356 THE POPULAR SCIENCE MONTHLY. 



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 ex- 

 periments 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 car- 

 ried 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 win- 

 dow 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. 



