less than on days when it is a duller blue, or when gray better describes its color. Some experiments which are at hand show a difference of over seventy per cent in the reflecting power of the northwest sky on two sunshiny days, and they were by no means extremes of the two conditions of atmosphere discussed. No experiments upon extreme states of the atmosphere are available, but it is safe to say that the reflecting power of the sky dome in this climate is one and a half times greater on some days than on others. Between the extremes are all possible variations.
Thus far the thought has been only of an unbroken expanse of sky, but if clouds float across the field they greatly change the conditions. A cumulus cloud piled high in great masses is carried past the window by the wind. It gleams beneath the sun's rays like a ball of cotton, and pours down a flood of light that may have as much as four times the intensity of the light from the sky directly beside it. At another time heavy thunder clouds will roll up from the horizon—a dark gray, unillumined by the sun. They obscure the sky and replace it by possibly ten per cent of its intensity of light.
Indeed, does it not seem as if there were no stability about the sky light? And yet, brushing the clouds aside, it will be found that the changes in any one day are not usually great. There is enough permanency in its reflecting power to make it serve as a practical standard of comparison—a standard not for the direct sunlight, which so far transcends any other light on the earth as to be unique, but for the vast variety of lights which crowd into the windows—the reflections from brick and stone, from wood and paint, from earth, water, and foliage.
Turning from the sky to the earth, a vast variety of reflecting surfaces is encountered. Each has its peculiar power of altering the light it reflects, both in intensity and quality. The amount of their influence upon window light is apt to be underestimated. Many rooms through the entire day and nearly all rooms for a portion of it have no direct sunlight, and all the light they do receive is entirely by reflection. Of this the portion coming from surfaces on the earth is a very considerable part.
It is true that most surfaces reflect but a small percentage of the light which strikes them, but when that light is the great flood from the sun itself the pittance which comes from them is
- In a number of cases carefully determined in city locations it has been found that the sky gave only from eight to forty per cent of the total light reaching a point inside a window on the ground floor. The remainder came from opposite building surfaces and from the street—this, with the sun shining upon these reflecting surfaces. At another time of day the sky value would be comparatively greater, yet not so much as might be imagined, because all the surfaces would not lie in shadow at the same time.