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BELL SYSTEM TECHNICAL JOURNAL 



in the crystal corresponding to the slow wave being one wave-length behind 

 the fast one. 



In Fig. 2.27 we show two AT plates resting on a reference surface. In the 

 first crystal the propagation is perpendicular to x, in the second crystal it is 



^D^cc 



How the kind of polarization changes with crystal thickness 



Fig. 2.27 — The normascope principle 



along X. If the reference surface is the reference table of a simple stauro- 

 scope the edge of the first crystal will appear dark because a = 0° or 90°; 

 the edge of the second crystal will appear bright because a is not 0° or 90°. 

 Actually this a for an AT plate can be ±35° d= any multiple of 90° because 

 we don't know whether z stands out to the right or to the left. Hence the 



