e, and blue with red. But we cannot get yellow
and blue to blend, nor red and green. When we try to get yellow and
blue to blend, by combining their appropriate stimuli, both colors
disappear, and we get simply the colorless sensation of white or gray.
When we try to get red and green to blend, both of them disappear and
we get the sensation of yellow.

Theories of Color Vision

Of the most celebrated theories of color vision, the oldest,
propounded by the physicists Young and Helmholtz, recognized only
three elements, red, green and blue. Yellow they regarded as a blend
of red and green, and white as a blend of all three elements. The
unsatisfactory nature of this theory is obvious. White as a sensation
is certainly not a blend of these three color sensations, but is,
precisely, colorless; and no more is the yellow sensation a blend of
red and green. Moreover, the theory cannot do justice either to total
color-blindness, with its white and black but no colors, or to
red-green blindness, with its yellow but no red or green.

The next prominent theory was that of the physiologist Hering. He did
justice to white and black by accepting them as elements; and to
yellow and blue likewise. The fact that yellow and blue would not
blend he accounted for by supposing them to be antagonistic responses
of the retina; when, therefore, the stimuli for both acted together on
the retina, neither of the two antagonistic responses could occur, and
what did occur was simply the more generic response of white.
Proceeding along this line, he concluded that red and green were also
antagonistic responses; but just here {221} he committed a wholly
unnecessary error, in assuming that if red and green were antagonistic
responses, the combination of their stimuli must give white, just as
with yellow and blue. Accordingly, he was forced to select as his red
and green elementary color-tones two that would be complementary; and
this meant a purplish (i.e., bluish) red, and a bluish green, with the
result that his "elementary" red and green appear to nearly every one
as compounds and not elements. It would really have been just as easy
for Hering to suppose that the red and green responses, antagonizing
each other, left the sensation yellow; and then he could have selected
that red and green which we have concluded above to have the best
claim.

A third theory, propounded by the psychologist, Dr. Christine
Ladd-Franklin, is based on keen criti