Oldal 175 [175]
COLOURS IN RIPPLED WATER tog
is a matter of common observation that this latter part of the
sky appears to be of a purer blue than the higher regions
overhead, which have a decidedly violet tinge. Now it is
naturally with the blue of the lower sky that we compare the
shadow, and by contrast it appears somewhat purple. This
may be verified by a simple experiment. We can take a
small mirror and lay it flat on the snow in the shadow of
some post or other convenient object. Looking straight down
into the mirror we find the deep colour reflected from the
overhead sky so much stronger than that of the surrounding
shadow that it is difficult to compare the two tones; but
stepping back and again looking into the mirror, we natur¬
ally see a lower part of the sky reflected in it, and we now
observe that the colour shown in the mirror is distinctly bluer
than that of the shadow. This will be most clearly noticed
if we draw back to such a position that, as we look obliquely
at the mirror, the tone of its blue reflexion is of the same
depth as the tone of the more purple colour of the shadow.
The mirror reflects light vegu/arly from a low and compara¬
tively pure blue part of the sky, whilst the shaded snow,
owing to its innumerable facets turned in all directions, re¬
flects light zrvegularly from all parts of the sky and very
largely from those dark and more violet regions overhead
that do not enter into the field of vision.
Thus the shadow of an object in bright sunshine almost
always receives a large proportion of blue or violet-blue
light, and inthis way we can explain the “cool” tones of
shadows under more ordinary conditions. It is often said
that “shadow is cooler than shade,” by which is meant
that the shadow cast by an object is cooler in tone than the
shaded side of that object. This is generally the case and
is due to the fact that the “shade” receives more warm light
from surrounding objects than the surface covered by the
ment in these shadows may therefore, as suggested by Prof. Threl¬
fall, be partly due to physiological causes depending on a fatigue
of the eye from looking at bright snow.