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COLOURS IN STILL WATER _ 73
(without previous knowledge) say whether the surface
were of white marble or of polished ebony. Or a piece
of coloured glass may be placed on a sheet of white
paper, and it will be found that, as the glass is turned
sideways, a position is reached in which its colour can
no longer be seen. In the same way, wet patches of
smooth sand left by the ebbing tide, when viewed
sufficiently obliquely, show only sky reflexion, and
nothing of the actual colour of the sand.
On the other hand, we get the opposite extreme,
if, standing at the edge of a muddy duck-pond in full
sunshine, we look directly at the near surface of the
water. The local colour, which resembles that of
green pea-soup, 15 now quite conspicuous, especially
if the pond has been lately stirred up by its tenants or
by a heavy shower of rain, whilst the reflexion-picture
at this abrupt angle is very faint, though it rapidly
asserts itself as we look more obliquely at the water.
The same thing is true in a less degree of the sea
on a calm day, when the greenish colour of the nearer
water is far less apparent from the beach than from
the top of a high cliff.’
1 The following simple experiment will serve as a practical
illustration of this principle. Take a plain white basin, tip it up
on one side, as in Fig. 26, and fill it as full of water as is possible
in this position. Now take a small object, such as a gold stud or
black button, attached to a piece of thread. Look vertically down
into the water, hold the button by the thread, so that it hangs
immediately beneath the eye, and lower it into the water. Before
it reaches the surface its reflexion will only be very faintly seen,
and after entering the water it will itself lose very little in brilliancy,
thus showing that in this position (z.e., when the line of vision 1s
vertical) the refracted ray reaching the eye is much stronger than