KINETIC OR MECHANICAl- MKW OF NATUUK. 53 



in!4 liere employed gave tlie ehie to all subseiiuent 

 attempts to deal with the ditticult problem of the inter- 

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

 sible alteration of the density or the rijj^iility (called the 

 elastic constants) of the ether when Hllinj^ the interstices 

 of transparent bodies ; of the meciianical differences which 

 make some bodies transparent for some and opa([ue for 

 other rays of light. ]\Iany possible modiKcations were 

 tlieoretically foreseen, giving rise to remarkaltle 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 suilicient, 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 tlie phenomena of })arlial 

 reflexion, refraction, dispersion, and partial or eomjilete 

 absorption. This more complicated problem in the 

 theory of elasticity had already presented itself in its 

 simpler form in the tlieory of the pendulum. To the 

 principle of optical consonance which had been employed 

 to explain the phenomena of absorption of liglit was 

 added, in order to explain the phenomena of tUsper- 

 sion, the principle of tlie free and forced vibrations 

 of a vibrating system.^ 



1 " If to the l)ob of a pendulum, Anomalous dispersion such as wiu 



executing horizontal vibrations, foreseen by Sellmciei- and Lord 



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



cuting vibrations of a slightly ansen and Kundt dejiends on the 



shorter [)eriod, the effect of the change of wave fre(|uency indei)en 



latter will be to increase the period 

 of the former and rice vcr.id " (see 

 A. S. Percival, ' Optics," 1 899, p. 1 81 ). 



dent of the change of wave length 

 in refracting media. 



