The perception of color is very complicated, and not at all well understood. Persons who do not have the same color perception as most of us are called color-blind, and by learning some things about color-blindness we shall best get an idea of color perception itself. About four men in one hundred have defective color sense; the proportion in women is only about a tenth as great. By far the commonest type is one in which neither red nor green is seen correctly, but both are seen as neutral tints, and in many cases look so much alike that the person cannot tell one from the other. The practical importance of knowing whether or not this defect is present is seen when we think that red and green lights are used more than any other colors in signaling, so that railroad men and others who work by signals must have normal color sense. It has been discovered that even people that have normal color vision are color-blind in the margins of the retina. This can easily be demonstrated by bringing a red or green disk slowly around in front of the eye of a person who, meanwhile, keeps looking straight ahead. He will see the disk out of the corner of his eye some time before he can tell what color it is. In fact, if a red or green disk is used, he usually will not be certain as to the color until it is almost straight in front. Blue or yellow disks can be told with certainty much farther out, but even these colors are not perceived clear to the edge of the field of vision. These experiments show that the retina becomes more and more highly developed as an organ for perceiving color as we get closer and closer to the center; at the extreme edges there is no color sense at all, but only the primitive ability to tell light from darkness; closer in blue and yellow are distinguished and not red and green; only in the central part are all colors clearly seen.