EXPERIMENTS ON PRESSURE OF LIGHT 47 



absorbing, and that the silver was perfectly reflect- 

 ing, and that the energy in the beam was P per 

 cubic centimetre, the pressures on these should 

 be 



B|B B|S S|S S|B 



P |P 2P 2 P 



But the black surface reflected a slight amount 

 which was estimated at 5%, and the silver surface 

 did not reflect the whole but an amount which was 

 estimated at 95%. We assumed that the black 

 radiated out O'95 as much as a full radiator, and 

 that the silver radiated out O'O5 as much as a full 

 radiator. With these values it can be shown that 

 the pressures should be 



B'B B|S SS S|B 



1-05 P 1-62 P 1-95 P 1-92 P 



Again, though we suspended the discs in a flask 

 evacuated as far as possible, there was still a small 

 radiometer action due to the residual gas. For 

 the front face of a disc was always a little higher 

 in temperature than the back face, higher by the 

 amount necessary to carry through from front 

 to back the energy emitted at the back. The 

 difference in temperature was greatest with the 

 black and black disc, and with that disc the 

 radiometer action was the greatest. It tended to 

 press the disc back from the source. 



Each disc was made of two thin circular plates 



