'Manchester Memoirs, Vol. Hi. (1908), No. 10. ii 



notion of atomic structure there is also another fun- 

 damental order of ideas, arising from the phenomena of 

 radiation, which must be included — the conception of an 

 atom or molecule as a vibrating system of some sort^ 

 complete in itself and reacting by resonance with such 

 waves of radiation as have periodic times adjacent to the 

 periods of its own free vibration. 



In the famous memoir " On the Theory of Light and 

 Colours " read by Thomas Young before the Royal 

 Society on November 12th, 1801, which, in the form of 

 a mass of brief and pregnant suggestions, lays the 

 foundation of modern physical optics, the view of the 

 refraction of light, as due to the reaction of natural free 

 vibrations of the constituent parts of the refracting 

 medium, had already been advanced. The passage 

 perhaps demands quotation.* After giving a correct 

 apergii of the mechanism of total reflection, as involving 

 and being supported by surface waves in the rarer 

 medium, he proceeds as follows : — 



^'■Proposition VII. If equidistant undulations be sup- 

 posed to pass through a medium, of which the parts are 

 susceptible of permanent vibrations somewhat slower than 

 the undulations, their velocity will be somewhat lessened 

 by this vibratory tendency ; and, in the same medium, the 

 more, as the undulations are more frequent. 



" For as often as the state of the undulation requires a 

 change in the actual motion of the particle which transmits it, 

 that change will be retarded by the propensity of the particle 

 to continue its motion somewhat longer ; and this retardation 

 will be more frequent and more considerable as the diiference 

 between the periods of the undulation and of the natural 

 vibration is greater." 



It is hardly possible to extract definite meaning from 

 this cryptic explanation : indeed, the dynamics of a com- 



. * " Lectures on Nat. Phil.," quarto edition, vol., ii., p. 623. 



