HOW LIGHT EXERTS PRESSURE 



II 



undergoing bombardment by the corpuscles, and it 

 was quite natural to suppose that the surface was 

 pressed back. 



We may illustrate the supposed mode of action 

 by fixing a vertical tin disc at the end of an arm 

 and suspending the arm by a fine wire, so that it is 

 free to turn round as in fig. i. 



Now arrange a funnel and a metal pipe, as in 



DISC 



FIG. 2. 



fig. 2, in which the disc is seen edgewise, and the 

 suspension is not shown. 



Pour some fine shot into the funnel, and they 

 run down the pipe and bombard the disc, pressing 

 it back. The shot acquire momentum. They 

 carry this momentum and give it up to the disc 

 when they^hit it. This giving up of momentum is 

 pressure. 



In the eighteenth century, when the Corpuscular 



