

A. 



A. Interference pattern obtained with a crystal of zinc blende, the direction of the 

 incident rays being parallel to a trigonal axis of the crystal. 



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B. 



I!. I liagram representing the pattern obtained with the same crystal when the 

 incident rays are parallel to a cubic axis. In this diagram the fainter spots have 



been made more distinct. 



Fig. 2. 



374] 



