ON LIGHT. 



379 



each 54 37', and a ray P Q, polarized in a plane 45 

 inclined to the plane of the section A B c D intromitted 

 perpendicularly at the face A B, so as to be reflected inter- 

 nally at Q on the side A c, (in which case, the reflexion 

 being at an angle of incidence 54 37' was total) ; and 



Fig. i<x 



again at R, at the same angle, on the opposite side D B, 

 it emerged from the face D c, along the line R s, circu- 

 larly polarized. In this case, the plane of reflexion 

 making an angle of 45, with that of original polariza- 

 tion, the reflected ray will consist of two equal rays, 

 oppositely polarized ; and of these the one in each act of 

 reflection has lost, in the other gained, an exact i6th of 

 an undulation, making an 8th difference at each reflexion, 

 or a quarter after both ; so as to emerge under all the 

 conditions of circular polarization. In consequence, 

 when analysed at its emergence by a tourmaline plate, 

 it is found to undergo no change of brightness on turning 

 the plate in its own plane, whereas the original ray, P Q, 



