Page:Popular Science Monthly Volume 44.djvu/367

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power, he will probably speak of the sky first as the great source of light to the window. Then, if he does not ignore them altogether, the objects on the earth are grouped in one class of comparatively little importance. This is not Nature's method. There is no sharp division between the strong reflecting surfaces and the weak, between the sky and the earth. We are prone to such a distinction, because the sky seems prominent as a reflecting surface, but it can be shown that circumstances arise when this is by no means true.

Imagine the window to look out upon a landscape. Earth, foliage, and sky all combine to reflect light into the room. The water may be very dark, from the shadow of overhanging trees, reflecting less than ten per cent as much light as comes from the sky, but there is a path across it where the sunlight is cast back far more brilliant than the sky itself. The beaten highway gleams in the sun so that it is a relief to look away from it into the blue above. Even the foliage, delightfully dark and cool in the shadows, may have a brightness where the sunlight strikes it which is fifty or eighty per cent of the intensity of sky light.

On turning from the country to the city view we find the lessened importance of the sky as a source of light especially emphasized. There the great value of space causes one building to encroach upon the sky light of another until frequently the patch of blue visible from the windows is limited to a mere streak, or may be cut off entirely. If the sky were the only means of lighting, the windows would be useless in such cases; but the fronts of buildings, the paved streets, and other surfaces combine to throw much light into them, and give a reason for their existence.

The reflecting power of the sky dome is due almost entirely to the particles of vapor contained in the atmosphere, and hence must be considerably affected by changes in its aqueous condition. Contrary to what might be imagined, the clearer the sky the less valuable is it as a reflector. The more haze that it contains within limits the more intense the light obtained from it. An observer will recall dreamy summer days when the sun has seemed to shine softly as through a gauze cast over its face and the shadows were mellowed and diffused, yet the sky was white with a radiance painful to the eye. There was little suggestion of ethereal blue in the white light sent down from this atmosphere charged with particles of moisture. Again, a clear day comes; the air fairly dances with brilliancy, and distant objects stand out in the sunlight as clear-cut as a silhouette. The sky is a beautiful Italian blue, but does it occur to one how really dark it is except in the immediate vicinity of the sun? Try to match it with a sheet of blue paper, and it is almost startling to discover what a somber surface the sky dome is. Its value as a source of light is greatly