294 McJYair — Method in Teaching Optical Mineralogy. 



ellipses have been numbered to correspond with those shown 

 in tiij. 1. The ellipse for the central ray of the cone being 

 drawn perpendicular to the C direction has for its semi-axes 

 a and b. 



If now we consider the series of ellipses belonging to rays 

 in the A-C, or axial, plane, represented in the section above, it 

 will he seen that as we leave the central ray, passing outward 

 to rays making a continually increasing angle with the central 

 one, the axis perpendicular to the A-C plane remains constant 



Fig. 2. 



and of the value 2b, while the axis in the axial plane decreases 

 from its maximum, 2a, toward the value 2c. Furthermore, 

 when the ray has the direction of the ray axis of the crystal 

 the ellipse obtained for it is a circle. Beyond this ray the axis 

 in the A-C plane is the minor axis of the ellipse. Thus it 

 appears that the ellipses on one side lie with the major axes at 

 right angles to the major axes of those on the other side, the 



