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



plate, and half of it covered by thin slices of paraffin-wax, 

 collodion, mica, glass, or fluorite so as to estimate the pene- 

 trating power of the radiation. A magnet was placed 

 between the target and photographic plate so as to deflect 

 from the latter any corpuscular radiation from the target. 

 The vacuum was made as low as possible by means of char- 

 coal and liquid air ; it was so low that no luminosity could be 

 detected between the anode and the target. The plates used 

 were Schumann plates ; the Paget plates were not sensitive 

 enough to detect the radiation from the slowest cathode 

 rays, though they gave good photographs when the rays 

 fell through an effective potential of more than 100 volts *. 

 The times of exposure, which varied from 1 minute to 2 hours, 

 were chosen so as to make the energy in the cathode rays 

 striking against the target during the time of exposure 

 constant: thus with cathode rays which had fallen through 

 20 volts, the time of exposure would be ten times that for 

 those which had fallen through 200 ; the latter gave quite 

 dense photographs with an exposure of 2 minutes. 



I have obtained photographs with cathode rays whose 

 energy ranged from 10 to 600 volts, and there would be no 

 difficulty in getting those corresponding to higher voltages 

 by using larger batteries to produce the main discharge. 

 These photographs are not due to ordinary light coming 

 from the discharge-tube, for (i.J they are not obtained when 

 the beam of cathode rays is deflected by a magnet from the 

 target, and (ii.) the rays which produce them are unable to 

 penetrate exceedingly thin films of glass. To test whether 

 they were due to a corpuscular radiation from the target two 

 methods were employed. First, a magnet was placed between 

 the target and photographic plate, so as to deflect the cor- 

 puscular radiation from the plate; this did not affect the 

 photographs. The second method was to place between the 

 target and the photographic plate a pair of parallel plates 

 similar to those used in the first experiment with positive 

 rays, and apply to them a potential difference of 1000 volts. 

 The intensity of the photographs was not diminished when 

 all the radiation which struck the plate had passed through 

 this strong electric field, which would have stopped any 

 charged particles. 



With regard to the penetrating power of this radiation, 

 when it is produced by cathode rays with less energy than 

 40 volts, I have never been able to detect any photographic 



* Since this paper was read I Lave, by using a more copious supply 

 of cathode rays, been able to get photographs at the lower voltages with 

 Paget plates. 



