ON  THE CHANGE OF REFRANGIBILITY  OF LIGHT.          269
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, consisting 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 different polarization.
3. On repeating some of Sir John Herschel's experiments, I was immediately satisfied of the reality of the phenomenon, notwithstanding its mysterious nature, that is to say, that an epi-polized 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 epipolic dispersion, if that term be 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. Never-
* 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.