44 EVOLUTION OF THE HUMAN EYE 



structure and connections, arc obviously not secreting cells, 

 but nerve end-organs. 



Numerous careful and accurate observers have recorded 

 how the visual purple is only to be found in the outer limbs 

 of the rods and not in the cones. I have myself been able 

 to confirm its absence from the cones in the retinae of frogs 

 and birds. 



If, then, the rods react to the stimulus of light through 

 the photo-chemical reaction of the visual purple, some other 

 explanation has to be found for the reaction of the cones. 

 A perception of light dependent entirely on changes in the 

 visual purple would become abolished when it was bleached 

 in bright light, and an animal with only rods in its retina 

 would be blind under such circumstances. 



The theory suggested independently by Max Schultze, 16 

 Parinaud and v. Ivries, 16 which assigns different functions 

 to the rods and cones, the so-called duplicity theory of 

 vision, helps to explain so many of the visual phenomena 

 met with in the human eye, both in health and disease, that 

 it may well be accepted as a working hypothesis for the 

 explanation of vision in other Vertebrata. 



According to this theory the rods come into action in low 

 degrees of illumination. They can only distinguish different 

 degrees of light, not form or colors. The cones, on the other 

 hand, conje into action in high degrees of illumination, are 

 capable of distinguishing the spectral colors, and of per- 

 ceiving clearly the details of form. As Parsons 17 very aptly 

 says: "Broadly speaking, vision with the dark-adapted eye, 

 i. e., scotopic vision, is monochromatic or tone-free. Vision 

 with the light-adapted eye, i. e., photopic vision, is poly- 

 chromatic or toned. In the former the threshold stimulus 

 intensity is low; in the latter relatively high." 



Prisoners who have been confined in dark dungeons for a 



