Study of Light Waves 



15 



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 test- 

 ed by this establishment and by the United States Bureau of 

 Standards, both in regard to the wave lengths which {hey 

 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 ani- 

 mals 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. The first (G38H) did not transmit waves 

 shorter than 475«*. It will be noted that protection was not af- 

 forded, although irradiation was carried out for 60 minutes at 

 a distance of only 9 inches (480 units; the protective dose being 

 about 2 minutes at 36 inches). Window glass which transmitted 

 rays as short as 334^ also obstructed the protective rays. Filter 

 G586A allowed the passage of a very small percentage of rays as 

 short as 313:^, and a very small percentage of 302^ rays. With 

 •this filter protection was afforded when long exposures were re- 

 sorted to. Pyrex glass which transmits a much larger percent- 

 age of rays of 313:^, as well as shorter rays, interfered but 

 slightly with the action of the mercury vapor lamp. 



From these experiments we may conclude that rays as long 

 as 334/A/>t have little or no protective action in rickets and that 

 the effective rays begin in the neighborhood of 310/^. The de- 

 grees of transmission of the various wave lengths will be dis- 

 cussed in detail in the full report of this work. 



The experiment with filter G86B is of special interest. This 

 filter transmits short rays of about the same wave lengths and 



4 Technologic Papers of the Bureau of Standards, No. 119 and 148, also 

 Scientific Paper No. 32o. 



