CRYSTALS UNDER THE MICROSCOPE 165 



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^ ob- 

 jective is substituted for another by turning the nosepiece it is 

 usually necessary to recenter the stage. A very convenient de- 

 vice 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 

 the other until extinction results; read the scale; the reading 

 upon each circle should be the same number of degrees. 



4. Testing the Graduated Circle upon the Circumference of 

 the Stage. Place at the center of the stage a preparation con- 

 taining long prisms of a salt exhibiting parallel extinction. With 

 the nicols crossed at zero, select a good crystal, center it and 

 bring its long prism edge coincident with a cross-hair. Now 

 turn polarizer and analyzer several degrees, each being rotated 

 an equal distance and therefore maintaining the relative positions 

 of crossed nicols. Read the graduated circle on the analyzer, 

 read the position of the stage and rotate the stage until the crys- 

 tal extinguishes. Read the stage circle. The angular rotational 



