50 Light and the Eye \1 : 4 



a sensation of violet when viewing ultraviolet. Persons with a lens do 

 not receive any appreciable energy at the retina at wavelengths shorter 

 than about 3,800 A. Thus, the lens (and cornea) limit the photons 

 reaching the retina to wavelengths greater than 3,800 A. 



On the long wavelength side, the water molecules in the cornea and 

 aqueous humor eventually absorb most of the energy at wavelengths 

 longer than 12,000 A. However, the eye pigments become very 

 insensitive to light above 7,000 A, and are almost unresponsive above 

 8,000 A. Technically, to find the long wavelength limit, one should go 

 to such high intensities that the eye is heated but not badly burned ; this 

 experiment is rarely performed. 



Thus, the filter action of the lens and cornea, plus the response 

 characteristic of the optically active pigments in the photoreceptors 

 tend to restrict the wavelength band, thereby reducing chromatic 

 aberration. In addition, the greatest acuity occurs in photopic vision 

 at the fovea. In this region, there are only cones which probably do 

 not respond to blue light. In this region also is a yellow pigment 

 believed by many to further eliminate the blue end of the spectrum. 

 Accordingly, the acuity at the fovea is greatest not only for objects 

 viewed with monochromatic green light, but also for those seen in 

 white light. 



REFERENCES 



1. Stuhlman, Otto, Jr., Introduction to Biophysics (New York: John Wiley & 

 Sons, Inc., 1943). 



2. Stevens, S. S., ed., Handbook of Experimental Psychology (New York: 

 John Wiley & Sons, Inc., 1951). 



a. Judd, D. B., "Basic Correlates of the Visual Stimulus," pp. 811-867. 



b. Graham, C. H., "Visual Perception," pp. 868-920. 



3. Glasser, Otto, ed., Medical Physics (Chicago, Illinois: Year Book Publishers, 

 Inc., 1944) Vol. 1. 



a. Luckiesh, Matthew, and F. K. Moss, "Light, Vision, and Seeing," 

 pp. 672-684. 



b. Sheard, Charles, "Optics: Ophthalmic, With Applications to Physio- 

 logic Optics," pp. 830-869. 



For a more thorough discussion of optics at an intermediate physics level, 

 see: 



4. Robertson, J. K., Introduction to Physical Optics. 2nd ed. (New York: 

 D. Van Nostrand, 1935). 



