Stranica 97 [97]
OCR
REFLEXIONS IN RIPPLED WATER :3 Fig. 15, which represents the view of P and its reflexions as seen from Q, we draw through P a horizontal line,’ we shall get as the reflexions of this line a succession of pairs of horizontal lines (one pair for each wave) through the points a and ő, c and d, 7 and £, / and 2? But, owing to the irregularity of the surface of the water, and the breaking up of the troughs by crossing waves into comparatively short depressions, these different reflexions of a horizontal line cannot continue in the water as horizontal lines. If we follow one of the pairs of theoretically horizontal lines on the side of a wave, we soon come to a place where the trough assumes such a shape that only one image, or none, can be formed; so that the two lines merge into one or vanish completely, perhaps to reappear a little farther on, and instead of two parallel lines we see a chain of loops or a series of disconnected rings.° Such rings are amongst the commonest features of gently moving water in the foreground of a picture. The reflexions of a boom or bowsprit, or of any conspicuous horizontal line, often assume this form (see On the third wave there is only one point at which P can be seen reflected, viz., E. ! Representing a straight line through P, Fig. 14, at right angles to the plane of the paper. ? And a single horizontal line through e. > A cup filled with water is all the apparatus necessary for the production of “rings.” Let the cup be placed so that a horizontal bar of the window frame is seen reflected in the still water. If the water be gently stirred round, its surface will become concave, and when the rotation has nearly ceased, the reflexion of the windowbar in the hollow surface will be seen to take the form of a ring.
