Rays of Positive Electricity. 433- 



it seems probable that they would be so deflected by the 

 intense electric forces in the neighbourhood o£ the cathode, 

 that they could not travel down the long narrow tube inter- 

 posed between them and the phosphorescent screen without 

 striking against the sides and so getting absorbed. 



I have measured also the velocity of the " retrograde 

 rays " — those positively electrified particles which travel 

 away from the cathode in the same direction as the cathode 

 rays — and find that, as in the case of the Canalstrahlen, this 

 velocity does not change when the potential- difference 

 between the electrodes undergoes wide variations : the 

 velocity of the " retrograde rays " is the same as that of the 

 Canalstrahlen. I was not able to study the "retrograde 

 rays " when the potential-difference between the electrodes 

 was anything like as small as 3000 volts. These rays are 

 not nearly so bright as the Canalstrahlen, and become too 

 faint to be observed when the pressure in the tube is high 

 enough to make the potential-difference comparable with 

 3000 volts. 



The direction in which the retrograde rays come off is 

 much the same as the direction of the cathode rays ; they are 

 much denser along the normal to the cathode than in any 

 other direction, and though they can be detected in directions 

 making an angle of a tew degrees with the normal, they 

 rapidly fall off in intensity as the angle increases and soon 

 become too faint to be observed. It would thus seem that 

 the electric field produces some kind of polarization in the 

 molecules which makes them eject the uncharged doublets 

 along the line of motion of the cathode rays in their 

 neighbourhood. 



I have tried a good many experiments to find the place 

 from which the Canalstrahlen and the retrograde rays start. 

 One of these was as follows : — The cathode was an aluminium 

 disk 2*5 cm. in diameter. A narrow thin piece of mica 

 coated with fused lithium chloride was fixed to a piece of 

 iron, and could be moved towards or away from the cathode 

 by a magnet. The chloride when struck by Canalstrahlen or 

 retrograde rays gives out a bright red light, and is thus a 

 good detector of the rays. Suppose we start with the mica 

 at a distance from the cathode less than about half the 

 thickness of the dark space, we find then that there is no red 

 light to be seen on the side of the mica next the cathode, 

 showing that no retrograde rays strike against the mica. 

 The anode side of the mica, however, is a brilliant red, 

 showing that plenty of Canalstrahlen are striking against it. 

 Let us now pull the mica further away from the cathode. 



Phil. Mag. S. 6. Vol. 19. No. 111. March 1910. 2 F 



