AN EXPERIMENTAL INVESTIGATION OF THE PRESSURE 



OF LKJHT. " 



Bv Peter Lebedew 



Kepler was prol)!i))ly the first (in 10Li») to attril)iite tlie form of 

 comets' tails to a repulsive force from the sun, and to explain this 

 repulsion as the I'esult of the pressure exerted by the sun's rays on the 

 matter in the tail. The same phenomenon also h'd Euler. in IT-ifi, 

 to ascribe a pressure to the solar radiation, and in ITri-l De Mairan 

 made the first attempts to test these ideas experimentally. 1)ut he did 

 not obtain a po.sitivt^ result. 



A dual theoretical basis for the pressure of radiation was indej)end- 

 ently and almost simultaneously given l)y Maxwell in isTo, as a con- 

 sequence of the magnetic theory of light, and l)v Bai'toli in IST*!, as a 

 consequence of the second law of thermodynamics. 



If a beam of parallel rays is normally incident on a plane surface, 

 the amount of the Maxwell-Bartoli light-pressure^^ is determined if 

 we know the quantity of energy ^received per second, the reflecting 

 p(nver p of the surface, and the velocity -" of light. For then 



where I) varies between for an absolutely black surface and 1 for a 

 perfectly reflecting surface. 



This pressure is very small; for solar rays falling normally at the 

 earth's distance uj)on a s([uare meter, the pressure exerted is 0.4 mg. 

 for a I)lack surface and 0.8 mg. for a mirror. 



In the experimental investigation of the pressure of light two large 

 sources of disturbance arise, the one due to the well-known radiometric 

 forces discovered l)y Crookes and the other to convection in the 

 residual gas. It is possil)le, however, to diminish these disturbing 

 forces and to make their efi'ect on the observations harndess, if the 

 experiments are made with very thin metal \anes and the exhaustion 

 is carri(Kl as far as possible by using a mercury pump and condensing 

 the mercury- \a})or by fi'cezing mixtures. 



" If eprinted, by penui.s.sion, from The Astropliysical .lournal, January, 1902. 

 Translated from an al)stract ooinmunicated l)y tlie author, of liiri extended paper in 

 Annalen der Physik, VI, 4:«-458, 1901. 



SM 1!>()2 12 177 



