746 VERHOEFF AND BELL. 



cut down the light received by patients whose eyes were for one reason 

 or another sensitive. This requirement resulted in the production 

 of glasses of more or less dark neutral tints and sometimes dark shades 

 of colored glasses commercially obtainable like green and blue. A 

 later phase of practice reflected the view common a quarter century 

 ago that red light was a thing producing in some unknown way spe- 

 cially bad effects and consequently was to V)e shunned. Hence it was 

 not uncommon to prescribe glasses of cobalt blue, green, and various 

 amethyst tints. It is interesting to note that such glasses, while they 

 reduce greatly the red of the visible spectrum still transmit quite 

 freely the nearer part of the infra red which carries a large amount 

 of energy (Coblentz ^^). In fact, of this nearer infra red such glasses 

 transmit almost as much as does ruby glass. Therefore, the earlier 

 protective glasses were not effective in cutting off heat radiation and 

 tended to transmit mainly light toward the blue end of the spectrum. 

 Perhaps the chief benefit of the agitation that has taken place within 

 the last decade on the possible, though as we have shown highly 

 improbable dangers of the ultra violet, has been the bringing into 

 prominence the new types of protective glasses. 



These, intended primarily for eliminating the ultra violet rays, 

 have tended to types of selective absorption which give advantageous 

 results in modifying the visible light, which is really the chief object 

 of concern to the ophthalmologist. There is a close kinship in the 

 absorption of most of this recent crop of glasses. The prototype 

 runs back nearly tw^enty years to the work of Fieuzal ^^^ who produced, 

 specially with a view to protecting the eye against glare in the high 

 mountains, a grayish green glass which cuts off the ultra violet very 

 completely and shades down the blue so as considerably to shorten 

 the spectral range of the rays transmitted. As one of us has already 

 shown ^^ the last line transmitted by this glass from the spectrum 

 of the quartz arc is 404.6 jjlh very faintly. Its absorption much 

 resembles (loc. cit.) that of ordinary amber glass, except that the latter 

 carries somewhat heavier absorption into the blue green accounting 

 for its yellowish rather than greenish tinge. Either of these glasses 

 is substantially as efficient as any of more recent origin in cutting 

 out the ultra violet. In more recent times Hallauer^^^ and Schanz 

 and Stockhausen ^°^, have discussed at length glasses protective against 

 the ultra violet and have brought out special protective glasses with 

 this point in view% the former known by the name of the inventor, 

 the latter under the trade designation " Euphos." At about the same 

 period the firm of Rodenstock produced a glass of similar type under 



