622 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
dating that of Hering. It was advocated by Helmholtz, and is 
generally known as the Young-Helmholtz theory. 
According to the Young-Helmholtz theory there are three photo- 
chemical substances, red, green, and blue, respectively, which are 
stimulated by the various rays of the spectrum as shown in figure 
4, Absence of stimulation produces black, and the simultaneous dis- 
association of all three yields white. Protanopia is interpreted as red 
blindness, due to deficiency of the red perceiving substance. Deutera- 
nopia is green blindness, and tritanopia is blue blindness. Since it 
would appear that the perception of white must be lost with the 
disappearance of one of the three elements, the theory has been vari- 
ously modified. In protanopia the red and the green substances may 
be so altered that each responds both to red and green light (Fick), 
R O Y G B V 
Fic. 3.—Diagram to illustrate Hering’s theory of color vision. The red- 
green substance, 7g, is vertically shaded; and the yellow-blue substance, 
y—b, is transversely shaded. 
or the red and the green substances may be imperfectly segregated, 
as assumed by Mrs. Franklin’s theory. The close relation between 
the red and green substances is shown in Koenig’s presentation of 
the Young-Helmholtz theory. (Fig. 5.) The absence of either 
would give rise to somewhat similar conditions, such as occur in 
protanopia and deuteranopia. The figure indicates that in trichro- 
matic vision, the colors from yellow to blue affect all three sub- 
stances to a certain extent, thus adding a small amount of white to 
the color sensation. In dichromatic vision the mixing of the two 
elements yields white. In case the red substance is absent, this 
white will appear nearer the blue than in case the green is absent; 
its position is indicated by the intersection of the blue with the 
green and red curves, respectively. In the absence of the blue 
substance, the white band is near the yellow. This accords with the 
