Oldal 74 [74]
42 LIGHT AND WATER
Fig. 17a may therefore be taken to represent the
general effect of waves travelling directly towards or
away from the spectator, and here the streak is very
clearly defined and far too long to appear in its
entirety on the greased glass. In Fig. 17c, which
illustrates the condition when the waves are running
at right angles to
the direction of
! sight, the streak is
W very short and ill¬
A defined. Thus we
see that when, as is
<> generally the case,
one is looking ob¬
liquely at the sur¬
> . —- . face of the water,
— the action of the
latter kind of waves
may almost be neg¬
lected in compari¬
_ son with that of the
3 | former. A combin¬
Fig. 17b. Glass turned round horizont¬
ally through 45°. ation of the two
forms of streak, that
is to say, the length of the vertical streak with the
width of the horizontal one, would give approxim¬
ately the effect produced if the wave-surfaces faced
equally in all directions. —
When the photographs shown in Plates IX and
X were taken, the wind happened to be blowing hard
from the north, and the ripples were travelling, as
may plainly be seen, across the direction of sight.
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