EXPERIMENTS ON PRESSURE OF LIGHT 35 



H 



]M 



that shown in fig. 13 was used. This was 

 suspended by a fine glass thread TH. HH 

 was a glass rod to which was fixed a mirror 

 M, which reflected a scale into a telescope to give 

 the position of the arrangement. On cross arms 

 were fixed discs of platinum foil each 0*5 cm. in 

 diameter, and each with its centre I cm. 

 from the vertical axis of suspension. 

 The upper pair were O'l mm. thick and 

 the lower pair o - O2 mm. thick. The 

 discs on the right hand were covered 

 on both sides with a layer of platinum 

 black, while those on the left hand were 

 polished. The globe was exhausted as 

 far as possible by means of a Sprengel 

 pump, and then the beam from an arc 

 lamp was directed on to one of the 

 discs, and the consequent retreat of 

 the disc was observed by means of the 

 motion of the scale reflected into the 

 telescope. The value of the force re- 

 quired to push the disc back by the observed 

 amount was determined by observing the times 

 of vibration of the system when loaded with a 

 body of known size and weight and when unloaded 

 the method always used to find the force needed 

 to twist a fibre or wire through any angle. 



With the high degree of exhaustion used there 

 was probably no bodily convection of the air in 

 C 2 



FIG 



