HOW LIGHT EXERTS PRESSURE 21 



by a beam of light was numerically equal to the 

 energy in a cubic centimetre of the beam. 



This, then, is Maxwell's explanation of light- 

 pressure on his electro-magnetic theory. The light 

 consists of tubes of electric force and tubes of 

 magnetic force, which are rushing sideways along 

 the beam and pressing sideways against each other 

 and against any surface which the beam strikes. 



But though we all now believe in the electro- 

 magnetic theory, and can hardly conceive that it 

 should be superseded, we must be warned by the 

 fate of the corpuscular theory, and recognize that 

 the electro-magnetic theory may have to go, if 

 some other still unimagined theory should account 

 for observed facts more completely and more 

 correctly. 



It is interesting then to know that whatever 

 kind of waves we imagine, so long as they have the 

 properties which we observe in light, these waves 

 must press against the surface from which they 

 start, and they must press against the surface on 

 which they strike. They must, in fact, carry 

 momentum with them just as surely as if they 

 were moving particles on the old corpuscular 

 theory. This was first pointed out by Bartoli in 

 1875, and the proof was put in a precise and simple 

 form by Sir Joseph Larmor. 



The fundamental idea of the proof is, that a 

 train of waves is somewhat like a compressed 



