POLARISATION. 



is allowed to sink below the plane of the paper ; and to the 

 left (B), if it is raised above the paper. The ellipses, which 

 represent angles of rotation between and 90, go through the> 

 range of transitions in regard to form and position between the> 

 two terminal members. The angle must evidently be repre- 

 sented by an ellipse, which may be situated thus o or thus 0, 

 according as m n is longer or shorter than the mean axis. The 

 transitions will therefore occur, in the first case, in such a way 

 that the ellipse rises with its right vertex and thus becomes 

 lengthened (Fig. 187 A, B, C), whilst in the second case a 



FIG. 188. 



gradually increasing inclination to the right is combined with a 

 similar extension (Fig. 188 A, B, C). 



We can just as easily determine the sectional surfaces which 

 come into play on rotation round any other given diameter in 

 the plane of the field of view. It is only necessary to compare the 

 known ellipse of the initial position with the equally known one 

 after a revolution of 90 ; the intermediary members then make in 

 form and position the transitions. 



