EFFECTS OF RADIANT ENERGY ON THE EYE. 747 



the trade name of "Enixanthos" and various approximations to and 

 imitations of these have from time to time appeared. A paper by 

 Hallauer ^^^ gives the result of a rather thorough spectrographic study 

 of all the protective glasses then in common use to which those inter- 

 ested in the subject may be referred. The composition of most of 

 these glasses is held as an unnecessarily solemn secret, but it is gener- 

 ally understood that they are essentially iron-lead glasses. They 

 run in color from a distinctly yellowish to a somewhat bluish green, 

 varying in tone and transmissibility according to the composition 

 of the various shades put on the market. An ordinary green glass 

 bottle gives in the spectrograph about the same absorption as the 

 medium densities of any of the glasses referred to. The deeper shades 

 of any of them cut off the spectrum completely just about at the 

 beginning of the ultra violet and weaken it well into the violet. The 

 lighter shades transmit to wave lengths 360 ^^u to 320 ix^x with cor- 

 respondingly less reduction in the general intensity. An^^ and all of 

 them are completely effective against the abiotic radiations, even 

 although the medium shades sometimes transmit very weakly in the 

 region 3200 jjlix to 3400 mm as well as in the visible spectrum. To render 

 the record of these recent protective glasses fairly complete there is 

 shown in Plate 7 the iron arc spectrum and its transmission through 

 the three ordinary grades of the Hygat glass of Rodenstock, an excellent 

 type of this general group. Figure 6 is here the iron arc spectrum, 

 5, 4, and 3, respectively the light, medium and dark Hygat glasses, 

 and Figure 1 a special protective glass of American manufacture 

 designed specifically to reduce the red end of the spectrum beside 

 cutting out the ultra violet and much of the violet and blue. In point 

 of effectiveness in cutting out the ultra violet the differences between 

 these various glasses of approximately the same shade are inconse- 

 quential and the choice between them lies mainly in the matter of 

 taste as regards their particular color and absorption in the visible 

 part of the spectrum. Of the recent glasses exploited in America the 

 so called Noviol glass is remarkable for its extraordinarily sharp cut 

 off of the spectrum in the blue. 



Voege^^^ in answering the question as to what spectral range of 

 radiation gave the most satisfactory results held that the light from 

 the clouds of a clear sky, being that light to which the eye through 

 evolution had become adapted, was on the whole to be preferred. 

 This would indicate the use of neutral glasses without selective absorp- 

 tion. Hertel and Henker ^^^ found that clouds and clear sky con- 

 tain very little energy below 310 mm and considered that the onlv 



