Human vision, like that of other primates, has evolved in an arboreal environment. In the dense,complex world of a tropical forest, it is more important to see well than to develop an acute sense of smell. In the course of evolution, members of the primate line have acquired large eyes while the snout has shrunk to give the eye an unimpeded view. Of mammals, only humans and some primates enjoy color vision. The red flag is black to the bull. Horse live in a monochrome world. Light visible to human eyes, however, occupies only a very narrow band in the whole electromagnetic spectrum. Ultraviolet rays are invisible to humans, though ants and honeybee are sensitive to them. Humans have no direct perception of infrared rays, unlike the rattlesnake, which has receptors tuned into wavelengths longer than 0.7 micron. The world would look eerily different if human eyes were sensitive to infrared radiation. Then, instead of the darkness of night, we would be able to move easily in a strange, shadows world where objects glowed with varying degree of intensity. But human eyes excel in other ways. They are, in fact, remarkably discerning in color graduation. The color sensitivity of normal human vision is rarely surpassed even by sophisticated technical devices.