The Antirachitic Effect of December Skylight and of December Sunlight Through Vitaglass.

In the preceding publication, 1 results were reported which indicate that December sunlight in the latitude of the City of Toronto possesses a definite antirachitic effect. Although the rats were exposed for 2 hours daily, they received only a daily average of 36 minutes of sunshine, due to the cloudy weather. It has been noted by Dorno 2 and Hill 3 that skylight contains ultraviolet rays, as well as direct sunlight. This raises the question as to how much of our antirachitic effect was due to the 36 minutes exposure to sunshine, and how much to the remaining 84 minutes exposure to skylight. Accordingly rats on McCollum's rickets producing diet No. 3143 1 were placed outdoors for 2 hours daily from 11 a. m. to 1 p. m., exposed to skylight, but protected from direct sunlight. After varying periods ranging from 1 week to 4 weeks, 2 rats at a time were killed, roentgenograms taken of the bones, the blood phosphorus determined, and the per cent ash in the bones estimated. The results which are given in Table I, indicate that December skylight possesses a definite antirachitic effect, which effect is almost as marked as that obtained by exposure to the available sunlight.

A clear glass which the manufacturer claims will transmit the shortest ultraviolet rays of sunlight (290 millimicrons) can now be obtained under the trade name of “Vitaglass”. We found that rays as short as 262 millimicrons, the source being an iron arc in air, will pass through this glass. To get some idea of its value from the clinical standpoint, we constructed from vitaglass, a glass box to fit over and around one of the cages.