INTRODUCTORY WORK 9 
identify the parts with an instrument at hand. Familiarity 
with the uses and names of parts greatly facilitates further 
work. 
Description.—The horseshoe-like support is the base. 
Directly above the base is a horizontal expansion—the 
stage; running upward from near the stage is a cylindrical 
column—the tube; the tube and stage are fastened to the 
base by a hinge-joint. In the center of the stage is an 
opening, the size of which is regulated by a diaphragm. 
This may be an iris diaphragm, opening and closing as the 
iris of the eye; it may be a metal disk with openings of 
various sizes which may be used by revolving the disk; or 
the opening may be regulated by the insertion into the 
stage of pieces of metal having apertures of various sizes. 
Below this opening in the stage is the mirror. This must 
be adjusted so that good light is reflected through the stage, 
the object to be examined, the tube, and to the eye which 
is at the upper end of the tube. At the lower end of the 
tube are the objectives, metal holders containing the lenses. 
Both objectives may be fastened to a metal piece—the nose- 
piece—so arranged that either objective may be turned be- 
low the end of the tube; or it may be necessary to unscrew 
one objective and screw on the other to make the change. 
The shorter objective is the low power, as it magnifies less 
than the longer, the high power. Inserted in the upper end 
of the tube is the lens known as the eyepiece. Neither of 
the eyepieces magnifies nearly so much as the objectives, 
but the shorter one has the higher power of magnification, 
There are two devices for raising and lowering the tube. 
One known as the rack and pinion, or coarse adjustment, is 
designed to move the tube rather rapidly. On either side 
of the tube is a screw-head for its manipulation. The fine 
adjustment is managed by means of a screw-head placed 
immediately behind the tube. 
Use.—First is the preparation of the object to be exam- 
