10 



Professor Sir J. J. Thomson 



[April 14, 



Ptonto:en rays produced by changes in the energy of the cathode rays 

 are but slight. 



This result has, I. think, a very important bearing on the origin 

 of Rontgen rays. If these rays were in the main produced dire.ctly 

 1 ly the stoppage of cathode rays, we should expect the quality of the 

 Rontgen rays to vary pari passu with the energy of the cathode rays. 

 We have seen that this is not the case ; the bulk of the rays seem to 

 keep of approximately constant quality, in spite of wide variations in 



£ART^ 



Fig. 2. 



the energy of the cathode rays. This result seems to me to prove 

 that the bulk of the Rontgen rays are produced indirectly and not 

 directly by the cathode rays. I take the view that the most important 

 part played by the cathode rays for this purpose is to ionize some of 

 the atoms in the target, expelling electrons from these atoms, and 

 tliat it is the subsequent return of electrons to ionized atoms which 

 gives rise to the greater part of the Rontgen radiation ; as long as 



