at it, the light of day falling directly upon the surface observed, it assumes a beautiful silvery blue colour. A green variety of fluor spar exhibits the same phenomenon. It is also strikingly shown by a weak decoction of the inner bark of the horse-chestnut tree, and by some other substances. A peculiar yellow glass, coloured by oxide of uranium, possesses like properties, but the light from it. in this case is a pale sea green.

of

(56.) Sir John Herschel called attention to these phenomena, in two papers published in the Philosophical Transactions.* He showed that the colours came only from a stratum of fluid of small but finite thickness at the surface, by which the light entered. He also proved that a ray light, having once passed through such a stratum, had lost the power of producing the same effect. This peculiar dispersion of light taking place at or near the surface of the body, was called by Sir John Herschel Epipolic; and a beam of light which, after having passed through a solution of quinine, was incapable of again producing the effect, he termed Epipolised light.

(57.) Sir David Brewster, in a paper read before the Royal Society of Edinburgh, in 1846f, drew attention to similar phenomena in a solution of the green colouring matter of leaves. These experiments were made by passing a beam of solar light, concentrated by a lens, into the solution or substance under observation. Upon examining the quinine solution in this way, it was found that light was dispersed, not merely close to the surface, but at a long distance within the fluid. These phenomena have been most thoroughly investigated by Mr. Stokes of Cambridge, to whose results attention must now be given. (58.) Without attempting any description of the numer

*On a Case of Superficial Colour, presented by a Homogeneous Liquid internally colourless, and On the Epipolic Dispersion of Light. Phil. Trans. 1845.

† On the Decomposition and Dispersion of Light within Solid and Fluid Bodies.

ous and beautiful experiments made by Mr. Stokes and described in his Memoirs*, which is not indeed necessary, I shall give a short account merely of the methods by which these phenomena of dispersion may be viewed, and copy Mr. Stokes' general conclusions.

If a ray of light concentrated by a lens is passed into a solution of sulphate of quinine, or a block of uranium glass, a cone of rays of a blue or a green colour, according as the first or the last is employed, will be seen penetrating the medium, these rays presenting an appearance essentially different from the rays of the ordinary spectrum.

If a pencil of light is refracted by a prism,and these refracted rays being first received on a good achromatic lens, the spectrum is thrown upon either of the above media, or any of those formerly mentioned, and we look through the sides in upon the image, we shall see all the least refrangible rays, rendered evident by moats in the fluid passing freely through; and if the spectrum is received on a screen, but little loss of light or colour will be perceived. From a certain point near the least refrangible violet rays—but this varies somewhat with the medium employed-small cones of light, of the peculiar colours of the light from the surface of the quinine solution, or the canary yellow (uranium) glass, will be seen passing into the solution or glass to various depths, and these will be found to extend with varying degrees of intensity beyond the violet and lavender rays. By this method it is shown that rays of high refrangibility, existing over the space usually included in the term of "Ritter's dark rays," are rendered visible. Hence, it has been inferred that, since the most active chemical rays exist within the limits comprehended by these rays, that they are rendered visible. This question will be discussed in a future section, after a full examination of the chemistry of the solar radiations.

On the Change of Refrangibility of Light. By G. Stokes, M. A., F. R. S. Philosophical Transactions, 1852.

Mr. Stokes has proposed the term Fluorescence to distinguish this peculiar optical phenomenon, since it exists in a marked manner in some varieties of fluor spar.

(59.) The results of his investigations, as given by Mr. Stokes, are the following:

"1. In the phenomena of true internal dispersion, the refrangibility of Light is changed, incident Light of definite refrangibility giving rise to dispersed Light of various refrangibilities.

"2. The refrangibility of the incident Light is a superior limit to the refrangibility of the component parts of the dispersed Light.

"3. The colour of Light is in general changed by internal dispersion, the new colour always corresponding to the new refrangibility. It is a matter of perfect indifference whether the incident rays belong to the visible or the invisible part of the spectrum.

"4. The nature and intensity of Light dispersed by a solution appear to be strictly independent of the state of polarisation of the incident rays. Moreover, whether the incident rays be polarised or unpolarised, the dispersed Light offers no traces of polarisation. It seems to emanate equally in all directions, as if the fluid were self-luminous.

"5. The phenomena of a change of refrangibility prove to be extremely common, especially in the case of organic substances, such as those ordinarily met with, in which it is almost always manifested to a greater or less degree.

"6. It affords peculiar facilities for the study of the invisible rays of the spectrum, more refrangible than the violet, and of the absorbing action of media with respect to them.

"7. It furnishes a new chemical test of a remarkably searching character, which seems likely to prove of great value in the separation of organic compounds. The test is specially remarkable for this, that it leads to the independent recognition of one or more sensitive substances in a mixture of various compounds, and shows to a great ex

INTERNAL DISPERSION OF LIGHT.

47

tent, before such substances have been isolated, in what menstrua they are soluble, and with what agents they enter into combination. Unfortunately, these observations require, for the most part, sunlight.

"8. The phenomena of internal dispersion oppose fresh difficulties to the supposition of a difference of nature in luminous, chemical, phosphorogenic rays, but are perfectly conformable to the supposition that the production of Light, of chemical changes, and of phosphoric excitement, are merely different effects of the same cause. The phosphorogenic rays of an electric spark, which, as is already known, are interrupted by glass, appear to be nothing more than invisible rays of excessively high refrangibility, which there is no reason for supposing to be of a different nature from the rays of Light."

(60.) The discovery of three or more new sets of rays, the extreme red, the lavender, and the fluorescent rays, still further involves the question of there being three primary rays, or a greater number. An extended series of experiments are still required to determine if it is possible by any means to decompose a ray of definite refrangibility and of pure colour into rays of another colour, or develope physical conditions of a new order, by new systems of refraction, reflection, absorption, or transmission.

(61.) It is essential that the condition of the prismatic image should be distinctly understood; I therefore return to the consideration. The sunbeam, passing through a hole into a darkened room, produces the sun's image on the floor, howsoever small or large that hole may be. If we interpose a prism we produce a spectrum by the refraction of the rays. If we isolate any ray, say the red ray, we find we have a red circular image of the sun; if the blue, we shall obtain a blue circular image, and so on ; therefore the spectrum must be regarded as an assemblage of images of different refrangibilities superposed on, and overlapping each other. The more we diminish the

angular diameter of the sun, the less will these prismatic images overlap, and consequently the purer will be the rays of the spectrum we shall obtain; but it will be evident that it is scarcely possible to obtain a spectrum in which we have not some error arising from this cause. In our future examination of the heating powers, and more particularly of the chemical action of the solar beam, the importance of attending to the existence of these conditions will be apparent, and the best arrangement for obtaining a pure spectrum described.

(62.) Howsoever pure the spectrum may be, it is not an uninterrupted line of light, but it is crossed by spaces giving no light-absolutely dark lines. These were first noticed by Dr. Wollaston*; but they were fully examined by Frannhofer, who, by the aid of his excellent instruments, discovered that the surface of the spectrum was crossed by these dark bands, or, as he terms them, "fixed lines," of which he observed upwards of 500, although in his drawing of the spectrum he inserted only 354. Sir David Brewster has been enabled to extend the number so largely as to comprehend 2000 lines in a spectrum which he delineated.

Seven of these lines, B, C, D, E, F, G, H, from their distinctness, are particularly distinguished. These are represented in the figure in the Frontispiece. B lies near the outer end of the red space; c beyond the middle of the red; D in the orange, and is a strong double line; E is in the green; F in the blue; G in the indigo; and н in the violet. Besides these, there are other lines, which from their decision require notice. At a is a well defined dark line, near the least refrangible edge of the red ray; half way between A and B is a group of seven lines, forming a black band; between в and c are nine lines, and b is a triple line in the green; between F and G are 185 lines, and between G and H are 190 of these dark spaces.

* Philosophical Transactions, 1802.