do reflect light, as all flash photographers know ... they just don't do it very
well. Our eyes look red in some photographs. But it's not a reflector. It's the
camera flash bouncing off the red blood vessels and red tissue in the back of
our eye. The distinctive "eyeshine" given off by wolves and raccoons
and crocodiles, among many other species, comes from the "tapetum lucidum",
a mirror-like layer of cells in or behind the retina. This structure is found
mostly in nocturnal animals, for whom it serves as a kind of light amplifier.
The retina captures some of the light that enters the eye, but some passes through.
The tapetum lucidum bounces it back at the retina, giving the animal a second
chance to "see" it. It's this ricocheting light that gives off the vaguely
eerie glow we've come to associate with scary tales ... all those sinister pairs
of eyes staring out from gloomy thickets.
about human ability to see at night? Cats are much better adapted for performance
under low light than we are. They have larger corneas and pupils than us to collect
more light in dim conditions. They also have the tapetum which reflects light
back out of the eye. This way, their retinae get two chances to capture each photon
of light. A cat's tapetum reflect 130 times more light than the human eye. This
is why cats' eyes glow in the dark in photographs.
low light levels the cats pupil must be able to open as wide as possible, but
also be able to contract to very small size to protect the sensitive retina in
bright sunlight. In human eyes, this size variation of the pupil is controlled
by a circular ciliary muscle, but this limits the amount of size variation. In
cats however, the same process is controlled by two, shutter-like ciliary muscles,
which gives the cat its characteristic slit-like pupil in bright light conditions.
All cats pupils are therefore elliptical, however some, notable the Big
Cats, appear more circular when dilated.
size of the cat's eye is relatively larger than those found in human, this enables
a larger pupil and therefore more light to enter the eye. Generally, the lens
is more curved enabling sharper focusing even at the edges of the lens. The size
of the anterior chamber and the curvature of the cornea is also greater, which
helps more light to be refracted onto the light-sensitive retina.
feature, which enabled the amount of light hitting the retina to be increased,
is the tapetum lucidum. This is positioned at the back of the eye, behind the
retina and acts like a mirror, reflecting light back onto the light sensor cells
in the retina. This gives the cats eyes the characteristic night-time glow when
they are caught in a beam of light.
are two distinct types of light receptor cell on the retina Cones, which
are sensitive to high levels of light, used in colour vision and Rods, sensitive
in low light conditions. In cats, there is a greater concentration of Rods, aiding
their night-time vision. As in humans, there is a greater concentration of receptor
cells at the centre of the eye, leading to the optic nerve. In cats, however,
these a concentrated along a broader, horizontal band. This gives the cat far
more sensitivity to movement along the horizontal axis and they are therefore
more able to detect prey movement along the ground at greater distances.
may say that humans are not night vision animals, but we must have some of the
best DAY time vision. Well, its hard to compete with the eagle. Eagle eyes are
approximately the same size and weight as human eyes, even though a full grown
adult bald eagle weighs no more than 14 pounds. But an eagle eye has a much different
shape. The back of their eye is flatter and larger than the back of our eye, giving
an eagle a much larger image than we can see. The back layer of the human eye
has a special area on the retina called the fovea where there is an enormous concentration
of vision cells. Our human fovea has 200,000 cones/mm which is an incredible number.
But the eagle easily outdoes us. The fovea of the eagle has about a million cones
per millimeter. This allows an eagle to see a mouse a mile away. That is far beyond