FLUORESCENCE, that property of cer tain bodies in virtue of which they become self luminous when exposed to light of certain wave lengths. All bodies reflect a part of the incident but fluorescence is more than a mere re flection, as may best be shown by an example. °Canary glass' (?lass colored slightly with oxide or uranium) exhibits a beautiful yellowish-green surface color when well illuminated, and for this reason it is much used for the production of ornamental effects. If a ray of sunlight be admitted into a darkened room through a piece of cobalt glass that is so dense that the feeble violet light that it transmits is barely visible, the canary glass shows its yellow-green color bril liantly when placed in its path. This shows that the phenomenon is not simple reflection, and further evidence of the same sort may easily be had. Glass that is tinged brownish-yellow by oxid of gold is almost perfectly transparent to the golden light from the canary glass, but if the violet light from the cobalt glass is caused to traverse the gold-oxide glass before striking the canary glass, the fluorescence is no longer ob served. Furthermore light that has passed through one piece of canary glass is incapable of exciting fluorescence in a second piece. These and other equally remarkable phenomena indi cate that a fluorescent substance absorbs a por tion of the light that strikes it, modifies it pro foundly in some manner, and then radiates it again. Stokes has shown that the modifica tion consists in increasing the wave-length of the incident light; and it is now known that fluorescent light invariably has a greater wave length than the primary light that excites the fluorescence. This fact has an important bearing upon many of the phenomena of physics. When it had been determined, for example, that the °X-rays' differ from ordinary light merely by having a very different wave-length, the ques tion whether their wave-length is longer or shorter was immediately answered by the fact they can excite brilliant fluorescence. Knowing that the X-rays are either too long or too short to affect the eye, and knowing also that fluor escent light always has a longer wave-length than the light that excites it, it follows at once that the X-rays have a shorter wave-length than ordinary light.
Many substances exhibit fluorescence to a greater or lesser degree. An aqueous infusion of horse-chestnut bark shows it brilliantly, and so also does a solution of sulphate of quinine. Certain of the coal-tar colors (q.v.) are con spicuously fluorescent, fluorescein taking its name from this fact. Kerosene is fluorescent, and sometimes strongly so. Most fluorescent substances cease to emit light almost instan taneously when the incident light is cut off from them. Some, however, such as calcium tungstate and the sulphids of calcium, barium, and strontium, continue to emit their rays for a sensible time, fading gradually away into dark ness after the incident light ceases to excite them. Instruments consisting of screens that are coated with some fluorescent material and protected from ordinary light by shields, or by enclosure within a light-tight box, are known as "fluoroscopes," and are used for studying the X-rays (or Röntgen rays), and the shadows cast by them. It is to be observed that fluores cence differs from phosphorescence (q.v.) not only because it is usually of very short dura tion, but primarily because it is induced by the exciting action of light-rays. Phosphorescence may be due to very different causes. The light emitted by phosphorus, for example, is probably due to the slow oxidation of that substance. That which is observed in the ocean at night, and in connection with various fungi and in sects, is due to causes which are more or less obscure; but in any event these various phos phorescent phenomena are quite different from true fluorescence. The name ‘fluorescence was coined by Sir George G. Stokes in 1852, from the fact that the mineral fluorite some times exhibits the phenomenon. Previous to 1852 fluorescence was known as uepipolic dis persion? Consult Stokes, 'Burnett Lectures on Light' (1884-87) ; Preston, iThe Theory of Light' (London 1901) ; Wood, Physical Optics> (1905).