The range of electromagnetic waves visible to the human eye.Our sense of vision is activated by radiant energy emitted or reflected from objects. This energy consists of electromagnetic charges traveling in waves at the rate of 186,300 miles per second. Electromagnetic radiation ranges from cosmic waves only ten trillionths of an inch in length to radio waves of many miles—but the human eye is sensitive only to an area about midway between these extremes. This area, constituting the visible spectrum, comprises waves measuring from sixteen to thirty-two millionths of an inch (380 to 760 millimicrons) in length.Wavelengths within this range activate the cells of the retina of the eye, and produce electrical impulses which are carried through the optic nerve to the brain. These impulses are sorted out, or “encoded,” in the occipital (rear) lobe of the brain, but the exact nature of this process is still unknown. We do know, however, that the retina contains two kinds of cells, one cone-shaped and the other rod-shaped. The cones, concentrated in the center, are responsive to wavelengths that produce color sensations; and the rods, distributed throughout the retina, are sensitive only to black and white. This indicates that some of the sorting out process occurs in the retina itself.For a long time scientists thought of the visible spectrum only in terms of the solar spectrum. Isaac Newton had discovered that sunlight can be broken up into its component wavelengths if it is passed through a prism, and it was assumed that the human eye was limited to this range. However, experiments have shown that the eye actually registers some colors that are not in the solar spectrum, especially at the red end where longer wavelengths are found. More recently, physiological psychologists have discovered that the eye can also discriminate more colors than had previously been expected. The average individual can discriminate about 350,000 different colors, but this ability, remarkable as it is, can be vastly improved through practice. Experimenters have found that when subjects are trained to notice differences of satura tion and brightness as well as hue, som< of them become capable of discriminat ing an estimated seven to ten millior separate surface colors.