338 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



for the perception of color (see p. 342). When a ray of light intermediate 

 between two of the fundamental colors falls upon the retina, the visual sub- 

 stances corresponding to these two colors will be affected to a degree pro- 

 portionate to the proximity of these two colors to that of the incident ray. 

 Since this effect is exactly the same as that which is produced when the retina 

 is acted upon simultaneously by light of two fundamental colors, we are incap- 

 able of distinguishing 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 ?t the 

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

 mixture of certain color sensations. 



Another color theory has recently been brought forward by Prof. G. E. 

 Midler, 1 who substitutes for Hering's antagonistic processes of assimilation and 

 dissimilation the conception of "reversible chemical actions" — i. e. actions 

 in which the products of a chemical change can be used for the reconstruc- 

 tion of the original substance. 



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- 



1 7a itschriftfiir Psyehologie und Physiologie der Sinnesorgane, 1875 and 1897. 



