. The Bell System technical journal . hem, hence opticaltwins of d. The b regions are small optical twins of c, and/ are flaws. between the a and c twins separates combined twins. Note also that the atwin bounds the b twin and the h twin bounds the c twin. Thus, a and b aretrue electrical twins, and b and c are true optical twins. 23 It is possible that growth conditions are such that combined twinning cannot occurbv itself without the presence of true optical twinning and true electrical twinning. Thatis, a region of given handedness and sense can not be entirely bordered by a region ofopposi
. The Bell System technical journal . hem, hence opticaltwins of d. The b regions are small optical twins of c, and/ are flaws. between the a and c twins separates combined twins. Note also that the atwin bounds the b twin and the h twin bounds the c twin. Thus, a and b aretrue electrical twins, and b and c are true optical twins. 23 It is possible that growth conditions are such that combined twinning cannot occurbv itself without the presence of true optical twinning and true electrical twinning. Thatis, a region of given handedness and sense can not be entirely bordered by a region ofopposite-handedness and same-sense. 46 BELL SYSTEM TECHNICAL JOURNAL Figure shows an unusual section which is mainly composed of leftquartz, 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 region a is an optical twin of d, and is uncommonly large for an opticaltwin (note: region a contains within it, two small optical twins). Since. Fig. —Regions c are electrical twins of the adjacent d regions, a is an electricaltwin 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 sameelectrical sense, thus exhibiting COMBINED-TWINNING. optical twins are usually very small (except for the one major surroundingtwin), it is seldom possible to cut them apart and use each twin , and were obtained by the means above described, andall sections shown in these figures (except Fig. and C) actually ex-hibited both electrical and optical twinning. Thus Fig. was obtainedfrom Fig. , and Fig. from Fig. , etc., by trimming the latter ETCH TECHNIQUE 47 named figures to give the sections simulated natural faces. Figures are of particular use in learning to distinguish between electrical andoptical twinning when examining e
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