372 FOURTH REPORT — 1834. 



was received upon a metallic reflector at an incidence greater 

 than 40° and less than 86°, the reflected light was partly depo- 

 larized. The eff"ect produced was greatest at an angle of about 

 74° ; and when the light underwent a second reflexion in the 

 same plane and at the same angle, it was restored to light po- 

 larized in a single plane. This new plane lies always on the 

 other side of the plane of reflexion; and its azimuth varies within 

 the limits 0° and 45°, being greatest for silver and least for 

 galena. It is evident, then, that the light produced by a single 

 reflexion cannot be common light. Neither is it plane-polarized 

 light, because it does not vanish in any position of the analysing 

 rhomb. Sir David Brewster concludes, that this light has re- 

 ceived a species of polarization hitherto unrecognised, interme- 

 diate between plane and circular polarization. He calls it elliptic 

 polarization, because the angles of reflexion at which this light 

 is restored to plane-polarized light, in any azimuth of the plane 

 of the second reflexion with regard to the first, may be repre- 

 sented by the variable radii of an ellipse ; while these angles are 

 equal in all azimuths in the case of light circularly-polarized. 



Sir David Brewster seems to have been led to employ the 

 term "elliptic polarization" in this manner, in his desire to 

 avoid as much as possible all reference to theory. The laws 

 which he has obtained, however, belong to elliptically-polarized 

 light, in the sense in which the term was introduced by Fresnel. 

 It appears, in fact, from the theory of the composition of vibra- 

 tions, as laid down by this author, that the vibration resulting 

 from the union of two rectilinear and rectangular vibrations, 

 will be in general elliptic; so that two oppositely polarized 

 pencils compound in general a pencil elliptically-polarized, — the 

 ellipse becoming a right line, when the difference of phase of the 

 two poi'tions is an integer multiple of 180°. When, therefore, 

 by the effect of reflexion, two such pencils are made to differ 90° 

 in phase (as Sir David Brewster has shown to be the case when 

 a ray polarized in the azimuth 45° is incident at the maximum 

 polarizing angle of the metal), a second reflexion in the same 

 plane, and at the same angle, will i-aise the difference to 180°, 

 and the resulting light will be plane polarized. In other parts 

 of his memoir, however. Sir David Brewster seems to acknow- 

 ledge that theory ; for he speaks of elliptic polarization as pro- 

 duced by the interference of two unequal portions of oppositely 

 polarized light, and even calculates their difference of phase for 

 any incidence. 



The identity of the light produced by metallic reflexion, with 

 the elliptically-polarized light of the wave-theory, seems to be 

 placed beyond all doubt by an observation of Professor Airy. 



