ON LIGHT. 149 



33 15' to the surface, and in this case the polarization is 

 complete, or the whole of the reflected light has acquired 

 the property in question.* If at any other obliquity ; it 

 is only partially polarized, or a portion only of the re- 

 flected light has acquired it. How this portion is to be 

 distinguished and separated from the impolarized portion, 

 we shall presently explain. Suffice it here to observe 

 that this latter portion bears a greater proportion to the 

 whole reflected beam, as the angle of incidence deviates 

 more from that above specified (which is called the pol- 

 arizing angle). The plane in which reflexion has been 

 made is called the plane of polarization ; and two rays 

 which have undergone reflection at the polarizing angle 

 in planes perpendicular to each other, are said to be 

 oppositely polarized. 



(128.) The angle of incidence 56 45' has this pecu- 

 liarity that if we consider the directions subsequently 

 pursued by the two portions into which a ray so incident 

 on glass is divided, the one pursuing its course by re- 

 flexion in the air, the other by refraction within the glass, 

 these two directions include a right angle as in the figure 

 overleaf, where A c is the incident, c B the reflected, 

 and c D the refracted rays, at the surface of a glass p Q. 

 When the angle A c P or B c Q is 33 15' Q c D is 56 45', 

 and D c B is a right angle. The law of polarization so 

 announced, as Sir David Brewster has shown, is general, 



* In point of fact the differently coloured rays are not all polar- 

 ized at exactly the same angle, so that this is rigorously exact only 

 for homogeneous light. But the difference is so trifling that it is 

 purposely here kept out of view. 



