386 Scientific Intelligence, 



effect in making the molecules set will get less and less, but the 

 waves will continue to be totally reflected until the negative part 

 of the specific inductive capacity due to the molecules is jnst 

 equal to the positive part due to the ether. Here the refractive 

 index of the mixture is zero. As the frequency of the force 

 increases, its effect on the molecules gets less and less, so that the 

 specific inductive capacity continually approaches that dae to 

 the ether alone, and practically coincides with it as soon as the 

 frequency of the force is a considerable multiple of that of the 

 molecules. In this case both the specific inductive capacity and 

 the refractive index of the medium are the same as that of the 

 ether, and there is consequently no refraction. Thus the absence 

 of refraction, instead of being in contradiction to the Rontgen 

 rays being a kind of light, is exactly what we should exp-ect if 

 the wave length of the light were exceedingly small. 



The other objection to these rays being a kind of light is, that 

 there is no very conclusive evidence of the existence of polariza- 

 tion. Numerous experiments have been made on the difference 

 between the absorption of these rays by a pair of tourmaline 

 plates when their axes are crossed or parallel. Many observers 

 have failed to observe any difference at all between the absorp- 

 tion in the two cases. Prince Galitzine and M. de Karnojitsky, 

 by a kind of cumulative method, have obtained photographs 

 which seem to show that there is a slightly greater absorption 

 when the axes are crossed than there is when the axes are parallel. 

 There can, however, be no question that the effect, if it exists at 

 all, is exceedingly small compared with the corresponding effect 

 for visible light. Analogy, however, leads us to expect that to 

 get polarization effects we must use, in the case of short waves, 

 polarizers of a much finer structure than would be necessary for 

 long ones. Thus a wire bird-cage will polarize long electrical 

 waves, but will have no effect on visible light. Rubens and Du 

 Bois made an instrument which would polarize the infra-red rays 

 by winding very fine wires very close together on a framework; 

 this arrangement, however, was too coarse to polarize visible 

 light. Thus, though the structure of the tourmaline is fine 

 enough to polarize the visible rays, it may be much too coarse to 

 polarize the Rontgen rays if these have exceedingly small wave- 

 lengths. As far as our knowledge of these rays extends, I think 

 we may say that though there is no direct evidence that they are 

 a kind of light, there are no properties of the rays which are not 

 possessed by some variety of light. 



It is clear that if the Rontgen rays are light rays, their wave- 

 lengths are of an entirely different order to those of visible light. 

 It is perhaps worth notice that on the electro-magnetic theory of 

 light we might expect two different types of vibration, if we 

 suppose that the atoms in the molecule of the vibrating substance 

 carried electrical charges. One set of vibrations would be due 

 to the oscillations of the bodies carrying the charges, the other 

 set to the oscillation of the charges on these bodies. The wave- 



