Phosphorescence

Phosphorescence is the property which some substances - e.g. sulphides of barium, strontium, and calcium - exhibit of shining in the dark. The body really absorbs light which falls upon it from some source, and gives back this light again when the source is removed. White light is not necessary for the exhibition of this phenomenon, it being chiefly caused by the violet and ultraviolet rays. Becquerel's Phosphoroscope has revealed the fact that a great number of bodies exhibit the property of phosphorescence, but only shine for an extremely short time after the light is removed. This instrument consists of two discs, joined one behind the other, each with four apertures, those of one disc being half-way between those of the other. The substance is placed between the two discs, the whole being kept in a dark cylinder and used in a dark room. Into a small hole at one end of the cylinder a beam of light is sent, and the eye is placed at a corresponding hole in the other end, the substance being in a line between the two holes. The discs are made to rotate, and in consequence of the alternation of the apertures in the two discs, the observer only sees the substance when it is not exposed to the light, If the body be at all phosphorescent, it will, after receiving light through one aperture, give out the light again a certain time after that aperture has passed on, and the speed of rotation can be so adjusted that the time during which the body imprisons its light is equal to an eighth part of the time of rotation. In that time one of the apertures of the other disc will have come opposite to the body, and so it will be seen by the observer. With this machine phosphorescence lasting only a ten-thousandth of a second can be detected. Ordinary phosphorus is luminous in the dark when rubbed, and this is the origin of the term phosphorescence. Animal phosphorescence is exhibited by both liting and dead natter. In the case of the latter it is easy to show that it is due to the slow oxidation or combustion of the animal tissues or to the development of bacteria. It is most typically shown in the case of the minute marine organisms such as the bacteria which grow around decaying fish, etc., or the infusoria which live on the surface, such as the tiny Noctiluca miliaris. These are only phosphorescent when irritated, as by waves or other disturbance of the water. Many more specialised invertebrates are also phosphorescent, such as e.g. the Jellyfish, Cyanea, Sea-pens or Pennatttlida? as Virgularia, Ascidians as Salpa, Starfish as the common Asterias, Ophiuroids as the Sand-stars, the Siphonophora as Physalia, Mollusca as Pholas, and worms such as Nereis. But some land animals possess the same property, e.g. the earthworm and the common English centipede (Scolopendra clectrica). The insects afford many of the most interesting examples, such as the Glowworm (Lampyris noctihica), which is the male larva of a beetle, the Fireflies (Elater), the Hemipterous genus Fulgora, the moth Noctua, and some species of Bonibyx, and the Mole Crickets. The cause of phosphorescence is still not fully explained; at one time it was thought to be due to slow combustion, similar to that which imparts phosphorescence to some inorganic bodies and some decaying animal matter. It is, however, now generally regarded as due to nervous energy; just as in the Torpedo or Electric Eel, nerve-power is transmuted into electricity, so in phosphorescent animals it turns intc light.