Abstract
In a communication presented a year ago we showed that rickets can be prevented in rats by daily exposures to direct sunlight for about fifteen minutes 1 . A similar result was reported at the same time by others 2 . When rats were placed in a box having flint glass windows it was found that the sun's rays, in traversing the glass, had lost their protective power. In a later communication it was shown that the pigment of the skin also hinders the action of the effective rays, that black rats require more radiation than do white rats 3 .
These experiments have been extended to ascertain more nearly the wave lengths which exert the protective action. For this purpose glass filters have been used. These filters are manufactured by the Corning Glass Works and have been tested by this establishment and by the United States Bureau of Standards, both in regard to the wave lengths which they transmit, and their percentage of transmission 4 . In this work white rats have been used which were fed the standard rickets-producing dietary (No. 84). Previous experiments have shown that rats on this dietary can be protected against rickets by daily irradiation for two minutes or less, by the mercury vapor quartz lamp at a distance of three feet with a voltage of 76 (a unit dose). The method of procedure was to interpose the various filters and to ascertain to what extent they altered the protective action of the light at exposures of varying intensity. The animals were radiographed after an interval of about 21 days, and killed after 28 days. The interpretation of rickets was based on a microscopic examination of the epiphyses.
The accompanying chart illustrates a series of experiments with various filters.
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