PROBLEM OF COLOR VISION—DANE. 621 
lighter shades of yellow which, toward the blue, becomes gray or 
white. This white shades into blue, which deepens toward the violet 
end of the spectrum. In deuteranopia, which is the normal condition 
of a peripheral zone of the retina, the red of the spectrum is not 
shortened. Red, orange, yellow, and green appear as lighter shades 
of one color, called red or yellow, and shade into a white or gray band 
which is a little nearer the red end of the spectrum than the corre- 
sponding band of protanopia. Blue is perceived normally. Tritan- 
opia is a rare form in which yellow and blue are not recognized. The 
spectrum presents red and green portions, separated by a white band 
in place of the yellow. <A dark green is seen in place of blue and the 
violet end of the spectrum is shortened. 
THEORIES OF COLOR VISION. 
Certain features of color blindness are ingeniously explained by 
Hering’s theory, illustrated in figure 3. It is supposed that the 
cones contain a photo-chemical substance which is disassociated by 
red rays, but which is built up by the green rays, giving rise re- 
spectively to the sensations of red and green. A second substance 
is broken down by yellow and built up by blue light. As shown 
in the figure, orange is a mixed sensation due to the simultane- 
ous partial destruction of red-green and the yellow-blue substances. 
Yellowish green and greenish blue are likewise mixtures, and violet 
is supposed to combine the partial construction of the yellow-blue with 
the destruction of the red-green, the latter being indicated by the 
broken line. There are four pure sensations, red, yellow, green, and 
blue. Color blindness may be due to the absence or deficiency of the 
red-green substance (protanopia and deuteranopia, the two forms 
being varieties of a single type), or to lack of the yellow-blue sub- 
stance (tritanopia). Hering further considered that there was a 
white-black substance, built up in darkness to give rise to the sensa- 
tion of black, but destroyed in varying degree by different colored 
lights, thus giving white. In monochromatic vision the retina con- 
tains only this white-black substance. The curve w of figure 3 shows 
that the maximum stimulation of white is in the yellow portion of 
the spectrum. Without considering the difficulties concerning the 
white-black hypothesis, it may be questioned whether both construc- 
tive and destructive chemical processes can produce color sensations of 
similar nature. Mrs. Franklin considered that her theory was sup- 
ported by the fact that the color sensations were all chemically 
destructive. Hering’s theory, moreover, calls for four primary color 
sensations, whereas physicists recognize that only three are necessary. 
Accordingly the physicist Young proposed a simpler theory ante- 
