THE OPTICAL EFFICIENCY OF TINTED GLASSES IN RELIEVING 



EYE STRAIN 



By D. S. Pratt 



{From the Laboratory of Organic Chemistry, Bureau of Science, 

 Manila, P. I.) 



One plate 



In the Tropics, and during certain seasons of the year in tem- 

 perate regions, the harmful effects of excessive bright sunlight 

 on the eyes have been more or less mitigated by employing tinted 

 glass. At the present time a large variety of colors and tints 

 are supplied in blanks from which plane surfaces or lenses of 

 any formula may be ground. The color or tint used has gen- 

 erally been a question of personal preference and skillful adver- 

 tising, rather than the result of any data regarding the quality 

 and quantity of light transmitted by the glass. 



It is well known that ultra-violet light exerts an extremely 

 harmful influence upon the retina of the eye due to its marked 

 actinic property, and that prolonged exposure to light rich in 

 these short wave lengths will cause blindness. Colorless glass is 

 sufficient to cut off the greater part of the ultra-violet spectrum 

 and give protection in special cases. 



The various portions of the visible spectrum differ markedly in 

 their physiological effect. The longer waves at the red end 

 are more or less converted into heat radiations within the eye 

 and act as an irritant, while those of shorter length comprising 

 the indigo and violet possess greater actinic property and resemble 

 the ultra-violet. The intermediate portion of the spectrum is, 

 therefore, most restful, and ideal protection would be afforded 

 by a glass capable of transmitting sufficient light throughout 

 this portion while at the same time reducing the intensity of 

 the red and eliminating the ultra-violet. 



The different types of optical glass investigated were samples 

 of various tints, carefully polished with approximately parallel 

 plane surfaces. 1 Light from a nickel-iron arc was employed, 

 and a Hilger spectrograph with special Cramer plates used in 



1 These samples were prepared and furnished by Messrs. Clark & Co., 

 Manila, P. I. 



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