It is, of course, obvious that when luciferin oxidizes, some oxidation products must be formed. Most observers have assumed the oxidation products of luciferin to be relatively simple and to represent a rather complete breaking down of the luciferin molecule. Carbon dioxide was mentioned by Phipson (1872) as being formed. We have just seen that no carbon dioxide is formed during the oxidation of _Cypridina_ luciferin and there is evidence that no fundamental change at all occurs. It is for this reason that I have called the oxidation product of luciferin _oxyluciferin_.[7] As we shall later see, the change luciferin oxyluciferin is to be compared to the oxidation of colorless dyes (leuco-compounds) to the colored dye. The chemical properties of oxyluciferin are similar to those of luciferin and the oxyluciferin can be readily reduced to luciferin again. [7] It is unfortunate that Dubois (1918 b) has used the term oxyluciferine in a quite different sense from the present use. He regards oxyluciferine as a substance still capable of giving light by autoöxidation, and represents the steps in luminescence as follows: "Co-luciférase + preluciférine = luciférine. Luciférase + luciférine = oxyluciférine. Oxyluciférine + oxygéne = lumiérè." I should represent them as follows: Luciferin + oxygen ⇆ oxyluciferin. The reaction proceeds to right with light production only in presence of luciferase. Finally, we have the fluorescent substance of _Pyrophorus_ and fireflies, which Dubois first called _pyrophorin_, but later, adopting McDermott's terminology, speaks of as _luciferesceine_. This Dubois regards as a substance intensifying the light and modifying its color by changing invisible into visible rays. As we have seen, this theory, while attractive, will not stand the test of critical examination.