396 



ON LIGHT. 



tourmalines instead of being crossed, are laid parallel, 

 the forms of the ovals are the same, but the colours 

 complementary, and the cross and curved branches 

 white. 

 (171.) The forms of these curves are governed by a 



Fig .17. 



Fig. 18. 



Fig. 19- 



very simple and elegant general law, common to all 

 biaxal crystals, and applicable to every angular separa- 

 tion of the axis: and when this separation is small, as 

 in the case before us, they may be regarded as " lemnis- 

 cates," of which the property is this, that for every point 

 in the circumference of each oval the product (not as in 

 an ellipse the sum) of two lines drawn to the two centres 

 or foci, is invariable ; and for successive ovals proceed- 

 ing outwards from either focus, these products increase 

 in regular arithmetical progression. When the two foci 

 coincide, that is to say, when the two axes of the crys- 

 tal coalesce, and it becomes uniaxal, the ovals pass into 



