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Sir William Thomson 



[Feb. 2, 



Cauchy was, I believe, the first of mathematicians or naturalists 

 to allow himself to be driven to the conclusion that the refractive 

 dispersion of light can only be accounted for by a finite degree of 

 molecular coarse-grainedness in the structure of the transparent 

 refracting matter ; and as, however we view the question, and how- 

 ever much we may feel compelled to differ from the details of mole- 

 cular structure and molecular inter-action assumed by Cauchy, we 

 remain more and more surely fortified in his conclusion, that finite 

 grainedness of transparent palpable matter is the cause of the dif- 

 ference of the velocity of different colours of light propagated through 

 it, we must regard Cauchy as the discoverer of the dynamical theory 

 of the prismatic colours. 



But now we come to the grand difficulty of Cauchy's theory.* 

 Look at this little Table (Table II.), and you will see in the heading 

 the formula which gives the velocity, in terms of the number of par- 

 ticles to the wave-length, supposing the medium to consist of equal 

 particles arranged in cubic order, and each particle to attract its six 



nearest neighbours, with a force varying directly as the excess of the 

 distance between them, above a certain constant line (the length of 

 which is to be chosen, according to the degree of compressibility 

 possessed by the elastic solid, which we desire to represent by a 

 crowd of mutually interacting molecules). If you suppose particles 

 of real matter arranged in the cubic order, and six steel wire spiral 

 springs, or elastic indiarubber bands, to be hooked on to each par- 

 ticle and stretched between it and its six nearest neighbours, the 

 postulated force may be produced in a model with all needful 

 accuracy ; and if we could but successfully wish the theatre of the 

 Royal Institution conveyed to the centre of the earth and kept there 

 for five minutes, I should have great pleasure in showing you a model 

 of an elastic solid thus constituted, and showing you waves propa- 



* For an account of the dynamical theory of the " Dispersion of Light," see 

 ' View of the Undulatory Theory as applied to the Dispersion of Light,' by the 

 Rev. Baden Powell, M.A., &c. (London, 1841.) 



