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.