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BELL SYSTEM TECHNICAL JOURNAL 



Figure 5.26 shows an unusual section which is mainly composed of left 

 quartz, of the electrical sense shown in D, region d. The region c is an elec- 

 trical twin of d. The region/ is a flaw in the quartz and is to be disregarded. 

 The region a is an optical twin of d, and is uncommonly large for an optical 

 twin (note: region a contains within it, two small optical twins). Since 



Fig. 5.25 — Regions c are electrical twins of the adjacent d regions, a is an electrical 

 twin of b, and a is also an optical twin of d. An uncommon condition of twinning is pre- 

 sented by the adjacent a and c regions which are of opposite handedness but the same 

 electrical sense, thus exhibiting COMBINED-TWINNING. 



optical twins are usually very small (except for the one major surrounding 

 twin), it is seldom possible to cut them apart and use each twin individually. 

 Figures 5.1, 5.2 and 5.3 were obtained by the means above described, and 

 all sections shown in these figures (except Fig. 5.2A and C) actually ex- 

 hibited both electrical and optical twinning. Thus Fig. 5. 3D was obtained 

 from Fig. 5.24A, and Fig. 5.2F from Fig. 5.24C, etc., by trimming the latter 



