Page:Elementary Text-book of Physics (Anthony, 1897).djvu/491

§ 389] periodic electromagnetic disturbance is supposed to be set up in it, such as would result from a rapid reversal of electromotive force at a point, and compared them with the observed properties of the ether, on the assumption that light is an electromagnetic disturbance. He showed that such a disturbance would be propagated through the medium in a way similar to that in which vibrations are transmitted in an elastic solid. He showed further that if light were such a disturbance, its velocity in the ether should be equal to $$v,$$ the ratio of the electrostatic to the electromagnetic system of units. Numerous measurements of the velocity of light and of this ratio show that they are very nearly equal.

He also showed that the indices of refraction of transparent media should be equal to the square roots of their specific inductive capacities. Measurements of indices of refraction and specific inductive capacities have shown that the relation which has been stated holds true in many cases. Hopkinson has shown, however, that there are many bodies for which it does not hold true.

The theory leads to the conclusion that the direction of propagation of the electrical disturbance and the accompanying magnetic disturbance at right angles to it is normal to the plane of these disturbances. By making the assumption, which is justified by Boltzmann's measurements upon sulphur, that an eleotropic medium has different specific inductive capacities in different directions, Maxwell showed also that the propagation of the electrical disturbance in a crystal will be similar to that of light. It has also been shown that the electrical disturbance will be reflected, refracted, and polarized at a surface separating two dielectrics.

Lastly, Maxwell concluded that, if his theory be true, bodies which are transparent to the vibrations of the ether should be dielectrics, while opaque bodies should be good conductors. In the former the electrical disturbance is propagated without loss of energy; in the latter the disturbance sets up electrical currents, which heat the body, and the disturbance is not propagated through the body. Observation shows that, in fact, solid dielectrics are transparent, and solid conductors are opaque, to radiations in the