Let us now turn to luminous organisms and consider the effect of temperature and of concentration of reacting substances (oxygen, luciferin and luciferase) on the luminescence. We have already seen that luminescence of a luciferin-luciferase mixture begins with an extraordinarily low oxygen tension and increases in intensity with increasing tension of oxygen, but that very soon an oxygen tension is reached where a maximum luminescence is obtained and further increase of oxygen tension gives no brighter light. In this respect the luminescence intensity--oxygen tension curve is no doubt very similar to the hæmoglobin saturation--oxygen tension curve. Hæmoglobin is about 50 per cent. saturated at 10 mm. oxygen pressure, 80 per cent. saturated at 20 mm. oxygen pressure and completely saturated at pressures of oxygen well below the pressure of oxygen in air (152 mm. Hg). As the optimum oxygen tension for luminescence of luciferin is also well below that of air, mixtures of luciferin and luciferase luminesce with equal brilliancy whether air or pure oxygen is bubbled through them. To obtain an excess of oxygen it is only necessary to keep the solution saturated with air and statements regarding concentration of luciferin and luciferase and intensity or duration refer to excess of oxygen. Investigators who have studied the effect of increase in oxygen pressure on luminous animals have come to the same conclusions. High pressures of air or oxygen do not increase the intensity of luminescence (Dubois and Regnard, 1884). The hydrogen ion concentration of crude solutions of luciferin and luciferase, made by extracting whole Cypridinas with hot or cold water is fairly constant, about PH = 9, determined electrometrically. Such solutions have a high buffer value and the PH does not change during oxidation of luciferin so that this variable is automatically controlled. Because of difficulties in measuring low intensities of light which are constantly changing, no figures on light intensities can be given, but it is easy to establish the following facts: The greater the concentration of luciferin or luciferase the more intense the luminescence. The greater the concentration of luciferin the longer the duration of luminescence and the greater the concentration of luciferase, the shorter the luminescence lasts. Thus, if we mix concentrated luciferin and weak luciferase we get a bright light which lasts for a half hour or more, gradually growing more dim. Concentrated luciferase and weak luciferin give a bright flash of light which disappears almost instantly. Concentrated luciferase and concentrated luciferin give a brilliant light which lasts for an intermediate length of time and weak luciferin and weak luciferase give a faint luminescence which lasts for an intermediate length of time.