CHAPTER VIII 



THE RELATION OF RETINAL STRUCTURE TO 

 VISUAL ACUITY 



Although there are a number of factors involved in the 

 matter of visual acuity, one of these, and an important one, 

 pertains to the size and numbers of the visual elements. 

 We are all aware from an elementary acquaintance with 

 photography that the fineness of detail which a photosensi- 

 tive surface can register depends upon the fineness of the 

 emulsion. If the receiving elements are fine and close 

 together, one obtains much greater detail than when the 

 particles are large and far apart. This condition is exactly 

 what one finds in the retina. A low visual acuity signifies 

 that the average distance between the active retinal ele- 

 ments is large, whereas a high visual acuity means that the 

 distance is small. Since this is true, it is obvious that verte- 

 brate retinas must vary greatly in this respect. Since also the 

 number of rods and cones is fixed structurally in any given 

 region of the eye, it is apparent that the number of elements 

 per unit area must vary functionally in order to mediate the 

 great variation in visual acuity accompanying changes in 

 illumination. This has been shown by a number of investi- 

 gators. 



Working with brightness discrimination Koenig (1894) 

 found that the whole range of intensities visible to the eye 

 is made in 572 discrete steps in intensity recognition. About 

 30 of these are mediated by the rods and the remainder by 

 the cones. Hecht (1928) has critically analyzed Koenig's 

 findings, adding physiological support to these data. 



With very low illumination, vision is a function of the 

 rods. As more and more rods reach their thresholds and 

 become functional, visual acuity increases. With further 



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