494 



Sir J. Conroy. 



[Mar. 3, 



which the light was reduced to a minimum, were observed ; two such 

 observations were made, and then the quarter plate turned through an 

 angle of 90° and two more observations made, so that the values for each 

 position of the polarising nicol are the means of four observations. 



The retardation produced by the plate not being equal to 90° for the 

 wave-length of the light employed, the resultant beam was ellipticaily 

 polarised, the major and minor axes of the ellipse being in the plane of 

 incidence, and in the plane perpendicular to it, or vice versa. The 

 relative position of the axes depending on the position of the polarising 

 nicol,* and on the amount of retardation produced by the plate (i.e., 

 whether it was less or more than 90°). 



An elliptical vibration being equivalent to two rectilinear vibrations 

 in the planes of the axes, differing in phase by a quarter of an undu- 

 lation, and of different amplitudes, this method affords a ready means 

 of producing with the same retarding plate a difference of phase of 

 exactly 90° between the vibrations in the plane of incidence and the 

 plane perpendicular to it with light of different wave-lengths. 



Hence the angle at which light polarised in this manner is reflected 

 by a metallic plate as plane polarised light is the true angle of principal 

 incidence ; the azimuth, however, of the reflected light is not the 

 principal azimuth. 



The azimuths are different, both in sign and in amount, for the two 

 positions of the polarising nicol, according as the major or the minor 

 axis of the ellipse is in the plane of incidence, the vibration in the 

 plane of incidence being the one retarded and reduced in amplitude 

 by the act of reflexion. 



The ratio of the semi-axes of the ellipse being called 7, and the 

 observed azimuths a-, and a 2 , the ratio of the amplitudes of the vibra- 

 tions in the plane of, and perpendicular to, the plane of incidence after 

 reflexion by the metallic plate (the amplitudes in the incident beam 

 being equal), or, in other words, the principal azimuth e> } is determined 

 by the equations — 



tan 

 tan«o 



which give tan a 1 tan a 2 =& 2 . 



The table contains the determinations made in this way with the 

 silver plate in air, water, and carbon tetrachloride ; the position of the 

 principal section of the polarising nicol, with reference to the plane of 

 incidence, being stated in the first column, and the mean values of the 

 principal incidences, and the values of the principal azimuths, calcu- 

 lated by the above-mentioned formula, in the last line of each portion 

 of the table — 



