transparency of the aether is replaced by approximate transparency, such as would involve ultimate decay of all structures existing in it.
11. The problem of the relative motion of the Earth and the aether was treated by Clerk Maxwell in 1867, in a letter to Sir W. Huggins which has been incorporated in the fundamental memoir of the latter on the spectroscopic determination of the velocity of movement of stars in the line of sight[1]. It is there pointed out that there are two independent subjects for examination. The Doppler alteration of the period of the light from a star is quite definite, and independent of the special details of the form of undulatory theory that may be adopted. But there is a second question as to whether the index of refraction depends on the orientation of the ray with reference to the direction of the Earth's motion, in which the observer and all his apparatus participate: this involves the physical nature of the undulations: here, as Fresnel had already remarked, the sources of light may just as well be terrestrial as astronomical. According to Arago's original experimental result, which had been closely tested by a more delicate arrangement by Maxwell himself working with homogeneous light, some years before this time, there is no influence on the index of refraction arising from the Earth's motion. As refraction depends solely on retardation in time owing to the smaller velocity of propagation in the refracting medium, the relative retardation must therefore be unaltered by the Earth's motion. If V be the velocity of a ray in air, and ν the velocity of the aether in air relative to the observer, and if V' be the velocity of the same ray in a dense medium and ν' the velocity of the aether in that medium relative to the observer[2], then across a thickness α of this medium the light is retarded with respect to air by a time
αV' + ν' − αV + ν'
