aberration as a phenomenon of relative motion. This explanation is absolutely valid if the light is something which travels in definite rays with finite speed, itself undisturbed by the motion of the Earth: on a corpuscular theory it thus requires that the luminous corpuscles are not sensibly affected by the Earth's attraction: on the undulatory theory it involves either that the luminiferous aether is not disturbed at all by the Earth's motion through it, or else that some special adjustment of its motion holds good which gives the same result. The explanation of the phenomenon of the aberration of light thus immediately opens up the whole question of the disturbance of the aether by the motion through it of material bodies like the Earth, and also of the manner in which the reflexion and refraction of light in our observing instruments is affected by their motion along with the Earth. It is merely one particular result, — more prominent because a positive result — in the field of the mutual influence of aether and moving matter.
7. It occurred to Arago, reasoning on the lines of the corpuscular explanation, that inasmuch as the velocity of light is different in glass from what it is in vacuum, the aberration of its path arising from the Earth's motion would also be different in the glass, and therefore the optical deviation caused by a glass prism would vary according as the light traversed it in the direction of the Earth's motion or in the opposite direction. With the achromatic prism which he employed for testing this conclusion, he calculated that this difference might be as much as a minute of arc. The outcome of the experiments showed no difference at all. Arago worked with star light for which the Doppler effect due to relative motion would make a real difference, excessively minute however and beyond his observational means: with light from a terrestrial source which (as Fresnel remarked) would do equally well for his test, the difference would be absolutely null. The significance of this result, as against the then current explanation of aberration, on corpuscular ideas, was fully realized by Arago: and he communicated the facts to Fresnel with a view to eliciting whether there was anything more satisfactory to be adduced on the basis of the wave theory, which he was then engaged in developing
