The Aether as an Elastic Solid. 177 



The great investigators who developed the theory of light 

 after the death of Fresnel devoted considerable attention to 

 the optical properties of metals. Their researches in this 

 direction must now be reviewed. 



The most striking properties of metals are the power of 

 brilliantly reflecting light at all angles of incidence, which is 

 so well shown by the mirrors of reflecting telescopes, and the 

 opacity, which causes a train of waves to be extinguished before 

 it has proceeded many wave-lengths into a metallic medium. 

 That these two attributes are connected appears probable 

 from the fact that certain non- metallic bodies e.g., aniline 

 dyes which strongly absorb the rays in certain parts of the 

 spectrum, reflect those rays with almost metallic brilliance. 

 A third quality in which metals differ from transparent bodies, 

 and which, as we shall see, is again closely related to the other 

 two, is in regard to the polarization of the light reflected from 

 them. This was first noticed by Malus ; and in 1830 Sir David 

 Brewster* showed that plane-polarized light incident on a 

 metallic surface remains polarized in the same plane after 

 reflexion if its polarization is either parallel or perpendicular 

 to the plane of reflexion, but that in other cases the reflected 

 light is polarized elliptically. 



It was this discovery of Brews ter's which suggested to the 

 mathematicians a theory of metallic reflexion. For, as we have 

 seen, elliptic polarization is obtained when plane-polarized 

 light is totally reflected at the surface of a transparent body ; 

 and this analogy between the effects of total reflexion and 

 metallic reflexion led to the surmise that the latter pheno- 

 menon might be treated in the same way as Fresnel had treated 

 the former, namely, by introducing imaginary quantities into 

 the formulae of ordinary reflexion. On these principles mathe- 

 matical formulae were devised by MacCullaghf and Cauchy^ 



*Phil. Trans., 1830. 



+ Proc. Roy. Irish Acad., i (1836), p. 2 ; ii (1843), p. 376 : Trans. Roy. Irish 

 Acad., xviii (1837), p. 71 : MacCullagh's Coll. Works, pp. 58, 132, 230. 



J Comptes Rendus, vii (1838), p. 953 ; riii (1839), pp. 553, 658, 961 ; xxvi 

 (1848), p. 86. 



N 



