﻿360 Prof. J. J. Thomson ok the 



are aetherial rays would also prove the finiteness of the 

 structure of the aether. 



The following experiments were made with the view of 

 determining the velocity with which the cathode -rays travel, 

 as it seemed that a knowledge of this velocity would enable 

 us to discriminate between the two views held as to the nature 

 of the cathode-rays. If we take the view that the cathode- 

 rays are aetherial waves, we should expect them to travel with 

 a velocity comparable with that of light ; while if the rays 

 consist of molecular streams, the velocity of these rays will be 

 the velocity of the molecules, which we should expect to be 

 very much smaller than that of light. 



The method I employed is as follows : — The discharge -tube 

 was sealed on to the pump, and the two electrodes were placed 

 at the neck of this tube. The discharge-tube was covered with 

 lampblack, with the exception of two thin strips in the same 

 straight line from which the lampblack was scratched : these 

 strips were about 10 centim. apart ; the one nearest to the 

 negative electrode was about 15 centim. from the electrode, 

 the other was 25 centim. from the electrode. They were chosen 

 so as to phosphoresce with, as nearly as could be judged, equal 

 brilliancy when the discharge passed through the tube. 



The light from the phosphorescent strip fell upon a rota- 

 ting mirror about 75 centim. from the tube. This mirror is 

 the one used by me in my experiments on " The Velocity of 

 Propagation of the Electric Discharge through Gases " (Proc. 

 Roy. Soc. 1890) , and is described in that paper. The only 

 change made in the mirror was to replace the single plane 

 strip of silvered glass which w 7 as used in the previous ex- 

 periments by six strips of mirror fastened symmetrically 

 round the axis. The mirror was driven by a large gramme- 

 machine. 



The images formed by reflexion from the mirror were 

 observed through a telescope, of which the object-glass was a 

 large portrait photographic lens of 4-inch aperture, the eye- 

 piece a short-focus lens : when the mirror was at rest the two 

 images of the phosphorescent strips were seen in the same 

 straight line, and the adjacent ends of the two images were 

 brought into coincidence by inserting between one of the 

 phosphorescent strips and the mirror a very acute-angled 

 prism. The point of the experiment was to see if the images 

 of the two phosphorescent strips remained in the same 

 straight line when the mirror was in rapid rotation. If, for 

 example, the cathode-rays travelled with the velocity of 

 sound, they would take about 33*00 °f a secon d to pass from 

 one strip to the next ; if the mirror were rotating 300 times 



