422 Prof. Lorenz on the Theory of Light. 



by accents, and/' put instead of /(a', /3', y), we obtain 



2e p*=4n<2 f — /f— /C, 



where 



V "i ft 7i ' 



But in accordance with the above formula for f(x, y, z), we have 



and therefore 



The values of Q 2 and Se/ can now be introduced into the 

 above expression for the mean refractive index. Since, how- 

 ever, we may suppose the perturbing effect of chromatic disper- 

 sion to be eliminated by calculation, we may entirely disregard 

 the small quantity a p , and only introduce the new magnitudes 

 into the equation 



a _0 2 __0* 2 

 r " Q 2 6Q 2 ^> 



where r accordingly is the value of the reduced mean refractive 

 index. 



We will now try to apply the results we have obtained. It 

 must not, however, be overlooked that the last equation gives 

 only an approximation to the value of r, inasmuch as it was ori- 

 ginally assumed that the magnitudes e p were small. I have 

 indeed succeeded in developing the exact value of r in a series 

 the first terms of which are the above, and I am ready to com- 

 municate this calculation in case it shall be called for; but the 

 above approximate formula may nevertheless be considered suffi- 

 cient for the present purpose. 



It is well known how the idea of the absolute refractive power 

 early arose in science, how it has been discarded by the prevailing 

 theory as useless, and how it nevertheless, claiming a certain 

 currency in spite of the theory, is perpetually reappearing. It 

 is only needful to cast a glance at the general survey of refrac- 

 tive powers of various bodies given by Albr. Schrauf in Poggen- 

 dorfFs Annalen, vol. cxvi., to be convinced of a certain regularity. 

 Especially it may be seen that the refractive power, freed from 

 the influence of dispersion, as it is expressed by the formula 



r 2 — 1 

 M= -> 



