4 M. Rudberg on the Refraction of the differently-coloured 



sition that the line H in the extraordinary spectrum was re- 

 duced to its minimum ol' deviation ; and the other lines not 

 only of this spectrum, but of the ordinary spectrum, were all 

 measured in the same position of the prism. 



As the two spectra always cover one another in part in 

 rock crystal, I employed, in order to be able to observe each 

 of them separately, a plate of tourmaline cut parallel to the 

 axis. When put before the aperture of the eye-glass, it al- 

 lowed only the light of the extraordinary spectrum to pass 

 when its axis was parallel to the axis of crystallization ; and 

 when its axis was in a position perpendicular to this, it allowed 

 only the ordinary light to pass. 



The angle of one of the prisms was 52°-94.0 or 47° 38' 46", 

 and that of the other 50°-372 or 45° 20' 5". These values 

 are the means of several observations, which giving imme- 

 diately the double angle do not differ more than 0°*()05 in 16". 

 The observations from which the following indices were cal- 

 culated, were made at a temperature of -f 18° centigrade. 



Extraordinary Spectrum. 



Ordinary Spectrum. 



The differences are evidently only errors of observation. 

 By taking the mean of the two results, we shall have the 

 following indices, which can scarcely err 0*00005. 

 Lines. Extr. Ray. Ord. Ray. 

 H 1-56772 1-55817 

 G 1-56365 1-55425 

 F 1-55894 1-54965 

 E 1-55631 1-54711 

 D 1-55328 1-54418 

 C 1-55085 1-54180 

 B 1-54990 1-54090 

 Malus had found for the extraordinary ray the index 

 1-55817, and for the ordinary ray 1-54843*, which being 

 both situated between E and F, correspond as well as could 

 be expected ; as in his time the fixed points or lines in the 

 spectrum were not known. 



• Mains found these two nnmbers to be the two indices, but he consi- 

 dered 1-54843 as the extraordinari/ index. It was M. Biot who discovered 

 that the larger index was the extraordinary one.— Ejuit. 



