EXPERIMENTS ON PRESSURE OF LIGHT 51 



known weight and observing the rate at which the 

 temperature of the silver rose, the device used by 

 Nichols and Hull (ante, p. 41). The energy was 

 such that the incident beam alone falling on a 

 perfectly black surface should have produced a 

 deflection of 13*6 divisions. In the table below 

 the first line gives the nature of the disc, and the 

 second line gives the ratios of the deflections 

 calculated on the supposition that the black 

 reflects 5 per cent, and that the silver reflects 

 95 per cent. The third line gives the deflections 

 which might have been expected if there had been 

 no other forces than light-pressure. They are 

 obtained by multiplying 13*6 by the ratios. The 

 fourth line gives the deflections actually observed. 



Discs B|B B|S S|S S|B 



Calculated ratios . . . 1*05 r62 1*95 1-92 



Calculated deflections . 14-3 22'O 26-5 26*1 



Observed deflections . 16*1 22*3 287 28'O 



The excess of 16*1 over 14-3 with the Black- 

 Black disc is no doubt due to a small radiometer 

 action still remaining. The nearness of the ob- 

 served and calculated deflections of the Black- 

 Silver disc appears to give conclusive evidence of 

 the back pressure of the radiation issuing from its 

 front surface. 



D2 



