There are some cases of luminosity on record in connection with man himself. (See Heller, 1854). Before the days of aseptic and antiseptic surgery, wounds frequently became infected with luminous bacteria and glowed at night. The older surgeons even supposed that luminous wounds were more apt to heal properly than non-luminous ones. We know that luminous bacteria are non-pathogenic, harmless organisms and the presence of these forms even on dead fish or flesh never accompanies but always precedes putrefaction. As recorded by Robert Boyle, no harm has come from eating luminous meat, unless it may also have become infected with pathogenic forms. A few cases of luminous individuals have been noted in which the skin was the source of light, especially if the person sweated freely. It is possible that here we are again dealing with luminous bacteria upon the accumulations of substances passed out in the sweat, which serves as a nutrient medium. There are also on record, in the older literature, cases of luminous urine, where the urine when freshly voided was luminous. If these observations are correct and they may, perhaps, be doubted, we are at present uncertain of the cause of the light. Bacterial infections of the bladder are not inconceivable although luminous bacteria are strongly aerobic and would not thrive under anaerobic conditions. I can state from my own experiments that luminous bacteria will live in normal human urine, but not well. In albuminous urines it is very likely that they would live better, and it is possible that the luminous urines reported are the results of luminous bacterial infection. On the other hand, the light may be purely chemical, due to the oxidation of some compound, an abnormal incompletely oxidized product of metabolism, which oxidizes spontaneously in the air. We know that sometimes these errors in metabolism occur, as in _alkaptonuria_, where homogentistic acid is excreted in the urine and on contact with the air quickly oxidizes to a dark brown substance. Light, however, has never been reported to accompany the oxidation of homogentistic acid, although it does accompany the oxidation of some other organic compounds. (See Chapter II.) Finally, we may inquire to what extent luminous animals may be utilized by man. Leaving out of account the use of tropical fireflies for adornment by the natives of the West Indies and South America and the use for bait, in fishing, of the luminous organ of a fish, _Photoblepharon_, by the Banda islanders, we find that luminous bacteria are of value for certain purposes in the laboratory. These methods are all due to Beijerinck (1889, 1902). He has, for instance, used luminous bacteria for testing bacterial filters. If there is a crack in the filter the bacteria will pass through and a luminous filtrate is the result, but a perfect filter allows no organisms to pass and gives a dark filtrate.