HELMHOLTZ IN BERLIN 



that of friction. Each molecule of the ether is thus 

 affected by ( I ) an elastic reaction of the ether ; 

 and (2) a force due to the ponderable medium, 

 which is supposed to be proportional to the relative 

 displacements of a molecule of ether and a molecule of 

 ponderable matter. The ponderable molecule, on the 

 other hand, is acted upon (i) by a force equal to, 

 and in the reverse direction of, the preceding ; (2) a 

 force due to the neighbouring ponderable molecules ; 

 and (3) a retarding frictional force proportional to 

 the rapidity of displacement. These conditions 

 mathematically expressed lead to the differential equa- 

 tions of motion. When there is no sensible absorp- 

 tion, the formulae indicate a normal dispersion, but 

 when great absorption takes place, theoretical results 

 are obtained in accordance with those observed in 

 anomalous dispersion. Thus, as expressed by Professor 

 Tait, Helmholtz's explanation 'depends upon an 

 assumption as to the nature of the mutual action 

 between the luminiferous ether and the particles of 

 the absorbing medium, coupled with a farther assump- 

 tion connecting the absorption itself with a species of 

 friction among the parts of each absorbing particle.' 

 These assumptions were first suggested by Allenmeier, 

 but they were fully applied by Helmholtz. 



This explanation was offered in 1874, but he re- 

 turned to the subject in 1892 and 1893, and endeavoured 

 to account for the fact in accordance with Clerk 

 Maxwell's electro-magnetic theory of light, by the 

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