More about the eye
The eye is the size of a table tennis ball. The eye's first
layer which functions, as the front window is the transparent
cornea. It starts the seeing process by bending light rays
towards the eye lens. Next, the pupil, an adjustable gateway
for light controls the amount of light entering the eye.
In bright light it is nearly closed; on a dark night it
is wide open. If you observe the pupils of a person who
walks from bright sunshine into a dimly lit room, you will
notice the widening of the pupils to accommodate more light.
Imagine what would have happened if the pupils were to malfunction.
On a bright day, too much light could enter the eye and
damage its insides. The importance of the pupil is apparent
while watching a total solar eclipse. When the eclipse is
in the total phase, that is the moon has covered the sun
completely, the amount of light drastically reduces. The
pupils widen to accommodate more light. When the total phase
ends, the limb of the sun makes a sudden appearance. The
pupils have not reacted to this sudden change of light and
are still wide open, thereby allowing more light to enter
the eye. The damage caused by this could result in blindness.
The lens is a little envelope of fluid, oval in shape and
is surrounded by a ring of tiny, very strong, unbelievably
hard-working muscles, the ciliary muscles. When they become
tense or contract, the lens fattens for seeing near things;
when they relax, it flattens for seeing distant things.
Continuous reading, or work requiring the eyes to focus
on close by objects keeps the ciliary muscles tensed. They
grow tired. Try focusing on some object very close by. Immediately
shift your focus on to something far away. Notice how fast
these muscles can act.
In front of and behind the lens, there are two fluid-filled
chambers. In front the fluid is like water; at the back
it is much thicker. The fluid helps maintain the shape of
the eye. Both fluids must be absolutely clear to permit
passage of light.
When you look at some object, the light passes through the
lens, which brings it into correct focus onto the retina.
The retina covers the rear two thirds of the interior. Covering
less than three square centimetres, the retina contains
137 million cells that are sensitive to light. Of these
130 million are shaped like rods. Rods provide for black-and-white
vision. The rest seven million are shaped like cones. Cones
are responsible for colour vision.
The rods are scattered all over the retina. When light falls
on these, it bleaches rhodopsin, a purplish-red pigment
in the rods. The bleaching process generates a tiny wisp
of electricity a few millionths of a volt, far too little
even to tickle a mosquito. This signal feeds into the straw
size optic nerve and is transmitted to the brain at a speed
of about 450 km per hour. The brain interprets the signals
flooding in and identifies the object. All of this intricate
electrochemical activity is completed in about 0.002 seconds.
The cones are concentrated in the fovea, a pinhead-size,
yellowish depression at the rear of the eye chamber. This
is the centre for acute vision reading, any close work,
say threading a needle and for colour. A leading theory
is that these cones, too, have bleachable pigments, one
each for red, green and blue. Like an artist mixing paints
on a palette, the brain blends these colours to make scores
of other hues. If anything should go wrong with this intricate
electrochemical process, one would be colour-blind as one
in eight men is to some degree. In dim light, activity of
the cones diminishes, colour sense vanishes. The rods remain
active and everything appears grey.