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only work of any consequence, on the influence upon the velocity of light of the motion of the medium through which it passes, is the experiment of Fizeau. He announced the remarkable result that the increment of velocity which the light experienced was not equal to the velocity of the medium, but was a fraction x of this velocity which depended on the index of refraction of the medium. This result was previously obtained theoretically by Fresnel, but most satisfactorily demonstrated by Eisenlohr, as follows:



Consider the prism AC in motion relatively to the ether in direction AB with velocity $$\theta$$. Suppose the density of the external ether to be 1 and of the ether within the prism, $$1+\Delta$$. In the time dt the prism will advance a distance $$\theta dt=BB$$. At