104 Dr Sec-beck on the polarizing angles, fyc 



Glass E, 57 38.5 57 41.0 +0 2.5 



F, 58 19.4 58 16.6 —02.8 



Pyrope, 61 7 .7 61 4.0 —03.7 



Yellow blende, 67 7 .0 67 8 .2 +01 .2 



The great agreement between the calculated and observed 

 results, as given in this table, confirms, in the case of singly 

 refracting substances, the accuracy of Dr Brewster's law, that 

 the tangent of the angle of polarization is the index of refrac- 

 tion, — a law with which observation will coincide only when 

 the reflecting surface has the same property as the rest of the 

 mass. 



What the cause is which produces the change upon the 

 outer surface of the glass has not been determined by my ex- 

 periments. That there is a chemical alteration produced by 

 the air only, particularly on the surfaces of glass, is highly 

 probable, yet I do not maintain that this is the principal cause 

 of the observed anomalies. Upon newly polished surfaces of 

 the glass C, where the difference was at first — 0° 1'.8, 1 found 

 the mean of twenty measurements of the polarizing angle to 

 be after six weeks 56° 54°.6 ; after fourteen zcee/es, 56° 51'.8 ; 

 after six months, 56° 54'.0, the differences + 0° 2'.6, — 0° 0'.2 

 and -}- 0° 2'.0 being within the limits of errors of observation, 

 so that the upper surface does not seem to have suffered any 

 change in that period. When 1 had the surfaces of the glass 

 E, which formerly gave angles which agreed with the calcula- 

 tion, again polished by an artist, they gave an angle of 58° 7'.0, 

 which gave a difference of + 0° 28'. 5, whereas the differences 

 were originally — 0° 23', and — 0° 38'. Hence it can scarcely 

 be doubted, that this grpat variation arose from some mechani- 

 cal cause, produced perhaps by the cleaning of the surface; 

 and it is probable, that the same cause produced the other 

 deviations from Dr Brewster's law. 



Observations by the Editor. 



The preceding observations of Dr Seebeck are undoubtedly 

 the most accurate that have been made on the polarizing angles 

 of particular surfaces, and are exceedingly valuable even as in- 

 sulated facts, independent of their confirmation of the law of 

 the tangents. This law, however, had been established by 

 many observations of my own, subsequent to the publication 



