3 i8 SCIENCE PROGRESS 



electromagnetic theory of light has received at the hands of 

 J. J. Thomson. On the electric theory of light as usually given, 

 it is tacitly assumed that the electric force is everywhere uniform 

 over the wave front, that there are no inactive spaces, and that 

 the front has no structure. Prof. Thomson suggests that the 

 energy in both Rontgen and light rays is localised in limited 

 regions, that the fronts of the disturbance are porous and possess 

 a structure which may be appreciated by the analogy of a 

 number of bright specks on a dark ground. 



The existence of a structure in the wave front implies a 

 structure in the ether, which it is assumed has disseminated 

 through it discrete lines of electric force in a state of tension ; 

 light travels along these lines as transverse vibrations, Rontgen 

 rays as pulses. Thus energy travelling outwards with the 

 wave is not distributed uniformly over the wave front, but is 

 concentrated in those parts which are, for the time being, 

 disturbed by pulses travelling in straight lines, with the speed 

 of light, along some or other of the lines of force. " The energy 

 is, as it were, done up into bundles or units, and the energy in 

 any particular bundle does not change as it travels along the line 

 of force." The rays diminish in intensity with increasing distance 

 owing to the wider separation of the bundles and not to the 

 enfeeblement of individual units. The distribution of energy is 

 thus very much like that contemplated on the old emission 

 theory of light, according to which the energy was located on 

 moving particles sparsely disseminated throughout space. 



Such a discontinuous wave front provides a ready explana- 

 tion of the small proportion of molecules ionised both by 

 ultra-violet light and Rontgen rays. If a unit by impinging on 

 a molecule makes it liberate a corpuscle, it will hand over to the 

 corpuscle its whole store of energy and the freed electron will 

 start with the same velocity, no matter where the encounter 

 occurs ; thus the velocity of the electrons would be independent 

 of the intensity of the light. And further, if the energy in the 

 bundle increases as the wave length or pulse thickness diminishes, 

 the secondary electron liberated by a short wave or thin pulse 

 would be expected to have a high velocity. Since the thickness 

 of a pulse (presumably less than io -8 cm.) is very small compared 

 with the wave length of even ultra-violet light (say about io -5 cm.) 

 we should for the same reason expect much greater energy in 

 the units in the 7 or Rontgen rays than in the case of ultra- 



