WIJVBOW LIGHTS AND THEIR VALUE. 359 



ordinary slate when held in the hand appears a very dark hlue ; 

 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 inter- 

 mediate 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 hori- 

 zon. 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 de- 

 gree. 



The pavement of city streets affords some interesting observa- 

 tions. 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 surpris- 

 ing to learn that dry flagging, when illuminated by sunlight, is 

 about the equal of the highly iDrized 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 trans- 

 lated, 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 reflec- 

 tion of white light from the water upon the surface. 



It has perhaps been noticed that thus far, in discussing reflect- 

 ing surfaces, attention has been confined to the changes they pro- 

 duced 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 



