50 PRESSURE OF LIGHT 



The plan of the apparatus is shown in fig. 20. 

 S was an Ediswan 5o-volt focus lamp, run steadily 

 at 60 volts. L x was a lens, and L 2 a second lens, 

 which threw an image of L x on whichever disc was 

 being used. B and C were a lamp a.nd scale, a 



So 



FIG. 20. 



mirror below the mica plate reflecting an image 

 of B on to C. 



The force corresponding to an observed deflec- 

 tion on the scale C was calculated in the usual 

 way from the time of swing of the suspended 

 system, first unloaded and then loaded with a 

 known mass. 



The energy of the incident beam was measured 

 by allowing it to fall on a blackened silver plate of 



