56 
ELEMENTARY CHEMICAL MICROSCOPY 
ference passes through the intersection of the cross-hairs but 
whose center is off to one side. Slowly rotate the stage until 
the mark has made a complete revolution, fixing in your mind 
the position of the center about which the particle has rotated. 
Now turn the stage until the particle or mark reaches its maxi¬ 
mum distance from the intersection of the cross-hairs and by 
means of the stage centering screws bring the particle to the 
center about which it has rotated. Move the slide on the stage 
with the fingers until the particle or mark again falls directly 
tinder the cross-hairs. Rotate the stage. It will now be found 
that the stage is nearly but not quite centered. Rotate again, 
noting as before the path of the mark or particle, and the position 
of the center of the circle through which the particle has moved. 
Bring the particle to this center and again test the accuracy of ■ 
the rotating stage. Absolutely perfect centering throughout an 
entire rotation of 360 degrees is seldom possible in the case of 
medium-priced instruments. Providing the centering is good i 
through a half rotation (180 degrees) satisfactory measurements ^ 
may be obtained. i 
Since microscopes are commonly provided with non-centering 
revolving nosepieces, centering the stage for one of the three | 
objectives will not answer for the other two. Each time one ! 
objective is substituted for another by turning the nosepiece it Ij 
is usually necessary to recenter the stage. A very convenient 
device for approximate centering is to have a disk diaphragm 
just fitting into the stage opening, the orifice of the diaphragm 
being a minute pinhole. To center the stage lay the diaphragm 
in place, focus upon the pinhole and bring the point of light 
exactly under the cross-hairs by means of the stage centering 
screws; or a circle of drafting ink, the exact diameter of the 
stage opening, can be drawn on thin ground-glass or tracing 
cloth with a dot at the center; this serves a purpose similar to 
that of the diaphragm. 
3. Testing the Graduated Circles upon Polarizer and Analyzer. 
— Although the zero points may be properly set, it may happen 
that the graduation in degrees of one of the nicols is incorrect. 
Turn one nicol a few degrees, note the scale reading, then turn 
