356 
upon the object; and, with a little practice, 
a very close estimate may be formed of the 
proportional size of the object, when it only 
extends over a part of a single division. This, 
for ordinary purposes, is by far the most con- 
venient mode of measurement. 
The camera lucida, however, which has been 
adapted to the microscope for the purpose of 
delineating representations of microscopic ob- 
jects, may be most advantageously applied 
also to micrometry. By this instrument (of 
the construction of which we shall presently 
speak) a highly magnified picture is projected 
upon a surface on which its outlines may be 
easily marked, and on which their size may, 
therefore, be determined with the greatest 
nicety. Here, as in former cases, the micro- 
meter object-glass must first be employed, in 
order to fix the standard. If one of these be 
placed in the focus of the microscope, and the 
camera lucida be so adjusted, that an image of 
its lines be thrown upon a piece of paper ata 
fixed distance from it, the distance of these 
may be marked with precision ; and subdivi- 
sion on the paper may be carried to any re- 
quired extent, so as to afford the means of at 
once ascertaining the size of an object placed 
in the field. Thus, if the magnifying power 
and the distance of the paper be so adjusted, 
that the lines which are really jth of an 
inch apart are projected upon it at five inches 
distance from each other, every inch on the 
paper will of course be equivalent to hth 
of an inch on the object. Lines of jth of 
an inch apart may easily be drawn on the 
paper; and the distance between each of these 
will represent y5J,5th of an inch on the ob- 
ject. In this manner the size of an object may 
be known with great nicety, and with less lia- 
bility to error than in the use of the screw mi- 
crometer. It is easy to increase the apparent 
size of the image thrown by the camera lucida 
to almost any required extent; so that even 
greater minuteness may be attained. The dis- 
tance between the eye-piece and the paper may 
be increased,—either vertically by placing the 
latter upon a chair or even on the floor,—or 
horizontally, by turning the prism or mirror 
a quarter round, and projecting the image in 
the direction of the side of the room, so that 
the range of distance is much increased. Such 
a plan is, of course, of no use in delineation ; 
but in micrometry it may be had recourse to 
with advantage, especially when comparing the 
relative sizes of similar objects, such as the 
blood-discs. For every magnifying power, 
whether gained by changing the objective or by 
increasing the distance of the screen, a determi- 
nate value must of course be ascertained for 
the divisions of the latter. 
The camera lucida of Dr. Wollaston is some- 
times applied to the eye-piece of the micro- 
scope for the purpose of delineation and micro- 
metry; but it is much inferior for these pur- 
to other plans which have been devised. 
Probably the best of these consists of a mirror 
composed of a thin piece of rather dark-coloured 
glass cemented on a piece of plate-glass, in- 
MICROSCOPE. 
clined at an angle of 45° in front of the 
glass. Of the light which passes out from 
latter, a sufficient quantity is reflected by 
mirror to give a distinct image; nade 
paper and pencil can be distinctly seen through 
the glass, though rather darkened co- 
loured glass, which thus serves to render the 
image more brilliant. A lens is placed below 
the reflector, which causes the rays from the 
paper and pencil to diverge at the same ans 
with those received from the eye-glass; 80 
both the object and the pencil are seen with 
equal distinctness. The use of a small 4 
polished steel mirror, fixed in the focus 
eye-piece, and inclined upwards towards the 
eye at an angle of 45°, is by some to 
this. The mirror being smaller than the pupil 
allows the rays from the paper to pass up into 
the eye around it; and thus the image is seen 
as upon the screw. In the use of either of 
these instruments, the chief difficulty (as in the 
use of the common camera lucida) is for t 
delineator to see both the i and 
with sufficient distinctness to enable him to 
make an accurate tracing of the former. Much 
will depend upon the advantageous adjustment 
of the amount of light upon the object and the 
per respectively. In drawing or m 
y lamp-light, we have found it useful to plac 
a small taper near the sereen, so that its dires 
rays may fall upon it, whilst the lamp is used 
for illuminating the object; and when the 
screen is illuminated by daylight it is prefer- 
able still to use the lamp for the other purpe 
The point of the pencil should be blackened. 
The micrometer eye-piece also may be em- 
ployed for drawing; its squares being repr 
sented by squares on the paper; and the por- 
tion of the object between each being delineated, 
in the manner commonly practised by artists. 
No assistance of this kind, however, can su 
that skill to the microscopic draughtsman whi 
is required for making finished delineations of 
any object. Accuracy of outline is all that 
they can ensure. ee 7 
Under this head it seems not inappropr 
to introduce a few remarks on the degree of 
minuteness in the structure of objects, which 
the magnifying power of the microscope ena- 
bles us to detect. ' th 
Much speculation has taken place amor 
philosophers at different times, relative to” 
possibility of detecting the ultimate riggs 
material, especially organic, substancé@s; 2 
saiesoscopists hase ocseulaaai hazarded state- 
ments in regard to their size, which an in-— 
creased knowledge has shown to be invalid. — 
It was a favourite theory about fifteen yee : 
He) 
since, that all organized bodies are made up of 
globules, which cannot be resolved into 
other kind of structure, the diameter of 4 
was stated at about gdgth of an inch. T { 
great improvements which have been recently 
made, however, in the microscope, and the 
general advance of knowledge on the Subject” 
of the ultimate constitution of organized st 
tures, have shown the erroneous nature of tl 
view, by proving that there is no body, 
