Page:Popular Science Monthly Volume 77.djvu/121

Rh of the Division of Chemistry of the Hygienic Laboratory, of the U. S. Public Health and Marine Hospital Service), in a study of the effects of various chemical agents on the emission of light by the common firefly of the country around Washington, Photinus pyralis K. In the progress of this work we had occasion to review the available literature quite thoroughly, and were struck with the lack of acquaintance of people generally with the theories which had been advanced to explain the phenomenon, and with the work which had already been done upon it. The results of this investigation will be published at an early date. In spite of this great amount of work which has been done, the firefly still preserves its secret of "the cheapest form of light," and seems likely to do so for some time yet.

Although the most common and brilliant manifestations of physiologic light are exhibited by the fireflies, this property is by no means confined to the animal kingdom. Various vegetable forms, from the lowest to the highest, have been reported as producing light. There are many varieties of luminous bacteria and molds, whose activity is seen in the luminous decay of fish and wood. Certain agarics and other of the higher fungi are luminous, and the light given by the underground rhizomorphs of fungus growths is among the first of these phenomena to be reported in scientific literature. Of the higher plants, the marigold, the nasturtium and other garden and wild flowers have been said to emit flashes of light—a circumstance attributed by Phipson to electricity. But, for the most part, the light of vegetable forms seems to be pale and often hard to discern, as compared with the brilliancy and glitter of the firefly and other animal forms.

To those not living on the sea-coast, the most common manifestation of the photogenic function is that produced by some variety of the firefly; but there are a large number of marine forms of varying degrees of organization which possess this property, and some of these are common on certain coasts. For the purpose of discussion, the animal forms will be grouped as marine and land forms.

The simplest marine form which emits light is the "Noctiluca" (Noctiluca miliaris), a tiny globule of protoplasm scarcely a millimeter in diameter, which when present—as it usually is—to the extent of millions upon millions, produces the appearance known as the "milky sea" or "phosphorescent sea." Many interesting studies have been made on this little organism, the principal importance of which lies in the fact that it seems to give practically the same reactions as other more highly organized luminous forms. Besides the Noctiluca, certain Beroe and other Ctenophores are often present in immense numbers, and give rise to the same appearance of the milky sea. Higher still, there are a number of Salpæ, and other marine forms which give light, and interesting studies upon them have been made