2 Prof. W. L. Bragg and Messrs. James and Bosanquet : 



velocity w through the angle at which some plane in it 

 reflects the X-rays, the theoretical expression for the total 

 quantity of. radiation E reflected states that 



I„ sin 20 mV„ 2 



= QrfV (2) 



In this expression : 



N = No. of diffracting units per unit volume, 



A = wave-length of X-rays, 



= glancing angle at which reflexion takes place, 



e = electronic charge, 



m = „ mass, 



c = velocity of light. 



The constant B, which occurs in the Debye factor e ~ Bshl20 , 

 was assumed to be 4*12, on the basis of an experimental 

 determination by W. H. Bragg*. 



The contribution of a single electron is represented by the 



factor — s-r in this formula. 



The factor F depends on the number and arrangement of 

 the electrons in the diffracting unit. At zero glancing-angle, 

 it has a maximum value equal to the total number of electrons 

 in the unit. As the glancing-angle increases, F falls off, 

 owing to interference between the wave-trains diffracted by 

 the separate electrons. 



2. In the case of a large crystal, the linear absorption- 

 coefficient, fi, of the rays in the crystal has to be taken 

 into account in calculating the intensity of reflexion. Two 

 special cases present themselves. 



In the first place, a narrow beam of rays may be reflected 

 from the face of a crystal cut parallel to the reflecting planes. 

 In this case, if the intensity I of the incident beam is defined 

 as the total amount of radiation falling on the crystal per 

 second, calculation shows that 



Ea> _ Q m 



"T ~ V C } 



where Q has the same significance as in equation (2). This 

 is the case dealt with in our previous paper. 



* W. H. Bragg, Phil. Mag. vol. xxvii. p. 897. 



