A sudden light causes pain to the eye.
Of what shape is the pupil of the eye?
In man, the pupil of the eye is circular.
What is the size of the pupil of the eye?
The pupil varies from the one-twentieth to the one-third of an inch in diameter, depending upon the brightness of the light entering the eye.
How else are our eyes protected from any sudden light?
By means of our eyelids, which may be closed, and thus shut out all the light.
When we pass from a well-lighted room into the open air, at night, why does it seem darker at first than it does afterwards?
Because the pupil is contracted at first; but it soon dilates, and allows more rays of light to pass into the eye, so that the night seems less dark.
Why does the pupil become larger in the twilight?