The Aether as an Elastic Solid. 181 



elastic solid by reason of the occurrence of the term in de z /dt. 

 But this is evidently a " viscous " term, representing something 

 like a frictional dissipation of the energy of luminous vibra- 

 tions : a dissipation which, in fact, occasions the opacity of the 

 metal. Thus the term which expresses opacity in the equation 

 of motion of the luminiferous medium appears as the origin of the 

 peculiarities of metallic reflexion.* It is curious to notice how 

 closely this accords with the idea of Huygens, that metals are 

 characterized by the presence of soft particles which damp the 

 vibrations of light. 



There is, however, one great difficulty attending this 

 explanation of metallic reflexion, which was first pointed out by 

 Lord Rayleigh.f We have seen that for ideal silver ^ is real 

 and negative : and therefore A must be zero and p negative ; 

 that is to say, the inertia of the luminiferous medium in the 

 metal must be negative. This seems to destroy entirely the 

 physical intelligibility of the theory as applied to the case of 

 ideal silver. 



The difficulty is a deep-seated one, and was not overcome 

 for many years. The direction in which the true solution lies 

 will suggest itself when we consider the resemblance which 

 has already been noticed between metals and those substances 

 which show "surface colour" e.g. the aniline dyes. In the 

 case of the latter substances, the light which is so copiously 

 reflected from them lies within a restricted part of the spectrum ; 

 and it therefore seems probable that the phenomenon is not 

 to be attributed to the existence of dissipative terms, but that 

 it belongs rather to the same class of effects as dispersion, 

 and is to be referred to the same causes. In fact, dispersion 

 means that the value of the refractive index of a substance 

 with respect to any kind of light depends on the period of 

 the light ; and we have only to suppose that the physical 

 causes which operate in dispersion cause the refractive index 



* It is easily seen that the amplitude is reduced by the factor e-* when light 

 travels one wave-length in the metal : K is generally called the coefficient of 

 absorption. 1" Loc. cit. 



