Abstract
Modern ultraviolet therapy has an interesting historical background. Following the observations of Huldschinsky, who in 1919 cured rickets in children by means of rays from the mercury quartz lamp, advancements have been rapid. It was shown in turn by various investigators that irradiation would produce in foods an antirachitic effect, that this induced property was destroyed by excessive irradiation, that the activatable substances in foods were ergosterol and a closely related compound associated with cholesterol and, to some extent, probably other sterols. A particularly valuable tool in the development of this knowledge has been the spectroscope. Considerable information has been accumulated about the spectroscopic properties of vitamin D and related compounds. It is well known that every chemical compound has a characteristic absorption spectrum. Heilbron and his collaborators<sup>1</sup>reported that the ultraviolet absorption spectrum of cholesterol has only a general absorption, which means that it has no bands of
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