202 



The values of the wave-length constants A and B for each ray, 

 independent of the medium, are taken from my Treatise (Undulatory 

 Theory applied to Dispersion, &c., Art. 270). Comhining these, we 

 obtain AD and BD' for each ray in the medium. 



Thence Sir W. R. Hamilton's formula (ib. Art. 237) gives for any 

 ray, 



the upper sign being used for rays above F, the lower for those 

 below. 



Ether. Dale and Gladstone. 



In both these media, of low dispersive and refractive power, the 

 accordances of theory and observation are sufficiently close. 



