It is a fact that light will cause the reduction of oxyluciferin. A tube of oxyluciferin exposed to sunlight for six hours, or the mercury arc for two hours, will be partially converted into luciferin. It will luminesce when luciferase is added, while a control tube kept in darkness shows no trace of luciferin. The action is more marked with the ultra-violet as a solution of oxyluciferin in a quartz tube showed more reduction than one in a glass tube when exposed for the same length of time to the quartz mercury arc. The reduction is not dependent on the formation of acid under the influence of light since two tubes of oxyluciferin, one kept in darkness and the other exposed to sunlight for six hours, had the same reaction, PH = 9.3. Of course some reducing substance might be formed under the influence of light but this is not very probable. We may therefore write the reaction for luminescence in the following way: darkness alkali luciferase luciferin (LH_{2}) + O ⇆ oxyluciferin (L) + H_{2}O (luminescence) perhydridase acid light Acid and light favor reduction while alkali and darkness favor oxidation in the luciferin ⇆ oxyluciferin reaction. Whether the luciferin be really oxidized by removal of H_{2} or whether by addition of oxygen is, of course, uncertain, but the analogy with methylene blue is striking and may serve as a working hypothesis until the composition of luciferin and its oxidation product are known. While I have not studied the properties of oxyluciferin as fully as those of luciferin, so far as I can judge, both substances give the same general reactions and possess identical properties. Both crude luciferin and crude oxyluciferin solution are yellow in color, but I do not believe that either pure luciferin or oxyluciferin are yellow in color, because an ether or benzine extract of _Cypridina_ is also yellow, although luciferase, luciferin, and oxyluciferin are insoluble in ether and benzine. The yellow pigment which can be observed to make up part of the luminous gland of _Cypridina_ is not luciferin or luciferase. It may be a pigment related to _urochrome_.