407 



come more and more faint as the incidence succeeds or falls short of 

 that angle. They appear best defined in light reflected from galena, 

 and from metallic lead, and with least distinctness in light reflected 

 from silver and gold. On examining the effect of successive reflexion 

 of the same ray by metallic surfaces, the author found that the quan- 

 tity of light which each polarizes in the plane of reflexion, increases 

 with every reflexion, and that in several cases the whole incident 

 pencil is completely polarized. 



The action of metals upon polarized light forms the subject of the 

 second section of this paper, in which he investigates the changes 

 which polarized light undergoes, according as it is reflected at diffe- 

 rent angles of incidence, and in different azimuths of the plane of 

 incidence with relation to the plane of primitive polarization. The 

 light experiences in these cases a physical change of a nature inter- 

 mediate between that of completely polarized light, and light wholly 

 unpolarized, neither does it possess the same characters -as that which 

 has passed through thin crystallized plates. Its constitution is ex- 

 ceedingly analogous to light which is circularly polarized ; that is, 

 which comports itself as if it revolved with a circular motion during 

 its transmission through particular media. But in the case of circu- 

 lar polarization, the ray has the same properties in all its sides, and 

 the angles of reflexion at which it is restored to simple polarized light 

 in different azimuths, are all equal, like the radii of a circle described 

 round the ray. In the case of metallic reflexions, the new phenomena 

 discovered by Dr. Brewster may be designated by the term elliptic 

 polarization, because the angles of reflexion at which this kind of 

 light is restored to polarized light may be represented by the variable 

 radius of an ellipse. In circular polarization the restored ray has its 

 plane of polarization always inclined 45 to the plane of the second 

 system of reflexion. In elliptic polarization, the inclination of the 

 plane of the restored pencil is always less than 45. In the former 

 case, this plane continues by successive reflexions to oscillate on 

 each side of the plane of reflexion, with a never- varying amplitude, 

 from -j- 45 to 45 ; while in the latter case the same plane os- 

 cillates with an amplitude continually diminishing till it is brought 

 to zero in the plane of reflexion. In steel, the polarization is highly 

 elliptical, and the amplitude of the oscillations of the plane of resto- 

 ration is quickly brought to zero ; but in silver, whose polarization 

 approaches nearly to circular, the oscillations diminish very slowly in 

 amplitude. The peculiar character of elliptic polarization shows itself 

 also in another manner in the variable position of the ellipses which 

 regulate its angles of restoration upon steel. 



In the third section of his paper, the author treats of the comple- 

 mentary colours produced by successive reflexion from the polished 

 surfaces of metals. 



He concludes by observing, that although we do not understand 

 the nature of the forces by which metals reflect the two oppositely 

 polarized pencils, yet we are certain they do not act exactly in the 

 same manner as the second surfaces of transparent bodies, when pro- 



