56 PRESSURE OF LIGHT 



diameter, was suspended in a case with glass sides, 

 by a quartz fibre, so that the disc was horizontal. 

 The pressure was reduced to I or 2 cm. of mercury. 

 A beam of light, AB, was directed, as in fig. 24, 

 at 45 on to a small area B, near the circumference 

 of the disc, the beam AB being in the plane 

 through the normal BN, and perpendicular to the 

 radius OB. The light force parallel to the surface 



FIG. 24. 



tended to push the disc round, through an angle 

 which we will call L. But the disc was heated by 

 the beam so that convection currents and radio- 

 meter action came into play. Unless the disc 

 were perfectly horizontal, a perfection which could 

 not be attained in practice, these would also turn 

 the disc round by an amount and in a direction 

 which would depend on the slope of the disc at 

 B. Let us call this angle of turn D. The total 

 angle of twist was therefore D + L, and this was 



