of singly refracting substances. 108 



In most of these eases, the differences are too great to be 

 ascribed to accidental errors of observation, and it was not pro- 

 bable that they arose from Dr Brewster's law being inexact, 

 or susceptible of modification, because the differences were in 

 some cases greater in two surfaces of the same body than 

 might be expected from accidental errors. I therefore deter- 

 mined to repeat the measurements on a newly ground and po- 

 lished surface of the glass E, where the differences were great- 

 est, viz. 23' and 38'. The observed angle of polarization, by 

 a mean of twenty observations, was now 57° 40/6, differing 

 only — °/.l from 57° 38'.5, the calculated angle. I now ground 

 and re-polished the surface of the same prism which had be- 

 fore given a difference of — 38', and found it to be. 57° 41'-4, 

 the difference being only -f- 0° 2'.9 



As these small differences are clearly within the limits of ac- 

 cidental errors of observation, I re-ground and polished all 

 the other substances in the preceding table, and having re- 

 measured the angle of polarization, I found it to agree so per- 

 fectly with Dr Brewster's law, that the slight deviations from 

 it could be ascribed only to errors of observation. The re- 

 sults of these measurements are given in the following table. 



The angle of polarization for the glass E, is the mean of 

 the two measurements already mentioned. The angle for 

 pyrope is the mean of twenty measurements upon one newly 

 ground face, which was 61° 3'.3, and of twenty measurements 

 upon another newly ground face, which was 61° 4'.8. The 

 angle for blende was observed in two natural cleavage planes ; 

 the mean of twenty observations in one being 67° 8'. 2, and of 

 twenty in the other, 67° 8M. The angle for the other sub- 

 stances was the mean of twenty observations upon surfaces 

 newly ground and polished. 



Colourless Fluor 



spar, 55 c 0'.7 Calc. 55 G'.7 Obs. CC.ODiffi 



Bluish Fluor spar 55 1.0 55 3.8 —03.2 



Common opal, 55 26.3 55 29-3 +03.0 



56 32 .2 50 36 .0 +0 3.8 



56 46.4 56 45.5 —0 0.9 



56 52.0 56 50.!' —0 1.8 



;>7 12.6 57 12.6 0.0 



