viii, a, 3 Pratt: Tinted Glasses 195 



from light of wave lengths below 3,000. The three shades of 

 blue show practically the same limits, but in addition cause a 

 reduction in the intensity of the red. This effect is so slight that 

 these tints possess little to recommend their use. The four sam- 

 ples of amber differ greatly in the light transmitted. The light 

 shade designated as No. 2 removes less ultra-violet than the blue, 

 while the darker tint represented by No. 6 cuts off all the blue 

 and indigo. This gives excellent protection from the short wave 

 lengths, but fails to moderate the remainder of the visible spec- 

 trum to any great extent, and results in sharp contrasts that are 

 not restful. Amber No. 4 is to be recommended when sharp 

 definition is desired with the greatest protection possible under 

 the conditions. In the great majority of instances this is not the 

 case, and a glass combining the good features of the amber with 

 a reduction of intensity throughout the visible spectrum is more 

 to be desired. 



The glass designated as shade No. 1, London smoke, shows a 

 general reduction in all regions of the visible spectrum with the 

 red eliminated to a slightly greater extent than the green and 

 blue. The amount of ultra-violet cut off is not greatly in 

 excess of that caused by clear glass, nor is the limit shifted to 

 much extent by the darker tints, although the effect on the visible 

 spectrum is progressively emphasized. The shade known as 

 No. 6, London smoke, affords the most complete protection, but 

 would hardly transmit sufficient light under ordinary circum- 

 stances. No. 4, London smoke, is probably the most satisfactory 

 of all the glasses examined. It does not distort the color balance 

 to which the eye is accustomed, as does the amber, but causes 

 more the effect of natural shade and is consequently very restful. 

 The larger proportion of short wave lengths transmitted by the 

 smoke tints is of less importance than the general and correctly 

 proportioned reduction in the visible spectrum. 



The two shades of amethyst resemble the blue in their limits 

 of transmission, and give selective absorption of so slight a char- 

 acter that they cannot be considered as supplying satisfactory 

 protection. 



It is hoped that this brief description of the optical behavior 

 of various tinted glasses available for eye protection may render 

 some assistance in choosing the most efficient color and shade 

 for the purpose desired. 



