58 PRESSURE OF LIGHT 



The gas action was so much less and so much 

 more regular in hydrogen that it was found possible 

 to obtain evidence for the tangential light force 

 even at atmospheric pressure. The following table 

 gives a series of values obtained for the deflection 

 in scale divisions due to the light force alone, the 

 angle we have denoted by L 



IN HYDROGEN. 



Pressure in Scale divisions deflection 



cm. of mercury. due to light force. 



0-04 5-9 



0-09 6' i 



0'2O 5 '2 



0'44 5*4 



074 5 '5 



1-2 57 



2'I 5'2 



3 -2 5'I 



6-0 5'5 



10-4 5-4 



22-4 5-2 



477 5'2 



73-6 4-4 



(2) When a beam of light is shifted parallel to 

 itself, it exerts a torque tending to turn the system 

 by which it is shifted. 



Let us begin with a mechanical illustration. If 



