HOW IT WORKS.

So that if a series of impressions follow one another more rapidly than the eye can rid itself of them the impressions will overlap, and give one of motion, if the position of some of the objects, or parts of the objects, varies slightly in each succeeding picture.²⁵

THE PLANE MIRROR.

This chapter may conclude with a glance at the common looking-glass. Why do we see a reflection in it? The answer is given graphically by Fig. 131. Two rays, A b , A c , from a point A strike the mirror M at the points b and c . Lines b N, c O, drawn from these points perpendicular to the mirror are called their normals . The angles A b N, A c O are the angles of incidence of rays A b , A c . The paths which the rays take after