Page:Popular Science Monthly Volume 77.djvu/114

108 of sound, but with a different velocity from that of transverse waves. But such longitudinal waves have no place in any optical phenomenon, and therefore constitute a difficulty for the theory. In order not to have them it was necessary for Green to suppose the ether incompressible. Thus the theory did very well for the propagation of light on free space. When light passes from free space to a transparent substance, however, it is partially reflected and partially refracted, travelling with a different velocity in the new medium. This change of velocity could be explained by a difference of either density or rigidity in the two media. Green chose one hypothesis, in fact the same as that of Fresnel, Neumann and McCullagh the other. This difference gave rise to a controversy over the direction of the vibration, as to whether it was in or perpendicular to the plane of polarization, a controversy vainly sought to be settled by experiment. Although reflection and refraction could thus be explained, there remained a very grave difficulty. The conditions to be satisfied at the surface between two different media are too many to be satisfied by a transverse wave alone, so that had there been originally only a transverse wave, it would give rise to a longitudinal wave on striking the surface limiting the media. To avoid this difficulty a mechanical theory was proposed by McCullagh, in which the elasticity was not like that found in any known substance, but was called into play when a portion of the medium was rotated, quite independently of whether neighboring portions were rotated or not. This theory gave a very satisfactory explanation of reflection and refraction, but long met with opposition on account of its postulating elastic properties not found in any substance.

Probably the person who took most seriously the view of the ether as having the properties of some familiar sort of matter was Lord Kelvin, who devoted a large portion of his life to the attempt to find a suitable mechanical representation of the ether. In fact he stated on the occasion of his jubilee that for forty years this question had not been absent from his mind for a single day. Lord Kelvin frequently uses the term "jelly" as typical of Green's elastic substance, and did finally, by a very ingenious assumption, succeed in assimilating the ether to such a substance. But in spite of all these attempts, we may agree with the opinion of Lord Rayleigh, who concludes that for many reasons "the elastic solid theory, valuable as a piece of purely dynamical reasoning, and probably not without mathematical analogy to the truth, can in optics be regarded only as an illustration."

Such was the condition of affairs at the close of what I may call the medieval period in optics, when, in 1864, Maxwell gave affairs an entirely new turn by the presentation of his famous paper on "A Dynamical Theory of the Electromagnetic Field." In this he was guided by the conjecture of Faraday that the same medium which is concerned