242 Dr. Hopkinson 



on the Refractive Index and 



Specific 



We have 

 whence 







n = 



dr _ 

 dn 



*+■ 



sin i 

 sin r 



sin 2 r 





whence 



sm i cos r 



±t dn _ 

 20S r dX 





By Cauchy's 

 whence 



Substituting, 



formula 

 we have 



we h 

 u = 



dn 

 dX 



ive 



2« 2 

 ~ \ a ' 





n cos i 

 or, finally, 



— ncos r = 



2(n - 



n (t = -±— 



2 «2 , 9 



\ 2 cosr K cos i cosr 

 - « a ) + n cos 2 r 





(8) 



If the angle i is small, the value of n will vary very little 

 Avith i ; consequently there will be a large number of circles, 

 all nearly achromatized. Under favourable circumstances as 

 many as one hundred rings have been counted, using an ordi- 

 nary lamp as source of light. 



The difference of path of the two pencils which produce 

 these rings in white light may exceed a thousand wave-lengths. 



XXIX. On the Refractive Index and Specific Inductive Capacity 

 of Transparent Insulating Media. By J. Hopkinson, D.Sc, 

 F.R.S* 



ONE of the deductions from Maxwell's electromagnetic 

 theory of light is, that the specific inductive capacity of 

 a medium is equal to the square of its refractive index. An- 

 other deduction is, that a body which is opaque to light, or, 

 more generally, to radiant energy, should be a conductor of 

 electricity. The first deduction appeared so clear an issue 

 that many experimenters have put it to the test. The results 



* Communicated, by the Physical Society, ha\ing been read at the 

 Meeting on February 26, 1882. 



