388 Royal Society. 



The following paper was also read : — 



" On the Change of Refrangibility of Light." By George G. 

 Stokes, Esq., M.A., F.R.S., Lucasian Professor of Mathematics, 

 Cambridge. 



The author was led into the researches detailed in this paper by 

 considering a very singular phenomenon which Sir John Herschel 

 had discovered in the case of a weak solution of sulphate of quinine, 

 and various other salts of the same alkaloid- This fluid appears 

 colourless and transparent, like water, when viewed by transmitted 

 light, but exhibits in certain aspects a peculiar blue colour. Sir 

 John Herschel found that when the fluid was illuminated by a beam 

 of ordinary daylight, the blue light was produced only throughout a 

 very thin stratum of fluid adjacent to the surface by which the light 

 entered. It was unpolarized. It passed freely through many inches 

 of the fluid. The incident beam, after having passed through the 

 stratum from which the blue light came, was not sensibly enfeebled 

 nor 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 John Herschel epipolized. 



Several years before Sir David 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 con- 

 densed 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 diflicult of explanation on such 

 a supposition, namely, that the whole or a great part of the dispersed 

 beam was unpolarized, whereas a beam reflected from suspended 

 particles might be expected to be polarized by reflexion. 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 

 David Brewster was led to conclude that e])ipolic was merely a par- 

 ticular 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 aftecting 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 deficiencies 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. 



y\ftcr having repeated some of the experiments of Sir David 



