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Vision

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.

全文翻譯：視覺

人類的視覺，和其它靈長目動物的一樣，是在叢林環境中進化出來的。 在稠密、復雜 的熱帶叢林里，好的視覺比靈敏的嗅覺更加重要。 在進化過程中，靈長目動物的眼睛變大， 同時鼻子變小以使視野不受阻礙。 在哺乳類動物中，只有人和一些靈長目動物能夠分辨顏 色。 紅旗在公牛看來是黑色的，馬則生活在一個單色的世界里。 然而，人眼可見的光在整 個光譜中只占一個非常狹窄的頻段。人是看不到紫外線的，盡管螞蟻和蜜蜂可以感覺到。與 響尾蛇不同，人也不能直接感受到紅外線。 響尾蛇的感覺器可以感受波長超過 0.7 微米的 光線。 如果人能感受到紅外線的話，這世界看上去將十分不同，而且恐怖。 到那時，將與夜的黑暗相反，我們能輕易地在一個奇異的沒有陰影的世界里走動。 任何物體都強弱不等 地閃著光。 然而，人眼在其它方面有優越之處。事實上，人眼對顏色梯度具有非凡的分辨能力。 普通人類的視覺感受色彩的靈敏程度，甚至連精密的技術裝備都很難超越。