Page:Popular Science Monthly Volume 44.djvu/369

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by no means contemptible. An illustration of this may be given. While reading on the shady side of a railway car one afternoon, a sudden flush of warm-tinted light seemed to cover the page and as quickly it was replaced by a cold gray light. The change was very noticeable and the cause easily revealed. At intervals along the track were stretches of rough-faced masonry, perhaps seven feet high. The stones looked in shadow a dull gray and a buff color, but when the sun struck them they glowed with a light that flashed into the car window. In spite of the fact that the area of sky light was far the greater of the two, that rough retaining wall determined in a great measure the character and intensity of the illumination on the paper.

Light-colored surfaces are the most valuable reflectors, and among them white paint and whitewash stand pre-eminent.[1] Even in shade, when illuminated only by other objects or by the sky, they will give sixty per cent or more of an average sky light. Sometimes they will run up to the full value of sky light, if a reflecting surface near by shines brightly upon them, or they may fall to twenty per cent or even less in deep shadow. But when the direct rays of the sun fall upon a newly whitewashed surface, the volume of light it reflects is almost blinding. Three hundred to four hundred per cent is not too large an estimate to place upon it in comparison with the sky light. On this account the well known expedient is used of whitewashing or painting in some bright tint the walls of a light-shaft or surfaces facing a window which is much shut in. A case is known of the rear of a house so treated being in summer time a source of great annoyance to dwellers on the next street, because of the blaze of light reflected into their rear windows. White marble is quite similar in its powers of reflection. A striking example can be seen in the spires of St. Patrick's Cathedral in New York. Rising far above the dust of the city, they are not yet covered with its grime, and their pure white glistens in beautiful contrast to the dark blue of the sky.

From the white of paint or marble there are many variations in building surfaces, all the way down to black. A large number are to be found among the granites. Some varieties are very dark in tone, reflecting little light unless polished, but the gray granites give considerable light. A freshly tooled gray granite will certainly yield one hundred per cent of sky light when in sunshine, and some varieties give far more. The writer has known the gray granite of an old building to give one hundred and forty

  1. The percentages stated in the following pages are based upon a large number of photometric measurements of the light from building surfaces, made by the writer at various times and places.