The human criterionfor 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 can spot a dime on the sidewalk while perched on ...

The human criterionfor 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 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 millimetre in its retina. And in water, humans are farsighted, while the kingfisher, swooping down to spear fish, can see well both in the air and water because it is endowed with two foveae – areas of the eyes, consisting mostly of cones that provide visual distinctions. One foveae 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 foveae 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 moving picture. Known as “bug detectors”, a highly developed set of cells in a frog’s 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.

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 kind 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 is not 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 mammals, only humans and some primates can enjoy pleasures of colour vision.

What does the passage mainly discuss?