significance. They must be taken account of in all arrangements of
colors and tints, for example, in dress, in the arrangement of flowers
and shrubs, in painting.

_Color-Mixture._ If, on a rotating motor, disks of different colors--say
red and yellow--are placed and rotated, one sees on looking at them not
red or yellow but orange. This phenomenon is known as _color-mixture_.
The result is due to the simultaneous stimulation of the retina by two
kinds of ether vibration. If the colors used are a certain red and a
certain green, they neutralize each other and produce only gray. All the
pairs of complementary colors mentioned above act in the same way,
producing, if mixed in the right proportion, no color, but gray. If
colored disks not complementary are mixed by rotation on a motor, they
produce an intermediate color. Red and yellow give orange. Blue and
green give bluish green. Yellow and green give yellowish green. Red and
blue give violet or purple, depending on the proportion. Mixing pigments
gives, in general, the same results as mixing by means of rotating the
disks. The ordinary blue and yellow pigments give green when mixed,
because each of the two pigments contains green. The blue and yellow
neutralize each other, leaving green.

_Visual After-Images._ The stimulation of the retina has interesting
after effects. We shall mention here only the one known as _negative
after-images_. If one will place on the table a sheet of white paper,
and on this white paper lay a small piece of colored paper, and if he
will then gaze steadily at the colored paper for a half-minute, it will
be found that if the colored paper is removed one sees its complementary
color. If the head is not moved, this complementary color has the same
size and shape as the original colored piece of paper. The negative
after-image can be projected on a background at different distances, its