APPENDIX I 663 
on a rack-and-pinion focusing adjustment having ample range 
to suit a variety of thicknesses of objects. A black-and-white 
metal stage plate serves for use as a contrast background. A 
more elaborate type of dissecting microscope of the twin objec- 
tive binocular type is shown in Fig. 491. This gives erect images 
and a stereoscopic effect. 
. THE CompounD MICROSCOPE 
A. Its CONSTRUCTION: 
The principal parts of a compound microscope are: 
1. The base, generally horseshoe shaped, which rests on the 
table. 
2. The pillar, an upright bar, which is attached to the base 
below, supports the rest of the instrument and incorporates the 
inclination joint. 
3. The stage, a horizontal shelf upon which is placed the 
preparation or slide to be examined. ‘The stage is perforated in 
the center for transmitting light reflected up by the mirror. On 
the stage are two clips for holding the glass slide. 
4. The mirror, situated below the stage, by which the light is 
reflected upward through the opening in the stage. The mirror 
generally has two faces; the one is plane for initial light intensity, 
the other concave for concentration of light on the object. 
5. The diaphragm, inserted in the opening of the stage or 
attached to its lower face, and used to regulate the amount of 
light reflected by the mirror. 
6. The body tube, a cylinder which holds the draw tube and 
lenses and moves up and down perpendicularly above the open- 
ing in the stage. The tube is raised or lowered either by sliding 
it back and forth with a twisting movement or by a rack and 
pinion mechanism. The latter is called the coarse adjustment and 
is used for finding the focus. 
7. The fine adjustment, a micrometer screw back of the tube, 
which, on being turned, produces a very slow motion of the entire 
framework which holds the body tube. It permits exact focus- 
ing of the higher power lenses. 
8. The oculars or eyepieces which slip into the upper end of the 
draw tube. Each of these consist of two plano-convex lenses, the 
