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BELL SYSTEM TECHNICAL JOURNAL 



Fig. 4.13 — This stone is full of blue needles, though onl}' those located in the focus 

 of the light beam are visible. See also Fig. 4.14. 



Fig. 4.14 — The same stone as in Fig. 4.13 shows other blue needles in a different ar- 

 rangement, by holding the stone differentU- with respect to the light. 



4.9 Effect of Interior Imperfections on Finished Plates 



The practical effect on the finished piezoelectric plates, of the various types 

 of interior defects, is one of the least understood factors directly related to 

 economic use of the strategic material, raw quartz. This is because of the 



