SPECIFYING QUARTZ CRYSTAL ORIENTATION 



255 



tern has been used to check the orientation but much grief has ensued due 

 to not recognizing one of its properties. This property is that, if z does not 

 lie parallel to the crystal boundary the center of the pattern is not per- 

 pendicular to the optic axis and a rather involved correction must be used. 

 This correction reduces the actual angle to about half the observed value. 

 This conoscope is an immersion instrument. The fluid is chosen to have 

 an index of refraction to match the "ordinary" one for quartz. When this 

 is done light is not bent in passing between fluid and quartz. When the 

 fluid does not match there is a bending and all readings are subject to a 



Fig. 2.36 — Crossed polarizers 



Fig. 2.37 — Rotation of the plane of polarizer 



correction. For example if we measure the angle of an AT plate in a fluid 

 that is too low by .0048 (since «o for quartz in green mercury light is 1.5462 

 this fluid has n = 1.5414), we will get a reading that is too high by a quar- 

 ter degree (the 35° angle will then appear low). A temperature rise of 12° C 

 will lower the relative refractive indices by this amount. 



Also the more perfectly the fluid matches, the more nearly will the rough 

 quartz surface disappear and seem smooth and clear. This greatly en- 

 hances the sharpness of the rings. 



The refractoscope (Fig. 2.39) was designed by G. W. Willard to tell when the 



