X-RAYS AND ORIENTATION OF QUARTZ CRYSTALS 



331 



from which to make the measurements. From equations (3.13) and (3.9) 

 we find that the direction cosines of the normals to these atomic planes in 

 terms of the A^T plate edges are 



/ .42561\ /-.01112\ 



(31-4) = ( -.90357] , (31-5) = f -.99294) , 

 \ .04972/f \-A\8l2/p 



/-.42618\ /-.00898\ 



(22-3) = I -.90421 I , and (52-5) = I -.99140 J 

 \ .02816/ p \-.13062/p 



Fig. 3.27 — Back reflection Laue photograph of an NT plate 



The orientation of the unit normal to the atomic plane (31-4) with respect 

 to the plate edges is shown in Fig. 3.28. Since .90357 = cos 25°22', the 



.04972 



angle between the unit normal N and the P2 axis is 25°22'. Since 



.42561 



tan 6°40' the angular component of the unit normal .V in the P1P3 plane 

 is 6°40' that is, the (31-4) spot should lie in a direction 6°40' from the long 

 axis of the film and should be at a distance from the center corresponding 

 to 25°22' as read from the special scale. 



The correct locations of the other three spots are similarly calculated. 

 These values are given below, with the values actually measured from the 

 Laue photograph of the NT plate and the errors determined from a com- 

 parison of the calculated and measured values. 



