The belief that sunlight is indispensable to human welfare has become strongly entrenched in modern hygienic propaganda. The scientific study of the possible basis for such views has lagged far behind the claims that have been advanced for the beneficent effects of light. Fresh air and sunshine are advocated the world over as desiderata in the campaign against disease; yet those who proclaim their virtues might often find it difficult to explain precisely how these climatic factors affect the human organism. Bacteriology has demonstrated the action of light in destroying various micro-organisms. In specific cases the effects of light rays of different sorts on the skin and the functions of the body as a whole are admitted; but at best the phenomena of heliotherapy are inadequately understood, and the influence of light is far from being clearly demonstrated in relation to the ordinary physiologic manifestations.
The marked contrast between the
THE COMPOSITION OF THE BLOOD IN SUNLIGHT AND DARKNESS. JAMA. 1919;73(19):1446. doi:10.1001/jama.1919.02610450042017
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