784 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



exactly the same as that which is produced when the retina is acted upon simul- 

 taneously by light of two fundamental colors, we are incapable of distinguish- 

 ing in sensation between an intermediate wave-length and a mixture in proper 

 amounts of two fundamental wave-lengths. 



When the retina is affected by two or more rays of such wave-lengths that 

 all three of the color visual substances are equally affected, the resulting decom- 

 position will be the same as that produced by the stimulation of the gray visual 

 substance out of which the color visual substances were differentiated, and the 

 corresponding sensation will therefore be that of gray or white. 



It will be noticed that the important feature of this theory is that it pro- 

 vides for the independent existence of the gray visual substance, while at the 

 same time the stimulation of this substance is made a necessary result of the 

 mixture of certain color sensations. 



Color-blindness. The fact that many individuals are incapable of distin- 

 guishing between certain colors i. e. are more or less " color-blind " is one 

 of fundamental importance in the discussion of theories of color vision. By 

 far the most common kind of color-blindness is that in which certain shades 

 of red and green are not recognized as different colors. The advocates of the 

 Young-Helmholtz theory explain such cases by supposing that either the red 

 or the green perceiving elements of the retina are deficient, or, if present, are 

 irritable, not by rays of a particular wave-length, but by all the rays of the 

 visible spectrum. In accordance with this view these cases of color-blindness 

 are divided into two classes viz. the red-blind and the green-blind the basis 

 for the classification being furnished by more or less characteristic curves repre- 

 senting the variations in the luminosity of the visible spectrum as it appears 

 to the different eyes. There are, however, cases which cannot easily be brought 

 under either of these two classes. Moreover, it has been proved in cases of 

 monocular color-blindness, and is admitted even by the defenders of the Helm- 

 holtz theory, that such persons see really only two colors viz. blue and yellow. 

 To such persons the red end of the spectrum appears a dark yellow, and the 

 green portion of the spectrum has luminosity without color. 



A better explanation of this sort of color-blindness is given in the Hering 

 theory by simply supposing that in such eyes the red-green visual substance is 

 deficient or wholly wanting, but the theory of Mrs. Franklin accounts for the 

 phenomena in a still more satisfactory way ; for, by supposing that the differ- 

 entiation of the primary gray visual substance has first led to the formation 

 of a blue and a yellow visual substance, and that the latter has subsequently 

 been differentiated into a red and a green visual substance, color-blindness is 

 readily explained by supposing that this second differentiation has either not 

 occurred at all or has taken place in an imperfect manner. It is, in other 

 words, an arrest of development. 



Cases of absolute color-blindness are said to occasionally occur. To such 

 persons nature is colorless, all objects presenting simply differences of light 

 and shade. 



In whatever way color-blindness is to be explained, the defect is one of 



