of singly refracting substances. 101 



of glass, with the view of reconciling Dr Brewster's law to this 

 class of bodies, by accurate measurements of the refractive 

 power and of the angle of polarization in the same pieces. 



(Dr Seebeck then proceeds to describe the divided instru- 

 ment by which he measured the angles of refraction, from 

 which he deduced the following measures of the refractive 

 power.) 



Colourless fluor-spar, - 1.4341 Index of refraction. 

 Greenish blue fluor-spar, 1.4343 

 Common opal, - - 1.4516 

 Glass A colourless Eng. plate glass, 1.5130 

 B colourless plate glass, 1.5266 



C English crown glass, 1.5321 



D do. do. - 1.5523 



E English flint-glass, 1.5783 



F do. do. 1.6206 



Tyrope, - - 1.8131 



Yellow blende, - 2.3692 



After describing the instrument for measuring the angle of 

 polarization upon these substances, which does not differ much 

 from those previously used for the same purpose, Dr Seebeck 

 gives the following table of his results. The surfaces were 

 mostly polished about six and eight months before, and the re- 

 flecting surfaceson which the polarizing angle wasmeasured were 

 those by which the ray was refracted when the refractive power 

 of the substance was determined. The differences between the 

 observed and the calculated angles seldom amount to above 

 20 7 , and in very few to above 30'. The calculated angle of 

 polarization given in the table is that deduced from Dr Brew- 

 ster's law. 



1 . Colourless fluor-spar. 

 Calculated angle of polarization 

 Observed do. on 1 surface 

 on 2 do. 



L'. Bluish Jhiorspar. 

 Calculated angle of polarization 

 I Hi ii rved do. 1 surface 



