II 



EXPERIMENTS ON THE PRESSURE OF LIGHT 

 FALLING PERPENDICULARLY ON A SURFACE 



WHEN Maxwell put forward his theory of light- 

 pressure and calculated the exceedingly small 

 value due even to full sunlight, he remarked that 

 probably "a much greater energy of radiation 

 might be obtained by means of the concentrated 

 rays of the electric lamp. Such rays falling on 

 a thin metallic disc, delicately suspended in a 

 vacuum, might perhaps produce an observable 

 mechanical effect." 1 



Twenty-seven years later Professor Lebedew of 

 Moscow described to the Congres International de 

 Physique 2 an experiment in which he detected the 

 pressure in precisely this way, and in which he 

 found a value agreeing very fairly with that given 

 by Maxwell's theory. 



In a large glass globe 20 cm. or 8 inches in 

 diameter, among other arrangements of discs, 



1 Electricity and Magnetism, 793. 



2 Rapports, vol. 2, p. 133. A fuller account is given in 

 Ann. du Physik, vi. 433, Nov. 1901. 



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