SPECIFYING QUARTZ CRYSTAL ORIENTATION 



247 



reading might be, for instance, -35°, +35", +55°, +125°, etc. This is 

 the principle of the normascope used to identify the .v direction for crystal 

 adjustment. 



Let us study these relative phase shifts at different angles near the optic 

 axis. Now quartz has an optical complication beyond that just described — 

 it rotates the plane of polarization of plane polarized light traveling along 

 the optic axis. This complicates our present attempt to build up a back- 

 ground sufficient for an understanding of the conoscope. But the cono- 

 scope finds the optic axis for other crystals that do not rotate the plane of 

 polarization, tourmaline for example; so we will ignore this rotation, to be- 



OPTIC 

 AXIS _ 



ANALYZER 



Fig. 2.28 — How the polarization changes with propagation direction in a crystal plate 



gin with, in order to arrive quickly at some useful conclusions. We will 

 later explain how optical rotation modifies these conclusions. 



Consider then the crystal z section shown in Fig. 2.28. A source 5 sends 

 monochromatic light through the polarizer which passes only vertical 

 vibrations. We will assume that the light passes in and out of the crystal 

 without a deviation of path. Since the vibration is in the plane of s and 

 pi (its direction of propagation) the ray does not break up inside the crystal 

 but is propagated as plane polarized light, unchanged. An analyzer set 

 for vertical extinction could then extinguish this ray. 



This is true for propagation from s anywhere in this vertical plane. 

 Also since the vibration is perpendicular to the plane of z and p2 the ray 

 P2 does not break up inside the crystal but passes through and out un- 



