434 On Rays of Positive Electricity. 



When the distance from the cathode becomes equal to about 

 one-half the thickness of the dark space, red light appears 

 on the cathode as well as upon the anode side of the mica, 

 showing that the mica is now struck both by Canalstrahlen 

 and retrograde rays. This continues until the mica reaches 

 the limit of the dark space, when the red glow disappears 

 from the anode side, showing that it is no longer struck 

 by the Canalstrahlen ; the cathode side continues to glow, 

 showing that it is still struck by the retrograde rays. We 

 can also get the same effect by altering the pressure without 

 moving the mica, if we put the mica at, say, about 1*5 cm. 

 from the cathode and start with the pressure fairly high, so 

 that the mica is outside the dark space : the cathode side of 

 the mica is red, but not the anode. If now the pressure is 

 gradually reduced so that the dark space increases, then, 

 when it reaches the mica, the red light appears on the anode 

 side as well as the cathode, and as the pressure is reduced 

 the red light remains on both sides until the pressure falls 

 so low that the mica is about in the middle of the dark space. 

 The red light disappears now on the cathode side, and this 

 side remains dark when the pressure is still further reduced, 

 the other side being covered with a brilliant red light. 



There can, I think, be no question that the Canalstrahlen 

 originate in the region near the outer boundary of the dark 

 space. The place of origin of the retrograde rays is more 

 difficult to fix. The experiment just described might suggest 

 that these rays arose from the portion of the dark space 

 beyond (i. £., further from the cathode) the place where the 

 red luminosity on the cathode side of the mica screen dis- 

 appears. Other experiments, however, have led me to the 

 conclusion that most of the retrograde rays originate close 

 to the cathode, and that the absence of them when the mica 

 is brought close to the cathode is due to the shadow cast by 

 the mica on the cathode. The experiments were as follows : — 

 If the cathode is in a large bulb and the mica screen is put a 

 little on one side of the cathode, the red lithium light can be 

 *observed on the cathode side quite close to the cathode even 

 though the dark space may be 5 or 6 cm. deep. 



Again, when, as in the first experiment, the tube is of one 

 bore throughout, as the mica is moved towards the cathode 

 across into the dark space the luminosity on both sides is 

 very bright, and remains so until the mica enters Goldstein's 

 first layer, then the luminosity becomes suddenly very much 

 fainter and in some cases disappears on the cathode side, at 

 the same time the shadow thrown on the cathode by the mica 

 becomes much more marked and suddenly increases in size. 



