CHAPTER III 



THE STRUCTURE OF VISUAL CELLS 



In 1866 Max Schultze made the fundamental discovery 

 that the vertebrate retina, and particularly that of man, 

 possesses two types of visual elements, namely, rods and 

 cones; and that these elements subserve different visual 

 functions. Further elaboration of his findings were made by 

 Parinaud and by von Kries, and led respectively to the so- 

 called Theory of the Double Retina of Parinaud and the 



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Fig. 29. Distribution of rod and cone thresholds, according to Hecht, 1928, 

 Arch. Ophthal., v. 57. 



Duplicity Theory of von Kries. ^ The duplicity theory of 

 vision, as we understand it, implies that the rods are con- 

 cerned with colorless vision at low intensities of illumination 

 (scotopic vision) and that the cones constitute the apparatus 

 concerned with vision at high intensities of illumination and 

 with color (photopic vision). The cones are not necessarily 

 assumed to be utterly useless at night, but are relatively 

 so because the majority of them possess thresholds which are 

 much higher than the highest thresholds of the rods (Fig- 

 ure 29). Conversely the rods, which have low thresholds, 



1 Also referred to as the " Duplexity Theory," v. Crozier and Wolf. Proc. Nat. 

 Acad. Sci., v. 24, p. 538, 1938. 



37 



