464 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
of Edinburgh in 1833*. It is described at length, as exhibited in the particular case 
of fluor-spar, in a paper communicated to the British Association at Newcastle in 
1838'!'. In Sir David Brewster’s experiments the sun’s light was condensed by a 
lens, and so admitted into the solid or fluid to be examined ; which afforded peculiar 
facilities for the study of the phenomena. On examining in this way a solution of 
sulphate of quinine, it was found that light was dispersed, not merely close to the 
surface, but at a long distance within the fluid ; and Sir David Brewster was led to 
conclude that the dispersion produced by sulphate of quinine was only a particular 
case of the general phenomenon of internal dispersion. On analysing the blue beam 
by a rhomb of calcareous spar, it was found that a considerable portion of it, con- 
sisting chiefly of the less refrangible rays, was polarized in the plane of reflexion, 
while the more refrangible of its rays, constituting an intensely blue beam, had a dif- 
ferent polarization. 
3. On repeating some of Sir John IIerschel’s experiments, I was immediately 
satisfied of the reality of the phenomenon, notwithstanding its mysterious nature, that 
is to say, that an epipolized beam of light is in some way or other qualitatively 
different from the light originally incident on the fluid. On making the observation 
in the manner of Sir David Brewster, it seemed no less evident that the phenomenon 
belonged to the class of internal dispersion Nevertheless, the singular phenomenon 
discovered by Sir John Herschel manifested itself even in this mode of observation. 
If indeed the vessel containing the solution were so placed that the image of the sun 
in the focus of the lens lay a little way inside the fluid, the phenomenon was masked, 
because the increase of intensity due to an increase of concentration in approaching 
the focus made up for the decrease of intensity due to passing out of the blue band. 
But when the vessel was moved so that the focus of the lens fell either further inside 
the fluid or else outside the vessel, the narrow blue band adjacent to the surface was 
seen as well as the blue beam which shot far into the fluid. Light which has been 
“ epipolized ” by transmission through a moderate thickness of the solution is indeed 
capable of undergoing further dispersion, but not epipoUc dispersion, if that term he 
restricted to the dispersion by which the narrow blue band is produced. It was no 
doubt of great importance to assign to the phenomenon its true place as a member 
of the class of phenomena of internal dispersion. Nevertheless the mystery was by 
no means cleared up ; rather, we were prepared to expect something of the same sort 
* Edinburgh Transactions, vol. xli. p. 542. 
t Eighth Report. — Transactions of the Sections, p. 10. 
I By this, I merely mean that, to take a particular example, the exhibition of a blue light by a solution of 
sulphate of quinine appeared to be a phenomenon of the same nature as the exhibition of a red light by a solution 
of the green colouring matter of leaves, although the latter does not manifest the same singular concentration 
as the former in the neighbourhood of the surface by which the light enters ; and the latter had already been 
observed by Sir David Brewster, and the phenomenon designated as internal dispersion. I make this remark 
because Sir David Brewster has applied this same term to another class of phenomena which are totally 
different. 
