IT HAS been established that light treatment is excellent for surgical tuberculosis and rickets, and the effects on many other diseases show that we have in this therapeutic measure a stimulant to general health. Skin diseases, chronic anemias, nutritional weakness and wasting in infants, chronic bronchitis, the debility following acute infectious disease, etc., are greatly benefited and the healing of refractory wounds of all sorts is accelerated. There is a lack of appreciation on the part of many as to what radiation embodies, of how the radiation of the quartz Hg lamp and of the C arc differ from that of the sun, of the importance of the intensity and duration of energy—its dosage. We can not always depend on the sun and so must have recourse to artificial sources. It should be obvious that if we are trying to copy sunlight we should know something about it as well as about the energy emitted by artificial sources. There is a common belief that the radiation of such sources is solely or mainly active because of the ultraviolet fraction. This has been demonstrated in but few instances, such as the treatment and prevention of rickets, and the formation of melanin in the skin. On the other hand, there is much evidence that the effects observed are due to the entire spectrum (Lo Grasso, Balderrey and Ewald). In some cases the evidence points to the visible portion as being particularly active (Sonne, Loewy and Dorno). In still others the action of certain wave lengths in the ultra-violet has been shown to be increased by the simultaneous action of the visible and infra-red. The appreciation of this activation, or intensification of the action, of ultra-violet by visible and heat rays is demonstrated on the Continent by the placing of a cluster of incandescent lamps around the hood of the quartz Hg vapor lamp. Comparison between sources shows the importance of specifying the percentage distribution of the energy emitted. Air-cooled quartz Hg lamps have a percentage distribution about as follows: 30 per cent ultra-violet, 53 per cent visible, and 17 per cent infra-red. In the water-cooled lamps the ultra-violet equals the visible. The energy of a 28 amp., 60 to 65 volt, flaming C arc is 15 per cent ultra-violet, 59 per cent visible, and 26 per cent infra-red. Sunlight at sea level contains from 1 to 4 per cent (usually not more than 1 per cent) ultra-violet, 42 to 53 per cent visible, and 57 to 43 per cent infra-red. Sunlight on Mount Wilson (1750 M.) contains from 2 to 5 per cent ultra-violet, 50 to 55 per cent visible, and 48 to 40 per cent infra-red; on Mount Whitney (4420 M.), 2 to 6 per cent ultra-violet, 54 to 55 per cent visible, and 43 to 39 per cent infra-red.