118 



The Propagation of Light in Biaxals. 



between the optic axes is 49° 50' and the radius of the circular 

 section is 4. 



Fte. 1. 



There is no difficulty in making a cardboard model of the 

 principal sections and Ihe circular sections of this quadric. 

 The ellipses can be drawn by the ordinary mechanical device 

 or by elliptic trammels. If the latter are not available it will 

 be probably found more speedy and accurate to set out the 

 curves on squared paper from the following tables (p. 320). 



