Abstract
It was first shown by Huldschinsky 1 that rays from the mercury vapor quartz lamp produced a definite antirachitic effect on rachitic infants. Hess and Unger 2 demonstrated that the sun's rays also prevented infantile rickets. This beneficial effect of the sun was confirmed with carefully controlled animal experiments by Powers, Park, Shipley, McCollum and Simmonds. 3 Hess and Weinstock 4 found that the rays which exerted this curative effect were shorter than 313 millimicrons. This means that the effective solar rays range only from 313 to 290 millimicrons in length, and constitute a very small percentage of the total solar radiation. As these very short rays are readily absorbed by smoke, dust and moisture in the atmosphere, the ultraviolet content of the sun's rays is markedly reduced in the winter months, when the sun is low in the sky, and the rays consequently have to pass through a greater distance of our atmosphere. 5 , 6 It is, therefore, reasonable to expect that the antirachitic effect of sunlight will be correspondingly reduced. This raises the question as to whether this reduction may not be so great as to practically eliminate any beneficial effect during the winter months.
The results here reported are some of those obtained in an experiment being conducted by this Department with the cooperation of the Department of Physics, and the Department of Physiological Hygiene of the University of Toronto, in an attempt to determine the variations in the ultraviolet content of the antirachitic effect of the sun's rays throughout the year. The experiment was commenced in September, 1926. Each week a series of young rats are placed on a rickets-producing diet and exposed daily to the direct rays of the sun.
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