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BELL SYSTEM TECHNICAL JOURNAL 



ample, CT slabs are to be examined the tiltable mounting-table is clamped 

 in the 38° position, and the slab placed crosswise on the table (X-axis normal 

 to line of sight, and beveled edge as shown). Upon moving the viewing 

 screen to position 1, only lamp 1 is lighted, and the slab is viewed by re- 

 flected light at a preferred angle. If the slab be twinned, one portion of the 

 slab will exhibit a bright sheen while the other portion is dull by contrast, 

 see two examples in Fig. 5.14, Test /. The twinning boundary is now pen- 

 ciled in. The viewing screen is then shifted to position 2 which lights only 

 lamp 2, and the cr\'stal moved to right or left so that only one twin is illu- 

 minated. On the screen^^ will be seen an etch-figure similar to one of the 

 four shown in Fig. 5.14, Test 2. If either of the two positive-cut figures are 

 observed the illuminated portion of the slab is usable, since the CT plate 



SIDE VIEW. SECTION 



FRONT VIEW 



Fig. 5.13 — The reflection TWINORIASCOPE for detecting twinning (using lamp 1 

 and no viewing screen, position 1) and for determining the orientation or sense-of-cut (using 

 lamp 2 and the viewing screen in position 2), of AT- BT-, CT-, or DT-cut slabs. The 

 "cut angle" is set for a CT slab. 



miist have a positive 38° orientation. The negative-cut, "golf-club", 

 figures are produced by the unusable portion of the plate. 



The same procedures are followed with the AT, BT and DT plates, in 

 each case resetting the table to the proper tilt, 35°, 49° and 52°, respectively. 

 The reflection view of Test 1 is the same for all cuts, and the etch-figures of 

 Test 2 are nearly the same (being almost identical for the negative-cut por- 

 tions of the slabs). However, in the case of AT and CT slabs the positive- 

 figures represent good portions (since these are positive cuts), and in the case 

 of BT and DT slabs, the negative-figures represent good portions. 



The basic principle of this mstrument is as described in section 5.41. As 

 here used, the two optical systems (including the eye and the slab) are so 

 disposed as to obtain the best reflection-contrast in Test /, and the most dis- 



1^ An excellent screen consists of two sheets of thin sandblasted cellulose acetate. 



