MICROSCOPE, 
magnifying power is employed; and the strain- 
ing of the eye, which is occasioned by their 
very minute aperture. Thus a triplet in our 
possession, which will show the most difficult 
test-objects, has a focal distance of only about 
3th of an inch, and an aperture through which 
the smallest pin would scarcely pass. But the 
first of these disadvantages is more apparent 
than real. The object should be always co- 
yered with talc, (which may be easily split into 
lamine of the ,};th of an inch in thickness,) for 
the purpose of protecting both it and the lens 
from injury by accidental contact; and the 
ifier should not be screwed into the arm 
which carries it, but loosely fitted, so that, if the: 
observer should happen to bring the arm too 
near the stage, he may not force down his lens 
upon the object. As Mr. Holland justly ob- 
serves, “ Should the proximity of the object 
to the lowest lens of the triplet be urged as a 
material objection to its usefulness, it may be 
answered that the whole microscope is a mass 
_of delicacies; consequently, it cannot be al- 
lowed that a line be arbitrarily drawn, beyond 
which every thing is to be considered as too 
delicate.” The second of the above objections 
must be obviated by never continuing the use 
of deep powers in a simple microscope for any 
length of time at one sitting, and by taking 
care to adjust the instrument in such a manner 
that the head may be as little inclined forwards 
as possible. 
The only other form of simple microscope 
which we shall notice is one commonly known 
under the name of the Coddington lens. The 
first idea of it was given by Dr. Wollaston, 
who proposed to cement together two plano- 
convex, or hemispherical lenses, by their plane 
sides, with a stop interposed, the central aper- 
ture of which should be equal to th of the 
focal length. The great advantage of such a 
lens is that the oblique pencils pass, like the 
central ones, at right angles with the surface; 
and that they are consequently but little subject 
to aberration. The idea was further improved 
upon by Mr. Coddington, who pointed out 
that the same end would be much better an- 
swered by taking a sphere of glass, and grind- 
; ing away the equatorial parts, the groove being 
then filled with opaque matter, so as to limit 
_ the central aperture. Such a lens gives a large 
phere of view, admits a considerable amount 
of light, and is equally good in all directions; 
but its powers of definition are by no means 
to those of an achromatic lens, or even 
adoublet. This form is very useful, there- 
fore, as a hand lens, in which a high power is 
not required, but has no particular advantages 
for magnifiers of short focus, nor for the object- 
glasses of a compound microscope.* 
_ It may be desirable to mention that a magni- 
fier, now known under the name of the Stan- 
hope lens, and much praised by those interested 
_* We think it right to state that many of the 
Magnifiers sold as Coddington lenses are not really 
(as we have satisfied ourselves) portions of spheres, 
but are manufactured out of ordinary double-con- 
vex lenses, and will be destitute, ‘therefore, of 
many of the above advantages. 
339° 
in its sale, is nothing more than a double con- 
vex glass, much thicker than ordinary, so that 
an object in contact with one of its surfaces 
shall be in focus to the eye placed behind the 
other. This is an easy method of applying 
rather a high magnifying power to scales of 
butterflies’ wings and other similar flat and 
minute objects; but the instrument is totally 
destitute of value as a means of scientific re- 
search, and must be regarded as an ingenious 
hilosophical toy. 
r Ceaonnd ‘Micro- 
scope. — The com- 
pound microscope es- 
sentially consists, as 
already stated, of two 
lenses, which are so 
disposed that one of 
them receives the rays 
of light from the object, 
and forms an image by 
its refraction of them; 
and this image is seen 
by the eye through the 
second lens, which acts 
upon it as asimple mi- 
croscope. The princi- 
ple of such a micro- 
scope will be at once 
understood from the 
adjoining diagram (fig. 
162). According to the 
laws already stated, if 
the object be ata less 
distance from the lens 
than its diameter of cur- 
vature (supposing it to 
be a double - convex 
lens, or twice that dis- 
tance of a plano-con- 
vex) the image will be 
larger than the object; 
and this in proportion 
as the latter is brought 
nearer to the principal 
focus, at which it can 
give rise to no image, 
as its rays after refrac- 
tion become parallel. 
Hence, by the use of 
the same object-glass, 
a considerable variety 
of power might be ob- 
tained ; for, if the image 
be formed near the lens, 
it will be small; but if 
the object be caused to 
approach it, the image 
will be thrown to acon- 
siderable distance, and 
will be proportionably 
magnified. The eye- 
piece would of course 
require, however, a cor- 
responding re-adjust- 
ment; and, in fact, 
the construction of 
the whole instrument 
would need modifica- 
Fig. 162. 
A B, the object, of which 
an amplified image is 
formed by the object- 
glass C D, in the con- 
trary direction, at A’ B’, 
by the convergence of 
the rays of the several 
pencils to afocus. These 
again diverging are re- 
ceived by the eye-glass 
LM, which gives them 
the nearly parallel di- 
rection necessary for 
them to enter the eye, 
and causes the apparent 
size of the image tobe E, 
z2 
