Oldal 26 [26]
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