Scattered Radiation from a Plate exposed to X-rays. 207 



(7) The values calculated for the relative absorption 

 coefficients for 7-rays agree fairly with the observed 

 absorption coefficients. The value of mass scattering co- 

 efficient of Al for 7-radiation, calculated by means of the 

 said formula, agrees very nearly with the value directly 

 observed by Ishino. 



(8) The number of the outer electrons in the lighter 

 elements seem to be the same for the element placed in 

 the same vertical row of the periodic system ; and the 

 distribution of electrons thus appears to be in close con- 

 nexion with the periodicity of the chemical qualities of 

 the elements as expressed by this system. 



Nobel Institute for Physical Chemistry, 

 Stockholm, June 1918. 



XVIII. Calcidation of Scattered Radiation from a Plate 

 exposed to a beam of X-rays. By Oskar Klein, Lie. pJiil.* 



IN a paper treating of the absorption and scattering of 

 7-rays, Ishino f has undertaken a few theoretical calcu- 

 lations of scattered radiation issuing from the layer of a 

 substance. Such calculations combined with measurements 

 of the radiation scattered within known solid angles would 

 lead to a determination of the scattering coefficient. 



Some time ago T. E. Auren, in connexion with his researches 

 on the absorption of X-rays, performed some experiments in 

 this direction. He measured the absorption coefficient in 

 different positions of the plate exposed to the primary 

 radiation. If the scattered radiation had been of any con- 

 siderable amount, he would have found different values for 

 the apparent absorption coefficient in the different positions. 

 Now the differences -were very small. Therefore it was of 

 interest to Auren to know how far this was in agreement 

 with the theory. For this purpose I have undertaken the 

 calculation below. As mathematical formulae are obtained 

 which may be used in numerical calculation, the results may 

 perhaps have a certain interest for those who treat these 

 questions experimentally. In this connexion it must be men- 

 tioned that Ishino in his paper just cited, has made an error 

 in reckoning, which has caused his final value to take the 



* Communicated by the Author. 



Since this paper was finished and sent to the Phil. Mag-, a quite 

 similar calculation has been published by Glocher, Phys. Zeitschr. xix. 

 p. 251 (1918). 



t Ishino, Phil. Mag. vol. xxxiii. p. 129 (1917). 



