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BIOPHYSICAL CHARACTERISTICS OF THE EYE 



cell layers. The inner segments of the cones in the fovea are closely 

 packed into a hexagonal pattern. This close-packed small elliptical 

 depression, whose horizontal and vertical diameters are about 0.3 and 

 0.2 mm respectively, is also the seat of most distinct vision. The cones 



-0.10 mm Adjacent to the choroid 



Fig. Ill— 13. Visual cone and rod cells of man. Cone cells: a, from the ora ser- 

 rata ; b, from fundus outside of macula lutea (yellow spot) ; c, from the margin of the 

 macula lutea; d, from fovea centralis. Rod cell e contains visual purple. 



in this depression are very much longer, as illustrated by d in Fig. 

 Ill— 13. They have the appearance of elongated rods. The number 

 of cones in the fovea is about 4000. The number rods in the human 

 retina totals about 130 million, the cones 7 million. 



Visual Acuity 



Acuity of vision is the ability to perceive the interspace between two 

 very close objects. It is defined as the reciprocal of the just resolvable 

 visual angle measured in minutes of arc. For the purpose of measure- 

 ment a test object, in the form of an equally spaced ruled grating, a 

 pair of parallel bars, or an incomplete circle or C-shaped figure, is 

 employed. 



Experiments show that the interspace between two parallel bars 

 must be increased as their distance from the eye is increased, and that 



