﻿Velocity of the Cathode- Rays. 363 



observed displacement of the images. Again, the two phos- 

 phorescent patches are as nearly as possible of equal bright- 

 ness, so that there can be very little difference in the intensity 

 of the cathode-rays falling upon them : it was for this reason 

 that both the phosphorescent patches were taken some dis- 

 tance down the tube. Again, I took a tube which was bent 

 so that that the cathode-rays fell more directly upon the patch 

 farther from the cathode than upon the other patch, so that 

 in this case the phosphorescence of the more remote patch 

 was brighter. The displacement of the images with this tube 

 was just the same as for the previous, t. e. the phosphorescence 

 commenced at the patch nearest the cathode sooner than at 

 the other patch ; whereas if the displacement of the images 

 was due to the interval between the arrival of the rays and 

 the beginning of the phosphorescence it should have com- 

 menced at the patch furthest from the cathode, as this was the 

 most exposed to the cathode-rays and phosphoresced with the 

 greatest brilliancy. 



I conclude, therefore, that the displacement of the images 

 is due to the time taken by the rays to travel from one patch 

 to the other. This displacement enables us to measure the 

 velocity of the cathode-rays. The amount of displacement 

 observed through the telescope is not constant : even though 

 the mirror is turning at a uniform rate, there are quite appre- 

 ciable and apparently irregular variations in the amount of 

 the displacement of the images seen in the course of a few 

 minutes. I think these are due to irregularities in the sparks 

 discharging the jar, and the consequent irregularities in the 

 electromotive force acting on the discharge-tube. 



When the mirror was rotating 300 times a second, the 

 bright edges of the two patches were on the average sepa- 

 rated by the same distance as the image of two lines 1*5 inillim. 

 from each other placed against the discharge-tube. Since the 

 distance of the discharge-tube which contained hydrogen from 

 the mirror is 75 centim., the mirror must, in the time taken 

 by the cathode-rays to pass from one patch to the other, have 



1*5 



turned through the angle whose circular measure is- — =^r. 



Since the mirror makes 300 revolutions per second, the 

 time it takes to rotate through this angle is 



1-5 1 



2x750x2ttx30O 6ttx10 5 ' 

 and since the distance between the patches is 10 centim., the 



