﻿358 Prof. J. J. Thomson on the 



within this region, but I estimate that within it the maximum 

 defect of pressure is less than that of a head of water, or 

 whatever other liquid the channel may contain, of one tenth 

 of the mean depth. 



The accompanying figure shows the form of the wave, only 

 half of it being drawn, however, as the wave is symmetrical 



about the crest. The thicker straight line indicates the bottom 

 of the channel, while the mean depth is shown by a finer line 

 to which the surface approaches asymptotically. 



XL. On the Velocity of the Cathode-Rays. By J. J. 

 Thomson, M.A., F.R.S., Cavendish Professor of Experi- 

 mental Physics, Cambridge*. 



THE phosphorescence shown by the glass of a discharge- 

 tube in the neighbourhood of the cathode has been 

 ascribed by Crookes to the impact against the sides of the 

 tube of charged molecules driven off from the negative elec- 

 trode. The remarkably interesting experiments of Hertz 

 and Lenard show that thin films of metal when interposed 

 between the cathode and the walls of the discharge -tube do not 

 entirely stop the phosphorescence. This has led some phy- 

 sicists to doubt whether Crookes's explanation is the true one, 

 and to support the view that the phosphorescence is due to 

 setherial waves of very small wave-length, these waves being 

 so strongly absorbed by all substances that it is only when 

 the film of the substance is extremely thin that any per- 

 ceptible phosphorescence occurs behind it. Thus on this 

 view the phosphorescence is due to the action of a kind of 



* Communicated by the Author. 



