318. On the other hand, metallic reflection produces very little chromatic effect, whilst the peculiarities of substances which produce selective reflection principally consist in chromatic effects. Moreover reflection from ordinary transparent substances, is considerably weakened by bringing them into optical contact with another having nearly the same refractive index; but in the case of quasi-metallic substances, the colours which they reflect, are brought out more strongly by placing them in optical contact with glass or water. 319. That there are certain substances, which strongly reflect light of the same periods as those which they absorb, is strikingly exemplified in the case of permanganate of potash. Stokes found', that when the light transmitted by a weak solution is analysed by a prism, there are five absorption bands, which are nearly equidistant, and lie between D and F. The first band, which lies a little above D, is less conspicuous than the second and third, which are the strongest of the set. If however light incident at the polarizing angle is reflected from permanganate of potash, and is then passed through a Nicol, placed so as to extinguish the light polarized in the plane of incidence, the residual light is green; and when it is analysed by a prism, it shows bright bands where the absorption spectrum shows dark ones. 320. Safflower-red or carthamine is another example of a substance which exhibits selective reflection. Stokes found, that this substance powerfully absorbed green light, but reflected a yellowish green light; and that when red light polarized at an azimuth of 45° was incident upon this substance, the reflected light was sensibly plane polarized, but when green or blue light, polarized in the same azimuth, was substituted, the reflected light was elliptically polarized. It further appeared, that the chromatic effects of this substance were different, according as the incident light was polarized in or perpendicularly to the plane of incidence; for when the incident light was polarized perpendicularly to the plane of incidence, the reflected light was of a very rich green colour, but when it was unpolarized the reflected light was yellowish-green. Similar results were obtained by using a compound of iodine and quinine called herapathite, which was discovered by Dr Herapath of Bristol, and which strongly absorbs green light. 1 Phil. Mag. Vol. vi. (18) p. 293. SELECTIVE REFLECTION. 301 321. The effect of bringing a transparent medium into optical contact with a quasi-metallic substance, may be illustrated by depositing a little safflower-red upon a glass plate, and allowing it to dry; when it will be found that the surface of the film which is in contact with air, is of a yellowish-green colour; whilst the surface in contact with glass, reflects light of a very fine green inclining to blue. Similar effects are produced with herapathite and platino-cyanide of magnesium. The latter crystal is one of a class of special optical interest, since it is doubly refracting, doubly absorbing, doubly metallic and doubly fluorescent. 322. Further experiments upon selective reflection have been made by Kundt', who found that it was strongly exhibited by the aniline dyes and other substances, which produce anomalous dispersion. The following table' shows the colour of the transmitted and reflected light, when the latter is viewed with the naked eye and through a Nicol's prism, adjusted so as to extinguish the component polarized in the plane of incidence. 323. Fluorescence. When common light is incident upon a solution of sulphate of quinine in water, it is found that the surface of the liquid exhibits a pale blue colour, which extends a short distance into the liquid; if however the light which is refracted by the substance, and has therefore passed through the thin coloured stratum, is allowed to fall upon the surface of a second solution of sulphate of quinine, the effect is no longer produced. The peculiar action which sulphate of quinine, as well as 1 Ante, p. 297, footnote. 2 Glazebrook's Physical Optics, p. 273. certain other substances, produces upon light, is called fluorescence. It was first discovered by Sir David Brewster' in 1833, who observed that it was produced by chlorophyll, and also by fluor spar. Sir J. Herschel' found that fluorescence was produced by quinine, but the subject was not fully investigated, until it was taken up by Sir G. Stokes in 1852. 324. To examine the nature of fluorescence produced by quinine, Stokes formed a spectrum by means of a slit and a prism, and filled a test tube with the solution, and placed it a little beyond the red extremity of the spectrum. The test tube was then gradually moved up the spectrum, and no traces of fluorescence were observed, as long as the tube remained in the more luminous portion; but on arriving at the violet extremity, a ghost-like gleam of pale blue light shot right across the tube. On continuing to move the tube, the light at first increased in intensity, and afterwards died away, but not until the tube had been moved a considerable distance into the invisible ultra-violet rays. When the blue gleam of light first made its appearance, it extended right across the tube, but just before disappearing, it was observed to be confined to an excessively thin stratum, adjacent to the surface at which the light entered. 325. This experiment shows that in the case of quinine, fluorescence is produced by violet and ultra-violet light, and also that it is due to a change in the refrangibility of the incident light. Stokes also found, that quinine was exceedingly opaque to those rays of the spectrum which lie above the line H, that is to those rays by which fluorescence is produced. This explains why light, which has been passed through a solution of quinine, is incapable of producing fluorescence, for the solution absorbs the rays which give rise to this phenomenon. 326. The effect may accordingly be summarized as follows. Quinine is transparent to the rays constituting the lower or more luminous portion of the spectrum, but it strongly absorbs the ultra-violet rays, and gives them out again as rays of lower refrangibility. The latter circumstance enables the eye to take cognizance of the invisible ultra-violet rays; for if this portion of 1 Trans. R. S. E. Vol. x. p. 542. 2 Phil. Trans. 1845. 3 Ibid. 1852, p. 463. FLUORESCENCE. STOKES' LAW. 303 the spectrum is passed through a fluorescent substance, it is converted into luminous rays, which are visible, and can be examined by the eye. By this method Stokes was able to make a map of the fixed lines in the ultra-violet region. 327. Fluorescence is also produced by a number of other substances, among which may be mentioned decoction of the bark of the horse-chestnut, green fluor spar, solution of guaiacum in alcohol, tincture of turmeric, chlorophyll, yellow glass coloured with oxide of uranium &c. It must not however be supposed, that the light produced by fluorescence is of the same colour for all substances, since as a matter of fact, it varies for different substances. Thus the fluorescence produced by chlorophyll consists of red light, showing that this substance converts green and blue light into red light. 328. As the result of his experiments, Stokes was led to the following law, viz.;-When the refrangibility of light is changed by fluorescence, it is always lowered and never raised. Whether this law is absolutely general has lately been doubted; and there appears to be some evidence, that exceptions to it exist. 329. We have already called attention to the fact, that the phenomena of dispersion, absorption and the like, are caused by the molecules of matter being set in motion by the vibrations of the ether. Now if the molecular forces depended upon the first power of the displacements, it would follow from Herschel's theorem, that the period of the forced vibrations would be equal to that of the force; if however the molecular forces depended upon the squares or higher powers of the displacements, Herschel's theorem would be no longer true, and under these circumstances Stokes suggested, that fluorescence arises from the fact, that the forces are such, that powers of the displacements higher than the first cannot be neglected. We have already pointed out, that ultra-violet light produces strong chemical effects. Now the molecules of most organic substances consist of a number of chemical atoms connected together, and forming a system of more or less complexity, which is stable for some disturbances but unstable for others. For instance, an ordinary photographic plate is fairly stable for disturbances produced by sodium light, but unstable for those produced by violet light. It is therefore not unreasonable to suppose, that the amplitudes of the vibrations communicated by ultra-violet light to the molecules, and to the atoms composing them, of a substance like quinine, should be of such far greater magnitude, than those communicated by light of less refrangibility, that the molecular forces produced under the former circumstances cannot be properly represented by forces proportional to the displacements. If this be the case, the period of the forced vibrations will no longer be equal to that of the force. 330. When a molecule is set into vibration by ethereal waves, the vibrations of the molecule will give rise to secondary waves in the ether. The periods of these secondary waves must necessarily be the same as those of the molecules by which they are produced; for Herschel's theorem applies to vibrations communicated to the ether, although it does not necessarily apply to vibrations communicated to the molecules. And if the periods of the secondary waves are longer than those of the waves impinging on the molecules, these waves will be capable of producing the sensation of light, provided their periods lie within the limits of sight, even though the periods of the impinging waves are too short to be visible. We can thus obtain a mechanical explanation of the way in which fluorescence is produced, but at the same time the following illustration will make the matter clearer. 331. The equation of motion of a molecule, which is under the action of molecular forces, which are proportional to the cube of the displacement, and which is also under the action of a periodic force, is The particular solution gives the forced vibration, whilst the complementary function gives the free vibration. The determination of the particular solution when F= Aept, where A is an arbitrary constant, would be difficult; but as the above equation. is given as an illustration, and not for the purpose of constructing a theory, we shall suppose that the force is represented by 2p3 The particular solution will then be found to be 2p y = cos pt. |