When all the vibration frequencies affect the eyes at the same time, we
see no color at all but only brightness. This is due to the fact that
certain vibration frequencies neutralize each other in their effect on
the retina, so far as producing color is concerned. Red neutralizes
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