REFLEXION AND REFRACTION OF POLARIZED LIGHT. 105 



the phases of the vibrations in the plane of reflexion Toeing more 

 retarded than in the perpendicular plane. The two oppositely 

 polarized portions, therefore, will differ in phase after reflexion, 

 and will therefore compound a pencil elliptically -polarized. Pro- 

 fessor Airy has observed a similar phenomenon when Newton's 

 rings were formed between diamond and plate-glass, the angle of 

 incidence being a few degrees less than the maximum polarizing 

 angle of diamond ; and he concludes that, for such incidences, 

 the nature of reflexion from diamond is analogous to metallic 

 reflexion. 



Sir David Brewster has extended his researches on the subject 

 of metallic reflexion to a great variety of cases, and has traced the 

 effects of successive reflexions in the same, or in different planes ; 

 and at the same, or different angles. When the light which has 

 been restored to plane-polarized light, by two reflexions in the same 

 plane, and at the maximum polarizing angle, undergoes a third 

 reflexion under the same circumstances, it becomes again ellipti- 

 cally-polarized. By a fourth reflexion it is again restored to plane- 

 polarized light, the plane of polarization being, however, brought 

 nearer to the plane of reflexion. This continued approach of the 

 plane of polarization to the plane of reflexion enables the author 

 to explain, according to his peculiar views, the effect of successive 

 reflexions upon common light. 



It remains, further, to extend the theory of Fresnel to reflexion 

 at the surface of a medium in which the elasticity of the ether is 

 different in different directions. All that we know on this interest- 

 ing subject we owe to the unwearied zeal of Sir David Brewster. 

 It had been supposed by Malus, and the opinion seems to have 

 passed current with succeeding philosophers, that the exterior sur- 

 faces of crystallized substances acted upon the reflected light 

 -exactly in the same manner as the surfaces of ordinary media ; or, 

 in the language of the theory of emission, that the reflecting forces 

 extended beyond the limits of the polarizing forces of the crystal. 

 Sir David Brewster was led to doubt this opinion ; and in the year 

 1819 he undertook an extensive series of experiments on the sub- 

 ject of crystalline reflexion. One of the first results at which he 

 arrived was, that the angle of complete polarization on the same 

 surface varies with the inclination of the plane of reflexion to the 

 principal section of the crystal ; being least when the plane of re- 

 flexion coincides with the principal section, and greatest when it is 



