'Subtle is the Lord . . .'
Maxwell's article Ether, written for the ninth edition of the Encyclopedia Britannica [M1], begins with an enumeration of the 'high metaphysical . . . [and] mundane uses to be fulfilled by aethers' and with the barely veiled criticism that, even for scientific purposes only, 'all space had been filled three or four times over with aethers.' This contribution by Maxwell is an important document for numerous reasons. To mention but three, it shows us that, like his contemporaries, Maxwell was deeply convinced of the reality of some sort of aether: 'There can be no doubt that the interplanetary and interstellar spaces are not empty but are occupied by a material substance or body, which is certainly the largest, and probably the most uniform, body of which we have any knowledge'; it tells us of an unsuccessful attempt by Maxwell himself to perform a terrestrial optical experiment aimed at detecting the influence of an aether drag on the earth's motion; and it informs us of his opinion that effects of the second order in v/c (v = velocity of the earth relative to the aether, c = velocity of light) are too small to be detectable. This last comment was prompted by his observation that 'all methods . . . by which it is practicable to determine the velocity of light from terrestrial experiments depend on the measurement of the time required for the double journey from one station to the other and back again,' leading to an effect at most of O((v/c)2) = O(10-8).
However, Maxwell still hoped that first-order effects might be astronomically observable. The example he gave was the determination of the velocity of light from the eclipses of Jupiter's satellites when Jupiter is seen from the earth at nearly opposite points of the ecliptic. If one defines the aether* in the sense of Maxwell, or, which is the same thing, in the sense of Augustin Jean Fresnel--a medium in a state of absolute rest relative to the fixed stars, in which light is propagated and through which the earth moves as if it were transparent to it-- then one readily sees that the Jupiter effect, if it exists at all, is of first order in the velocity of the solar system relative to this aether.____________________