BELL. — OPACITY OF GLASSES FOR THE ULTRA-VIOLET. 679 



total energy received by Figure 1, Plate 2, and well exhibits the effect- 

 iveness of the absorption produced by the glasses considered in the 

 whole ultra-violet region. Without going here into the matter of any 

 specific effects due to the action of the ultra-violet rays upon the eye 

 it is interesting to compare the results here obtained with those 

 derived from some of the other protective glasses recently brought 

 to notice. From the very interesting studies of Hallauer ^ it 

 appears that the so-called enixanthos glass shows absorption very 

 similar to that of the Fieuzal, while Hallauer's own glass possesses 

 similar characteristics in the ultra-violet, but like the amber glass 

 here described carries a stronger absorption into the blue. The Gonin 

 rose glass apparently resembles the pinkish glass of Figure 6, Plate 1, 

 while the blue glass is still more transparent to the shorter wave 

 lengths. Hallauer's gray glass apparently corresponds quite closely to 

 the smoke of Figure 3, Plate 1. The commercial red glass tested by 

 Voege^° cut off the ultra-violet completely in a thickness of 2.9 mm. 

 Broadly the red, amber, yellowish and yellowish-green glasses which 

 absorb noticeably in the blue and violet also carry increased absorption 

 through the ultra-violet, while the blue and green glasses which trans- 

 mit somewhat freely in the blue and violet also transmit a perceptible 

 amount of ultra-violet. This latter class of glasses is undesirable for 

 protective use for other reasons than those connected with the ultra- 

 violet, however, since a predominant blue tone in the transmitted 

 light is both of uncomfortably low luminosity and greatly reduces the 

 contrasts in most objects viewed. Of the amber and greenish glasses 

 all will transmit, when of light tint or very thin, a limited amount of 

 the more refrangible rays, including some ultra-violet, as would 

 readily be found by a very prolonged exposure. The ultra-violet 

 radiations transmitted by any of them when of sufficient thickness or 

 density to be of service in softening the intensity of the visible rays is 

 extremely trivial in amount, so small as to be utterly negligible in 

 practice, as the spectrograms given show, and all of them, even the 

 lightest in tint, suppress very thoroughly the only radiations in the 

 ultra-violet that are certainly known to have a specific harmful effect. 

 To sum up this matter of protection against the ultra-violet : All 

 ordinary glasses, even the clear optical glasses, suppress all the ultra- 

 violet radiations certainly known to have a specific harmful effect 

 upon the human eye. As to the rest of the ultra-violet, say from w. 1. 

 300 fifi to the visible rays, there is little clinical evidence of injurious 

 effect, and even this little does not gain in convincing quality from its 



^ Loc. cit. ^^ Loc. cit. 



