7O PRESSURE OF LIGHT 



force, but it is a mere nothing compared with 

 the pull of the Sun on the Earth due to its 

 gravitation, a force 47 million million times as 

 great. While the Sun, then, is pushing against 

 the Earth with its light, it is pulling it in with 

 its gravitation enormously more. 



But if the size of the body acted on is less, the 

 proportion of light-pressure to gravitative pull 

 becomes greater. For let us imagine that the 

 earth were divided into equal spheres, each with 



FIG. 32. 



half the radius of the actual earth. There would 

 be eight such spheres. If these eight spheres were 

 set facing the sun, as in fig. 32, they would ex- 

 perience in all the same total gravitative pull, but 

 they would expose in all twice the surface to his 

 light and would experience twice the light-pressure. 

 The gravitative pull would, therefore, be only 23^ 

 million million times the light-pressure. If each 

 of these small spheres were again divided into 

 eight equal spheres of half the radius the surface 

 would again be doubled, and the gravitative pull 



