Theory of X-Ray Reflexion. 683 



the breadth of the good reflexion is 6" and i£ we calcu- 

 late the intensity corresponding to a slit of the supposed 

 breadth at 3" from the centre, we find it to be a little less 

 than that at the centre. So we conclude that a slit of 5" 

 will hardly give full reflexion. As the breadth is only re- 

 quired for the purpose of a rough estimate, we shall take it 

 at 8". This is the narrowest slit which can be used if the 

 intensity of reflexion of monochromatic rays is not to suffer. 



5. White Radiation. 



We next find the intensity of reflexion of white radiation. 

 Let the intensity of the incident radiation at distance R from 



If 00 

 the source be y- 2 J udk. Reflexion only occurs for values 



of k near those which satisfy the equation ka sin 6 = nir — q . 

 These values will be denoted by k n . A value of k near kn 



can be expressed in the form k = k n \l-\ ). The centre 



\ mrj 



of best reflexion for k is at an angle 6— -q where 



ka sin (0 — rj) = nir — q , 



and it follows from this that x = ka cos 6 . rj, so that x is the 

 same as v in (6). The intensity of reflexion thus is 



p 2 n "nTrlj^ix-Vxi-qy * J q (x+^x'-q^ ■ Ji 



or 1 v k n 8 



-y 2, u n — - T, q. 

 p" H7T 6 x 



If we introduce the value of q and the factors for polari- 

 zation and temperature, and if we express the result in terms 

 of the quantity E A where E A d\ = udk, we have 



1 lG 1 + | COS 26 i XT .. ^ 1 -l -^(2»7T)2 



p^r 2 Nat n ^ e ° (l/|E,\)„ (12) 



6. Composite Crystal. 



When the crystal is composite the complete discussion of 

 any special case is rather more complicated. If, for example, 

 the alternate planes are i different in character, we obtain a 

 set of four difference equations involving two different types 

 of T's and S's. If three of these are eliminated we obtain a 

 single difference equation for the fourth, the solution of 



