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. Horses 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 honeybees 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 shadowless world where objects glowed with varying degrees of intensity. But human eyes excel in other ways. They are in fact remarkably discerning in color gradation. The color sensitivity of normal human vision is rarely surpassed even by sophisticated technical devices.

人类的视觉，和其它灵长目动物的一样，是在丛林环境中进化出来的。 在稠密、复杂的热带丛林里，好的视觉比灵敏的嗅觉更加重要。 在进化过程中，灵长目动物的眼睛变大， 同时鼻子变小以使视野不受阻碍。在哺乳类动物中，只有人和一些灵长目动物能够分辨颜色。红旗在公牛看来是黑色的，马则生活在一个单色的世界里。 然而，人眼可见的光在整个光谱中只占一个非常狭窄的频段。人是看不到紫外线的，尽管蚂蚁和蜜蜂可以感觉到。

与响尾蛇不同，人也不能直接感受到红外线。响尾蛇的感觉器可以感受波长超过 0.7 微米的 光线。如果人能感受到红外线的话，这世界看上去将十分不同，而且恐怖。 到那时，将与夜的黑暗相反，我们能轻易地在一个奇异的没有阴影的世界里走动。任何物体都强弱不等地闪着光。然而，人眼在其它方面有优越之处。事实上，人眼对颜色梯度具有非凡的分辨能力。普通人类的视觉感受色彩的灵敏程度，甚至连精密的技术装备都很难超越。

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