After noticing various cases of solid and liquid bodies, which disperse light of a colour quite different from the transmitted tint, and Sir John Hersche's explanation of the phenomenon in the solution of acid sulphate of quinine, which, although itself colourless, disperses light of a bright blue, the author considers—

1. The internal dispersion of fluor-spar. By employing a more intense light than Sir J. Herschel did—namely, a condensed beam of the sun's light—he conceives that he has proved that the dispersion is not, as Sir J. Herschel supposes, superficial or epipolic, but, on the contrary, belongs, in some specimens at least, to every part of the crystal. The phenomenon is only seen in the green fluor of Alston, and in some pink and bluish-yellów varieties from Derbyshire. Some specimens are formed of strata alternately dispersing and non-dispersing. The author has not found the same appearances in any other mineral; but he has observed similar phenomena of dispersion in several kinds of glass, and even in some colourless glasses, which disperse a fine green tint.