

EXPERIMENTS ON PRESSURE OF LIGHT 39 



method was somewhat like that of Lebedew, but 

 it differed in very important details. Two circular 

 microscope cover glasses, CD (fig. 15), each I2'8 

 mm. in diameter, and 0*17 mm. thick, were hung 

 up in a partially exhausted glass vessel by a 

 quartz fibre. They were sil- 

 vered on the front side and 

 brilliantly polished, m was 

 a small mirror, in which the 

 reflection of a scale could 

 be viewed by a telescope. 



A beam of light was di- 

 rected on to a disc, and the 

 force which it exerted was 

 measured by the deflection of 

 the scale in the telescope. 



Nichols and Hull made 

 use of the fact not yet ex- 

 plained, that when the pres- 

 sure of the air in the vessel 

 is reduced to 2 or 3 cm. of. 

 mercury, say somewhere 

 about an inch, the convection 

 effects are enormously reduced. Further, these 

 effects take time to develop, for the discs take 

 time to heat up. They therefore put the beam on 

 to the disc for 6 seconds only, one quarter of the 

 time of a complete torsional swing of the suspended 

 system. The light-pressure h^s its full value in* 



FIG. 15. 



