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32 LIGHT AND WATER their normal directions ; the reflexion of the upper edge of the picture has advanced to D, that of its lower edge to C, and at A and B are seen parts of the wall beneath. Now when we look at rippled water, we see surfaces tilted at many and ever-varying angles, so that a mirror floating at any point (as supposed in Fig. 12) NN Nea ee \> RI if Fig. 13. Here the observer is supposed to be looking from a considerable distance on to the surface of the rippled water (in the direction of the right-hand arrow, z.e. at an angle of about 40° with the horizon) and the lines of vision may therefore be represented by paralle/ straight lines. If the water were still these lines would also be parallel after reflexion, taking the direction shown by the arrow on the left. The crests and troughs of the waves are marked by short vertical lines. The figure shows how few, comparatively of the lines of vision (even at this abrupt angle of incidence) strike the far sides of the waves, and how some of those that do so, as at ő and e, undergo a second reflexion from the near side of the next wave. Thus there are very few lines inclined de/ow the normal direction of reflexion (as represented by the left-hand arrow) compared with those tilted up towards the sky. would be perpetually changing its inclination, swaying to and fro as the waves passed by; and therefore if we imagine the mirrors in Figs. 11 to oscillate between their positions in the second and third diagrams (though not necessarily in unison) we get an idea of
