To defective transparency, or rather to the different degrees of it, we must attribute, in part, the colours of permeable media. Thus, a glass or fluid appears yellow to the eye, because it has the property of admitting the permeation of a larger quantity of the yellow rays than of any others;--red, because the red rays pass it with the greatest freedom; and so on for every other colour. In most cases the powers of transmission and of reflection are similar; but it is not so in all; a variety of fluor spar, which, while it transmits green light, reflects blue, and the precious opal, are striking instances to the contrary. Some glasses, which transmit yellow light have the singular power of dispersing blue rays from one surface; and a solution of quinine in water acidulated with sulphuric acid, although perfectly transparent and colourless when held between the eye and the light, exhibits, if viewed in a particular direction, a lively cerulean tint. These effects being supposed to be due to the conditions of the surface, have been called _epipolic_ phenomena.[92] The careful investigation of these phenomena has made us acquainted with some very interesting facts, and indeed discovered to us a set of luminous rays which were previously unknown. The dispersion of blue light from the surface of some yellow glasses--such as have been coloured by the oxide of silver--is of a different order from that which takes place with the solution of sulphate of quinine, or with the fluor spar. The first depends upon a peculiar condition of the surface, while the latter phenomena are due to a dispersion which takes place _within_ the solid or fluid. In addition to the sulphate of quinine, and the fluor spar, we obtain the same results in a very marked manner by a canary yellow glass, coloured with the oxide of uranium, and by a decoction of the inner bark of the horse-chesnut tree. Mr. Stokes, who has investigated this class of phenomena, and proposes to call it _Fluorescence_, from its being naturally seen in fluor-spar, has shown that the peculiar internal dispersion, and the consequent alteration of the colour of the ray, is due to an alteration in its refrangibility. Whether this hypothesis prove to be the correct one or not, it is certain that there exists a set of rays of far higher refrangibility than those seen in the ordinary Newtonian spectrum. This may be shown in the following manner: taking either of the solutions named, or a block of uranium glass, throw upon one face, by means of a prism, a very pure spectrum. On looking _into_ the glass or fluid there will be seen, commencing amidst the most refrangible rays, a new set of spectral rays, struggling to make their way through the absorbent medium. These are of a blue colour in the quinine or chesnut solution, and green in the uranium glass, and are seen extending themselves far beyond the most refrangible rays of the ordinary Newtonian spectrum. This is the space over which those rays which