330 



BELL SYSTEM TECHNICAL JOURNAL 



and substituting the values of 701.2 and 701. 1 from Table I, 



90' 



32°25' + 51°47' , aoM 



+ 



aoi-2 



= 47°54' + 



aoii — a;oi.2 



(3.47) 



For more complicated cuts, such as the NT cut, four spots are chosen, 

 two close to the line ('(' and two close to the line w'w' . (See Fig. 3.25). 

 The positions these spots should occupy on the film for a correct NT cut 



_P 



_x 



_p 



Fig. 3.26 — Engraved glass plate for measuring Laue photographs 



are computed and compared to the spot-positions on the measured film. 

 From this comparison the following errors are determined: 



Cw error in rotation around the ww axis of the plate from the average 



displacement along t'C . 

 et error in rotation around the (I axis of the plate from the average dis- 

 placement along iv'w' . 

 et error in rotation around the / axis by the average angular displace- 

 ment of the four spots about the center of the film. 

 The first step in determining the position of a spot for any given atomic 

 plane in a correctly cut plate is the determination of the direction cosines 

 of the normal to that atomic plane with respect to the plate edges. This 

 procedure has been described in Section 3.8. 



Figure 3.27 is a Laue photograph of an NT plate. Taking this as an 

 example, we may choose the four spots (31-4), (31-5), (22-3) and (52-5) 



