Phosphorescence. Strictly speaking, the term is applied to the phenomenon, exhibited by certain bodies, of remaining luminous in the dark for some time after being exposed to a strong light. Certain preparations, such as calcium sulphide (see LUMINOUS PAINT), indurated limestone, &c., possess this property in a very high degree. With the great majority of phosphorescent bodies, however, the duration of the phenomenon is very short, rarely more than a small fraction of a second. Becquerel, who studied this phenomenon with great care, invented a very ingenious instrument for the purpose, called a phosphoroscope. The body to be tested is placed in a small drum, which has an opening at each end. In this drum there revolve two discs, mounted on the same axle, and pierced symmetrically with the same number of holes. They are so adjusted that when a hole in one disc is opposite to the hole in the corresponding end of the drum the second disc closes the hole at its end of the drum, and vice versa. Light is admitted by one of the holes in the drum so as to fall on the object, and it is examined through the other hole. It is obvious that, when the discs are made to revolve, the object is alternately exposed to light and presented to the eye. By a train of multiplying wheels, these alternations may be made to succeed each other as rapidly as the observer pleases, and thus the object is presented in the dark to his eye as soon after its exposure to light as may be desired. Almost all bodies are found to be phosphorescent; for instance, some kinds of pink rubies when exposed to sunshine in this apparatus appear to glow like live coals in the dark. When phosphorescence is continuous, bodies receiving light and giving off radiation of a lower refrangibility, we have the phenomenon observed by Brewster and Herschel in quinine and certain crystals of fluor-spar, and thence called Fluorescence. The green colouring matter of leaves, a decoction of the bark of the horse-chestnut, and the common canary glass (coloured with oxide of uranium) are bodies which exhibit this phenomenon very well. Perhaps the most striking method of studying the phenomenon is to receive in a darkened room the solar Spectrum (q.v.) on a sheet of white paper, and to pass over the coloured spaces a brush dipped in a solution of sulphate of quinine with sulphuric acid. No change is produced on the less refrangible rays, but in the blue and indigo spaces a strange change of colour is at once apparent where the liquid has been spread. This appears more strongly in the violet, and vividly in the spaces beyond the violet, where rays fall which excite no luminous sensation in the eye. By this experiment the visible length of the spectrum may easily be doubled. By using the electric light, which is peculiarly rich in these highly-refrangible rays, a prism of quartz, which allows them to pass very freely, and various fluorescent substances, Stokes has obtained spectra six or eight times as long as those otherwise visible. The characteristic of all these rays is that they are less refrangible than those from which they are pro- duced. The entire phenomenon is, as Stokes first showed, identical in principle with Leslie's photometer, in which light was measured when changed into heat by absorption in the coloured glass, of which one of the bulbs of his differential thermometer was formed. Ordinary phosphorus (from which the phenomenon took its name) becomes luminous in the dark by slight friction; whence the common trick of drawing self-luminous figures on doors and walls with a stick of phosphorus, or an ordinary lucifer match wetted.
PHOSPHORESCENCE IN ORGANIC BEINGS.—The emission of light by minerals after insulation and the cognate phenomena of luminosity on heating, friction, cleavage, crystallisation, &c. are of quite different nature from the production of light by organisms, the special subject to which we now turn.
Luminosity due to Putrefaction or to Disease.—The fact that many organic substances (especially fish) become luminous when decaying has long been known, and has often been erroneously adduced as the chief cause of the phosphorescence of the sea. It is only quite recently that the discovery of luminous bacilli has rendered possible any general explanation of these facts, and even yet its wide applicability remains to be proved. They have been shown to exist in several instances which will be mentioned below, and the constant association of these low forms of life with putrefactive processes at once suggests the hypothesis that luminosity under such circumstances may be due to their presence. Perhaps they may also account for the few instances in which the human body has been recorded as phosphorescent during life.
Luminosity of Healthy Living Organisms.—(1) In the vegetable kingdom the instances of the occurrence of this property are but few, and the majority of these belong to the algae and fungi, the bacilli or bacteria above mentioned being referable to the former. Most of the fungi are Hymenomycetes (Agaricus, spp.): in some cases the mycelium (root-like threads) gives out the light; in others, as in A. olearius, not uncommon at the roots of olive-trees, it is the under surface of the mushroom-like head. The light is only seen while growth is progressing; it ceases so soon as the fungus is mature. A moss (Schistostega osmundacea), some grasses, a Euphorbia (E. phosphorea), a lily (Lilium bulbiferum), a poppy (Papaver orientale), and a nasturtium (Tropæolum majus) have all been recorded as luminous. The last instance is worthy of note as having been observed by the daughter of Linnaeus in 1762.
(2) In the animal world there is not one of the larger groups, up to and including the fishes, which does not afford some good examples of this phenomenon. A complete list of these would be outside the scope of such an article as the present; it must suffice to mention some of the best instances in each class, indicating briefly the nature of the photogenic mechanism. Among the Protozoa the small spheroidal Noctiluca miliaris is perhaps the most widely spread instance of this property, this organism, or others allied to it, abounding at certain times around the coasts of the greater part of the world. The light is emitted from the general protoplasm of the body; a pocket-lens shows some points to be brighter than others, whilst a higher magnifying power shows these to be really groups of smaller points, just as a nebula is gradually resolved into stars by increasing powers of the telescope. On the high seas Noctiluca seems to be replaced by species of Pyrocystis, one of the discoveries of the Challenger expedition; these may possibly, however, be stages in the life-history of very similar forms. Among the Porifera (sponges) the only recorded luminous form is the larva of a species of Reniera. The poverty of this class is, however, more than compensated by the wealth of the Cœlenterata. The common hydroid colony Obelia geniculata, often growing upon the fronds of Laminaria (sea-tangle), is a familiar instance. Numerous Medusæ (jelly-fishes) must be added to the list; and here the light is variously emitted in different species: (a) from the whole surface; (b) from the marginal corpuscles; (c) from the radial canals; (d) from the ovaries. Sometimes the same genus includes both luminous and non-luminous forms. Most Pennatulidæ (sea-pens) furnish instances of the possession of this property; such are the long, reed-like Funiculina found in Loch Torridon (Ross-shire) and on other parts of the Scottish coast, and Pennatula, the subject of classic researches by Pauceri, where the light emanates from eight bands or tracts of specially modified tissue situated in the wall of the stomach. Alcyonarians, Siphonophores, and Ctenophores also furnish contributions to the list; amongst these last the small ovoid jelly Beroë was shown by Allman not to give out its light until it had remained for some time in the dark.
A few species of Ophiuroids (brittle stars) and the deep-sea asteroid Odinia constitute the only known instances of luminosity among the Echinodermata, but the worms furnish a larger array. More than one case has been noticed among earthworms, the most complete description being due to Professor Giard. This worm (Photodrilus) is to 2 inches long, and the luminous material is due to a series of glands opening into the œsophagus. In these forms the luminosity ceases immediately after sexual congress. Among the marine Chætopoda the power resides in the dorsal scales (elytra) of the Polynoidæ, the tentacles, dorsal tubercles, &c. of Chætopterus, and the bunch of cephalic tentacles of Polycirrus.
Of phosphorescent mollusca the small pelagic Phylliroë bucephala is of importance, because the transparency of its tissues permits of its easy study and the successful localisation of its luminosity, which is found to reside in nerve ganglia, as well as in certain small rounded cells situated at the ends of the nerve twigs. The rock-boring bivalve (Pholas) is one of the longest known instances of animal phosphorescence, being recorded by Pliny, who noticed that if any one chews the animal the whole interior of his mouth becomes luminous. In this form the light is emitted from five definite patches all situated within the mantle cavity—(1) an arc corresponding with the anterior margin of the mantle, (2, 3) two triangular patches near the opening of the branchial siphon, (4, 5) two long parallel bands within the same siphon. The luminous material is secreted by the epithelial cells covering these portions of the body, and has been shown to consist essentially of two substances, a white crystalline body (luciferine) and a ferment (luciferase); by the mixture of these two in a test-tube it is possible to reproduce the light at will, without the presence of living matter. The ferment is most likely produced by a certain bacillus, which occurs in large numbers in small pits in the walls of the siphon.
Interesting observations upon bacilli as the cause of phosphorescence have also been made on Crustacea of the genera Talitrus and Orchestia (sand-hoppers) in a state of disease; the malady, and the consequent luminosity, can be transferred to healthy specimens by inoculation, and the germs can be reared in artificial media in the usual manner. It would appear that the sand-hoppers infect themselves with the bacilli from dead fish on which they habitually feed. Many other Crustacea, however, are luminous under normal conditions, as, for instance, some Copepoda (Sapphirina), and more particularly the Schizopoda, one of which,
Nyctiphanes norvegica, is not uncommon in the deep waters of Loch Fyne as well as on the Norwegian coast. Definite organs (photosphæria) are here present for the production of light; one in the stalk of each eye, one at the root of each first, and one at the root of each penultimate, thoracic limb, and one under each of the first four segments of the abdomen. Each organ, as has been proved by microscopic sections, is in reality a miniature bull's-eye lantern, only equalled in complexity by the organs of certain fishes. Several phosphorescent Ascidians are known, the most conspicuous being the pelagic colony Pyrosoma, in which each individual zooid has two rounded luminous organs. The researches of the recent deep-sea expeditions have revealed quite a large number of phosphorescent fishes, with specialised organs of many different types. Some of these appear to be glandular, whilst others are optical instruments of great complexity, with spheroidal and parabolic reflectors to send the light in definite directions and with lenses to concentrate it. In some of the abyssal Lophioids (angler-fish) the modified dorsal fin-ray which acts as a lure has a phosphorescent extremity, no doubt to render it more attractive.
There still remain for consideration the phosphorescent insects, which belong almost exclusively to the Hemiptera (bugs) or the Coleoptera (beetles). To the former belong the lantern-flies (Fulgora and allied genera), though it must be mentioned that the possession of this power by these insects has been denied by more than one good observer. Luminous beetles appertain to the families Lampyridæ and Elateridæ. The glow-worm (Lampyris spendidula) and the Italian firefly (Luciola italica) are good examples of the former, and have been often described. In both cases the organs, which are situated in a certain number of the posterior segments of the abdomen, consist of two layers, of which the dorsal contains large quantities of uric acid salts, and the ventral clear cells, which are arranged in cylindrical lobules. In some species both sexes are luminous, in others only one. In the Mexican firefly or Cuceyos (Pyrophorus noctiluca), which belongs to the Elateridæ or click-beetles, the organs are three in number, one in each upper and hinder angle of the prothorax, and one much larger occupying the centre of the ventral surface of the first abdominal segment. This last is invisible while the insect is at rest, but shines forth when it spreads its wings and raises its abdomen in flight. Both the eggs and larvæ of this species are luminous.
As regards the physical peculiarities of the light, it varies in colour in different cases, being blue in the mycelium growing in rotten wood, in Cunina, Beroë, Pyrosoma, and Lampyris; green in a species of Agaric, Pteroeides, in Ophiuroids, and in Pyrophorus; yellow in Noctiluca, Bolina, and with a reddish tinge in a species of Cestum; purple in Funiculina and Fulgora; whilst in Pyrosoma atlanticum and an Appendicularia observed by Giglioli it has been stated that the light varies in colour. In all cases in which the matter has been investigated the spectrum has been found to be continuous; the amount of heat given off in connection with the process seems to be quite infinitesimal, though actinic rays have been shown to be present. The phenomenon would appear to consist in a definite vital process, which, in these cases, leads to the production of radiant energy just as in other instances to the evolution of heat or of electricity.
The uses of this property to its possessors may, so far as is known at present, be grouped under four heads: (a) It enables the sexes to find each other, as in Luciola and earthworms; (b) it is possessed by so many stinging Cœlenterates that in them we may fairly regard it as a warning signal, and it may have been adopted by stingless forms for the same purpose by mimicry; (c) it may serve to attract prey, as in the phosphorescent lure of the deep-sea angler-fish (Melanocetus, &c.); (d) it may illuminate the surrounding regions and enable the light-producing animal to find its way, as in Pyrophorus, Nyctiphanes, and many deep-sea fish. So many deep-sea animals are possessed of luminous qualities that it has been assumed that these living lights play an important part in the economy of life in those regions; this hypothesis is commonly known as the 'abyssal theory of light.' See the articles FIREFLY, GLOW-WORM, and works there cited.