Note 420 — Chapter IV · THE SOURCES FOR THE PERIOD 1100–1250 A. D.

Duhem (Système, vol. iii, 1915, pp. 219–223) shows, conclusively the writer believes, both from external and internal evidence, that this work was by the twelfth-century Gerard of Cremona and not by the thirteenth-century Gerard of Sabbionetta, with whom the former was often confused. Boncompagni in his important work on Gerard (cited above, p. 399, note 39) made the mistake of attributing the Theorica planetarum to Gerard of Sabbionetta, in which error he was followed by the writer of the article on Gerard of Cremona in the Encyclopaedia Britannica, 11th edit.