These facts can all be explained by regarding luciferase as a catalyzer which accelerates the oxidation of luciferin and by assuming that intensity of luminescence is dependent on reaction velocity, _i.e._, on rate of oxidation. Contrary to the condition for phosphorus and for pyrogallol there appears to be no optimum concentration of luciferase or luciferin, but the luminescence intensity increases gradually with increasing concentration of luminous substances up to the point where pure (?) luciferin and pure (?) luciferase, as secreted from the gland cells of the animal, come in contact with each other. This, the maximum brightness, is not to be compared with the light of an incandescent solid, but is nevertheless visible in a well-lighted room, out of direct sunlight. The effect of temperature on _Cypridina_ luminescence also bears out the preceding conclusions. For a given mixture of luciferin and luciferase the light becomes more intense with increasing temperature up to a definite optimum and then diminishes in intensity. The diminution in intensity above the optimum is due to a reversible change in the luciferase so that its active mass diminishes. This change becomes irreversible in the neighborhood of 70° (depending on various conditions), where coagulation of luciferase occurs. Light will appear at 0° but it is far less intense than light at higher temperatures and it is more yellow in color. The light of optimum temperatures is quite blue. The weaker light at temperatures above the optimum is also more yellow in color. I believe this difference in color is a function of the slowed reaction velocity, for a mixture of luciferin and luciferase which gives a bluish luminescence at room temperature, will give a weaker and yellowish luminescence if diluted with water. Dilution with water will slow the reaction velocity. If the difference in color were not real but due to change in color sensitivity of the eye with different intensities of such relatively weak light (Purkinje phenomenon), the weaker light should appear more blue. As the weaker light appears more yellow, I therefore believe the color difference is actual and not subjective. A minimum, optimum, and maximum temperature for luminescence is observed in all luminous organisms. The minimum is usually very low. Luminous bacteria will still light at -11.5° C. The power to luminesce under ordinary conditions is not destroyed by exposure to liquid air, for, on raising the temperature, light again appears (Macfayden, 1900, 1902). Almost all organisms will luminesce at 0° C., and the luminescence minimum probably represents the point at which complete freezing of the luminous solution occurs. It is very low with bacteria because they are solutions in capillary spaces of very small size, a condition tending to lower the freezing point.