新东方必背英文每日一篇：第四十六天
VisionHuman vision like that of other primates has evolved in an arboreal
environment. In the dense complex world of atropical forest, it is more
important to see well than to develop an acute sense of smell. In the
course ofevolution members of the primate line have acquired large eyes
while the snout has shrunk to give the eye anunimpeded 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 intowavelengths longer than 0.7 micron. The world would look eerily
different if human eyes were sensitive to infraredradiation. Then instead
of the darkness of night, we would be able to move easily in a strange
shadowless worldwhere 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 bysophisticated technical devices.
视觉
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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