4O PRESSURE OF LIGHT 



stantly, and keeps that value through the whole 

 time of exposure. The convection effect only 

 gradually creeps up to its full value, and has not 

 become serious in 6 seconds. From the deflection 

 produced by the 6 seconds incidence of the beam 

 its full effect with long-continued incidence could 

 be calculated. 



To eliminate the radiometer action the beam 

 was first directed on to the front face of the disc. 

 That face would be slightly the hotter, and the 

 radiometer action would join with the light pres- 

 sure in pushing the disc back. The beam was 

 then directed on to the back face of the disc, 

 and the light pressure was reversed. The beam, 

 passing through the transparent glass, would still 

 be incident on the silver on the front. This 

 would still therefore be the warmer face, and the 

 radiometer action would be in the same direction 

 as before. That is, it would now be opposed to 

 and lessen the apparent effect of the light-pressure. 

 The mean of the two deflections observed would 

 therefore give the light-pressure, and eliminate the 

 radiometer action. 



The beam was almost totally reflected by the 

 silver, and so the pressure was nearly twice as great 

 as if the beam had been absorbed. The actual re- 

 flecting power of the silver was determined, and its 

 slight defect from total reflection was allowed for. 



To determine the energy in the beam it was 



