urable amounts of heat certainly shows that if it is indeed a combustion, it is a most remarkable one and one which differs from any analogous process known to us. The view that the light might be the result of oxidation has, however, long been held. Robert Boyle made experiments on this point in 1667, and concluded that the light produced by shining wood and fish was not affected by the absence of air, and was therefore not what we now call a combustion or oxidation. Spallanzani, as the result of his studies on luminous sea forms, came to the opposite conclusion, in which he was opposed by Macartney and Carradori. More recently this phase of the subject has been studied by Dubois, Watasé and Townsend, all of whom have published very interesting observations. As a result of these several observations the conclusion must be drawn that oxygen is essential to the process of the production of physiologic light, and that we have in this phenomenon a true but remarkable form of combustion. Of the mechanism of this process we are still very ignorant. Dubois's theory is that the light is produced as the result of the action of an "oxidase" (oxidizing ferment), to which he has given the name "Luciferase," upon a substance of unknown composition, which he calls "Luciferin," the latter being oxidized by the atmospheric oxygen through the agency of the ferment. It is a little early to accept this hypothesis finally, although it certainly presents some analogy to known processes—for example, the production of the black pigment melanin through the action of the oxidase tyrosinase upon the organic compound tyrosin. Phipson had already described a substance he called "Noctilucin" as the active principle of physiologic light; it seems possible that Phipson isolated and analyzed a culture of photogenic bacteria.
In this connection the structure of the light organs of various animal forms has been given special attention. In general, the results of studies on those forms having special photogenic organs have been essentially similar. Briefly, the luminous organs appear to be masses of cells of some special kind, possibly a fat-derivative, or according to Macaire and Kölliker, an albuminous substance penetrated by a network of trachea (tracheoles), and as the result of some chemical action, apparently oxidation, taking place in these tissues, the light is produced. Whether these tracheoles are in life filled with air or with a liquid seems doubtful; the evidence is contradictory so far as given, but it seems quite probable that they convey air.
What is the purpose of this production of light? Of what value is it to the forms which possess it? This is another side of the "secret of the firefly," which has yet to be solved. Quite probably the function bears some relation to the reproductive life of the insect. The females of the local species (Photinus pyralis K.) give a very much less bright light than, and are quite rare as compared with, the males; one female
