Page 176 [176]
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 con¬
trast the red or purple which is complementary to the blue¬
green of the surrounding water in sunshine.
This is merely thrown out as a suggestion. It is an un¬
doubted 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