Page 71 [71]
REFLEXIONS IN RIPPLED WATER 39
Plate XI shows the streak just before its disappear¬
ance and about as clearly defined as is possible in the
open sea.
In considering the formation of such a streak of
light, we started by assuming the spectator to be
looking stvazght across a succession of very regular
waves, which, it was then shown, would cause a
luminous point to appear in the water as a vertical
line. But it might be argued that very regular waves
travelling at right angles to the direction of sight
would give a horizontal line in the water, and that
therefore, if the sea were rough, with the sides of the
waves facing in all directions, we should get, as the re¬
flexion of a luminous point, no streak at all, but only
a mere patch of light." That these cross waves are,
however, of little account as compared with those
" The statement on page 36, that the points a, 4, c, d, etc., in Fig.
14, all lie in the plane of the paper, is based on the supposition,
made for the sake of argument, that the waves are perfectly regular,
with the lines of their crests at right angles to the plane of the
paper. But as a matter of fact, the waves are often very far from
regular, and the motion of the water is generally exceedingly com¬
plicated, different wave-systems crossing each other in all directions,
so that the normals to the surface are inclined in every possible
direction within a certain angle from the vertical. The point P
would under these circumstances be seen reflected at points on the
surface of the water to the right and left of the vertical plane con¬
taining P and.Q, and the images thus formed will lie to the right
and left of the vertical through P (Fig. 15), so that, as we have
said, the combined effect of all the images will be a vague streak
rather than a well-defined line. In order to meet the objection
stated above, viz., that unless the movement of the waves towards
or from the observer were much more marked than in other direc¬
tions, the vertical streak would entirely disappear, let us consider
the opposite extreme and suppose the waves to be moving across