570 STUDIES IN ADVANCED PHYSIOLOGY. 



to them repeatedly they are sometimes enabled to recognize 

 Ihe red by a slightly different appearance which it shows to 

 them in their single color. 



Monochromatic color-blindness is not very general. Rel- 

 atively few cases have been scientifically noted. The ex- 

 planation of the phenomena of monochromatic eyes will be 

 explained in terms of each theory further on. 



Dichromatics . Dichromatic eyes see but two colors. 

 These are blue and yellow. Of course black is not counted 

 as a color in any of the eyes, black being simply the absence 

 of light, and in this sense it would be correct to say that a 

 totally blind individual could still see black. These dichro- 

 matic eyes see the blue about as we do, but instead of green, 

 yellow and red, they see but yellow alone. 



1. Green- Blindness. Two distinct varieties of dichro- 

 matic eyes occur. The varieties are so marked off, that 

 gradations from one into the other do not seem to exist. 

 One variety of dichromatic eyes sees yellow where we see 

 red, but sees no definite tint where we see green. Such per- 

 sons are called green-blind. They seem to their friends to 

 be perfectly able to recognize the red, but the red is to them 

 really an intense yellow, which they have learned to call red 

 because it has always been pointed out to them as red, 

 while green they are not able to recognize except as a paler 

 yellow, and this inability to readily pick out green objects 

 has given them the name of green-blind persons. 



2. Red- Blindness. In the second variety of dichro- 

 matic eyes (a variety which is more numerous than either 

 of the others) the yellow is seen most intense where we see 

 green, while our red is to them a very indistinct color some- 

 thing like a pale yellow. They readily pick out green ob- 

 jects, because these appear to them a bright yellow, calling 

 them green, however, for reasons explained above. Such 

 persons not being able to clearly distinguish the red are 

 called red-blind persons. 



