MECHANICS AND OPTICS. 225 



that the recoil of the source has for its counterpart the 

 forward motion of the transparent matter that is in 

 contact with the source ; a Httle later the recoil of 

 this same matter has for its counterpart the forward 

 motion of the transparent matter a little further off, 

 and so on. 



Only, is the compensation perfect ? Is the action of 

 the Maxwell-Bartholi pressure upon the matter of the 

 transparent medium equal to its reaction upon the 

 source, and that, whatever that matter may be? Or 

 rather, is the action less in proportion as the medium 

 is less refringent and more rarefied, becoming nil in a 

 vacuum? If we admit Hertz's theory, which regards 

 the ether as mechanically attached to matter, so that 

 the ether is completely carried along by matter, we 

 must answer the first and not the second question in 

 the affirmative. 



There would then be perfect compensation, such as 

 the principle of the equality of action and reaction 

 demands, even in the least refringent media, even in 

 the air, even in the interplanetary space, where it 

 would be sufficient to imagine a bare remnant of 

 matter, however attenuated. If we admit Lorentz's 

 theory, on the contrary, the compensation, always 

 imperfect, is inappreciable in the air, and becomes nil 

 in space. 



But we have seen above that Fizeau's experiment 

 does not permit of our retaining Hertz's theory. We 

 must accordingly adopt Lorentz's theory, and conse- 

 quently ^zW up the principle of reaction. 



(1,777) 15 



