e (since the confusions of the color-blind eye might lead to
mistaking signals in navigation or railroading), takes on additional
significance when we discover the curious fact that _every one is
color-blind_--in certain parts of the retina. The outermost zone of
the retina, corresponding to the margin of the field of view, is
totally color-blind (or very nearly so), and an intermediate zone,
between this and the central area of the retina that sees all the
colors, is red-green blind, and delivers only blue and yellow
sensations, along with white, black and gray. Take {211} a spot of
yellow or blue and move it in from the side of the head into the
margin of the field of view and then on towards the center. When it
first appears in the margin, it simply appears gray, but when it has
come inwards for a certain distance it changes to yellow. If a red or
green spot is moved in similarly, it first appears gray, then takes on
a faint tinge of yellow, and finally, as it approaches the center of
the field of view, appears in its true color. The outer zone gets only
black and white, the intermediate zone gets, in addition to these,
yellow and blue, and the central area adds red and green (and with
them all the colors).

[Illustration: Fig. 35.--Color cones of the retina. F is the fovea, or
central area of clearest vision. (Figure text: all colors, white-black &
yellow-blue, white-black)]

Now as to the question of elements, let us see how far we can go,
keeping still to the sensations, without any reference to the
stimulus. If a collection of bits of color is presented to a class of
students who have not previously studied this matter, with the request
that each select those colors that seem to him elementary and not
blends, there is practically unanimous agreement on three colors, red,
yellow and blue; and there are some votes for green also, but almost
none for orange, violet, purple, brown or any other colors. {212}
except white and black. That white and black are elementary sensations
is made clear by the case of total color-blindness, since in this
condition there are no other visual sensations from which white and
black could be compounded, and these two differ so completely from
each other that it would be impossible to think of white as made up of
black, or black of white. Gray, on the other hand, appears like a
blend of black and white. In the same way, red-green blindness
demonstrates the reality of yellow and bl