Oldal 175 [175]
OCR
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 naturally 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 comparatively pure blue part of the sky, whilst the shaded snow, owing to its innumerable facets turned in all directions, reflects 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. Threlfall, be partly due to physiological causes depending on a fatigue of the eye from looking at bright snow.
