THE MICROSCOPE. 25 
The eye-piece consists of a short tube, which slips into 
the body. The lens at the end next to the objective is 
called the field-glass, and that next the eye the eye-glass. 
The eye-glass magnifies the image formed in the eye-piece 
by the field-glass. Eye-pieces may be either deep or shallow. 
The deeper the eye-piece, the more it magnifies, and the 
shorter it is. LEye-pieces are usually numbered 1, 2, &e» 
No. 1 being the shallowest ; but sometimes they are lettered 
A, B, C, &., in which case A is the shallowest. 
The magnifying power of a microscope may be varied 
in three ways: 
1. By changing the objective. 
2. By changing the eye-piece. 
3. By altering the length of the body by means of the 
draw-tube. The longer the body the more the mi- 
croscope will magnify. 
If the magnifying power is increased either by pulling out 
the draw-tube, or by putting on a deep eye-piece the light 
is at the same time diminished. For as an objective can 
only admit a certain quantity of light, the more the image 
it forms of an cbject is magnified, the more the light is 
spread out, and consequently the less must be the amount 
on an equal area. If, therefore, a large increase in mag- 
nifying power is required, a higher objective, which admits 
more light, must be used. 
The STAGE. The object to be examined is almost always 
placed in a drop of water, or other fluid, on a glass slide, 
generally made three inches long by one broad. In order to 
prevent the curved surface of the water affecting, by its 
refractive power, the distinctness of the image, it is usual 
to cover the object with a cover-glass, made expressly for 
this purpose, of very thin glass. 
