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THE A MEHH WX .V.l TURALIST [Vol. XLI 



colors of which we have any sensation. It may naturally be asked : 

 Do we not see objects of other colors such as roses, grass, violets, 

 oranges, and so on ? The answer is that we do see all these things 

 but that they do not give us the color sensation correctly belonging 

 to them; their colors appear to us as varieties of the other color 

 sensations which we are able to receive. Take for example the 

 color red. A soldier's coat or a stick of sealing wax conveys 

 to me a very positive sensation of color, by which I am perfectly 

 able to identify, in a great number of instances, bodies of this hue. 

 But when I examine more closely what I really see, I am obliged 

 to conclude that it is simply a modification of one of my other 

 sensations, namely yellow. It is in fact a yellow shaded with 

 black or gray, a darkened yellow or yellow brown." 



Dichromatic vision occurs in three forms, in two of which red 

 and green are not differentiated from one another. The three 

 forms are named protanopia, deuteranopia, and tritanopia respec- 

 tively. In protanopia the red end of the spectrum is shortened; 

 that is, a portion which to the normal person is red, appears black. 

 The remainder of the red, the orange, the yellow, and the green 

 appear as successively lighter shades of yellow which, toward the 

 blue, becomes gray or white. This white shades into bhie which 

 deepens toward the violet end of the s])e( tnim. \n deuteranopia, 

 which is the normal condition of a jHTipheral zone of the retina, 

 the red of the spectrum is not shortened. Red, orange, yellow 

 and green appear as lighter shades of one color, called red or 

 yellow, and shade into a white or gray band which is a little nearer 

 the red end of the spectrum than the corresponding band of protan- 

 opia. Blue is perceived normally. Tritanopia is a rare form in 

 which yellow and blue are not recognized. The spectrum presents 

 red and green portions, separated by a white band in place of the 

 yellow. A dark green is seen in place of blue and the violet end 



