RONTGEN RAYS 317 



intensity or quantity of the rays calls for first consideration. 

 This result, standing alone, has often been interpreted as indi- 

 cating that the corpuscles from the metal are not expelled by the 

 electric forces in the wave front, but by a sort of radioactive 

 explosion of some of the molecules which have been put in an 

 unstable state by the incidence of the light, which on this view 

 merely acts as a trigger to start the explosion : thus both the 

 energy and the material of the corpuscular radiation are furnished 

 by the atom. We know that radioactive substances emit high- 

 speed electrons, and indeed there is other evidence to support 

 this theory of atomic disintegration. J. J. Thomson (1908) 

 measured the maximum velocity of the secondary cathode rays 

 which are produced when primary cathode rays move through 

 gases ; he found it to be independent of the gas and of the speed 

 of the primary rays, although the latter was widely varied. 

 Fuchtbauer (1906) got almost the same value for the velocity of 

 the secondary cathode rays from a number of different metals 

 subjected to primary cathode rays or canal-strahlen. 



But, coming back to the case in point, we have now to explain 

 why, as set forth above, the speed of the electrons is not 

 independent of the quality of the primary rays. Why should 

 the speed be great when the light-waves are short and the pulses 

 thin, if their only effect is that of a trigger action ? And further, 

 why should the electrons pursue, in the main, the original direc- 

 tion of propagation of the pulses ? Nor do the difficulties end 

 here. If the electrons owe their velocity to atomic disintegration, 

 we should expect their speed to vary from atom to atom much 

 more than experiment indicates, and as it does in the case of the 

 radioactive substances. Thus the explosion theory cannot be 

 said to score many successes, and, on summing up, it must be 

 conceded that the energy of the secondary electron does not 

 come, at any rate entirely, from the atom. 



We are left to suppose that the energy is imparted by the 

 Rontgen or light rays, since it varies in a continuous manner 

 with their quality. Now it can easily be shown that the energy 

 of a pulse, if spread over an ever-widening space, is utterly 

 insufficient to provide the energy required for the secondary 

 electron, and thus the pulse theory as we have been regarding it 

 requires modification. 



The discontinuous wave front and pulse theory. — We are led to 

 the development which not only the theory of pulses but the 



