1 84 POPULAR LECTURES AND ADDRESSES. 



matter is the cause of the difference in the 

 velocities 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. 1 Look at this little table 

 (Table II.), and you will see in the heading the 



TABLE II. VELOCITY (F) ACCORDING TO NUMBER (N] OF 

 PARTICLES IN WAVE-LENGTH. 



formula which gives the velocity, in terms of the 

 number of particles to the wave-length, supposing 

 the medium to consist of equal particles arranged 



1 For an account of the dynamical theory of the " Dispersion of 

 Light," see Vieiv of the Undulotory Theory as applied to the Dis- 

 persion of Light, by the Rev. Baden Powell, M. A., &c. (London, 

 1841.) 



