Oldal 174 [174]
OCR
I08 LIGHI AND WATER in looking at such an object the eye receives light from all sides by reflexion in the countless small surfaces that go to make up the whole. Itis evident therefore that the apparent colour of all such unpolished objects will be affected by the colours of surrounding objects—a fact with which all artists are familiar. This modification of the normal colour of an object by its surroundings is most readily observed in the case of a white surface, such as a piece of white drapery or a whitewashed wall, which in the neighbourhood of well-illuminated coloured objects, may show a wonderful variety of hues and shades. In the case of the shadows on the snow we have practically only two tones to deal with—the pure white of the snow in sunshine, and the deep blue reflected from the sky above—so that the conditions are as simple as possible. Such blue shadows, seen at their brightest in the clear atmosphere of high altitudes, are well known to visitors to winter Alpine resorts. Now the shadow on the snow of a colourless object, such as a wooden post, appears blue, because the direct white light from the sun is cut off by the post and the shaded part receives only light from the sky above it, consisting largely of blue rays. For general purposes it is near enough to say that the shadow is blue, though as a matter of fact it appears slightly more violet in hue than the sky with which one compares it. : This purplish or violet tone is probably due to the fact that in the shadows is seen chiefly light reflected from the higher parts of the sky, which contains a larger proportion of violet rays than that from regions near the horizon.’ It :! Dr. Shelford Bidwell has observed that the “‘after-image” of a bright white object, when projected upon a gray background, appears under certain conditions to be purple and not merely a darker gray, which phenomenon he has attributed to the fact (pointed out by Rood) that the green colour-sensation is more readily fatigued than the others. The disproportionate weakening of the green sensation would make a physically gray object appear purple, for the red and blue-violet elements would predominate. The purplish ele
