Manchester Memoirs, Vol. xli. (1897), No. 15. 11 



vacuum ? I do not think we are. I believe it passes 

 the power of art to produce a perfect vacuum. You 

 always have a little residue of which you cannot abso- 

 lutely get rid, and some of Lenard's own figures show 

 the effect of the residual gas. He isolated by screens a 

 small part of the cathodic discharge in the second tube, 

 and received it on a phosphorescent screen. He repre- 

 sents the phosphorescent light in the tube as consisting 

 of a bright nucleus surrounded by a less bright halo. 

 The bright nucleus was such as would be produced if 

 the cathodic rays were rays of light, provided that that 

 light were incapable of diffraction. But, then, how do 

 you account for the halo ? The blue light by which 

 the cathodic rays are seen under ordinary circumstances 

 is due, I believe, to an interference of the projected 

 molecules with the molecules of the gas. In some of 

 Lenard's experiments he received the cathodic rays in 

 the first tube into the air, and a considerable amount 

 of this blue light was seen. The appearance was much 

 as if you had admitted a beam of light into a mixture 

 of milk and water. To my mind this fainter halo in 

 the most refined of Lenard's experiments, lying outside 

 this well-defined nucleus, was evidence that the vacuum, 

 in spite of all the skill and time expended upon it, was 

 not perfect. And for aught we know to the contrary — 

 I believe, indeed, it is the case — the cathodic rays in 

 the second highly-exhausted tube were really streams of 

 molecules coming from the residual gas in the tube. 

 But now comes a difficulty with regard to the passage 

 of the cathodic rays through an aluminium plate. If 

 the cathodic rays were something going on in the ether 

 we might very well understand that an aluminium plate 

 might be transparent to them although opaque to ordi- 

 nary rays of light. But if the cathodic rays are really 

 streams of molecules, how can we imagine that they get 



