POLARIZATION OF LIGHT. 155 



to the ray ; being refracted ordinarily, when the plane of 

 polarization coincides with the principal section, and extra- 

 ordinarily when it is perpendicular to it. In other cases it 

 gives rise to two pencils, which vary in intensity according 

 to the position of the principal section. 



II. It suffers no reflexion at the polished surface of a trans- 

 parent medium, when this surface is presented to it at a cer- 

 tain angle, and in a plane of incidence perpendicular to the 

 plane of polarization ; while it is partially reflected, when the 

 reflecting surface is presented in other planes of incidence, or 

 under different angles. 



The apparatus best fitted for the exhibition of these phe- 

 nomena is that devised by M. Biot. It consists of a tube, 

 furnished at its extremities with two graduated rings, which 

 are capable of revolving in a plane perpendicular to its axis. 

 Each of these rings carries a plate of glass set in a frame, 

 and held by two projecting arms. These plates are capable 

 of revolving round a transverse axis, so that their incli- 

 nation to the axis of the tube may be varied at pleasure ; 

 and a small graduated circle, attached to one of the arms, 

 measures the inclination. The whole apparatus is connected 

 with a vertical pillar, by a moveable joint, so that the tube 

 may be inclined to the horizon at any angle. In this form of 

 the apparatus, it is arranged to exhibit the properties of polar- 

 ized light dependent on reflexion ; in order to show the other 

 properties, one of the glass plates may be removed, and a double- 

 refracting prism, or a plate of tourmaline, substituted in its place. 



(168) The angles of incidence at which light is polarized 

 are called the angles of polarization. They are in general dif- 

 ferent for different substances : thus the angle of polarization 

 of glass is 57, and that of water ^ 53. 



The connexion between the polarizing angles and the other 

 properties of substances with regard to light was discovered 



