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BELL SYSTEM TECHXICAL JOURNAL 



HF). Stronger etching is not deleterious. Very strong etching gives mod- 

 erately good ligures with sawn or coarse ground slabs. For Test 1, alone, 

 weaker etches would suffice. Under properly controlled conditions of slab 

 preparation and instrument operation Test 2 might be eliminated, for 

 under such conditions the negative-cut portion of the slab is bright, the 

 positive-cut portion is dark. Under shop conditions this means of detecting 

 sense of cut appears to be not reliable, especially with untwinned slabs (which 

 are either all bright or all dark). The addition of Test 2, however, gives 



EYE 



Z - AXIS f 



TURN LJ TABLE 



Fig. 5.15 — The direction and sense of the electric axes of a sand-blasted and etched raw 

 quartz stone may be determined by reflection of light from the 0.21 facets. These same 

 facets are utilized in Test / of the twinoriascope, Figs. 5.13, 5.14. 



complete reliability, for if etch-figures are obtained the sense of cut is ob- 

 vious, if no figures are obtained the slab can be returned for further etching. 

 The principle of Test 7, above, has been applied by W. L. Bond to a lab- 

 oratory instrument for determining the direction and sense of the X-axes in 

 raw quartz stones prepared with a sand-blasted and etched surface. With 

 the stone mounted rotateably about its Z-axis (previously determined by 

 conoscope or inspectoscope), and a light beam properly projected onto the 

 stone, reflection of the light beam to an eye piece or viewing screen will occur 

 whenever the 01.2 facets come into proper angular position, see Fig. 5.15. 

 The approximate direction and sense of the electric axis, or the sense of cuts 



