POLARISATION. 



lines joining the points m n and s t (Fig. 195), in which the 

 ellipses corresponding to the two plates intersect, this supposition 

 is so far true that the difference between the major axis m n and 



the minor s t diminishes continuously, in 

 conformity with the interference colour, 

 when e increases, and finally becomes nil 

 when e 90 ; presumably, therefore, it 

 represents also the combined action of a 

 pair of plates and a selenite plate. The 

 position in which the imaginary ellipse of 

 elasticity is situated similarly to that of 

 the selenite plate will here also produce 

 an acceleration colour, and when the 

 FIG. 195. homologous axes intersect at right angles 



a retardation colour is produced. Thus 



in fact it actually behaves, as may be seen from the following 

 table : 



