We have just seen that the effect of cooling a _Cypridina_ extract containing luciferin and luciferase and luminescing with a bluish light, is to reduce the intensity and change the shade toward the yellow. Velocity of oxidation must be lowered and with the same concentration of luciferase lowered velocity means more light of the longer wave-lengths. A very instructive experiment on color of the light can be carried out with animals having different colored lights and so closely related that their luciferins and luciferases will interact with each other. Such a case is presented by the American fireflies, _Photinus_ and _Photuris_. _Photinus_ emits an orange light, while _Photuris_ emits a greenish yellow light. The difference in color is especially noticeable when the luminous organs of the two forms are ground up in separate mortars. As shown by Coblentz, the difference in color is real, the spectrum of _Photinus_ extending farther into the red than that of _Photuris_ (see Fig. 8). We can easily prepare luciferin and luciferase from the two fireflies and make the following mixtures: _Photinus_ luciferin × _Photinus_ luciferase = reddish light. _Photinus_ luciferin × _Photuris_ luciferase = yellowish light. _Photuris_ luciferin × _Photuris_ luciferase = yellowish light. _Photuris_ luciferin × _Photinus_ luciferase = reddish light. Thus the color of the light in these "crosses" is that characteristic of the animal supplying the luciferase. To bring this fact in line with what we have already said regarding reaction velocity and luminescence, we must believe that the _Photinus_ luciferase oxidizes at a slower rate than the _Photuris_ luciferase. In this connection it is of interest to recall that the _Photuris_ light as emitted by the insect becomes reddish at high temperatures, or if the insect is plunged into alcohol, both conditions which bring about partial coagulation of the luciferase and reduce its active mass. BIBLIOGRAPHY A few of the enormous number of papers on luminescence are included in the list below. The attempt is made to list only those dealing with the structure, chemistry or physiology of luminous animals and the physical nature of their light, together with a small number of general interest. More complete works on light and luminescence come first and original articles follow. Authors' names are arranged alphabetically, their papers chronologically. A fairly complete list of literature covering the whole field of Bioluminescence is given by Mangold, 1910. The 1913 paper of Dubois gives a bibliography of his own contributions up to this date so that only those papers to which special reference is made are included below. BOOKS AND GENERAL WORKS BECQUEREL, E.: 1867, La Lumière. DAHLGREN, U.: 1915, The Production of Light by Animals. _Jour. Franklin Inst._, vols. 180 to date. DUBOIS, R.: 1914, La Vie et La Lumière. Alcan, Paris. GADEAN DE KERVILLE, H.: 1890, Les Vegetaux et les Animaux Lumineux. Paris.