602 ADDITIONS. 



mediums. I have, at the end of Chapter iii., given the reasons which 

 prevent my assenting to the assertion of a special analysis of light by 

 absorption. In the same manner, with regard to other effects pro- 

 duced by media upon light, it is sufficient for the defence of the theory 

 that it should be consistent with the possibility of the laws of pheno 

 mena which are observed, not that it should explain those laws ; for 

 they belong, apparently, to another province of philosophy. 



Some of the optical properties of bodies which have recently 

 attracted notice appear to be of this kind. It was noticed by Sir John 

 Herschel, 1 that a certain liquid, sulphate of quinine, which is under com- 

 mon circumstances colorless, exhibits in certain aspects and under cer- 

 tain incidences of light, a beautiful celestial blue color. It appeared 

 that this color proceeded from the surface on which the light first fell ; 

 and color thus produced Sir J. Herschel called epipolic colors, and 

 spoke of the light as epipolized. Sir David Brewster had previously 

 noted effects of color in transparent bodies which he ascribed to inter- 

 nal dispersion : 2 and he conceived that the colors observed by Sir J. 

 Herschel were of the same class. Professor Stokes 3 of Cambridge 

 applied himself to the examination of these phenomena, and was led 

 to the conviction that they arise from a power which certain bodies 

 possess, of changing the color, and with it, the refrangibility of the 

 rays of light which fall upon them : and he traced this property in 

 various substances, into various remarkable consequences. As this 

 change of refrangibility always makes the rays less refrangible, it was 

 proposed to call it a degradation of the light; or again, dependent 

 emission, because the light is emitted in the manner of self-luminous 

 bodies, but only in dependence upon the active rays, and so long as 

 the body is under their influence. In this respect it differs from phos- 

 phorescence, in which light is emitted without such dependence. The 

 phenomenon occurs in a conspicuous and beautiful manner in certain 

 kinds of fluor spar : and the term fluorescence, suggested by Professor 

 Stokes, has the advantage of inserting no hypothesis, and will probably 

 be found the most generally acceptable. 4 



It may be remarked that Professor Stokes rejects altogether the 

 doctrine that light of definite refrangibility may still be compound, 

 and may be analysed by absorption. He says, "I have not overlooked 

 the remarkable effect of absorbing media in causing apparent changes 



1 Phil. Trans. 1845. 2 Edinb. Trans. 1833. 



1 Phil. Trans. 1852 and 1854. * See Phil. Trans. 1852. 



