118 BIOPHYSICAL CHARACTERISTICS OF THE EYE 



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The crystalline lens is opaque to wavelengths below 3200 A, although 

 young lenses have shown transmissions from 3150 to 3300 A. With 

 increased age there is some absorption in the spectral region extending 



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from 4000 to 4200 A. The lens absorbs powerfully the ultraviolet 

 radiations lying between 3000 and 3800 A roughly; it also fluoresces 

 when this group of rays strikes it. The absorbed radiation is re-emitted 

 as scattered light of longer wavelengths, and is therefore useless for 

 image formation. This scattered light confuses the vision and should 

 be externally absorbed, before entering the cornea, by means of a sheet 

 of Crookes A glass. 



The vitreous humor, since it is composed chiefly of water with a slight 

 addition of salts, is comparable to a layer of water 1.46 cm thick, and 



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like water it is transparent to short violet light around 2300 A. It has, 



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however, an absorption band reaching from 2500 to 2800 A. But 



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2800 A is not transmitted by the cornea or by the lens; hence, the mini- 



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mum wavelength reaching the retina must be 3500 A, or longer. 



The combined tissues of the " normal eye " probably do not transmit 



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violet light below 4000 A, a good practical dividing line between the 

 visual spectrum and the ultraviolet. 



In the development of artificial sunlight illuminants, Luckiesh [1930] 

 has shown that, when the outer membrane of eyes was exposed to 

 moderate intensities of illumination, even though wavelengths as short 



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as 2800 A were present in abundance, no conjunctivitis was developed. 

 For instance, reading 3 hours from a book illuminated to an intensity 

 of 300 ft-candles caused no inflammation of the conjunctiva. Intense 

 direct ultraviolet radiations, however, will produce conjunctivitis. 



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Therefore it seems that 3100 A is a safe lower wavelength limit for inclu- 

 sion in artificial sunlight, as used for general lighting. A 1-mm soda 

 glass screen is ample protection against the inflammatory radiations 

 emitted by lighting devices. 



Infra-Red Transmission 



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The near-infra-red region extends from about 7000 to 14,000 A. On a 

 clear day with the sun at zenith and normal atmospheric pressure an 

 intensity of 8540 ft-candles can be recorded. Of this intensity 44 per 

 cent lies between 4000 and 7600 A and 36 per cent in the short-wave infra- 

 red region. (See chapter on absorption.) The most efficient artificial 

 producers of this radiation are high-temperature solids, as, for instance, 

 the metal filaments in incandescent high-wattage tungsten-filament 

 lamps. 



Visible radiation will penetrate great depths of water, but the trans- 



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mission factor of water falls off rapidly from 7600 A towards the longer 



