326 BELL SYSTEM TECHNICAL JOURNAL 



SO that we may write 



sin d fi Ar\ 



sin g2 = sin gi = ^— — (3.45) 



sin (7 — A2) 



As an example consider a. BT plate for which A2 — —49°, corrected around 

 the WW axis (Fig. 3.12) from the (02-3) plane. Table I gives us ^023 ^ 

 34°05', 7023 = 40°16' whence 



^1 = 34°05' + 40°16' - 90° + 49° = 33°21' 



g3 = 90° + 34°05' - 49° - 40°16' = 34°49' 



. -1 sin 34°05' -,o_ ., 

 ^^ = ^^ = ^" siiTWia' = '' '^ 



For an yir plate of A2 — +35°15' corrected around the WW axis from 

 (01 -T) we find, under (01 -1), that 6 = 13°20', 7 = 180 - 51°47' whence 



^1 = 13°21' + 180° - 51°47' - 90° - 35°15' = 16°18' 



g3= 26- gi= 10°22' 



. -1 sin 13°20' 12001/ 



A complete determination of the errors in a quartz plate includes the 

 measurement of angle errors about three mutually perpendicular axes such 

 as, for example, the plate edges. Therefore, in correcting a plate we must 

 use three different X-ray "shots"; usually two on the major surface with 

 the plate rotated 90° between the two "shots" and a third on a surface 

 normal to the major surface, commonly called an "edge". If the plate is 

 thin, the intensity of reflection from the "edge" is low and this measurement 

 becomes difficult. 



The goniometric procedure described above is best adapted to the routine 

 measurement of errors in large numbers of plates which are known to be 

 close to the correct orientation. If the error in a plate is large, however, 

 or if the cut is a rare one or the orientation of the plate completely unknown, 

 analysis by the goniometric method may be extremely difficult. 



A simple, direct method of determining the orientation of such plates is 

 that of the Laue camera. 



3.10 Use of Laue Photographs in Determining the Orientation of a 



Quartz Plate 



In the original Laue photograph the X-ray beam passed through the 

 crystal and was diffracted so as to give a spot pattern on a photographic 



