122 Dr. K. Domalip on Alternating 



These induced currents may be conveniently observed by 

 employing- a tube in which the " dark space ' ' is much extended. 

 If, then, the glass wall at any point of this dark space be placed 

 in connexion with the earth, the luminous currents which are 

 formed between the point touched and the nearest electrode 

 can be easily observed, appearing to issue from the electrode 

 and approach the point touched in the form of an arc. But 

 when we employ a tube in which the vacuum has reached such a 

 point that fluorescent light appears at the negative pole, the 

 negative light of the induced current fills the whole section of 

 the tube and strikes the opposite wall, when it sometimes pro- 

 duces a fluorescent surface. 



These two induced currents differ from each other in the 

 same way as the direct currents in the two well-known 

 Crookes's bulbs, which serve to show the different distribution 

 of the current in a space in which the air is moderately ex- 

 hausted, and in one in which it is very highly exhausted. 



Reitlinger* first observed certain phenomena with such 

 tubes — first with a bromine-tube, and then with other tubes. 

 When the finger, or any other conductor, is brought near the 

 part of such a tube in which the brush-discharge is visible, 

 there is seen on the glass wall opposite the finger or conductor 

 a sharply-defined luminous green surface. If the finger is 

 brought nearer, this bright green light assumes the form of a 

 line thick in the middle and pointed at the top and bottom. 

 Reitlinger believes that this is not a case of fluorescence, but 

 of some other peculiar production of light in the glass of the 

 Geissler's tube. He supposes this to be confirmed by the fact 

 that often, when the light in the tube is stratified, it shows no 

 change, but follows its course along the centre of the tube 

 undisturbed by the approach of the finger, causing the green 

 light on the opposite wall. The magnet may even be used to 

 drive the luminous column of gas up against the finger, and 

 nevertheless the green light on the glass of the opposite wall 

 shows no change; so that it cannot be produced by any fluo- 

 rescent action of the luminous column of gas. 



Goldsteinf has further examined this phenomenon, and 

 observed that it may be completely explained by assuming 

 that the inner surface of the portion of the glass wall touched 

 by the conductor behaves exactly like a kathode, and conse- 

 quently emits light possessing all the properties known to 

 characterize the kathode-light. 



The fluorescent phenomena observed by Reitlinger and 

 Urbanitzki are to be regarded as excited by the kathode-rays 



* W. Sitzber. vol. lxxiii. p. 691. 

 t W. Sitzber. vol, lxxiv. p. 463. 



