Chapter III 



BIOPHYSICAL CHARACTERISTICS OF THE EYE 



t The preceding chapters have discussed the biophysical properties of 

 very high-frequency radiant energy, in the form of x-rays and gamma 

 rays, but man does not possess the necessary sense organs with which 

 to identify such radiations. Comparatively lower frequencies lying in 

 the spectral wavelength band extending from 4000 to 7000 A, however, 

 can be identified by a unique receptor mechanism, the eye, which is 

 sensitive to changes in frequency and energy of this visible radiation. 



The expression " we see " means that we experience a sensation 

 which begins as a photochemical reaction in the retina, provided that 

 the intensity of the radiant energy is adequate and its frequency lies 

 within the limits set by the transmission properties of the ocular media, 

 photosensitivity of the retina, and propagation characteristics of nerve. 



A process such as vision in man represents a complicated series of 

 events. As a form of consciousness and its directive function in behavior, 

 vision must be studied by the methods of psychology. As an activity 

 depending on the anatomical structure of the human eye and its nervous 

 mechanism, vision is a physiological problem. So far as the processes 

 involved rest on the physical responses of the ocular media to radiant 

 energy, they relate to phenomena pertaining to the physical sciences. 



Merging the last two groups of facts into a single group of interre- 

 lated biophysical phenomena permits a better organized approach to 

 the problem of vision. ' 



Optical System of the Eye 



The optical system of the eye consists of those structures which 

 together focus an image of an external object on the retina; they are 

 the cornea, the aqueous humor, the crystalline lens, and the vitreous 

 humor. The media are more or less inhomogeneous, and no satisfac- 

 tory system of spherical refracting surfaces has been adapted that will 

 replace them exactly. Of the many schematic models suggested, Gull- 

 strand's (Table III— 1) seems to possess the most acceptable qualities. 

 Figure III— 1 shows such an eye at rest, with its principal focal plane 

 coinciding with the retina and with the refracting surfaces formed so 

 that parallel rays are brought to a focus on the retina. 



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