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
(5)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
(10)is endowed with two foveae – areas of the eye, 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
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
(15)motion 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
(20)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
(25)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.