Beneath the cornea is the iris, the colored part of the eye
which we refer to when we say a person has brown or blue eyes.
In the center of the iris is the pupil, which is the opening of
the iris. The iris functions like a shutter, adjusting pupil
size to control the amount of light entering the eye. Located
behind the iris is the lens, which works together with the
cornea and vitreous to focus light. Like the lens in a camera,
it adjusts light rays as vision shifts between nearby and
distant objects in a process called accommodation. Light then
passes through the vitreous, the gelatinous substance that fills
most of the eye and gives it its shape. The back of the eye is
lined with a thin layer of tissue containing millions of
photoreceptor (light-sensitive) cells. This is the retina, where
light rays focus into an upside-down image. In the center of the
retina is the macula. Less than 1/4 of an inch in diameter, the
macula is responsible for clear central vision. The retina
converts the image into an electri