290 FLUORESCENCE no longer marked the course of the beam, nor did they appear in a second or third medium of the same kind into which the beam was successively passed; whence it was evident that at a certain depth the beam had lost the power of exciting them. Herschel therefore proposed for the phenomenon the name of epipolic (surface) dispersion. The character of the change was not understood until in 1852 Prof. Stokes submitted the subject to a more careful investigation. He reasoned that the facts observed by Brewster and Herschel were the same, the rays which produced the red dispersed light possessing the power of penetrating to a greater depth before being exhausted than did those producing the blue. The latter he found to be exhausted within a film about -fa of an inch thick, but the blue light to which they gave rise traversed the liquid with perfect freedom ; hence there must be a difference of nature between the produ- cing and the produced rays. Such differences could, probably, only be explained by polariza- tion or change of refrangibility ; but the sup- position of polarization was found untenable, and the case was not one of phosphorescence. In order to test the remaining hypothesis, Stokes obtained a pure luminous spectrum by means of an achromatic lens and two or more flint-glass prisms, and in place of receiving the colors on a screen held the quinine solution in these successively. In the less refrangible colors no effect was observed ; but about the middle of the violet space the blue diffused light made its appearance at the entering sur- face, as if the liquid medium had there become self-luminous. This result appeared in all parts of the upper violet, and until the tube had been carried to some distance into the or- dinarily dark space beyond, occupied by the chemical rays. The depth of the stratum thus luminous at first exceeded the thickness of the vessel used, but it rapidly diminished in the upper parts of the space to a minute fraction of an inch. The blue light, turned aside and again dispersed by a prism held obliquely in its course, yielded in some degree rays having various refrangibilities, with color corresponding, the higher colors being most abundant. By other experiments, also, the blue dispersed light was separated from the inducing violet rays ; and it was found that the former always corresponded to a band of cqlors below the place of the latter. The light thus acted on, then, had its refrangibility always lowered. Thus the remarkable con- clusion was arrived at, that by passing light through particular media certain rays belong- ing to the violet space have their refrangi- bility, and of course their color, let down in the scale, while portions of the invisible chemi- cal rays in like manner become let down so as to Hall within the range of visibility, and to appear as colored light. In the undulatory theory, these results are explicable only by an increase of the wave length and time of vi- bration, with a consequent diminution of the velocity of the rays thus affected. The case is one of degradation of light : in the chlorophyl solution there is a fall from higher colors to red ; in the quinine solution, from invisible or violet to a mixture whose predominant hue is blue ; in canary glass, colored yellow by oxide or salts of uranium, from invisible or violet to green. The striking feature in these results is the conversion of the unseen ray power, which ordinarily induces chemism only, as in the de- composition of carbonic acid and fixation of carbon within the green leaves of plants, and in the blackening of the photographic plate, into common light, thus proving the intimate relation, if not the identity, of the two. Stokes gave to the phenomenon the name of fluor- escence, as having been seen in fluor spar ; and this name, conveying no theory of the case, is preferred. It is conveniently observed by pen- cilling over, by candle light, a sheet of white paper with the quinine solution, or by tracing with it letters on the paper : nothing unusual is observed on the paper, which is as white as before, until it is brought into some light well supplied with chemical rays, and not too brightly luminous for witnessing the effect (as into a beam in an otherwise dark room), when fluorescence appears ; and when in such a room the beam is decomposed, the luminous spectrum hidden from the view, and the paper brought into the ultra-violet space (which is of itself, of course, dark), its sudden lighting up with a pale blue radiance is an effect ap- parently little short of the supernatural. Other fluorescent media are infusion of horse-chest- nut bark, or its active principle, assculine, the infusion of seeds of datura stramonium, tinc- ture of turmeric, &c. Gas and candle light excite little or no visible fluorescence ; hence these are poor in actinic rays. The flames of hydrogen and of sulphur burning in alcohol give very distinct results ; hence these abound in those rays. But so rich in this respect is the light of the voltaic arc from metallic points, that it produces fluorescence through a space six or eight times the length of the lu- minous spectrum. It is worthy of remark, however, that the fluorescent space can be de- tected to any considerable distance above the violet only when the prisms employed are of quartz. Glass at once cuts down the effect within narrow limits, proving that it is highly opaque to the chemical rays, for which quartz serves as the true glass. In 1858 Mr. Robin- son of Armagh found the light of the aurora borealis to produce, for its intensity, very marked fluorescence; another fact favoring the electric origin of that phenomenon. M. Niepce the younger claimed in 1859 that he had preserved during six months the photo- genic power of light, in card paper impreg- nated with tartaric acid or nitrate of uranium, exposed for half an hour to sunlight, and then at once sealed up in a tin tube. It is certain that at the end of this time this card, removed
