KINETIC OK, MECHANICAL VIEW OF NATURE. 53 



ing here employed gave the clue to all subsequent 

 attempts to deal with the difficult problem of the inter- 

 action of the ether and ponderable matter ; of the pos- 

 sible alteration of the density or the rigidity (called the 

 elastic constants) of the ether when filling the interstices 

 of transparent bodies ; of the mechanical differences which 

 make some bodies transparent for some and opaque for 

 other rays of light. Many possible modifications were 

 theoretically foreseen, giving rise to remarkable unex- 

 pected phenomena, and these were frequently verified by 

 subsequent experience. The whole theory of light 

 entered upon a new phase as it became more and more 

 evident that the study of the vibrations of the elastic 

 medium was not sufficient, but that it must be supple- 

 mented by that of the interaction of two vibrating 

 systems, the ether and the molecules of the ponderable 

 substance, which give rise to the phenomena of partial 

 reflexion, refraction, dispersion, and partial or complete 

 absorption. This more complicated problem in the 

 theory of elasticity had already presented itself in its 

 simpler form in the theory of the pendulum. To the 

 principle of optical consonance which had been employed 

 to explain the phenomena of absorption of light was 

 added, in order to explain the phenomena of disper- 

 sion, the principle of the free and forced vibrations 

 of a vibrating system. 1 



1 " If to the bob of a pendulum, Anomalous dispersion such as was 



executing horizontal vibrations, foreseen by Sellmeier and Lord 



another pendulum be attached, exe- Kelvin and discovered by Christi- 



cuting vibrations of a slightly i ansen and Kundt depends on the 



shorter period, the effect of the 

 latter will be to increase the period 

 of the former and vice versd" (see 

 A.S. Percival, 'Optics,' 1899, p. 181). 



change of wave frequency indepen- 

 dent of the change of wave length 

 in refracting media. 



