advanced by Planck,[1] is that the two conditions of Stokes's theory—namely, that the motion of the aether is to be irrotational and that at the earth's surface its velocity is to be the same as that of the earth—may both be satisfied if the aether is supposed to be compressible in accordance with Boyle's law, and subject to gravity, so that round the earth it is compressed like the atmosphere; the velocity of light being supposed independent of the condensation of the aether.
Lorentz,[2] in calling attention to the defects of Stokes's theory, proposed to combine the ideas of Stokes and Fresnel, by assuming that the aether near the earth is moving irrotationally (as in Stokes's theory), but that at the surface of the earth the aethereal velocity is not necessarily the same as that of ponderable matter, and that (as in Fresnel's theory) a material body imparts the fraction (μ2 - 1)/μ2 of its own motion to the aether within it. Fresnel's theory is a particular case of this new theory, being derived from it by supposing the velocity-potential to be zero.
Aberration is by no means the only astronomical phenomenon which depends on the velocity of propagation of light; we have indeed seen[3] that this velocity was originally determined by observing the retardation of the eclipses of Jupiter's satellites. It was remarked by Maxwell[4] in 1879 that these eclipses. furnish, theoretically at least, a means of determining the velocity of the solar system relative to the aether. For if the distance from the eclipsed satellite to the earth be divided by the observed retardation in time of the eclipse, the quotient. represents the velocity of propagation of light in this direction, relative to the solar system; and this will differ from the velocity of propagation of light relative to the aether by the component, in this direction, of the sun's velocity relative to the aether. By taking observations when Jupiter is in different signs of the
