y.

From the point of view of physics, there is no reason for considering
any one color more elementary than any other. Every wave-length is
elementary; and if sensation tallied precisely with the stimulus,
every spectral color-tone would be an element. But there are obvious
objections to such a view, such as: (1) there are not nearly as many
{219} distinguishable color-tones as there are wave-lengths; (2)
orange, having a single wave-length, certainly appears to be a blend
as truly as purple, which has no single wave-length; and (3) we cannot
get away from the fact of red-green blindness, in which there are only
two color-tones, _yellow_ and _blue_. In this form of color vision
(which, we must remember, is normal in the intermediate zone of the
retina), there are certainly not as many elementary responses as there
are wave-lengths, but only one response to all the longer waves (the
sensation of yellow), one response to all the shorter waves (the
sensation of blue), one response to the combination of long and short
waves (the sensation of white), and one response to the cessation of
light (the sensation of black). These four are certainly elementary
sensations, and there are probably only a few more.

There must be at least two more, because of the fact that two of the
sure elements, yellow and blue, are complementary. For suppose we try
to get along with one more, as _red_. Then red, blended with yellow,
would give the intervening color-tones, namely, orange with reddish
and yellowish orange; and red blended with blue would give violet and
purple; but yellow and blue would only give white or gray, and there
would be no way of getting green. We must admit _green_ as another
element. The particular red selected would be that of the red end of
the spectrum, if we follow the general vote; and the green would
probably be something very near grass green. We thus arrive at the
conclusion that there are six elementary visual responses or
sensations: white and black, yellow and blue, red and green.

It is a curious fact that some of these elementary sensations blend
with each other, while some refuse to blend. White and black blend to
gray, and either white or black or both together will blend with any
of the four elementary colors or with any possible blend of these
four. Brown, for {220} example, is a grayish orange, that is, a blend
of white, black, red and yellow. Red blends with yellow, yellow with
green, green with blu