INFRA-RED TRANSMISSION 



119 



wavelengths, as seen in Fig. 111-10. Since the optical system of the 

 eye may be reduced, according to Luckiesh [1921], to an equivalent 

 layer of water 2.28 cm thick, it is possible to predict the transmission of 

 the optical system by analyzing the infra-red transmission curve of 

 water of this thickness. The justification for this water equivalent lies 

 in the fact that the cornea is composed of about 90 per cent water. The 

 cortex of the lens is about 92 per cent water, falling to 84 per cent in 

 the center. It will be noted in Fig. III-10 that the per cent transmission 



o 



of water falls off rapidly, reaching a very low value for 10,000 A and 

 dropping to opacity at 14,000 A. 



-100 



80 



-60 



40 



20 



-Visibl 



2.28 cm 



Near infra-red- 



0.06 cm water 





I \ 

 I \ 



A 



_L 



1^ 



/ V 



_L 



4,000 8,000 12,000 16,000 



Wavelengths in A 



20,000 24,000 



Fig. Ill— 10. The spectral transmission of water in the near infra-red. Retina 



o o 



is sensitive to about 8350 A. Radiant energy, however, is transmitted up to 12,000 A. 

 Retina does not respond to region between 8000 and 12,000 A despite the high trans- 

 missivity of the medium. Values of the transmission of water at various thicknesses 

 were obtained from Aschkinass, Ann. Physik, 55, 401 (1895). Values for 2.28 cm 

 reproduced through the courtesy of M. Luckiesh [1921]. 



The visible limit at the red end of the spectrum under the most 



o 



favorable conditions has been found to be near 8350 A; under ordinary 

 circumstances it is difficult to go beyond 8000 A, although some ex- 



o o 



perimenters place this limit at 7600 A. At 10,000 A the media of the 

 eye are about 40 per cent transparent; although the transparency rises 

 to 65 per cent at about 11,000 A, it rapidly drops to opacity near 12,000 



o o 



A. Thus those rays having wavelengths greater than 8000 A do not 

 excite a response in the retinal structure. 



The question is often raised as to the efficiency of protection by eye- 

 glasses in the near infra-red. The spectral transmission of glass decreases 

 quite rapidly beyond 30,000 A, but glasses are fairly transparent in the 



