CH. IV.] 



LENARD RAYS 



29 



Lenard extended Hertz's discovery in a remarkable 

 way by skilfully constructing a tube with an outer 

 window of very thin aluminium, so arranged as to be 

 able to stand the atmospheric pressure outside. He 

 then directed the cathode ray bombardment on to this 

 window or aluminium film, and showed that the rays 

 can penetrate it and actually come outside into the 

 ordinary atmosphere, where they are called Lenard 

 rays, in honour of this indefatigable investigator, a 

 friend and disciple of Hertz. (See Fig. 1.) 



FlG. 1. Lenard tube for the production of Lenard rays, which were 

 discovered before Rontgen rays. C is a cathode in high vacuum ; the 

 anode A is a metal cylinder behind it ; the whole is screened in metal, 

 and the cathode rays impinge on a minute hole W covered with ex- 

 ceedingly thin aluminium foil, through which it would seem the rays 

 emerge into the air, radiating in all directions from the aluminium 

 window as Lenard rays L, where they are rapidly diffused and absorbed. 



These Lenard rays make the air phosphoresce and 

 produce the other effects which cathode rays can 

 produce, but they are stopped within a moderate 

 range by the immense obstruction they meet with 

 from a substance of the density of ordinary air. 

 Substances seem to stop them simply in proportion 

 to the quantity of matter which they encounter, 

 without regard to its nature. A thick layer of air 

 would be about as opaque as a layer of water -^^ as 

 thick ; and even if the body put in their way is 



