252 



PROFESSOR POWELL'S RESEARCHES TOWARDS ESTABLISHING 



Sir J. Herschel has justly characterized as data of the utmost value in the theory 

 of light*. These values are as follows : 



With these values I have gone through every case of a refractive index for a defi- 

 nite ray at present known, that is, for every one of these seven definite rays in each 

 of the ten substances whose refractive energy for the different rays was examined by 

 Fraunhofer. 



The following tabular statement gives the comparison between the refractive index 

 for each ray in each medium, as given by Fraunhofer's observations in the first 

 column, and as resulting from the formula of theory (adopting his independent de- 

 terminations of the values of X,) in the last ; whilst in the intermediate columns the 

 elements of the calculation are exhibited. 



See Treatise on Light, art. 751. 756. 



