EXPERIMENTS ON PRESSURE OF LIGHT 45 



face. The black surface should be pushed back 

 by the issuing radiation. But the experimental 

 difficulties in the way of this direct method appear 

 to be quite insuperable. 



Dr. Barlow and the author have made an experi- 

 ment 1 showing the back pressure in a rather less 

 direct way, the disc being heated by allowing a 

 beam of light to fall upon it. The temperature of 

 the disc rose till it attained the steady state in 

 which the energy emitted as radiation was equal 

 to the energy absorbed. The effect was therefore 

 due in part to the pressure of the incoming radia- 

 tion, in part to the pressure of the outgoing 

 radiation, and the two parts of the effect had to 

 be disentangled. 



The nature of the action to be looked for may 

 be explained by considering ideal cases. Let us 

 suppose in the first case that a beam with energy 

 P per cubic centimetre is allowed to fall normally 

 on a very thin disc, perfectly black or fully absorb- 

 ing on both sides, and let the disc be suspended in 

 a perfect vacuum so as to be quite free from air 

 disturbances. The disc is heated and rises in 

 temperature till it gives out as much energy as it 

 receives. If it is very thin it is practically at the 

 same temperature on the two faces, and it gives 

 out half the energy from each face. The pressures 

 of the issuing radiations are therefore equal and 



1 Proc. Roy. Soc. A., vol. Ixxxiii. p. 534, 1910. 



