VACl'UM TUBES. 149 



friction against the tube, fading out almost instantly. It 

 does not matter how the tube is held, the only condition being 

 that the motion shall be rapid and light, several times a 

 second, the hand leaving the tube after each stroke. The 

 glow fills the whole interior of the tube, but is usually more 

 intense at the point of rubbing. The condition of the atmos- 

 phere, the matter of insulating or grounding the tube, heat 

 and cold, have no apparent effect upon the light. Of the 

 various substances used as rubbers nothing was found to 

 answer better than the hand. 'The intensity of the light 

 depends to some extent upon the state of the vacuum, as some 

 tubes respond more readily than others. Tests with the photo- 

 graphic plate show that the light possesses but feeble actinic 

 power. In order to see the glow it is necessary that the room 

 be absolutely dark. 



"The existence of the light probably depends upon the 

 production of electricity, although its apparent indifference to 

 atmospheric conditions, insulation, heat and cold, is difficult 

 to reconcile with this view." 



The phenomenon, however, remained without satisfactory 

 explanation. 



Some time after the above was published there appeared in 

 the Literary Digest of September 30th, 1905, a translation 

 from Cosmos ( Paris ) describing some quite similar experi- 

 ments by Professor Sommer, of the University of Giessen, 

 Germany. The phenomenon described in this article is appar- 

 ently the same thing as that observed by the writer, although 

 the production of the light is attributed to a physiological 

 rather than physical cause. The translation referred to is as 

 follows : — 



"Taking hold of the bulb of a small electric lamp, he 

 observed one night that whenever his hand touched the bulb 

 the latter showed a light like a luminous mist which illumin- 

 ated certain parts of the bulb as well as his fingers, even 



