50 Proceedings of the Royal Society of Edinburgh. [Sess. 
approach the K bands for the metals actually investigated — copper, silver, 
tin, and lead. But a numerical investigation shows that the proportion 
of this increase due to the free period of the K electron must be a small 
one ; the increase is nearly all due to interference between the different 
electrons of the atom according to formula (3). 
Equation (7) is not quite accurate, because there is a resistance to the 
motion due to the reaction of the radiation on the electron. This 
resistance introduces an additional term 
2 e 2 d 3 x 
on the left side of the equation (e in electrostatic units). This additional 
term diminishes the scattering ; owing to the reaction of the radiation the 
electron neither moves so far nor so fast. But the additional term is 
a small one, and does not become appreciable until the wave-length 
diminishes to 10“ u cm. 
I am greatly indebted to Professor Barkla for pointing out an important 
error in my first calculations on this subject. 
{Issued separately March 25, 1920.) 
