148 MK. G. W. C. KAYE ON THE 



p. 405) that the effect of scattering on the transmission of a homogeneous beam is to 

 convert an exponential absorption into one in which the coefficients of absorption 

 (reckoned on an exponential basis) diminish with the thickness of the screen. The 

 larger the ratio of the energy scattered to the energy spent in ionisation of the 

 absorbing substance the more will the absorption of the rays depart from an 

 exponential law. 



With a coil discharge cathode rays of widely varying velocities are incident upon 

 the anticathode. From observations of the magnetic spectrum it appears that a 

 considerable proportion of them have a common maximum velocity, at any rate when 

 the potential on the tube is not very high. For a large mimber of particles we might 

 therefore expect a typical mode of arrest, with, features arising from the corpuscular 

 groupings of the atoms of the anticathode. This being so, pulses of a definite type 

 peculiar to the metal of the anticathode will form a large part of the Rontgen rays 

 emitted. 



It would be expected that pulses thus generated would suffer very little scattering 

 on encountering further layers of a metal which presents atomic structures similar to 

 those of the anticathode, and that the consequent diminution in intensity would be 

 chiefly the result of ionisation. It must be remembered, too, that the rays are sifted 

 to some extent before they emerge from the surface of the anticathode. When the 

 atoms of the screens provide different corpuscular groupings to those in which the 

 Rontgen pulses were generated, then the scattering effect will be pronounced, and 

 the law of absorption of the beam would not be an exponential one, even if the beam 

 were wholly homogeneous. 



If the potential on the tube is increased, faster cathode particles impinge upon the 

 anticathode, harder Rontgen rays are generated, and it will be these which remain 

 when the thickest screens are used. Now the importance of the scattering term 

 compared with the loss of energy due to ionisation of the absorbing substance 

 increases with the hardness of the rays, and we thus have an explanation of why, 

 even when screen and radiator are alike, the absorption falls away from exponential 

 with higher potentials and thick screens. 



It appears from the results that there is a considerable range in the hardness of the 

 rays from an anticathode, for which a screen of the same metal will show an 

 exponential absorption. 



The fact that the intensities of similar hard radiations from different anticathodes 

 follow the order of atomic weights and not that of densities, indicates for the cathode 

 rays encounters which are dependent on the properties of individual atoms, and not 

 on their behaviour en masse as a material. With heavy atoms a larger proportion of 

 cathode rays would be expected to undergo arrests sufficiently abrupt to produce 

 Rontgen rays than would be the case with lighter atoms. 



It is not one of the objects of this paper to discuss the " neutral pair " theory of 

 the Rontgen rays recently put forward by Prof. BRAGG, but seems to the writer 



