of the Plane of Polarization of Light. 325 



plane of polarization which the glass plate shows upon excita- 

 tion of the magnetic field thus varies, no doubt, in magnitude 

 with the angle of incidence, but always takes place in the same 

 direction as the electromagnetic rotation which the rays suffer 

 in the plate. 



If, on the other hand, the plane of polarization of the light 

 before its entry into the plate is at right angles to the plane of 

 incidence, if we adopt the same symbols, and if the electro- 

 magnetic rotation is again small, then the rotation x, suffered 

 by the plane of polarization upon emergence from the plate, is 

 given by the formula 



„,',. sin z sin r cos (i— r) 



mux = — zcbcl tan 7 j-. — \ 



cos(i-l-r) 



If i + r < 90, x is negative ; but if i + r > 90, x is positive. 



For i + r — 90, that is, for the polarizing angle, .£ = 90; but 

 in this case the amplitude of the light is zero. 



The case considered above was examined experimentally in 

 the following manner : — The poles of the large electromagnet 

 were adjusted at a distance of about 3 centim. apart. A glass 

 plate, the walls of which were not accurately parallel, so that 

 the rays reflected from the back surface were well separated 

 from those reflected at the front surface, was placed against 

 the vertical side-faces of the poles so that it connected them. 

 The plane of incidence of the light was horizontal. The 

 angles of incidence were read off upon a large divided circle. 



The following double rotations were obtained upon exciting 

 the electromagnet at different angles of incidence for the rays 

 reflected at the back surface. 



The signs = and _L denote, as before, that the plane of 

 polarization of the incident light is parallel or at right angles 

 to the plane of incidence. The rotations are given in degrees. 

 Angle of incidence. Rotation. 



o o 



50-5 +0-8 



61-3 +1*6 



70-0 +2-5 



80-4 +3*4 



o -L o 



30-7 - 0-7 



39-5 - 1-8 



50-5 - 4*8 



53-8 -27-4 



60-4 +24-1 



61-3 +12-0 



70-0 + 6-2 



80-4 + 4-2 



