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XXX. On the Change of Refrangihility of Light. By G. G. Stokes, M.A., F.R.S., 
Felloiv of Pembroke College, and Lucasian Professor of Mathematics in the 
University of Cambridge. 
Received May 11, — Read May 27, 1852. 
1. XhE following’ researches originated in a consideration of the very remarkable 
phenomenon discovered by Sir John Herschel in a solution of sulphate of quinine, 
and described by him in two papers printed in the Philosophical Transactions for 1845, 
entitled ‘On a Case of Superficial Colour presented by a Homogeneous Liquid 
internally colourless,’ and ‘ On the Epipolic Dispersion of Light.’ The solution of 
quinine, though it appears to be perfectly transparent and colourless, like water^ 
when viewed by transmitted light, exhibits nevertheless in certain aspects, and under 
certain incidences of the light, a beautiful celestial blue colour. It appears from the 
experiments of Sir John Herschel that the blue colour comes only from a stratum of 
fluid of small but finite thiekness adjacent to the surface by whieh the light enters. 
After passing through this stratum, the incident light, though not sensibly enfeebled 
nor coloured, has lost the power of producing the same effect, and therefore may be 
considered as in some way or other qualitatively different from the original light. The 
dispersion which takes place near the surface of this liquid is called by Sir John 
Herschel epipolic, and he applies the term epipoUzed to abeam of light which, having 
been transmitted through a quiniferous solution, has been thereby rendered incapable 
of further undergoing epipolic dispersion. In one experiment, in which sun-light was 
used, a feeble blue gleam was observed to extend to nearly half an ineh from the 
surface. As regards the dispersed light ^itself, when analysed by a prism it was found 
to consist of rays extending over a great range of refrangihility : the less refrangible 
extremity of the speetrum was however wanting. On being analysed by a tourma- 
line, it showed no signs of polarization. A special experiment showed that the 
dispersed light was perhaps incapable, at any rate not peculiarly susceptible, of being 
again dispersed. 
2. In a paper ‘On the Decomposition and Dispersion of Light within Solid and 
Fluid Bodies,’ read before the Royal Society of Edinburgh in 1846, and printed in 
the 16th volume of their Transactions, as well as in the Philosophical Magazine for 
June 1848, Sir David Brewster notices these results of Sir John Herschel’s, and 
states the conclusions, in some respects different, at which he had arrived by operating- 
in a different way. The phenomenon of internal dispersion had been discovered by 
him some years before, and is briefly noticed in a paper read before the Royal Society 
MDCCCLfl. 3 o 
