hat our three-dimensional diagram would
finally take the form of a double cone, with the most intense white,
like that of sunlight, at the upper point, with dead black at the
lower point, {209} and with the greatest diameter near the middle
brightness, where the greatest saturations can be obtained. The axis
of the double cone, extending from brightest white to dead black,
would give the series of neutral grays. All the thousands of
distinguishable colors, shades and tints, would find places in this
scheme.

[Illustration: Fig. 34.--The color cone, described in the text.
Instead of a cone, a four-sided pyramid is often used, so as to
emphasize the four main colors, red, yellow, green and blue, which are
then located at the corners of the base of the pyramid. (Figure text:
white, black, R, B, G, Y)]

Simpler Forms of the Color Sense

Not every one gets all these sensations. In _color-blindness_, the
system is reduced to one or two dimensions, instead of three. There
are two principal forms of color-blindness: total, very uncommon; and
red-green blindness, fairly {210} common. The totally color-blind
individual sees only white, black, and the various shades of gray. His
system of visual sensations is reduced to one dimension, corresponding
to the axis of our double cone.

_Red-green blindness_, very uncommon in women, is present in three or
four percent of men. It is not a disease, not curable, not corrected
by training, and not associated with any other defect of the eye, or
of the brain. It is simply a native peculiarity of the color sense.
Careful study shows that the only color sensations of the red-green
blind person are blue and yellow, along with white, black and the
grays. His color circle reduces to a straight line with yellow at one
end and blue at the other. Instead of the color circle, he has a
double saturation series, reaching from saturated yellow through
duller yellows to gray and thence through dull blues to saturated
blue. What appears to the normal eye as red, orange or grass green
appears to him as more or less unsaturated yellow; and what appears to
the normal eye as greenish blue, violet and purple appears to him as
more or less unsaturated blue. His color system can be represented in
two dimensions, one for the double saturation series,
yellow-gray-blue, and the other for the intensity series,
white-gray-black.

Color-blindness, always interesting and not without some practical
importanc