490 ON THE ABSORPTION OF LIGHT 



alone would utter, the resonance of its tone is feeble, and 

 beyond a certain interval becomes inaudible. 



(15.) The dynamical principle on which these and 

 similar phasnomena depend is that of " forced vibrations/* 

 as it is stated in the Essay on Sound above referred to, 

 or, more generally, in a more recent publication (Cab. 

 Cyclop., volume on Astronomy), in terms as follow : 

 " If one part of any system, connected either by material 

 ties or by the mutual attractions of its members, be con- 

 tinually maintained by any cause, whether inherent in 

 the constitution of the svstem or external to it, in a state 

 of regular periodic motion, that motion will be propa- 

 gated throughout the whole system, and will give rise in 

 every member of it, and in every part of each member, 

 to periodic movements, executed in equal periods with 

 that to which they owe their origin, though not neces- 

 sarily syncJuwtous with them in their maxima and mini- 

 ma." The general demonstration of this as a dynamical 

 theorem is given in the Essay on Sound already referred 

 to, and its applicability to the transmission of light 

 through mateiial bodies is indicated in a note thereto 

 appended. 



(16.) The mode, then, in which we may conceive tlie 

 transmission of light through gross media to be per- 

 formed, so as to bring the absorptive pheenomena wdthin 

 the wording of this principle, is, to regard such media 

 as consisting of innumerable distinct vibrating parcels 

 of molecules, each of which parcels, with the portion ot 

 the luminiferous aether included within it (with which it 

 is connected, perhaps, by some ties of a more intimate 



