To understand this well, you must know (a) that the quantity of rays entering the eye increases in inverse proportion to the squares of its different distances, and that if an object is two inches from the eye, for example, nine times more rays will enter it than if it were six inches from it; (b) that as a determinate quantity of rays is necessary to disturb the retina sufficiently and to make us see objects, the opening of the pupil would have to diminish as the rays increase; (c)