For an image to reach the optic nerve, it has to pass into the eye, whose function is to allow us to see and then interpret the shapes, colors, and dimensions of objects by processing the light that they reflect or give off. (It is because the eye sees objects only by means of the light emitted by their surfaces that, while the eye is able to see in dim light or bright light, it cannot see objects when light is absent.
What happens is shown in the diagram of the eye: (1) Light striking the eye passes through the eye's "window," the transparent cornea, which is also the eye's main focusing element. (2) Light is bent or refracted by the cornea and directed through the pupil, the central opening in the ringlike iris behind the cornea, which admits light into the interior of the eye. (The iris gives our eyes their color and regulates the size of the pupil, making it larger when the light is low and smaller when the light is bright.) (3) Light passes through the lens, a clear, flexible focusing element immediately behind the pupil, and is projected onto the retina, the light-sensitive portion of the eye at the back of the eyeball. (4) The retina contains specialized cells (called photoreceptors) that convert the image from light rays into electrical signals and send them through the optic nerve to the brain, which interprets these signals as visual images.
| ©1998 The Museum of Modern Art, New York |
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