Prevention and treatment of rickets with ultraviolet light has become a well established therapeutic factor since Huldschinsky1 in 1919, first pointed out the curative action of ultraviolet rays from mercury-vapor lamps. A short time later, Hess and Unger2 demonstrated the same effect brought about by sunshine. Recently Tisdall and Brown3 stated that even skyshine, sun rays filtered and reflected by clouds, has still a marked antirachitic action. The same authors, however, proved by experiments the fact, well known through experience, that during the winter months the power of sunlight is relatively very small. As a substitute for the invigorating power of bright summer sun, mercury lamps are now used more and more during the darker months.
Photosensitivation can be regarded as another method of bringing about the visible effects of sunlight on the skin, but this method and its capacity are as yet very little known. Compared