Ill 



EXPERIMENTS ON THE PRESSURE OF LIGHT 

 AGAINST THE SOURCE FROM WHICH IT 

 ISSUES. THE RECOIL FROM LIGHT 



WE have seen that theory leads us to suppose 

 that waves of light carry momentum forward 

 momentum with them as surely as if they were 

 particles shot out from the source, and that they 

 receive this momentum from the source. The 

 loss of momentum by the source should be mani- 

 fested as a back pressure against it. In fact, just 

 as a gun recoils from a bullet which it sends forth, 

 so a luminous body should recoil from the waves 

 of light which it pours out. 



The most satisfactory and most direct method 

 of experiment would no doubt consist in suspend- 

 ing a disc, black on one side and silvered on the 

 other, in as perfect a vacuum as possible. Inside 

 the body of the disc should be a coil of wire. 

 An electric current should be introduced by the 

 suspension to heat the coil. The heat would be 

 given out as radiation almost altogether by the 

 black surface, and hardly at all by the silver sur- 



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