Focal Points
:
DRAWING AND ITS UTILITY
The focal point of a circle is its center, and is called the
focus. But an ellipse has two focal points, called
foci, represented by F, F in Fig. 148, and by B, B in Fig. 149.
A
produced line is one which extends out beyond the marking point. Thus in Fig. 148 that part of the line K between F and G represents the produced portion of line K.
Fig. 149.