“Li quali, secondochè nel _Libro dell’ Aggregazione delle Stelle_ epilogato si trova, dalla migliore dimostrazione degli astrologi ....” (_Conv._ II. vi. 133-136). Although the usual name for the book of Alfraganus in Latin was _Elementa Astronomica_, the version for which mediæval students were indebted to the indefatigable translator Gerard of Cremona bears in the MSS. the title _Alfragani liber de aggregationibus Scientiæ Stellarum et de principiis cœlestium motuum_. It is this book, therefore, that Dante means, though he has translated the title somewhat inaccurately.[97] ------------------------------------------------------------------------ [97] See Schiaparelli’s letter in Lubin’s _Dante e gli Astronomi Italiani_. The name is, however, also used as a sub-title in the printed edition of Christmann, Frankfort, 1590, which was based not on the translation of Gerard but of Johannes Hispalensis of Seville; and Toynbee thinks that this Frankfort edition represents most nearly the version of Alfraganus used by Dante. It is the only one of the five printed editions which gives the same figure for the diameter of Mercury as that quoted by Dante. See Toynbee, “Dante’s Obligations to Alfraganus” in _Romania_ xxiv. 95, and Moore, _Studies in Dante_ iii. p. 3, _note_. ------------------------------------------------------------------------ There were several other books known to Dante from which he must have gleaned information about Ptolemy’s system, but Alfraganus we may regard as his standard reference book. Many versions of Alfraganus are still extant, and it is pleasant to feel that we can hold in our own hands Dante’s text-book on astronomy. The best known is the printed edition of Golius, published in Amsterdam in 1669: it is in both Latin and Arabic, and opening in the middle we may turn the pages backwards to see the beautiful Arabic letters of Alfraganus’ own language, which is written from right to left, or forwards to look at the Latin in which he became known to Dante.