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篇章阅读25：视力研究 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 feet away. 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. 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.-areas of the eye, consisting mostly of cones. that provide visual distinctions. One fovea permits the bird, while in the air. 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 frogs 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 frogs eyes responds mainly to moving objects. So, it is said that a frog sitting in a field of dead bugs wouldn t see them as food and would starve. | 
