6 Sir D. Brewster's Obsei-vations on M. Rudberg's Memoir. 



The indices given by Mains are 1-654-3 and l'4-833. Calling, 

 as before, n' the index of the ordinary ray, and 7i" that of the 

 extraordinary ray, we have the following values of the ratio 

 n' 

 '^- H 1-12385 



G 1-12154 



F 1-11891 



E 1-11750 



D 1-11582 



C 1-11440 



B 1-11400 

 From which we see incontestably the mcrease of the double re- 

 fraction with the individual refra7igibility of the colour. 



[To be continued.] 



II. Observations on the preceding Memoir. By Sir David 

 Brewster, K.H. LL.D. F.R.S. V.P.R.S. Ed. 



TT is impossible to estimate too highly the value of the ob- 

 -■■ servations contained in the preceding memoir. To the 

 mineralogist, as well as to the optical philosopher, such obser- 

 vations are fixed data of the highest utility. 



The only general conclusion, however, which M. Rudberg 

 has drawn from his experiments in the first part of his me- 

 moir, is, that the double refraction increases with the refrangi- 

 bility of the coloured ray ,- or, to express the same fact in other 

 words, that the dispersion produced by the extraordinary 

 refraction is greater in proportion to the mean refraction than 

 that produced by the ordinary refraction, — or that doubly re- 

 fracting crystals have two different dispersive powers. 



This discovery was made by myself so long ago as 1812, 

 and an account of it published in my Treatise on New Phi" 

 losophical Instruments, Edinb. 1813, p. 312-315 : in the Philo- 

 sophical Transactions, J 813, p. 107; and in the article Optics, 

 in the Edinburgh Encyclopedia, vol. xv. p. 544. The fol- 

 lowing is the account of it, which I have published in the first 

 of these works. 



" The most singular result, however, which is contained in 

 the following table (Table of Dispersive Powers) relates to the 

 dispersive powers of doubly refracting substances. The first 

 experiment which I made upon crystals, was to determine the 

 dispersive power of Iceland spar; and from a cause merely 

 accidental, I corrected the colour of the least refraction. The 

 result thus obtained was 0-026, considerably below water, 



