object-glasses or metals, and attached to a sliding
scale, with fine divisions, a nonius, &c, by which very mi-
nute quantities can be measured : this kind of micrometer is
the best for measuring the fineness of wool, hair, silk, cotton,

ON MICROMETERS. 45

and the diameters of a variety of similar objects, but cannot
easily be applied to engiscopes having objectives of very short
focus.

I believe it may be pretty safely asserted, that there is no
sort of micrometer for engiscopes superior to those usually
made for astronomical telescopes, consisting of moveable cob-
webs, working in the field of view, exactly in the focus of the
eye-glass, and in the plane of the image ; only in the engis-
cope it will be advisable to apply these to a negative eye-glass,
instead of a positive one ; at least if the goniometrical part, or
that which measures angles as well as distances, is not re-
quired*. Thus slits can be cut through the eye-tube and eye-
piece of the engiscope, and the apparatus contained in a
moveable slide, inserted exactly in the locality of the field-
bar, and in this manner made to suit several eye-pieces, which
I never suppose to be of such short focus as to preclude this
arrangement ; when withdrawn, a common fi<?ld bar may be
inserted in lieu of them, or by turning the eye-glass a little
round, the slits will be covered by the eye-tube, and light and
dust prevented from entering.

As screws are made having a thousand threads to the inch,
it is easy to see that such micrometers may easily be made to
indicate very minute quantities indeed, by dividing the revo-
lution of the screw into a vast number of parts, by means
of a divided circle of considerable diameter attached to it ;
indeed, there is hardly any limit to the exactness which may be

* For the purpose of measuring angles, of course there is nothing supe-
rior to the apparatus here mentioned, but it will require a positive eve-
piece ; in short it must be in all respects similar to that applied to the best
astronomical telescopes: there is, however, a very simple method of
measuring the angles of microscopic crystals, &c, which is, to attach a
camera lucida eye-piece to the body of the microscope when in a hori-
zontal position, and make a drawing on paper of the crystals, or other
bodies whose angles we wish to measure, which of course can always be
done with a protractor, the legs of the angles being sufficiently produced
for the purpose.

46 THE MICROGRAPHIA.

obtained in this way, in principle at least : unfortunately,
however, screws, when much used, contract a degree of loose-
ness which prevents them from causing a movement in the
cobwebs the instant they are turned round, especially when
moved in an opposite direction to that in which they have pre-
viously been worked. This defect may apparently be re-
medied by the action of springs bearing against the screws ;
but the natural drunkenness or unevenness of the threads of
the best screws, and its effect on the accuracy of the results
given by the instrument, must for ever remain the same.

I am myself much disposed to doubt whether, if we had
micrometers capable of indicating, with the greatest accuracy,
the millionth part of an inch, we could with certainty measure
such a quantity with them. I have frequently tried to measure
a smaller quantity than the 40 or 50,000th part of an inch
with an engiscope having an objective of 2-1 Oths of an inch
focus, and a deep eye-glass, used with one of Troughton's best
micrometers, which, under the circumstances, indicated quanti-
ties far less, but I do not think that any reliance could be placed
on measurements of smaller quantities than those I have men-
tioned, made with such an instrument : but when we consider
that achromatic triplet objectives are now made of not more
than l-18th of an inch focus, perhaps a very strong practised
eye might, with their assistance, when attached to a long body,
measure even so small a quantity as the 1 -200,000th part of
an inch with certainty ; but 1 question if it could proceed
much further. The two edges of a cobweb frequently are not
parallel to each other, which is apt to cause a slight inaccu-
racy in mensuration ; a cobweb seems to me not to be a round,
but rather a flattish thread, so that if it becomes at all twisted,
it must, like a piece of tape, present different apparent widths
in different places ; moreover, as substances of animal origin,
they are subject to decomposition, from the action of air and
moisture upon them. To remedy these evils, I have proposed*

* Vide Quarterly Journal of Science.

ON MICROMETERS. 47

to construct artificial cobwebs from a thick solution of caout-
chouc in the purest rectified spirits of turpentine or naphtha :
this substance (Indian rubber) being, it is well known, very
little affected by the action of either air or water, extremely
strong and elastic, and capable of being drawn out into
threads of any degree of fineness, when dissolved as aforesaid :
and if the operation is carried on from a homogeneous solution
of the Indian rubber, perfectly round and parallel threads may
be obtained. These will stretch to double their length with-
out snapping, are as opaque as the web of the spider, and, like
it, resist the action of the solar rays, when condensed by an
object-glass.

When measurements of opaque bodies are to be executed
with an engiscope, or of certain transparent ones, when the
power is high, the field of view is apt to grow too dark to
allow us to see either cobwebs or divisions sufficiently well ;
in these dilemmas a little accession of light may become very
acceptable, and can be obtained by inserting a diagonal stop
or field-bar, gilt but unpolished, into the body of the engi-
scope, between the field and object-glass, a hole being made
in the side of the body to permit the light of a lanthorn or
taper, &c. to be reflected towards the micrometer (as in astro-
nomical instruments in a similar predicament), by the said gilt
diagonal stop.

As to other species of micrometers, there are, perhaps, none
superior to those executed on thin slips of mother-of-pearl,
provided they are made with long divisions at every ten, and
shorter at every five strokes, something like those on a foot-
rule, and are not too broad. I do not much admire micro-
meters on glass myself, even when the strokes are deep enough
to allow black lead to be rubbed into them, to render them
visible in a commodious manner; for they do not admit of
leading strokes to enable us to count them easily (at least I
never saw any executed in this way on glass), and unless we
can count the divisions with certainty, they are, I think, of

48 THE MICROGRAPHIA.

little use. Mother-of-pearl micrometers, divided into 100
parts in an inch, and from thence to 1000, will be found to
give satisfaction in most cases where single or compound
magnifiers are employed ; and so will divisions on steel, as far
as 5 or 10,000 parts to an inch. When a micrometer having
only 200 divisions to an inch is inserted into the field-bar of an
engiscope, it may easily be made to indicate so small a quantity
as the 10,000th of an inch, provided the body is pretty long
and the object-glass deep; and if the eye-tube is so constructed
as to admit of being thrust in till the total length of the body
is shortened to one half, the divisions in the aforesaid
micrometer will, in that state, have their value doubled ; that
is, they will become equal to the 5000th of an inch. More-
over, by having divisions engraved on the eye-tube, and
ascertaining their value, we gain the advantage of a variable
scale, of every dimension between the 10,000th and 5000th
part of an inch, which is a great convenience in measuring
microscopic objects. This leads me to that part of the present
subject which treats

OF ASCERTAINING THE VALUE OF THE DIVISIONS OF THE SCALE
OF MICROMETRICAL EYE-PIECES FOR ENGISCOPES.

I consider it the bounden duty of every optician who sells
an engiscope having a micrometer in its field of view, to
ascertain the value of its divisions with every object-glass
belonging to it ; also the alteration made by pulling out the
eye-tube to the various divisions marked on it ; and if there is
more than one eye-piece having a micrometer belonging to it,
then the value of the divisions of this also. As, however, it
is part of my subject, [ shall give a few examples of the way
in which this is done, for the benefit of observers.

Example I.
I have a mother-of-pearl micrometer, having 200 divisions

6N MICROMETERS. 49

to an inch, in the field-bar of an eye-piece. I wish to know
what is the value of them with a particular body and object-
glass ?

I place a micrometer, having 1000 divisions in an inch*,
on the stage, and find that one of its divisions seems just
equal to one on the field-bar ; the value of which is therefore
1-I000th part of an inch. In order, however, to be quite
certain that it is so, I compare ten of the divisions of the one
against ten of the other; and now I find that my observation
was not quite right, for ten of the divisions of the micrometer
on the stage are only equal to nine and a half of those on the
eye-piece ; so, as fractions are very troublesome things, I just
draw out the eye-tube a little, until the two sets of divisions
exactly subtend each other, making a mark on the eye-tube
for adjustment, when I use the object-glass in executing
measurements-|-.

Example II.

I have a cobweb micrometer also ; I wish to know the
value of a revolution of its wheel, and the divisions upon it,
with the same object-glass, eye-piece, and body, as before :
I therefore insert it in the place of the field-bar. Now the
screw of the said micrometer is supposed to have a hundred
threads to an inch ; its wheel also is divided into a hundred
parts. On observing a set of divisions on the same stage
micrometer used before, I find that five revolutions of the
wheel cause the cobwebs to open to somewhat more than ten
divisions of the aforesaid micrometer ; I therefore push in the
eye-tube till the space between the cobwebs is exactly equal
to the ten divisions : it is evident now that one revolution of
the wheel is equal to l-500th of an inch, and each of the

* This will be found a very convenient scale for the purpose.

t I presume I need scarcely observe that when the value of the divisions
has been determined, the length of the body can in no way be altered
without affecting the said value,

E

to cxl

f*"« CD

;

50

THE MICROGRAPH1A.

divisions of the wheel to 1-lOOth of l-500th of an inch, i. e.
1 -50,000th of an inch.

Example III.

With another object-glass, ceteris paribus, I find that one
division" on the stage is equal to two and a half divisions on
the mothei'-of-pearl micrometer, or ten of the stage micrometer,
to twenty-five of that in the eye-piece ; therefore, with this
object-glass, the value of one of the divisions in the field is
the l-2500th of an inch. On drawing out the eye-tube until
ten divisions of the stage micrometer exactly cover thirty of
the field one, the value of the latter becomes equal to the
l-3000th of an inch ; further, until they cover thirty-five,
l-3500th; further, until they subtend forty, 1 -4000th of an
inch : the value, being obtained in every interval, be it what
it may, by multiplying 100 by the number of divisions in the
eye-piece covered by ten of those on the micrometer on the stage,
which I always suppose to be 1000 parts to an inch. In this
way the value of any divisions on the eye-tube may be
computed.

Example IV.

I now have recourse again to the cobweb micrometer and
a deep object-glass, every thing else being as already stated.
I find that 1 must turn the wheel round fifty times* before the
cobwebs open sufficiently to include ten divisions of the
micrometer on the stage, reckoning from the centre of the said
divisions, which are now so much magnified as to make this
precaution very necessary : therefore the value of the revolution
of the wheel is now rendered equal to l-5000th of an inch,
and the divisions on its circumference to 1-1 00th of l-5000th,

* There usually is in cobweb micrometers, on the superior part of the
field, a set of teeth, the interval of which is equal to that of the threads of
the screw, to shew us how often the wheel has revolved, the said teeth
commencing from the immoveable cobweb, or zero of the scale.

ON MICROMETERS. 51

or l-500,000th of an inch, a quantity which I am afraid the
eye can never estimate or discern with any power so as to
measure it correctly, though the micrometer certainly may
indicate it. It would perhaps be advisable to have the tire of
the wheel made broad enough to admit of four sets of divisions,
over which the indicator should extend with a sharp edge :
one set dividing the circumference into 100 parts, the second
into 50, the third into 20, the fourth into 10 only. We
should then have the option of reading off the divisions either as
500,000ths of an inch, 250,000ths, 100,000ths, or 50,000ths ;
the last quantity being one which can be measured with perfect
certainty.

It is sometimes customary to have the bar of a microscope
or engiscope converted into a micrometer for measuring the
depth of cavities, the thickness and foci of lenses, &c. by
having proper divisions executed upon it, with a nonius
attached to the socket of the stage ; and sometimes when the
stage has a transverse rack or screw movement, it is also
divided, in like manner, to serve the purpose of a micrometer.
I do not much admire the latter expedient; the former may
frequently be useful.

As natural micrometers, grains of sand, human hairs, and
the lines on the scales of butterflies and beetles, previously
measured, may be used in cases of emergency, where better
tools cannot be had.

DESCRIPTION OF A NEW MICROMETER,

I shall now proceed to describe a new kind of micrometer
for simple and compound microscopes, which it appears to me
has been long; a desideratum in instruments for viewing; real
objects, and which, consequently, do not admit of a real
micrometer placed in the same plane with tliem, so that the
observer can seldom or never see his measure, and what he
measures in focus at the same instant, unless with low powers.

52 THE MICROGRAPHIA.

Let a mother-of-pearl or hair micrometer be attached to the
end of a tube, about six inches long, and an inch in diameter,
furnished with stops, to prevent any light from being reflected
by its sides. At the other end a small achromatic object-
glass, adapted for parallel rays, of about half an inch focus,
and a quarter of an inch in diameter, is to be placed. In default
of such object-glass, the double object-glass of an engiscope,
or two combined, may be used with the convex lens or lenses
turned towards the micrometer. Let this apparatus be inserted
into the interior of a slider-holder, or fixed underneath the
stage of a microscope, the object-glass having a motion up
and down in the tube by means of rack-work, and it must be
evident that it will be easy to bring the image of the micro-
meter at the end of the tube, which the object-glass will form,
into the same plane with that in which the object happens to
be situated when in focus, by merely adjusting the screw of
the rackwork which moves the object glass ; so that whereas
in the compound microscope we see an image of the object,
but a real micrometer in the same plane with it, in our field
of view, we shall in the simple microscope see the real object
and an image of the micrometer in the same plane, at the
same instant*.

It is, moreover, obvious that whatever degree of delicacy
and fineness the micrometer at the end of the tube may
possess, the image of it, formed by a line of so short a focus
as half an inch, placed at six inches distance from it, will
always be far smaller and more delicate, and that the pro-
portion it will bear to the real micrometer can always be

* If it should be objected that, in the case of this micrometer, the
image of the divisions or hairs will be always behind the object, whether
opaque or transparent, and that therefore we shall not see them well unless
with very minute or very translucent objects, and with opaque ones not at
all ; the answer is, that all other micrometers applied to microscopes will
be just in the same predicament ; the great advantage possessed by the
micrometers of engiscopes is, that being before the image of the object, they
are effective under all circumstances.

ON MICROMETERS. 53

determined by regulating the length of the tube ; while the
value of its divisions may always be ascertained by trying
them against those of a real micrometer placed on the stage.

If the achromatic object-glass employed to form the image
is a bad one (and be it observed no common lens will do at
all), the image it gives will of necessity always be indistinct,
so that we shall hardly see delicate hairs or cobwebs with it.
Moreover;, it will condense light in the plane of the object,
which will cause great indistinctness in the vision of many
highly diaphanous bodies., such as very delicate animalcules ;
but this defect may always be cured by placing a coloured
glass (blue or neutral tint) over the end of the tube, to reduce
the intensitv of the illumination to the requisite standard.

It is of course not necessary for me to remark that this sort.
of micrometer may also be used with engiscopes, but I do not
recommend it, because that in their field of view is a real, and
consequently a better, thing. It is, moreover, effective upon
all sorts of opaque objects.

A very simple, cheap, and good kind of micrometer, may
be made in the following manner : — Get a scale made on a slip
of wood, six feet long, similar to that of a foot-rule ; let the
inches be divided into tenths. This you may easily execute
for yourself, on strips of drawing paper pasted together, and
afterwards to the slip of wood. Fix this to the wall of an
apartment, just in the axis of an engiscope or microscope
placed on a table with its body or lens in a horizontal
position; take a piece of string and measure off such a
distance from the wall, for the site of the engiscope, as shall
cause its field of view to appear to be of about six feet
diameter ; and consequently, when you look with both eyes open,
to make the scale appear to cross the field just as a real
micrometer would, placed in the field-bar ; there must be a
kind of arm, with an eye-hole in it, attached to the eve-piece
of the engiscope or the magnifier of the microscope, as a sight
for the eye which views the scale on the wall; and care must

54 THE MICROGRAPHIA.

be taken that this spectacle is placed parallel to to it when an
observation is made ; moreover, the piece of string mentioned
above must be carefully preserved, and constantly used, to
measure off the place for the eyes, and determine their
distance rigorously from the scale when measurements are to
executed, and the instrument is set up for such purposes.
The value of the divisions on the scale is to be determined
precisely in the same way as of those really placed in the field
of view, on which subject I have already fully treated. By
removing the instrument nearer to or farther from the wall,
the value of the scale is increased or diminished at pleasure.

We thus get a scale of divisions which may be executed
with the greatest nicety, being on a large sccde, though it does
the work of a small one ; which may also be numbered and
divided by leading strokes, in a way which will enable us to
read it with the greatest facility, however dark the field of view
may happen to be. Moreover, with a simple or compound
magnifier of the highest power, it will always be distinctly
seen when the object is perfectly in focus.

It would be an improvement to place the scale across a
hoop covered with white paper, of the size of the field of view,
and so placed as apparently to cover it ; for then the image
would not be confused by extraneous objects seen by the eye
employed to look at the scale, and the effect of the whole
would be very like that of a solar microscope.

I need scarcely add that the body of the engiscope and the
magnifiers used with this micrometer must be altogether fixed
and motionless ; all the necessary adjustments must be given
to the stage; — on this account the species of micrometer just
described is well adapted to the Amician reflector and to hori-
zontal achromatics.

APPLICATION OF MICROMETERS.

I shall now proceed to explain the use of micrometers in
ascertaining the foci and power of lenses : my more learned

ON MICROMETERS. 55

readers will excuse me it* I am more explicit than there is any
occasion for on their account, because I hope ladies, and per-
haps even schoolboys, will do me the honour to read this
treatise.

It is not quite so easy to measure the focus, either solar or
visual, of a lens as it may appear at first sight. Nothing,
indeed, appears more simple if we have a microscope with a
bar very finely divided, and a nonius attached to the
socket of the stage, than to lay a lens upon the stage, and
reflecting the image of a distant object upwards, by a plane
mirror, to measure the distance of the plane of the stage from
the locality of the said image ; and also that of the plane of the
stage from some minute body placed on the upper surface of
the lens, which will be equal to its thickness ; and then to de-
duct one half of the thickness from the distance of the plane of
the stage from the place of the image, which many persons
suppose gives the correction for the thickness. This, how-
ever, is not the case, for there is no form of lens usually made
whose focal distance can be measured correctly from the mid-
dle of its thickness, though the curves of such a one might
indeed be calculated : the true correction for the thickness
cannot be got without a deeper knowledge of the science of
optics than can reasonably be expected from the generality of
the persons who read this book : such as are curious on the
subject I therefore beg to refer to Mr. Coddington's work on
the " Refraction and Reflection of Light," pp. 89 — 102, which
will give them every, information requisite on the subject.
There are, however, other methods of arriving at the true foci
of lenses, which are altogether practical, and to which the
most fastidious and exalted mathematician can hardly raise
any objection ; namely, by measuring the power they give to
a telescope ichen used as eye-glasses to it. A small Newtonian
reflecting telescope is the most proper for this purpose, because
we shall have no thickness in the objective part to disturb us
in measuring its true solar focus, which may be effected with

56 THE MICROGRAPHIA.

great accuracy by removing its plane metal, and receiving the
image it forms of the sun on a disc of white paper placed in
its axis, the distance of the solar image from the centre of the
metal being the true focus, which must be very accurately
measured : if this should turn out to be some round number
it will be very convenient, but it is hardly to be expected.
The method of operating then is as follows : — The plane metal
is to be replaced, (which, if of accurate figure, will cause no
alteration in the focus,) and the lens whose focus we wish to
explore, must be attached as an eye-piece, and the instrument
adjusted to distinct vision on a star, or some very distant ob-
ject : then, with a dynameter, measure the size of the visual
pencil, (which is, in fact, an image of the object metal or glass
seen beyond the eye-piece,) and see how often it is contained
in the acting diameter of the metal : this gives the power.
You have then only to divide the focus of the metal by the
power, and you obtain the true focus of the eye-glass, or lens,
whose focal length you wanted to measure. Thus, if the
length of the focus of the metal is 3 feet, and the power turns
out to be 36, you may rest assured that the focus of the lens
must be 1 inch, and so on.

As under ordinary circumstances the quantities in this
method of mensuration are most likely to occur in a fractional
form, I here give an example of that kind : —

Q. — The true sidereal focus of an object-metal, or achromatic
object-glass, is 29| inches, its aperture 2 to inches, and the
size of its visual pencil, measured by one of Ramsden's dyna-
meters*, 19-10,000ths of an inch : what is the solar focus of
its eye-glass?

* This elegant little instrument (now rarely made) is composed of a
divided plano-convex lens, about ,'g inch focus, attached to a micrometer
screw, with a nonius dividing an inch into 10,000 parts. On turning the
head of the screw (previously adjusted to 0), the pencil instantly begins to
separate into two semicircles of light; and when their extreme edges are
brought into contact, the divisions may be read off in 10,000th of an inch.
I may observe that mathematicians say the powers of telescopes are nut obtained

ON MICROMETERS. 57

I". 59 m. 21

F.29i=¥ A2i=T0

Dividing the aperture by the visual pencil, we have

21 . 19 21 10000 210000 5

10 • 10,000 — 10 X 19 — 190 — * 19

power.

Dividing the focus of the object-glass by the power, we get

59 • 21000 59 19 1121

2" ~~T9~ = "2 X "21000" = 42000 the focuS °f the
eye-glass for parallel rays being about l-38fh of an inch.

Now, madam, you will say you do not care a straw what
focus the lens has, — you only wish to know exactly how much
it magnifies. I regret to acquaint you that I am afraid we
shall never be able to tell you this with anything like accuracy,
because the standard of human sight varies so extremely in
different individuals, and again in the same person at different
periods of life; so that unless your ladyship will be content to

with absolute accuracy by this method, though it gives the ratio of one power to
another with perfect exactness ; therefore, if it is thought worth while, the
power of the telescope may be got by actual measurement, thus: — Select a
well-built smooth brick wall, and set off an accurately divided scale upon
it, in white paiut, or with a white chalk crayon : let the divisions be about
3 inches apart, or of such magnitude that they can be distinctly seen 40 or
50 yards off. Place the telescope at this distance from the wall, and adjust
it upon one of the divisions ; then, looking with one eye at the scale on
the wall, and with the other at the magnified division in the telescope,
see how many of the divisions on the scale it subtends ; and if it does not
exactly subtend a certain number, increase or diminish the distance of the
telescope from the wall until it does : the number subtended is, of course,
the power of the telescope upon a near object. It will now be necessary
to measure the acting focus of the instrument under the existing circum-
stances ; or to ascertain how much its focus has been elongated from what it
was on an infinitely distant object, and to add this quantity to the sidereal focus.
Then, dividing the focus so obtained by the power, we must have the focus
of the eye-glass as nearly as it is possible to ascertain it : and having once
got the correction for the dynameter with one power, it can be easily
applied to any other,

58 THE MICItOGRAPHlA.

know the power it gives to your own eye, or that of some other
person in particular, your curiosity will be likely to remain
unsatisfied; but all this shall be duly settled hereafter. In
the meantime, allow me to remark that it were much to be
wished that microscopists and opticians would leave off prating
about the powers of their microscopes altogether, and confine
themselves to speaking of their foci ; or if of engiscopes, to the
foci of single lenses having the same power : we should then
know what they were talking about. Thus, a zoologist would
express himself very like a man of sense and science if he
would be pleased to acquaint us that he was observing such
an object with a doublet of l-30th of an inch solar focus, or
with an engiscope whose power was equal to that of a single
lens of l-30th of an inch focus, &c. &c ; mentioning also,
perhaps, the angle of aperture his instrument had ; — this is
speaking point-blanc. But if, instead of this, he merely says
he employed a power of 300 diameters, or 30,000 in super-
fices, or 9,000,000 in solidity ; unless he also mentions the
standard of sight it is his will and pleasure to compute by, we
are left in the dark as to what the focus of his instrument
might have been, and consequently cannot estimate its real
character. As to the puerile custom many observers have, of
giving the power of their instruments in superficial measure-
ment instead of diametric, as it is intended ad captandum
vidgus, so it answers no other purpose. Let any man endea-
vour to give some tolerable reason why the powers of telescopes
should not be computed by the number of times they magnify
the diameter of an object, but by the square of this number, or
its cube, and I will admit the propriety of the practice as
applied to microscopes.

But to return from this digression : I here proceed to give
a few examples of the method of finding out the solar focus
of other lenses when we have once duly ascertained that of
one.

Convert the standard lens, supposed to be of one-inch focus,

ON MICROMETERS. 59

into the object-glass of an engiscope having a micrometer in its
field of view, and observe to how many of its divisions one of
another micrometer placed on the stage of the instrument is
equal*.

Now let us suppose, for example's sake, that one of the
divisions of the micrometer on the stage happens to cover or
subtend five of those on the field-bar, or that you can make it
do so by pushing in or drawing out the eye-tube a little : well ;
affix the lens whose focus you want to know exactly in the
place of the one inch lens aforesaid, taking great care that it is
exactly at the same distance from your field-glass as the former
lens teas, and the body precisely of the same length as before.
Let us suppose, on trying it, one division of the same micro-
meter before used on the stage subtends ten divisions of that
in the field-bar : here must clearly be a lens of twice the
power, or one-half the focus of the one inch,, — that is to say,
half an inch. In like manner, had it subtended twenty
divisions, it must have been l-4th of an inch focus ; or if 40,
l-8th; if 80, 1-I6th ; if 160, l-32d; and so on.

The solar focus of compound magnifiers may be tried pre-
cisely in the same way, taking care that their second or third
glass, or that placed nest the eye, is in the place of the object-
glass ; and the distance from it to the field-glass the same as
when the standard lens was used.

More frequently, however, it happens that things do not go
on so smoothly as I have here supposed ; and the lens whose
focus you want to measure will produce an image of the
division of the micrometer on the stage, which is equal to some
fractional number which is no multiple of the number of the
divisions given by the standard lens. Thus, suppose it should

* When a deep object-glass is used, it frequently occurs that the lines
on the micrometer placed on the stage are so much magnified as of them-
selves to subtend several of those in the field-bar ; in this case care must
be taken to reckon from the middle of them, as nearly as may be, in com-
paring them against the others, or the error will be very considerable,

60

THE MICROGRAPHIA.

subtend 17| divisions, you will only be able to arrive at the
focus by a proportion, or little rule-of-three sum in vulgar
fractions, thus : —

Div.

As 171 :

Div.
5 :

T

Inch,

: 1

171 =

35

5 ' I • 35 5 2 10 2

Q 1

In another case, using a lens of — or — inch focus for the

10 5

standard lens ; the image of the one to be measured subtends

2

50 divisions, while the _ , or standard image, subtends 14 :

Div. Div. Inch.

A 50 14 1

As — : : : _

1 1 5

Inch.

14 x l __ 14 __ 50 _ . 14 ] _. 14 _ 4 Am

1 5" ~ 5 • 1 " 5~ 50 ""* 250 *" 125

or nearly — of an inch.
J 18

If you use the focus of a very deep lens to measure that of
a shallow one, then, of course, the longer the focus the fewer
will be the divisions its image will subtend on the micrometer
of the field-bar, compared with that of the short one : thus, if
the image of a division, given by a l-60th inch lens, sub-
tends 120 divisions on the field-bar, a 1-inch lens will only
cover two with its image : where fractional quantities occur,
the focus must be obtained in the same way as before. Ano-
ther method of getting at the true solar foci of lenses, is to
make them the object-glasses of a diminishing telescope, in-

ON MICROMETERS. 61

stead of an engiscope, and measure the size of the image of
some comparatively distant object with them, such as a win-
dow, or one of its panes, exactly in the same manner, and
upon the same principle, as before, by means of a micrometer
in the eye-piece of the miniature telescope ; but this method
1 do not recommend — it is not so accurate as the other, because
we have to work on a smaller scale, and is only applicable,
perhaps, to lenses of excessive depth, which give so dark or
so indistinct an image in the engiscope, that the divisions on
the micrometers or their cobwebs cannot be well seen. I
shall, however, append one example of this mode of measure-
ment*.

Example I.

A lens, whose known focus is 1-1 9th of an inch, happens
to make the image of a window seen from a distance of three
yards, subtend 131 \ divisions on a micrometer in the field-bar
of an erecting eye-piece, employed to examine the image :
another lens, at the same distance, gives a spectrum which sub-
tends only 43f division : what is the focus of the latter ?

1311 = *?

434=4
263 175 , . J_

T X" 19

175 1.175 - 263 __ 175 ^ _ 350 _

~T 19 " ' 76" T T 76 263 "" 19988

175 i 1 <• • i j

or nearly — ot an inch. — Ans.

9994 " 57

* When plano-convex lenses have their foci measured in this way, the
convex side must be turned towards the parallel rays, or their foci will
come out too long.

62 THE MICROGRAPHIA.

I suppose I must now give some statement or other about
the power of lenses, or my readers will not be content ; but
how am I to determine the standard of sight upon equitable
principles ? This is, in fact, the only obstacle, but a proper
tough one it is ; and a stumbling-block it must remain to the
end of the world.

I have, then, made many experiments upon young persons in
the full vigour of health and strength, whose eyes were per-
fect and highly keen and good, and I find that the average of
their standard of sight upon minute objects is much less than
ten or even eight inches (the usual standards), and approaches
nearly to five inches on the average : many are able, with a
little practice, to see most perfectly at about four, and I have
repeatedly seen school-boys read with their noses touching
their books, who were not in the least short-sighted, (for I of
course exempt all long and short-sighted persons from any
voice in determining a standard of sight) ; my own distance is
six inches, at the age of forty ; twenty years ago it was four
inches and a half.

I am not in the least short-sighted, on the contrary T can
see distant objects with the quickness of a savage. I fancy
that I have seen several double stars which are very far sepa-
rated with my naked eye, and I can almost always see the shot of
a piece of ordnance when I fire it myself. I shall therefore con-
struct a table of magnifying powers, for a standard of sight of
5 inches (which, as a comparative estimate, will be useful), and
they who do not like it, may double the quantities and standard
of sight, and then I hope they will have power enough. Much
good may it do them.

First, however, I must premise, that lenses of all foci what-
soever, 100 feet for example, magnify somewhat, and so do
even thick plates of refracting substances having plane sur-
faces ; because, when placed before the eye, they increase the
refraction of the rays of light impinging upon it, and thus

ON MICROMETERS. 63

enable us to view objects under a larger angle than we could
otherwise do.

What may be called the visual focus of a lens, or its dis-
tance from an object upon which we have adjusted its focus as
a magnifier diffei's always widely from its focus., when used to
collect the parallel rays of distant objects into an image of
them, after the fashion of the object-glass of a telescope ; yet
these two kinds of foci have always been confounded together
by opticians, and it is general considered, as a matter of course,
that a lens which brings parallel rays to a focus two inches
distant from it, will, when used as a magnifier, preserve a
visual focus of two inches, with a little allowance for dif-
ferent sights ; but nothing can be more erroneous, as both
theory and experiment well demonstrate.

METHOD OF FINDING THE VISUAL FOCI OF LENSES, THEIR FOCI

FOR PARALLEL RAYS, AND THE FOCUS OF THE

EYE BEING GIVEN.

In order to discuss this subject in its right point of view, we
must begin by considering the naked eye itself as a magnifier,
having poiver equal to that of a lens of its own shortest visual
focus ; and when a magnifier is "placed before it, it must be
treated in the light of the anterior lens of a doublet, and the
focus and power of the combination must be found precisely
in the same manner, viz., we must multiply the shortest
visual focus of the eye by the sidereal or solar focus of the
lens whose power we wish to learn, and divide the product
by the sum of the two foci, minus the distance between them,
which gives the focus of a lens whose power is equal to the
combination, which may then be ascertained by seeing how
often the said focus is contained in the visual focus of the
eye. This method will be found to agree extremely well
with experiment ; but if we consider the lens and the cornea

64 THE MICROGRAPHIA.

in contact, and make no allowance for distance, as we shall
most likely think ourselves justified in doing, the focus will
turn out somewhat too short, and the power consequently too
high. If we consider them in absolute contact, and the
distance between = 0, we must of course suppose them di-
vested of thickness, or one of them at least, which can never
be ; the thickness, therefore, must always be reckoned as
so much distance : besides which we well know that we can-
not endure any lens to touch the cornea of the eye, even if
the eyelashes and eyelids would permit it, therefore the dis-
tance must always be equal to something, though, for my
own part, I do not think it worth while to take it into con-
sideration.

I shall give an example or two of this method of compu-
tation „

Q. — What is the power of a lens of seven inches solar focus
used by a person the shortest, visual distance of whose eye
is six inches ?

6 X 7 = 42 Distance — 0

6 + 7 = 13

.a . , o 42 o 3 Inches, the focus of the eye and lens

~~ 13 Vi combined, or the visual focus.

6-42 6 ^ 13 78 1 e . .

— zz — X — ~ — — "7 times.' — Ans.

1 13 1 42 42

Q. — What is the power of a lens of 1-inch focus, computed
by a standard of six inches ?

6X1 = 6
6 + 1=7

Inch.

_ therefore is the visual focus.

7

66_67 42 „ .. .

- — - — - X-— — — 1 times. — Ans.
17 7 6 6

ON MICROMETERS. 65

In this manner any person may settle the power of a lens,
as far as his own •particular eye is concerned, with conside-
rable exactness, but he must not measure his neighbour's corn
by his own bushel.

I must observe that if the power of a lens is low, as, for
example, two inches focus or more, it is not easy to find its
visual focus very exactly, even with such a delicate object
as a small globule of quicksilver ; for there will appear to be
considerable space over which it may be moved without causing
the star to swell out into a disc, either ivilhin or without the
focus : in making the experiment, I have, therefore, always
selected the shortest distance at which the star seems perfectly
sharp and free from coma, just before it swells out when placed
within focus; and I ascertain the visual focus of the eye in
the same way. I request my reader particularly to recollect,
that in the following table the power is given according to
the visual focus of the lens, not its sidereal or solar one, and
that the power of a lens, rated by its solar focus, is always
under the truth, especially if the focus is a long one. I be-
lieve it wholly impracticable to ascertain the power of lenses
without reference, to a standard of sight, as may be done with
telescopes, and likewise in the case of the solar microscope.

66

THE MICROGRAPHIA.

TABLE

OF THE rOWERS OF LENSES OF THE FOCI GENERALLY MADE

BY OPTICIANS, RATED BY A STANDARD OF SIGHT

OF FIVE INCHES.

Visual Focus.

Power
in Diameters.

Visual Focus.

Power
in Diameters.

Inches.

Inch.

2

%\

16

80

n

n

17

85

n

2f

IS

90

|5

0T3

T<5

95

I1

sj

25

100

n

317T

25

125

n

4

33

150

4^

33

175

1

5

33

200

I

5|

33

225

3
5

6?

35

250

S

5

8

33

275

L
2

10

B3

300

3

s

13|

B3

325

I

4

20

TO

350

I

8

40

73

375

1
TO

50

53

400

1

55

55

425

A

60

53

450

1

13

65

53

475

1

70

1

T35

500

*

75

ON MJCR031ETERS. 67

THE POWER OF ENGISCOPES

Is very easily ascertained : we have only to place a micro-
meter on the stage, having the same divisions as that in the
field-bar, and to observe how many of the divisions of the
latter are subtended by one of those of the micrometer on the
stage (which of course shews the number of times the image
is magnified), and to multiply the number of divisions by
the power of the eye-glass, whether single or double, as-
certained by the preceding rules : in this case the field-glass
or glasses (for there are sometimes two) must not be associated
icith the eye-glass when its power is calculated, because the
field-glass co-operates with the object-glass in forming the
image, which is magnified solely by the anterior lens or lenses.
Example. — In an engiscope, with a particular object-glass,
one division of a micrometer on the stage subtends 13| of
those on the field-bar ; and the visual focus of the double eye-
glass is found to be one inch and a quarter : required the
total power, computing by a standard of sight of five
inches —

, x _ 5 5.5_54_20_/t .

l± — - _ — - zz. - X — = z= 4 power oi eve-

4 1-4155

jlass.

Theu 13i X 4 = 54 Total power.— Ans.

Second Method — Treating the Engiscope as a Single Lens.

View the image of some distant object, a window for ex-
ample, formed in the visual pencil of an engiscope, with a
dynameter of very delicate divisions, and see how many of

OO THE MICROGRAPH I A.

them it subtends : view the image of the same object formed
by some lens of known focus, in the same manner with the
same dynameter, taking care that the object-glass of the en~
giscope and the lens are at the same distance from the object,
and see how many divisions the latter image subtends ; then,
by comparing them against each other, we learn to what single
lens the engiscope is equal in focus and power.

Example. — The image of a window in the visual pencil of
an engiscope subtends 14 divisions ; that of the same object,
formed by a lens of l-5th of an inch solar focus, 50 ; what
is the power of the engiscope rated by a standard of sight of
five inches ?

As 50 14 1

1 T : : 5

14 1 14 50 _ 14 1 14 __ 7

TX5"~5""~"T~='5" X 50 ~ 250 " 1~25

solar focus of a single lens of the same power as the engiscope.
We have now, therefore, to find the visual focus of a lens
having a solar one of 7-125ths, by the preceding rules.

7 5 35
125 X T — 125

7 4- * - 5-L -
125 ^ 1 ~ 125 —

632
125

35
125 "

632 35 125 4375
• 125 ~~ 125 X 632 " 79000 ""

35

visual focus.

632

35

Five Inches (standard of sight) divided by r=

6 632

5 632 3160 nnZ.: .

- v -?^r — — 90- times. Answer.

1 A 35 35 7

ON MICROMETERS. 69

Third Method. — The principle of this is, that the diameter
of the visual pencil of an engiscope bears the same propor-
tion to the focus of a lens equal in power to the engiscope,
that the diameter of the object-glass of the said engiscope
bears to its anterior conjugate focus.

Thus let us suppose we know that a certain object-glass, or
metal, or a series of object-glasses, used with a certain stop,
has 18^° of aperture (for the method of measuring the angle of
aperture, vide " Microscopic Cabinet") : this, in other terms,
denotes an object-glass whose aperture is equal to l-3rd of its
anterior acting focus ; and that we find, by measurement with
a dynameter, that with a certain eye-piece its visual pencil is
3-200ths of an inch in diameter; — required the power of the
engiscope ? Here we must multiply 3-200ths of an inch by
three, which gives 9-200ths of an inch for the solar focus of a
lens equal in power to the engiscope ; the visual focus of which
must be ascertained according to the examples already given,
and compared with the standard of sight.

If the angle of aperture had been 27g°, then the diameter of
the visual pencil must have been multiplied by two ; had it
been 55*, the diameter would have been equal to the solar
focus of the equivalent lens, so that in this case we should
have merely to find its power.

The length of the anterior focus of an object-glass, with a
body of any determined length to which it may be attached,
may be got at as follows : — Take out the eye-piece, and pre-
senting the body with its object-glass in its place towards the
sun by tilting it to its proper angle, ascertain the solar focus of
the said object-glass by placing a flat surface on the stage of
the engiscope to receive the image of that luminary. When
the stage has been duly adjusted to this focus, make a mark
on the bar, against the socket of the stage, with a needle ;
then attach the body to the object-glass, and adjust its focus

70

THE MICROGRAPHIA.

upon the same flat surface which received the sun's image. It
ivill now become longer, and a fresh mark must be made
against the side of the socket ; then measure very carefully the
space between the two marks, and add it to the length of the
solar focus of the object-glass found by some of the preceding
methods ; — the sum is the length of the anterior conjugate
focus of the object glass, <$°c. When this is ascertained, by
measuring the exact size of the stop behind the object-glass,
or its interior diameter, if it has none, and making a diagram
on paper, the angle of aperture may be ascertained, and the
power of the instrument measured by the third method given
above, without having recourse to a regular instrument for
measuring angles of aperture.

OF MEASURING THE SIZE AND ANGLE OF THE FIELD OF VIEW OF
ENGISCOPES.

1st Method. — Having ascertained the visual focus of the eye-
glass, or eye-glasses, if a double one is used, measure the
diameter of the field-bar, and multiply it by the power of the
eye-glass.

2d Method. — Hold a foot-rule at your own shortest visual
focus against the body, and look at it with one eye, while
you look through the instrument with the other, and note
where the edges of the field cut the divisions on the rule.

3cZ Method. — Attach a camera lucida, or a diagonal bit of
speculum metal, or thick glass, to the ocular end of the en-
giscope, and placing the body in an horizontal position, fix a
sheet of paper underneath it at your particular visual focus ;
then with a pencil mark the edges of the field on the paper,
and measure the distance between them. In order to obtain
the angle, or number of degrees your field takes in, make a
diagram on paper, thus : — Let the width of your field, taken
at any distance from the eye-piece by this third method, form

ON MICROSCOPES. 71

the base of the triangle, bisect this, and erect a perpendi-
cular on it, marking a point for the apex of the triangle, at the
distance of the axis of the body or visual pencil from the base;
then complete the legs, and measure the angle they form with a
protractor : it would be best always to speak of the number
of degrees taken in by any instrument, instead of the absolute
width of the field of view, because the angle is not affected by
difference in the visual focus, and is therefore a constant
quantity.

Another very simple method of ascertaining the angle of
the field is by holding the eye-piece up to one eye, and
looking against a wall with the other ; then noting the size of
the circle of light, and making two marks by driving two
nails into the wall, denoting the diameter of the field ; then
take a bit of twine, tie its ends to the two nails, and stretch the
strings, bringing them together precisely at the point of sight
you used in taking the breadth of the field of view ; measure
the angle they form, with a protractor, and you have the angle
of vision : or the same thing may be effected by taking a
sector or foot-rule, applying the joint to your eye, and open-
ing the legs out till they come into a line with the two nails,
when the angle is to be measured as before.

This operation may be performed on the small scale by
taking the exact width of the field-bar, and constituting it the
base of a triangle, of which the apex is to be the end of a
line at right angles to the middle thereof, of the exact length
of the visual focus of the eye-glass : the angle thus formed is
to be measured like the rest.

It is easy to chalk a scale on a brick wall, or it may be
made in the small way on a slip of wood, which, from a par-
ticular point of sight, will shew at a glance the angle of vision,
by looking at it with one eye, while the eye-piece is held to
the other, so that its field shall cover the scale.

As many persons may like to know how to ascertain the
solar focus of a lens, from knowing its radii and the refrac«

72 THE MICROGRAPHIA.

tive power of the glass it is made of, and its radii from
knowing its focus, &c. I append a method of ascertaining the
same, from the inestimable work of Mr. Coddington on Re-
flexion and Refraction, p. 293. For this purpose we have
only to divide the radius by the index of refraction, minus 1.

Example. — Required the I focal length of a plano-convex
lens, made of crown glass, having a refractive index of 1.525,
the radius being | inch.

1-525 — 1 = -525
g = -750 dec.

•750 _ -525 = 1-428 focus.— Ans.

Example 2. — An equi-convex lens is desired of the same
focal length (1*428), made of the same glass ; required the
radius. In this case the index of refraction, minus 1, is mul-
tiplied by 2, and by the focus.

Thus, -525 X 2 X 1'428- = 1-643 radius -Ans.

; i . . ■ ■ m ■■ ' ; •' '

I have now, I trust, given the public at large the means of
determining the power of microscopes and engiscopes, with a
much greater degree of accuracy than has hitherto been done ;
perhaps with all the exactness practicable, though, as I have
already said, it would be better to abstain from discussing
their powers altogether, and to speak only of their solar foci,
which are constant quantities.

C. R. G.

CHAPTER III.

ON

MONOCHROMATIC ILLUMINATION.

To the illustrious Sir David Brewster we are indebted for the
scheme of illuminating microscopes by monochromatic or ho-
mogeneous light, in order to render their chromatic aberra-
tion insensible, which, it must be confessed, appears a priori
extremely plausible ; nevertheless, there are many objections
to it. It is evident that all sorts of monochromatic illumina-
tion whatsoever must be far less intense than the ordinary
kinds, inasmuch as they must consist of one of the seven con-
stituent prismatic colours abstracted from the rest ; for if a
beam of light is divided into seven parts, and only one of
them is taken (though it should be the brightest of the set), it
must follow that it will be far fainter than the combination of
the whole.

It is well known to practical men, that the most vivid white
light which can be procured (barring always that of the sun),
is by no means too brilliant for opaque objects ; it can need,
therefore, no argument to shew that, at all events, monochro-
matic light can never answer effectively for these, however
much it may be condensed ; with certain transparent ones,
seen with low powers and large apertures, it performs tole-
rably well, and is extremely useful when we wish to examine
the spherical aberration of uncorrected lenses, divested of
the chromatic species. Monochromatic light must of course

74 THE M1CR0GKAPHIA.

always be of some colour or another, which circumstance
must necessarily produce a very fallacious appearance in the
vision of many objects, which will compel the observer to ex-
amine them afresh with white light, in order to rectify their
apparent tints. I myself am convinced that a real absolute
monochromatic light is yet to seek, and shall here state the
experiments on which I found my opinion. When white light
is subjected to the action of a prism, it is well known that it is
dispersed or resolved into seven prismatic rays, leaving a
number of colourless lines, when the experiment is made with
due delicacy, in the method prescribed by Fraunhofer. Now
I think if there is any such thing as monochromatic light, a
good prism ought to be able to produce it, and accordingly
I have chosen light so generated for my experiments.

The white light of the sun, reflected by a plane mirror, is
able to shew most of the coarser proof-lined objects in a rough
manner : indeed, I have sometimes succeeded in bringing out
the straight lines on the podura with it, and this too with an
instrument which had not the power of shewing them with any
other species of illumination. I therefore choose this class of
objects in my trials with solar light, for they are, upon the
whole, exhibited better in proportion by it than any other
bodies. In order to obtain from the solar beam a monochro-
matic illumination, I made the following arrangements : I
placed an engiscope with its body in a horizontal position,
and excluded all white extraneous light from it, as follows : I
attached a tube, six inches long, to the stage, and at the end
of it fixed a diaphragm l-8th of an inch in diameter ; in the
prolongation of the axis of this tube, and at about a foot dis-
tance from it, I hung up a piece of black silk velvet ; I like-
wise drew a shade over the objective end of the microscope
reaching close down to the slider, so that no light should get
into the body of the instrument by reflection. By these means
when its power was equal to that of a l-20th of an inch lens,
my field of view was perfectly dark and invisible ; I then in-

MONOCHROMATIC ILLUMINATION. 75

troduced a prism* (the edges and other parts of which not em-
ployed to refract light, were carefully blackened, that no white
light might be reflected or radiated by it), between the piece
of black velvet and the tube, so as to refract the light of the
sun from a mirror into the microscope. The prism was placed
vertically, so that either of the seven colours it produced could
be brought to bear upon the small aperture at the end of the
tube, to the exclusion of the rest, the spectrum being of con-
siderable breadth, owing to the distance of the prism from the
diaphragm, which was about three feet. Now by this appa-
ratus I could very easily select any one of the colours I
wanted, by merely moving the microscope a little to the right
or left, and illuminate the object with it. The object-glass
which I made use of was a common plano-convex of half an
inch focus, and a quarter of an inch of aperture, which, with
the white light of the sun, was just able to shew the lines upon
the scales of the Menelaus butterfly in a very imperfect man-
ner, and was much disappointed and surprised to find that no
amelioration in the vision was produced by using any of the
prismatic colours, instead of the white light. Moreover, I
found that not one of the colours produced apparent achroma-
tism, unless it truly perforated the axis of the microscope, any
obliquity of the light instantly causing prismatic fringes to
appear, modified by the tint of the coloured ray used, much in
the same manner as white light would have been, if merely
passed through a piece of coloured glass. Now the lined
objects always require oblique light for their manifestation,
and consequently never could be shewn without exhibiting
other colours beside those of the primary ray employed ; but
other objects which agree with direct light could be shewn
destitute of prismatic fringes, though very confused and indis-

* This was capable of shewing Fraunhofer's lines very well with the
angle I used.

76 THE M1CH0GRAPHIA.

tmct. I may observe that the light of the sun treated in this
way is by no means too intense for microscopic purposes.
I even thought that the violet ray was not intense enough.
The blue ray with an achromatic object-glass performs pretty
well on the lined objects : I like it better than any of the rest,
for it most resembles day-light, though each is capable of
exhibiting the lines, if not procured in a state of too great in-
tensity, by being taken too close to the prism. I repeated
these experiments with the flame of a wax taper in a dark
room, in the following manner : — I used the prism as before,
and adjusted the wax-light to a proper height to suit it ; I then
carefully shaded the candle, so that none of its direct light
should get into the microscope, and threw the spectrum formed
by the prism into the body of the instrument, taking care to
use it at such a distance from the prism that the light of the
taper should be fairly dispersed. I did not use the long tube
in these experiments, thinking it of no use, as the room was
dark, and I could not see my field of view until the spectrum
of the prism was directed into it. The light afforded by the
taper was too faint for the power I used with the solar light;
I was therefore obliged to reduce it one-half : even then the
blue ray had not sufficient power ; the yellow one had, and
seemed little different in its effect from the taper itself in the
same state of intensity. Suffice it to say, that I got the same
results as before ; that is, when I obtained the light of the
prism perfectly direct, I obtained achromatism ; when it came
obliquely, the coloured fringes instantly appeared, modified by
the tint of the ray, and much in the same way as if I had
looked through a coloured eye-glass at the image produced by
common light. Wishing to sift this phenomenon to the
bottom, I then used the light of the taper in its natural state,
and found that there is one particidar position in which it may
be placed behind the stage, so as to give apparent achromatism
with any sort of microscope whatsoever, however chromatic it

MONOCHROMATIC ILLUMINATION. 77

may be. This appears to be as nearly as possible in the axis
of the microscope ; if it is placed ever so little out of it, the
coloured fringes instantly shew themselves.

I am confident that I could make an inexperienced person
believe in the achromatism of any microscope whatever, by
making this arrangement, and a most fruitful source of decep-
tion it must prove as to the achromatism of these instruments.
I may observe that perfectly direct day-light also gives appa-
rent achromatism with any common uncorrected lenses.

As prisms and lenses have a great analogy to each other in
their operation, I then experimented on the dispersed solar
light with them, and it always appeared to me that where I
could succeed in getting the light from the diaphragm thown
perfectly direct through a prism, no sensible secondary dis-
persion was produced ; but when it came obliquely, the aper-
ture was completely fringed by prismatic tints in their natural
order. As it seemed to me that a piece of rubbed glass,
placed over the diaphragm, caused the light to be equally
diffused over the interior of the tube, whether it came obliquely
or not, I tried the effect of the prism upon it, but the light
always seemed to be dispersed in this way as before. (For
these experiments I used the apparatus first described, only I
took out the object-glass and eye-glass of the microscope, and
looked at the diaphragm through a small prism, held in my
hands at the ocular end of the body.) In order to have made
these latter experiments with the prism properly, I should
have required a heliostat moved by clock-work, for the motion
of the earth subverted the direction of the solar beam imme-
diately I had got it truly in the axis of the body of the
microscope, and the power I possessed of counteracting this
was only by means of adapting the prism to the motion of
the light with my hands : I should have liked to have dis-
persed the colour a second time, and fairly produced a spec-
trum of it on paper, under the circumstances of direct and
oblique incidence.

78 THE MICROGRAPHIA.

The impression produced in my mind, by reflecting on the
facts stated, was, that a certain quantity of white light enters
into the constitution of each of the seven-coloured rays, as water
does into the constitution of alcohol, and may be rendered ma-
nifest by the dispersive power of prisms and lenses, under the
circumstances I have described : and this view of the subject
seems to acquire conformation from the " New Analysis of Solar
Light," by Sir D.Brewster, (vide Edinburgh Journal of Science,
Vol. V. p. 197,) published subsequently to my paper on Mo-
nochromatic Light, in the said Journal, (of which the present
tract is a corrected and modified second edition.) See also
the original paper, and his remarks upon it, Vol. V. pp. 52,
and 143.

Fortunately, however, notwithstanding all that I have said
of the impossibility of obtaining monochromatic light from
the action of the most perfect prisms, some kinds of lamps will
produce it in a sufficiently pure state for practical purposes,
though by no means deserving of the appellation of mono-
chromatic, in strict scientific language. I shall merely men-
tion two kinds : — The first is reckoned by Sir D. Brewster
the best yet discovered ; it consists in burning diluted alcohol,
saturated with muriate of soda, in any suitable apparatus, ap-
plying heat at the same time to the solution ; a faint golden-
coloured flame, nearly invisible when presented against the
light of a common candle, or of the sky, is the result: its in-
tensity may be somewhat increased by using a double con-
centric Argand wick, or a long flat one, presented sideways,
so as to have the effect of several lights placed in a row. I
myself do not approve of using a multiplicity of lights, though
adjusted in a line, for microscopic purposes, for very obvious
reasons.

This golden-coloured flame, when analyzed by the prism, is
perfectly monochromatic, except that it sheivs green, blue,
indigo, and purple light, as the flames of other substances in
combustion do ; but there is a remarkable want of the red and

MONOCHROMATIC ILLUMINATION. 79

orange light. It is too faint to allow us to perceive any of the
green, blue, indigo, or violet rays, which it certainly has in its
constitution, when reflected or radiated from coloured objects,
all of which appear to be some compound of black (which
implies a negation of colour) with yellow, except that white
bodies appear white, or nearly so ; thus a printed book, or
the dial-plate of a watch, appear just as they do when viewed by
faint candle-light : the white ought, according to theory, to
appear yellow, or rather gold-coloured ; but we must consider
that, nothing in this world goes on just as it should do.

This species of lamp gives a very unsteady flame, which
scintillates and sputters very disagreeably ; I cannot say that
I ever saw a proof-object with it.

Another sort may be made by dissolving borax in the
strongest alcohol, (of a specific gravity not greater than 0.815)-
Borax usually imparts a green colour to the flame of alcohol,
but treated in this way it gives a golden light extremely si-
milar to the other ; it does not require heat to be applied
to it, and burns much more steadily. It is, however,
somewhat capricious : thus, if burnt in a common thimble, its
flame is pretty much like that of neat alcohol ; if in a spirit
lamp, it sets off" at first with a flame, which shews traces of red
and orange light, but as the wick becomes consumed, (which
should be allowed to become pretty long, and be but little
trimmed,) it gives the golden flame like the other, shewing
only green, blue, indigo, and violet light, when subjected to
the action of the prism : the best way, however, of burning it,
is in a little brass cup, not more than 3-10ths of an inch wide,
and the same depth, without any wick at all • this gives a
strong flame of the right sort, but unfortunately it is soon
expended : a lamp should thei-efore be contrived, having a
fountain to keep this little burner constantly replenished; and
in order to render the lamp really effective, an apparatus for
feeding the flame with oxygen gas must be associated with

80 THE MICROGRAPHIA.

it*. This can easily be made by any workman accustomed to
manufacture chemical implements : the light will then acquire
the intensity requisite for microscopic purposes, and though it
may prove of somewhat too expensive and troublesome a nature
for ordinary use, will be of infinite service to the optician
who wishes to be released from the trouble of contending with
chromatic aberration, and may probably promote discoveries,
by allowing us to use object-glasses in which the spherical
aberration has been more exquisitely corrected than can easily
be done where the foci of the convex and concave glasses
must constantly be kept in a certain ratio to each other, which
circumstance is a great drawback on the exertions of the work-
man, and is perpetually interfering with every step he takes
for the destruction of indistinctness. It is well known that
in all compositions of convex with concave lenses of flint glass,
the curves which correct with a moderate aperture will over
correct with a larger one ; that, in short, the contrary aberra-
tions of these two kinds of lenses can only be exactly balanced
with small apertures : thus, when a very large angle of aper-
ture is to be obtained, we are obliged to combine two, three,
or even four pairs together : by thus dividing the refraction
the evil is diminished, but a radical cure is evidently impos-
sible. Now, where achromatism is not a desideratum, the
concaves of flint glass may be dispensed with ; for by com -
bining a meniscus of particular curves with another lens,
having a minimum of aberration, the spherical error may be
got rid of equally well. To Sir J. Herschel we are indebted
for the dimensions of two species of lenses of this description.
— Vide Phil. Transactions, for 1821.

It must, however, be understood that the meniscus will only
correct to a certain angle of aperture, (about the same that a

* Vide Mr. Talbot on Homogeneous Light; Phil. Magazine, for 1833.
Vo!. ITT. p. 35.

MONOCHROMATIC ILLUMINATION. 81

concave of flint glass will,) after which the error begins to appear
as before, but is of a contrary nature ; viz. with a large aper-
ture the composition is under corrected for spherical aberration :
it would, however, seem probable, a priori at least, that by
combining one of Sir J. Herschel's doublets with a lens con-
sisting of a convex associated with a concave, a correction of
the error of sphericity to a very large angle ought to be
obtained. This experiment is well worthy of a trial ; at all
events, several pairs of Herschelian doublets would probably
be associated with effect, since we know that regular achro-
matic combinations may, which will also act separately. A
monochromatic light, therefore, being once obtained in a suffi-
cient state of intensity for practical purposes, bids fair to con-
duct us to the highest perfection of which aplanatic object-
glasses and magnifiers are susceptible.

According to the experiments of P. C, detailed in the
" Records of General Science," for 1835, pages 34, 108, 172,
and 351, the true monochromatic or primitive colours are red,
(N.B. not crimson^ green, and violet: these, when pure andun-
diluted with white light, form the supposed primitive colours,
crimson, blue, and i/elloiv, as follows : —A union of violet and
red, crimson ; Green and violet, blue; green and red, yellow.
Pei-haps we may one day obtain green or violet-coloured
glasses of a certain tint and intensity of colour, which will
render the chromatic aberration of telescopes and microscopes
insensible ; though if this should be effected there is a chance
of the remedy proving as bad as the disease.

C. R. G.

CHAP. IV.

ON SOLAR ENGISCOPES,

AND ON THE EXHIBITION OF TESTS BY THEM.

By a Solar Engiscope of course is meant an engiscope applied
as the magnifier of a solar microscope, with a suitable appa-
ratus. Persons of a contemplative turn of mind, familiar with
optical studies, are seldom much gratified by the ordinary
exhibitions of solar microscopes, even when equipped with
achromatic magnifiers of the best construction. They see at
a glance that the immoderate amplification of the image of
these instruments is produced by permitting the rays to spread
themselves, at the expense of the sharpness and light of the
picture ; and that power so gained is not effective, or capable
of extending our insight into the nature of things, as it might
do if obtained in a legitimate manner ; that, m short, it only
serves to raise a feeling of stupid wonder and admiration in
the vulgar, and beyond its power of captivating them it has
no real use whatever. Individuals of this cast will naturally
turn towards the solar engiscope as an instrument capable of
affording real gratification to a man of science, and lover of
the fine arts : here is found magnifying power fairly obtained ;
a picture whose sharpness and clearness is a maximum, com-
bined with a field of view of considerable extent. Whatever
transparent objects can be seen distinctly with the optical part
of a solar engiscope, when applied to the eye, can, with very

SOLAR ENGISCOPES. 83

few exceptions, be seen also in (he picture in the camera of
the solar engiscope ; whatever traits are visible in the one
method of using it are more or less visible also in the other ;
and as these may be contemplated by two or three persons
at once, where any dispute or difference of opinion may happen
to occur as to their real character, it may be cleared up satis-
factorily by their joint scrutiny. The pictures of opaque
objects formed in the camera are, however, certainly rather
pleasing and beautiful than useful ; for the amplifying power
with them can never be carried to the extent necessary to
shew their minutiae satisfactorily, from the want of light, or
the danger of burning the object if it is rendered sufficiently
intense to create a strong image. Nevertheless, a vast
number of the most remarkable features of these bodies may
be exhibited ; and I have no doubt that many persons, well
entitled to the appellation of philosophers, will consider the
pictures of opaque objects as the most interesting part of the
display.

I have used several species of stands or mountings for solar
engiscopes ; the original one was altogether different from
that now presented to the public : in this the light of the sun,
first reflected from an heliostot, was thrown downwards by a
plane oval mirror, the bar and body being vertical, and the
image formed in a camera below. This answered very well
with ordinary objects, and moderate powers, and was very
easily convertible into the form of an ordinary engiscope ; but
when very high powers were used with test objects, the tremor
from the top heavy construction of the instrument could not
be subdued, and utterly marred the vision of very delicate
lines. I then used the same construction in an horizontal
position, by which means I got rid of the tremor, and likewise
precluded the necessity of introducing the primary reflection
from the heliostot ; but in order to get the oval mirror to reflect
a round spot of light, I was obliged to expose the instrument

84 THE MICROGRAPHIA.

to the full rays of the sun, which was inconvenient on many
accounts ; more especially as the head and face of the observer
were of course also exposed, and could not easily be shaded.

The construction I shall now describe is free from these
inconveniences, and upon the whole, I think as good as any
which can be contrived ; and will moreover act as an ordinary
solar microscope, when required. Fig. 1, in the annexed
engraving, is a geometrical elevation of the instrument, one;
tenth of the real size. The various parts are represented as
if formed of transparent matter, displaying all their internal
structure, which I consider a more useful and intelligible
method of drawing than that of giving regular sections. («) is
a strong frame of woodwork, resting upon four legs ; it has a
large hole cut out in it, into which the instrument is fixed, by
means of the two screws (ff), in the same way that ordinary
solar microscopes are to a hole in a window-shutter. The said
frame is of sufficient height and width to protect an observer from
the rays of the sun. It will be seen that there is considerable
resemblance throughout between this instrument and the best
kind of solar microscopes usually made. (6) is the plane
mirror, made of considerably greater length than they usually
are, in order to insure a round spectrum of light from the sun
when low. (c) is an arm (which must be made to take off, for the
convenience of packing), moving on a pin affixed to the side of
the mirror, and likewise on a joint attached to a strong round
wire (e), which slides backwards and forwards in the tube ((/),
having a spring in its interior, and a pinching screw to regulate
its position. The wire (e) being thrust out or drawn in causes
the mirror to alter its inclination, corresponding to the position
of the sun ; when drawn home the mirror is at an angle of 45°,
(being the smallest angle which can ever be necessary under
any circumstances.) The mirror has also the usual circular
movement, given by an indented wheel and pinion, the thumb-
screw of which is seen at (g). At (A) is placed the illuminating

86 ON MICROMETERS.

lens, of one foot focus, which must not be of less than five
inches diameter, and moreover, ought by all means to be
made achromatic. For this purpose it will not require a piece
of flint glass of that goodness indispensable to make a telescope ;
it will be sufficient if it is homogeneous as to specific gravity :
a liquid concave lens of the sulphuret of carbon might perhaps
be used with advantage. If it should be found impracticable
to get glass of sufficient thickness to make a double achro-
matic lens of so large an aperture, two, of suitable focus, may
be combined, either close together, or separated, as in the com-
pound illuminator for opaque objects in the Amician engiscope.
It is absolutely necessary that the illuminator should have a
large angular aperture, for the intrinsic brightness of the
image of the sun formed by it will depend upon this circum-
stance. If its aperture remains five inches, but its focus is
lengthened say to two feet, instead of one, in order to suit the
convenience of the workman, its effect will be very greatly
impaired, for its illuminative power will be reduced to one
quarter ; because the light refracted by it, when drawn to a
focus, will be found to give an image of the sun of twice the
diameter of that given by the lens of one foot focus, and (the
superficial contents of circles being to each other as the square
of their diameters) the original quantity of light being diluted
and spread over a surface four times more extensive than
when the focus was only one foot, can only have one quarter
of its original intensity. » I have said this much, because it is
well known that the light of a telescope with any given power
is always in the ratio of the square of the aperture of its object-
glass, without any regard to its angular aperture j and many
persons may think that the illumination of the lens of a solar
microscope, which also acts upon parallel rays, is in a like
predicament. Be it known that I never have myself used an
achromatic illuminator ; but I have so constantly felt the
want of one, that I do not scruple to recommend it as almost

SOLAR ENGISCOPES. 87

as necessary to the perfection of the instrument as that the
object-glasses themselves should be achromatic : for in order
to get the instrument to do its best on test-objects, it is fre-
quently necessary to cause the spectrum of the illuminator, or
its focus, to fall exactly upon the object-glass ; and in this
case only a small portion of the middle of the spectrum is
sufficiently free from colour to be used, the edges being highly
prismatic ; therefore, the instant the axis of the illuminator
and that of the object-glass cease to coincide, either from the
motion of the spectrum of the sun, or from its being thrown
obliquely through the object, the whole field of view becomes
full of colour, and the achromatism of the optical part is, in a
certain sense, thrown away.

It may be thought by some that I have here made out rather
too strong a case in favour of achromatic illumination ; it is
certainly a matter of taste and opinion whether it is as needful
as I suppose or not, for a chromatic spectrum does not operate
upon the traits of an image as a chromatic object-glass does,
causing a blur or confusion in them, but merely as prismatic
light would upon any other sort of picture on a white surface ;
that is to say, tinting it with all sorts of rainbow hues where
it should appear of its natural colours ; just as some of our
modern artists, for the sake of eifect, and more particularly to
catch and strike the senses of the vulgar, daub their pictures
with the most insane and ridiculous colouring, and that with
no sparing hand, but with the pallet-knife itself. Such virtuosi
would probably think that I am recommending rather a dete-
rioration than an improvement on the simulachra of solar
microscopes, in wishing to render them achromatic at all points;
or, in other words, to cause them to exhibit every thing in its
true and proper colours.

Indeed, I think no harm whatever, but much good, might
be effected by curing the spherical as well as the chromatic
aberration of the illuminator ; for in this case such a conden-

88 THE MICROGRAPHIA.

sation of its light at the focal point may be expected to take
place as will preclude the necessity of employing a condensing
lens for the purpose of rendering the light of the illuminator
still more intense and effective upon proof-objects. This is a
point of great importance, for if a common uncorrected lens is
used with an achromatic one for this purpose, it will greatly
impair the achromatism of the marginal part of the primary one,
though still, on the whole, it may do good.

The body next the illuminator is of a conical form, but has
a cylindrical joint with a bayonet catch at (/), to receive that
part of the main tube which carries the stage and the rack-
work appertaining to it : this consists of a tube moving within
the exterior one, by means of the rack and pinion seen at
(n in): the end of this tube is closed, and upon it is mounted
an ordinary slider-holder, (o,) which is fixed to its place by
means of a stud passing through a niche in what may be
called the stage, and turned a little way round, just upon the
principle of that in the Aplanatic Engiscope, described in the
" Microscopic Illustrations :" on the reverse of the stage is
placed a condensing lens, of about two inches focus, and an
inch and a half of aperture*, the frame of which is furnished
with a sliding wire, passed through a hole in the stage formed
by a piece of tube sprung at the sides in the ordinary manner,

* This condenser ought of course to be achromatic, or it will require
the most unwearied attention to keep the field free from colour when it is
employed (which, indeed, it is but barely possible to effect) ; however, I
believe it may be altogether dispensed with, if the primary illumina-
tion is achromatic. As to those lenses usually placed at the back of the
object in the ordinary solar microscope, for the purpose of enlarging the
circle of light on the screen, I am happy to say I never felt the want
of them with solar engiscopes, which is fortunate, for had they been indis-
pensable, there would have been the same necessity for making these
achromatic also as the others. If, however, it should be thought proper
to retain the said field-glasses for the sake of the instrument, when
converted into a solar for a room, they can of course be inserted
between the plates of the slider-holder, immediately behind the
object.

SOLAR ENCISCOPES. 89

so that the lens may be made to swing from one side of the
tube to the other, and also to approach or recede from the
stage. The upper part of the tube, to which the stage is
affixed, is cut away on each side, in order to permit the said
lens to be moved to-and-fro. The slider-holder is made with
double plates, but with two pillars only, that it may admit of
a slider or glass trough of the largest size it can receive, in
order that aquatic insects may be immersed in a sufficient
quantity of fluid to protect them from the heat produced by the
condensation of the calorific rays of the sun, and that large
sliders may have sufficient space to be moved about in, &c.

The stage is also furnished with the usual apparatus for
confining bodies flat to it, consisting of a pair of arms with
wires attached to them, moving in sprung tubes beneath, and
also with aquatic and dry live-boxes, &c. I may observe
that, instead of having one tube to work within another for
the purpose of adjusting the focus, the stage might be made
to move upon a bar in the ordinary way, and perhaps this
may be the better arrangement. It will be remarked in the
drawing, that a second tube of smaller diameter, slit open at
the sides, is screwed upon that in which the stage moves : this
is marked p p p : into this again the optical part of the en-
giscope (q) is made to slide ; the object-glass of which is shewn
at (&.) The focus may of course be roughly adjusted, by sliding
the body backwards and forwards in its containing tube, be-
fore it is attached to the camera (fig. 11) : but when this has
been done, it must of course remain immoveable. I look
upon it as a principle in the solar microscope, that the mag-
nifier or object-glass should not be moved, but always remain
at a fixed distance from the illuminator ; and as 1 have already
observed, when it is intended that the illumination should be
a maximum*, the focus of the illuminator should fall upon the

* If (the diameter of an object-glass being given) we wish to know
what kind of an illuminating lens we should apply to it, so that the light
of the picture formed should be a maximum, and bear the utmost possible

90 THE MICROGRAPHIA..

object-glass or magnifier, for then the whole of the light will
be sure to pass through. If it is intended to reduce the intensity
of the illumination, then the magnifier, &c. should be placed
within the focus according to circumstances, and any portions
of the edges of the spectrum may thus be cut off at pleasure.

If the tube (p p p) is unscrewed and removed, another ap-
propriate mounting may be used in place of it, and to
this the various object-glasses may be affixed without the body
or eye-glasses. The instrument then becomes a solar micro-
scope of superior construction, and the image may be thrown
upon the wall of an apartment, the whole apparatus being of
course removed from the frame («), and attached to a window-
shutter in the usual way, and the room darkened.

In the case of the engiscope, however, the end of the body
(q) is pushed into its receiver at (r, fig. 2), which communicates
with a conical tube of brass, having a rectangular prism with
its reflecting side silvered, 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, or on a
surface of plaster of Paris duly curved to suit its shape. The
camera is constructed with windows (v v), so as to admit two
persons to look at a time, and might also be made for three ;

amplification, we have only to select a lens which shall farm an image of the
sun of the same diameter as the object-glass, and give it the largest angle of
aperture piosible ; or if a condenser is employed, we may use a lens of long
focus and large aperture in the first instance, and condense the image
of the sun by a smaller one into the required compass. I should appre-
hend that an illuminator of twelve inches aperture, and two feet focus,
would, as a single glass, give a maximum of light with the lower powers
used in common solar microscopes ; for the higher ones a condenser would
be necessary, to enable the whole body of light to clear the small apertures
of the object-glasses, and proceed to the picture.

We. may draw the following inference from what has been here ex-
plained, viz. that it would be very easy to make a solar instrument on a gigan-
tic scale with an illuminator of enormous actual aperture, which, nevertheless?
would have no more light or intrinsic brightness in its image than a much smaller
one, scientifically made.

SOLAR ENGISCOPES. 91

but I am afraid this would cause the eyes of the observers to
be somewhat too far off from the image to see its minutiae
with sufficient exactness ; (w w) are two pieces of wood carved
out to fill the slope of the upper part of the face ; one is repre-
sented in a plan separately (at 4) in which the aperture (y) is
seen, and that part of the block which closes round the nose.
I have found it necessary to exclude the breath from entering
the camera, as it dims the eye-glass of the engiscope, and thus
spoils the image.

The sides of the said camera (11 u) are made to remove at
pleasure, to permit the observer to draw the image; some
black drapery being hung on in place of the sides, to exclude
extraneous light, while the hand is admitted through a cuff
made in it. The legs of the camera (x x) are made to unscrew
at pleasure, others of greater length being used in lieu of
them, when it is necessary to elevate the head of the camera to
the height of the body of the engiscope when equipped with
the opaque box ; but blocks of wood with holes in them may
of course also be used for the same purpose. The whole of
the exterior of the conical brass tube and camera should be
well blacked, or lined with black silk velvet. And the totality
of the apparatus of the solar engiscope should be placed on a
very firm solid table, in an apartment on a ground flooi-, in a
situation where the tremor occasioned by the passing of car-
riages, &c. cannot be felt.

Fig. III.

Is the opaque apparatus of the solar engiscope, differing but
little from that commonly made ; when used, it applies to the
conical part of the body shown in Fig. I, by means of the
bayonet catch at (I), as the transparent apparatus did ; (r) is a
plane mirror govevned by the adjusting screw (s), to throw up
the light of the illuminator to the object (o), which is adapted
to the focus of the object-glass (k), by means of the adjusting

92 THE MICROGRAPHIA.

screw (t in), which causes the stage which carries the object
to move backwards and forwards in a socket, framed in the
back of the opaque box. However, I believe it will be found
that with such powers as can be used to give images of opaque
objects, no adjusting screw will be necessary, for the object
may be pushed to-and-fro with the finger with sufficient ex-
actness. The stage is formed by a piece of cork covered with
black velvet, to which insects may be fastened by means of the
pins used to keep them in their Cabinets ; as may also other
objects similarly mounted, viz., on corks having pins passed
diagonally through them : discs also of blackened wood may
be made to fit into the stage, in place of the cork one, for re-
ceiving insects ; and a sort of stage will be found very useful
which merely consists of a little round shelf placed rather
below the axis of the optical part ; on this any thing may be
laid for exhibition.

I have supposed the little door of the opaque box to be re-
moved in the drawing, or to be transparent, in order to let the
apparatus be seen.

(p p p) is merely the tube into which the body of the en-
giscope (§) is inserted as before. It may be removed, and a
simple object-glass affixed in an appropriate mounting (atm);
the instrument then becomes a solar opaque microscope, and
is managed as such.

METHOD OF MANAGEMENT EXHIBITION OF TEST OBJECTS, &C.

It will be remarked, that the necessary directions for ma-
naging the instrument are mixed up with the description of its
various parts : I think, therefore, little more need to be said,
except a few remarks on the exhibition of transparent proof-
objects, for little can be done in the way of shewing opaque
ones.

The management of the illumination is, perhaps, more easy

SOLAR ENGISCOPES. 93

in the solar engiscope than in any other ; for the light of the
sun seems to have the power of bringing out all the lines and
markings in tests when thrown perfectly direct through the
axis, or., where this does not happen to be exactly the case,
the natural motion of the sun, if left to itself, soon renders
the illumination oblique, and if the lines happen to he in a
•proper disposition, with respect to it, brings them out as a
matter of course. The line in which the light travels may
always be discovered by putting the object out of focus, ac-
cording to the directions I have given for the management of
proof-objects in the Amician catadioptric engiscope ; and by
turning the slider about, the lines of some of the scales may
always be placed at right angles to the direction of the rays
For this purpose, however, it will be advisable that the slider
should be very short, and have only one hole in it, otherwise
there will not be room for it to turn round in the tube (i).
The condensing lens behind the stage being moved a little on
one side, also gives oblique light.

When an object-glass of rather long focus is used, say of
two inches, which is a very pleasing magnifier for ordinary
objects, the slider-holder should be removed, the object or
slide laid flat against the stage, and confined to it by means
of its arms ; because, in this case, the object-glass may still
remain nearly in the focus of the illuminator, which will be at
one foot distant from it : if, on the other hand, the body of
the engiscope is drawn farther out to adapt itself to the slider-
holder, a great loss of light will be the consequence, because
the rays from the illuminator will have begun to diverge be-
fore they reach the object-glass.

I have frequently been surprised at the great condensation
of the solar rays to which the transparent proofs may be ex>
posed, without burning or apparently affecting them in any
degree. I have in the winter, when the sun was low, con-
densed the whole of the light furnished by the illuminator of
five inches aperture and one foot focus, by the other behind

94 THE MICROGRAPH IA.

the stage of about two inches focus ; and very nearly in the
focus of the latter I have kept the podnra for half an hour
together, which seemed to resist its action just as cobwebs
will do, being, I suppose (like cobwebs) too fine and delicate
to arrest a sufficient portion of the calorific rays of the sun to
produce combustion. I do not recollect ever burning a trans-
parent proof-object, though I have the slider which held it :
but I hardly ever began to exhibit opaque objects without
meeting with some accident of the sort ; — they can seldom be
well shewn without being placed in jeopardy ; for it seems to
be necessary, in order to get a good picture of these bodies,
that the focus of the illuminator should fall exactly upon them :
it will not answer to place them either within or without the
focus, for the purpose of securing them against the danger of
combustion ; we must, therefore, ascertain, a priori, what
degree of heat the object we are about to exhibit will be likely
to endure, and adapt that of the spectrum to it, by cutting off
some of the aperture of the main illuminator, by means of
rings of pasteboard placed between it and the mirror. It
may sometimes also be advisable to have recourse to the
same expedient with transparent bodies, instead of placing
them within the focus ; and it is, perhaps, the more scientific
mode of proceeding of the two, though in the latter case both
may be had recourse to. It would, however, be still better to
have a spare achromatic illuminator, of the same aperture, but
twice the focal length of the other, with an extra joint adapted
to lengthen the body, when it was used either on very inflam-
mable opaque objects, or in very hot weather, or in a hot
climate, on delicate transparent ones.

With good object glasses of short focus and large angular
aperture, of from l-10th to l-18th of an inch focus, and about
55° of aperture, a picture of all the markings on the podura
may be distinctly exhibited, and likewise the longitudinal
lines and their cross striae on the brassica ; but I have never
yet seen a picture of the diagonal lines on the latter, though I

REFLECTING SOLAR ENGISCOPE. 95'

do not altogether despair of doing so. I have carefully stu-
died the pictures formed by deep achromatic triplet object-
glasses acting with Huygherian eye-pieces, and when the said
object-glasses were really good, have found their images,
according to my oivn taste in such matters, better finished
and more perfect than those given by common doublets and
triplets of equivalent power. The latter, however, perform
very well also, and give the more luminous image of the two ;
and so will also single magnifiers. I have tried against each
other a very fine l-60th of an inch lens of glass, (equi-convex,)
a doublet of l-70th, and a triplet of l-50th, and excepting the
difference in the size of the image, could not say that 1 per-
ceived any sensible superiority in either. The engiscope I
have mentioned with the object-glass of l-18th inch focus,
though its light was inferior charged with the same powers,
by its mere achromatism instantly effected a great improve-
ment in the picture for the better; it moreover resolved several
of the scales of the podura which the doublets and triplets did
not, and shewed the cross striae on the brassica in a much
more decided manner than they did ; the said objective, not.
withstanding, was far from being a perfect glass, having a con-
siderable quantity of uncorrected aberration of the convex
kind, and rather too short a focus to be achromatic.

REFLECTING SOLAR ENGISCOPE.

It is obvious that a reflecting solar engiscope on the
Amician principle may very easily be constructed, by applying
the body and bar of the catadrioptric instrument, described
already in this work, to the top of the camera (fig. 2) in lieu
of the conical brass tube, the illuminating apparatus remaining
as before, and the aperture in the side of the tube containing
the metals being posited precisely where the object-glass of

96 THE MICROGRAPHIA.

the refractor is, or, more strictly speaking, where a single
object-glass would be, if the instrument was in action as an
ordinary solar ; I have used an instrument made precisely in
this way, only the body and bar were not attached in any way
to the illuminating apparatus, but only adjusted so as to coin-
cide well with it. It thus makes a very pleasing instrument ;
but will only shew the most easy lined objects, from want of
power in the objective part, which, as it is well known, cannot
be made of less than 3-10ths of an inch focal length, for it
requires a very great depth and poiver in an object-glass, or
metal, as well as a very considerable degree of perfection, to
lender the niceties of proof-objects visible in a picture formed
of them ; moreover, the reflecting engiscope is well known to
be much darker than an achromatic instrument of equivalent
power and angular aperture ; and there is another unfortunate
circumstance attached to it (the reflector) when used as a
solar engiscope, viz.'the hole in the side of the tube, which is
always much smaller than the object metal ; for in order that
the illumination should be a maximum, the spectrum of the
illuminating lens must fall upon this hole, and clear it, as it
does the object-glass in the refractors, which are of course of
much larger diameter than the said hole, and therefore can
admit a much greater quantity of light.

Opaque objects can hardly be depicted at all by the re-
flector; and as the metals of long focus are usually made of
small angles of aperture, merely to create space between the
sides of their containing tubes, for the purpose of illuminating
opaque bodies, it would be advisable if a regular reflecting
engiscope was tried to make them with as large angles of
aperture as possible, because they can, in the said instrument,
only operate on diaphonous subjects: thus, that of two inches
focus should have an inch at least of aperture, instead of half
an inch ; that of an inch focus, half an inch, and so on, in
order to gain as much light as possible. After all that could

HYDRO-OXY ENGISCOPE3. 97

be done, however, I am afraid a refractor would be sure to
beat it hollow*; therefore I shall take my leave of the subject,
as I cannot conscientiously recommend such an instrument.

OXY-HYDROGEN ENGISCOPES.

These instruments are constructed by applying the optical
part of an achromatic engiscope, with the camera already de-
scribed, in lieu of the magnifiers and screen used with the
ordinary oxy-hydrogen microscope ; but retaining the illumi-
nating and other apparatus belonging to the latter instrument :
in short, the construction must be so obvious to the artist,
that I conceive it perfectly superfluous to say any thing far-
ther on the subject. Whatever advantages are possessed by
solar engiscopes over solar microscopes, will, in like manner,
be retained by oxy-hydrogen engiscopes, or oxy-hydrogen mi-
croscopes ; but there is a milkiness, or ivant of serenity and
clearness, in the light produced by the action of the oxy-
hydrogen blowpipe on lime, which renders it very inferior, in
the exhibition of the superior order of tests, to that of the sun.
How, indeed, can we expect it should be otherwise ? I am
afraid this would still be the case if the light of the lime
could be rendered as intense and brilliant as that of the great
luminary at his proper distance from us.

Much has been said about the superiority of the hydro-oxy
instruments, in enabling us to dispense with the uncertain sun
of our northern climates, and about solar microscopes re-
quiring our confinement during the finest part of the day, &c. ;

* A friend of mine, who was perfectly qualified to make such experi-
ments, assured me that he had constructed a solar microscope, with
metals on the Amician principle, used without a body or eye-glasses, and
exhibiting an image in the usual way on the wall of an apartment, and
found that it exhibited a variety of test objects in a highly satisfactory
manner. I have myself never tried the metals in this way.

H

98 THE MICROGRAPHIA.

but we must not forget that in warm climates the light of the
sun is nearly certain for months together ; and so fiercely is
shed " the intolerable day/' that we are kept at home under
penalty of a knock on the head from this mighty body.
Under these circumstances, more especially where every pud-
dle swarms with life, and the whole atmosphere with the most
brilliant and curious insects, I humbly think a solar micro-
scope or engiscope, managed by a couple of slaves, who might
easily be trained to exhibit it, would have as many charms as
a siesta; unless, indeed, the heat should prove so oppressive
as to incapacitate us for all rational amusement, and compel
us to sleep, whether we will or not. People cannot have
their cake, and eat it too ; and if they will sleep by day, they
must lay awake at night ; I therefore recommend the inha-
bitants of warm regions, whether black, brown, whitey brown,
toad-coloured, red, yellow, or white, to betake themselves to
these instruments, and try their eifect in killing time during
the heat of the day, in conjunction, of course, with cards,
hookahs, dancing girls, &c. &c, and never to forget their
obligations to me for suggesting to them a new species of
pleasure.

C. R. G.

CHAPTER V.

ON TRYING

MICROSCOPES AND ENGISCOPES

AGAINST EACH OTHER ;
WITH RULES FOR ASCERTAINING THEIR COMPARATIVE MERITS.

Trying instruments against each other may be thought a
mighty simple affair;, and that very little can be said or written
with propriety on such a subject. As much may be asserted
of running horses against each other : nevertheless, horse-
racing has its rules and regulations, established by the joint
consent of the sporting world, many of whose ordinances may
appear at first sight not a little unjust and absurd. Thus,
that two horses matched against each other should not carry
equal weights on account of a difference in their age, and that
such weight shall be determined by a scale nearly or alto-
gether arbitrary ; that a horse shall be considered beat because
his opponent happened to come in first by some small fraction
of the length of his head; or that the most trifling loss of
weight in the rider, such as that of his whip, shall cause the
horse to lose, though he was to distance his adversary by a
hundred yards at the goal, and the like, certainly must be
considered very vexatious laws ; and though they are as fair
for one person as another, it may be truly said they are fair for
nobody. However, when such consequences attend the loss
or winning of a race as the loss of perhaps twenty or thirty

100 THE MICROGRAPHIA.

thousand pounds, and the utter ruin of one man to the making
of the fortune of the other, it is evidently necessary that the
most minute circumstances should be taken into consideration,
and that every possible contingency should be determined in
some way or other, so as to place the matter beyond contro-
versy or appeal, or every race-course would present nothing
but a scene of angry contention, and of quarrels which only
an appeal to arms would settle.

Now let us suppose that a couple of microscopists were to
pit their microscopes or engiscopes against each other, to
shew some particular proof object ; and in order to increase
the interest of the trial, should bet sums of money on the event;
how, and in what manner, are we to determine which instru-
ment shews the said object best, and which of the parties is to
lose his money on the occasion? We cannot for a moment
suppose that either of the parties will have such excessive
candour and complacency as to give up the point when it
should be given up : no, every man is almost sure to make an
idol of his oivn instrument, be it what it may. If the case
resolves itself into a matter of opinion, one man's is at least
as good as another's ; and an ounce of a man's own wisdom is
always worth a tun of his neighbours. Is it not most notorious
that all savans, inventors, and projectors, have, time imme-
morial, regularly puffed their own productions, and decried
every thing else with scarcely a grain of candour ? The most
trifling mote in the works of others they can see, and expose
and magnify ; but they cannot and will not see the beams in
their own.

Under all these circumstances I think we must begin by
laying it down as a rule that no man's judgment is worth a
straw, relative to his own instrument, even in a case where
there is no money to be lost or won by it. In the case pro-
posed above it would be necessaiy to appoint two indifferent
persons as judges ; and if they cannot agree, to refer the
matter to a third, as umpire, whose judgment should be final,

ON TRYING MICROSCOPES AND ENGISCOPES. 101

and without appeal. If it should be asked, of what description
should such persons be — should they be profound opticians or
microscopists ? — I should answer, that any individuals whose
sight is perfect are competent to the task ; the more so, per-
haps, if they never looked into a microscope in their lives, for
then they are likely to be without prejudices of any kind. I
was myself, when I first began to reform microscopes, very
much in the habit of taking the opinion of an actress on them ;
and her decisions were anything but agreeable to me, though
I believe they were perfectly correct. Whatever objections
there may be to the owners or inventors of particular instru-
ments being allowed to have any voice in determining their
specific merits, they are the most proper persons in the world
to have the management of them, as they will be sure to
display their powers and properties to the greatest advantage,
and make them so put forth their whole mettle as certainly as
a man will who rides his own horse, and has betted money in
his favour on the race-course : therefore it is not necessary for
the judges to understand a tittle of the method of managing
microscopes or engiscopes, all of which is to be left to the
commanders of them : the said judges have nothing to do but
to look at the object when duly prepared for their scrutiny.

Having pitched upon judges, it remains for us to ascertain
by what laws they shall be governed in their decisions ; for it
is impossible to give a sound judgment in any case without
reference to certain principles ; even a common game at cards
cannot be lost or won but according to certain rules.

I shall lay down what 1 consider the laws by which micro-
scopes and engiscopes ought to be judged beaten or victorious ;
they who do not relish them can establish others, which they
may consider more equitable ; for laws there must be, of some
kind or other, before we can proceed a step in our decisions.

102 THE MICROGRAPHIA.

SECTION I.

CODE.

Art. 1. — That instrument is the best which shews the dif-
ferent details of objects most decidedly, and with the greatest
clearness and perspicuity* ; no matter of what nature or kind,
or whether it is chromatic or achromatic, planatic or aplanatic,
in or out of adjustment, or whether its lenses are well or ill

* The capability of counting accurately the number of any particular
traits or objects, developed both in telescopes and microscopes, has been
adduced as decided evidence of the goodness of these instruments, and
their power of shewing things in a, decided and forcible manner. I know
not exactly how the truth may be ; of this I am certain, that I have seen
objects as I should say with the very intensiva of distinctness, and yet I am
quite confident I could not have counted them truly : perhaps I have not
acquired the habit of doing so, or may have some natural incapacity on the
subject ; for I have frequently found myself unable to count aright the
number of sheep in a meadow, when not amounting to more than three-
score. I have heard graziers assert that considerable practice is neces-
sary in order to be able to count sheep and herds of cattle accurately.

With respect to lined objects, I have, in the ''Microscopic Cabinet,"
p. 179, given it as my opinion, that they are seen to the greatest advantage,
and in the most perfect style, when the spaces between the lines are/ilear,
and the lines themselves dark and strong, as if drawn with a pen and ink
on white paper. When scales and feathers are exhibited as opaque bodies —
for example, a scale of the diamond beetle — with a high power, the lenses
should appear as furrows, or with distinct ridges, dark one side and light
on the other, as a ploughed field seen when the sun is low in the horizon,
and its beams play across the furrows. Where several systems of faint
lines cross each other in an irregular manner, the result will be an appear-
ance similar to the watering of silks and moreens. This may be verified
by applying two wire sieves, or two pieces of worsted gauze, to each other,
and holding them up to the light. The markings on some scales of the
podura seem to be of this description ; others seem mere points, arranged
so as at first sight to give the appearance of a system of lines, or of two
crossing each other. The studs on the skin of a boiled pullet, seen as
opaque bodies, removed to some distance from the eye, are not an inapt
illustration of this sort of optical deception ; those of the feathers near the
pinions, in particular.

There is certainly a very great difference in the degree of facility with
which different specimens of the podura may be resolved into lines, as

ON TRYING MICROSCOPES AND ENG1SCOPES. 103

worked,, or polished, or centered. If a microscope made out
of the lens in the eye of a stinking whiting would shew me some-
thing which I could not see ivith any other*, 1 should say it was
the best, and had beaten every thing else out of the field.

Art. 2. — The same identical object must be applied to each
microscope and engiscope, tried against each other : supposing
the objects to be the scales of insects, a drawing must be made
of the configuration of them, that we may be able always to
pitch upon one particular specimen.

Art. 3. — Casteris paribus, I should say, that instrument
which will shew an object perfectly with the lowest power is
the best : thus if one instrument, A, shews an object distinctly
and satisfactorily with a power of 200, while another, B, will
shew all its minutias equally well, though on a smaller scale,
with 100, I should say that B was the best: in this case I
suppose that when the power of each is made equal to 100,
the performance of both is not equal, but that B has the
advantage. But if the power of each was raised to 200, and
then A had the advantage, I should still say that B was the
best. I have often insisted on this point in my writings, and
assigned what I consider sufficient reasons for my assertions.

well as in the strength and plainness of their markings. A curious phe-
nomenon sometimes presents itself in some choice pet scales, having
straight lines from end to end, and two systems of oblique lines also, both
apparently grooved in a decided manner, as is the case with many I have
seen ; viz. one of these systems may be seen by looking directly, and the
other, without any alteration in the illumination, by looking obliquely into the
instrument. The same circumstance occurs with some other tests,

* There is a very ordinary object, which I can never see with any
instrument, which certainly we should expect to be visible by mere mag-
nifying power, with the common microscope ; — I allude to the edge of a
very sharp razor, presented to the axis in a line bisecting the thickness
of the back, so that the thickness of the said edge, or its apex, should
be truly shewn and measured, as it may be in a dull case-knife. I suspect
the edge of a very sharp razor is the nearest approach to a mathematical
line with which we are acquainted ; it seems to resist magnifying power
altogether, or at least such as we can apply to it.

104 THE MICROGRAPHIA.

Art. 4. — If two instruments, C and D, shew the lines and
markings on an object equally well ; but 0 shews the edge of
the scale or feather with the same adjustment of the focus
which serves best to bring out the lines, so that the outline and
the lines are simultaneously visible, and D does not ; then C
is the best.

Art. 5. — If one instrument shews the lines on any particular
test as if composed of an aggregation of dots or globules ; or
exhibits them broken, interrupted, and ragged, while another
shews them clearly made out, as veritable lines or stripes
drawn with a pen and ink ; the latter is the best. In the first
paper I wrote on test objects, I have given as many as five
degrees of illusive or false vision produced in the appearance
of the feather of the Morpho Menel'dus, by viewing them with
an object-glass deficient in defining and penetrating power. —
Vide Quarterly Journal, Vol. xxii. p. 265*.

Art. 6. — If two instruments shew certain lined objects as
transparent bodies equally well in all respects, but one will shew
them more or less evidently as opaque objects also, while the
other will not, then it has clearly the advantage over the other.

N.B. — One of the best methods of exhibiting scales as
opaque bodies is to take away the disc of talc next the eye,
replacing the ring ; and then to attach a black wafer, or a bit
of black paper, to the reverse of the remaining piece of talc, to
which some of the scales will generally adhere. In this case
I have supposed the scales under consideration to have been
mounted in the ordinary way, in slides or circlets, as trans-
parent bodies ; and that we wish to be sure of seeing some
of the same scales we have seen as transparent bodies, as
opaque ones also. If the upper piece of talc is allowed to
remain, it will prove a considerable detriment to vision ; but
still it will be as unfavourable to one instrument as to another.

* See also " Illustrations of the Effects of Aperture,'' &c. &c. in this
work.

OF TRYING MICROSCOPES AND ENGISCOPES. 105

Art. 7. — If two instruments should prove equal in all other
respects, but one is achromatic and the other not, that which
is achromatic has of course the advantage, as it will shew the
objects perfectly free from false colouring.

Art. 8. — Tf two instruments seem to shew ti*ansparent bodies
equally well, but one of them, when tried upon opaque ones,
has a slight fog of the diffused kind over its whole field, or a
penumbra or nebulosity encircling numerous points, to a
certain distance from them, while the other is free from this
illusion, that which shews the opaque objects best is the
winner, for opaque objects are still more severe tests than trans-
parent ones.

Art. 9. — Whenever one instrument happens to shew some
marking or feature in an object not visible in another, and the
owner of the latter attempts to get rid of this, by asserting that
it is an illusion produced by some defect of his antagonist's
instrument, let him be made to 'prove his assertion to the satis-
faction of the judges, or the umpire ; and if he cannot do so,
let him be turned adrift as a scamp and blacklegs. Let us
suppose, for example, that the trial was the wheel animalcule
(vorticella rotatoria) ; and that in the instrument said to
exhibit an illusion two points were visible near the head of
the animal, when in its grub form, with the wdieels withdrawn,
(supposed by some naturalists to be the eyes of the animalcule)
I should say that the illusion here was in the instrument, which
did not shew them ; for a false evidence may be given by
suppressing facts, as well as by coining false ones.

There are several minor points which might be insisted upon ;
such as that instruments which have a strong natural light,
combined with a large field of view, free from distortion, and
equally good all over, have the advantage over such as cannot
pretend to these pi'operties ; but as such instruments no not
seem to possess any absolute power of shewing anything
more or better than others, I shall lay small stress on these
properties.

106 THE MICROGRAPHIA.

When we wish to ascertain which instrument is most perfect
in point of figure, achromatism, or adjustment, &c. I have
laid clown abundant rules for this purpose in Chap. XIX. of
the " Microscopic Cabinet," and shall merely therefore remark,
that if we wish to ascertain which instrument is constructed
on the best principle, and what kind of microscopes or
engiscopes are made on the most effective plan, in this case
the instruments to be tried against each other should have their
object-glasses or metals of exactly the same angular aperture
and focus ; their bodies also should be of the same length, and
their eye-glasses of the same nature, description, and magni-
fying power. When doublets, or triplets, or single magnifiers,
are tried against each other, their power and angular aperture
must also be scrupulously equalized ; or if they are tried
against engiscopes, their power and aperture must also be
made equal to theirs.

In my paper in the " Microscopic Cabinet," page 202, line
22, nearly a whole paragraph is wanting ; which unfortunately
changes the sense of the context, and makes it read very ill.
This being part and parcel of my present subject, I shall here
introduce, as well as I can, from memory, not having the
manuscript by me.

" It must however be recollected that single and compound
magnifiers., the diameters of which exceed that of the iris of the
eye, for the time being, cannot have their aberrations, either
chromatic or spherical, estimated in the manner detailed above ;
because the said iris, or moveable curtain, of the eye, cuts off
their aperture to its own standard, so that only a certain central
portion of them, equal to it in diameter, can be tried. When
their foci are too short to be tried on opaque objects, their
defects and aberrations may be looked into by viewing the
penumbra of transparent objects put out of focus ; or rather
the penumbra of some body which is not transparent by inter-
cepted light proceeding from behind it." Then should follow
the concluding sentence of the paragraph.

ON TRYING MICROSCOPES AND ENGISCOPES. 107

Since writing the paper I have alluded to, in the " Cabinet,"
I have discovered an object which will be found very useful
as a test of aberration, when the focus of an object-glass, or
of a single or compound magnifier, is too short to allow us to
see a globule of quicksilver with it as an opaque body.

Stick two pieces of talc together with a little Canada balsam,
in such a manner as to include some small globules of air
between the plates ; this will be very easily effected : when
these are viewed by intercepted light, they appear as very small
discs perfectly black, having a luminous point in the centre,
which has almost the appearance of an artificial star, only
not so bright : by putting this within and without the focus
(always taking care that the illumination, which should be day-
light, is reflected by a plane mirror, is thrown directly through
it, otherwise the luminous spot will not be centrical), the state
of an object-glass or magnifier, as to chromatic and spherical
aberration, adjustment, and centricity, &c. can be as well
looked into as by a globule of quicksilver, and the pretensions
of different constructions scrutinized and brought to the grind-
stone.

Another object of a similar nature can be produced by
mixing together some oil and water in an aquatic box, and
agitating the two fluids until the water is dispersed into very
minute globules, surrounded by the oil ; in this state they
closely resemble the air bubbles, surrounded by the balsam.

SECTION II.

It will probably be expected by the public that I shall now
take upon me the disagreeable and invidious task of favouring
the world with my own particular notions of the comparative
merits of the different sorts of microscopes and engiscopss ; an

108 THE M1CROGRAPHIA.

office which I am afraid I shall execute to the satisfaction of
no one human being, while I shall be certain of putting the
noses of a great many completely out of joint : I would as
soon undertake to give a comparative estimate of the beauties
and perfections of a dozen different women, to the satisfaction
of the ladies themselves and their lovers. I really think I have
gone far enough in laying down a code of laws on the subject,
which others may put in execution, (at least if they like.)
If, however, I may be permitted to proceed to judgment, ac-
cording to my own code, I will endeavour to do so : I will not
hazard any mere opinions, if I can help it, but merely state facts,
and draw {if possible) legitimate inferences from them ; and
if they should happen to be at variance with statements here-
tofore published by me, 1 request my reader to consider those
now to be given by me as my most mature conclusions, revised
and corrected, with the whole attention of which my mind is
capable, according to the most recent improvements in the
different instruments. Though, as I have already asserted,
there is no microscopist living who may be expected to give
a perfect and absolutely unbiassed criticism on the matter in
question, yet I may be as well qualified for the task as most
others ; for I have been principally concerned from the very
beginning in carrying iuto effect the various reforms in the
structure of instruments which have made them what they
now are, and may therefore regard them all with equal favour.
If I have never interested myself much about compound mag-
nifiers, it was because I conceived that a very great man had
taken up the subject before me, and exhausted it, leaving op-
ticians nothing to do in the branch, but to carry his theories
and combinations into effect to this best of their ability.

As the subject is one on which much stress is laid by many,
I shall here take into consideration the comparative illumina-
tive power, or intrinsic brightness, or light, of various instru-
ments ; though I think it will be found in the sequel not to
merit much attention, as far as microscopes are concerned.

ON TRYING MICROSCOPES AND ENGISCOPES. 109

If we take a piece of any well-polished plane glass, placing
a piece of black velvet, or some other dark substance, behind
it, to prevent the image from being confused with the rays
which it refracts from surrounding objects — or, what is still
better, a piece of black glass, having the same reflecting power
as the white, and view the picture which it forms of a land-
scape, or any thing else reflected, as nearly as possible at
right angles by it — we shall be surprised at the quantity of
light arrested by a single surface only, for in the image may be
distinctly recognized every object seen in the landscape by the
eye, together with its colour ; only every thing is a vast deal
darker than in nature, the difference being apparently about
as great as the light of a twilight evening is to that of meridian
day. Now, when we reflect that eyery polished glass surface
reflects an equal quantity of light, we may wonder how we
see objects at all in an instrument composed of many glasses,
and may be very much inclined to think that the fewer glasses
we see through, the more perfect must be our vision ; that, in
short, single lenses must inevitably be the best instruments
after all ; and such was the opinion of Leeuwenhoek and all the
early microscopists. Lenses formed of gem, especially dia-
mond, must be much darker than glass ones, from the greater
reflective power of such substances, which defect, however,
ought (if there be any truth in analysis) to be much more than
compensated for by their high refractive and low dispersive
power. Jewelled simple microscopes, then, (a priori at least),
should seem, upon the whole, to stand upon the top of the
scale ; but as we here only consider illuminative power, they
certainly occupy only the second rank, or, if coloured, as is
the case with many of them (sapphire and garnets, for example),
a still lower one. Doublets must of course arrest twice as
much, and triplets three times as much light as single lenses
iormed of the same glass ; and an engiscope, consisting of four,
five, or six glasses, &c. four, five, or six times as much, under
any given illumination.

110 THE MICROGRAPHIA.

The question of the comparative light of reflecting and refract-
ing instruments has been often agitated, and generally deter-
mined in a very unfair way by the champions on both sides.
The amateurs and makers of refractors, led away by their
hatred and contempt of the reflecting tribe, have gone the
length of asserting that an achromatic engiscope, having a
sextuple, or two triple object-glasses, has fully four times as
mnch light as a reflector of the Amician construction, having
the same power and angular aperture ; and that another having
an octuple object-glass, composed of four double achromatics
not cemented together, has still more light, ceteris paribus,
than the sextuple one* ; a quantity of light being, I suppose,
gained, not lost, by the refractions of the two extra glasses.
I merely quote these opinions to shew to what lengths a
bigotted zeal for any particular kind of instrument may carry
even very clever opticians. The opposite faction, exasperated
at the quantity of dirt and calumny thrown in their teeth, will
naturally take up the cudgels for their darling principle, and
retort in the same style upon the imperfections and blindness
of the refractors, until the affair becomes a contention for
victory instead of truth.

Let us see if there is no way of determining this question in
a natural and philosophic manner.

The late Mr. Charles Tulley, of Islington, (whose like we
shall not see again in a hurry, and than whom I never met
with a more candid, gentlemanly, and scientific optician, ever
ready to do justice to every thing and every body), assured me
that he had been at great pains, and made many experiments,
to determine the comparative light of reflectors and refractors,
the results of which I shall state. His favourite method of ex-
perimenting (and I do not see how a better or fairer can be
employed), was to charge a reflector and a refractor with the
same power, and to direct both to a printed bill, placed at an

* A discovery of the late Mr. W. Tulley.

ON TRYING MICROSCOPES AND ENGISGOPES. Ill

equal distance from each, on a clear evening; and as the light
gradually diminished, to see how long he could read it in
each : in an instrument whose light or penetrating power was
superior to the other, he could distinguish the letters long
after the other had become too dark to permit him to do so :
but where the light of both was equal, or made so by re-
ducing the aperture of the brighter one, the power of reading
in both faded away at the same time as the twilight gradually
deepened.

After many trials made by himself in company with other
persons, he arrived at the conclusion, that a refractor having
a double object-glass of five inches aperture had as much
light as a reflector of the Newtonian construction of eight
inches aperture, ceeteris paribus ; and the light being in the
ratio of the square of the apertures was therefore as 25 to 64.
Thus, where the apertures of a reflector and refractor are equal,
the light of the former is represented by 25, that of the latter
by 64 : or it may be expressed thus, — the light of the reflector
is to that of the refractor as 1 to 2". Now, it can scarcely
be necessary for me to observe, that the Amician catadioptric
engiscope is a Newtonian telescope, made on a very small
scale, reversed, and that its light must therefore be the same,
with this difference, that the diagonal metal being much larger
than in the Newtonian, arrests much more light. Its pi-opor-
tional diameter varies in the Amician in different sets of metals,
and with it the size of the blot in the middle of the visual
pencil. In the pair of 55° of aperture, and in that of 13°, it is
one-third of the diameter of the elliptic one ; but in that of 36°,
one-half. Mr. Cuthbert has sometimes made metals of 41° of
aperture, with a focus beyond the tube ; in such the size of the
small metal is necessarily increased, so much as to leave only
the mere margin of the concave in operation : on this account
they are very dark indeed, as much so as their worst enemies
can wish ; but metals of this aperture ought always to be made
on the same plan as those of 55°, in which the slider is passed

112 THE MICROGRAPHIA.

through the tube, because in this construction the size of the
diagonal can be kept somewhat under a third of that of the
elliptic one.

I have thought it right to allude to these circumstances, be-
cause many would think it perfectly correct to pitch upon such
a pair of metals as I have described, as a fair specimen of the
whole. " Ab lino dit.ce omnes."

In the Newtonian telescope the diameter of the diagonal is
generally one-seventh of the other : thus, in one of seven inches
aperture the plane is one inch ; it therefore only arrests l-49th
part of the whole light, and this is of course considered in Mr.
C. Tulley's estimate aforesaid ; and if we want to know how
much darker the Amician engiscope is than the Newtonian
telescope, we have only to calculate hoiv much more light is
arrested by the diagonal in the one case than in the other. Now
where the plane is l-3d of the diameter of the concave, it
arrests l-9th of the whole ; but in the extreme case, where
it is one half, it stops one quarter: therefore, where the light
of a Newtonian is rated at 25, the light of the Amician will in
the first case be represented by 221, and in the latter by 18|.
But in order to obtain the light of the Amician, we must de-
duct the l-49th from the l-9th and l-4th of the light stopped
by the diagonals of the engiscope, and the account will stand
thus : —

1 1 _ 40

9 " 49 "~ 44f

1 J_ 45
4 "~ 49 "" 196

Therefore of rr 2U! deducted from 25, leaving

441 1

22f|f, represents the light of an Amician in an ordinary case,

compared with a refractor of the same aperture having its

light estimated at 64 ; and of — z= 5 7^, deducted

S 196 1

ON TRYING MICROSCOPES AND ENGISCOPES. 113

from 25, leaving a residue of 19-^ , represents its light in an

extreme case. Now if we compare the aforesaid numbers
against 64, the representative of the light of the refractor, the
results are, that the light of the reflector in an ordinary case,
compared with that of a refractor, is as 1 to 2 ~ ; in an ex-
treme one, as 1 to 3 — : and be it remembered, this is when
it is compared with a refractor consisting only of one double
object glass ; but there is a confounded deal of difference be-
tween the light stopped by four surfaces and that arrested by
twelve or sixteen, as will soon be discovered by any one who
experiments fairly ; and even when an object-glass consists of
two or three double ones, with their inner surfaces cemented
together, there is still a loss of light to a very considerable
extent beyond that occasioned by one uncemented double one*.
They who think I have not darkened the reflector suffi-
ciently can experiment for themselves, by trying refractors
and reflectors against each other in a clear evening, taking care
to place the instruments in the same aspect, so that the light
whereby they are illuminated shall come from the same part
of the sky which is always darkest at the zenith, and lightest
in the west, low down in the horizon, where a rich saffron tint

* According to the calculations and experiments of Sir W.Herschel, in
his paper " On the Power of penetrating into Space by Telescopes, &;c." —
(Phil. Trims, for 1800, p. 65,) out of 100,000 incident rays, 94,825 only
will be transmitted through a lens of glass of the ordinary thickness;
hence if two lenses are combined, only 89,918, which, subtracted from
100,000, leaves 11,082, the quantity which will be arrested by every
double object-glass, being somewhat more than one-ninth part of the
whole of the incidental light : therefore we must deduct l-9th for the light
lost by every additional double glass employed, which is a quantity equal
to the whole stopped by the small metal ofthe Amician, under order ordi-
nary circumstances. Sir W. calculates, that where a double reflection
takes place at right ayigles, as in the Gregorian telescope, out of 100,000
rays, only 45,242 will be reverberated, a loss of more than the half.
This statement agrees tolerably well with that of Mr. C. Tulley, but makes
the reflectors rather more luminous than he does.

114 THE MTCROGRAPHIA.

frequently remains for a long time after sunset. No illumi-
nating lenses or mirrors must be used ; the object-glasses or
metals must be directed to the same point of the heavens, and
the natural light of the atmosphere uncondensed and unaltered
by either refraction or reflection, the sole illumination used.
If these conditions are not fulfilled, the experiments made will
be good for nothing, as must be evident.

For my own part I consider the question of the comparative
light of reflecting and refracting microscopes and engiscopes
not worth agitating; and I think the following experiment
quite conclusive on the subject : — Take an Amician, equipped
with one of its darkest sets of metals and its deepest eye-
glass ; mount it for viewing some transparent test, or at least
something or another requiring great distinctness for its mani-
festation ; illuminate it by reflecting the light of the sky from
its plane mirror (an operation which of course reduces the
light), and you will find that dark as the instrument is, it is
more luminous than consists with perfect distinctness, and that
you may greatly improve the vision by using diaphragms under
the stage, reducing the light of the field several shades loiver.
Again, opaque objects we can brighten at pleasure, by com-
pound illuminators, and lamps fed by oxygen gas, &c. &c.

In telescopes the case is totally different, any defalcation of
their light being sure to be attended with a serious defect in
their performance upon all objects requiring penetrating power,
because we have no means whatever of increasing their intrinsic
brightness.

It will be found that a certain angle of aperture in metal is
quite equal to the same aperture in glass, as to its penetrating
power in developing the tissue of all sorts of lined or proof
objects, when aided by a proper illumination ; and I need not
say, that with respect to telescopes, such a position is entirely
false : therefore there is a wide difference between the relation
of the refracting and reflecting principles to each other, when
applied to these different instruments ; and the performance of

ON TRYING MICROSCOPES AND ENGISCOPES. 115

reflecting engiscopes is proportionally far superior to that of
reflecting telescopes * though it is most fully admitted that
their natural light or intrinsic brightness is greatly inferior to
them. The worst point about reflecting engiscopes, in my
opinion, is, not their darkness, but the brownness of their light :
this is very perceptible in reflecting telescopes also, when used
with high powers, and is in both instruments remedied to a
certain extent, by using eye-glasses of crown glass instead of
plate.

In the Amician, used with day-light, it causes objects, or
rather their images, to appear just as if they were depicted on
brown academy paper ; and the effect is very disagreeable,
compared with vision by refraction, which always shows
them as if executed on white paper. A connoisseur may dis-
cover merit in a drawing on brown or violet-coloured paper,
but the vulgar will hardly look at it - the prevailing tint dis-
gusts them at once.

It is, however, a most fortunate circumstance, that this
brownness in the light of reflecting instruments wholly disap-
pears, or at least becomes insensible, when they are used with
artificial light ; in consequence, they are thus seen to the
greatest advantage, and as we now have lamp-light brightened
to perfect whiteness by oxygen gas, for microscopic purposes,
and nearly equal to day-light for such objects as agree best
with it, the reproach of a brown image will be in a manner
done away with altogether from reflecting engiscopes.

* Whoever will be at the pains to look into the state of the figure of
the metals of reflecting telescopes, and compare it with that of the elliptic
metals of engiscopes, will find the latter (if they have been well executed)
far more perfect than the former ; for their figure being very nearly allied
to that of a sphere, and moreover having a great depth of curvature, can
always be kept from degenerating into that state of flatness at the edges
which produces over-correction by giving the outside rays too long — a de-
fect which, seems almost inseparable from reflecting telescopes.

116 THE MtCROGRAPHIA,

As Dialogue is that mode of writing in which a variety of
contending opinions may be most naturally brought forward
and answered, I have selected it as the best way of discussing
the comparative merits of instruments : if the reader must
know the names of the parties, I beg to inform him, they were
Tobias Oldbuck, Esq. the naturalist, and Mr. William Putty,
the optician. Lest I should be accused of putting nonsense
into die former gentleman's mouth, I must state that he only
expresses opinions current among the observers of this en-
lightened age, and which I have frequently encountered in
society.

I must positively introduce my personages to the reader.
Mr. O. is an old bachelor of about 60 (a descendant of the
Oldbucks, of Monkbarns). He always wears a very natty
wig, made by an eminent artist ; his clothes are of the true
scientific brown-study colour ; he has not altered the cut of
them, or the model of his hat, or any of his opinions on any
subject, for the last ten years, and declares he never will,
thinking it beneath his dignity either to learn or unlearn any
thing at his time of life. Having described his costume — that
outward and visible sign of a man's character, and which, in-
deed, conjointly with the periwig (which I have sufficiently de-
scribed), constitute the man — 1 do not think it necessary to
say any thing more about him, except that at one period of his
life he was far gone, and most intently engaged in constructing
achcromatic object-glasses for microscopes, which, however,
he abandoned as an impracticable bad job : when another
person afterwards succeeded in producing them, Mr. O.'s
pride was wounded in the tenderest place, for he considered
himself a perfect giant in all sorts of microscopic science ; in
consequence, the sight of an engiscope ever after turned his
stomach.

Mr. William Putty is an enthusiastic young optician, whose
talents are of the first order ; he has originated many capital
improvements in microscopes, of which instruments he is very

ON TRYING MICROSCOPES AND ENGISCOPES. 117

fond ; thinks we live in the golden age of these matters, and
that we have left our ancestors far behind in all things
pertaining to them. His character the reader will find is a com-
plete contrast to that of Mr. Oldbuck.

Now it came to pass that one wet day Mr. O. was walking
up and down in his study, occasionally surveying his wig in
a mirror, and somewhat ruffled in his temper by the imperti-
nence of the junior members of a learned body to which he
belonged, who worried him, till (much against his will) he was
forced to look into a very fine achromatic engiscope, which
showed objects too well to please him. To vent his spleen he was
indulging himself in the following soliloquy : " What occasion
have I, or any man, for any other microscopes than single ones ?
vision by these is so perfect and satisfactory to me, that 1 want
nothing better. Was there ever any thing so ludicrously pre-
posterous, as forming the image of an object, and vieiving it
by a species of microscope, called an eye-glass, instead of look-
ing at the object itself with an eye-glass ? Are we not content
with seeing nature itself? would any rational being rather
behold a picture of a beautiful woman or landscape, than see
the reality ? What a monstrous perversion of all taste, to
prefer art to nature ! it seems to me little short of insanity
itself. Away, then, with this nonsensical trumpery, and let me
alone with my good, old-tried, staunch, simple microscopes.'' —
Here Mr. Putty was introduced, who had brought home
some trifling job ; he met with rather a frosty reception, but
instantly took up the word thus :

"But, my good sir, admitting the justness of your observa-
tions, these simple microscopes of yours are very imperfect
things, generally equi-convex lenses of plate glass ; by making
them of other substances of higher refractive and lower dis-
persive power, they may be greatly improved."

"I shall not deny the truth of what you say, Mr. P., theo-
retically speaking at least, but I have looked through lenses
made of diamond and other precious stones, but cannot satisfy

118 THE MICROGRAPHIA.

myself that I see any better with them than with those made
of* glass. Here,, examine this opaque object with a single equi-
convex lens of glass, having an angular aperture of 55°, set in
a small silver cup ; its sidereal focus is only one-thirtieth of an
inch ; or take this, it is one sixtieth of an inch, set in the
same way, and has an aperture of one-sixtieth of an inch or
55° also ? did you ever see any thing more distinct : the object
is the scales of the diamond beetle, and would, from its bril-
liancy, be sure to show the aberration of the lens, if it had
any, but you see the scales perfectly clear and distinct; will
you pretend that such vision can be improved ?"

" The vision is more clear and satisfactory than I could a
priori have thought possible ; hut I do not see the lines on
the scales well made out; on many of the scales, begging
your pardon, I cannot see them at all.''

" That is not the fault of the lens, but of the method of
illumination, which is not of the kind required to bring out
lines."

" But illumination has so wonderful a power in modifying
the vision of many objects, especially those of the test kind, that
it will sometimes make the better instrument appear the worse,
and the worse the better ; now, I could show you the said
lines much better in an engiscope, because I can illuminate
objects in such an instrument (especially opaque ones) any
how I like ; though I grant that I do not show you the lines
themselves, but only a picture of them ; so exact, however,
that" —

" It will not impose upon me ; I do not choose to trust to
pictures, especially when I am exploring unknown objects j
your engiscope (as you call it) may do vastly well to exhibit
known ones to a parcel of women and children."

" Then you admit it to have the power of showing known
objects ; how and in what way can you ascertain that even
your dear single magnifier can be trusted upon unknown
objects, save by the experience you have had of their per-

ON TRYING MICROSCOPES' AND ENGISCOPES. 119

formance upon those you are acquainted with? Your dog-
matizing is an insult to reason and science, and unworthy of a
reply."

"Well, I will admit (just to put you in humour again) that
an engiscope may show such things as the lines in question
rather better than my single lenses, but 1 should be glad to
know if you can show me any transparent object better than
they do, with any other sort of instrument; (now I have you
on the hip.)"

" I perceive you are too much prejudiced against engis-
copes, from a priori considerations to do them justice ; they
ought to be inferior to single magnifiers you think, and there-
fore 3011 will have it they must be so, and are ; but have you
ever used compound magnifiers ? they show you the real
object as well as the single ones ; and when scientifically
made, are thought, by excellent judges, to perform much
better, inasmuch as their aberration is far lower than that of
equi-conves or even plano-convex lenses ; in short, they would
seem to afford us the ne plus ultra of perfect vision in con-
templating minute objects under large angles."

i( I have not used them much ; they are things of great an-
tiquity, and have been both in and out of fashion with natu-
ralists many a good time and often, before now : when their
power is high they are very inconvenient in the ordinary course
of microscopic researches, from the closeness of them, when in
focus, to the object ; indeed, this circumstance precludes their
use on opaque bodies altogether, and I candidly tell you, that
when an instrument will not show opaque objects well, I am
apt to reject its use altogether."

" In this you are very unjust. It is a law in optics and
most other things, that when we gain an advantage one way,
we must in general be content to lose it in another ; it only
remains for us to choose of what kind the advantage shall be ;
we must have one kind of instrument for one set of objects,
and another for another, if we wish to avail ourselves to the

120 THE MICROGRAPHIA.

utmost of every good within our reach. To reply point-blanc
to your former question, will you be content to admit the su-
periority of doublets and triplets, if I show you something
with them in a decisive manner, which you will hardly be able
to see at all with one single lens out of twenty, and then only
in a most imperfect manner ?"

" I don't know : I am so confident of the excellence of my
own lenses, that if you were to shew me any thing with ano-
ther instrument, which I cannot see with them, I should be
disposed to set it down as an illusion of some sort or other ;
and if you were to shew me any thing in a different manner
from what they do, I should be very much inclined to think
that they were in the right, notwithstanding.''

" I thank you for your candour ; this is precisely the way
that every man thinks with respect to his favourite instrument,
be it what it may, though he may not dare to speak out as
you have done ; it is s poor human nature all over.' But, sir,
this method of reasoning — I cannot see such a thing in my
microscope, ergo it is an illusion ; or, another man's micros-
cope shews an object different from mine, ergo it shews it
wrong, cannot be tolerated; — it may become a fine lady to
talk in this manner, but in a pretender to science it is insuf-
ferable."

" Well, what is the wonder you are going to shew me? I
suppose it will turn out to be the oblique lines on the brassica,
you are always making such a fuss about, — an isolated in-
stance, supposing the said lines actually to exist. I certainly
have seen a few of the lines you allude to, with a doublet of
about l-20th of an inch focus ; but, in general, doublets and
triplets shew no more of them than single lenses do."

" I see plainly I shall have to fight the ground inch by inch
with you ; but I shall not despair of gaining the victory at
last, ' if I have a fair field, and no favour ;' at least in the opi-
nion of impartial and unprejudiced bystanders, if any such
there are. You say you have seen the lines in question, or

ON TRYING MICROSCOPES AND ENGISCOPES. 121

the ghost of them at least, with a doublet : now, have you
seen as much with any of your adorable single magnifiers?"

" I have seen them occasionally, when they have been of
very superior quality, and exquisitely set with a large aper-
ture ; but, 1 freely own, not with sufficient distinctness to
convince me of their existence. The appearance somewhat
resembles the tissue of a coarse piece of canvas, with the
fibres running diagonally, when it is placed too far off from
the eye to allow us to recognize the threads distinctly. The
doublet makes this difference, that a thread here and there
seems stronger and coarser than the rest, and therefore more
distinct; but, as I have already observed, I am strongly dis-
posed to think the whole an illusion, produced by oblique
light."

" Have you ever seen the French brassica ? The traces on
it are still more difficult to exhibit, or fainter at least, than
those on the English variety, particularly on that kind which
is of the form of a heart, or like the head and tail of the other
hind, abruptly joined together without the intervening por-
tion ; these may be said to make two other objects of the same
kind. And now please to prove to me that all these are illu-
sions : the onus probandi rests with you, and what arguments
you are to use for that purpose I cannot imagine, unless they
are such as will equally serve to disprove the existence of any
other delicate traces on tests in general: beware of proving
too much ; many a logician has overreached himself in this
way."

" I was trying against each other the other day three mag-
nifiers, a triplet of l-40th of an inch, a doublet of l-70th of
an inch, and a single lens of l-60th; with none of these could
I see the diagonal lines in question : all other lines and ob-
jects they shewed admirably well, and their performance
seemed to me precisely alike, except, of course, as to power :
all were made with the utmost care by a workman of the high-
est eminence, and were indisputably of the very first quality ;

122 THE MICROGRAPHIA.

the apertures of all were fully 55° ; the triplet I think more,
60° about. Now, sir, I candidly tell you, that I cannot be-
lieve that these three capital magnifiers would all agree in
passing over unheeded the lines we are arguing about, if they
really did exist, and allow a doublet, and a single lens of
I-20th inch focus, to have the merit of shewing them. It is
this circumstance which staggers me — the conflicting testi-
mony of various instruments and lenses ; I am puzzled, as I
should be were two highly respectable witnesses to aver they
had seen such and such an event or thing, and two more, at
least equally worthy of evidence, to assert that the actors in
the affair were, to their knowledge, twenty miles distant from
the place where it was said to have occurred at the time."

te You will frequently find a similar discrepancy in the
performance of very excellent telescopes upon certain objects ;
the pole-star, for example. I have seen many excellent instru-
ments of this class with large apertures, which shewed double
stars, nebula, and clusters, in perfection — nevertheless, as blind
as young puppies on this particular object — which a good
achromatic of only two inches aperture will often shew ; yet
no astronomer has ever had the hardihood to deny the existence
of the small star near polaris, or term it an allusion : a fine
pickle, truly, would astronomy be in, if every observer had
the privilege of denying the existence of every heavenly body
he could not see in his favourite telescope !"

" Well, sir, suppose I concede to you this point in favour
of compound magnifiers, what else have you to urge on their
behalf?"

" I conceive it to be a just inference that they do actually
exhibit all sorts of lined and ordinary objects better than single
ones, though, from the coarseness of many of them, and the
want of a requisite delicacy of apprehension in our eyes, we
are not able duly to appreciate their performance on all occa-
sions alike. It certainly happens to be the case, that the
diagonal lines we are talking about are a sort of solitary in-

ON TRYING MICROSCOPES AND ENGISCOPES. 123

stance of the superior penetration of doublets ; but if they
had chanced to exist in the whole tribe of tests, then most
certainly would the single lenses pass them over in all alike,
and the doublets bring them out ; at least, when of the requisite
goodness. Let us suppose a telescope to be gifted with a
sufficient power of penetrating into space to be able to shew
the sixth and seventh satellites of Saturn, and that there hap-
pened to be no other bodies requiring an equal degree of
illuminative power for their manifestation ; then, perhaps,
would the invidious assert that they were very probably some
illusive appearance or another, as the telescope was not capa
ble of shewing any other objects better than ordinary. I need
not say how senseless, unscientific, and unphilosophical, such
a.conclusion would be; however, there is, as I think, another
mode of demonstrating the superiority of compound magnifiers,
which you will be satisfied with, sceptic as you are."

" Sir, I am all attention ; what can this be ?"

" Will you admit that the fact of our being able to discern
some difficult test with a very low power is a proof of the
goodness of the instrument we employ, and the lower the
power the better the instrument?"

" It is certainly a very great convenience to be able to do
so at any rate, and I should not hesitate to say myself, that
an instrument which will shew me every thing I want to see,
without forcing me to employ high powers, is, at least, a most
useful and valuable implement for a naturalist, for a thousand
obvious reasons, whether actually better or not than another
which requires a higher charge of power for the same
purpose."

" Well, then, here is a doublet made upon truly scientific
principles, being the compound aplanatic lens of one kind of
glass of Sir J. Herschel ; its focus is one-sixth of an inch.
Compound magnifiers, believe me, are at present far from
that state of perfection to which they are capable of being
brought, because the makers of them are generally pleased

1 24 THE MICROGRAPHIA.

to vie with each other in trying who can insult science the
most grossly in the principles which they adopt, though I grant
that they work excellently well, according to them. Now,
with this magnifier I think you will find that you will see tests
which cannot be discerned with any sort of single lens of the
same focus and aperture, nor with this either, if you choose
to reverse it, or turn either of the glasses the wrong way, so
as to subvert the principle on which it is made."

" I like this sort of test a great deal better than the diagonal
lines on the brassica ; the name of Herschel carries with it
weight and authority. I will verify the facts you state at my
leisure ; but I must give full loose to my scepticism in the
meantime, and assert that I believe microscopes may be very
good, and perfectly to be depended upon for making disco-
veries in all the ordinary branches of natural history and mi-
croscopic research, which will hardly shew any of these
precious things you have set up as tests : a plague upon
them ! are we to be eternally bored to death with the dust of
butterflies' wings and the scales of beetles ? for my part I
don't value them a rope's end?"

" Then you deserve to be made to appreciate the uses and
properties of a rope's end at their full value ; nevertheless, I
will myself so far agree with you that I consider the whole
family of lined tests as very exclusive sort of objects : it is
very difficult to find others requiring the same penetrating and
defining power ; therefore the individual who first discovered
and introduced them to the notice of the public, associated
with them others, especially some difficult ones of the opaque
class, which I hardly think any reasonable person can refuse
to admit as proper tests for all microscopes, of what nature or
kind soever may be the researches in which they are to be
employed : such are the minutiae of a fly's foot, the serratures
on a human hair, the little pits on a mouse's hair, the tissue
of the moss hypnum, and to these must certainly be added
the eves of several animalcules. If an instrument cannot

ON TRYING MICROSCOPES AND ENGISCOPES. 125

shew these, I suppose you would reject it as unworthy of
confidence ?"

" My dear sir, you now begin to talk like a rational being ;
I cordially agree with you : all these tilings single lenses will
shew in •perfection. Do you know I had such satisfaction the
other day in shewing a young puppy the eyes of a wheel-
animalcule with a single equi-convex lens: he thought, for-
sooth, to have come over me with his thingamy, his bang-his-
cope, as I think he called it.

ce You must not forget, that even the objects you admit as
indispensable proofs or tests are not shewn, except by instru-
ments likewise capable of shewing the majority of the lined
ones, and that their goodness would be as well demonstrated
by the lined objects as by a fly's foot, &c. ; whatever shews
the one will be sure to shew the other."

" Well, well, be it so ; I am glad to find that we can go
along with each other to a certain extent ; and I hope you will
appreciate my long experience and long habits of research,
with regard to the minutiae of natural history, as worth some-
thing, and that I have not adopted the opinion that single
lenses are, upon the whole, the most useful and effective
working microscopes, rashly. I shall always be ready to ad-
mit theoretically the supei'iority of compound magnifiers; but
I believe it is only when they are of a certain focus and
aperture that the eye really derives any advantage from the
use of them."

"Have the goodness to explain yourself; I really do not
comprehend your drift."

" Why, simply this : here are some equi-convex lenses,
(veiy useful ones I find them), of If inch, 1 inch, and | inch
focus ; I mean to say that 1 find no use in employing them
with a larger aperture than that of my iris ; and when thus
reduced, I see no aberration about them ; a little colour only
is perceptible with an oblique illumination ; they are, in short,
as good as need be, — the finest doublet or triplet of the same

126 THE MICROGRAPHIA.

focus and aperture seems no better : if I even use an achroma-
tic glass corrected by a concave lens, my vision does not seem
to be improved, because the aberration of the equi-convex lens
was already quite insensible to me : you see what a deplorable
animal you have to deal with ! You set before me a salmis de
becasses a la supreme, and a bottle of clos vougeot to wash it
down ; and I think a beefsteak and a pot of stout as good or
better food."

" There is some shew of reason in what you say, as we
generally like any kind of food to which we have been ha-
bituated^ however coarse it may be, — so do our eyes become
coarse and indelicate in their perceptions, and satisfied with
very ordinary glasses, from the effects of long habit ; but of
course, when we come to high powers, you will then admit the
superiority of the compound glasses.

" Not at all : when we come to lenses of l-30th, I-40th,
l-60th of an inch, and upwards, it appears to me that their
aberrations are so very trifling as to be insensible, even when
only equi-convex. You saw how distinctly the l-60th of an
inch lens shewed the scales of the diamond beetle in the out-
set of our conference : I do not believe any practicable doublet
would act any better ; but it would, as you know, be impos-
sible to get a doublet of this power to shew an opaque object
at all, so the experiment could not be fairly tried. I have
already told you the result of my trial of the doublet, triplet,
and single lens, on the diagonal lines of the brassica ; and
to conclude, I verily believe that it signifies very little when
we come to these high powers, whether they are obtained by
single or compound glasses."

" Then do you actually mean to deny the superior efficacy
of compound magnifiers altogether in all and every case : this
is too bad !"

" Not exactly; that would drive you frantic altogether:
I do think that there is a certain range of power, from about
| of an inch to about l-20th, at which the eye is capable of

ON TRYING MICROSCOPES AND ENGISCOPES. 127

appreciating their superiority, and consequently of deriving
advantage from it, (though it may not be much). When a
lens, having a large angular aperture, is not of greater diameter
than the pupil of the eye, so that all its marginal rays shall
operate fully, it will, if free from aberration., make its good-
ness felt and recognized, provided, as I have already said, it
does not run to an extreme in point of depth : now, in the
aforesaid range of power, these conditions will occur ; and it
may perhaps be considered that the circumstance of the dia-
gonal lines being best shewn by doublets of about \-1\)t1t of
an inch focus, and rarely by deeper ones, as well as the
striking efficacy of the compound aplanatic of l-6th of an inch
focuSj are a sort of confirmation of my hypothetical notions,
on which, however, I shall not insist.''

"Well, I am glad to be able to extort this unwilling con-
fession from you, at last; I suppose you would consider
single lenses, made from substances which naturally give as
low an aberration as doublets and triplets, in the same predi-
cament with them ; and that you have no better opinion of
diamond or sapphire lenses, or even of compound magnifiers
made of them: indeed you said as much just now."

" How can I ? I must of course be consistent in my prin-
ciples ; you will have a confounded hard task to prove their
superiority in any other way than by reference to analysis,
believe me."

" With such bigotted, obstinate, sceptics as yourself. But
let us drop the subject of compound magnifiers : will you come
and look at a beautiful aplanatic engiscope ? "

"'* A what ? stuff and porridge ! prythee, leave off this ridicu-
lous slangs and speak plain English."

" Well, then, a compound microscope, — but of wonderfully
improved construction."

" I have no desire to see it : in the outset I told you my
mind about these awkward unwieldy things ; T can assure you

128 THE MICROGRAPHIA.

it has long been made up on the subject : and though I fully
admit the magnitude of the improvements made of late on these
instruments^ and their superiority over the old ones. I consider
them useless ; except, perhaps, for the purpose of executing
micrometrical measurements, — for which end I do not deny
that they are somewhat more convenient than simple micro-
scopes : however, we can measure the size of objects perfectly
well without them."

" If you will not allow them any merit in an optical sense,
you must admit that they are vastly more convenient than
simple or compound magnifiers, for a variety of other purposes,
as well as in executing micrometrical measurements, owing
to the room they create between the object and the glass : —
for dissecting ; viewing bodies placed at the bottom of cavities ;
looking through the sides of a glass vessel containing polypi,
chara, and aquatic insects ; examining small parts of large
bodies, not detached from their wholes, as opaque objects, and
the like ? "

"All these things can be done with single lenses also, of
the ordinary ivorhing poivers : I repeat, their assistance can
be dispensed with."

"What say you to their large field of view? Is their
achromatism also without merit?''

" I say that their field is no larger than that of a single lens,
provided it is so set that it can be brought very close to the
eye ; for then, by turning the eye about, an equal number of
degrees of field can be examined : the only difference seems
to be, that in the compound microscope or engiscope, as you
are pleased to miscall it, the whole field is taken in at one
swoop."

" That makes a vast difference, let me tell you : but will
you not admit that the engiscopes fatigue the sight less than
single magnifiers, when the power is high ? "

" That is entirely an affair of habit ; my eye is never

ON TRYING MICROSCOPES AND ENGISCOPES. 129

fatigued with the deepest single magnifiers : whatever instru-
ment shews an object best will fatigue the sight least, be it
what it may, as must be self-evident."

"I see you have rolled yourself up like a hedgehog, and
present nothing to me but a ball of prickles, let me turn you
over which way I like : however, if you are not afraid of
meeting with an instrument you may chance to like better than
your own, pray look at these scales of the podura in the
engiscope."

" Afraid, indeed ! a likely joke."— [He looks.]

" Can you say you have seen this object as well in your
single, or I will say compound, magnifiers? say, on your
honour !"

"I cannot say, upon my honour, that [ have seen the lines
so dark, and so decidedly made out ; but the circumstance is
easily accounted for by the superior darkness of the engiscope,
which I need not say must be much greater than that of a
single lens of the same power and angular aperture, having, of
course, an opening equal in size to the visual pencil of the
engiscope."

" Well, sir, that circumstance need not distress you ; if
making instruments darker will cause them to perform better, we
can, God be praised, darken your single lenses for you to any
required degree, with coloured glasses ; but 1 will defy you to
shew the object with them as you have seen it just now, do what
you will with them ; not only are the lines shewn darker, and
the spaces between them clearer, than ivith your lenses, but
many which with single or compound magnifiers, of high
powers, seem ragged and dotted, or rather an aggregation of
dots, are shewn as veritable lines ; moreover, you see the out-
line of the scale best at the same adjustment which serves also
to bring out the lines*."

* My coadjutor, whose experience is very great, assures me, that this
circumstance depends upon the state of correction in the objective as to
sphericity, — the aberration of which must be overbalanced to exhibit the

130 THE MICROGRAPHIA.

" That is because the magnifiers shew them right, and the
engiscope wrong ; I do not believe that the lines on the scale
are in the same plane with the margin of the scale, though they
may be nearly so."

" Here is the old object, the diagonal lines on the brassica ;
look how strongly they are made out, and how many more of
them are visible than ivith the best doublets or triplets. Can
you doubt of them, really, now ? Look, how beautifully,
by a slight change of the light, all the cross stria come
out."

<l I told you before, engiscopes must not be trusted for
exploring unknown objects, or making discoveries ; the whole
is an illusion."

"■ You say engiscopes cannot be trusted to make discoveries,
behold ! they make them for you, and you will not believe in
them.

" I see your patience is nearly worn out, I will not plague
you any more, except to view the diagonal lines on the brassica

JN THE INSTRUMENT WHICH DISCOVERED THEM THE REFLECTOR

of Amici ; they are shewn so decisively in this instrument,
that it would be as reasonable to doubt their existence as that of
ruled lines in a copy-book : doublets and achromatics sometimes
shew them, sometimes not, but the reflectors never conceal
the truth : moreover, observe the perfectness of the longitudinal
lines ; no dots !"

" Pooh, pooh, man ! of course the reflectors being still
darker than the refracting engiscope, naturally shew these
and all other objects darker; and this you call shewing them
better."

Here Mr. Putty, who was one of those persons who can, on
ordinary occasions, laugh in their sleeves without moving a
muscle, and who had been long much tickled with Mr. Old-

grooves and contour together : again, a variation occurs according to the
side of the scale which is viewed,

ON TRYING MICROSCOPES AND ENGISCOPES. 131

buck's humours, was fairly upset, and burst into a long scornful
laugh ; which Mr. Oldbuck, who mortally hated and dreaded
any thing like ridicule, resented, as a gross piece of ill breeding,
and an affront to his dignity ; and so the conference termi-
nated, both parties being unconvinced by each other's argu-
ments, and remaining in their original opinions ; Mr. Putty
considering his engiscopes a signal instance of the glorious
triumph which it is possible for art to achieve over nature, by
the destruction of aberration ; thus creating an instrument
which, though it operates through the medium of an image,
but of almost inconceivable perfection, is nevertheless capable
of exhibiting all objects more perfectly than can be done by
the best magnifiers ; at least when the artist has been so fortu-
nate as to produce a perfect system of object-glasses, with
their aberrations exactly balanced, or a pair of metals exqui-
sitely figured : Mr. Oldbuck, on the other hand, regarding
them as a mere waste of labour, and prostitution of talent,
quite unfit for philosophical purposes. — C. R. G.

POSTSCRIPT.

It only remains for me to say, as an epilogue to this little
scene, that I have expressed my own sentiments in the
character of Mr. Putty ; and though I have formerly, in a
paper in the Quarterly Journal of Science, given it as my
opinion that it would be utterly inconsistent with the laws of
nature and optics that engiscopes should ever equal the per-
formance of magnifiers applied directly to the object, yet I am
now convinced by facts, that those now made not only equal,
but are capable, in a few rare instances, of surpassing them,
especially if of the reflecting kind. The only instance I can
point out in which an engiscope fails to do its duty, or what I

132 THE MICROGRAPHIA.

suppose to be such, is this : — we all know that a globule of
quicksilver, like a convex mirror, forms a miniature image of
every thing it is exposed to ; a window for example : we know
also, that we may, with a powerful engiscope, view, and
magnify, and examine, this minute image, pt~ovided the globule
is not too small; if it is, the engiscope makes a blot of light of
it (like the spurious disc of fixed stars seen with a powerful tele-
scope), instead of developing the features of which it is com-
posed, for there can be no doubt that the most minute globule
forms an image. A reflecting instrument, indeed, gives a
smaller spurious disc than a refractor, and will also define the
image of a smaller globule, but still, it fails at last. Now,
whether magnifiers of high power would do the same thing or
not, we have no very accurate means of knowing ; for they
approach so very near, when applied to objects, as totally to
overshadow them, and thus preclude the formation of an image
on the globule, except by means of a cup ; and the illumination
of the globule being accomplished in this way, of course we
can expect to see nothing but the image of the cup, which may
easily be mistaken, when on a very small scale, for a spurious
disc. Now I have sometimes thought that this property of
engiscopes causes them to give that appearance of dots or
specks so visible in the tissue of the lines and markings on the
grooved proof objects, especially the brassica, when viewed
with aplanatic object-glasses of great depth ; so that a small
effort of the imagination would easily enable us to fancy that
we actually saw the constituent particles or atoms of which
the body was composed. This phenomenon may, with old
specimens of scales, be, in some measure, owing to decompo-
sition ; at the same time reflectors cannot be said to shew it,
but there is no knowing what they might do if their objective part
could be rendered of the same depth as that of the refracting
instruments. As to compensating for this deficiency, by forcing
the power with eye-glasses of very short focus, it is a practice
I have always reprobated, as it dilutes the image till, in my

ON TRYING MICROSCOPES AND ENGISCOPES. 133

opinion, there is very little reliance to be placed upon its
demonstrations. The best doublets, triplets, and lenses of gem,
seem to shew the dots and points much in the same way as
the refracting engiscopes, but perhaps not so strongly marked.
As for the reason why refracting instruments so rarely shew
the oblique lines on the Picris Brassicce, and when they do,
perform their task in so slovenly a manner, I must confess I
am as much at a loss as why metals, having a large angle of
aperture, should so invariably exhibit them ; unless the reader
is pleased to lay the circumstance to the account, of the superior
defining power of the reflecting instruments. Perhaps there
may be some analogy between this object and the small star
near Polaris, which is frequently shewn by small telescopes
possessing great defining power, and passed over by others of
large aperture, which shew all other telescopic objects per-
fectly, such as the minute stars composing clusters, and the
like, which we should suppose a priori quite as difficult to
bring out. This star seems also to melt away under the
action of high powers, whereas there are others, to all appear-
ance vastly like it, which cannot be seen till the field of view is
rendered very dark, by a high power (vide Sir W. Herschel's
Catalogue of Double Stars, in the Transactions.) I may
observe, that I never saw these oblique lines with a power
exceeding that of a l-40th of an inch lens : all the very deep
object-glasses and doublets I ever happened to use were totally
blind with respect to this very remarkable object.

C. R. G.

CHAP. VI.

ON THE SPHERICAL AND CHROMATIC

ABERRATION, &c. OF EYE-PIECES.

It being still stoutly maintained that engiscopes and erecting
eye-pieces, having foci in front of their anterior glasses, can be
rendered achromatic by merely arranging their component
lenses at particular intervals ; and that " the intellectual and
manual labour which has of late years been expended on the
construction of achromatic object-glasses has been, in conse-
quence, unprofitably directed* ;" it is the province of the present
work, and the bounden duty of its authors, to demonstrate the
falsity of such doctrines, and to vindicate the truth of the

* Vide a Paper in the " Philosophical Magazine, and Annals of Phi-
losophy," for August, 1831, No. 56, p. 112, — On the Theories of Achro-
matization, by the Rev. H. Coddington, in reply to a paper published by
myself, " On the Chromatic and Spherical Aberration of Eye-pieces," in
Sir D. Brewster'' s Edinburgh Journal of Science; in which, as it was in sepa-
rate circulation at the time, Mr. C.'s answer certainly ought to have
appeared, unless it was intended to be read by itself, without reference to the paper
it pretends to refute. Mr. C. yields but one point, viz. the theory of the
Gregorian telescope, constructed with mirrors of glass, which he admits to
be somewhat imperfect. I think it will generally be found that if a man
has an imperfection in his understanding, or method of analysis, which will
cause him to be in error on one optical subject, it will operate on a vast
number of others. The only favour I have to request of opticians is, that
they will read the paper I have alluded to, or the present one, in con-
junction with Mr. C.'s candid and ingenuous answer. In the former there
are many typographical errors, which confuse my meaning in some places.

ABERRATION, &C. OF EYE-PIECES. 135

opposite principles, lor the honour of the improvements lately
introduced into use. If, indeed, the said dogmas and theories
had been laid down by obscure individuals without influence,
it would have been the wisest course to have allowed them to
rest in peace and contempt ; but, unfortunately, their authors
are persons of the highest consideration and influence in the
scientific world, by whom they are supported with all the parade
of mathematical demonstration ; and it may not without reason
be expected that a large majority of that class of savans, who
dare not, and perhaps cannot, think for themselves, will con-
ceive it to be much more creditable to be in the wrong with
men of such high eminence, than in the right with mere
practical opticians. Yet it must be admitted that optics is both
an experimental and demonstrative science, and it is well that it
is so ; and though experience ought certainly never to dictate to
analysis, (which would be placing the cart before the horse, with
a vengeance), yet, the testimony of the senses, and of those
organs which are to use optical instruments, must not be wholly
rejected. A telescope, and what is usually termed a compound
microscope, or engiscope, pass into each other by insensible
degrees, so that it is somewhat difficult to define the one from
the other otherwise than by drawing a line of demarcation
between them by reference to the length of the anterior and
posterior focus of the object-glass : thus we may sav that a
telescope has its anterior conjugate focus, or the distance of
the object from the object-glass^ longer than the posterior, or
that next the eye-piece. In an engiscope the case is reversed ;
perhaps, therefore, a definition of an engiscope, as distinct from
a telescope, may be given thus : — it is a telescope made on a
very small scale, having its object-glass, and the conjugate foci
thereof, reversed. In support of the analogy between the two
instruments, it seems proper here to remark, that an engiscope
will act with a concave eye-glass just as a Galilean telescope,
and that its focus can be adjusted by moving the eye-glass
alone, instead of the whole body, as is usually practised.

136 THE MICROGRAPHIA.

The line of separation between them is where the two foci
are equal : such being the case, we may make an object-
glass of short focus, say four inches, gradually pass through
all the gradations from a telescope to a compound microscope,
without making the least change in the principle on which the
image is formed. Now no optician has ever yet dreamed of
making an achromatic telescope by arranging a system of
glasses at particular intervals ; though, if a compound achro-
matic microscope or engiscope can be so made, we ought
surely to be able to construct a telescope, of some sort or other,
on the same principle ; the two instruments, as before stated,
being so similar in their action of forming an image. It is a
consideration also worthy of being attended to, that the first
practical opticians in Europe have now all adopted the method
of making achromatic engiscopes precisely on the principle of
telescopes ; that is, by correcting the objective part by means
of concaves of flint glass. Such men as Fraunhofer, Utzneider,
Amici, Selligue, Chevalier, Dollond, Tulley, would scarcely
have all adopted so difficult a task as that of making these
minute achromatic glasses, if it had been possible to obtain a
perfect correction of chromatic and spherical aberration without
them. They are all in the habit of making so-called achro-
matic erecting eye-pieces for spy-glasses, which are, in fact,
nothing but compound microscopes ; and it is hardly credible
that they should not have perceived the application of them as
such, if they were bona fide achromatic. But the fact is, these
artists all know the fallacy of these things, and how, and under
what circumstances, they are achromatic. I propose to shew
that an engiscope cannot be made achromatic in any other
manner than as a telescope is, and that the new achromatics
are, consequently, not superfluous.

The angle of aperture of the erecting eye-pieces of spy-
glasses is usually very small, not exceeding that of the object-
glass they operate with, or about five or six degrees ; the
chromatic aberration, therefore, of what may be considered

ABERRATION, &C. OF EYE-PIECES. 137

their objective part, is nearly insensible to vulgar eyes, on
account of this small aperture. For, cceteris paribus, the
chromatic aberration of lenses is in the direct ratio of the
square of their aperture, or the actual quantity of light they
admit ; — if, therefore, the size of the pencil is restricted to
perhaps one tenth of an inch (the usual diameter of the stop
between the first and second glasses of an eye-piece), it is
wholly impossible that much colour can be generated. But if
a lens of thirty inches focus has an aperture of five or six de-
grees, its diameter will be about two inches, and the quantity
of colour generated by it eighty times stronger than in a lens
of two inches focus with an equivalent opening. Moreover, it
is well known that all working opticians are in the habit of
over-correcting the colour in the object-glasses of those tele-
scopes which are expressly made for viewing terrestrial objects.
It is supposed by some, that the predominance of blue and
purple rays, which is occasioned by this over-correction, tends
to exhibit terrestrial objects better than they would be seen if
the instruments were perfectly achromatic — an absurdity which
I shall not take the trouble of refuting. The real manner in
ivhich a beneficial operation is produced by this over-correction
for colour, is by its effect on the eye-piece, which it greatly
ameliorates and improves, by neutralizing the red and orange
light which would otherwise greatly preponderate in it, so that
the totality of the instrument may be considered sufficiently
achromatic for practical purposes ; nevertheless, under certain
circumstances, the uncorrected colour of the eye-piece renders
itself very manifest — as, for example, when we view some dark
object, such as the rigging of a vessel, or a weathercock
against the light of a clear sky ; for by placing the object
at the edge of the field of view, while we look obliquely at it
from the opposite side, so as to catch an eccentrical pencil, the
uncorrected colour is instantly felt.

Now, as I am contending for the principle of achromatism,
I shall proceed to shew that all erecting eye-pieces of the

138 THE MICR0GRAPH1A.

ordinary construction, though they may, under certain circum-
stances, and in a practical point of view, pass for achromatic, yet
have, in good earnest, no achromaticity about them ; and of this
I think any man may convince himself by the following experi-
ment : — there is a small circular disc of brass, with a hole in
it, placed between the two bottom glasses of all erecting eye-
pieces, which, when the telescope is in action, serves only to
exclude the false light which is reflected by the long tube : the
diameter of the hole in it is usually about one-tenth of an inch,
or just what will allow the pencil of light from the object-glass
to clear it : the telescope will be just as achromatic without it
as with it; when, however, the eye-piece is removed from
the telescope, and tried by itself, as a compound microscope,
the said diaphragm acts a very important part, and, in fact,
reduces the angle of aperture of the eye-piece nearly to that
of the object-glass of the telescope along with which it acted,
being, as I said before, perhaps six degrees. Now it is no
wonder that with so small an opening, though unassisted by
the over-corrected object-glass, the eye-piece should still pass
for achromatic with those who know not how to look into its
defects, — just as we see Mulattoes and people of colour, who
have not much black blood in their veins, pass for whites.
Let, however, the said stop be knocked out, or, what will
amount to the same thing, let it be opened out with a broach,
until it will admit the whole of the pencil of a perfectly achro-
matic object-glass, having a large angular aperture — for
example, one belonging to an opera-glass, or perspective, or
or some of those dumpy achromatics which Ramsden was in
the habit of making — this will probably cause an enlargement
of the perforation of the said stop, from 1-1 Oth to 3-4th of an
inch, if the focus of the first glass is two inches. It may be known
to have attained its proper size when the ratio between the
diameter of the visual pencil and that of the object-glass truly
represents the magnifying power of the telescope, or, when it
is found by examining the pencil with a magnifier, that none

ABERRATION, &C. OF EYE-PIECES. 139

of the aperture of the object-glass is cut off by it. On trying
any telescope thus mounted, it will soon be seen whether the
eye-piece is achromatic or not : or the said eye-piece may be
tried as a compound microscope, by itself; and I can only say,
that a man who is not able to perceive colour in it must be
either utterly blinded by preconceived opinions, or by a com-
plete incapacity to perceive the faults of optical instruments*.

Nevertheless, there is a construction for an erecting eye-
piece which is bona fide achromatic to any angle of aperture,
as it appears to me, namely, a simple reduplication of the
Huyghenian eye-piece ; that is, using one to erect and form the
secondary image, and another to view it, which will give a com-
pensation to each part ; but this construction is never employed,
at least I never met with it, because, I presume, if we use a
Huyghenian eye-piece to form a secondary image, the com-
ponent lenses will be nearly of the worst figure, and in the
worst position for giving distinctness-]-, — which is a quality to
be preferred even to achromatism. I need not observe that
such a construction could never be used by itself, as an
engiscope, or compound microscope, because its focus being
negative, or between the glasses, it can only be made to operate
on an image already formed.

In the year 1815, being then a student in the University of
Edinburgh, I began my career as a reformer of microscopes,
from reading the article " Telescope," in the Encyclopaedia Bri-
tannica, written by Professor Robinson, and chiefly from the
consideration of a passage in it, which I shall here quote.

<fWe have examined trigonometrically the progress of a

* Vide Chap. XIX. p. 191, and particularly p. 200, of the "Microscopic
Cabinet." Every thing I have there said concerning the art of looking
into the defects of microscopes and engiscopes applies to erecting eye-
pieces, and also to telescopes, with vei'y little modificaton. He that hath eyes
to see, let him see for himself, and be made a dupe no longer, either by
fine-spun theories or great authorities.

t The ordinary construction rather resembles an Huyghenian eye-piece
reversed, or inverted, which, of course, utterly subverts the achromatism.

140 THE MICROGRAPHIA.

red and a violet ray, through many eye-pieces of Dollond's
and Ramsden's best telescopes, and we have found in all of
them that the colours are united on, or very neai-, the field-
glass, so that we presume that a theory somewhat analogous
to our's has directed the ingenious inventors. We meet with
many made by other artists, and even some of theirs, where a
considerable degree of colour remains, sometimes in the natural
order, and often in the contrary order : this must happen in the
hands of mere imitators, ignorant of principle. We presume
that we have now made this principle sufficiently plain. Fig.
20, No. 2, represents the eye-piece of a very fine spy-glass, by
Mr. Ramsden ; the focal length of its object-glass is 8| inches,
with 1-jJjj of aperture, 2'05° of visible field, and 15*4 magnifying
power; the distances and focal lengths are of their proper
dimensions, but the apertures are a half lai-ger, that the progress
of a lateral pencil might be more distinctly drawn. The
dimensions are as follows : —

Focal lengths, Aa = 0-775, Bb = 1 025, Co r= 1 10, Del — 0-79.
Distances, AB = 118, BC — 1-83, CD — IT 05.

'f It is perfectly achromatic, and the colours are united, not
precisely at the lens (C c), but about l-20th of an inch nearer
the eye-glass.

" It is obvious that this combination of glasses may be used
as a microscope, for if, instead of the image formed by the
object-glass at F G, we substitute a small object illuminated
from behind, as in compound microscopes, and if we draw the
eye-piece a very small way from this object, the pencils of
parallel rays emergent from the eye-glass D will become con-
vergent to very distant points, and will there form an inverted
and enlarged picture of the object, which may be viewed by a
Huyghenian eye-piece, and we may thus get high magnifying
powers without using very deep glasses. We tried the eye-
piece of which we have given the dimensions in this way, and

ABERRATION, &C. OF EYE-PIECES. 141

found that it might be made to magnify 180 times with very-
great distinctness. When used as the magnifier of a solar
microscope, it infinitely surpasses every thing we have ever
seen. The picture formed by a solar microscope is generally
so indistinct that it is fit only for amusing ladies ; but with this
magnifier it seemed perfectly sharp. We therefore recom-
mend this to the artists, as a valuable article of their trade."

I caused that distinguished artist, Mr. Adie, of Edinburgh,
to execute for me a microscope similar to that recommended
by the Professor, on which no expense was spared ; — it of
course gave an erect image. I was not content with it, its
construction being too complicated, and its length inconvenient ;
moreover, though its angle of aperture was no greater than
that usually given to eye-pieces, its chromatic aberration
forced itself occasionally on my unwilling conviction in a
manner which I could not resist, in spite of my faith in
analysis. I naturally, therefore, imputed this defect to Mr.
Adie's imperfect execution, and set him to work afresh, to
make another microscope, on the plan of an erecting eye-piece
of the ordinary construction, gaining the different powers by
the application of Huyghenian eye-pieces to the image, formed
by the two bottom glasses. I was very much pleased by the
distinctness of this microscope, and so were all who examined
it; still, however, it was not achromatic, so I set Mr. Adie
down, at the time, for a bungler, who could not adjust the foci
and intervals of the glasses in a proper manner, and deter-
mined to have the thing done effectually, if possible, by
London artists, which resolution I executed afterwards. In
the course of these experiments I found the utility of getting
the power at the objective instead of the ocular end of the
instrument, as a much sharper image was thereby obtained ; 1
accordingly deepened what I shall call the object-glasses, as
far as one-fourth of an inch focus, and obtained a very superior
microscope, an account of which has already been laid before
the public.

142 THE MICROGRAPHIA.

As to achromatism, however, I regret to state that the
London workmen succeeded no better than Mr. Adie, though
I thought a feeble compensation was produced sometimes
when the curves of the two plano-convex lenses which com-
posed the object-glass were turned towards each other in the
manner they have been engraved. I moreover once thought
that I had got a combination that was achromatic upon opaque
objects ; and being determined to leave nothing to be done by
any man who should take up the subject after me, I resolved to
apply to Mr. Tulley, of Islington, to have his opinion and
advice on the subject. He soon convinced me of the impossi-
bility of obtaining achromatism in any case where an image
was to be formed from a real object otherwise than by the
action of concaves of flint glass, and that my ignorance of the
art of seeing the flaws and imperfections of optical instruments
was the sole cause of my supposing I had ever obtained
achromatism in any degree.

Now as my respect for truly exact science is not surpassed
by that of any man, I must confess that all these circumstances
created in my mind some doubts and misgivings as to the
accuracy of Professor Robinson's statements and theories, not
very much unlike what 1 have observed in farmers in the
country, when unable to reconcile the actual state of the
weather with the predictions of Moore's Almanack, in which
their faith is altogether inflexible.

These theories of Professor Robinson seem to have been
resuscitated by Professor Airy and Mr. Coddington. The
work of the latter gentleman, " On the Refraction and
Reflection of Light," contains the substance of Professor Airy's
papers on these subjects ; and whether it is considered as a
work of originality, or as a compilation from the writings of
our first opticians, it is admitted to be the best publication
of the kind at present extant, whatever defects it may possess,,
and wants no feeble testimony of mine in its favour.

I presume, however, that the most staunch advocate for the

ABERRATION, &C OF EYE-PIECES. 143

dignity of exact science, while he will insist on the vast supe-
riority of its evidence over that afforded by the senses, will
never contend that it is to run diametrically opposite to their
testimony. 1 shall venture, though well aware of what I am
doing, to impugn its infallibility, in the case of these theories
of achromatism of Professor Robinson, Professor Airy, and
Mr. Coddington, on the ground that no artist is able to make
an achromatic instrument according to them. I shall oppose
the said theories by what we call, in common language, facts;
for I must state that when I view the subject theoretically I
am utterly unable to detect the least flaw in them. It is not
to be expected, indeed, that a poor sciolist like myself should
be able to school men of such high mathematical acquirements
as the aforesaid savans ; the task must evidently be left to
mathematicians whose calibre is still larger than theirs*. I
have already partly expressed my own views concerning
achromatism, and I shall endeavour now to expand them into
the following propositions, which rest on the basis of the
evidence of the eyes, or experience.

1. When achromatism is obtained by the adjustment of
lenses to particular intervals, as in the case of the Huyghenian
eye-piece, such achromatism is absolute and perfect, and not

* Analysis may, I conceive, be subject to the following defects : —

1st. It may be founded on correct principles, but false in its details.
2d. It may be founded on false principles, but correct according to
them, and also in its details.

3d. These defects may be combined.

I should suppose the theories of achromatization I have impugned
to be in the second predicament, and the false principle assumed,
to be, that the compensation for dispersion may take place any how, so
that it is effected on the whole system ; and that the double correction I insist
•upon is unnecessary ; and it must be confessed that, viewing the subject
a priori, purely in a theoretical maimer, nothing can be more consonant to
right reason than that one compensation should suffice, always supposing that it
is possible to effect it on the whole, without giving a correction to the objective and
ocular part separately, which, however, does not happen to be the case.

144 THE MICR0GRAPH1A.

like that effected by the combination of a concave lens of flint
glass with a convex of plate or crown glass, which never
effects a complete neutralization of the chromatic aberration,
as is well known ; therefore, if engiscopes and telescopes could
be made by the adjustment of a system of lenses to particulai"
intervals, their achromatism ought to be decidedly superior to
that obtained by the action of concaves of flint glass, and
equal to that of a reflecting instrument.

2. The only kind of achromatization produced by convex
lenses, which is known in practice, is when two are adjusted to
an interval equal to one half the sum of their focal distances,
or thereabouts. These conditions are not rigorously neces-
sary, as the lenses may be placed somewhat nearer than that.
Moreover, the intervals seem to differ a little according as the
eye-piece is adapted to an engiscope or to a telescope ; at least
if the power of the former is low, and the tube short.

3. Many modifications of this combination may be made, —
as by doubling or tripling what are usually termed the eye
and field glasses, so that the compensation for a double or
triple eye-glass may be thrown upon a double or triple field-
glass : also, there may or may not be an interval between the
lenses composing the eye and field glasses : still, however, the
mode of compensation is the same.

4. In order to form a truly achromatic erecting eye-piece,
or engiscope, there must be a compensation both in that part
which erects and forms the image, and in that ivhich views it ;
therefore no achromatic erecting eye-piece, or engiscope, can be
made with so few as three lenses, because either the objective or
ocular part must inevitably be without compensation.

5. An erecting eye-piece, therefore, can only be made really
achromatic, if we do not employ concaves of flint glass, by
combining two Huyghenian eye-pieces, or some modification
thereof, together, in such a way that one shall erect and form
the image, while the other is made to view it. As to the

ABERRATION, &C OF EYE-PIECES. 145

interval between them, or what may be termed the second and
third glasses, if the eye-piece consists of four, it may be greatly
varied — for example, from six to eighteen inches, and more —
without sensibly affecting the achromatism''' ; the first and third
intervals only requiring to be determined with precision.

6. Such an eye-piece could only be used for viewing an
image, and could never be employed as an engiscope, because
it would have no external focus in front of the bottom glass ;
therefore, an achromatic engiscope can only have a double
compensation in the same way as a telescope has it, of which
latter instrument it is nearly a modification ; and as it is
exceedingly difficult to procure perfect flint glass lenses of
large diameters for the object-glasses of telescopes, it is
devoutly to be wished that, by the omnipotence of analysis,
we may at length be enabled to construct them absolutely
achromatic, of one kind of glass only, by adjusting a system of
lenses to particular intervals ; while with proper attention to
the forms of the lenses, their spherical aberration ought, at
least, to be reduced to insensible quantities, using only a
small angle of aperture, though it could not, perhaps, be
wholly removed.

Now, on comparing these propositions with those contained
in Mr. Coddington's work, under the head of Achromatism,
page 262 to 269, it will be seen how completely theory and
practice contradict each other.

According to Mr. C.'s theories the following is a specimen

* Thus saith experience: behold what theory says, — supposing the first
and third intervals, a and i, to be ascertained, and given to find the second,
e, this will be represented exactly, of course, by the following singular
expression (the four nuinbered/s being the foci of the four lenses in the
eyepiece, namely, 3, 4, 4, 3, vide p. 266) : —

\*(* + i)-<fi+ft)\ f>fa- \oai+flf,-'i{ifl+af)\ \f+f\

€ = ■

3 f*Cfi+/.)+«(/.+/0 \ -**-*(fx+fj (f3+f

If a = 4 i = 5 then e = \\ = 6.5454.
L

146 THE MICROGRAPHIA.

of the dimensions of an achromatic erecting eye-piece, con-
sisting of four glasses ; vide pages 168 and 266.

Inches.

Focal lengths 3 — 4 — 4—3.

Intervals 4 — 6 — 513.

when the axis of the incident pencil is parallel to that of
the lenses : when it is inclined to them, the following variation
in the intervals will occur, if I rightly understand Mr. C. :

4 — 6-558 — 5-43.

I suppose, with due submission to better men, an achromatic
eye-piece ought to be achromatic: let any optician execute
this ; if it proves to be achromatic, I shall make the amende
honorable to Messrs. C. & Co.

If the lenses in the aforesaid eye-piece have the best possible
figures, as ascertained in p. 79, the spherical aberration will be
reduced so low as 0.003 ; that of the single equivalent lens of the
least aberration being 0-75. (vide 179.) I wish the makers of
the achromatic object-glasses had in general the luck to reduce
their aberration as low as this, with any large angle of aperture.
I conceive that another false principle has been assumed by
Messrs. C. and their colleagues, relative to the spherical aber-
ration ; viz. that, like the chromatic, it does not require a
correction both in the ocular and objective part, but that the
compensation for it can be effected upon the ivhole any how.

It will be a memorable instance of the fallability of analysis,
even when conducted by minds of the very first order, if the
said theories are found to be false and erroneous, as I am
perfectly satisfied they are — '* for by their fruits we may know
them to be so." I have always found that good sound theories
will produce instruments which satisfy the eye ; and though
it must be admitted that this organ is exceedingly coarse, and
easily pleased in the generality of mankind, it is nevertheless,
in many instances, capable of acquiring a refined and accurate
taste in optical instruments, by a good education, just as it
does in pictures and other works of art.

ABERRATION, &C. OF EYE-PIECES. 147

I once had the honour of looking through a telescope of the
Gregorian form, but constructed with mirrors of glass, by the
late Mr. Tulley, according to the theory of Professor Airy.
Mr. T. had expended the whole of his skill upon the instru-
ment, and had worked the said mirrors over and over again, a
great number of times, till his patience was exhausted ; yet
the confusion in the vision produced was so great, that it was
but just possible to recognize objects.

I think when such an artist as Mr. T. cannot bring a tele-
scope something near to distinct vision, when working to the
utmost of his ability by the theory laid down for him, it is not
unjust to conclude that such a theory must be rotten at the
very core. Can it be said that such sophistries are useful as
serving to exercise the understanding? I should say that
they can only serve to habituate it to fallacious hypotheses,
unsound subtleties, and delusions of every description. We
had better remain in the dark altogether, than be misled by
such ignes fatui. If books cannot be written even on the
exact sciences, without being full of errors, what is to become
of those on other subjects — such as politics, medicine, religion,
cosmography, &c. where men's passions are perpetually in-
terfering to warp their judgments, and lead them astray from
the truth ? Such considerations certainly ought to teach the
proudest philosopher a little true humility.

In the second volume of Mr. Hume's Essays, section 12, en-
titled " Of the Academical or Sceptical Philosophy," Part
Second, there are some curious reasonings on the subject of
mathematical demonstration, which the reader would do well
to consult.

I may observe that Mr. Coddington has been at the pains
of falsifying his own theories, practically at least, to the best
of his abilities, by presenting to opticians a compound micro-
scope, termed achromatic, which is constructed according to
them. It is described in his treatise on the " Eye and Optical
Instruments," pages 60 and 61, under the head of Achromatic

148 THE MICR06RAPHU.

Object-Glass, and is represented in plate 13, figure 190. He
has implicated articles 205 and 210 with the expressions in
pages 261 and 263, in the Treatise on the "Refraction and
Reflexion of Light/' in the construction of this instrument ;
and if he had succeeded in making it achromatic, there would
be good reason for asserting that it is unnecessary to correct
the dispersion of its object-glass separately ; since, if his theo-
ries were accurate, a much better and purer achromatism ought
to be obtained by the simple adjustment of a system of lenses
to proper intervals. I assert point blank that his instrument
is as complete a failure as any thing of the sort I ever at-
tempted myself. I have examined one of these instruments
of the latest and most improved construction, (which, however,
differed not materially from that described in the Treatise on
Optical Instruments, except in having a double field-glass,)
and can, 1 think, be positive that both the chromatic and
spherical aberration of the objective part was ivholly un-
touched, and that the eye-piece, consisting of four glasses, was
achromatic* . Nothing can surpass the beauty of the field of
this microscope ; but if I may be allowed the expression, not-

* I hope I shall be excused for quoting a part of a note to the first
tract I wrote on the Amician reflecting engiscope ; in which I have in a
manner described Mr. C.'s instrument beforehand, as the said note was pub-
lished in 1826, before his work and microscope were introduced to public
notice : —

" I cannot here refrain from protesting against those preposterous ac-
cumulations of eye-glasses which we find in the best common compound
microscopes (as they are called). It would appear that the worthy
glaziers, who preside over the destinies of these unfortunate instruments,
have not yet discovered the right end of a microscope from the wrong
one ; at least they have vented their rage for improvement entirely on the
eye-piece ; having first doubled the anterior eye-glass, then tripled it, and
finally interposed a body-glass of long focus between the field-glass and
object-glass (making the eye-piece to consist, in fact, of five lenses) ;
they sit down contentded with their large flat field of view, and imagine
they have arrived at the very extreme verge of perfection. 1'he object-
glass is allowed to remain a pitiful double convex lens, being, I suppose,
either above or below their art.''

ABERRATION, &C. OF EYE-PIECES. 149

withstanding; its extent and flatness, nothing more can be made
to grow in it than in that of any ordinary compound micros-
cope having- a well-figured object glass of the same power and
angular aperture, used with an Huyghenian eye-piece also of
equal power with that applied to the instrument in question.

The dispersive and spherical error of small lenses is un-
doubtedly small, as I have frequently remarked, but it is not
insensible, even with a very small aperture, to those who know
how and where to look for it. Some persons see it at the
first glance ; others, only if it is pointed out to them ; and
some not at all, if it happens to contradict their hypothetical
assumptions and preconceived opinions ; for there is no blind-
ness greater than that generated not in the organ of sight,
but in the mind which receives its impressions. We are, in-
deed, so accustomed to tolerate aberration in microscopes, that
it frequently is unable to stimulate our senses.

Now I shall not condescend to discuss the point whether it
is right or not to correct the dispersion even of small lenses
and object-glasses : but how is it to be done by an achromatic
eye-piece ? as in the case of Mr. C.'s microscope. If a man
chooses to tell me that those little rainbows and fringes of
colour which appear in objects viewed by chromatic instru-
ments, are, in his opinion, highly ornamental, and not at all
injurious to vision, I can only say I wish him a better taste.

If people like to eat dirt, they shall meet with no opposition
from me : much good may it do them.

The only case, I think, in which colour can be said to be cor-
rected by an eye-piece, is when an object-glass, over-corrected
for colour, is made to act with a chromatic eye-piece, as I have,
indeed, before stated : having once caused the object-glass of
an engiscope to be over-corrected, because it was to act with
an erecting eye-piece of the ordinary construction, the bene-
ficial effect of this arrangement was very sensible, and it might
either be said that the eye-piece corrected the object-glass, or
the object-glass the eye-piece. I am of opinion that though it

150 THE MICROGRAPHJA.

would be highly advisable to get rid of the chromatic aber-
ration of simple and compound magnifiers, it is not sufficient
to produce any real detriment to vision, unless with exceed-
ingly minute and delicate objects, beyond that of giving a
false colouring, especially if oblique light is used. This,
however, we soon learn to allow for ; it is, moreover, con-
siderably concealed by their spherical aberration.

As to the spherical aberration of object-glasses and metals,
inasmuch as it is sensible to vision, I am persuaded, from
reiterated experiments, that the power which an eye-piece pos-
sesses of modifying or altering it is exceedingly feeble, at all
events. A case, however, in which I can be positive that it
does actually correct it, is that of the Gregorian telescope, in
which, if the primary metal is very nearly right in point of
figure — for example, a little too much inclined to be spherical
or parabolical — the error may be corrected by giving the se-
condary one an hyperbolical figure. Moreover, in the Cassa-
granian telescope, it is well known that the aberration of the
two metals (supposing them to be both spherical) is equal to
the difference between the aberration of the convex and the
concave, which is a clear proof that an eye-piece may correct
to a certain extent, where a secondary image is formed, (for
I consider the small metals and eye-glasses of the Gregorian
and Cassagranian telescopes in their combined action, as
neither more nor less than eye-pieces*, which operate by form-
ing a secondary image, like the erecting ones of common spy-
glasses). I should, moreover, conceive that the same con-
ditions would occur, if achromatic glasses of similar construc-
tion and effect, and, in equivalent states of correction, were
substituted for the metals of the Gregorian and Cassagranian
telescopes, and made to operate against each other on the
same principle*. I have in vain endeavoured to find an in-

* An inverting eye-piece for either telescopes or engiscopes may be
formed of a concave lens, in place of the two first glasses of an erecting

ABERRATION, &C OF EYE-PIECES. 151

verting eye-piece of the ordinary construction, which would
exert an influence over the state of spherical aberration of the
object-glasses and metals of engiscopes, ivhen no secondary
image ivas formed. My way of judging was to form an arti-
ficial star by their unassisted operation as magnifiers in a solar
opaque microscope, making the posterior conjugate focus to
be of the same length as in an engiscope, that is, eight or nine
inches ; and having carefully examined the state of aberration
in the picture, I then have attempted to modify it (when formed
at the field-bar of an engiscope), by various eye-glasses, but
so far as my eyes are capable of affording a criterion, 1 never
succeeded in any degree : the image appeared as intractable
as a picture formed by human hands, and remained inflexible
both in its defects and excellencies, whatever they might be :
the various eye-pieces I employed gave a better or worse field
of view, were or were not achromatic, &c, as regarded their
own intrinsic operation : for example, they might give the
oblique pencil in a better or worse state ; but if the said
oblique pencil came in a confused or distorted state from the
object-glass, I never could make them improve it. At the
same time I think it proper to observe, that I never tried
thoroughly the effect of positive over-corrected achromatic eye-
pieces.

All these experiments, however, have convinced me that a
compensation for spherical aberration cannot take place upon
the totality of an erecting eye-piece or engiscope, unless both
the objective and ocular part have a separate correction, for
they cannot correct each other. I therefore believe that Messrs.
C. and Co.'s nostrums for the cure of this evil are just as

one, combined with an Huyghenian eye-piece, which I consider an equi-
valent to the concave metal of a Cassagranian acting along with Its
proper eye-piece, and capable of exerting a similar action on the primary
object, metal or glass ; that is, of reducing its aberration in the ratio of
the contrary aberration of the concave lens.

152 THE M[CROGRAPHIA.

good as those for chromatic aberration. — Vide p. 168 and 179
of the Treatise on the Refraction and Reflection of Light.

1 here give the construction of an erecting eye-piece or en-
giscope, which is both achromatic and aplanatic, to an aperture
of about 27°, and having, moreover, a reasonable good cor-
rection of the oblique pencil.

Let there be two achromatic lenses of suitable focal length to
give the required power, and let the anterior or first glass have
its focus about half the length of the second : a stop may
be placed between them in the solar focus of the first, to
regulate its aperture, if necessary : to these let an Huyghe-
nian eye-piece be applied at a convenient distance. It is not
necessary that either of the lenses should be perfect in itself;
they may be framed so as mutually to correct each other's
aberrations, both spherical and chromatic. If they have their
internal curves in contact, and are cemented together, there
will be no sensible loss of light in this eye-piece beyond that
in the ordinary ones ; and in this case also the correction for
the spherical aberration may take place in Mr. Lister's method,
which will considerably facilitate their construction.

I have already stated that I consider the ordinary erecting
eye-piece sufficiently perfect for practical purposes ; but I think
it would be a great improvement on spy-glasses if they could
be shortened nearly one-half, so that they should be easily held
and directed by the hand, when charged with a power of thirty
or forty. I think an object-glass of two inches and a half, or
three inches aperture, does as much upon terrestrial objects,
at least in the day-time, as any other; the focal length of which
is generally not less than thirty or thirty-six inches, which ren-
ders the glass too long to be easily used without a stand. If
the focal length was reduced to fifteen inches, the angular
aperture would thereby be doubled, and with it the defining
power also (provided it could be perfectly executed). In this
case the erecting eye-piece I have given would be indispen-

ABERRATION, &X. OF EYE-PIECES. 153

sable, for the colour of an ordinary one would become very
sensible.

Trading opticians may not much relish my recommenda-
tions, but I say, and will maintain, that terrestrial telescopes
will not have received their finishing touch, or have arrived at
their ne plus ultra of perfection, until their secondary image
is just as perfect as their first, and it can only be rendered so
by being formed by regular achromatic glasses. Moreover,
when we apply an erecting eye-piece to the object-glass of an
engiscope, so as to obtain a low power, and for that end
thrust it far into the body, so as to approach very near to the
object-glass, (in which case what I shall call the posterior angle
of aperture becomes considerable,) a truly achromatic construc-
tion is also highly requisite.

A positive inverting achromatic eye-piece for astronomical
telescopes, or for engiscopes used with micrometers, may be
made of two double achromatic glasses, having their convex
lenses next the eye, with a certain interval between them, for
the purpose of improving the oblique pencil. They mav be
either achromatic and aplanatic in themselves, or by mutual
correction, and their foci may be varied at pleasure, provided
that of the second or field-glass is not shorter than that of the
first. I am sorry to say that I have not been able to procure
any combination which gives so flat a field of view as T could
wish, consistent with perfect distinctness. If it is thought
worth while to obtain a large field at the expense of centrical
perspicuity, we have only to reverse the position of the glasses,
and that purpose will be accomplished.

It only remains for me to say somewhat about oblique pen-
cils ; they are very much mistaken who suppose that they
have not received quite as much consideration as they deserve
from those who have interested themselves of late in the im-
provement of the microscope. It is well known to every man
practically acquainted with optics, and ought to be much
better by those who know the science theoretically, that in all

154 THE MICROGRAPHIA.

constructions whatsoever there is a perpetual system of gain-
ing advantages in one direction, and losing than in another,
just as in mechanics, — what is gained in power is lost in time,
&c. It is not permitted to man, assisted by the most power-
ful analysis, to combine together every advantage he may
desire : he must content himself with what is attainable, ac-
cording to the laws of the refraction and i*eflexion of light. I
have ever found that all those constructions which give a very
large angle of aperture, combined with great distinctness,
invariably produce what is called a bad field of view ; that is
to say, the vision is only perfect in the centre. In refracting
instruments a good deal may be done by combining several
object-glasses together, more especially if they have intervals
between them, (which, however, produce great inconvenience,
by shortening the anterior conjugate focus, and thus precluding
the use of high powers). As the action of metals is very sim-
ple in comparison to that of lenses, I shall give an illustration
of the mischief occasioned by what is called improving the
oblique pencil in the Amician reflecting microscope. It must
be evident, that as the angular aperture is increased, the error
of the oblique pencil must also be augmented, for it will come
more and more oblique ; now Mr. C. has actually recommended
opticians to render the concave metals of the aforesaid instru-
ment spherical, because in this case every pencil becomes equa-
lized, and may be said to be equally bad or good, distinct or in-
distinct, throughout the whole field of view, though, so far as the
objective part of the instrument is concerned, the whole instru-
ment becomes utterly worthless, if this figure is adopted.
They who work the metals of this instrument well know what
infinite pains they have to get rid of this spherical figure, and
to attain the true one* ; and that when they have done so,

* I have known Mr. Cuthbert to have been employed a whole week in
getting rid -of nothing but the spherical error of one of his metals of 3-10ths
of an inch focus and the same aperture, and not able to succeed at all in

ABERRATION, &C. OF EYE-PIECES. 155

the point is effected at the expense of a certain want of per-
spicuity about the edges of the field of view, which is altogether
irremediable with low powers, which take in a large portion
of the marginal part of the image.

If Gregorian and other telescopes, having a large angular
aperture, are charged with a low power, the same defect is
also very perceptible in them.

The Newtonian and Herschelian telescopes having very
small angles of aperture, will admit of concave metals with
spherical figures, because in this case the aberration occa-
sioned by such a figure will be quite insensible. I do not,
however, believe it possible to preserve a perfect spherical
figure in metals having very small angles of aperture : for, in
the process of polishing, it is sure to pass into the parabola
at least, if it does not get beyond it, in spite of the utmost
efforts of the workman. All that I ever examined are in the
predicament of inclining to a hyperbolical figure, instead of a
spherical one.

Of a piece with the performance of a spherical metal is
that of a globular or bird's eye object-glass, which is con-
sidered to produce an excellent correction of the oblique
pencils, because they come exactly like the centrical ones,
which are utterly uncorrected, for the aberration of a sphere
is to all outward demonstration much the same as that of
an equi-convex lens, and requires as strong a concave to cor-
rect it.

When the omnipotence of analysis shall point out some
truly achromatic and aplanatic construction, ivhich will give
an oblique pencil as perfect as a centrical one, then will the
labours of mathematicians have assumed a proper direction.
At present I am afraid they have been hallooing before they
are out of the wood. I trust, however, that the time will

certain states of the weather, supposing always that a perfect figure was
to be combined with a perfect polish.

156 THE MICROORAPHIA.

come when the subject (which seems to me both useful and
important) will be taken up in the right point of view, or it
will be likely to remain a standing opprobrium mathemati-
corum.

It is quite plain that compound microscopes and ei-ecting
eye-pieces are only part and parcel of the subject of teles-
copes, and worthy of the same attention. Nothing which
concerns the exact sciences can in my opinion be frivolous
or unimportant: a mere subtlety or conundrum having the
charm of exactitude about it, must always be respectable,
although it may be of no apparent utility.

1 shall leave it to others to judge how far the subject of
microscopes is insignificant, or easily discussed ; and I should
recommend those who consider it theoretically, in future not
to disdain to receive every assistance which experience and
practical men can give them, before they embark in it u out
of sight" like Pantagruel and his companions in quest of the
oracle of the bottle ; and to remember the Laputan tailor, who
being too great a mathematician to avail himself of any of the
practical implements of mensuration belonging to his trade here
in Europe, and working entirely by analysis, always sent home
his customers' clothes damnably ill made at all points. It was
in vain that he swore, down their throats, that their clothes
mmt be well made, and were so, for he could not err in his
calculations, &c. ; he was only answered by derision, slaps on
the chops, and kicks on the breach.

If we examine into the amount of the obligations opti-
cians and optical instruments have derived from the science
of mathematics, we shall find them fewer and less onerous
than we might expect. Sir I. Newton certainly preconceived
the possibility of correcting the chromatic aberration of lenses
by the opposite refraction of a concave medium of greater
density ; but this discovery would have been of small use, if
the spherical aberration could not have been corrected by the

ABERRATION, &C. OF EYE-PIECES. 157

same means : which was an idea which does not seem to have
struck him, but was excogitated by a practical man, the elder
Dollond, who seems to have made excellent telescopes long
before any theory was in existence sufficiently correct for an
optician to work by.

The late Charles Tulley, (certainly one of the first opticians
of his day) assured me, that he never could find any theory,
English or foreign, by which a really good telescope could be
made* ; all the calculations of curves he ever saw being
more or less inexact, and requiring after correction by trial,
before an instrument made according to them could be 'per-
fected. The best he ever met with are those of Sir J. Herschel,
but these are faulty, and have too much aberration in the
concaves, which he was obliged to reduce before he could
make a perfect instrument by them. To Sir J. however, we
are much indebted for some excellent aplanatic combinations
for eye-pieces and magnifiers, which seem to leave nothing to
be wished for or desired.

The Huyghenian eye-piece, a most valuable invention, was
partly the result of experiments ; Huyghens having purposed
only to correct the spherical aberration of an eye-piece by
his combination, which was afterwards found to be capable of
correcting the chromatic also.

It cannot be denied, moreover, that we have been favoured
with many excellent theorems and recipes for correcting the
distortion produced by lenses, and obtaining what is called a
good flat field of view : this is a subject on which the mathe-
maticians have been more successful than any other. To them
we likewise are indebted for the knowledge of the value of
the aberrations of lenses of different figures, according to the

* The article Telescope, in Rees's Cyclopaedia, was written by Dr.
Pearson, but the materiel was furnished by C. Tulley. It shews how-
telescopes actually are made, and may be perfectly relied on, by the

artist.

158 THE MICROGRAPHIA.

refractive and dispersive powers of the substances of which
they are made, &c.

We have seen what theory has done for us in the case of
erecting eye-pieces and engiscopes, &c. With respect to
achromatic object-glasses for diverging rays, Euler has re-
commended us to reverse those of a triple object-glass made
on a small scale — a wretched expedient, which will never
make a good instrument. Probably in the course of about
another century, long after the said object-glasses have been
perfected by practical men, some pompous analyst will arise,
who will buckle on some sort of semi-exact theory to them.

C. R. G„

CHAP. VII.

ILLUSTRATIONS OF THE ALTERATIONS

PRODUCED IN THE VISION OF CERTAIN MICROSCOPIC OBJECTS,

BY USING INSTRUMENTS HAVING VARIOUS ANGLES OF

APERTURE, BUT A FIXED POWER.

Mr. J. Murray, of Albemarle-street, having, with the consent
of Mr. Brande, allowed Mr. Pritchard the use of a plate for-
merly published in the 22d vol. of the Journal of the Royal
Institution, with a paper of mine on Achromatic Objectives
and Test Objects, I shall take advantage of the liberality and
kindness of these gentlemen to present to the public a few
illustrations of the said plate, as I have never written any
thing on the subject of the present tract, except in the afore-
said paper. I must observe that when it was published,
Achromatic objectives were quite in their infancy, the podura
was unknown, and the diagonal lines on the brassica were
undiscovered ; nevertheless, with proper allowances on these
points, the said plate will serve my purpose indifferently
well.

As the subject is now pretty generally understood by the
observers of the present day, I shall be brief in what I say,
which must be understood as adapted to the calibre of the
rising generation, and those readers who have not yet made
microscopic science their study.

] 60 THE MICROGRAPH1A.

Fig. 1 is a very tolerable representation of an excellent and
very beautiful test, a feather from the wing of Morpho Me-
nelaiis, (being the first object in which I observed the very
remarkable property of the lines as tests), as shewn by a triple
object-glass of 2-10ths of an inch focus, and l-10th of an
inch of aperture, or 27|°. The cross striae are not so nu-
merous as they appear, when the object is illuminated ex-
pressly for the purpose of shewing them, when, indeed, it
appears like a piece of brick-work ; but in this case the longi-
tudinal lines are rendered much fainter; therefore a kind of
medium illumination was employed.

The seven circular discs are supposed to pourtray a small
portion of the said scale, seen with an object-glass of half an
inch aperture, and 9 lOths* of an inch focus, used with a ne-
gative eye-piece of l-4th of an inch, and having five different
circlets or stops applied to it, so as to cause its aperture to
vary from 1-1 0th of an inch (the usual aperture given to a lens
of that focus in the old compound microscopes) up to half an
inch, by a gradual increase of the size of the perforation of
the stop.

Disc No. 1, Aperture 1-1 0th. — The scale appears quite dark
all over ; not a vestige of its lines is visible ; even its blue
colour is scarcely perceptible.

No. 2, Aperture 3-20th. — Very little improvement ; the
colour is lighter, the blueish tint more apparent ; we may
contrive, by looking with all our might and main, to fancy
we see some indications of lines, or rather scratches.

No. 26, Fig. IV. — Aperture l-5th of an inch, colour lighter
and brighter ; traces of irregular scratches are now clearly
perceptible here and there : peradventure we shall see what
we shall see, by and by, as the Mussulmans say.

No. 28, Aperture 3-10ths of an inch. — Land begins to be

* Fig. 13 represents the curves of this object-glass, but of twice their
real radius, for tbe sake of greater accuracy.

ANGLES OF APERTURE. 101

seen from the mast head : a practised eye will now recognise
the nascent lines, but they seem like an aggregation of dots,
and are interrupted and broken : the spaces between them
are still very dark.

No. 3, Aperture 4-10ths of an inch. — The lines are at length
resolved, but not fairly ; the)' are very faint, and seem ragged,
as if still composed of dots and points, but more closely con-
glomerated than before : the spaces between them are still far
too dark.

No. 4, Aperture 5-10ths, or without a stop. — The lines are
as much resolved as they can be by an object-glass of so long
a focus. They now appear in their true character, that is,
pretty fairly drawn as if by a pencil of deep black lead on
tolerably white paper, or, more correctly speaking, by a pen
with some blue pigment on light violet-coloured paper ; for
their bluish tint is now abundantly manifest.

No. 5, Aperture naked as before, but the object turned
one quarter round, in order to permit the illumination to
operate on the cross striae, which now become perceptible,
though as a minimum visible, being by no means shewn so
strong as in the plate : if the observer should not have seen
them before with a deeper object-glass, he might be expected
to consider their existence very dubious.

Such are the effects of various apertures on the object in
questiou, the power remaining the same. For the information
of such as are but young in these matters, it behoves me to
remark that, loith so small cm aperture as 27J°, not one of
the phenomena I have mentioned will be seen at all, unless
the illumination is oblique. Vide Figs. II. and III. shewing
a plan and elevation of the relative situation of the object,
and of the taper employed to shew it, when the illumination is
conducted in a proper manner. If, however, the aperture is
very large, and the object-glass deep, then the lines and cross
striae will be developed any how, or in spite of the most direct
light which can be used, and this with a moderate power.

M

162 THE MICROGRAPHIA.

Figs. V. VI. VII. are examples of tests which are invi-
sible with 27 1° of aperture, even when assisted with the most
favourable light, and the greatest amplifying power: the
regular microscopist will probably recognise in them the
pieris brassier, two specimens of feathers from the clothes
moth, and the scales of the diamond beetle (which are sup-
posed to be here shewn as transparent objects): the minimum
of angular aperture they require is about 364°, consequently
they can scarcely be shewn by any single object-glass.

Fig. V. is, of all other objects, that best adapted for a test, on
account of its singular uniformity of structure. Mr. Pond, our
late worthy astronomer royal, has noticed a remarkable feature
in it, which is, that it is sure to be invariably of the same
length ; now I suppose if we were to collect all the leaves from
any tree whatever, we should scarcely find two of them exactly of
the same length, when subjected to strict micrometical measure-
ment : and I think the same would hold good with respect to
the scales of almost any other insect, and indeed to the other
scales of the brassica also ; I mean those not of the particular
character here alluded to.

The effects of various apertures on the markings of this
capital test are as follows* : — Up to 27 J°, either in glass or
metal, nothing can be observed about it with certainty ; its
surface appearing so nearly uniform, that no one who saw it
for the first time, and did not know there actually were lines
or markings upon it, would suspect their existence, whatever
power may be employed. Between 27|° and 36|° faint glim-

* In this, as in all other cases, I always suppose the power used to be
equivalent to the manifestation of the lines or markings, that is, inasmuch
as a certain power is necessary to render the human eye capable of dis-
cerning them ; but the object of this paper is to impress on the mind of
the reader the effect of aperture, or, what is the same, thing under another
name, ■penetrating power as distinct from magnifying power ; which latter is
no doubt able to accomplish a great deal, but never can supply the place
of a large visual pencil of light, which, as it is the ultimate effect of a large
aperture, is another term nearly synonymous with it.

ANCLES OF APERTURE. 1C3

merings of marks of some kind begin to manifest themselves ;
with 36h° lines of a ragged, broken, uneven contour, are
plainly seen, as well as cross striae : if the objective is a metal
of good figure, a few oblique lines will also be developed.

If the aperture is above 36f°, and approaches to 55° in
glass, (whether in achromatic engiscopes, compound mag-
nifiers, or simple ones), the lines will be more strongly made
out, but will appear to consist of an aggregation of points .
the cross stria? will be shown closely packed together all over
the scale, and occasionally a few oblique lines will be faintly
exhibited : if the aperture is as much as 60° or 70°, and the
object-glass or magnifier at the same time very deep, nothing
but a tissue of dots and points will appear, occupying the site
of the longitudinal lines.

The effect of 55^° of aperture in a well-figured metal of
3-10ths focus, is very different : the lines and cross stria? will
not in this case be resolved into dots or points, but will appear
in, what I suppose to be, their proper character ; and the two
sets of diagonal lines will be shewn with a force and effect
which will leave no doubt of their existence in the mind of a
candid observer ; the various lines, the longitudinal, the cross
striae, and the two sets of diagonals, being all observable,
successively, by a slight change of the illumination, though
we can rarely see two of the systems well at the same instant.

Being aware that the invidious would be likely to assert
that the scales and feathers of insects were objects sui generis,
and that it was of little consequence whether microscopes and
engiscopes, devoted to the examination of other objects, would
shew them or not, I from the beginning associated others with
them, which no naturalist can pretend to assert should not be
properly exhibited by all instruments fit for philosophical in-
vestigation of the minutige of nature. Accordingly, fig. 9 is a
bat's hair shewn as an opaque object; fig. 10 (a), a large
mouse hair, shewn as an opaque object also; and fig. 12, a

164 THE MICROGRAPHIA.

fly's foot ; (b) fig. 10 is a small mouse hair, seen as a trans-
parent object; fig. 8, the fragment of a leaf of the species of the
moss hypnum.

Now none of these can have their minutiae properly shewn
without an aperture of 27|° perfectly free from aberration* ;
and indeed, if the aperture is much greater, it will be found
vastly acceptable., by facilitating vision. The bat's hair suffi-
ciently explains itself; the large mouse hair has pits in it, just
like the interior of a tea-cup, and which ought to be made out
as distinctly : the small mouse hair (which is shewn by trans-
mitted light) has fine longitudinal lines connecting its joints,
which are the most difficult parts to develop. The hypnum
should have all its component lozenges distinctly made out :
object-glasses will frequently shew the Menelaus, and other
lined objects of a similar class, and yet not define these : re-
flectors are apt to boggle at them. I may observe that Mr,
Pond has discovered, (by using object-glasses and lenses of
very large aperture and high power), that what I have termed
the lozenges of the hypnum are, in fact, hexagons, having two
opposite sides longer than the rest.

The fly's foot has the following notabilia about it : — the lines
on the hairs about the ancle at (5 b b b b) ; the scales on the
pastern (c c) ; the grooves on its claws (a) ; the white points
(d d) on the soles of its feet, and the fine fringe (e, e), which
surrounds them.

As to the said drawing of the fly's foot, it is a wretched
affair ; because the said foot was fastened to a black cylinder
with gum, and thereby sadly distorted, one of the claws being
entirely sunk and invisible in the gum, yet it happens to shew
all the minutisB pretty well. Copper-plate engraving is not at
all adapted to exhibit this object well ; a wood-cut does much

* This is a condition which I always suppose associated with aper-
ture ; as without it the mere opening out of a glass to a large diameter
does as much harm as good.

ANGLES OF APERTURE. 165

better: and in the succeeding volume (the first of the new
Series of the Journal of the Royal Institution1), the reader will
see one which gives a better idea of it.*

Now., in order to illustrate my position about the value of
aperture, in exhibiting so very ordinary an object — the foot of
a common blue-bottle fly — I hope it will not give offence if I
refer the reader to the celebrated Bauer's drawings of the same
object, in the Transactions of the Royal Society for 1816,
p. 146, belonging to a paper, entitled, " Some Account of the
Feet of Animals whose progressive motion can be carried on in
opposition to Gravity," by Sir E. Home ; in which all the
minutiae I have detailed are totally passed over. In the wide
world, where shall we find an artist who can delineate objects
so beautifully and so faithfully as Bauer? What is the reason,
then, that he has committed so many sins of omission in this
particular instance ? I answer, that the fault lay not with him,
but with the instrument he used, which, though it might be one
of the best which could be procured at the time he made the
drawings in question (at which period achromatic objectives
were unknown), wanted the penetrating power or aperture ne-
cessary to develop this object. Mr. B. drew exactly what he
could see with certainty — nothing more or less, as was his
bounden duty.

W hy have I, in my drawings of the wheel animalcule, in the

* I have forgot which of the fly's feet I have drawn ; they differ con-
siderably from each other ; but I think this was a hind foot. I have been
told more than once that my drawing is unfaithful, because all the minutm
I have represented can never be seen at once, as in the drawing. What could I
do 1 This object requires a power of l-30th inch for its development : as
its surface is quite uneven, only a point can be in focus at once, or per-
fectly exhibited. Ought I to have made twenty drawings of it to shew
it just as it appeared at every different adjustment of the focus of the in-
strument ? The true way to draw it is surely to represent it just as it
would have appeared had it been of the size of the drawing, and seen as
ordinary objects are — a cat's paw, for example ; i. e. without any instru-
ment, or magnifying power at all.

166 THE MICROGRAPHIA.

" Cabinet/' omitted to give the eyes, and a variety of other
minutise since discovered in that animal? I answer, I had
nothing but uncombined triple object-glasses to use, which,
though they were fine things in their day, were utterly infe-
rior in penetrating power to the triple systems of double achro-
matics now made. I drew just what I could see, nothing
more or less.

To what are we to attribute the astounding discoveries of
Professor Von Ehrenberg, in the organization of the Infusoria,
in comparison with which all other microscopic revelations
seem to shrink into insignificance, even those of Leeuwenhoek
(supposing them to be correct) ?

That creatures generated without any primitive organic sub-
stance, and so minute that we were formerly content to see
little more than their outline, and many of which were sup-
posed to be as simple in their structure as an hydatid, should
be found to possess all the essential organs of the larger ani-
mals, and to be as perfect in their kind — to have the details of
their whole anatomical structure laid open to us in so satis-
factory a manner that we cannot entertain a doubt of their ac-
curacy*— must be allowed to reflect eternal honour on the
modern improvements on engiscopes ; no less than on the skill,
perseverance, and profound talent for microscopical and ana-
tomical investigation, possessed by him who wielded their
energies. Professor Von E. could not, I think (had he pos-
sessed no better tools to work with than the old microscopes),
have succeeded in unravelling the construction of the Infusoria,
however great the resources of his splendid genius may be : a
good workman, it is said, never quarrels with his tools ; but
the subjects he had to work upon are totally invisible with in-

* Vide Organization, in der Richtung des Kleinsten Raumes. Dritter
Beitrag. Von C. G. Ehrenberg : Berlin, 1834. Also, tbe Edinburgh
New Philosophical Journal, for January 1836, p. 42.

ANGLES OF APERTURE. 16'7

struments which do not possess enormous 'penetrating power,
combined with the requisite amplification ; therefore, when we
compare the illustrious Professor's drawings of the Infusoria
with those of the same objects by Muller and the earlier ob
servers, we are presented with a veiy fair illustration (making
an allowance for the effects of colouring matter introduced
in the systems of the animalcules) of the difference in vision
produced b y large and small apertures ; for in the article of
magnifying power, the old instruments were quite equal to
those of the present day.

C. R. G.

CHAPTER VIII.

ON THE

CONSTRUCTION AND MANAGEMENT

OF

SOLAR AND OXY-HYDROGEN GAS MICROSCOPES, &c *

The opinions respecting the apparent magnitude of bodies
seen through optical instruments in general, are so conflicting,
and often deviate so widely from the truth, that it is of great im-
portance to be enabled to have recourse to any plan by which
their admeasurements can be accurately determined. With
the solar and gas microscopes, whose several properties I
propose to consider, a method of doing this is readily sup-
plied : for, by the contrivance of a screen, upon which the
images or pictures of objects are formed, their superficial con-
tents may be ascertained with the same precision as if they
were the real objects themselves. In this respect, therefore,
we have a remedy against those optical deceptions in the ap-
pearance of things, which frequently occasion no inconsider-
able inconvenience to an observer. Nor is this facility of com-

* This Tract was originally intended to have preceded Chap. IV. ; but
it was found more convenient to place Dr. Goring's papers together, as
his proofs had to be sent to North Devon for correction, and the engraver
had not then completed the wood-cuts.

SOLAR AND GAS MICROSCOPES. 169

puting correctly how much an object is magnified the only ad-
vantage peculiar to them ; for, with these and similar instru-
ments, a number of persons can view a living object at the same
instant of time, and remark upon its particular organization,
functions, and habits, with great exactness, and satisfaction to
themselves and to one another. The force of this observation
will be sufficiently manifest, if we reflect that most diminutive
living objects are constantly in motion, and that with the table
miscroscope or engiscope, before a second observer can possi-
bly note any peculiarity mentioned by the first, the creature
will most probably have changed its position, and the part to
which his attention was especially directed may be entirely
concealed from his view. Persons of weak sight,, who are un-
accustomed to look through a table microscope, (for it does not
occur to those who are well versed in these matters,) some-
times experience a little fatigue from the eye being intently
fixed for any considerable time upon the same object, and sub-
jected to the stimulating action of the intense light often
thrown upon it for its illumination. The inconvenience arising
from this does not apply to the solar and gas microscopes, the
absence of which may therefore be enumerated very fairly
among the advantages peculiar to them.

For the invention of the solar microscope we stand indebted
to the good genius of Dr. Leiberkhun, who, about the year
1738, contrived an apparatus, by means of which a distinct
resemblance of a magnified object;, strongly illuminated by the
condensed rays of the sun, (hence its denomination solar
microscope,} was thrown upon a white screen, and rendered
visible to any number of persons conveniently placed in a dark-
ened room. To this gentleman's ingenuity we owe the valu-
able application, also, of the concave silver speculum, for con-
centrating the light upon opaque objects viewed through the
table engiscope, as well as other improvements in that instru-
ment About the year 1774, Benjamin Martin published his
description of the solar microscope, and its use in examining

170 THE MICROGRAPHIA.

opaque objects ; subsequent to which, it underwent various
skilful alterations in the hands of Messrs. Cuff and Adams, the
latter of whom has given us a very minute account of the in-
strument, with drawings, in his quarto work on the Microscope,
published in the year 1787, — omitting, however, it may be
remarked, every particular respecting its optical construction.
Since that time it has remained, until lately, quite stationary ;
and no practical work whatever has been written upon the
subject.

In all the solar microscopes hitherto described, the optical
part consists of a simple convex lens placed between the object
(at a little more than its sidereal focal distance from it) and the
screen upon which the image is designed to be thrown : hence
it necessarily follows that they all have a considerable quan-
tity of aberration, arising both from the figure of the lens, and
the chromatic dispersion of the light. The former of these de-
fects Dr. Robison in a great degree remedied, by substituting
Rarnsden's eye- piece in the place of the common lens ; and Dr.
Goring, by the application of achromatic lenses to the instru-
ment, may be said to have effectually corrected them both*.

The oxy-hydrogen microscope, so attractingly exhibited in
the present day, and unquestionably meriting all the encourage-
ment that can possibly be bestowed upon it by the promoters
of rational instruction, may be defined to be a mere modifica-
tion of the solar, adapted to receive, and employ to the great-
est advantage, the rays of an artificial light diverging from a
central point, instead of the parallel rays from the sun. In
the year 1824, Dr. Eirkbeck delivered two lectures on optical
instruments at the London Mechanics' Institution ; in one of
which he took occasion to delineate on a screen, by means of

* Mr. Benjamin Martin mentions in his works the adaptation of achro-
matic lenses to the solar microscope ; but not being acquainted with the
value of angular aperture, he only gave them the same as was usual with
common lenses; hence they were laid aside as no better than the latter,
and may justly be said to be re-invented.

SOLAR AND GAS MICROSCOPES. 171

a large magic lantern, representations of magnified objects in-
tensely illuminated by the light emitted during the combustion
of lime by hydrogen and oxygen gases*, and to indicate the
practicability of applying successfully this method of illumina-
tion to the microscope. I would not omit, however, to men-
tion, that, about the same time, Mr. Woodward instituted some
experiments with the phantasmagoria, where the light was ob-
tained in the same way-]-. In the interval between that
and the present time, various amateurs and artists have studi-
ously exercised their talents in perfecting the several parts of
the instrument, which, like the solar, assumes its name from
the source whence the light requisite to its action is derived.

In the present treatise I intend to lay before the reader a
practical illustration of the construction of both the above-
mentioned instruments, commencing with such parts of them
as are common to the two, and then treating of those which
are pecvdiar to each, — remarking also, as I proceed, on the
various improvements they have recently undergone, and con-
cluding with such instructions for the management of them as,
I trust, will tend to remove those difficulties which have hitherto
obstructed their being brought into more frequent use.

METHOD OF ASCERTAINING THE MAGNIFYING POWERS
OF SOLAR AND GAS MICROSCOPES.

In all cases where the real dimensions of any object to be
viewed with these instruments is known, the magnifying power
may be readily computed, by simply measuring the length or
breadth of the image or picture formed on the screen, and di-
viding them by the length or breadth of the object itself. Thus,
for instance, suppose you had an object whose real length was
the tenth of an inch, and whose image on the screen measured
five feet in length, or sixty inches, the lineal magnifying power

* Mr. Cooper assisted Dr. Birkbeck in this 'experiment.

t Lieut. Drummond has ingeniously applied this light to Light-Houses
and to Geodretical operations ; Phil. Trans, for 1830, p* 383. See also the
same work for 1826, p. 324.

172 THE MICROGRAPHIA.

of the instrument would be 60 multiplied by 10, or 600. But
since the object has been magnified to the same extent in
breadth as well as in length, the superficial magnifying power
will be six hundred times six hundred, or 360,000 ; or, in other
words, it would require 360,000 of the original objects to cover
its magnified image. Some ingenious persons, whose design
I suppose it is to astonish the woi'ld, carry their estimate to
a much larger extent than this, and give to these instruments
the marvellous power of exhibiting the solid as well as the
superficial magnified representation of an object ; but as it is
clear that the superficies alone, without any portion of the
thickness of the object, can be delineated on the screen, the
superficial magnitude must suffice. The solid content is known
(but cannot be seen) by multiplying the superficies by the dia-
meter or lineal magnification. Another mode of determining
the magnifying power is to measure the distance of the screen
from the lens of the instrument, and divide that by the distance
of the object from the lens. Thus — suppose the screen to be
placed twelve feet distant from the lens, and the object one
inch from it ; divide twelve feet by one inch, and the quotient
is 144, the lineal magnifying power of the instrument : conse-
quently, the superficial magnifying poAver will be the square
of 144, or 20,736. This result will not be accurately true,
although sufficiently so for ordinary purposes : for in prac-
tice it is a point of difficulty to determine the focal centre of a
lens from which the measurements ought to be taken, inas-
much as the thickness, form, and position (even of a single
lens) must be considered, before the acting focal length can
be strictly ascertained. In a combination of lenses, the mat-
ter of course becomes much more complex. I recommend,
therefore, adopting the first method, and throwing the image
of a micrometer upon the screen. In all cases where the mag-
nifying power is not great, a disc with a circular aperture of
known dimension, placed as an object, will readily determine
its extent.

SOLAR AND GAS MICROSCOPES. 173

THE SCREEN.

The next thing in common, connected with the two instru-
ments, is the Screen, upon which the image of the objects is
displayed. In constructing this, every care should be taken
to render its surface as smooth, white, and opaque, as it can
be made; the chief consideration being, that it should reflect
the greatest possible quantity of light, and absorb the least.
The material usually selected for the purpose is a sheet of can-
vass properly stretched upon a frame, and painted with two or
three coats of white paint ; in doiug which, some attention is
required to smooth the surface between each layer with
pumice-stone, or any other suitable substance, so that, when
finished, it may be as plane and free from prominences or cavi-
ties as pains can form it. It is scarcely necessary to mention,
that the purest white lead should be obtained, inasmuch as the
brilliancy and perfectness of the picture will greatly depend
upon the whiteness, and the sharpness of its outline upon the
smoothness of the screen.

A screen of a superior kind may be produced by spreading
a thin coat of plaster of Paris upon the flattened surface of a
well-constructed walk This, of course, will be a fixture ; nor
do I know that the last material can be used conveniently in
any other way, unless on a small scale, when it may be attached
to a moveable board or frame. The form of the screen will be
regulated chiefly by the height and size of the room, and in
some degree by the nature of the exhibition ; but, in all cases
where a circular one of sufficient dimensions can be contrived,
it will succeed the best : it must be situated at right angles to
the axis of the instrument. I may remark also, that, with the
exception of the screen, the whole interior of the apartment
should be made as black and sombre as possible, in order to
produce a good effect.

174 THE MICKOGRAPHIA.

The position of the screen with regard to the instrument is
of considerable importance ; for if its centre is not placed op-
posite the axis of the instrument, and its surface in a plane
every way at right angles to it, the picture will be a distorted
representation of the object, its parts being magnified differ-
ently, according to their distance from the instrument.

CONSTRUCTION.

The optical construction of the solar and gas microscopes
may be divided into two distinct parts : First, that which is
necessary for magnifying the object, and depicting its image
on the screen — which is, strictly speaking, the microscopic
part ; and, secondly, that by which the light is condensed
upon the object — which may be termed the illuminating part.
The microscopic part may be the same in both these instru-
ments, and, by a suitable arrangement in the mounting, in the
Table Engiscope also ; but the illuminating part must of ne-
cessity be different. Pursuing the order, therefore, adopted
at the commencement, I shall first describe that which is, or
may be rendered, common to both — the microscopic part.

If we take a common convex lens, as shewn at A (Fig. 1),
and place an object strongly illuminated at B, sp that an image
of it may be projected at C, that image of it will be an in-
verted magnified representation of the object. By a little consi-
deration it will be evident, that all that portion of the light
emanating from B, which conduces to form the picture at C,
must pass through the aperture of the lens A, — the surplus
rays, as indicated by the figure, being entirely obstructed and
cut off. Now, in proportion as the picture C is enlarged, the
available portion of the light will be spread over a larger sur-
face, and consequently the picture will be more and more di-
luted. To remedy this defect, it would seem that you have
only to extend the aperture of the lens by substituting one of

SOLAR AND GAS MICROSCOPES.

Fig. I.

cq

175

greater diameter, which shall transmit an additional quantity
of light ; but, by doing this, the following evils will arise : —

I. The aberration occasioned by the spherical figure of the
lens will be greatly increased, and the image will be less de-
fined.

II. The dispersion arising from the unequal refrangibility of
the light will produce a strong colouring throughout tbe image.

III. The aberration of the oblique pencils will cause indis-
tinctness and colour around the edges of the field of view.

176 THE MICROGRAPHIA.

IV. The image being formed in a caustic, and not in a plane,
it will not be distinct in all its parts on a flat screen, with the
same adjustment of the instrument.

I shall now consider how these defects are to be obviated,
taking them separately in the order I have noticed them.

1 . The first defect arising from the aberration occasioned by
the form of the lens may be lessened by employing, in the
place of a common convex lens, another of such a substance
and form as shall cause the least aberration. To determine
this, you must first ascertain the refractive power of the glass
or gem of which the lens is to be made, and the distance at
which the screen is to be placed from it; for it may be coi-rect
enough at one distance, but will not be equally so at another.
It may be mentioned here, that, with ordinary glass, the form
in most cases should be a plano-convex ; the flat side being
placed towards the object. But since the aperture in a single lens
of the best figure cannot be greatly enlarged, it has been found
expedient to have recourse to a combination of lenses. The
most efficient combination I have met with for practical uses
is a modification of Ramsden's eye-piece, before alluded to.
When this is employed for high powers, it becomes of course
the doublet of Dr. Wollaston, as described in the Microscopic
Cabinet, p. 162. This combination, indeed, if properly exe-
cuted, is the most efficacious that has hitherto been used
for deep powers in the gas microscope. The form of the lenses
and their arrangement is shewn at Fig. II. For further particu-
lars respecting spherical aberration, see Microscopic Cabinet,
p. 175, §4, and p. 197.

Fig. 2,

SOLAR AND GAS MICROSCOPES.

177

II. The second defect, chromatic dispersion, arising from
the unequal refrangibility of the light, may be remedied, as
before mentioned, by substituting in place of a single lens an
achromatic combination of lenses, which may be so formed
as to obviate spherical aberration also. In the solar micros-
cope, this combination has been used with considerable advan-
tage ; and there can be little doubt, that, for all scientific pur-
poses where the utmost definition and correctness are required,
it is by far the most perfect construction of any that has been
hitherto discovered. It might probably be inferred from
what has been said, that an achromatic object-glass completely
rectified for a table engiscope would of necessity be adapted

Fig. III.

178 THE MICROGRAPHIA.

for the solar ; and so it would, as far as the two instruments
correspond: but since the angle of the field of view in the
former is generally much less than that in the latter, an achro-
matic object-glass suitable to the one may be wholly unfit for
the other, and thus be found quite defective. For instance,
let A (Fig. III.) be the object-glass, B the object, and c c the
diameter of the field-bar of a table engiscope : now it is evi-
dent, that so much only of the object can be seen in the engis-
cope as occupies the space c c: but the illuminated disc, or
field of view, on the screen of a solar microscope, should sub-
tend a larger angle than that subtended by c c, viz. that under
G G : all the marginal rays, therefore, beyond c c may pass un-
corrected, and consequently may produce a distorted, coloured,
and undefined picture.

It may be remarked here, that the field-bar of a table engis-
cope generally subtends an angle of from five to ten degrees,
and seldom or ever exceeds fifteen ; whilst the solar and gas
microscopes should admit of an angle of nearly thirty degrees ;
indeed, I have seen some which subtended 45°. As, therefore,
the area of the field of view in both instruments varies as the
square of their diameters, in the most favourable case the latter
will be four times the extent of the former.

The construction of achromatic object-glasses having been
amply treated of in our other works, it would be improper to
repeat it here : there are, however, a few observations con-
nected with them that suggest themselves, which, if completely
carried into effect, would probably render the solar microscope
the most valuable instrument which the naturalist can possess,
enabling him to obtain larger angles of aperture, and greater
space between the object and magnifiers, — the first giving in-
creased means of penetrating into the structure of bodies, and
the latter affording practical facilities in the admission of ob-
jects, the want of which renders the best reflecting microscopes
useless for the examination of a numerous class of objects. —
First, then, the posterior focal distance of the object-glass in

SOLAR AND GAS MICROSCOPES. 179

a solar microscope being longer than in an engiscope, enables
us with it to obtain a larger angular aperture without any
augmentation of its real aperture, while it is now well
ascertained that, in proportion as we reduce the length of the
body, (that is, shorten the posterior focus of the object-glass or
distance A c, Fig. III.) we must reduce its aperture to obtain
all the necessary corrections. Thus, for example, suppose
we have an inch-triple achromatic object-glass, and employ it
with a body ten inches long, we may obtain an angle of aper-
ture of 18° ; but if we shorten down the body to five inches,,
then, to obtain the requisite corrections, it will be necessary
to reduce its aperture to about 14°. Hence, in solar micros-
copes, where we have the posterior focus elongated twenty or
thirty times, it is probable that a much larger angle of aperture
might be obtained; and thus, with an object-glass of no shorter
focal length than one quarter of an inch, we might probably
obtain an aperture sufficient to penetrate into the structure of
every object we are now acquainted with.

I should not omit to notice, that when the bodies with deep
object-glasses, say one-tenth of an inch focus, are reduced one-
half, their angular aperture remains nearly the same ; and the
reason is obvious. In the case of the inch object-glass we
reduce the proportion between its anterior and posterior foci as
low as one to five ; but, with the same reduction of body and the
one-tenth object-glass, the proportion remains as one to fifty.

I may remark, as a caution to those who fit up solar micro-
scopes with achromatics, that the lenses must not be cemented,
as they may in use be soon spoiled by the heat of the sun.

The practical rules for determining how far any combination
of lenses is free from chromatic dispersion, are amply detailed
in the Microscopic Cabinet, p. 200 : see also Test Objects,
chap 16.

III. The third defect, the aberration of the oblique pencils
of rays, will be better explained by referring to Fig. IV.,
where B represents the object: from any point B'not in the

180

THE MICROGRAPHIA.

axis of the lens, let a pencil of rays be incident upon the lens,
and refracted to form an image at that point at F. From the

Fig. IV.

spherical figure of the lens, the extreme or marginal rays
of the pencil will undergo greater refraction than those repre-
sented by a a, nearly coinciding with the axis of the pencil,
and will meet at m, the focus of marginal rays, whilst those
near the axis of the pencil will proceed on to c, the focus of
central rays ; consequently, an indistinct circular represen-
tation of the point B will be found between b and m, the longi-

SOLAR AND GAS MICROSCOPES.

181

tudinal aberration of the oblique pencil. A similar effect will
take place in all the other pencils passing through the lens
from every point of the object B.

This aberration of the oblique pencils may be corrected by
a proper form and combination in the achromatic object-glass ;
for although it is a manifest and visible error, it would not be
advisable, in practice, to construct a form exclusively to remedy
it, unless, indeed, when very low powers only are to be em-
ployed ; in which case it can in a great degree be done by a
right adjustment as to distance of the lenses shewn at Fig. II.,
and giving their surfaces the necessary curvature, so that the
defect will be imperceptible.

Fig. V.

IV. The last defect I have to notice, will also be better un-
derstood by a direct reference to Fig. V., where A is the lens,

182 THE MICROGRAPHIA.

B the object, and C its image. As the several portions of the
object B are at unequal distances from the centre of the lens A,
the image C, instead of being formed on a plain surface, will
assume a curve, as represented at C. To remedy this, it is only
necessary that the screen should be constructed of a corre-
sponding curvature ; but, in practice on a large scale, this -will
be extremely difficult : moreover, with every change of magni-
fying power, or variation of distance between the lens and the
screen, a new curve for the latter would be required. By
adopting the converse method, however, and mounting the
object between two surfaces of the proper curvature, such as
watch-glasses, an image will be seen equally distinct through-
out the entire field : thin sections of vegetables, specimens of
gauze, lace, and other flexible substances, are viewed extremely
well when mounted in this way, especially when the angle of
the field of view exceeds 30 degrees ; although it is not ad-
visable to employ a larger angle if it can be avoided.

Mr. Coddington, who appears to have been the first person
to investigate this error in microscopes, has succeeded to a
great extent in correcting it, by substituting spheres of glass
in place of other lenses. This contrivance is shewn at Fig. VI.,
where the aperture of the sphere is defined by a groove cut
about its equatorial parts.

Fig. VI.

" The only point of consequence is," says Mr. Coddington,
" that the rays which pass through this magnifier should tra-
verse both the refracting surfaces without any obliquity, by
which means the whole field of view is equally distinct." The
two refracting surfaces here alluded to are, of course, portions

SOLAR AND GAS MICROSCOPES. 183

of the same sphere., and, if continued, would meet. I have
made this remark simply because, in all the Coddington lenses
that I have examined, there is a great fault in the making of
them : few, if any, do form perfect spheres, if thus continued.
Mr. Coddington has recommended me to make these spheres
of rock crystal ;* and Sir D. Brewster considers that they
would be quite perfect, if composed of garnet, and used in
homogeneous light.

FOCAL LENGTH OF MAGNIFIERS USED IN SOLAR AND
GAS MICROSCOPES.

The range of magnifiers suitable for either of these instru-
ments depends upon the dimensions and arrangement of their
illuminating apparatus. In the larger ones, which are by far
the most instructive and amusing, and best adapted for public
exhibitions, a great extent of surface can be intensely illumi-
nated, and therefore very feeble magnifiers can be employed ;
the lowest having a sidereal focal length of about three inches,
and thence proceeding upwards to a quarter of an inch. If
they are much shorter than a quarter of an inch, unless the
field of view be small, and the instrument very accurately con-
structed in every part, the image on the screen will be little
more than a gigantic coloured shadow of the object. Indeed
I may safely assert, with respect to the gas microscope at
least, that, with its most extravagant magnifying powers,
none of the minutiae of an object have been displayed which
could not be seen through a simple microscope with a single
lens or doublet of a quarter of an inch sidereal focus. With
the solar, a magnifier as high as the sixteenth of an inch may be
advantageously employed, although half that power will be

* I find, with this medium, that a slight obliquity produces double re-
fractiou.

184

THE MICROGRAPHIA.

found amply sufficient for most scientific purposes. I am
aware that, in Germany, the polygastric sacs, or stomachs, in
the smallest genus of animalcules, the Monads, are said to
have been rendered distinctly visible with the solar microscope ;
to effect which, a power of the sixteenth of an inch, if not a
deeper one, would certainly be required. The most splendid

Fig. VII.

SOLAR AND GAS MICROSCOPES. 185

solar that has ever been produced in this country had a set of
achromatic lenses, whose foci ranged from an inch and a half
to two-tenths of an inch ; but, in some that I have made with
achromatics of two inches focus, even their superiority over
common lenses was decidedly manifest ; and, as the powers
become deeper, the advantages to be derived from the achro-
matic construction increase in a high ratio.

ILLUMINATION FOR SOLAR MICROSCOPES.

Transparent Objects.

Fig. VII. will illustrate the construction of the illuminating
apparatus of a solar microscope. D is the large convex lens for
condensing the parallel rays of the sun ; F the sidereal focus of
the lens D. The condensed rays of light, after crossing at the
point F, will divei'ge towards the screen 1 K M, and may be
supposed to occupy the space I K. Now, if the instrument
be so constructed as to require the object to be placed at
the point B, it is evident that the central portion of the object
through which the condensed rays must pass, will be power-
fully illuminated. But if the microscopic part of the instru-
ment, viz. the magnifier, will take in a larger portion of the ob-
ject than is thus illuminated by the lens D, it is evident that
we shall not obtain so extensive a field of view as the micro-
scopic part of the instrument will admit of. To supply this
deficiency, an additional lens E, as shown in the drawing be-
neath, may be introduced at the dotted line, the effect of which
will be to make the rays converge to a point much nearer to
the lens D than is its sidereal focus F, and consequently to
increase the angle of divergence I F K, — thereby occasioning the
rays to illuminate an enlarged space, 1 M on the screen, by which
means the field of view is greatly extended. If the object B be
now placed close to the field lens E, every portion of it will be
illuminated. As, however, different sized objects require differ-

]86 THE MICR0GRAPH1A.

ent magnifying powers to display them properly, it is neces-
sary, in like manner, that the field lenses be suitable to the
powers employed.*

Since it is necessary that the sun's rays should pass directly
through the lens, in order to form a proper disc upon the
screen, it will appear evident, either that the axis of the con-
denser D must be directed towards the sun, or that a plane
mirror be so placed behind D as to reflect those rays along
its axis. The latter method is usually adopted, as being by
far more convenient and simpler than to be continually chang-
ing the position of the instrument and of the screen. It is im-
portant that the mirror be made as perfect as possible, lest a
considerable portion of the light be absorbed, or otherwise in-
jured. For this purpose we generally employ a silvered look-
ing-glass, (a speculum of sufficient size being too costly,) which
should be of thin Dutch plate-glass, as free from colour as it
can be procured : if it be not thin, the images of the sun re-
flected from each of its surfaces will be so far separated as to
be visible on the screen, projecting a double image of the ob-
ject, the one overlapping the other.

The intensity of the light condensed upon the object by the
lens D, Fig. VII., (if there were no light lost in reflection from
the mirror, or from the surfaces of the condenser, and none
absorbed in passing through it,) will be in the proportion of the
squares of their respective diameters. Thus, if the diameter
of the condenser were six times that of the object, the latter
would be illuminated by a light 36 times as intense as that
direct from the sun. If it were wished therefore, to magnify,
the same number of times, two objects of unequal size, which
should be equally illuminated, it is clear you would not require
so large a condenser for the smaller object as for the other :

* In all solar microscopes, the construction requires the illuminating
rays from the object to be converging ; but it is probable, if they were di-
verging, that a superior definition would be obtained, as is found to be the
case with delicate objects under an engiscope.

SOLAR AND GAS MICROSCOPES. 187

hence, for a solar intended to exhibit large objects, where a
considerable surface of condensed light is absolutely necessary,
or where very high magnifying powers are to be used, which
greatly extend the disc over which the light is to be spread,
a condenser of comparatively large diameter will be required.

The diameters of condensers for solar microscopes vary
from an inch and a half to about six or seven inches, — those
of four or five inches being the most convenient and generally
useful. If they are much smaller than that, they will not suffi-
ciently illuminate large objects, and are consequently suited
only for a limited class of them ; and when they exceed seven
or eight inches, the heat becomes so intense, that it will scorch
and burn up the objects placed in or near the focus, and even
fuse the magnifiers themselves.

Having determined on the diameter of your condenser, the
next point to be considered is its focal length, which should
be such that the prismatic dispersion may interfere as little as
possible. If the focal length be too short, the image will be
coloured, notwithstanding you have an achromatic object-glass ;
and if it be too long, it will occasion great strain upon the in-
strument. Those I have found to answer the best have been
in about the proportion of four or five times the diameter. The
foci and diameters of the field lenses (see E, Fig. VII.) must
be decided upon by trial, after the magnifiers or object-glasses
have been selected.*

Under any circumstances it will be evident, that whenever
a single condensing lens is used, some portion of the light will
of necessity be dispersed ; but if there were substituted in its
stead a portion of a paraboloid reflector, as proposed by the
Rev. Mr. Packman, this defect would be entirely obviated, and
you would therewith collect an intense spot of pure white
light.

* See p. 87.

188

THE MICROGRAPHIA.

Fig. VIII.

Fig. VIII. represents a section of such a reflector, with its
apex cut off; F is its focus for parallel rays, within which the
object is to be placed. By a principle of the parabola, all
the rays of the sun impinging upon its surface will be reflected
towards F, without aberration or dispersion ; and thus the ob-
ject intercepting them will be intensely illuminated without
any coloured fringes.

Opaque Objects.

The usual method of constructing a microscope for display-
ing this boundless class of objects, in all their varieties and
beauties, augmented by the reflection of the solar light con-
densed upon them, is shewn at Fig. IX., where S represents
the condenser, B the object, N a small reflector, and A the mag-
nifier. Since, in this case, the light is to be reflected from the
object, and not transmitted through it, a plane reflector is placed
at N, in order that the condensed rays falling upon it may be
thrown upon the object, and thence proceed, as in the case of
transparent objects, to depict its image on the screen. The
reflector so used is generally a piece of looking-glass, acted
upon by an adjusting screw from behind, so as to vary its inclina-

SOLAR AND GAS MICROSCOPES.

189

tioiv, and suit the different objects to be viewed ; but here a small
speculum would be an improvement worth the additional cost*

Fig. IX.

ap

Another method of illuminating opaque objects is by means
of four smaller condensers arranged in a circle, from which the
condensed light, ere it arrive at their focal points, is received
upon four small reflectors so situated as to throw the whole of
the light upon the object. This plan will be readily understood
by a reference to Fig. X., where d d represent two of the four

* See Fig. III. p. 85.

190

THE MICROGRAPHIA.

condensers, n n their corresponding mirrors, and A the position
of the magnifier. An instrument thus constructed has been
made in America, and found to answer very well.

Fig. X.

*H

The following plan, contrived by myself, has many
advantages. It consists of one condenser (the same as is
used for transparent objects) and a silvered concave spherical
reflector, or cup, with an aperture through its centre to admit
of the condensed light proceeding to the magnifier. This ar-
rangement is given at Fig. XI., where S is the condenser, B
the object, R the concave reflector, by means of which the
rays, partly condensed by passing through the lens S, are col-
lected and thrown upon B, and thence proceed through the
aperture to the magnifier or object-glass a.

Fig. XI.

R

-^

¥

By observing the preceding construction, it will be evident,
that if the concave reflector R were of equal diameter with the

SOLAR AND GAS MrCROSCOPES. 191

condenser D, the latter might be wholly dispensed with : this,
doubtless, would be the simplest method of illuminating opaque
objects, and at the same time would possess the additional
advantage of exemption from refrangibility. But since, in prac-
tice, it is not advisable to construct a solar microscope that
will not exhibit transparent as well as opaque objects, and since
the condensing lens conduces effectually towards illuminating
them both, it would be a wasteful expense to make the reflector
of such laroe dimensions.

ILLUMINATION FOR GAS MICROSCOPES.

It has been assumed (which, however, is notstrictly true,
although sufficiently so for practical uses), that the rays of light
emitted from the sun are parallel to each other, and that it
belongs therefore to the illuminating portion of the solar
microscope merely to divert them from their parallel course,
and make them converge toward the object intended to be
illuminated. In the case now under consideration, the rays
emanating from an artificial light placed at a short distance
from the condenser are divergent, and all except the central
ones fall obliquely upon the surface of the lens ; hence a double
operation must be performed upon them before they can be
made, as in the former instance, to converge upon the object
interposed for illumination : that is to say, it is necessary first
to bring them parallel, and then, as in the instance of the solar,
to converge them toward the object they are intended to
illuminate. This, however, may be effected, as we shall pre-
sently see, with a single reflector also. In both cases the
main object to be attained is to collect the greatest possible
number of rays that can be taken up ; to accomplish which
with a lens, the surface next the light should be concave, or at
least a piano, otherwise the rays nearest its margin will, owing
to their great obliquity, be reflected from and not refracted

192 THE MICROGRAPHIA.

through it. From the numerous experiments I have made in
constructing gas microscopes, I find the best arrangement for
the illuminating part, when lenses are employed, to be similar
to that shewn in figure 12 ; where a plano-convex lens, D, is

first placed with its flat surface next the light, G, and at such
a distance from it as to bring the divergent rays nearly parallel '■>
and then, in close connexion with it, a double convex lens, 'D,
to condense them upon the object, B. In some instances I
have used three plano-convex lenses, but I do not think the ad-
vantage obtained by thus dividing the refractions at the surfaces
compensates for the loss sustained by the introduction of an
extra lens*. The contrivance given at figure 12 answers so
completely that it leaves little room for any improvement to be
effected by a combination of lenses, an angle of light of between
50 and 60 degrees being taken up.

In order to increase the quantity of light, it has been pro-
posed to place a reflector behind it, but this cannot be made
to produce the desired effect unless its centre of curvature
coincides with the radiant; for the reflected rays not having
the same divergence as those proceeding direct to the lens
from the light itself, cannot be taken up so as to afford any
additional advantage in the illumination. If, however, a simple

* In cases where the diameters of the condensing lenses are great, and
the focus short, it might be an improvement to use fluid lenses ; or even,
in some cases, Sir D. Brewster's polygonal lenses might be serviceable,,

SOLAR AND GAS MICROSCOPES.

193

concave reflector properly constructed be used without any
lenses, the result will be quite equal, if not superior, to that ob-
tained by the preceding management, and at the same time will
obviate one of the greatest defects there is to contend with,
viz. that arising from refrangibility. This construction is given
at figure 13, where R is the reflector, G the ignited lime, and

Fig. 13.

B the object : a field lens, F, is introduced in the diagram,
and might be useful in some cases.

In gas microscopes constructed as above, with only a
reflector for illumination, on removing the magnifiers in front
the illuminated portion of the screen will not be a complete
circle, the lime, jets, &c. interposing between the two, shewing
their shadow upon it. I am informed by a person who has
experimented with a reflector in this way, that we cannot
obtain so great a range of distance between the screen and
instrument as when lenses are used, so that a certain radius of
curvature is suited to one distance only : of this circumstance
I am unacquainted, not having observed it when making my
experiments about four years ago.

Before proceeding to the consideration of the illumination of
opaque objects, it will be advisable to introduce a few remarks
on the above.

It has been stated in the Microscopic Cabinet, that objects

194 THE MICROGBAPHIA.

are best defined when viewed by diverging rays. In the solar
microscope this is difficult to accomplish, but it will be seen
that in the gas microscopes we have divergent rays ; and I have
little doubt but for large objects it would be a vast improvement
to introduce them close to the condensing lens, on the side
next the light. This arrangement would have the advantage
of enabling us to reduce their diameter — a point of no small
importance, as the thickness of glass in large deep lenses is
considerable, and the loss of light appreciable.

In the solar microscope, the Rev. J. B. Reade has recently
introduced astronomical slides, and those of the phantasmagoria,
between the large reflector and condensing lens, with effect ;
and indeed all the best instruments of that kind should be pro-
vided with the means of such adaptation.

I cannot omit to notice a remarkable error which is to be
found in all the optical treatises from Gravesande's down to
those of the present day, respecting the construction of the
phantasmagoria and magic-lanthorn. In all of these, the
slide containing the figures to be projected on the screen is
described and drawn as placed between the plano-convex bull's-
eye condenser and the magnifier — that is to say, in converging
rays, whereas, in practice, the slide is placed next the light,
and close behind the bull's-eye, so that in fact they are in
diverging rays, and the bull's-eye condenser and magnifier
act as a doublet, and the distortion arising from the sphericity
of the condenser is thus greatly diminished.

In my lucernal microscope, where a moderately magnified
picture of the object is thrown upon a plate of gi-eyed glass, for
drawing its outline, &c, or for enabling several persons to
view it at the same time, the principle of the optical construc-
tion which I adopt is similar.

Opaque Objects.

The simplest method of illuminating opaque objects in gas
microscopes is by means of a concave reflector only, as shewn

SOLAR AND GAS MICROSCOPES. 195

in fig. 13; in which case, however, the object and the light, B
and G, are required to change places, and the reflector must be
provided with a sufficient aperture in its centre to admit of the
rays of light proceeding freely from the object toward the mag-
nifier behind it. By inspecting this figure with the alteration
proposed, it will be manifest that the cone of rays incident upon
the mirror will of necessity be very small, and consequently
that large objects only can be sufficiently illuminated by it.

Another mode is to receive the rays at B, (fig. 12) upon a
concave reflector, similar to that represented in fig. 1 1 ; but it
may be remarked, that in no instance has it been practicable
to introduce, for displaying opaque objects, magnifiers of equal
powers with those used for transparent ones, and solely on
account of a deficiency in the illumination.

Though the description of an apparatus answerable for the
purposes of containing the gases and regulating a due supply
of them for combustion, should in strictness fall within the
province of the chemist rather than the optician, still, as the
subject seems to be almost naturally brought under our con-
sideration, a few observations respecting it will contribute, I
think, towards rendering this tract so much the more complete.
Before I proceed, therefore, with the rules necessary to be
observed in the management of the instruments themselves, I
shall take this opportunity of stating some particulars as to the
regulation of the gases. It is scarcely requisite to mention
the propriety of having every part of a gas apparatus, together
with all the pipes, stop-cocks, joints, &c. used in connexion
with it, perfectly air-tight, both as far as safety is concerned,
and to prevent the waste and loss of a valuable commodity.

Fig. 14 exhibits the form of an apparatus suitable for the
purpose. O and H are two cylindrical vessels of sufficient
capacity to contain gases for one or more representations ;
P P are two pipes, with stop-cocks at p p, and apertures to
admit of their discharging themselves into the bottom of the

196 THE MICROGRAPHIA.

vessels O and H ; T represents a tank or reservoir of water, to
Fig. 14.

supply an uniform pressure or force to expel the gases from
the vessels ; o and h are exit pipes fitted to communicate with
the jet, where the ignition is to take place. The action of
this apparatus is as follows : the vessels, O and H, being filled
with the gases, and the closed tank, T, with water, the pipes
o and h are to be attached by means of union-joints to the
feeding pipes of the jet. If the stop-cocks of the tank are now
opened, the funnels at P P will be filled with water, on the
principle of the common bird fountain ; the lower cocks, at
p p, may then be opened, when the water issuing from the
apertures at the bottom of the pipes will ascend in the vessels,
and so compress the gas that it will exert itself to escape.
The pressure of the water against the under surface of the gas
will be equal to that of a column of water of the height of the
water in the funnel above that to which it can ascend in the
vessel ; the pipes, P P, must therefore be of sufficient length,
viz. 20 to 30 inches, to admit of the pressure overcoming the
resistance occasioned by the gas passing through the tubes,
and to discharge it from the mouth of the jet with sufficient

SOLAR AND GAS MICROSCOPES. 197

force against a body of lime prepared for combustion.* The
stop-cock at h may now be opened, and the hydrogen gas
issuing from a proper aperture in the jet must then be inflamed.
In like manner, the stop-cock at o being turned, a due supply
of the oxygen will also be furnished.

The mode of filling the vessels with the gases may be thus
explained : the stop-cocks at o h and P P being opened, the
water from the tank will flow freely into the vessels O and H ;
as soon, then, as they are filled, let all the stop-cocks be closed.
The caps of the induction orifices or shallow tubes situated at
I, should next be unscrewed, and the extremity of the small
pipe through which the gas is to pass inserted at the induction
orifice : as the gas rises to the top of the vessel, and there dis-
places the water, the latter will continue to flow out at I,
until the vessel is filled with gas, when, the caps being screwed
on tightly, the apparatus will be ready for use.

I would here remark, that each of the vessels should be
furnished with a glass gauge-pipe, communicating with the
interior, for the purpose of indicating the quantity of gas in the
vessel, and how much is consumed in a given time. It may
also be stated, that if there be no gas in the vessels or feeding
tubes, the water will be liable to be expelled from the jet,
destroying the lime, and otherwise damaging the apparatus.
The tank should be constantly filled with water, which may
readily be done, whilst the lower cocks are closed by means
of the funnel at the top. The great advantage, however, of
my closed tank over an open cistern is, that the pressure upon
the gases is regulated by the length of the pipes, P P, without
any reference to the variable height of the water in the tank.
This is a point of vast importance, inasmuch as the intensity
and steadiness of the light will depend entirely upon an uniform

* Tt will be found in practice much better to employ only one funnel
and stop-cock to the tank, T, and make the pipe, P, branch oft' to each
vessel : I have recently learnt that Mr. Maugham has proposed the latter
plan, but in place of the tank, T, he uses a ball-cock to regulate the supply
of water.

198

THE W1CR0GRAPHIA.

pressure : with a varying pressure the brilliancy of the image
on the screen will be continually varying also.

Fig. 15 illustrates a method of constructing the jet, and how

Fig. 15,

the lime is disposed for combustion : o and h represent the
feeding-pipes of the jet, which are to be attached to the corre-
sponding tubes of the vessels O and H, designated by the same
letters. At S are situated two of Hemming's safety-tubes,
containing bundles of fine copper wire, gauze, or asbestos, to
cool the gases, and prevent explosion, should any accident
occasion the ignited gas to return towards the vessels. When
speaking of the return of the gases in the direction of the
vessels, and thus forming in the tubes, or vessels themselves,
an highly explosive mixture, there is one point to which
I have not yet alluded, but which 1 conceive to be of
the greatest possible importance ; and it is this — viz. that
the areas of the bases of the vessels H and O should bear
exactly the same proportion to one another as that in which
the hydrogen and oxygen gases, in reference to volume, are
required to be used for consumption. The necessity of this
will be sufficiently manifest, since more than twice the quantity
of hydrogen to that of oxygen is expended in producing

SOLAR AND GAS MICROSCOPES. 199

the requisite light. Whilst the gases are being consumed,
their under surfaces, in the vessels H and O, by the pro'
posed arrangement will be continually kept at equal alti-
tudes ; and since the water ascends to these under surfaces,
and the pressure there exerted is dependent upon these altitudes,
as before mentioned, the pressure in both vessels will remain
uniformly the same. By admitting, therefore, twice as much
hydrogen as oxygen to pass through the stop-cocks of the
feeding-pipes of the jet, the two gases, in their due proportion,
will arrive at the mixing-chamber, C, pressed forward by equal
forces ; nor will there be any disposition in the one to over-
come the force of the other, and thus to repel the mixed gas
from the chamber, C, back into either of the vessels, but it
will proceed steadily on to the mouth of the jet, and so much
of the gases only will be admitted to mix as can be contained
in the small chamber at C. R represents the rod upon
which the lime, in the form either of a sphere or cylinder,
is sustained — (the latter form for ordinary uses is generally
preferred.) In either shape the lime, by means of clock-work,
or the band, should be made to revolve, and thus present a
new surface to the ignited gas, otherwise it will be liable to
burn away unequally, and to burst. The lime cylinder is some-
times placed horizontally, and the flame brought to play upon
its base ; but this arrangement does not afford so steady a
light, and a cavity being soon produced by the combustion, a
strong shadow will be thrown upon the screen.

I next proceed to describe an apparatus of a different con-
struction, and possessing the immense advantage over the other
of requiring but a comparatively small quantity of water for its
use. I conceived the plan of this from reading an account of
one constructed in Turkey, on a similar principle, by Mr.
W. H. Barlow.* Mine differs from that gentleman's, however,
in two material respects : 1st, I am enabled to dispense with a
vast proportion of the water requisite for his apparatus ; and,
2dly, by substituting weights in the place of water, I give it
* See Phil. Mag. vol. viii. p. 240, 3d Series.

200

THE MICROGRAPHIA.

additional steadiness, by bringing down the centre of flotation
to a much less elevated point. Let A, fig. 16, be a section of

Fig. 16.

a cylindrical vessel composed of copper or tin, and connected
at the bottom to a small cylinder in the situation of B. A
third cylinder, D, of equal length with A, and about three
inches less in diameter, is so placed within the vessel A that
it can be moved up and down with perfect freedom, and admit
of a small quantity of water being contained between its outer
surface and the inner surface of A. The cylinder D is fur-
nished with a diaphragm, situated at such a distance from its
lower extremity that when D shall descend to the bottom of the
vessel A, the diaphragm may coincide with the upper extremity
of the cylinder B. The cavity represented at B will be of
nearly the same dimensions as are required in the vessel D, to
contain a volume of gas sufficient for a specified time. The
operation of this apparatus is as follows : a small quantity
of water is poured into the vessel A, and D is then inserted
into it. The stop-cock at g being opened, as the diaphragm
and cylinder D descend to B, the atmospheric air issuing
through the orifice at G will pass along the pipe, and make
its escape at g. If the gas be now admitted at g, it will
occasion the floating vessel, D, to ascend in A, until the space
G is taken up by the quantity of gas required. The stop-
cock, g, may then be closed, and the water in the vessels will
stand at the same level, m, within and without the floating

SOLAR AND GAS MICROSCOPES. 201

vessel, D. If weights are now placed at W, in the cylinder D,
and additional water be poured into the vessel A, it will rise on
the outside above m to some height, M, to be determined by
the requisite amount of pressure. On opening the stop-cock,
an uniform supply of gas will then be discharged at g, and as
the cylinder D descends, the water will remain constant at
the levels M and m. When it is necessary to replenish the
vessel with gas, the weights at W must be removed by the
hand, or otherwise, and the operation conducted in the order
previously described. In Mr. Barlow's apparatus, the spaces
represented at B and W, the part of D situated above its dia-
phragm, are occupied with water, which, in case of leakage,
may be exceedingly troublesome, and must be always incon-
venient, from the necessity of removing the whole of that con-
tained in D, whenever the vessel is to be refilled with gas.
Further, in order to obtain the requisite pressure with water,
the portion of D over the diaphragm must be nearly filled with
it, when its centre of gravity will be somewhere at C ; but in
the case of weights, when sand, stone, or metal of greater
specific gravity than water, is used, the centre of gravity will
be somewhere at W, which being below M will render the
flotation far more steady than if it were above it, at C.

Since the gases must be kept apart from each other, the like
construction will be required for the use of each; and the
cylindrical vessels of the two apparatus being made of the
same elevation, should be so proportioned with respect to their
diameters, that the same height of gas may be consumed from
the one as from the other in a given time.

The description of jet used by Mr. Barlow is shewn at fig.
17: it is commonly known as Daniell's jet. It consists of two
tubes, a and O, the one inserted within the other, in the manner
represented by the figure. The outer tube [a) has a diaphragm
situated at a, through which a portion of the inner tube (o) is
admitted into a mixing chamber at c. This diaphragm is per-
forated with small holes, to allow the hydrogen gas to pass
from the feeding-pipe (h) through the holes a into the chamber,

202 THE MICROGRAPHIA.

c ; and that portion of the tube O situated within the chamber,
Fig. 17.

c, is perforated also, to admit the oxygen from the tube O to
mix with the hydrogen in the chamber, c, and, in that condition,
to proceed to the mouth of the jet. I prefer this arrangement
to the one given at iig. 15, because it is not so liable to be
injured, should any water, as will sometimes unavoidably
happen, be forced through the pipes into the jet. As the
gases burn much better when dry, it might be desirable to pass
them through a vessel containing atdiydrous muriate of lime, to
free them from moisture.

I shall now describe a portable apparatus, in which the
gases are contained in bladders, or bags made of cloth or
leather, rendered air-tight by a coating of caoutchouc ; and
the pressure is managed entirely without water, by the appli-
cation of sand-bags or weights*.

Figure 18 represents a side view of the apparatus. It con-
sists of a square frame of wood, running on castors, and
furnished with horizontal partitions or shelves upon which the
bags or bladders, O and H, containing the gases, are to be
placed ; the upper shelf being usually preferred for the oxygen
gas, in order that the stop-cock at o may be the more readily
adjusted, and the lower shelf for the hydrogen. Small pipes,
with stop-cocks, as shewn at o, a, i, and h, are annexed to the
different bladders, and made to communicate with the jet, J.
On the upper shelf stands a purifier, hereafter to be described,

* Hydrogen being transmissible through most bodies, the bags or
bladders should not be filled until wanted for use.

SOLAR AMD GAS MICROSCOPES,

203

for freeing the gases of their impurities, so that, without passing
through any other intermediate vessels, they may, as soon as
generated, be conveyed at once into the bladders, O or H, to

Fig. 18.

be used when requrired. W W are the weights, or sand-bags,
pressing, by means of inclined boards, upon the distended
bladders, and thus, in conjunction with the stop-cocks, acting
as regulators for the steady and proportionate supply of the
gases. It is scarcely worth mentioning that, during the time
the bladders are being filled, the weights and boards must be
of course removed*.

* The method by which the pressure on the gases is obtained in the
apparatus figure 18, has been objected to, as it varies with the inclination
of the boards to which the weights are attached : to remedy tbis incon-
venience, the gas-holders might be constructed similar to the receivers of
organ-bellows. See Edinburgh Encyclopasdia, vol. xv. p. 676, plate 447,
figs. 5 and 7,

204

THE MICROGRAPHIA.

The addition of a few words more on the practical method
of generating and purifying the requisite gases, will, I think,
be in strict conformity with the design of the present tract, and
suffice to render this part of the subject tolerably intelligible to
the generality of readers, It has been already noticed, that a
bottle, termed a purifier, is placed, for convenience sake only,
on the stand at P ; and, on the other side of it, not discernible
at a side view, is stowed a leaden bottle also, adapted for the
purpose of generating the hydrogen gas. Fig. 19 represents

Fig. 19.

these two bottles, taken from their respective situations, and
now supposed to be employed in the order they are exhibited.
Some water and granulated zinc are first put into the leaden
generator, G, (about a pint of water and a pound of zinc will
be sufficient,) and the purifier, P, is about two-thirds filled with
water. The bottles are then securely corked, and a communi-
cation is established between them by tubes perforating the
corks, and brought into connexion with each other by an union-
joint at u. In like manner the tube at p is made to commu-
nicate successively with the pipe, 1 or 2, of the several bladders
intended for use. If a small quantity of sulphuric acid (about
half a wine-glassful) be now poured into the funnel at c, a
portion of the water will be decomposed, and the hydrogen gas
speedily evolved, and pass through the purifier into the bladder
or bags, H : when the evolution becomes languid, fresh acid

SOLAR AND GAS MICROSCOPES. 205

may from time to time be added, until the needful supply of
gas is obtained*.

The mode of generating and purifying the oxygen gas is
extremely simple, and may be very summarily explained.
If an iron retort partly filled with lump manganese be inserted
into a strong fire, as soon as the manganese attains to a red heat
it will part freely with the oxygen it contains ; and if a com-
munication be made between the retort and the purifier, P,
by means of a long tube, the oxygen will pass over into the
purifier, through 2 and a, into the bladders at O, in the manner
before described for the hydrogen. Thus the same purifier
will serve for both gases : too much care, however, cannot be
taken to retain the gases quite separate from each other, and
not to risk the possibility of an explosion. I would caution
also against the purifier being more than two-thirds filled with
water, lest any of the water should be conveyed to the bladders
and destroy them. The quantity of oxygen that can be con-
tained in the bladders suited for the apparatus I have now
described will sustain a light for one hour ; that of the hydro-
gen for about half that time ; but since the latter can be very
readily procured, little, if any, delay will be thereby occasioned.

ON THE MANAGEMENT OF SOLAR AND GAS MICROSCOPES.

In selecting a room for a solar microscope, the first con-
sideration should be, that the aspect be towards the south, so
that from the window, in fine weather, you may command a
view of the sun for the greater portion of the day. This room
must be provided with the appropriate white screen, con-
structed on the wall immediately opposite the window ; ex-

* The apparatus, figure 18, would be found useful for many chemical
experiments, and especially for exhibiting the combustion of metals by
the mixed jjases.

206

THE MICROGRAPHIA.

cepting which, every other part, ceiling, walls, &c. should be
rendered, as before mentioned, as dark and sombre as possible.
On this arrangement, and the brilliancy of the day, will very
much depend the beauty of the representation. The size of
the room should be in a great measure regulated by the nature
of the exhibition : where many persons are to assemble, it
should be well ventilated and large ; and in no case should the
distance from the window to the screen be less than ten or
fourteen feet, as this will have much to do in determining the
extent of the illuminated disc upon which the image is to be
thrown, and the magnitude of the image itself. The diameter
of the disc should be about one half the distance between the
window and the screen. The window must be closely fitted
with a firm shutter or board, in the centre of which must be
cut an aperture suitable for the reception of the instrument :
thus all the light will be excluded but that which is to pass
through the instrument and illuminate the screen. If, however,
the shutter be furnished with an additional opening, which may
be readily closed when necessary, it will be found extremely
convenient for the purposes of ascertaining the position of the
clouds, the state of the weather, &c.

Figure 20 represents the external appearance of a solar

Fig. 20.

microscope : — A, a side view of a square metal plate, which is
to be firmly fixed to a window shutter ; IJ, the tube containing
the condensing lens : between this lens and the plate, A, is an
aperture for the admission of large phantasmagoria and other
slides ; near the end of the tube is attached a bar, with a rack

SOLAR AND GAS MICROSCOPES. 207

for carrying the slider-holder and magnifiers ; C, the said slider-
holder, with its field-lens ; and D the arm for holding the
magnifiers ; R is a plane mirror, which is exposed to the sun,
and capable of being adjusted to its azimuth and altitude by
means of the milled heads, e and jf.

This adjustment requires adroitness, in order to keep pace
with the varying position of the sun: the great object of it is
that the sun's rays may be reflected from the mirror directly
along the axis of the instrument, so as to form, after passing
through the condensing lens, a circular disc upon the screen :
when this is obtained, it will be a certain test that the adjust-
ment of the reflector is correct. Some little care is requisite,
in showery weather, that the reflector be not injured from
becoming wet. At a convenient distance should be stationed
a suitable table, or shelves, for containing the various parts of
the apparatus, vessels containing objects, &c. which should be
disposed in such order that they may come to hand readily in
the dark. If, however, it can be easily obtained, a small
inclosure, just sufficient for one person to manage the instru-
ment, will be found of great advantage. This may be fitted up
with every convenience, and a little light being admitted into
it will not affect the appearance in the room itself. If the
magnifier be now inserted into the tube D, and the arm moved
backwards or forwards until the disc be entirely illuminated,
the object may then be placed within the slider-holder, C, and
adjusted to the magnifier at D, and thus a distinct image, or
picture of it, will be thrown on the screen.*

The sliders for inclosing the various living objects must be
kept clean and dry ; and all the objects, whether animate or
inanimate, should be carefully arranged in the order they are
to be viewed. A few small white basins or saucers, with some

* I am informed by the Rev. J. B. Reade, that it is preferable to place
the field-lens beyond the focus of the condenser, by which means the intense
heat produced at that point is avoided, and the objects or magnifiers pre-
served from injury. Mr. Reade has had so much practice with the solar
microscope that his opinion is valuable.

208 THE M1CROGRAPHIA.

feathers, &c. for transferring the living objects into the sliders,
will be useful. For a few minutes previous to the exhibition,
the room should be retained in total darkness ; or if a little
light be necessary, it should be supplied from a common lamp ;
the reasons for which are these : in darkness the pupil of the
eye dilates, and becomes capable of discerning the minutiae of
objects with greater facility and exactness ; and the dull yellow
flame from a common lamp, by its contrast with the resplen-
dent whiteness of the sun's light, serves but to heighten the
effect.

The selection of objects should greatly depend upon the
taste of the company to whom they are to be shown. When
amusement and general instruction are principally aimed at,
large living objects, such as aquatic larvae,* &c. will answer
the purpose very well ; also the splendid wings of foreign
butterflies; thin sections of fossil woods, charcoal, jet;
the circulation in animals and plants ; solutions of salts
made to crystallize by evaporation : and silver from a state of
solution precipitated upon small pieces of copper wire, thus
forming what is called the silver tree ; — all these will prove
highly interesting when viewed under the miscroscope. And if
to these be added the various manufactured fabrics, such as lace,
blond, muslin, and the like, and the contrast displayed between
them and the transverse sections of different woods, plants,
&c, I know of no exhibitions that will serve better to elucidate
the perfection and pre-eminence of Nature's structures over
the most ingenious contrivances of art, and to engraft upon the
youthful mind impressions, not soon to be obliterated, of the
Almighty hand that created them.

To the more scientific observer, objects in detail will be of
deep interest : for instance, the proboscis of the fly, its feet,
&c. ; the lancets of some insects ; the tongue of the bee ; the
singular and delicate forms of the different infusoria, and the
arrangement of their internal organization ; the markings on
* See Microscopic Illustrations.

SOLAR AND GAS MICROSCOPES. 209

the scales of butterflies, moths, &c. ; sections of fossil woods,
and longitudinal sections of woods of recent growth : in short,
myriads of specimens may be procured, both of vegetable and
animal structures, accommodated to the intellect of every hu-
man being, and calculated to advance the most profound philo-
sopher a step onward in the great path he is journeying, toward
comprehending, as far as the mind of man can comprehend,
the immensity, beauty, design, and order of God's works.

As much, however, will necessarily depend upon the selec-
tion of magnifiers suitable for particular objects, it is advisable
to arrange the objects according to their size, commencing
with the greater ones, which require only shallow magnifiers,
and gradually proceeding to those which are to be viewed with
the deepest powers. Notwithstanding, it will leave a good
impression, ordinarily, to conclude with representing a col-
lection of large living objects under the lowest power of the
instrument. Whilst changing the object or slider, a screen
should be placed before the instrument for the purpose of ex-
cluding the extra light which would be otherwise admitted, and
tend in a great degree to destroy the effect, by causing the
pupil of the eye to contract.

Although the preceding remarks may seem to refer especi-
ally to the solar microscope, still, as both that instrument and
the oxy-hydrogen gas microscope resemble each other so very
closely in most particulars relating to management, it would be
almost a repetition of words to give them, in this respect, a
distinct consideration. If, then, we substitute the regulation
of the gases, the adjustment of light as to distance from the
condenser, and the arrangement of the lime- ball, or cylinder,
for the employment of the sun's light by means of the plane
reflector, little more will remain to be said upon the subject.
As very much of the effect will always depend upon the quali-
ties of the lime, I would recommend the trial to be made of all
the various descriptions of lime that can be easily procured ;
and after they have been well baked, let that be selected which,

p

210

THE MICROGRAPHIA.

without bursting, affords the steadiest and most brilliant light.
In putting the gas apparatus into action, the hydrogen should
be turned on first, and the light instantly applied to it ; after
which, by a due supply of oxygen, the dull bluish-red flame
will gradually change its character until the lime situated near
the mouth of the jet shall become entirely ignited, when it will
be succeeded by an exceedingly intense white light. If too
much oxygen be admitted, the light will be totally extinguished.
A very little experience, however, with some attention, will
enable any one to form a true judgment as to the correct
proportions.

I shall conclude this little treatise by appending a few cur-
sory observations. — The solar microscope is admirably adapted
for tracing with a pencil a magnified likeness of any minute
object, which may be done with great facility and precision by
placing a sheet of white paper in front of the screen, and throw-
ing the image directly upon it. The exact proportions of the
different parts, and their relative situations, by far the most
difficult points to be attained in drawings of this nature, (see
Microscopic Illustrations, on this subject,) may thus be achieved
by merely the mechanical motion of the hand. When living
objects are to be drawn, what are termed angular sliders should
be provided, to retain them ; for, when placed in these, they
will soon, by their restlessness, thrust themselves into situations
of restraint well suited for the purpose. This form of slider
will also be found extremely convenient for showing the circu-
lation in the water-newt, or any small fish. If the drawing of
objects be principally aimed at in the use of the instrument,
a small moveable screen, expressly constructed for it, of just
sufficient dimensions to take in the projected image, may be
interposed between the instrument and the oi'iginal screen ;
also, its texture being appropriate, the image will be seen
more distinctly behind the small screen than in the front of
it, in which position there will arise no obstruction to the draw-
ing, from the shadow of (he hand.

SOLAR AND GAS MICROSCOPES. 211

EXPERIMENTS ON THE POLARIZATION OF LIGHT WITH THE
SOLAR OR GAS MICROSCOPES.

In most of the different arrangements for viewing the won-
derful and oftentimes beautiful phenomena displayed by various
bodies, when submitted to the action of polarized light, the
quantity of light which can be obtained is so extremely small,
that very faint impressions consequently result from it. In
solar and gas microscopes, where an abundance of light is
readily supplied, all these properties may be exhibited with
exceeding brilliancy ; for which purpose a slider must be so
contrived, that the crystal, or other bodies to be examined,
may be placed between two plates of tourmalines, or two single
image calc prisms, bundles of glass plates, &c.

When a transparent object is placed in the solar or gas mi-
croscope, the illuminated disc, upon which the image is viewed,
is made up not only of the light which is transmitted through
the object itself, but of that portion also which, not having
come into contact with it, passes more freely through the in-
strument. This extraneous light, in cases where a delicate
object is exhibited, has so great a tendency to subdue the
image, that but a very faint impression of it is produced upon
the screen. Opaque objects, the representations of which are
seen entirely by the light reflected from them without the inter-
ruption of any extraneous light, have all their beautiful mark-
ings and colours displayed in the greatest possible perfection.
From the intense heat, however, always accompanying the
condensation of solar rays, very minute objects are liable to be
too seriously injured (if not absolutely burnt up) to be risked
in solar or gas microscopes : those of moderate size, therefore,
are necessarily used, many of which, such as small flowers,
shells, &c, and the basso relievo on medals, will be found to
be highlv interesting.

212 THE MICROGRAPHIA.

In exhibiting the internal structure, circulation, pulsation,
and peristaltic motion of the alimentary canal and other organs
of aquatic larvae, &c, * specimens should be selected immedi-
ately subsequent to the shedding of their skins, in which con-
dition the exterior covering is much thinner and more trans-
parent than at any other period of their existence. If the same
living objects are to be preserved for a second use, care must
be taken that they be provided, on being removed from the
instrument, with fresh water of a temperature not much below
that of the water which is contained in the slider ; for, by sud-
denly immersing them in cold water after they have been heated
by the sun, you would inevitably destroy them.

If the carnivorous aquatic larvae of insects be kept hungry
in clear water for a short time pi-evious to their being viewed,
and then provided with appropriate food, or, in its absence,
with the eels found in sour paste, for which they have a relish,
their peculiar mode of satisfying their voracious appetites
will be a subject of curious examination. The eels in paste,
and many of the infusoria, are often so exceedingly crowded
in the vessels that contain them, that additional clear water is
absolutely necessary before you can obtain a distinct repre-
sentation of them. This operation will be performed with good
effect whilst they are under the instrument ; or, what perhaps
may be more convenient, let a slider furnished with a due
quantity of clear water be first placed within its holder, and
then add a drop of the infusion including the animalcules.

Among aquatic larvas, the most beautiful and delicate are
those of the numerous species of gnat, but more especially that
of the straw-coloured one (Chironomus plumosus) described
and drawn in the Microscopic Illustrations ; the larva of some
May flies, such as that of the Ephemera marginata, given in the
same work. For these creatures a deep vessel or slider should

* The " Microscopic Cabinet" and " Illustrations" contain descrip-
tions and drawings of avariety of the most choice living objects for these
instruments.

SOLAR AND GA.S MICROSCOPES. 213

be obtained;, and the instrument should admit of it moving up-
wards and downwards, as they rise occasionally for respiration.
I have just hinted at the disadvantageous consequences, in
reference to the use of these instruments, resulting from the
association of heat and light ; in short, that numerous objects
would be totally destroyed, if subjected to that intensity of
heat which universally accompanies the acquisition of a suffi-
ciency of light for illuminating solar and gas microscopes.*
The inference to be drawn from this consideration is, that these
two instruments are necessarily much restricted in their use,
because, requiring, as they certainly do, a far more powerful
light than other microscopes in order to give an equal effect,
a multiplicity of objects in every way adapted to other instru-
ments are thereby wholly excluded from them. To do away
with this obstruction to extending the power of these micro-
scopes to their utmost capability, I have interposed transparent
media between the condensed light and the objects, such as a
large slide, filled with clear water, placed immediately behind
another containing delicate living objects, and thereby in a great
degree reduced the heat. It appears, however, from the va-
luable experiments recently made by M. Melloni " On the free
transmission of radiant lieat through different solid and liquid
bodies," -J- that heat and light may be separated to a very great
extent ; for some bodies, while they transmit nearly all the
heat, do not transmit any light; and for our purposes there are
several diaphanous substances which transmit very little heat,
as alum, the salts of soda or potash, fiuor spar, &c. Thus,
out of 100 incident rays, the following pass through the same
thicknesses of —

Ice very pure (diaphanous, colourless) 6

Sugar melted, (do. yellowish) 7

* In the gas microscopes, a plate of mica placed between the light and
the first condenser will prevent the latter from breakage by the heat.

t Annales de Chimie et de Physique, t. 53, p. 1, and t. 55, p. 337.
Also see an excellent translation in the first part of the Scientific Memoirs.

214 THE MICROGRAPHIA.

Alum, . . 9

Citric Acid, . . (colourless) 11

Gum, . (diaphanous, yellowish) 18

White topaz, (do. colourless) 33

Glass, (do. do.) 39

Fluate of lime. (do. greenish) 46

Rock salt (do. colourless) . ... .92

A. P.

CHAPTER IX.

I. — On Cuviers Method of Dissecting Microscopic Sub-
jects under Fluid ; with Additions.

II. — On a new Method of obtaining a delicate Adjust-
ment to the Focus of Microscopes.

III. — On an improved Mode of Supporting a Candle or
Lamp for Microscopes.

1. — cuvier's method of dissecting microscopic subjects in
fluids, with additions.

Being requested some years ago to make a dissecting appa-
ratus to a microscope, similar to that usually employed by
Cuvier, which consists of a metallic trough, similar to fig. 22,
it occurred to me that for many purposes, where the subject
under dissection was not quite opaque, a glass bottom to
the trough would be an improvement, and accordingly I made
some, and have found them very useful, as sufficient trans-
mitted light is often obtained. The addition also of coarse
micrometer lines, cut on the disc of glass, renders it more
complete. One of these troughs is shown at fig. 21 : they
are attached by a bayonet-joint to (he common stage, or fit

216

THE MICROGRAPHIA.

into the moveable one at o, fig. 1, plate 1, and can thus be
turned in any direction.

Fig. 21.

Fig. 22.

M. le Baron Cuvier's method of using: these troughs is as
follows : — Take a composition of bees'-wax and Venice turpen-
tine, or Canada balsam, and line the trough with it while
warm ; then lay in the subject to be dissected, first having
dried the parts that are intended to be fixed to the compo-
sition, and when the whole is cold, the dissection may be com-
menced^ the trough being first filled with water*.

By fixing a subject in this manner its parts are more readily
separated, and being covered with a fluid, the adventitious por-
tions are easily washed away with a camel's hair pencil.

The knives used for dissecting microscopic subjects, are
usually made similar to those employed by oculists for opera-
tions on the eye, and the scissors, described in the " Cabinet,"
page 243, are highly useful. When the subjects are minute,
T have found a great advantage in having the small cutting in-
struments fitted into handles, in which their lengths and
consequent elasticity can be varied, and for these handles,

* Microscopic dissections are of great value; they are now beginning
to be appreciated, and ought to be encouraged. It is impossible to de-
termine, with accuracy, the organization and functions of the smaller
tribes of animals and vegetables (which, from their numbers, often pro-
duce serious consequences to man), without careful dissection of them.
In my humble opinion, it is to be regretted that the Society of Arts, in
place of rewarding Cuvier's troughs, which are not new, had not re-
warded the candidate for his industry and patience in making microscopic
dissections, arid thus have directly promoted this valuable and important
branch of natural history.

ON DISSECTING MICROSCOPIC SUBJECTS. 217

needles of various kinds, especially the leather-workers', when
ground on a line hone, afford very useful instruments for dis-
sections.

II. — ON A NEW AND SIMPLE METHOD OF OETAINING A DELICATE
ADJUSTMENT OF THE FOCI OF MICROSCOPES.

In Chapter XV. of the Microscopic Cabinet, I have de-
scribed the various methods by which the adjustment of the,
focus of microscopes is effected ; such as the rack-and-pinion,
the screw, the bent lever, and the excentric*, with their various
advantages and defects. I have also minutely described a plan
of mine, by which a coarse and fine movement may be readily
obtained. This is the primary object to be accomplished. Now
although that method is unobjectionable for the microscope,
where single lenses or doublets are used, yet it is not so when
applied to an engiscope (that is, compound microscope), the
size and weight of the body to be moved rendering it unsteady.
To remedy this defect, various ingenious contrivances have
been devised, retaining the rack-and-pinion movement, for
coarse adjustment, and applying the finer one to the stage
which carries the object. All these methods, however, where
the desired effect has been obtained, have been so complex,
that it is not likely they will come into general use. Under
these circumstances I was led to construct the following, which
it is not probable will be surpassed in simplicity or effective-
ness, and, what is of great importance, cannot be easily de-
ranged. Figure 23 is a perspective view of it.

It consists of a plate attached to the stage of the instrument :
on this plate is fixed a socket for holding a fine steel screw
with a conical point, which latter acting against a block

* See Treatise on Optical Instruments, p. 36, fig. 36.

218 THE MICR0GRAPH1A.

carrying the object held in a safety slider-holder, is forced
upwards by the conical end of the screw acting as an inclined
plane, while a spring is so arranged as to keep it down to its
bearing. The milled-head of the screw, if required, might
be divided,, by which means the elevation or depression would
become a measured quantity.

Fig. 23.

III. ON AN IMPROVED MODE OF SUPPORTING A CANDLE OR LAMP

FOR MICROSCOPES.

Observers with the microscope are fully aware of the im-
portance of the proper illumination necessary to make that
instrument produce the best effect : much depends upon the
direction of the illuminating source, and when once properly
arranged, any accidental alteration destroys the effect, and the
whole must be commenced afresh. From these considera-
tions I have been induced to attach the holder for carrying the
candle or lamp to the stand of the instrument, as shown in
the sketch below, by which the not unfrequent movement of
the instrument, especially when more than one person is ob-
serving, will not affect the direction of the illumination, as all
will move together. I am convinced those who have much
employed a microscope will appreciate this trifling improve-
ment, and approve of its introduction in this work.

ON DISSECTING MICROSCOPIC SUBJECTS.

219

A is the stem of the stand of a microscope ; on this the
socket slides to tbe required elevation, and the arms allow of
the light being placed in any direction. B is a shade, to take
off the direct li°:ht from the observer.

A. P.

Fig. 24.

APPENDIX.

No. 1.

ON MAKING DRAWINGS OF MICROSCOPIC
SUBJECTS ;

In a Letter, by FRANCIS BAUER, Esq. F.R.S. F.L.S.
Sfc. fyc.

Dear Sir,

I feel great pleasure in complying with your request, by ex-
plaining my method of making correct magnified drawings with
microscopes. I often wished to have an opportunity to publish
such an account ; and I think your forthcoming e< Micrographia"
woidd certainly be the most proper for promulgating such a
useful subject.

The requisite apparatus for ascertaining with a microscope
the correct dimensions and proportions of any minute object,
are two glass micrometers, each of forty divisions to an inch,
and crossed or squared over their whole surface, similar to
those you made for me. One of them ought to be pretty
sharply engraven, on a very thin and clear plate of glass : this
micrometer is fitted into the eye-piece in the focus of the eye_
lens ; the other micrometer, which is used on the stage, ought

222 APPENDIX.

to be very strongly engraven, and its lines well blackened,
that they might be distinctly seen when viewed through the
micrometer in the eye-piece. When my microscope was thus
armed and adjusted, I ascertained the acting magnifying powers
of its objective lenses, which is effected by placing the stronger
engraved micrometer on the stage, and viewing and carefully
observing, through the micrometer in the eye-piece, how many
divisions are occupied by one division, or fortieth part of an
inch of the micrometer on the stage : I found that with the ob-
jective, No. 2, it occupied precisely ten divisions each way,
and consequently divides one linear inch into 400 parts, and
one square inch into 160,000 squares.

Having thus ascertained that every division of the micro-
meter in the eye-piece is equal to a 400th part of au inch on
the stage, and every square on that micrometer represents a
1 -160,000th part of a square inch, I now retain this adjust-
ment of the microscope permanently, and have no more occa-
sion to use any micrometer on the stage for future operations.

When I wish to make a magnified drawing of a very minute
object, I trace on my drawing-paper a number of squares,
similar to those on the micrometer, of which each division is an
inch (linear), which consequently is 400 times longer than a
400th lineal division on the micrometer in the eye-piece (see
fig. 1, plate 3), I then place the minute object, which I
wish to measure, on the stage, view it through the squared
micrometer in the eye-piece, and move the object on the stage,
till it comes into a proper position to be easily examined ;
that is, till one extremity of the object touches one line of the
square on the micrometer in the eye-piece. I then trace on
my ruled drawing-paper a correct outline of the object, as
infig. 1, A, B, C, D, E, and find that A is 2 £ -400th parts of
an inch long, and about l-900th part of an inch in breadth or
thickness. B is 2-400th parts of an inch ; C, l-400th ;
D, 1 -800th part: and the three very minute fossil animalcules
at E, about l-1200th of an inch in length, and proportional

APPENDIX. 223

in breadth. The globular fungi at F are l-1600th part of an
inch in diameter, and the very minute globules of blood at G are
each about l-2400th or 1 -2500th part of an inch in diameter.
Having thus secured correct outlines of all the objects under
examination, I finish the drawing by reviewing the objects
with another microscope of higher power (perhaps 300 or 400
times,) by which I am enabled to introduce and mark cor-
rectly all the minute parts of the objects. All the objects re-
presented at fig. 1 are magnified 400 times linear, and
160,000 times superficial.

I must here remark, that in all my microscopic drawings
I have used the English measure.

I must also notice, that in adjusting the microscope for
ascertaining its magnifying power, great care must be
taken that the lines of the square of the micrometer on the
stage exactly join and encompass the lines on the squares on
the micrometer in the eye-piece, which is not always the case,
or with some objectives the image may perhaps be found to
occupy nine divisions and a fraction, which would occasion
much calculation and trouble to produce a correct drawing* ;
but this difficulty is easily obviated, when the microscope is
constructed with sliding tubes, for by elongating or shortening
the tubes, the instrument can be adjusted to the greatest ex-
actness, and on that adjustment depends the correctness of the
whole.

When I intend illustrating objects which are larger than
l-40th part of an inch, and would be uselessly increased if
magnified 400 times linear, I rule the drawing-paper into
squares or divisions of half an inch, and proceed in every
respect as in the former case : the drawing thus produced will
be magnified 200 times linear, or 40,000 times superficial,
(see fig. 2), and again, for drawing larger objects, I make
the divisions of my paper of one-fourth of an inch, which

* See Chapter II.

224 APPENDIX.

will produce a figure magnified 100 times linear, or 10,000
times superficial, (see fig. 3, plate 3).

When I have wished to investigate and draw large opaque
animal substances, such as the inner coats of the stomach, the
papillary vessels of the tongue, or the internal structure of the
lungs, or the spleen, &c. &c. (all such objects must be ex-
amined under water, and cannot be brought under the micro-
scope, and no glass micrometer can be used) — I conceived
and used another method, viz., I had several silver plates (of
about a square inch, and about the thickness of a shilling),
constructed, and in the middle of them regular and accurate
square holes or perforations cut out, from half an inch to a
1-1 0th of an inch linear, (see fig. 4, plate 3.) When I wished
to examine such opaque objects under water, I placed one of
these plates upon a chosen spot of it, and tracing on my pre-
pared drawing-paper, (divided in squares of the size re-
quired); for instance, when a perforation of l-10th of an inch
is employed, I made my squares on the drawing-paper three
inches, by which the perforation, and all that is encompassed
within it, is magnified thirty times lineally, and 900 times su-
perficially ; and tracing and portraying correctly on my paper
the image of the perforation, the drawing will be magnified
30 times linear, and 900 times superficial.

I often wished to improve this silver plate micrometer, by
dividing, with some fine silver wires, the orifices into smaller
divisions ; for instance, the quarter part of an inch might
easily be divided into four squares, and the half of an inch
into sixteen squares, which would greatly facilitate the opera-
tion and the production of correct drawings ; but I could
never meet with any workman who could execute it properly ;
but I have no doubt you would soon effect this, or any other
improvement of this useful instrument. This apparatus should
always accompany a complete microscope.

I have now adopted and practised these methods for more
than thirty years, and I do not hesitate to state, that, in my

APPENDIX. 225

opinion, they are the simplest, the easiest, and perhaps the
best, for producing correct magnified microscopic drawings.

I avail myself of this opportunity to acknowledge and
correct an error into which I had inadvertently fallen, when
(in the year 1816) illustrating the particles or globules of
the human blood : I was then provided with very indifferent
optical instruments, and stated the diameter of a globule
of the human blood to be 1-I000th part of an inch : but
on a subsequent investigation, with a somewhat improved
apparatus, I corrected that error, and noted the diameter
of these globules to be l-2000th part of an inch; but
having since obtained an improved achromatic microscope,
and repeated the measurement with that instrument, can
now state with certainty, that each of the globules of the
human blood is l-2500th part of an inch in diameter. I am
therefore very anxious to give this explanation, as I perceive
that my former erroneous statement is still quoted in some
recent publications. In the Penny Cyclopasdia, Vol. V.
page 4, a table of the size of the globules is given. The
author concludes his account of the human blood thus : —

" All observers are agreed that the size of these particles,
as long as they retain unimpaired the form they possess on
escaping from the blood-vessel, is perfectly uniform ; but
their real magnitude is variously estimated ; the size of the red
particle of human blood is, according to

Bauer 1-2000 part of an inch.

Wollaston 1-5000 do.

Young 1-6060 do.

Kater 1-4000 do.

Prevost and Dumas . . 1-4076 do.

Hodgkin and Lister . . 1-3000 do. "

This table proves how difficult it is to ascertain that point ;
but it also proves, that however erroneous my statement of

Q

226 APPENDIX.

l-2000th part of an inch then was, it was nearer the truth than
the other measurements.

In hopes that the above will prove satisfactory,
I remain, dear Sir,

Most sincerely yours,

Francis Bauer.

Kew-Green, Nov. 1836.

To Andreiv Pritchard, Esq.

EXPLANATION OF PLATE III.

Figure 1. — A, B, C. D, E, are magnified outlines of Fossil
Infusoria, composing Tripoli, which has lately been disco-
vered at Franzenbad, in Bohemia*. F is the outline of four
globules or sporules of the fungus Uredo fcetida, which pro-
duce the disease in wheat, called smut-balls or pepper-brand ;
and G represents globules of human blood. All the drawings
in fig. 1 are magnified 400 times linear, or 160,000 times
superficial, so that the real dimensions of the above bodies
are easily obtained by a direct measurement of their outline
in the drawing.

Figure 2. — A represents the outline of a grain of pollen of
the Passiflora quadr annularis ; and B, a grain of the (Eno-
thera Lindleyana. The objects in this figure are magnified
200 times linear, or 40,000 times superficial : that is, each
square space represents l-40000th part of a square inch.

Figure 3. — A is a capsule of fern, Aspidium trifoliatum ;
and B the same, burst open, with its sporules or seed thrown
out. These objects are magnified 100 times linear, or 10,000
times superficial.

Figure 4 represents the silver plates, with apertures of
different sizes.

* These remains of the Infusoria are the silicious shells of animalcules.
Some genera exhibit series of delicate transverse markings : they are best
seen as transparent objects, in spirits of wine or Canada balsam. They
belong to the division Bacillaria. — See my Natural History of Animalcules,
page 59 ; also § 71 (94), 85, 89, &c— A. P.

No. 2.

ON

A NEW METHOD OF ILLUMINATING
MICROSCOPIC OBJECTS.

By the Rev. J. B. RE ABE, M.A. of Cains College, Cambridge.

In Dr. Goring's valuable Memoir on the Verification of
Microscopic Phenomena, it is observed, that " the verification
of the real nature, form, and construction, of a vast variety of
objects which elude the sense of touch by their extreme
minuteness, can only be made out by an attentive study of
their appearances, under a variety of methods of illumination*."
The methods of illumination at present adopted are four in
number, and consist in the application of direct and oblique
refected light, and direct and oblique transmitted light.

The first two methods are applicable to opaque objects,
but for the examination of transparent objects all the methods
are available. The two latter, however, it is well known, are
those most commonly used.

Now, when microscopic objects, not opaque, are viewed
with oblique reflected light — the flame of the candle being
placed higher than the stage of the instrument, and its light
condensed upon the object — it is invariably found that the

* Microscopic Cabinet, p. 183, § 16.

228 APPENDIX.

maximum of condensed light which can be obtained by
this method is insufficient for the full development of many
important characters. If, again, transmitted light, either direct
or oblique, be substituted for reflected light, obstacles of a
still more serious nature greatly interfere with accurate
investigation. Delicate tints are lost ; colours naturally bright,
or even brilliant, are all but absorbed ; the texture and con-
struction of objects are erroneously represented ; and, in fact,
nothing is seen, in many cases, but a magnified image of the
object in mere black and white. Nor is this all ; for besides
this defective representation, the eye of the observer is always
subject to much painful excitement, arising from the intense
illumination of the ivhole field of view. And here, in fact, lies
the great practical inconvenience of the present method ; for,
to take a common case — an object about l-300th of an inch in
diameter being placed in the middle of the field of view, the
diameter of which is about 1-1 2th of an inch, and consequently
being l-625th part of the area of the field of view, the eye has
to contend with 624 parts of bright light, which are not
brought to bear upon the illumination of the object. Hence,
a method by which this intense glare shall be wholly removed,
and that without the loss of a single effective ray, must evidently
be superior to the one usually employed, in the ratio of at least
600 to 1.

Being lately engaged in the examination of a few test objects,
I happened to notice that the feathers of the Lycana argns,
when held above the flame of a candle, exhibited at a certain
angle all their peculiar tints, and at the same time the flame was
not visible to the eye. It then occurred to me, that by pre-
serving the same angle under the microscope, the advantages of
amplification would also be accompanied by the natural colours
of the object. The requisite angle was readily obtained by
making the axis of the microscope coincide with the line from
the object to the eye, while the candle and the object retained

APPENDIX. 229

their relative positions. The result accorded with my antici-
pation, and I was gratified by the exhibition of the most
brilliant diamond tints, sparkling with exquisite lustre on a
jet-black ground. This new method of illuminating micros-
copic objects, it is at once apparent, consists in obtaining
oblique refracted light.

On submitting a series of objects to the same illumination,
I was soon convinced of the value of the discovery ; and I
scarcely know which to admire most — whether the very natu-
ral appearances of objects, adorned, as they invariably are, by
the presence of their most delicate colouring, or the personal
comfort of the observer, arising from the absence of all super-
fluous light. To illustrate the two methods by a reference to
the telescope, it may be observed, that the discomfort of view-
ing spots on the sun not unaptly corresponds with the view of
miscroscopic objects on an illuminated field ; while the re-
moval of all inconvenient and ineffective light from the field
of the miscroscope corresponds with the clear and quiet view
of stars on the dark blue vault of the firmament.

The most practicable mode of obtaining the illumination
now described is to fix the object on the stage of the miscros-
cope, in the usual way, the axis of which must be inclined to
the table, at about an angle of 45°, and then to place the
candle about two inches below the stage, and about one or
two inches to the right or left of it ; but this lateral distance
must be varied, according to the nature of the object and the
angle of aperture of the instrument. It must be carefully borne
in mind that the illumination will not be correct unless the
field of view be wholly darkened.

To obtain this kind of illumination with facility and effect, it
will be necessary to make some alterations in the construction of
the instrument : as, for instance, in order to apply condensed light,
the arm of the condenser must be placed in a ball-and-socket joint,
or some similar contrivance must be adopted ; for when it is

230 APPENDIX.

perpendicular to the axis of the microscope, its introduction di-
verts the course of the rays from the candle to the stage, and
not unfrequently illuminates the field of view. The mirror
also cannot be made available in its present position, for this
kind of illumination, because light, when reflected from it,
must of necessity illuminate the field. It must therefore be
fixed on an extended and jointed arm ; and when so con-
structed, microscopic objects may be viewed even in the day-
time by oblique refracted light. Again, a very remarkable
microscopic effect will be produced by giving a small vertical
angular motion either to the body of the instrument or to the
stage, as in Goring's Engiscope*. By this means, the plane
of the object which, owing to the present construction, is of
necessity parallel to the diameter of the object-glass, may be
inclined to it at different angles ; and we shall thus obtain
oblique vision as well as oblique illumination. These two
conditions are absolutely necessary for obtaining, in many in-
stances, the true effect of coloured objects even with the naked
eye, and the introduction of magnifying powers between the
object and the eye does not render these two conditions a whit
the less necessary.

The effect of this new method of illumination may be tried
with advantage on various subjects of the larger kind, as cut-
tings of wood, scales of fish, and wings of insects-]-. We may
also apply it, with peculiar interest, to the investigation of the
elementary organs of plants; animal tissues- mosses; coral-
lines ; crystals ; and the scales of insects of the orders Lepi-
doptera and Thysanura. In each and all of these some striking
and hitherto unperceived character will be developed, and
the observer will rise from his pursuit with a more thorough

* See Microscopic Illustrations, page 39, fig. 1 and 9.

t Among the various objects which shew the superiority of this kind of
illumination over transmitted light, the spiral vessels of the hyacinth and
the pollen of the convolvolus major are the most decided.—- A. P.

APPENDIX. 23 1

persuasion that the Being whose word is power, and by whom
his own body u is fearfully and wonderfully made/' has equally
exhibited the matchless efforts of his skill in the exquisite
polish of an insect's joints ; in the opening of a leaf ; and
the pencilling of a flower. — To be a theoretical atheist is
impossible.

Peckham, Nov. 1836.

THE END.

Wilson and Son, Printers, 57, Skinner-Street, London,

Mtari (/rci/ifiM , lJLat.ell ■

■ ■

'

'

, ■ -marie Street.

/O

L

/ I i

Y:f*.

B

■: !

E

!i

%.i.

G

/V-2.

- 1 A

Fig. 4 .

Fig. 3.

-~„

)

„-

=

/

Ja^

U-

Y "

>— ^

^

xi

i. • \

*

4 .x5

v—

\

X

<

■'\ /

-A

bto

B

"P

fe\

■:. '

j-1

-4%;

'~r

.

\ -~*

f

'

Ut

1

1. 1

London.. W/utta,kei" bC C?<fa.n,./<?3f .

: kern