144 J. W. Gibbs— Electric Theory of Light. 



ent bodies. This indicates that in metals the displacement of 

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

 compared to anything of the kind in transparent bodies, where 

 it indeed exists, but 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 

 publication 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 opti- 

 cal 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 any- 

 thing which existed in the time of Fresnel, Cauchy, and Green. 

 The third wave, for example, is no longer something to be got-* 

 ten 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 Cauchy's work on the subject in 1830, and of 

 Green's in 1837, many attempts have been made by many workers to find a dy- 

 namical foundation for Fresnel's laws of reflexion and refraction of light, but 

 all hitherto ineffectually." Sir William Thomson, loc. citat. 



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

 ing 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 Eayleigh, Phil. Mag., Septem- 

 ber, 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 different 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 oc- 

 cupy any such fundamental position in the theory. It is in fact only another 

 way of saying that two of the constants 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. 



