14 BELL SYSTEM TECHNICAL JOURNAL 



stone into separate parts when the twins are large enough to be utilized 

 separately. Further, at some stage before reaching the finished plate all 

 twin portions but one must be cut away.^ 



In this connection it is important to note a size and form difference be- 

 tween electrical and optical twins. Fig. 5.2 shows the appearance of twin- 

 ning boundaries when only ELECTRICAL TWINNING is present. Note 

 that electrical twins are commonly large, hence may often be separated ap- 



Fig. 5.1 — Examples of ELECTRIC.\L and OPTICAL twinning, as exhibited at the 

 etched surface of Z-cut sections. These examples are tj-pical of an appreciable portion 

 of the quartz that is cut up for quartz plates. 



proximately along a boundary and both portions utilized. Fig. 5.3 shows 

 the appearance of twinning boundaries when only OPTICAL TWINNING 

 is present. Since optical twins are commonly small and in the form of thin 

 laminations, it is seldom possible to cut optical twins apart and use both 

 parts separately. 



The conventions here used, regarding handedness and axial sense, are 



^ See Section 5.7 for the possibility of utilizing partially twinned finished plates. 



