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 The ether adheres, or is fixed, to the molecules of the body, and, consequently, shares in the motion that may be imposed on the body;

Or, the ether is free and independent, and is not carried along by the body in its motion;

Or, finally, a third hypothesis which borrows from each of the two above, in which only a portion of the ether would be free, while the other portion would be fixed to the molecules of the body and would solely share in its motion.

This last hypothesis, postulated by Fresnel, was conceived in order to satisfy at the same time the phenomenon of abberation, and a famous experiment by Arago, in which he had shown that the motion of the Earth did not have any effect upon the refraction value of starlight in a prism. These two phenomena could be explained through Fresnel's hypothesis with admirable precision. However, Fresnel's hypothesis is not regarded today as absolute truth, and the relationships between ether and ponderable matter are still generally considered as uncertain and hard to understand. This is because Fresnel's mechanical conception seems too unusual to be accepted without direct proof, or perhaps because it seemed equally possible to satisfy the observed phenomena with either of the two other hypotheses. Perhaps, finally, as other physicists have thought, certain results from this theory seemed contrary to experience.

The following considerations have led me to try an experiment, the results of which, I believe, should clarify this matter.

It is possible in the three hypotheses enumerated above that, if the body is in motion, the velocity at which light will go through it will be different from that observed if the body were at rest. For each of these hypotheses, the motion of the body would have a different effect upon the light velocity.

Thus, if ether is supposed be fixed to the body during the latter's motion, the velocity of light will be augmented by that of the body, if the 2