391 



values of 26 in succession with that of 20o, we get for 2)3„ 

 tlie series of values 32° 38', 33° 28', 34° 30'; the differences 

 between which are to be attributed to the causes above 

 stated. The mean value of 2fto thus found is 33° 32'; while 

 Its value, as given by M. de Senannont, is only 28° 41'. 

 The difference 4° 51' is the value of 0' - 6", which, divided 

 by the tangent of 2/3„, gives 7° 19' for the mean value of e, 

 the error of the mica-plate corresponding to that part of the 

 spectrum which was observed at the incidence of G0°. 



At incidences nearer the angle of maximum polarization, 

 the errors are probably much greater. Beyond that anglj 

 they again diminish, and in some cases they almost vanish. 

 Thus, at the incidence of 85° upon steel, with the value 

 of 200 and the value of 29 corresponding to a = 20°, we get, 

 by computation, a value of 2/3„ which differs only by a few' 

 minutes from that given by M. de Senarmont. Nearly the 

 same thing happens at the same incidence when we take 

 a = 25°. In these cases therefore the results belong to that 

 particular ray for which the thickness of the pfate was 

 exact. 



The observations of M. de Senarmont on speculum metal 

 were not carried beyond the incidence of 60°. He states 

 that he was unable to observe at higher incidences, on ac- 

 count of the uncertainty arising from the dispersion of the 

 metal; but though this cause operated in some degree, his 

 embarrassment must have been really occasioned by the in- 

 creasing magnitude of the difference 0'-r, as he approached 

 the angle of maximum polarization ; that difference being 

 perhaps twice as great as in the case of steel. My own eZ 

 periments on speculum metal were all made, as has been 

 seen, at incidences greater than 60°. 



The experiments of M. de Senarmont do not at all agree 

 with the formulas; and therefore I have been obliged to Ana- 

 lyse his method of observation, and to show that it could 

 not lead to correct results. It is to be regretted that his 



VOL. 11. O L 



