Manchester Memoirs, Vol. xli. (1897), No. 15. 13 



as regards deflection like an electric current. Again, if 

 you have highly-charged molecules in the neighbourhood 

 of a positively or negatively statically-charged body, they 

 will be attracted or repelled, and the deflections of the 

 rays are precisely what was to be expected according 

 to that theory. I think we may assume that the cathodic 

 rays are really streams of electrified molecules which 

 strike against the opposite wall of the tube, or, as I 

 will now call it, the target. Now, when a molecule, 

 coming in this way from the cathode, strikes the target, 

 how does the molecule act ? It may act in two ways. 

 It may act as a mass of matter, infinitesimal though 

 it be, by virtue of its momentum — by virtue of its 

 mass and velocity — and it may act also as a charged 

 body, a statically-charged body. What the appropriate 

 physical idea is of a statically-charged body is more 

 than I can tell you. I was talking not long ago to 

 Lord Kelvin about it — and he is a far higher authority 

 in electrical matters than I am — and he considers that 

 the physical idea of a statically-charged body is still 

 a mystery to us. Well, if these charged molecules 

 strike the target we may think it exceedingly probable 

 that by virtue of their charge they produce some sort 

 of disturbance in the ether. This disturbance in the 

 ether would spread in all directions from the place of 

 disturbance, so that each projected molecule would on 

 that supposition become, on reaching the target, a source 

 of ethereal disturbance spreading in all directions. Well, 

 what is the character of such a disturbance ? The 

 problem of diffraction, dynamically considered, may be 

 supposed to reduce itself to this. Suppose you have 

 an infinite mass of an elastic medium, and suppose a 

 small portion is disturbed in the most general way 

 possible, what will take place ? A wave of disturbance 



