R. Milson (2000) "An Overview of Lie’s line-sphere correspondence", pp 1–10 of The Geometric Study of Differential Equations, J.A. Leslie & T.P. Robart editors, American Mathematical Society ISBN 0-8218-2964-5 , quotation pp 8,9 ↑ Thomas Hawkins (1996) Historia Mathematica 23(1):92–5