The Intensity of Reflexion of X- Rays by Rock- Salt. 3 



The second case is illustrated by fig. 1. A beam o£ 

 "homogeneous X-rays is obtained by reflexion by the first 

 crystal C. This beam is reflected by a second crystal 



K*. 1. 



plate S, which is cut so that the reflecting planes are at 

 right angles to its surface,, and is turned with angular 

 velocity co. 



In this case, all the rays have to traverse a depth t secO 

 in the crystal, where t is the thickness o£ the plate. Calcu- 

 lation shows that 



5?-«*Q.««-", (4) 



where t = t o sec0, and Q and I have the same meaning as in 

 equation (3). 



B2 



