green, blue neutralizes yellow, violet neutralizes yellowish green,
orange neutralizes bluish green.

All variations in vision as far as color and brightness are concerned
are due to variations in the stimulus. Changes in vibration frequency
give the different colors. Changes in intensity give the different
brightnesses: black, gray, and white. All explanations of the many
interesting phenomena of vision are to be sought in the physiological
action of the eye.

Besides the facts of color and light and shade, already mentioned, some
further interesting visual phenomena may be mentioned here.

_Visual Contrast._ Every color makes objects near it take on the
antagonistic or complementary color. Red makes objects near appear
green, green makes them appear red. Blue makes near objects appear
yellow, while yellow makes them appear blue. Orange induces greenish
blue, and greenish blue induces orange. Violet induces yellowish green,
and yellowish green induces violet. These color-pairs are known as
antagonistic or complementary colors. Each one of a pair enhances the
effect of its complementary when the two colors are brought close
together. In a similar way, light and dark tints act as complementaries.
Light objects make dark objects near appear darker, and dark objects
make light objects near seem lighter.

These universal principles of contrast are of much practical
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 rot