230 INTERFERENCE OF POLARIZED LIGHT. 



circles, as in the former, but will form curves which are sym- 

 metric with respect to the lines drawn from the eye in the 

 direction of the two axes ; the points of the same band 

 being those for which the interval of retardation of the two 

 waves is the same. Now this interval is proportional to the 

 product of the sines of the angles which the normal to the 

 front of the waves makes with the optic axes (235 ) ; and 

 these sines are, very nearly, as the distances of the points of 

 emergence (measured on the face of the crystal) from the 

 projections of the optic axes. Hence the product of these 

 distances will be constant for all the points of the same 

 curve. The curve formed by each band is therefore the lem- 

 niscata of James Bernouilli, the fundamental property of 

 which is, that the product of the radii-vectores, drawn from 

 any point to two fixed poles, is a constant quantity. 



The exactness of this law has been verified, in the most 

 complete manner, by the measurements of Sir John Herschel. 



The constant varies from one curve to another, being pro- 

 portional to the interval of retardation, and increasing there- 



