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



greatest, the ray F was brought to a minimum of deviation, 

 and in the other spectrum the ray H. 



Theprisvi A, No. 2. — Refracting angle 51° 4.8' 31". Temp. 

 + 18° cent. The rays F in the two spectra were brought to 

 a minimum of deviation. 



The following are the indices of refraction for the spectrum, 

 whose plane of polarization is perpendicular to A. 



Prism A, 

 No. 2. 



. 1-54225 



. 1-53885 



. 1 •53478 



. 1-53265 



. 1-53011 



. 1-52822 



. i -52751 



These differences are evidently errors of observation, and 

 the invariability of the velocity of the ray polarized perpen- 

 dicularly to the axis A is consequently well established. With 

 respect to the other ray, its velocity cannot be constant accord- 

 ing to theory ; and this is proved by observation, as is shown 

 by the two following measures in the spectrum whose plane 

 of polarization passes through A. 



Prism A, Prism A, 



No. 1. No. L'. Diff. 



1-70996 . . 1-70590 . . 0-00406 

 , 1-69502 . . 1-69128 . . 0*00374 

 No. 1.— Refracting angle 36° 13' 30". Temp. 

 + 1 8°. In the spectrum with the greatest deviations, the ray 

 F was reduced to a minimum of deviation, and in the other 

 the ray H. The same was the case in 



Prism B, No. 2. — Refracting angle 40° 1 2' 3". Temp, -f- 

 The following were the indices of refraction. 



Diff. 

 = 0-00001 



— 0-00005 

 + 0-00002 



— 0-00001 

 + 0-00004 



— 0-00004 



— 0-00004 



H 



F 



Prism 



18= 



The differences here, though greater than in prism A, owing 

 to the difficulty of cutting a face exactly perpendicular to the 

 plane of the optical axes, are yet sufficient to prove the invari- 

 ability of the velocity of the ray polarized perpendicularly 



