INTERFERENCE OF POLARIZED LIGHT. 



227 



The visible portion of the emergent beam will form a cone, 

 AEB, whose summit coin- 

 cides with the place of the 

 eye, and axis, EG, with the 

 axis of the crystal. The ray 

 which traverses the crystal 

 in the direction of the axis, 

 POE, will undergo no 

 change whatever; and con- 

 sequently will be reflected, or not, from the analyzing plate, 

 according as the plane of reflexion there coincides with, or is 

 perpendicular to, the plane of the first reflexion. But the 

 other rays composing the cone will be modified in their pas- 

 sage through the crystal ; and the changes which they 

 undergo 'will depend on their inclination to the optic axis, 

 and on the position of the principal section with respect to 

 the plane of primitive polarization. 



Let the circle represent the section of the emergent cone 

 of rays made by the second surface of the crystal ; and let 

 MM 7 and NN X be two lines drawn 

 through its centre at right angles, 

 being the intersections of the plane 

 of primitive polarization, and of 

 the perpendicular plane, respec- 

 tively, with the surface. Now 

 the rays which emerge at any 

 point of these lines will not be di- 

 vided into two within the crystal, 

 nor will their planes of polarization 



be altered, because the principal section of the crystal, for 

 these rays, in the one case coincides with the plane of pri- 

 mitive polarization, and in the other is perp endicular to it. 

 These rays therefore will be reflected, or not, from the ana- 

 lyzing plate, according as the plane of reflexion there coin- 



Q2 



