Manchester Memoirs, Vol.lii.{igo?)),No.\Q. 13 



closely attuned to it, so as to induce strong sympathetic 



vibration in the parts of the medium which they traverse, 



the radiant energy emitted by these active vibrators must 



be supplied from that of the exciting train of waves. 



But it appears that, in following out this chain of ideas 



on the nature of optical absorption, it had not occurred to 



either Stokes or Thomson to consider the reaction 



exerted by the vibrating absorber on the train of waves ; 



and the elucidation of the dispersion of colours in light as 



due to the induced vibrations of the molecules, which 



had naturally presented itself to Young at the beginning 



of the century, remained to be enforced by Maxwell, 



Rayleigh,* Ketteler, and especially Sellmeier, having 



been as it seems definitely perceived and sought for 



experimentally, in the form of anomalous dispersion near 



a region of absorption, by the latter, as early as 1866. As 



the matter presented itself in luminous brevity to Lord 



Rayleigh, the medium is capable of free standing 



vibrations after the manner of a plucked string, executed 



in a periodic time which is determined for each type of 



vibration, i.e., wave-length, by its elasticity and inertia 



alone, provided it contains no independent internal 



vibrators that could be excited cumulatively by this 



motion : but where it contains structures that can set 



themselves vibrating in sympathy, the circumstances are 



analogous to those of an oscillating string on which light 



free pendulums with periods of their own, near those 



under consideration, are hung : the regular swing of the 



system now takes place in a modified time, and the 



velocity of propagation, determined by the ratio of wave- 



* See an early paper by Lord Rayleigh, " Scientific Papers," pp. 14X — 6 

 (1872) — prior to the publication of Sellmeier's demonstration, but, as he now 

 thinks, possibly written with recollections of Maxwell's ideas — in which the 

 practical inadequacy of mere differences of passive optical density is 

 insisted on. 



