Deprived of sunshine the human skin assumes a pale greenish hue, like the skin of the people in the polar regions. This is probably due to the absence of the blue and ultra-violet rays of the sun which dilate the blood vessels and bring the blood to the surface, and to the absence of the infra-red rays which are associated with heat. Sunshine promotes transpiration and perspiration, and thus favors the elimination of toxic products through the skin. Bright, sunny days lessen the strain on the kidneys. The amount of blood and the per cent. of hemoglobin are increased by sunlight and decreased by darkness. Oerum, in an experiment on animals, found that light increased the quantity of blood 25% in 4 hours. Likewise Grawitz and Graffenberger have observed a diminution in the quantity of blood in animals kept in the dark, while Marti found under similar conditions that the number of red blood corpuscles was lessened (13). There is thus a loss of red coloring matter in the blood of animals kept in the dark. The well-known baker’s anemia points to the same impoverished state of the blood in night workers (Gardenghi, 13). Not only so: in an examination of 29 persons Finsen (7) found less hemoglobin in the winter than in the summer, presumably because of the diminished sunshine in the winter time. Diesing suggests that the lack of sunlight in northern countries is an essential factor in the causation of rachitis. The rays of the sun--specifically the blue and ultra-violet rays--thus exercise a tonic or stimulating action on the organs of circulation, transpiration and elimination, and very probably also directly stimulate the nervous system. Finsen has shown that benumbed or apparently dead insects have been revitalized by the application of ultra-violet rays. It is doubtful whether this stimulating effect is due to increased electrical potential, as argued by Dexter (5), because the more intense the sunshine and the higher the temperature the lower is the potential (14). The stimulating effect is probably due to the chemical action of the short rays of light. These rays probably penetrate the deeper tissues of the body just like the X-rays. It is well known that persons suffering from chronic joint disease, particularly from joints affected with tubercular sinuses, can be greatly benefited or entirely cured by constantly exposing the affected limbs to the direct rays of the sun. Guggenbuhl, it will be recalled, attained some success in the treatment of cretinism simply by removing the cretins from the dark Alpine valley to the sunlit summit of the Abendberg.