Page:Popular Science Monthly Volume 7.djvu/470

454 1870, so many of the deep-sea animals were found to be phosphorescent, that Prof. Thomson has suggested that the light necessary to the development of the eyesight which some of the specimens possessed may have had its origin in that source. In animal phosphorescence, as in all her works, Nature exhibits an immense variety in the forms in which she displays her power; in one case, the luminosity will be visible in a fluid secretion; in another, it will manifest itself through the action of a minute and complicated organ; one species of animal will shine with a yellow light; a second, with brilliant green; a third, with pale lilac; and we are acquainted with one instance in which the light changes successively to the chief colors of the solar spectrum. The causes which produce these phenomena are still very obscure. Although many forms of life are known in which the luminous quality is present, scientific men are not yet agreed on what the quality depends; and the purposes which the light serves in the animal economy are also little understood. But the phenomena themselves are often very remarkable.

Some strange theories were advanced to account for the phosphorescence of the sea, before the real cause was discovered. In 1686, an ecclesiastic, named Tachard, suggested that the ocean absorbed the sun's light by day, and emitted it again at night. About the same time, a better-known philosopher, Robert Boyle, endeavored to account for the same phenomenon by the friction which, he supposed, the rotation of the earth upon its axis caused between the water and the atmosphere. The problem was finally solved in 1749, by the discovery of luminous animalcules in the water of the Adriatic Sea; and a large proportion of the lower classes of marine animals are now known to be phosphorescent to a greater or less degree. Let us take the invertebrate divisions of the animal kingdom in their regular order, and briefly glance at one or two examples in each. Beginning with the simplest forms of life, the Protozoa, we find, in addition to certain Infusoria, the little jelly-like organism to which naturalists have given the name of Noctiluca, the phosphorescence of which is largely demonstrated around our coasts.

The radiated class of sea-animals possess high phosphorescent qualities. Star-fish, sea-pens, jelly-fish, sea-fans, sea-rushes, may be mentioned as cases in which the luminous quality is present among the radiata. We will take our examples from among the specimens captured during the expeditions of the Porcupine. On some occasions when the dredge was hauled up late in the evening, the hempen tangles which were attached to it came up sprinkled over with stars of the most brilliant uranium green; little stars, for the phosphorescent light was much more vivid in the younger and smaller specimens. The light was not constant, nor continuous all over the star, but sometimes it struck out a line of fire all round the disk, flashing, or one might rather say glowing, up to the centre; then that would fade, and a defined