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 divide 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) dividing 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.六级查询成绩

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

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