ordinary slate when held in the hand appears a very dark blue; little light is reflected from the interior, and that little is mainly blue, while the very small amount of white light reflected from the surface is not enough to pale the interior hue to any great extent. But turn the slate so as to reflect the light differently, and then there is such an excess of surface reflection as to give even a silvery tone to what was a moment before a dark material, A twist of the wrist will easily make a difference of two hundred per cent in the reflecting power. The extremes and all the intermediate conditions can be seen in slate roofs, and these are in a position where comparison is easily made with the sky. One roof will look almost black, as it stands out against the sky; another that reflects the sunlight will gleam like a white sail on the horizon. At another time it may melt away with a hue and shade that are not to be distinguished from those of the blue sky back of it. The same effect is obtained with shingles, but in a less degree.
The pavement of city streets affords some interesting observations. It is ordinarily looked upon as only fit to be trodden under foot of man, and very little credit is given its reflecting power. Paving stones appear of a rather somber color when held in the hand, and ordinary blue flagstone is similar. Hence it is surprising to learn that dry flagging, when illuminated by sunlight, is about the equal of the highly prized sky as a reflector of light. A considerable number of experiments, taken at various times and places, go to show this. Under ordinary conditions of dirt the pavement may be slightly darker than the bluestone flagging, but they will both give approximately one hundred per cent of sky light. Moreover, their hue is not unlike that of many skies. The idea is perhaps difficult to grasp, in view of their condition; but if a section of New York street could be purified and translated, it would quickly be lost to sight, ascending in a sky of its own color. When in shadow, fifteen to twenty per cent represents the amount of light that the street will give as compared with the sky. If the stones are wet, not more than ten per cent of light comes from them, except where there may be a powerful reflection of white light from the water upon the surface.
It has perhaps been noticed that thus far, in discussing reflecting surfaces, attention has been confined to the changes they produced in the intensity of light. These are not, however, the only alterations that occur. Quality as well as intensity is affected by the surfaces which throw light into the window. Leaving now the intensity of reflecting power, let us endeavor to realize what is taking place in every city street. It may be a thoroughfare filled with the noise of travel, but the air is crowded with silent lines of light. Back and forth they fly in all directions, every
