PRESSURE OF LIGHT 



of known size and weight, had a small hole bored 

 in it, and a small thermometer was inserted. A 

 screen with a hole D, just the size of a disc, was 

 placed in front, and the beam was directed on to 

 D, and passing through, fell on and warmed C. 

 The rise in temperature in a 

 given time showed how much 

 energy the beam brought up 

 in that time, and so the 

 energy brought up in a 

 second was determined. 



If V is the velocity of 

 light, and if H is the heat 

 developed in C per second, 

 measured in ergs, then H is 

 the energy in length V of 

 the beam, and H/V is that 

 in a length of one centimetre. 

 The force on a black disc, 

 the size of the hole D, should 

 therefore be H/V. The heat 

 measurements agreed with the force measurements 

 within about one-fifth of the whole. 



While Professor Lebedew was making this ex- 

 periment, Professors Nichols and Hull were also 

 experimenting on the subject, and in 1903 l they 

 published the full account of their work. Their 



1 Proc. American Academy of Arts and Sciences^ vol, 

 xxxviii. p. 559. April 1903. 



FIG. 14. 



