The iris is the screen of the eye. Just as the photographer uses a
screen with a large opening when he wants more light to enter his camera
and a small opening when he requires less, so Nature arranges that the
iris automatically contracts or dilates to make a larger or smaller
pupil opening, according to the amount of light needed within the eye
for purposes of vision. When the light is very bright less is needed in
the eye. Thus in brilliant artificial light at night one’s pupil is
small. On the other hand, when the light is waning, as in the dusk or
semi-darkness, the pupil is enlarged by the iris contracting down to a
narrow ring under the outer circumference of the cornea.