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6 LIGHT AND WATER % illustration of the law. Referring again to Fig. 1 let us now suppose AB to represent the cushion of the table; if the player’s ball is at P, and he wishes to hit another ball at D off the cushion, he must play his ball so that it strikes the cushion at C; if he wants to send his ball to G, he must play for F, and so on. In practice, owing to the imperfect elasticity of the rubber cushions (unless it have a counteracting "side" or spinning motion) the ball will travel slightly more nearly parallel to the cushion after impact than before. Fig. 2. Image of a point in a horizontal mirror. In Fig. 2 we assume that there is a mere point of light at P, and that the eye is placed at E. ABCD is the perspective view of a horizontal mirror. There will be two rays of light going from P to E, (i) the direct ray PE, and (ii) the ray PFE, which reaches E after reflexion on the surface of the mirror at F: so that, in addition to the actual luminous point P, we shall see a second point of light I in the direction E F, the distance FI being equal to the distance FP. This second point of light, which appears at I, exactly as far beneath the surface of the mirror as P is above it and in the same vertical line with P, is called the
