EXPERIMENTS ON PRESSURE OF LIGHT 37 



disc than with the thinner five times greater 

 since the disc is five times thicker. If, for instance, 

 the deflection is 18 divisions with the black disc, 

 O'l mm. thick, and 13 divisions with the black disc, 

 O'O2 mm. thick, we get a reduction of 5 divisions 

 for a reduction of '08 in thickness. A further 

 reduction of 4 O2 would lessen the deflection by 



5 x - ( = - = 1*25. That is, an exceedingly thin 

 *Oo 4 



disc should give a deflection of 13 1*25 = ir/5 

 divisions, and an exceedingly thin disc should 

 have the same temperature on the two sides, and 

 give no radiometer action. In this way, by obser- 

 vations with the same beam, first on the thicker, 

 and then on the thinner black disc, the radiometer 

 action was eliminated. 



On the bright discs the radiometer action was 

 too small to be measurable. 



On the black discs the beam was absorbed, and 

 gave its full pressure P = E. On the bright discs 

 the beam was partly reflected, and the reflected 

 beam was also pressing against the surface. The 

 fraction of light reflected was determined by some 

 subsidiary experiments. Suppose it to be r, then 

 the pressure was P (i + r). 



To test the theory of light-pressure it was neces- 

 sary to measure the energy E in a cubic centimetre 

 of the beam. One way in which this was done is 

 illustrated in fig. 14. A blackened copper block C, 



