each other.

A point of experimental technique: in _mixing colored lights_ for the
purpose of studying the resulting sensations, we do not mix painter's
pigments, since the physical {215} conditions then would be far from
simple, but we mix the lights themselves by throwing them together
either into the eye, or upon a white screen. We can also, on account
of a certain lag or hang-over in the response of the retina, mix
lights by rapidly alternating them, and get the same effect as if we
had made them strike the retina simultaneously.

By mixing a red light with a yellow, in varying proportions, all the
color-tones between red and yellow can be got--reddish orange, orange
and yellowish orange. By mixing yellow and green lights, we get all
the greenish yellow and yellowish green color-tones; and by mixing
green and blue lights we get the bluish greens and greenish blues.
Finally, by mixing blue and red lights, in varying proportions, we get
violet, purple and purplish red. Purple has no place in the spectrum,
since it is a sensation which cannot be aroused by the action of any
single wave-length, but only by the mixture of long and short waves.

To get all the color-tones, then, we need not employ all the
wave-lengths, but can get along with only four. In fact, we can get
along with three. Red, green and blue will do the trick. Red and green
lights, combined, would give the yellows; green and blue would give
the greenish blues; and red and blue would give purple and violet.

The sensation of white results--to go back to Newton--from the
combined action of all the wave-lengths. But the stimulus _need_ not
contain _all_ the wave-lengths. Four are enough; the three just
mentioned would be enough. More surprising still, two are enough, if
chosen just right. Mix a pure yellow light with a pure blue, and you
will find that you get the sensation of white--or gray, if the lights
used are not strong.

[Footnote: When you mix blue and yellow _pigments_, each absorbs part
of the wave-lengths of white light, and what is left after this double
absorption may be predominantly green. This is absolutely different
from the addition of blue to yellow light; addition gives white, not
green.]

{216}

Lights, or wave-lengths, which when acting together on the retina give
the sensation of white or gray, are said to be _complementary_.
Speaking somewhat loosely, we sometimes say that two _colors_ are
complementary when they mix to pro