A correspondent lately described in a letter to a London newspaper what he believed to have been “a luminous owl,” which was seen flying about at night in Norfolk. He mentioned the well-known fact that the dense greasy patch of feathers on the breast of the heron is said to be luminous by many trustworthy observers. It is very probable that it was some carnivorous or fish-eating bird, which was thus seen in a luminous condition at night. The occurrence is much more in accordance with known facts than most people would suppose to be the case. Light, even strong light, is produced by many natural objects without the accompaniment of heat. We usually expect not merely fire where there is smoke, but heat--in fact, great heat, where there is light or flame. Yet there are many instances to the contrary, and the word “phosphorescence” is used to indicate a production of light without heat in reference to the fact that phosphorus is luminous, even when covered with water, although no appreciable heat accompanies the light such as we are accustomed to observe in ordinary “combustion” or burning. There is more than one kind of phosphorescence. We separate the phosphorescence which is due to the oxidation of peculiar fatty matters in the bodies of plants and of animals (such as glow-worms) from that which is caused by the breaking or heating of crystals (white arsenic and apatite), or by longer or shorter exposure to the sun’s rays (luminous paint), or by radio-activity, or by electrical discharges in vacuum tubes.