Closing Years of the Nineteenth Century. 413- 



advanced by Planck,* 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,f 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 ponder- 

 able matter, and that (as in Fresiiel's theory) a material body 

 imparts the fraction (ju 2 - l}/ju 2 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 seent that this velocity was originally determined by 

 observing the retardation of the eclipses of Jupiter's satellites. 

 It was remarked by Maxwell 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 



* Of. Lorentz, Proc. Amsterdam Acad. (English ed.), i (1899), p. 443. 

 t Archives Neerl. xxi (1886), p. 103 : cf. also Zittinsgsversl. Kon. Ak. Amster- 

 dam, 1897-98, p. 266. 



J Cf. p. 22. Proc. R. S. xxx (1880), p. 108. 



