XXXII. On the Effect of the Motion of a Body upon the Velocity with which it is traversed by Light.

By M. H. Fizeau [1].

MANY theories have been proposed with a view of accounting for the phænomenon of the aberration of light according to the undulatory theory. In the first instance Fresnel, and more recently Doppler, Stokes, Challis, and several others have published important researches on this subject; though none of the theories hitherto proposed appear to have received the complete approval of physicists. Of the several hypotheses which have been necessitated by the absence of any definite idea of the properties of luminiferous æther, and of its relations to ponderable matter, not one can be considered as established; they merely possess different degrees of probability.

On the whole these hypotheses may be reduced to the following three, having reference to the state in which the æther ought to be considered as existing in the interior of a transparent body. Either, first, the æther adheres or is fixed to the molecules of the body, and consequently shares all the motions of the body; or secondly, the æther is free and independent, and consequently is not carried with the body in its movements; or, thirdly, only a portion of the æther is free, the rest being fixed to the molecules of the body and, alone, sharing its movements.

The last hypothesis was proposed by Fresnel, in order at once to satisfy the conditions of the aberration of light and of a celebrated experiment of Arago's, which proved that the motion of the earth does not affect the value of the refraction suffered by the light of a star on passing through a prism. Although these two phænomena may be explained with admirable precision by means of this hypothesis, still it is far from being considered at present as an established truth, and the relations between æther and matter are still considered, by most, as unknown. The mechanical conception of Fresnel has been regarded by some as too extraordinary to be admitted without direct proofs; others consider that the observed phænomena may also be satisfied by one of the other hypotheses; and others, again, hold that certain consequences of the hypothesis in question are at variance with experiment.