LUMINOSITY. Sea water, in the deep, is of a deep violet-blue, but often in the ocean are seen luminous sparks or points of light ; also a soft, liquid, general, and wide-spread effulgence. Occasionally are moon-shaped patches of steady light and instantaneous recurrent flashes, and a milky sea is often seen. There are many minute ocean creatures, Entomostraca and others, which are luminous at night. Often the globular Noctilucm are to be seen ; they are to of an inch in diameter, and Pyrosoma are also supposed to be causes. The cause of a milky sea is not known, but a bucket of water brought up from one had a small Entomostraca, Megalopas, minute Medusa, small Porpita, Pteropods, Annelids, Globigerinm, etc., and all night the Crustacem gave forth bright spots of luminous light. It is the small Crustacean (Entomostraca) and small Medusm (Medusidm) which seem to exhibit the more prominent lumin ous properties ; not the larger Medusm (Lucer naridm), as Aurelia. Pelagia, Rhizostoma, etc.; the Physophoridm, the Porpita or Velella, nor the Physalia, or Portuguese man-of-war. The Pro tozoa, Noctilucm, however, retain their luminosity so long as they retain organic contractility. In the majority of cases of luminous annelids, the light manifests itself in scintillations along the course of the muscles alone, and only during their con traction. The Squalus fulgens of the South Seas gives forth a bright phosphorescent light, resem bling that of the Pyrosoma. The tunicated mol lusc Pyrosoma gives forth a livid greenish phos phorescent glow. The Salim, Cleodora, and other points or dots are luminous. On the night of October 30, 1772, Dr. Foster saw a very beautiful exhibition of this sort of sea-light off the Cape of Good Hope, at a few miles' distance from the shore, and while a fresh gale blew. Upon examin ing the water on which it was displayed, he was convinced that it proceeded from living animal cules. Dr. Sparmann, in the years 1772 and 1775, observed Mollusca and Medusa in such masses near the surface of the ocean, and moving with such a rising and falling motion, as seemed perfectly adequate to the production of the phenomenon. The Noctilucm of the ocean are
so minute, that seventy of them ranged in a lino would only make an inch, and millions could be contained in a wine-glass.
Luminous appearances at sea, observed by Dr. Bennett in the South Pacific, were caused by Medusm, species of Salpa. Pyrosoma, Cancer, and Scopelus. The molluscs, Nereis noctiluca, Medusa pelagica, rar. B, and the Monophora noctiluca, when alive, during pleasure emit a weak phos phoric light, generally of a bluish colour. In July 1853, between lat. 12° and 13° N., and long. 50° and 65° E., when the Peninsular and Oriental Steamer Madras was on her voyage to Aden, with the sea very high during a gale, at midnight the horizon was visible all round. The sea was of the faintest green colour, almost like milk. The luminous Scopelus stellatus, Bennett, occurs in the Pacific.
Luminous shark is Squalus fulgens, Bennett. In a dark apartment at night the entire surface emitted a vivid and greenish phosphorescent gleam, which faded away after the shark died.
Luminosity of land animals is exhibited in the fire-fly, the glow-worm, among the Myriapoda, and the luminous centipede, Geophilus fulgens. In Australia it occurs in a species of Agaricus, which gives out a pale livid light. With the glow-worm (Lampyris) there is neither combus tion nor phosphorus, but the light is the product of a nervous apparatus, and dependent on the will of the animal. A species of Agaricus of Australia emits light sufficient to show the time on a watch. —Humboldt in Jam. Ed. Jour. v. p. 328 ; M. de Quatrefages, Kolliker, quoted by Collingwood ; Bennett's Gatherings ; iYiebuhr's Travels, i. p. 441 ; Hartwig.