Page:1851 Fizeau en.djvu/6

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on considering the circumstances of that experiment, I have remarked a cause of compensation which must render the effect of the motion imperceptible. This cause consists in the reflexion which the light undergoes in that experiment; in fact it may be demonstrated, that when two rays have a certain difference of course, that difference is changed by the effect of the reflexion upon a mirror in motion. On calculating separately the two effects in the experiment of M. Babinet, it is found that they have values sensibly equal with contrary signs.

This explanation renders still more probable the hypothesis of an alteration of velocity, and an experiment made with water in motion appears to me completely appropriate to decide the question with certainty.

The success of the experiment seems to me to render the adoption of Fresnel's hypothesis necessary, or at least the law which he found for the expression of the alteration of the velocity of light by the effect of motion of a body; for although that law being found true may be a very strong proof in favour of the hypothesis of which it is only a consequence, perhaps the conception of Fresnel may appear so extraordinary, and in some respects so difficult, to admit, that other proofs and a profound examination on the part of geometricians will still be necessary before adopting it as an expression of the real facts of the case. — Comptes Rendus, Sept. 29, 1851.