1912] on t'ifdl Epds of Railiuiti ami other Bdys. 295 



niys upon the skin, I may mention that Fveund found that a tube 

 so high as to give no fluorescence on the screen caused the hair to 

 fall out, and also that with a tube having the electric current passed 

 in the reverse direction, so as to produce only very weak primary 

 ./;-rays, similar results were obtained. It would be interesting to 

 construct a tube so as to employ, for therapeutic purposes, these 

 secondary rays alone. 



I have made comparative measurements of these rays by means of 

 the fall of the gold-leaf electroscope. The simple instrument called 

 an electroscope, which is merely a strip of gold leaf or aluminium 

 leaf attached to a support, is discharged by the action of x-rays, which 

 makes the air a temporary conductor, and it is a most convenient 

 method to measure the degree of conductivity produced, the time 

 taken by the gold leaf to fall through a given distance being taken in 

 each case. Measured by the fall of the electroscope leaf, I found that 

 with a " high tube " giving very penetrative x-rays, if the action of 

 the primary rays were taken as 1, the action of the secondary rays 

 would be ^, and that with a low tube, or a tube giving x-rays of a low 

 order of penetrability, if the primary rays were again taken as 1, the 

 second rays would be \. 



Professor Silvanus Thompson several years ago showed that the 

 cathode stream, after impinging upon the target and thus giving rise 

 to the primary x-rays, was reflected and impinged upon the glass 

 walls of the tube, causing a green fluorescence. He called these re- 

 flected cathode rays paracathodic. Whether they produce x-rays upon 

 this second impact or not does not appear to have been proved, but 

 as Barkla and Sadler and others have demonstrated that x-rays out- 

 side the tube, impinging upon solid matter, give rise to secondary 

 rays, it seems certain that x-rays, in passing through the walls of the 

 tube in which they are generated, must give rise to secondary x-rays, 

 and it may well l)e the case that the green fluorescing glass of an 

 x-ray tube emits two sets of x-rays — one, as we have mentioned, pro- 

 duced by the primary x-rays in their impact on passing through it, 

 and the other possiltly by reflected cathodal rays. Be that as it may 

 — and this is a matter for the physicist — I feel sure that their physio- 

 logical action upon the skm must be considerable, es])ecially as they 

 are much more readily absorbed than the primary x-rays. In illus- 

 tration of this, I can show you a photograph which has been pro- 

 duced by them. As far as I am aware, the physiological importance 

 of these rays seems to have been entirely overlooked, and I am sure 

 they present a field worthy of immediate investigation. 



[Secondary rays from glass of tube shown by 

 fluorescent screen, the primary rays having been cut 

 oflF by a mass of lead. 



Diagram 1, showing production of secondary rays 

 from tube. 



