602 LIGHT Talbot ; and he discovered its real nature. But the first published notice of such phenomena is due to Le Roux. Christiansen and others have since greatly extended our knowledge of the subject, and Helmholtz and Ketteler have given theoretical explanations of it. Fox Talbot s experi ment, though the earliest on record, is one of the easiest to perform, and we therefore quote his own account. The experiment was made about 1840, and the following account is from the Proc. Roy. Soc. Edin., 1870-71. Talbot s " I prepared some square pieces of window glass, about an inch experi- square. Taking one of these, I placed upon it a drop of a strong ment. solution of some salt of chromium, which, if I remember rightly, was the double oxalate of chromium and potash, but it may have been that substance more or less modified. By placing a second square of glass on the first, the drop was spread out in a thin film, but it was prevented from becoming too thin by four pellets of wax placed at the corners of the square, which likewise served to hold the two pieces of glass together. The glasses were then laid aside for some hours until crystals formed in the liquid. These were neces sarily thin, since their thickness was limited by the interval between the glasses. Of course the central part of each crystal, except the smallest ones, was bounded by parallel planes, but the extremities were bevilled at various angles, forming so many little prisms, the smallest or them floating in the liquid. When a distant candle was viewed through these glasses, having the little prisms interposed, a great number of spectra became visible, caused by the inclined edges. Most of these were no doubt very imperfect, but by trying the glass at various points, some veiy distinct spectra were met with, and these could with some trouble be isolated by covering the glass with a card pierced with a pin-hole. It was then seen that each prism (or oblique edge of crystal) produced two spectra oppo sitely polarized and widely separated. One of these spectra was normal ; there was nothing particular about it. The colours of the other were very anomalous, and, after many experiments, I came to the conclusion that they could only be explained by the supposition that the spectrum, after proceeding for a certain distance, stopped short and returned upon itself." Iodine Le Roux in 1860 l discovered that vapour of iodine, vapour, which allows only red and blue rays to pass, refracts the red more than the blue. He, like Talbot, did not at first venture to publish his result, and it appeared only in 1862. Among the many convincing proofs of its accuracy he shows that the dispersion by an iodine-vapour prism can be nearly achromatized by a glass prism which gives refraction in the same direction. He also states that the dispersion in iodine-vapour is- less as the temperature is higher. Fuchsine. Christiansen s 2 earliest determinations were made in 1870 upon an alcoholic solution of fuchsine (one of the powerful aniline colours). This solution gives a dark absorption band in the green ; and it was found that the refractive index rises (as in normal bodies) for rays from the red to the yellow. But all the rest of the transmitted light, consisting of the so-called " more refrangible " rays, is less refracted than the red. Kundt and others shortly afterwards greatly extended these observations. The explanation of this phenomenon, which has been advanced by Helmholtz, 3 depends upon an assumption as to the nature of the mutual action between the lumini- ferous ether and the particles of the absorbing medium, coupled with a further assumption connecting the absorp tion itself with a species of friction among the parts of each absorbing particle. In 1879 De Klerker 4 made a very curious observation, which shows that the whole subject is still obscure. He employed two hollow prisms of equal angle, turned opposite ways, and filled with alcohol. Through such a combination light passes (as we have seen) without refraction or disper sion. When a few drops of the fuchsine solution were added to the contents of one of the prisms, the yellow, orange, and red rays (in the order named) began to separate themselves from the others. This process could be carried on until the solution was so strong that it trans mitted no visible light. All this time the blue and violet 1 Comptes Rendus, lv., 1862. 3 Pogg. Ann., civ., 1874. Pogg. Ann., cxli. Comptes Rendus, 1 879. rays remained apparently unrefracted the yellow, orange, and red showing continually increasing refraction. The conclusion from this, on either theory of light, is that the addition of fuchsine to alcohol alters the velocity of propa gation of the (so-called) less refrangible rays, but not per ceptibly that of the more refrangible. Fluorescence. The singular surface appearances presented Fluor- by " canary " glass, by some specimens of fluor spar, and escenc by certain liquids, such as a solution of sulphate of quinine acidulated with sulphuric acid, had been the source of much speculation long before their true nature was traced by Stokes in 1852. 5 By a series of well-contrived experi ments, one or two of which will presently be described, he put it beyond doubt that the cause of these phenomena lies in a change of refrangibility of the light which has Chang< been absorbed by the upper layers of the medium, and of re ; then given off again. In every case the fluorescent light ^"A 1 " appears to belong to a less refrangible part of the spectrum Yiglit. than does the incident light which gave rise to it, thus affording an instance of dissipation, or degradation of energy. The yellowish-green surface-colour of canary glass (coloured with oxide of uranium) is well known, as the substance is, mainly on account of this property, very commonly used for ornaments. If we admit a ray of sunlight (or light from the electric lamp) into a dark room, through a cobalt glass so dark that the feeble violet- coloured light it transmits is scarcely visible, we find that the canary glass shows its yellow-green colour vividly when placed in the track of the ray. Striking as this experiment Stokes is, it is not quite conclusive as to the true cause of the ex P eri - appearance. But if we take another piece of glass, slightly m tinged of a brownish-yellow (by oxide of gold), we find that it is quite transparent to the brilliant light from the canary glass ; if, however, we place it in the track of the violet rays before they fall on the uranium glass, it prevents the production of the phenomenon altogether. That is, rays which cannot pass through the glass coloured with gold are rendered capable of freely passing through it after incidence on the canary glass. That the phenomenon is due to rays which are stopped by the uranium glass itself is proved by the fact that a second piece of the glass, placed in the track of the rays which have passed through the first, does not show the phenomenon. Unless, indeed, the source of light be very bright, the appearance is confined to a mere surface- layer of the first piece of canary glass. The phenomenon is very well shown by an aqueous infusion of horse-chestnut bark. Some specimens of paraffin oil exhibit it most brilliantly. To find the rays which are most effective in producing the fluorescence of any substance, we have only to place it in a pure spectrum of sunlight (or, preferably, of the electric light), prisms and lenses of quartz being used for producing the spectrum, because that material is found to be far less opaque than glass is to the violet and ultra violet rays. When this is done with uranium glass we find scarcely a trace of effect until the substance reaches the blue rays, and the effect persists through all the higher colours, and even very considerably beyond the bounds of the visible spectrum. Stokes in fact used it as a means of studying the otherwise invisible, but far extending, spectrum of the ultra-violet rays of the electric spark. The mechanism of the process by which these extra ordinary results are produced is still somewhat obscure, and we cannot attempt to explain it here. The duration of fluorescence is so very short that it is Phos- only by specially devised methods that we can make certain poro- that it persists for any measurable time after the exciting sc< 5 Phil. Trans., " On the Change of Refrangibility of Light."
