344 
drawing out of the eye-piece until the end is 
accomplished answers the object most simply 
and effectually. In the use of the micro- 
metric eye-piece, also, which will be presently 
described, the capability of adjusting the mag- 
nifying power to a certain definite amount will 
be found of very great utility. It is to be 
borne in mind, however, that for giving the 
highest effect to the achromatic objectives, a 
certain fixed distance of the eye-piece is neces- 
sary; this is usually adjusted by the maker; 
but it may be easily determined by trial, since 
at any other the want of correction of the chro- 
matic aberration will make itself apparent 
reaver slightly) by the presence of coloured 
inges around the images. 
The degree of perfection in the construction 
of the optical part of a microscope, whether 
simple or compound, is judged of by the dis- 
tinctness and comfort (by which we mean the 
freedom from strain or effort on the part of the 
observer) with which it exhibits certain objects, 
the details of which can only be made visible 
by combinations of lenses of high magnifying 
eg and a near approach to correctness. 
uch are called test objects. They are of 
various degrees of difficulty. For testing the 
penetrating powers of a microscope, the lined 
scales on the wings and bodies of certain in- 
sects are commonly employed. The scales 
from the wings of many Lepidoptera are so 
coarsely marked, that a good ordinary com- 
pound microscope, or deep single lens, will 
make the lines apparent. But there are many 
others, which, under such magnifying powers, 
only show a flat unmarked surface, requiring 
lenses of large angular aperture to make their 
lines visible. One of the most beautiful of 
these, and at the same time most easily re- 
solved, is the scale of the Menelaus butterfly ; 
its longitudinal strig may be seen in the best 
ordinary microscope, but they require a cor- 
rected object-glass to be well made out; and 
its transverse markings are only to be seen 
distinctly with a superior instrument. A scale 
with more delicate lines than these is the 
larger of those of the Lycena Argus; the smal- 
ler one will be presently noticed. The most 
difficult of the test-scales, however, is that of 
the Podura plumbea, or common spring-tail ; 
and a microscope which will distinctly exhibit 
its markings may be regarded as, for this class 
of objects, of the highest order. For defining 
power, however, another class of objects is 
needed. Among these one of the best is the 
small scale of the Lycena Argus (or battledore), 
which, with an inferior instrument, appears 
covered with coarse longitudinal lines; but 
these, when more perfectly defined, are found 
to be resolvable into separate circular or oval 
dots, arranged ina linear manner. The curi- 
ous hair of the Dermestes, and that of the 
Bat, require a microscope of good defining 
power to represent their forms with clearness 
and accuracy. We have ourselves been accus- 
tomed to employ the branching hairs of the 
common bee as tests of the correctness of a 
microscope of moderate power; for they have 
- . e * 
colour, under most of the ordinary 
illumination, unless there is a perfect freedom 
from chromatic aberration. The spiral fibres 
lining the trachee of many insects, also, will 
be found good tests of the defining powér, 
and freedom from aberration, of a mi e. 
For still lower powers, we consider the glan- 
dular dots in the peti = a =~ resinous 
trees (especially those of the order Conifers) 
as on veetcectten tests. They ought to be 
rendered distinctly visible with an object-glass 
of half an inch focus; and the depression in 
the centre should be clearly made out, with a 
perfect freedom from colour. We have seen 
objectives of English construction (in which a 
large aperture for showing opaque objects was 
the chief point aimed at) so defective in this 
respect, as to be far inferior in their exhibition 
of these and similar objects to French acro- 
matics of much smaller aperture. We may 
here repeat the general rule, that those micro- 
scopes are, ceteris paribus, the best, which will 
show the most with the lowest magni 
power. It will sometimes happen al- 
though the details of an object may be made 
out with tolerable clearness, there is a sort of 
thin fog or mist over the whole field. This 
fault may proceed from the too great enlarge- 
ment of the aperture of the objective, or from 
a faulty mode of illumination; or it may result 
from the imperfect extinction of the rays re- 
flected within the body of the microscope from 
the surfaces of the lenses of the iece, 
and from the interior of the tube itself;*— 
a fault which may be obviated by carefully 
coating the inner surface with a black ore 
adapted to absorb all the false light. B 
velvet may be advantageously used for this 
paiaens. If the aperture be too the 
ult may generally be corrected by the use of 
stops beneath the stage, by which it may be 
diminished as required. This plan, whieh will 
be presently described more in detail, will be 
found very much to increase the applicabi- 
lity of low-power achromatic lenses of that — 
large aperture which is desirable for opaque 
objects, ay 
II. Or THE MECHANICAL ARRANGEMENTS © 
OF MICROSCOPES. 
Having now described, with as much detail 
as the nature of this article permits, the prin- 
«3 ‘ 
ciples on which the operation of the micro= 
scope depends, we shall next proceed to con- 
sider the means of arranging the eae por- 
tion of the instrument, so as to confer upon it 
the best and most varied application, keepi 
especially in view, however, the wants of the 
anatomist and physiologist. 
We shall begin — 
by stating what, in any form of mi aa 
we regard as the essential conditions to be 
attained in its construction. : - 
1, Steadiness and firmness in all its parts, 
* [Or it arises, as suggested to the Editor by 
Mr. Powell, the eminent optician, from imperfection 
in the correction of the objectives.—ED. sie 
1 
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