246 COMPARISON OF THE ELECTRIC THEORY OF LIGHT 



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,! but only to apply the laws furnished 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 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 dynamical 

 foundation by 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 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. 



f 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 occupy 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. 



