SOME EFFECTS OF LIGHT 



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that the lamp has sixteen candle-power, for light decreases 

 in density as the square of its distance from the source, 

 That is, if you are twice as far from a lamp as some one 

 else, you are getting (at least directly from the lamp) 

 only one-fourth as much light, which is easy to understand 

 when you remember that the light is radiating out in every 

 direction. So we see that an electric lamp which, at four 



FIG. 87. Diagram illustrating how candle-power may be measured, as explained 

 in text. 



feet, produces the same intensity of light or shadow as a 

 candle at one foot, has a candle-power of four squared, 

 that is, sixteen. 



Photosynthesis. You have noted the great importance 

 to man of certain effects that light produces on him. Yet 

 far more important to man than any of these is an effect 

 which it does not produce on him; an effect which it pro- 

 duces directly upon green plants, but which, indirectly, 

 is of vast importance to all other living creatures. 



Blindness is one of the greatest of misfortunes, and yet 

 one can live without seeing; that is, he can live without 

 witnessing all the effects of light which we have been con- 



