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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 sur¬
faces 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) com¬
pared 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