393 



the formula is in fault ; but I am inclined to think that the 

 naetal which I employed had really a low reflective power. 

 Its angle of maximum polarization was certainly much less 

 than that of the speculum metal used by Sir David Brewster 

 (Phil. Trans. 1830, p. 324), who states the angle to be 7G°, 

 whereas in my experiments it was only about 73|°; and any 

 increase in this angle, by increasing the value of M, raises 

 the reflective power. On the other hand, the maximum va- 

 lue of /3 (when a = -15°) was greater than that given by Sir 

 David Brewster, namely, 32° ; and any increase in /3 tends 

 also to increase the reflective power. Now it is not unrea- 

 sonable to suppose that the highest values of both angles may 

 be most nearly those which belong to the best specula ; and 

 accordingly if we take 76° for the incidence of maximum po- 

 larization, and retain the maximum value of /3, namely 34-° 37', 

 which results from my experiments, we shall get M = 3.68, 

 X = 66° 16', and the value of i at the perpendicular incidence 

 will come out equal to .662, which scarcely differs from the 

 number given by Herschel. 



It is clear from what precedes that the optical constants 

 are different fordiflferent specimens of speculum metal, and 

 this is no more than we should expect, from the circum- 

 stance that the metal is a compound, and therefore liable 

 to vary in its optical properties from variations in the pro- 

 portion of its constituents ; but I am disposed to believe that 

 the same thing is generally true, though of course in a less 

 degree, of the simple metals, so that in order to render the 

 comparison satisfactory, the measures of intensity should 

 always be made on the same specimen which has furnished 

 the values of m and y« There is one metal, however, with 

 respect to which there can be no doubt that the experiments 

 of different observers are strictly comparable, when it is pure, 

 and at ordinary temperatures ; I mean mercury. For this 

 metal Sir David Brewster states the angle of maximum pola- 



