the organ of sight. It is our most important organ for finding out about the world
around us. We use our eyes in almost everything we do--reading, working, watching
movies and television, playing games, and countless other activities. Sight is our
most precious sense, and many people fear blindness more than any other disability.
The human eyeball measures only about 1 inch (25 millimeters) in diameter. Yet the
eye can see objects as far away as a star and as tiny as a grain of sand. The eye
can quickly adjust its focus between a distant point and a near one. It can be accurately
directed toward an object even while the head is moving.
The eye does not actually see objects. Instead, it sees the light they reflect or
give off. The eye can see in bright light and in dim light, but it cannot see in
no light at all. Light rays enter the eye through transparent tissues. The eye changes
the rays into electrical signals. The signals are then sent to the brain, which
interprets them as visual images.