48 DISPERSION. 



of the water prism is to be varied, until the beam emerges 

 colourless. The angle of the water prism being then measured, 

 the ratio of the differences of the refractive indices, and thence 

 that of the dispersive powers, will be given by the formula 

 of the preceding Article. 



(60) We now proceed to the physical explanation of the 

 foregoing phenomena. 



To account for dispersion, the modern advocates of the 

 theory of emission have been forced to assume that the mo- 

 lecules of light are heterogeneous, and that the attractions 

 exerted on them by bodies vary with their nature, being 

 in this respect analogous to chemical affinities. This sup- 

 position, as Young has justly observed, is but veiling 

 our inability to assign a mechanical cause for the pheno- 

 menon. 



According to the principles of the wave-theory, the 

 colour of light is determined by the frequency of the ethereal 

 vibrations, or by the length of the wave ; the longest 

 waves producing the sensation of red, and the shortest 

 that of violet. Now, observation proves that the refractive 

 index, or "the ratio of the velocities of propagation in the 

 two media, is different for the light of different colours. 

 The velocity of propagation in a refracting medium, there- 

 fore, varies with the length of the wave. Here, then, we en- 

 counter a difficulty in this theory, which was long regarded 

 as the most formidable obstacle to its reception. Analysis 

 seemed to indicate that the velocity of wave-propagation 

 depended solely on the elasticity of the medium as com- 

 pared with its density, and should therefore be the same 

 for light of all colours, as it is for sound of all notes ; and 

 that, consequently, all rays should be equally refracted. It 

 will be necessary to enter, in some detail, into the considera- 

 tion of this difficulty. 



