There are many biological effects of light, which pose a variety of problems and entail a variety of methods of approach; however, there are certain common principles involved in all. This discussion is confined largely to the effects of sunlight on the skin. Wave-length Variable Apart from certain rare diseases (in which other parts of the spectrum are concerned) and experimental use of extraneous photosensitizers, the principal effects of sunlight on human skin are produced by wave lengths shorter than about 0.32 μ (0.32 μ = 320 mμ = 3,200 A). There are certain important diseases of the skin in domestic animals that are produced by other wave lengths, but, for physiological or dietary reasons, these do not have their counterpart in human skin. 1 Wave lengths shorter than 0.32 μ, which comprise only a tiny fraction of sunlight (i. e. 0.01%), produce the various responses of sunburn, antirachitic action, and cutaneous