54 Trans. Acad. Sci. of St. Louis. 
being at the intersection of this element with the plane, is at 
infinity. 
In Fig. 4, where Z,Z, is taken so as to cut a hyperbola 
from the cone, the focus F, is obtained by producing the line 
So, in the opposite direction till it intersects Z, Z,,. 
FIGURE 4. 
When the plane of the section is — to the axis, the 
construction becomes even simpler ; 
for then 
s=0°, 
and : 
tan @,=tan 6,=sin a 
