So that more rays of light may enter the eye, whereby objects may be more distinctly seen.
Why can an owl or a bat see at night?
Because the pupil of the eye in them is quite large, and admits much light.
Why do they stay in dark places through the day?
Because the bright light of day hurts their eyes.
May the image of an object be retained in the eye after the eyelids are closed?
It may for a very short time, as any one can prove by first looking at some bright object, and then closing the eyelids.
Why does a burning coal moved rapidly around, seem like a circle of fire?
Because the light from it is retained a short time by the eye, thus seeming to form a complete circle.
Why cannot we count the posts in a fence, when riding rapidly in a car?