4 2 



PRESSURE OF LIGHT 



forward against the front plate. These two kicks 

 neutralized each other. Another action, due to 

 emission of particles from the absorbing surface 

 itself when heated, was at the same time elimin- 

 ated, for particles thus shot out would give equal 

 impulses, back against the disc and forward 



FIG. 17. 



against the plate. Hull found that with such an 

 arrangement the light-pressure was measurable 

 even at a pressure of 70 mm. of mercury. It is 

 interesting to note that Sir William Crookes ! had 

 found that radiometer action nearly ceased even in 

 full sunshine with a similar arrangement of an 

 enclosed disc. 



Phil Trans. 170, 1879, p, 88, 389. 



