EXPERIMENTS ON PRESSURE OF LIGHT 41 



allowed to fall on a blackened silver disc of known 

 size and weight, and the rise in temperature was 

 determined by a thermo-electric method, into which 

 we shall not enter. The rise in a given time gave 

 the heat developed, and thence the energy per 

 cubic centimetre of the beam could be determined. 



Allowance was made for departure in various 

 ways from the ideal of the perfectly reflecting disc, 

 and ultimately Nichols and Hull found that the 

 pressure observed agreed with the value of the 

 energy per c.c. in the incident beam within less 

 than one per cent. 



When we consider the minuteness of the force to 

 be measured and the magnitude of the disturbances, 

 we must recognize this experiment as one of the 

 finest achievements of our time. 



FIG. 16. 



Professor Hull has made some interesting ex- 

 periments, 1 in which the receiving disc was enclosed 

 between two parallel glass plates, fixed respectively 

 in front and behind the disc, with a small interval 

 between, as shown in fig. 16. The radiometer 

 action was then eliminated. For if a molecule 

 gave an extra kick back against the absorbing 

 disc, it rushed forward and gave an extra kick 



1 Physical Review ', XX. May 1905. - 



