Q. Why can a thousand persons see the same object at the same time?
A. Because it throws off from its surface an infinite number of rays in all directions; and one person sees one portion of these rays, and another person another.
Q. Why is the eye pained by a sudden light?
A. Because the pupil of the eye is burdened with rays, before it has had time to contract.
Q. Why does it give us pain, if a candle be brought suddenly towards our bed at night time?
A. In the dark the pupils of the eyes dilate very much, in order to admit more rays. When a candle is brought before them, the enlarged pupil is overladen with rays, and feels pained.
Q. Why can we bear the candle-light after a few moments?
A, Because the pupil contracts again almost instantly, and adjusts itself to the quantity of light which falls upon it.
Q. Why can we see nothing, when we leave a well-lighted room, and go into the dark road or street?