When a vessel is ploughing through masses of these animals, the effect is extremely brilliant. An American captain states that when his ship traversed a zone of these animals in the Indian Ocean, nearly thirty miles in extent, the light emitted by these myriads of fire-bodies, of which he estimated there were thirty thousand in a cubic foot of water, eclipsed the brightest stars; the milky way was but dimly seen; and as far as the eye could reach the water presented the appearance of a vast, gleaming sea of molten metal, of purest white. The sails, masts, and rigging cast weird shadows all about; flames sprang from the bow as the ship surged along, and great waves of living light spread out ahead,--a fascinating and appalling sight. The enormous quantity of _Noctilucæ_ in the water explains the intensity of the light. In experiments made at Bologne, one-seventh to one-half of a given amount of water taken up consisted of these minute light-givers, and Rymer Jones found thirty thousand in a cubic foot. According to Quatrefages, the light of _Noctilucæ_ in full vigor is a clear blue; but, if the water is agitated, it becomes nearly, if not quite white, producing rich silvery gleams sprinkled with greenish and bluish spangles. Regarding the intensity of the light, a tube fifteen millimetres in diameter, containing a bed of _Noctilucæ_ at the surface twenty millimetres thick, emitted light sufficient to see the face of a watch and read the figures; and, if the little creatures were agitated, time could be ascertained at a distance of a foot. M. Quatrefages found that the most delicate thermometer was not affected by the light; and he assumes that it is not combustion from the fact that oxygen gas, when introduced, does not restore the light after it has disappeared at the death of the animal. His conclusion is, that the light is produced by the contracting of the interior mass of the body; and that the flashes, or scintillations, are due to the rupture and rapid contraction of the filaments of the interior. The fixed light he explains as resulting from the permanent contraction of the contractile tissues adhering to the inner surfaces of the general envelope. Giglioli is especially enthusiastic over the light of the _Noctilucæ_ and other forms; and to show its general distribution he says that in fifty-five thousand marine miles traversed by the “Magenta,” the Italian exploring-ship, in four hundred and thirty-nine days, phosphorescence was observed more than half of the time. He met _Noctilucæ_ in the Bay of Naples, at Rio, in the Straits of Banca, while in the east coast of Asia; and at Port Jackson “the same milky uniform light was seen, without any green or bluish tint,” and again at Valparaiso. He observed, including _Noctiluca miliaris_, three luminous forms, all differing in the color of their light. The one observed on the Asiatic coast emitted a green light, and is called by M. Giglioli, _N. homogenea_. The Pacific form, _N.