66 



PRESSURE OF LIGHT 



chamber communicated with a side cylindrical 

 chamber bored through the block and 3-25 mm. in 

 diameter. In this, nearly closing it, was a small 

 metal piston P, 2-85 mm. in diameter. This piston 

 was at one end of a torsion rod suspended from T 

 by a quartz fibre, and on the rod was a mirror by 



FIG. 31. 



which the deflection was read by a telescope on a 

 scale 5 metres away. The rod passed through a 

 small hole in the side of the cylindrical cavity, 

 just large enough to give it sufficient play. The 

 whole apparatus represented in the figures was 

 enclosed in an outer gas-tight case. 



If the piston had fitted the cylindrical cavity 

 exactly and had been entirely free from friction, 

 when the beam passed through G there would have 



