It will be found that the vibration rates of the ether are very great. It is only within a certain range of vibration frequency that sunlight affects the retina. Slower rates of vibration than that producing red do not affect the eye, and faster than that producing violet do not affect the eye. The lightness and darkness of a color are dependent upon the intensity of the vibration. Red, for example, is produced by a certain vibration frequency. The more intense the vibration, the brighter the red; the less intense, the darker the red. 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 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.