with the support of Arago's powerful advocacy. The phenomenon to be accounted for was that the motion of the Earth does not affect the laws of reflexion and refraction of light. In Fresnel's reply[1], which is one of the fundamental documents on the present subject, he pointed out that a simple answer was possible, namely to assume that the surrounding aether is carried along completely by the Earth so that all relative phenomena would be the same as if the Earth were at rest: but he went on to say that this view could not be entertained on account of the facts of astronomical aberration, of which he could form no intelligible conception except on the hypothesis that the aether remained absolutely stagnant as the Earth moved through it. On this latter hypothesis the velocity of light outside a transparent body must have the normal value: and it was an easy problem to find whether it was possible for any law of modification of the velocity of light inside the body, arising from its motion, to make the laws of refraction and reflexion relative to the moving body the same as for matter at rest, as Arago's experiment required. It appeared that there is such a law, the conditions being all satisfied if the absolute velocity of light inside a transparent medium of index μ is increased by the fraction 1 - μ-2 of the velocity of the medium resolved in its direction. This supposition, adopted on the above grounds by Fresnel, keeps the paths of the rays relative to the moving bodies unaltered, and at the same time satisfies the facts of aberration. The attempt made by Fresnel to provide for it a dynamical foundation suffers from the same kind of obscurity as did his later dynamical theory of crystalline refraction: and though the subsequent views of Boussinesq and Sellmeier, on the part played by the matter in the mechanism of refraction and dispersion, allow a valid meaning to be read into Fresnel's explanations, yet they perhaps form no very essential part of his achievement in this field. Afterward Sir George Stokes showed in detail that Fresnel's hypothesis not only left the relative paths of rays unaltered, but the phenomena of interference as well, some of which had been urged against it by Babinet.
- ↑ See Appendix D.
