Stranica 131 [131]
OCR
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
