Let A B, _fig. 20._, be the cushion, and C D the direction in which the ball moves towards it. If the ball and the cushion were perfectly inelastic, the resistance of the cushion would destroy the motion of the ball, and it would be reduced to a state of rest at D. If, on the other hand, the ball were perfectly elastic, it would be reflected from the cushion, and would receive as much motion from D to C after the impact, as it had from C to D before it. Perfect elasticity, however, is a quality which is never found in these bodies. They are always elastic, but imperfectly so. Consequently the ball after the impact will be reflected from D towards C, but with a less motion than that with which it approached from C to D. Now let us suppose that the ball, instead of moving from C to D, moves from E to D. The force with which it strikes D being expressed by D E′, equal to E D, may be resolved into two, D F and D C′. The resistance of the cushion destroys D C′, and the elasticity produces a contrary force in the direction D C, but less than D C or D C′, because that elasticity is imperfect. The line D C expressing the force in the direction C D, let D G (less than D C) express the reflective force in the direction D C. The other element D F, into which the force D E′ is resolved by the impact, is not destroyed or modified by the cushion, and therefore, on leaving the cushion at D, the ball is influenced by two forces, D F (which is equal to C E) and D G. Consequently it will move in the diagonal D H. (91.) The angle E D C is in this case called the “angle of incidence,” and C D H is called “the angle of reflection.” It is evident, from what has been just inferred, that the ball, being imperfectly elastic, the angle of incidence must always be less than the angle of reflection, and with the same obliquity of incidence, the more imperfect the elasticity is, the less will be the angle of reflection. In the impact of a perfectly elastic body, the angle of reflection would be equal to the angle of incidence. For then the line D G, expressing the reflective force, would be taken equal to C D, and the angle C D H would be equal to C D E. This is found by experiment to be the case when light is reflected from a polished surface of glass or metal.