HOW LIGHT EXERTS PRESSURE 15 



Convection currents, then, disturb the experiments 

 when we work in air, and radiometer action disturbs 

 them when we work in a vacuum. We shall see 

 later how it is possible to steer between Scylla and 

 Charybdis and reveal the true pressure due to light. 



It is just a hundred years since Thomas Young 

 killed the corpuscular theory of light and founded 

 in its place the theory that light consists of waves, 

 a theory soon accepted by every one. But there 

 was no reason at that time to suppose that the 

 waves could press, and so experiments to detect 

 light-pressure ceased for nearly a century. 



In 1873 Clerk Maxwell put forth the Electro- 

 magnetic Theory of Light, a theory now universally 

 accepted. On this theory light still consists of 

 waves, waves of electric and magnetic disturbance 

 just like the waves used in wireless telegraphy, 

 but microscopic in length instead of being yards 

 or miles from crest to crest. He showed, too, 

 that such waves should exert a pressure, but just 

 half that exerted according to the discarded 

 Corpuscular Theory. He calculated with the data 

 he took that strong sunlight falling perpendicularly 

 against a black surface exerts a pressure of rather 

 less than one two-hundred-thousandth of a grain 

 on a square inch, or rather less than one twenty- 

 thousandth of a milligramme on a square centi- 

 meter. It only amounts to two and a half pounds 

 weight on a square mile. 



