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THE HUMAN EYE The human eye works much like a movie camera. It receives light from objects, focuses the light, projects it onto a sensitive screen, and then converts the images into electrical impulses that are sent to the brain via the optic nerve. 

On the surface of your eye, the transparent cornea covers the colored iris, and the nearly black pupil. The pupil, a round opening in the iris, allows light into the eye. The pupil dilates, in low light conditions, thereby allowing more light to enter the eye. In bright conditions, the pupil contracts, allowing less light in. 

The space between the cornea and lens is filled with a fluid called aqueous humor, which provides nourishment for the cornea and lens, and keeps the eyeball round and firm.  Just behind the iris and pupil is the crystalline lens, which converges light rays to focus on the retina. The shape of this elastic lens changes with the contractions and relaxations of muscles in the eye. The large chamber behind the lens is filled with a fluid known as vitreous humor. At the back of the eye is a thin membrane called the retina. It contains rods, used primarily for night vision, and cones, used for daylight vision. When light reaches the rod or cone cells, it causes a chemical change in light-sensitive pigments found within the cells. The chemical change results in an electrical impulse, which is transmitted to nerve cells. The nerve cells branch into many fibers, which form the optic nerve. The impulse is carried from the optic nerve, to the visual centers of the brain, where we perceive the image.

FOCUSING  Near-sightedness vs. Far-sightnedness

When the eye views distant objects, the crystalline lens becomes flat, as it is pulled by the suspensory ligaments. When viewing closer objects, the ciliary muscle contracts, reducing the pull of the suspensory fibers, and allowing the lens to bulge at the center. The rounder lens has a stronger focusing power, allowing objects to come into sharp focus on the retina. This increased focusing power, necessary for near vision, is known as accomodation. As we grow older, accomodation is gradually lost, as the lens begins to harden. By the forties a person may require eyeglasses for reading, and other close work.