PHYSIOLOGIC LIGHT i 1 5 



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 physi- 

 ologic 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 mari- 

 gold, the nasturtium and other garden and wild flowers have been said 

 to emit flashes of light — a circumstance attributed by Phipson to elec- 

 tricity. 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 manifesta- 

 tion 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 com- 

 mon 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 milli- 

 meter in diameter, which when present — as it usually is — to the ex- 

 tent 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 Salpse, and other marine forms 

 which give light, and interesting studies upon them have been made 



