Construction of a Conic
Diagram showing how to construct a conic when given the focus and the auxiliary circle. If the focus is outside the circle, we get a hyperbola. If it's inside the circle, we get an ellipse. If the auxiliary circle is a straight line (radius is infinite), we get a parabola.
Arthur Latham Baker The Elements of Solid Geometr with Numerous Exercises (Boston, MA: Ginn & Company, 1893)
Courtesy the private collection of Roy Winkelman