¶ Descartes
Index Entry
Descartes:
“Certainly, Descartes discovered the 720° all right, but he didn’t realize it was the tetrahedron. . . . and that it could be turned inside and outside.”
(RBF Comment on reading letter from H.S.M. Coxeter to Barry Farrell dated 28 Sept. 1971, in which Coxeter says: “The theorem about 720°, so charmingly described on page 9 (Nehrd Speech) in terms of flattening out a tiger skin, was discovered by Descartes (see my Regular Polytopes, Macmillan, New York, 1963, p.23).”)
“. . . The tetrahedron can become invisible; it has an internal invisible tetrahedron of concave angles 720° or less.”
-Cite RBF to EJA in Chicago, Blackstone Hotel, 31 May 1971
