164 Dr. Silvanus P. Thompson on 



emission of *r-rays, and increased the continuity and brilliancy 

 of the luminescence. Various amounts of current were sent 



Fig. 3. 



through the platinum, the most effective result being obtained 

 by currents which heated the surface to visible redness. 

 Whether the effect is a direct one or an indirect owing to the 

 driving out of occluded gases is not yet determined. 



4. When watching with the luminescent screen the emis- 

 sion of #-rays within bulbs connected with the pump, some 

 observations were made of direct importance with regard to 

 the state of exhaustion that is best. The degree of vacuum 

 which suffices for the production of kathodic shadows is 

 known not to be sufficient for the production of ^-rays. It is 

 also known that when evacuation is pushed very far the 

 internal resistance of the bulbs rises very high, so that they 

 become almost non-conductive. If a bulb is exhausted, and 

 heated during exhaustion, and the vacuum pushed almost to 

 non-conductivity, and if a little air is again admitted and the 

 tube again exhausted, the high degree of vacuum is again 

 very soon reached, probably because during the first ex- 

 haustion the gases absorbed upon the walls of the bulb were 

 mostly removed. After three or four repetitions of this 

 process the transition from the low state of vacuum to the 

 high state is exceedingly rapid. If a bulb in such a condition 

 is examined by the luminescent screen while the pump is at 

 work, scarcely any trace of #-rays can be noticed so long as 

 the vacuum is such that the resistance is low. A pair of 

 discharging-points arranged as a shunt to the tube serves as 

 an approximate gauge. Kathodic shadows can be seen when 

 the resistance is so low that the discharge- points do not spark 

 even when placed 3 millimetres apart. When the resistance 

 rises so that the spark- points must be put 20 or 30 millimetres 

 apart #-rays begin to be given off ; and are given off both 

 from the back and from the front of the antikathode. The 

 bulb, as seen upon the adjacent screen, shows two pale lunii- 



