DISPERSION . 45 



are of the utmost importance in optical investigations. On 

 account of the sharpness of their delineation, their position 

 may be observed with an accuracy equal to that of astro- 

 nomical measurements, and the refractive indices of the rays, 

 to which they correspond, thus determined with the utmost 

 exactness. 



The refractive indices of the definite rays in the glasses 

 vary with their composition. Those for distilled water are 

 the following. They may be taken, as proposed by Sir John 

 Herschel, as standards to which the refractive indices of other 

 substances may be referred : 



Kay. Kefractive Index. 



B . . 1-33098 



C . . 1-33171 



D . . 1-33358 



E . . 1-33585 



F . . 1-33779 



G . . 1-34129 



H . . 1-34418 



(55) The dispersion of a ray which passes nearly perpen- 

 dicularly through a thin prism is easily expressed. 



If Si and S 2 denote the deviations of the extreme red and 

 violet rays, jui and ^ the refractive indices of the prism for 

 those rays, and A its refracting angle, we have 



Si = Gm -1) A, S 2 = Ou 2 -l)A; 

 whence 



$2 ~ $1 = (jU 2 - jUi) A. 



Accordingly the dispersion, in this case, is equal to the angle 

 of the prism multiplied by the difference of the refractive 

 indices. 



