38 
NATURE FOR ITS OWN SAKE 
The law of 
shudows. 
Electric- 
light 
shadows. 
the illuminated upper atmosphere—counting 
out 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 
