﻿Velocity of the Cathode-Rays. 361 



a second it would, in the interval taken by sound to pass 

 from one strip to the next, rotate through about 33° ; the 

 displacement of the image produced by a rotation one 

 thousandth part of this could easily be detected. 



When the phosphorescence was produced by the discharge 

 of an ordinary induction-coil, the images seen in the tele- 

 scope after reflexion from the revolving mirror were drawn 

 out into very faint ribands of light without definite begin- 

 nings or ends; so that it was impossible to say whether or 

 not there was any displacement of one image relative to the 

 other. 



I tried a considerable number of phosphorescent substances 

 in the hope of obtaining sharp images, but without success. 

 The substances I tried were ordinary German-glass, uranium- 

 glass, lead-glass, the cyanide of magnesium and platinum, 

 asaron, aesculine, and Schuchardt's " Leucht-farbe." 



The gradual fading away of the phosphorescence after the 

 exciting cause has been removed, is one reason why one of 

 the edges of the image formed by the revolving mirror should 

 be indistinct : this cannot be remedied unless some substance 

 can be found which ceases to phosphoresce immediately the 

 incidence of the negative rays ceases. I was unable to find 

 any substance possessing this property : of the substances I 

 tried, uranium-glass was the one whose phosphorescence died 

 away most quickly. 



I tried several experiments with a specimen of asaron. This 

 substance was found by Lenard to cease to phosphoresce 

 so quickly after the cessation of the phosphorogenic rays that 

 he could not detect its duration in his very sensitive phos- 

 phoroscope. My specimen of asaron must, I think, have been 

 impure, as it phosphoresced a coral-pink instead of violet as 

 described by Lenard, and its phosphorescence showed a very 

 appreciable duration ; in addition to this, it did not give out 

 nearly enough light to be of any use in experiments of this 

 kind. 



But even though the images of the phosphorescent strips 

 have one edge (that corresponding to the end of the phos- 

 phorescence) indistinct, we can get the information we require 

 about the velocity of the cathode-rays if we can get the 

 image of the edge corresponding to the beginning of the 

 phosphorescence sharp and distinct. 



After unsuccessful attempts with several methods, I found 

 that this could be done in the following way, using the oscilla- 

 tory currents produced by the discharge of a Leyden jar : — 

 The electrodes of the discharge-tube were connected with the 

 ends of the secondary coil of a transformer, whose primary 



