38 



NATURE FOB ITS OWN SAKE 



the illuminated upper atmosphere counting 

 oat for the present the faint if direct light of 

 the stars. Were it possible for a tree or a 

 house to be in the far upper space where there 

 is no air, its sunlit side would be intensely 

 brilliant and its shaded side coal-black ; but on 

 the earth the shadow of a tree or house is illu- 

 minated by the atmosphere surrounding it, and 

 by the side reflections thrown upon it. It is 

 the diffused light, produced by atmosphere or 

 otherwise, that makes a shadow luminous, and 

 it is the sharp, direct light that makes a shadow 

 dark. One may state a general rule in these 

 terms : The greater the diffusion of light, the 

 greater the expansion and illumination of shad- 

 ows ; the sharper and more direct the light, the 

 more contracted and the darker the shadows. 



We can see this well exemplified almost any 

 night by studying the light of the electric arc- 

 lamp. It is the strongest and the most direct 

 artificial light we possess ; moreover, it is a 

 white light, with much of blue and violet in 

 it, and the shadows produced by it are very 

 dark and clear-cut. Seen at night, these 

 shadows cast by the bare limbs of a tree upon 

 pavement or upon snow are precisely edged, 

 have little penumbra, and are almost inky in 



