of Light and the Theory of a Quasi-labile ^ther. 253 



the sether is resisted by a strong elastic force, quite enormous 

 compared to anything of the kind in transparent bodies, where 

 it indeed exists, but it is so small that it has been neglected 

 by most writers, except when treating of dispersion. We can 

 make these suppositions, but they do not correspond to any- 

 thing which we know independently of optical experiment. 



It is evident that the electrical theory of light has a serious 

 rival, in a sense in which, perhaps, one did not exist before the 

 pubhcation of Sir William Thomson's paper in November 

 last'^. Nevertheless, neither surprise at the results which 

 have been achieved, nor admiration for that happy audacity 

 of genius, which, seeking the solution of the problem precisely 

 where no one else would have ventured to look for it, has 

 turned half a century of defeat into victory, should blind us 

 to the actual state of the question. 



It may still be said for the electrical theory, that it is not 

 obliged to invent hypotheses f, but only to apply the laws fur- 

 nished by the science of electricity, and that it is difficult to 

 account for the coincidences between the electrical and optical 

 properties of media, unless we regard the motions of light as 

 electrical. But if the electrical character of light is conceded, 

 the optical problem is very different from anything which 

 existed in the time of Fresnel, Cauchy, and Green. The 

 third wave, for example, is no longer something to be gotten 

 rid of quocunque modo, but something which we must dispose 

 of in accordance with the laws of electricity. This would 

 seem to rule out the possibility of a relatively small velocity 

 for the third wave. 



* '' Since the first publication of Oauchy's work on the subject in 1830, 

 and of Green's in 1837, many attempts have been made by many workers 

 to find a dynamical foundation for Fresnel's laws of reflexion and refrac- 

 tion of light, but all hitherto inefi^ectually." Sir William Thomson, 

 loc. cit, 



" So far as I am aware, the electric theory of Maxwell is the only one 

 satisfying these conditions (of explaining at once Fresnel's laws of double 

 refraction in crystals and those governing the intensity of reflexion when 

 light passes from one isotropic medium to another)." Lord Rayleigh, 

 Phil. Mag. September 1888. 



t Electrical motions in air, since the recent experiments of Professor 

 Hertz, seem to be no longer a matter of hypothesis. We can hardly 

 suppose that the case is essentially diflerent with the so-called vacuum. 

 The theorem that the electrical motions of light are solenoidal, although 

 it is convenient to assume it as a hypothesis and show that the results 

 agree with experiment, need not occupy any such fundamental position in 

 the theory. It is in fact only another way of saying that two of the con- 

 stants of electrical science have a certain ratio (infinity). It would be 

 easy to commence without assuming this value, and to show in the course 

 of the development of the subject that experiment requires it, not of 

 course as an abstract proposition, but in the sense in which experiment 

 can be said to require any values of any constants, that is, to a certain 

 degree of approximation. 



