One of the most extraordinary things regarding luminescence in general is the small amount of material necessary to cause a visible emission of light. To take an extreme case, the flash of light resulting from the impact on ZnS of a single α particle, a helium atom, is visible to the naked eye. Addition of one part in a million of some heavy metal to pure CaS will confer phosphorescent properties on the latter. We are forced to believe that the heavy metal enters into some reaction during illumination which is reversed with light emission after illumination and a very small amount of heavy metal is necessary. Pyrogallol in water, 1:5,000,000 (m/512,000), can be oxidized with light production by K_{4}Fe(CN)_{6} and H_{2}O_{2} (Harvey, 1917) and m/100 pyrogallol + H_{2}O_{2} will give a visible light with colloidal platinum in 1:250,000 concentration (Goss, 1917). Luciferin and luciferase from _Cypridina_ will also luminesce in exceedingly small concentration. If one grinds a single _Cypridina_ in a mortar with water and dilutes the extract to 25,600 c.c., light can be observed if luciferin is added to this dilute luciferase solution. By determining the volume of the luminous gland of _Cypridina_ and even assuming that this volume is all luciferase, one can calculate that one part of luciferase in 1,700,000,000 parts of water will give light when luciferin is added. Likewise, a similar dilution of luciferin will give visible light when luciferase is added. The sensitivity of our eye is largely responsible for the detection of so small an energy change. As we have seen, recent determinations have proved that the dark adapted eye can detect 18 × 10^{-10} ergs per second. From the heat of complete oxidation of pyrogallol it is possible to calculate the amount of pyrogallol necessary to give 18 × 10^{-10} ergs if completely oxidized. This quantity is infinitesimally small. When pyrogallol is oxidized by K_{4}Fe(CN)_{6} and H_{2}O_{2}, it is not completely oxidized and probably only a small amount of the energy is converted into light; otherwise we should be able to see the luminescence of a very much weaker concentration of pyrogallol. As the reaction luciferin ⇆ oxyluciferin is so easily reversible, very little energy must be liberated, and, as experiments indicate, very little heat, if any, accompanies light production. Even though this be true, it is still possible for a very small amount of luciferin to produce a very large amount of light. A very small amount of luciferase only is necessary because it behaves as an enzyme and follows the general rule that catalysts act in minute concentrations.