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 circumstances of the experiment, I have noticed the existence of a compensating cause that disguises the effect of motion. This cause is the reflection suffered by the light in this experiment. In fact, it is possible to show that when there is a difference in the velocity of two rays, this difference is changed when the rays are reflected by a mirror in motion. A separate calculation of the effects in Babinet's experiment shows that they have fairly equal values, but with opposite signs.

This explanation renders even more probable the hypothesis of the change of speed, and it has seemed to me that an experiment made in moving water should help to decide the question with certainty.

I believe that the success of this experiment should bring about the adoption of Fresnel's hypothesis, or at least of the law he discovered that explains the change in the speed of light because of motion of bodies. Since this law is true, it constitutes a very strong proof in favor of the hypothesis, of which this law is only a consequence. It might be that Fresnel's conception will seem so extraordinary, and so hard to admit in certain respects, that additional proofs might still be required. A thorough examination by the geometricians is required, before this can be considered to be valid.