Scattering of X-rays and Atomic Structure. 225 



were easily determined. (The corrections necessary for 

 absorption were obtained from subsidiary absorption experi- 

 ments.) 



The reason for the choice of this particular direction for 

 the scattered radiation measured is simply that it in all 

 probability gives us the truest measure of the whole energy 

 of radiation scattered. An " extra radiation " *, the bulk of 

 which is included within a cone of small angle in the forward 

 direction, is not included in the scattered radiation measured 

 in these experiments, for this extra radiation cannot be 

 explained on the simple theory of scattering, and in all 

 probability is due to processes other than those considered. 



Method (b). 



In the second method thick sheets of scattering sub- 

 stance were used, so thick as to transmit no appreciable 

 portion of the primary radiation. The intensity of scattered 

 radiation appearing through the face of incidence of the 

 primary X-radiation was compared for various scattering 

 substances placed successively in the same position. Measure- 

 ment and standardization were effected as before. It is 

 easily shown that the effective thicknesses of material from 

 which the radiations emerge are inversely proportional to 

 the coefficients of absorption (X) in the various substances 

 of the radiation used, provided that radiation is homogeneous. 

 Thus with primary rays normal to the surface, the intensity 

 of primary radiation at a depth x is given by l = I e~ kx . 

 If a fraction s dx is scattered in passing through a thickness 

 of matter dx, and k is the fraction of scattered radiation 

 proceeding in such a direction as to enter the measuring 

 electroscope, then if this direction is approximately normal 

 to the surface the total intensity entering the electroscope is 



i 



2i\ 



The same expression holds if the primary and scattered 

 beams make equal angles with the surface of scattering 

 substance, as was the case in these experiments. I and k 

 were the same for different radiators, X for the emerging- 

 scattered X-radiation was determined from absorption 

 experiments, consequently relative values of s, the scattering 



* Barkla & Ayres, Phil. Mag. Feb. 1911 ; Crowther, Camb. Phil. Soc. 

 Proc. 1911, and Roy. Soc. Proc. 1911 ; E. A. Owen, Camb. Phil. Soc. 

 Proc. 1911. 



Phil. Mag. S. 6. Vol. 31. No. 183. March 1916. R 



