25f) OBSERVATIONS ON" VISION. 



ffciieniUv iiicaiit that they confute cohn-s rather than aro actually 

 l)lind to any. and indeed they confuse pigments only, while pure 

 spectral colors are distinouish(>d by them almost as well as by persons 

 of normal vision. 



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

 several noteworthy differences, which appear best in the observation 

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



(1) A^liile normal eyes distinguish red, yellow, green, blue, and 

 violet, to the color-blind the l)lue-green zone appears Avithout color, 

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

 coloi'-blind). 



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

 as compared with the l)lue. 



(3) To color-blind })ersons a mixture of red and blue produces 

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



The yellow and blue ])arts of the spectrum appeal- as vividly to 

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

 and blue ]Mgments. P'rom this it follows that the color vision of the 

 color-blind is effected by 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 c<mes it is doubly difficult to assign 

 causes for 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 vision, the thought forced itself to 

 my attention that the peculiai'ities of color-blindness might be ex- 

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

 region of direct vision where normal eyes have only cones contains 

 rods as well. 



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

 following exi)eriment to ]n"ov(^ that the peculiarities of color-blindness 

 are found in i)eri])heral vision with noi-mal eyes when care is taken to 

 perceive them : 



Probably few ])ers()ns have ever carefully examined a bi'ight spec- 

 trum under obli(|ue vision. Fpon thus observing a long stretch of the 

 sjH'ctrum thei-e is seen, in fad, as I had expected, a colorless brilliant 

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

 more, the red end of the spectrum is shortened, and only reddish yel- 

 low and blue a[)pear as vivid colors. 



Hut a further consequence follows from the hypothesis that color- 

 l)liiid peisons have both rods and cones all over the retina, includ- 

 ing the fovea centralis. For if this is the reason why the color-blind 

 see a mixture of red and blue as white, normal eyes ought to receive 

 the same impression with oblicpie vision. Such is indeed the case 



