256 OBSERVATIONS ON VISION. 



generally meant that they confiiHo colors rather than are aetually 

 blind to any, and indeed they confuse ])iginents only, while pure 

 spectral coloi-s are distinguished by them almost as \yell as by persons 

 of normal yision. 



Similar as the two systems may be in many respects, there are yet 

 several noteworthy differences, which ai^jjear best in the observation 

 of a pure, lirilliantly colored, and strongly lighted spectrum. 



(1) AMiile noi-mal eyes distinguish red, yellow, green, blue, and 

 violet, to the color-blind the lilne-green zone appears Avithout color, 

 merely having a gray white luminosity C"' neutral " zone of the 

 color-blind). 



(2)- To color-blind persons the red end of the spectrum is shortened 

 as compared with the blue. 



i'^) To color-blind persons a mixture of red and blue produces 

 white, Ayhereas such a mixture is rose-red to normal eyes. 



The yellow and blue j)arts of the spt'ctrum appear as yividly to 

 color-blind persons as to those of normal vision, and so also do yellow 

 and blue pigments. From this it follows that the color vision of the 

 color-blind is effected 1)V the cones like the color vision of normal 

 eyes. In view of the fact that we do not know what are the color 

 differentiating organs of the cones it is doubly difficidt to assign 

 causes few these various differences in color vision. Do these differ- 

 ences perhaps depend on a different structure of the cones? 



Struck by the coincidence of the '' neutral " zone with the color of 

 maximum sensitiveness for rod yision. the thought forced itself to 

 my attention that the peculiarities of color-blindness might be ex- 

 plained by supposing that the retina of a color-blind person in the 

 region of direct yision where normal eyes have only cones contains 

 rods as well. 



Without ascertaining whether this idea is new or old, T present the 

 following experiment to jH-oye that the peculiarities of color-blindness 

 are found in i)eri]^heral yision with normal eyes when care is taken to 

 perceiye them : 



Probably few persons have ever carefully examined a bright si)ec- 

 trum under oblique vision. I'pon thus obserying a long stretch of the 

 spectrum there is seen, in fact, as T had exi)ecte(l, a colorless brilliant 

 Avhite " neutral " zone lying in the place of the blue-green. Futher- 

 niore, the red end of the sjxH'trum is shoi-(en((l. and oidy i-eddish -yel- 

 low and blue ai)pear as yivid colors. 



r»ut a further cousecjucnce follows from the hypothesis thai color- 

 blind persons lunc both I'ods and cones all oyer the retina, includ- 

 ing the fo\-ea centralis. I^'or if this is tlu" reason why the color-blind 

 see a mixtui-e of red and blue as white, normal eyes ought to receive 

 the same injpression with oblicpie yision. Such is indeed the case 



