1. Simplicii in Aristotelis De Coelo Commentario, ed. Heiberg, Berlin, 1904, 7a, 33.
2. Die Coss Christoffs Rudolffs. Durch Michael Stifel, Königsberg, 1553, fol. 9; H. P. Manning, Geometry of Four Dimensions, New York, 1914, p. 2, 3; H. Wieleitner, Isis, vol. 7, 1925, p. 486.