622 Sir J. J. Thomson on the Production of Soft Rontgen 



effect is due, I think, to the fact that the positive rays, 

 except at the very lowest pressures, do not; remain constantly 

 positively charged, but alternate from the charged to the 

 uncharged condition : thus a few of them might remain 

 without charge all the time they were between the plates 

 and thus escape deflexion ; after passing through the plates 

 they might re-acquire a positive charge before striking 

 against P. I think this is more probable than that the 

 residual effect is due to ordinary Rontgen rays or ultra-violet 

 light produced in A, for the rays, as we shall see, are too 

 •easily absorbed by white fluorite to be ultra-violet light, and 

 by thin mica or collodion to be ordinary Rontgen rays. 

 Another proof that the effect is not due to stray radiation 

 from A, is that it disappears entirely if C is made anode 

 instead of cathode. 



One of the effects of the impact of positive rays against a 

 metal plate is to make the plate emit slow cathode rays, and 

 it might be thought that the effect on the photographic plate 

 was due to these rays, starting from P and travelling down 

 to the plate. If this were the case, then putting a potential 

 difference of 1000 volts between the plates in M ought to 

 stop the effect entirely. I find, however, that the photo- 

 graphs are just as dark when 1000 volts are on the plates as 

 when they are at the same potential. This seems a conclu- 

 sive proof that the radiation which affects the plate is not a 

 corpuscular radiation or a form of positive rays, but is 

 analogous to light or Rontgen radiations, that in fact Rontgen 

 radiation is produced by the impact of positive rays against 

 a solid. 



This radiation is unable to penetrate even the thinnest films 

 I have been able to procure of substances such as collodion, 

 mica, paraffin-wax, aluminium, or white fluorite: when part 

 of the slit was covered with one of these films it entirely 

 stopped the radiation through that part of the slit. This 

 radiation can be reflected, for if a slit of the kind shown in 



Yicr. 2. 



/ 



fig. 2 is put in front of the photographic plate, it is found 

 that the plate is affected not only underneath A but also on 



