EFFECTS OF RADIANT ENERGY ON THE EYE. 789 



name of Enixanthosglas was offered as well as a variety of the modi- 

 fications by glass manufacturers. Here again the color was not satis- 

 factory. 



Schanz and Stockhausen^°^, after finding that electric ophthalmia 

 could be produced through 18 mm. of common glass as has been men- 

 tioned, began to study glass manufacture in the hope of producing a 

 colorless glass of high ultra violet absorption power for general use. 

 In 1909 they produced and patented a glass of higher absorptive power 

 than hard flint and called it "Euphosglas." At first it was made in 

 grades 1, 2, 3, and 4, but recently other grades have been added. It 

 has a light yellowish green tinge and fluoresces in ultra violet light. 



Birch-Hirschfeld^^ in 1909 studied photometrically the absorption 

 power of these and other glasses with considerable accuracy. In the 

 same year Vogt ^°* compared a new and very hard flint glass produced 

 by Schott with his absorptive solutions and was surprised to find that 

 it had about the same efficiency, beginning to absorb at 405 jUju and 

 giving practically complete absorption below 396 mm- 



In 1909 Hallauer^^^ measured photometrically the absorptive power 

 of the various protective glasses available at that time. The thick- 

 ness varied from 1 to 3 mm. and exposure time of one minute. The 

 average results follow. 



Euphos gray glass " " 390 



Euphos green glass " " 390 

 Hallauer glass ^64 " " 420 



As glasses became available to cut out various wave lengths the 

 question arose as to what spectral range constituted the best illumina- 

 tion. 



Voege's^^^ answer to this question in 1908 raised a considerable 

 controversy. He maintained that the light from the clouds or clear 

 sky had been for ages the normal illumination for the eyes, but never- 

 theless contained a considerable amount of ultra violet light as low as 



