464 
POPULAR SCIENCE REVIEW. 
distances of tlie two, produces a necessity for a tube or body 
in the compound microscope, which the single instrument does 
not require. 
But in the completer form of compound microscope another 
lens is employed, which is inserted between the object- 
glass and the image produced by it. This glass serves the 
important purpose of so changing the 
course of the rays as to diminish the 
size of the image, thus allowing the 
whole of it to come within the range of 
the eye-glass, and enlarging the field of 
vision. Hence it is called the field-glass. 
It has, however, an important relation to 
the eye-glass, which, together with it, is 
called the eye-piece. (Fig. 1.) The most 
useful eye-piece consists of a plano- 
convex field-glass and a similar eye-glass, 
the plane surfaces of each being towards 
the eye, and a diaphragm or stop being 
Fig. 1. interposed between them, so that only 
the more central rays are allowed to 
pass, the distortion arising from spherical and chromatic aber- 
ration being thus avoided. 
The general optical principles concerned in the construction 
of the compound microscope being understood, the various 
modifications and matters of detail must be mastered at leisure, 
being, in fact, not the sudden and easy application of principles 
long known, but the laborious and careful result of ceaseless 
experiment and research. But there are other principles to 
be applied, of a secondary nature it is true, but still scarcely 
less necessary for the efficient working of the instrument than 
those to which brief reference has been made. These are the 
mechanical arrangements, the stability of the whole apparatus, 
its capability of accurate adjustment, and the convenient mutual 
relation of all its parts. These, perhaps, will be best illustrated 
by a description of the various parts of an ordinary compound 
microscope, such as that of which a drawing is given at fig. 2. 
In this microscope it is first to be observed that the tripod 
stand affords a firm basis of support, so that there is no fear of 
the instrument being readily overturned, or even moved by any 
ordinary manipulation. The two pillars upon which the micro- 
scope itself works are solid, and with the feet are altogether of 
a considerable weight, thus preventing any movement or vibra- 
tion in the instrument, which is securely screwed between them. 
This is extremely important, since any vibration is communi- 
cated to the object in an increased proportion, according to the 
magnifying power used. The microscope itself consists of two 
