224 SCIENCE AND METHOD. 



repels them just as would a shower of projectiles 

 coming from the Sun. The mass of these particles is 

 so small that this repulsion overcomes the Newtonian 

 gravitation, and accordingly they form the tail as they 

 retreat from the Sun. 



Direct experimental verification of this pressure ot 

 radiation was not easy to obtain. The first attempt 

 led to the construction of the radiometer. But this 

 apparatus turns the wrong way, the reverse of the 

 theoretical direction, and the explanation of its rota- 

 tion, which has since been discovered, is entirely 

 different. Success has been attained at last by creat- 

 ing a more perfect vacuum on the one hand ; and 

 on the other, by not blackening one of the faces of 

 the plates, and by directing a luminous beam upon 

 one of these faces. The radiometric effects and other 

 disturbing causes are eliminated by a series of minute 

 precautions, and a deviation is obtained which is 

 extremely small, but is, it appears, in conformity with 

 the theory. 



The same effects of the Maxwell-Bartholi pressure 

 are similarly predicted by Hertz's theory, of which I 

 spoke above, and by that of Lorentz, but there is a 

 difference. Suppose the energy, in the form of light, 

 for instance, travels from a luminous source to any 

 body through a transparent medium. The Maxwell- 

 Bartholi pressure will act not only upon the source at 

 its start and upon the body lighted at its arrival, but 

 also upon the matter of the transparent medium it 

 traverses. At the moment the luminous wave reaches 

 a new portion of this medium, the pressure will drive 

 forward the matter there distributed, and will drive it 

 back again when the wave leaves that portion. So 



