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62 LIGHT AND WATER air into water, or vice versd, is a matter of everyday experience. It is a well-known fact that a coin just out of sight at the bottom of a cup can be made visible by filling the cup with water. A ray of light from a point C on the coin (Fig. 23) cannot travel , in a straight line from C to the eye at A, but must follow 0 eae a bent path, as in Fig. 21. The ray CO is bent downwards on leaving the water, and thus enters the eye; and eee the eye, assuming that light travels in straight lines, sees the coin in the direction AO at c, so that the water appears to be shallower than it actually is.* In the same way a stick thrust obliquely into water seems to be broken or sharply bent upwards at the point ! The image of a point formed by refraction is not stationary, as in the case of one formed by reflexion, but varies with the = position of the eye. In Fig. 24 (taken , a”% from Prof. W. Watson’s " Text Book we ae Physics,” Longmans, Green and a d = 7 Co., 1899) the points 2, 2, 2’, which ey lie on a “‘ caustic curve,” mark the difpi ferent positions of the image of the point P, corresponding to the different positions of the eye, a, a and a’. So that the more obligue the direction of vision, the shallower the water Fig. 24. appears to be. A sunken rock, that can easily be passed over in a boat, appears, as we approach it, to reach within a few inches of the surface of the water; but looking vertically down, the water seems some three-fourths of its actual depth.
