Sir George Gabriel Stokes. 207 



the fluid. The incident beam after having passed through the stratum 

 from which the blue light came, was not sensibly enfeebled or coloured, 

 but yet it had lost the power of producing the usual blue colour when 

 admitted into a solution of sulphate of quinine. A beam of light 

 modified in this mysterious manner was called by Sir J. Herschel 



Several years before, Sir D. Brewster had discovered in the case of 

 an alcoholic solution of the green colouring matter of leaves a very 

 remarkable phenomenon, which he has designated as internal dispersion. 

 On admitting into this fluid a beam of sunlight condensed by a lens, 

 he was surprised by finding the path of the rays within the fluid 

 marked by a bright light of a blood-red colour, strangely contrasting 

 with the beautiful green of the fluid itself when seen in moderate 

 thickness. Sir David afterwards observed the same phenomenon in 

 various vegetable solutions and essential oils, and in some solids. He 

 conceived it to be due to coloured particles held in suspension. But 

 there was one circumstance attending the phenomenon which seemed 

 very difficult of explanation on such a supposition, namely, that the 

 whole or a great part of the dispersed beam was unpolarised, whereas 

 a beam reflected from suspended particles might be expected to be 

 polarised by reflection. And such was, in fact, the case with those 

 beams which were plainly due to nothing but particles held in 

 suspension. From the general identity of the circumstances attending 

 the two phenomena, Sir D. Brewster was led to conclude that epipolic 

 was merely a particular case of internal dispersion, peculiar only in 

 this respect, that the rays capable of dispersion were dispersed with 

 unusual rapidity. But what rays they were which were capable of 

 affecting a solution of sulphate of quinine, why the active rays were so 

 quickly used up, while the dispersed rays which they produced passed 

 freely through the fluid, why the transmitted light when subjected to 

 prismatic analysis showed no deficiences in those regions to which, with 

 respect to refrangibility, the dispersed rays chiefly belonged, were 

 questions to which the answers appeared to be involved in as much 

 mystery as ever." 



Such a situation was well calculated to arouse the curiosity and 

 enthusiasm of a young investigator. A little consideration showed 

 that it was hardly possible to explain the facts without admitting that 

 in undergoing dispersion the light changed its refrangibility, but that if 

 this rather startling supposition was allowed, there was no further 

 difficulty ; and experiment soon placed the fact of a change of refrangi- 

 bility beyond doubt. "A pure spectrum from sunlight having been 

 formed in air in the usual manner, a glass vessel containing a weak 

 solution of sulphate of quinine was placed in it. The rays belonging 

 to the greater part of the visible spectrum passed freely through the 



