period that is enormously large in comparison with that of the light-vibrations. Since observation shows that the specific inductive capacity obtained differs when different rates of alternation are employed in the experiment, it may readily be supposed that, if alternations were used which were as rapid as those of light, values of the specific inductive capacity would be obtained which would confirm the theory. Since this cannot be done, the only method of comparison possible is to calculate, as well as it can be done, the index of refraction for light of very long period; this is done by the aid of a formula for the dispersion of light derived by Cauchy, from which the index of refraction of infinitely long waves is calculated. The agreement obtained by this method is still very far from good, but this may easily be explained by supposing that Cauchy's formula, which rests, after all, only on an empirical basis, and has been tested only within narrow limits, does not apply to waves of very long wave length, or that, in other words, the waves of long wave length exhibit anomalous dispersion. The experiments which have been made to test the relation between the specific inductive capacity and the index of refraction of electromagnetic waves show in very many cases an exceedingly good agreement, and in no case a disagreement, with the theory.
While there are still difficulties to be overcome and questions to be answered, it is yet highly probable that the true theory of light is the electromagnetic theory or some extension of it. We may therefore view magnetic, electrical, and luminous actions as actions occurring in the ether, and arising in some way from the interactions between the ether and matter, by which the energy of matter is transformed into energy in the ether, and this energy in the ether transferred through it to other matter.
