774 SIGHT. 



fibre transmitting three distinct kinds of impulses ; or we may suppose that 

 the visual substances are three in number instead of six, the changes in each 

 substance provoking a primary sensation. 



666. Such are the two main theories of color vision ; and much may be 

 said in favor of both of them ; at the same time both of them present many 

 difficulties. To discuss them fully is a task beyond the limits of this book, 

 and to discuss them in any but a full manner would be unsatisfactory. We 

 must be satisfied, therefore, with the foregoing simple statement of the two 

 views. Independently of any theory, however, we may remember (1) 

 that all the sensations which we experience under the action of light of 

 whatever kind may be reduced to six white, black, red, yellow, green, and 

 blue ; and (that these may be all reproduced by various mixtures of three 

 standard sensations, if black be allowed to indicate the absence of all sen- 

 sation. These are matters of fact ; what is at present debated is whether 

 the six fundamental sensations are the outcome of three primary sensa- 

 tions or whether they represent six distinct conditions of the visual 

 apparatus. 



667. Color-blindness. Persons vary much in their power of appre- 

 ciating and discriminating color, i. e., in the intensity and accuracy of their 

 color sensations. Some people regard as similar, colors which to most peo- 

 ple are glaringly distinct ; the former are said to be " color-blind." The 

 most common form of color-blindness is that of persons unable to distinguish 

 green and red from each other. As in the case of Dalton, they tell a red 

 gown lying on a green grass plot, or a red cherry among the green leaves, 

 by its form, and not by its color. They confound not only red, green, and 

 certain forms of brown, but also rose, purple, and blue. Such persons are 

 often spoken of as " red blind." On the Hering theory they lack the red- 

 green visual substance ; hence, all the color sensations they possess must be 

 those of yellow and blue free from all mixture of red or green ; and such 

 accounts as have been given of their sensations by those persons who are 

 " red blind " in one eye, but possess normal vision with the other, accord 

 with this conclusion. On the Young-Helmholtz theory, such persons lack 

 the primary red sensation ; and hence the sensations which they have must 

 be mixtures of green and blue alone, our yellow appearing to them a bright 

 green, and our green-blue a kind of gray, 



All such red-blind people ought, on either theory, to be less affected than 

 are persons with normal eyes, by the red end of the spectrum ; this ought 

 with them to be shortened and obscure. In a certain number of persons 

 who confoun^ red and green, this is the case ; but in some instances no such 

 lack of appreciation of the red end of the spectrum can be ascertained. 

 Such cases have been supposed to be green blind, that is, lacking the pri- 

 mary sensation of green. According to the Hering theory green blindness 

 apart from red blindness is impossible, the only two possible color defects 

 being red-green and blue-yellow blindness. And the existence of distinct 

 green blindness has been held to contradict that theory. On the other 

 hand, the Hering theory admits the possibility of total color-blindness, i. e. t 

 the inability to see anything but white and black ; and this on the Young- 

 Helmholtz theory, is impossible, since for vision to exist at all, one of the 

 three primary sensations must be present ; a man to see at all must see 

 things in various shades of either red, or of green, or of violet, though he 

 may confound this single-colored vision with the normal vision of white of 

 different intensities. But, indeed, a full examination of color-blindness 

 rather increases than diminishes the difficulties of deciding between the two 

 rival theories. 



668. Influence of the pigment of the yellow spot. In the macula lutea, 



