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OCR
110 LIGHT AND WATER shadow, which, in all probability, faces directly towards the blue sky. For instance, the shadow of a red-brick chimney falls upon a red-tiled roof. The shady side of the chimney reflects a certain amount of light from the blue sky, but also a good deal from the warm red tiles of the surrounding roof, whilst the shaded portion of the roof, being turned towards the sky, reflects little but blue light and thus appears cooler in tone. In the case of the cloud shadow on the lake, however, something more than the mere blue or violet-blue colour of the light falling on the shaded surface is needed to account for the purplish tone of the shadow. The shadow may still be pink or purple, even when the greater part of the sky is covered with white clouds. But in shade there is less light reflected from floating particles than in sunshine, so that the shadow hides the colour of the water and at the same time renders the surface reflexion (from blue sky or white clouds, as the case may be) more prominent; and we may account for the apparent colour of the shadow on the supposition that the surface-reflected light there tends to assume by contrast the red or purple which is complementary to the bluegreen of the surrounding water in sunshine. This is merely thrown out as a suggestion. It is an undoubted fact that when two different colours are brought into contact their apparent difference is increased, each seeming tinged with the complementary colour of the other. A white or neutral gray patch on a bright green ground looks purplish, a blue patch on green tends to violet. And it has been noticed that, in the case of cloud shadows on water, the greener the water the pinker the shadow appears to be. A similar question arises with regard to the pink patches caused by submerged seaweed referred to on page 98. Are we to ascribe the colour entirely to the effect of the double absorption of light—in its passage (i) through the water, and (11) through the surface layer of the seaweed—so that the rays which finally emerge from the water give the sensation
