Analogue for the JEther. 655 



a wide range of dispersive qualities is provided for. If 

 a system of vibrators of the type suggested here were in 

 steady motion through the fluid, it has been demonstrated * 

 that there would be a certain convection of the fluid in 

 the region occupied by the vibrators ; and a convection 

 -coefficient has been obtained in agreement with that required 

 by the theory given by H. A. Lorentz T in 1895. 



We come now to consider the motion of solid bodies 

 at various speeds through water in its bearing upon the 

 effects observed in the case of motion of bodies at increasing- 

 speeds through the aether. In this connexion the analogy 

 between the aether and an ordinary fluid has been very 

 fully discussed, especially with reference to the theory 

 advanced by Sir George Stokes in 1845 to account for 

 aberration. It is not the intention meantime to discuss 

 Stokes's theory except in its direct relation to the parti- 

 cular form of the fluid analogue for aether with which we 

 are at present concerned. Stokes assumed that the Earth 

 and the planets in their motion would set the aether in 

 motion, just as any solid in translationai motion within 

 an incompressible fluid would communicate motion to the 

 fluid. Later writers have referred to Stokes's theory as 

 directly applicable to all solids in motion. But, from the 

 standpoint of the present comparison between aether and 

 water, it does not appear to be consistent with the analogy 

 to suppose that all actual solids would set the aether in 

 ■motion in the manner described. In particular it seems a 

 more natural comparison to make between the aether and an 

 ordinary fluid, if we regard all solids which freely transmit 

 luminous and electromagnetic waves as being more or less 

 freely permeable also by the aether. This would appear 

 to exclude from the application of Stokes's theory all 

 transparent bodies and certain dielectrics, and it would 

 remove one of the outstanding difficulties in the way of 

 the acceptance of Stokes's theory %. 



An important objection to Stokes's theory has been 

 pointed out by H. A. Lorentz §. If the aether be an 

 incompressible fluid, its irrotational motion would be com- 

 pletely determined by the normal component velocity alone 



* See It. A. Houstoun, " Fizeau' s Experiment and the JEther," 

 Phil. Mag. vol. xxxvii. p. 214 (1919). 



f ' Versueh einer Theorie der Electrisehen und Optischen Er- 

 scheinungen in Bewegten Korpern,' p. 101 (1895). 



\ The difficulty referred to is to account for the experimental result 

 obtained by Rowland, Rontgen, and Eichenwald. 



§ Archives Neerl. xxi. p. 103 (189G). 



