788 WALKER. 



light, also Birch-Hirschfeld did the same while working with the 

 Schott-light. However, Vogt ^^^ in 1908 dilated his own pupil and 

 after producing erythropsia by observing a sunlit snow field found no 

 decrease in the red vision when he went into a room illuminated only 

 by a light whose red rays had all been screened out by an Erioviridin 

 filter. Vogt 3" in 1908 and also Wydler ^^2 -^^ 1912 considered ery- 

 thropsia as the red phase of the after picture of the intense white 

 surface. Best^^ in 1909 agreed with Vogt's view and considered 

 erythropsia due to visible rays since he could produce it by looking at a 

 snow surface through a yellow glass cutting out rays below 400 nij. 

 but not with a blue uviol glass. However Birch-Hirschfeld^^ could 

 not consider that ultra violet rays were relieved of responsibility 

 entirely since the wide pupil alone of the aphakic eye could hardly 

 account for erythropsia, as Vogt held, by admitting a greater quantity 

 of light. He therefore concluded that invisible as well as visible rays 

 were active in the etiology of erythropsia. 



Rivers ^^^ in 1901, advanced a theory as to the red color based on 

 an observation made by Briiche in 1851 that the eye under normal 

 conditions is more or less completely adapted for red. Rivers attri- 

 butes the color in erythropsia to the blood in the anterior retinal 

 layers. Schoute ^^* objected to this theory on the basis that Pur- 

 kinje's experiment of the eutopic vision of retinal vessels shows that 

 light is absorbed by the blood and they give dark shadows. 



Protective Glasses. 



In 1900 Schulek ^^° first studied the means of protecting the eyes 

 against ultra violet rays and found that solutions of Triphenylme- 

 thane in xylol and Nitrobenzol in Alcohol had the highest absorptive 

 power of the transparent media examined. These liquids absorbed 

 practically all rays below 396 ju/z. He suggested that these solutions 

 be enclosed in flat oval shaped glass chambers made to fit the eyes 

 and to protect them from injuries due to ultra violet radiations. 



Stearkle^*^, Vogt^^^ and Hallauer ^^^ studied the absorptive prop- 

 erties of blue uviol, yellowish and smoky gray glasses. The last 

 named worker produced a glass mixture by a secret process the so 

 called "Hallauerglas." After a similar study, Fieuzal ^^^ produced 

 in like manner Fieuzelglass, which however, was not greatly used on 

 account of its color. Also a yellow-green glass patented under the 



