Theory of the Striated Discharge. 995 



above the line E = V&<? where V* is the 

 •adiation potential. Light of this kind would thus be 

 distributed over wider patches than that due to ionized 

 atoms or molecules. A careful study of the distribution 

 of the different spectral lines throughout the striated dis- 

 charge w r ould probably throw much light on questions of 

 this kind. 



The light from the ionized atoms or molecules arises from 

 particles which are moving down the tube with a velocity 

 approaching that of sound through the gas. If we could 

 isolate the light due to a thin layer of these particles, the 

 front and rear of the patch of. light to which it would give 

 rise would appear to be moving towards the cathode with 

 this velocity. If the discharge were absolutely steady, the 

 light would remain quite constant at each part of the tube, 

 and ^Ye could not detect this movement by observing the 

 appearance of the discharge in a rotating mirror. We might 

 expect, however, to get evidence of it in this way if the 

 discharge were flickering, and especially if it were inter- 

 mittent. 



Movements of the striations towards the cathode have 

 been detected by Spottiswoode (Proc. Roy. Soc. xxv. p. 73, 

 1876), also by Aston and Kikuchi (Proc. Roy. Soc. xcviii. 

 p. 50, 1920)/and by Kikuchi (Proc. Roy. Soc. xcix. p. 257, 

 1921). The method employed in each case was to observe 

 the discharge after reflexion from a rapidly rotating mirror ; 

 the appearance of the discharge showed that some of the 

 striations w r ere moving towards the cathode. Spottiswoode, 

 who used an induction-coil to produce the discharge, did 

 not give any estimate of the velocity. Aston and Kikuchi^ 

 who studied the discharge through neon, used both an 

 induction-coil and also a large number of small storage 

 cells ; they estimated the velocity as about that of the velocity 

 of sound through the gas. The effects observed witli a 

 revolving mirror are somewhat complicated, and include 

 a good deal besides the movements of the striations. Tims 

 Spottiswoode, using the discharge from an induction-coil, 

 found, in addition to the moving stria?, luminous patches 

 moving from the anode towards the cathode. In some 

 experiments which I made manv years ago (Proc. Roy. 

 Soc. xlix. p. 84, 1891) with a' tube about 40 ft. long, 

 through which the discharge from an induction-coil was 

 sent, I found that luminosity following the inception of 

 the discharge travelled down the tube towards the cathode. 



I think we can understand why there should be effects of 

 this kind if we remember that when the tube lias sen led 



