SPECIFYING QUARTZ CRYSTAL ORIENTATION 



251 



instead of the 1.546 of quartz. The dial is graduated into degrees and a 

 vernier allows readings to tenths of degrees. 



If a crystal plate is held against the glass reference surface one may read 

 the angle between the optic axis and the surface normal. One should 

 occasionally check the instrument (against slippage of the dial) by reversing 

 the crystal and recentering the pattern. If the readings are not identical, 

 the dial should be adjusted till they are. Even if the readings are not iden- 

 tical the mean value should be correct. If one is using the method of ring 



10 20 30 40 50 60 



R 



Fig. 2.33 — The ring eccentricity correction chart 



centering, the correction for eccentricity from Fig. 2,33 should be applied to 

 this mean value. 



The carriage may be slid back and forth and for very small crystals the 

 carriage should be placed so that the crystal is near the center of the tank 

 so that very little of the light cone by-passes the crystal. By the use of a 

 block a thin crystal can be examined by viewing through its edge or length. 



The carriage can be removed and "raw" crystals examined. The optic 

 axis is plainly visible and quite accurate orientations can be made if there 

 is not too much opaque material on the crystal. Excessive optical twinning 



