Stranica 174 [174]
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 ob¬
ject by its surroundings is most readily observed in the case
of a white surface, such as a piece of white drapery or a white¬
washed 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 prac¬
tically 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 com¬
pares 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, ap¬
pears 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 sen¬
sation would make a physically gray object appear purple, for the
red and blue-violet elements would predominate. The purplish ele¬