  Newton illustrates the value of Proposition 6 with a series of examples, the two most important of which involve motion in an ellipse. If the force center is at a focus S of the ellipse, then the limit of (QR/QT2) is everywhere equal to half the constant latus rectum of the ellipse, and hence the force varies as 1/SP2, or 1/r2. But if the force center is at the center C of the ellipse, the force turns out to vary as PC, that is, linearly with r. This contrast raises an interesting question. What conclusion can be drawn in the case of motion in an ellipse for which the foci are very near the center, and the center of force is not known to be exactly at the focus? Newton clearly noticed this question and supplied the means for answering it in the Scholium that ends Section 2.