420 



INTENSITY OF LIGHT. 



the earth a diminution of its light takes place, but this 

 is usually not even noticed. 



The farther away a surface is from the source of 

 light the less is the intensity of the light which the 

 surface receives. This is easily seen from fig. 240, in 

 which three surfaces, A, B, C, are placed at the distances 



FIG. 240 (an. pry'.). 



: ] f ' 



LA,LB,LC from the flame Z, the distance LB being 

 twice LA, arid LC being three times as great as LA. 

 The same amount of light that is cast upon A would be 

 cast upon B, a surface four times as great ; therefore the 

 intensity of light there is one-fourth of what it is at A. 

 Again, the same amount of light that is cast upon A 

 would be cast upon (7, a surface nine times as great ; 

 hence the intensity there would be one-ninth of what it 

 is at A. The distances being therefore expressed by 

 the numbers 1, 2, 3, &c., the intensities will be ex- 

 pressed by the numbers 1, , -J-, &c., or, generally, the 



