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 reflection must make angles with b N and c O respectively equal to A b N, A c O. These are the angles of reflection. If the eye is so situated that the rays enter it as in our illustration, an image of the point A is seen at the point A1, in which the lines D b, E c meet when produced backwards.