52 DISPERSION. 



mula, pointed out by Professor Powell, that as s diminishes, 

 or the wave-length in vacuo increases, the value of ^ approxi- 

 mates to a fixed limit, given by the equation 



which, therefore, defines the limit of the spectrum on the side 

 of the less refrangible rays. This limiting index corresponds 

 to a point not greatly below the red extremity of the visible 

 v spectrum. 



