Manchester Memoirs, Vol. xli. (1897), No. 15. 9 



the tube. The appearance, then, may be analogous to 

 that of a sunbeam coming from a hole in the clouds. If 

 it were not for the slight amount of dust and suspended 

 matter in the air, the sunbeam would be invisible if you 

 looked across it. But as the air is never free from 

 motes, you see the path of the sunbeam when you look 

 across it by the light reflected from these motes. Some- 

 thing of the same kind may be conceived to take place 

 with regard to the cathodic rays if they are some pro- 

 cess going on in the ether. But there are very great 

 difficulties in the way of this second hypothesis, and 

 especially as regards certain properties of the cathodic 

 rays. In the first place, they act mechanically. In 

 Crookes' experiments he succeeded in causing a light 

 windmill, if I may so describe it, to spin rapidly under 

 the action of the rays. And when they were received 

 on a very thin film of blown glass, the glass was actually 

 bent under them as they fell upon it. But that is not 

 all. These cathodic rays appear to proceed in a normal 

 direction from the cathode, and ordinarily proceed in 

 straight lines. But — and this is the important point — 

 they are capable of being deflected in their path both by 

 electro-static force and by magnetic or electro-dynamic 

 force. Nothing whatever of the kind occurs with rays 

 of light, and there are enormous, almost insuperable, 

 difficulties in the supposition of any such deflection 

 occurring if the cathodic rays are a process going on 

 in the ether. I will not go into all the arguments for 

 and against the two views, especially as the cathodic 

 rays only enter incidentally into the subject I have 

 chosen to bring before you. I will confine myself to 

 one or two of the chief difficulties in the way of the 

 supposition that the cathodic rays are streams of mole- 

 cules. In his admirable experiments Lenard produced 

 the cathodic rays in a tube which was highly exhausted, 



