258 OBSERVATIONS ON VISION. 



We will distin<i:uish these rods as ''eoiitnxl "' and ''bright-vision " 

 rods to distinguish them from '' peripheral " or " obscure-vision '' 

 rods. In order to prove their existence there is necessity for physi- 

 ological experiments of a kind suited to distinguish the differences 

 that occur between normal and color-blind persons in direct central 

 vision of fields of different size. The Purkinje phenomenon and the 

 displacement of the " neutral " zone with diminishing light intensity 

 must be more accurately studied with different individuals in order 

 to recognize how far the rods extend toward the fovea centralis in 

 normal eves, and Avhere the " central " rods of the color-blind begin 

 to be capable of adaptation and become '' peripheral." 



Perhaps this hypothesis might also be tested by anatomical re- 

 searches on the retinas of color-blind persons, although it would be 

 difficult to distinguish the '' central " rods from the cones, for it 

 appears that the cones approach more nearly to the rods in structure 

 the nearer they lie to the center. Besides this the anatomist must be 

 informed whether he is examining normal or color-blind eyes. 



AVhen I found that this (possibly not original) hypothesis had 

 not as yet been carefully tested, I thought it worth publishing even 

 if hereafter found to be false, since the careful experiments which 

 its testing demands can not l^ut throw light on this interesting 

 subject. There appears to be no logical or zoological argument 

 opposed to it, for since there are aninuils with more cones than 

 rods (birds) and animals and men with rods only (animals living 

 in darkness and the totally color-blind), and since persons of nor- 

 mal vision lie between these classes and possess more rods than 

 cones and one spot entirely rod-free, why may there not be indi- 

 viduals having rods in this small spot, too? And if this is the 

 case, then it is not strange that these central rods by continued 

 usurpation of the ]3lace of the cones should have become bright- 

 vision organs, and have lost their adaptive quality. The fact of 

 the great number of color-l)lind persons (3 per cent of males and 

 one-fourth of 1 per cent of females) supports the hypothesis, for 

 it readily explains the great variations between individuals. For, 

 according to the number of " central " rods, the degree of their 

 sensibility, the extent of the area over which they have lost adapta- 

 bility, and the degree to which they have supplanted the cones, by 

 such gradations may normal color perception degenerate into the 

 various degrees of color confusion, or even into total color-blindness. 



