He then says:
In other media than air the velocity V is inversely proportional to the product of the dielectric and the magnetic inductive capacities. According to the undulatory theory, the velocity of light in different media is inversely pro portional to their indices of refraction.
There are no transparent media for which the magnetic capacity differs from that of air more than by a very small fraction. Hence the principal part of the difference between these media must depend upon their dielectric capacity. According to our theory, therefore, the dielectric capacity of a transparent medium should be equal to the square of its index of refraction.
In Maxwell's theory we accordingly find the dielectric medium of Faraday identified with the luminiferous ether. But the elasticity of the luminiferous ether which is involved in the transmission of all known forms of radiation must be of the nature of rigidity, and a fluid dielectric such as Maxwell's assumption of contracting lines of force seems to require does not possess rigidity.
It would accordingly seem that while Maxwell's method of calculating the velocity of light from purely electrical experiments seems to prove beyond question that the luminiferous ether is the medium of electric and magnetic induction, the assumption as to the contraction of this medium in the direction of the electrical lines of force and its expansion in all directions at right angles to these lines may require modification.
It is plain that this assumption that an electrical charge is merely one aspect of a stress in the ether is equivalent to a denial of an electrical substance per se. It is difficult to see, however, how from the assumption of a mere contraction of the dielectric between two conductors the surfaces of the conductors could be put in qualitatively different electrical conditions such as are known to distinguish positively and negatively electrified bodies. Both aspects of such a stress would appear to be exactly alike, just as the stresses at the opposite ends of a stretched elastic cord.
It accordingly became necessary to make some further assumptions to account for the difference in the positive and negative electrical surfaces. Here recourse was again had to Faraday's notion of a polarizable medium; that is, a medium made up of particles having opposite electrical properties at two opposite extremities. The ether accordingly came to be regarded by many of Maxwell's successors as made up of particles or "cells" holding positive charges on one side and negative charges on the opposite side, very much as the current magnetic theory regards a magnet as made up of molecules having a north magnetic pole on one face and a south magnetic pole on the opposite face. The polarization of the medium in electrical induction was supposed to consist in the orientation of these hypothetical particles so that their charges of the same kind were turned in the same direction.
