EXPERIMENTS ON PRESSURE OF LIGHT 43 



This principle of enclosure may be used to 

 show the pressure of light to an audience. Fig. 17 

 represents an arrangement which I have found to 

 work very well. 



A silvered disc S and a blackened disc B are 

 fixed inside a thin mica rectangular box suspended 

 by a quartz fibre in a metal case with plate glass 

 front ; w is a side window through which a beam of 

 light is sent on to a mirror m and reflected thence 

 on to a scale. The pressure is reduced by a pump 

 to about I or 2 cm. of mercury indicated by a 

 small barometer G. When the light is directed for 

 a short time on to S, S is pushed back. When it is 

 directed on to B for the same time, B is pushed 

 back, but not so much as S. 



