17. This form, attributed to Poincaré, is close to one he gave in the Bull. Soc. Math, de France, 15, 1887, 203-16 = Œuvres, 11, 79-91. The model described here seems to have been given first by Joseph Wellstein (1869-1919) in H. Weber and J. Wellstein, Enzyklopädie der Elementar-Mathematik, 2, 1905, 39-81.