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发布时间:2019-10-11 10:38:00源自:互联网作者:admin阅读()

  The human criterion for perfect vision is 20/20 for reading the standard lines on a Snellen eye chart without a hitch. The score is determined by how well you read lines of letters of different sizes from 20 feetaway. But being able to read the bottom line on the eye chart does not approximate perfection as far as other species are concerned. Most birds would consider us very visually handicapped. The hawk. for instance,has such sharp eyes that it can spot a dime on the sidewalk while perched on top of the Empire State Building+ It can make fine visual distinctions because it is blessed with one million cones per square millimeter in its retina(视网膜). And in water, humans are farsighted. while the kingfisher, swooping down to spear fish. can see well in both the air and water because it is endowed with two foveae.(凹窝)-areas of the eye, consisting mostly of cones. that provide visual distinctions. One fovea permits the bird, while in theair. to scan the water below with one eye at a time. This is called monocular vision. Once it hits the water, the other fovea joins in, allowing the kingfisher to focus both eyes, like binoculars, on its prey at the same time. A frog's vision is distinguished by its ability to perceive things as a constant motion picture. Known as "bug detectors", a highly developed set of cells in a frog's eyes responds mainly to movingobjects. So, it is said that a frog sitting in a field of dead bugs wouldn’t see them as food and would starve.

  The bee has a "compound" eye. which is used for navigation. It has 15,000 facets that pide what it sees into a pattern of dots. or mosaic. With this hind of vision, the bee sees the sun only as a single dot, a constant point of reference. Thus. the eye is a superb navigational instrument that constantly measures the angle of its line of flight in relation to the sun. A bee's eye also gauges flight speed. And if that isnot enough to leave our 20/20 "perfect vision” paling into insignificance. the bee is capable of seeing something we can't-ultraviolet light. Thus,what humans consider to be "perfect vision” is in fact rather limited when we look at other species. However, there is still much to be said for the human eye. Of all the mammals, only humans and some primates can enjoy the pleasures of color vision.1. The Snellen eye chart measures one's eyesight by__________________.

  A) the number of lines he reads

  B) the distance he stands away from the chart

  C) the speed at which he recognizes the letters

  D) several integrative factors

  2. How is hawk's eyesight better than ours?

  A) It can identify small items more quickly than we do.

  B) It can see ai a longer distance than we do.

  C) It has more cones in che retina than we do.

  D) It has bigger eyes than we do.

  3. Monocular vision enables the kingfisher to__________________.

  A) avoid farsightedness

  B) be able to see underwater

  C) move one eye at a time

  D) focus its eyes on the prey

  4. A bee finds its direction by_______________________.

  A) piding what it sees into thousands of dots

  B) constantly gauging its flying speed

  C) using the sun as a constant point of reference

  D) measuring the angle of the sun

  5. What is the main idea of the passage'?

  A) The differences between human eyes and other species' eyes.

  B) The advantages of other species' eyes in comparison with human eyes.

  C) The factors that make other species' eyes better than human eyes.

  D) The standard that determines me "perfect vision” of human eyes.







  [A]首段第2句中的lines of letters和第3句中的bottom line表明视力的好坏是依靠看到视力表的行数来决定的,选项A为本题答案。首段第2句表明测验视力时距离是预先设定的,选项B不正确;选项C属于无中生有,原文并未提及该内容;文中只提到视力的好坏取次于能看到多少行字母这一因索,并没有提到其他因素,故D项错误。






  [B]首段笫5句中的while perched on top of the Empire State Building表明本句强调的是鹰能看到距离很远的物件,由此可见,选项B为本题答案。从首段第5句中的dime一词可以推断鹰可以看到很小的物件,但原文并没有暗示它辨认物件的速度,更没有在这方面与人眼作比较,因此选项A不正确;首段第6句只是指出鹰眼的视网膜每平方毫米有100万个圆锥细胞,但并没有对比人眼和鹰眼所具有的圆锥细胞的总数,因此选项C不正确;原文并来提及与选项D有关的内容。






  [C]首段第9句中的This表明前一句(即第8句)就是monocular vision的定义,很明显,选项C为本题答案。首段第7句表明翠鸟在水里能看得清楚,不会像人类那样远视,是因为它眼睛里有两个凹窝区域,并非因为它具有单眼视觉,因此选项A和B都不正确;根据该段第10句可知两个凹窝区域同时发挥作用才能使翠鸟把目光聚焦到猎物身上,因此选项D不正确。






  [C]第2段第3,4句描述了蜜蜂利用太阳导航的机制,将选项与这些细节对照可以知道选项C为正确的说法。选项A是蜜蜂的成像机制,虽然眼睛必然与导航有关,但单靠成像并不能起到导航的作用,因此选项A不是蜜蜂寻找方向的方法:选项B中的gauging its flying speed也跟蜜蜂寻找方向无关:第2段第4句表明蜜蜂测量的是飞行路线和太阳的夹角,而不是测量太阳的角度,因此选项D不正确。










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