Theorems Connected with Inversion

Extract

The square of the tangent from a point P to a circle S, divided by the diameter of S, will, in what follows, be denoted by (PS).

If S open out into a straight line, it is easily seen that (PS) equals in the limit the perpendicular distance of P from this line; accordingly the length of the perpendicular from a point P on a straight line S will be denoted by the same symbol (PS).