EFFECTS OF RADIANT ENERGY ON THE EYE. 791 



cloud light and entirely within the spectrum obtained from ordinary 

 Welsbach gas mantle lights with ordinary clear glass shades. They 

 used a variety of lights including flaming arc and mercury vapor arc 

 lights. 



The absorptive power of various other glasses proposed for this 

 purpose, by their inventors was determined by use of the most ac- 

 curate available method. In the visible part of the spectrum the 

 absorpti^•e power was determined by use of a polarizing spectrophoto- 

 meter with crossed Nicol's prisms, while in the ultra violet region the 

 same instrument was used with optical parts of quartz glass. The 

 results are tabulated below in terms of percentage of penetrability of 

 various wave lengths. The absorption power is obtained by simply 

 subtracting these results from 100. In all these cases a thickness of 

 glass of 1 mm. was used indicated by A', except in the case of the 

 very dense Neutralglas which was measured in thicknesses of 0.1 mm. 

 indicated by A*''^ 



Concerning protective glasses for the eyes, Hei'tel and Henker 

 believe the same criterion should be followed, namely, that the best 

 glass is the one which will reduce the spectrum of the particular light 

 to which the eyes are exposed to the closest possible approximation 

 to the spectrum of cloud and sky light. For observation of the 

 strongest arc lights at close range, the condition under which certain 

 workmen are placed, they consider the Neutralglas F 3815, of Schott to 

 be the best. With this glass, in layers thinner than any other glass, 

 one may observe directly the bare 20 amp. arc light at 50 cm. without 

 injury since the spectrum is about the same as that of cloud light 

 minus the ultra violet portion. The thickness of Hallauerglas No. 64 

 necessary to give the same results as Neutralglas of 0.8 mm., was 9.0 

 mm. ; and of Euphosglas No. 4 was 38 mm. 



Next to Neutral glas, for this purpose stood the smoky or Rauch- 

 glas No. 276 and Sonnenglas No. 66 of the Fredener glass works. 

 After these came Hallauerglas No. 66, while Hallauerglas No. 62 and 

 No. 64 and Euphosglas Nos. 1, 2, 3 and 4, were not strong enough in 

 absorptive powei*. 



Schanz and Stockhausen^^^ at once criticised the above work, 

 objecting particularly to the premises on which the decisions were 

 based, namely that skylight or cloud light can be taken as the ideal 

 light. Against this view was cited particularly the work of Hand- 

 mann showing that a very large group of cataracts begin in the quad- 

 rant of the lens most exposed to the light of the sky during life. Since 

 it was definitely known and admitted by all that certain injuries to 



