SLRIACE LAYERS OF CRYSTALS 



143 



\()lume of quartz than are rays reflected from the disturbed layer. The 

 stront^est of these "white" reflections from the main plate is that from the set 

 of atomic planes most nearly parallel to the surface of the plate, the planes 

 that reflected the characteristic radiation in the zero position. 



NO. 3, 10 SECONDS 



NO, 3, 20 SECONDS 



ZERO 



NO. 3, 40 SECONDS 



Fig. 6.— Sijcctrometer photographs showing effect of etching a 303^ emery-lapped BT 

 quartz plate with 48"^ h>drotiuoric acid 



In Fig. 5, the central line (indicated by the arrow) resulting from the char- 

 acteristic radiation reflecting from the disturbed quartz, is distinctly present 

 through the 4° exposure but not in higher-angle exposures, indicating that in 

 the 3(lU emer}'-lapped surface from which the X-ra\-s were reflected there 

 was not enough quartz misaligned by more than 4° to reflect a beam that 



