The projective and kinematic geometry of closed polygons (1914)
Hebbert, Clarence Mark · leaf 46
ose centers lie on a fifth circletangent to the fifth fixed line of Theorem XII. Theorem XIV. The centers of the four circles in Theorem XIIIlie or four parabolas confocal with the given parabolas, whosevertices are the mid-points of the sides of the quadrilateralformed by the vertices of the given… … 17. Hence we have nine confocal parabolas, five fixed lines, andfive infinite sets of circles tangent to the five fixed lines andconcurrent at the common focus of the parabolas. These seem toexplain the apparent (though only apparent) dualism of the theoremsrelating to quadrilaterals having an…
