Photographic Action of Stationary Light-Waves. 103 



question open as to whether the principal part of the mag- 

 netic moment is due to orbital motions in the atoms or to the 

 motions of the constituent atoms in the molecules ; though it 

 suggests strongly the latter alternative. In that case there 

 will usually be a differential magnetic action of the field as 

 between these moving atoms ; but the magnetic actions on 

 positive and negative ions will be by no means equal and 

 opposite, ns is true of the electric actions. Thus, for example, 

 in the limiting case of two equal and opposite ions revolving 

 round each other, the elements of the equivalent ionic con- 

 vection-currents will be at each instant parallel, and there will 

 be no differential magnetic forcive at all ; there will also be no 

 magnetic moment ; but the electric differential action will 

 retain its full force. 



It is well understood, and in accordance with this explana- 

 tion, that the energy of chemical combination of atoms into 

 molecules is almost entirely that of electrostatic attraction of 

 their atomic charges. In fact the electric attraction between 

 them diminishes according to the law of inverse square with 

 increasing distance, their magnetic attraction according to the 

 law of the inverse fourth power : if these forces are of the 

 same order of magnitude in the actual configuration of the 



o © 



atoms in the molecule, the work done by the former during 

 their combination must be almost indefinitely greater than 

 the work done by the latter. 



If we contemplate the purely dynamical basis which must 

 underlie the descriptive explanations of the electric theory of 

 light, it is difficult to see how there can be any place for a 

 theory of the aether loaded by the material molecules, which 

 dynamical views usually associated with Fresnel's theory 

 demand. There could be no polarity in the inertia of a 

 mere load, such as the present consiJerations require. On 

 the other hand, the presence of electrically polarized mole- 

 cules is effectively a diminution of the elasticity of the luini- 

 niferous medium ; and I have tried to show elsewhere * that 

 the principles of MacCullagh's theory of optics are in sub- 

 stantial agreement with all the general features of our 

 electrical and optical knowledge. 



It is definitely implied in the electromotive, as distinguished 

 from an electrodynamic, character of the electric theory of 

 light, that the atomic charges vibrate in unison with the 

 light-waves, quite unimpeded by any material inertia of their 

 atoms. This hypothesis is conceivable and natural, indepen- 

 dently of any particular explanation, on the theory that the 



* Lot: cit. Phil. Trans. 1894. 



