THEORIES OF COLOR VISION 129 



The fundamental color sensations according to some theories are red, 

 green, and blue; according to others, they are red, yellow, green, and 

 blue. 



Theories of Color Vision 



It is doubtful that any subject in science has given rise to so much 

 speculation as the cause of color vision. The earliest historical period 

 of speculation on how color is experienced, 640 B.C. to a.d. 1671, con- 

 tains such great names as Pythagoras, Epicurus, Aristotle. The second 

 great period, 1671 to 1801, of color theory was dominated by Newton's 

 corpuscular theories. During the third period, 1801 to 1874, the con- 

 tributions of the great minds of Young, Helmholtz, and Maxwell were 

 added. The fourth period, which began with Hering's outstanding 

 contribution, extended from 1874 to the early nineteen hundreds and 

 culminated in the quantum interpretation of the photoelectric effect 

 and applications of the quantum theory to photo-chemistry of the 

 photoreceptor processes in the retina. 



The Young-Helmholtz theory, as Troland [1920] points out, is pre- 

 ferred by physicists because it lays emphasis primarily upon the stimuli 

 to vision, while the Hering theory receives more attention from the 

 psychologists because its fundamental conceptions are derived from 

 introspective analysis. 



These theories postulate the existence in the photoreceptors of the 

 retina of a number of specific chemical substances which are acted 

 upon by light. According to the Young-Helmholtz theory, there are 

 three of these substances which are decomposed at a maximum rate 

 by red, green, and blue light, respectively, and less so by the remainder 

 of the visual spectrum. The rates of photochemical decomposition of 

 these three chemical substances are supposed to be reported individually 

 to the brain via the optic nerve, and the ratio between the three decom- 

 position products determines the nature of the sensation. This ratio 

 accounts satisfactorily for the laws of " color mixture " for normal, and 

 also for some forms of color-defective vision, but it does very little more 

 than this. It fails completely to explain the changes in the nature of 

 the colors when the combination of red and green forms yellow, or when 

 the combination of yellow and blue forms white. 



The theory of Hering also postulates the presence in the entire retino- 

 cerebral apparatus of three substances. One of these substances is 

 decomposed by light of all frequencies, although to the greatest extent 

 by yellow-green light. This substance is then supposed to accumulate 

 during the absence of light, owing to a reversible chemical reaction. 



