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 

 under 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 

 through a half rotation (180 degrees) satisfactory measurements 

 may be obtained. 



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 

 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 



