154 POLARIZATION OF LIGHT. 



(166) But a polarized ray possesses other characters. 

 "When a ray of light, proceeding directly from a self-luminous 

 body, is received upon a reflecting surface at a given angle, 

 the intensity of the reflected beam will be unaltered, whether 

 the surface be above or below, on the right or on the left of the 

 incident beam. The case however is different, when, instead 

 of the direct light of a self-luminous body, we submit to the 

 same trial light which has been already polarized. It is then 

 no longer indifferent on which side of the ray the new sur- 

 face is presented. The inclination of the reflected or trans- 

 mitted ray will, indeed, remain unaltered, on whatever side 

 the surface be presented, but its intensity will be very dif- 

 ferent ; and a ray which is reflected most intensely when the 

 new surface is presented at one side, under a certain angle, 

 will be wholly transmitted when it is offered to another, all 

 other circumstances being identical. 



It is evident then, that the ray which has suffered reflexion 

 at the first surface, in this experiment, has in consequence 

 acquired properties wholly distinct from the original light. 

 The latter is equally reflected in every azimuth of the plane 

 of reflexion ; while, on the other hand, the intensity of the 

 twice-reflected ray diminishes, as the angle between the re- 

 flecting planes increases ; and it vanishes altogether, and the 

 ray is wholly transmitted, when the plane of reflexion at the 

 second surface is perpendicular to that at the first. These 

 sides, or poles, once acquired, are retained by the ray in all 

 its future course, provided it undergoes no further modifica- 

 tion by reflexion or refraction ; for, whether the plates be an 

 inch or a mile asunder, the phenomena are the same, 



(167) A polarized ray, then, is distinguished by the fol- 

 lowing characters : 



I. It is not divided into two pencils by a double-refracting 

 crystal, in two positions of the principal section with respect 



