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 or some other similar displacement which could be doubled by reversing the direction of the flow. This displacement certainly could not have escaped observation.

Following are the results of the calculation according to Fresnel's hypothesis:

$\Delta=4L\frac{u}{v}\left(m^{2}-1\right)=0,0000001367$

$\frac{\Delta}{\lambda}=0,0002325.$

This is a displacement of 2/10,000 of the width of a fringe which is unobservable, even if it were 100 times greater. Thus, Fresnel's theory explains the apparent immobility of the fringes in the experiment made with air in motion. The displacement of the fringes is not really zero, but it is so weak that it cannot be noticed.

After obtaining this negative fact, I still continued to search for an explanation for the hypotheses relative to ether that would, at the same time, explain the phenomenon of abberation and Arago's experiment. It seemed to me that it was necessary to admit with Fresnel that the motion of bodies produces a change in the speed of light, and that the magnitude of this change of speed depends upon the energy with which the different media refract light. It is fairly large for highly refractive bodies, and very small for those that refract very little, such as air.

This resulted in the fringes not being displaced when light traveled through the air in motion. An appreciable displacement should have been obtained if the experiment were performed in water, whose refractive index is much greater than that of air.

One experiment performed by Babinet and mentioned in Volume 9 of the Academy's Comptes Rendus, seemed to contradict the hypothesis of the speed change in conformance with Fresnel's law. But upon considering the