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 to develop an acute sense of smell. In the 41 ______ course of evolution members of the primate line has acquired large 42 ______ eyes while the snout has shrunk to give the eye an unimpeding view. 43 ______ Of mammals only humans and some primates enjoy color vision. The red flag is black to the bull. Horses live in a monochrome world. Light visible to human eyes, therefore, occupies only a very narrow band in the whole 44 ______ electromagnetic spectrum. Ultraviolet rays are invisible to humans though ants and honeybees are insensitive to them. Humans have no direct perception 45 ______ of infrared rays like the rattlesnake that has receptors tuned into 46 ______ wavelengths longer than 0. 7 micron. The world would look eery different 47 ______ if human eyes are sensitive to infrared radiation. Then instead of the darkness 48 ______ of night, we would be able to move easily in a strange shadowless world where objects glowed with varying degrees of intensity. But human eyes excel in other way. They are in fact remarkably discerning in color gradation. 49 ______ The color sensitivity of normal human vision is rarely surpassed even 50 ______ sophisticated technical devices.