Engler — Foci of Sections, Cone of Revolution. 53 
or, by (1) and (2) the lines So, and So, are the elements of 
the focal cones for all sections parallel to that cut by the plane 
Z4,Z, and their intersections with Z,Z, give the foci for that 
section. 
The correctness of the above construction is also evident 
from the consideration that the points 0, and 0, are the points 
of contact with the sphere of planes parallel to Z7,Z, and are 
therefore points on the focal cone. 
FIGURE 3. 
In Fig. 3, where Z,Z, is taken parallel to an element of the 
cone so that the section is a parabola, the focus which is at an 
infinite distance is obtained from the construction by observ- _ 
ing that the point 0, falls on that element of the original cone _ 
to which the plane Z,Z, is parallel, and that therefore F,, ae 
