On the other hand Dante quotes twice quite correctly from the _Tetrabiblios_, which was more widely known.[94] He may, therefore, possibly have read it, and he alludes to it in the _Convivio_, though by a curious slip he does not give its name, thinking he has done so already: “Tolommeo dice nello allegato libro....”[95] This is in the fourteenth chapter of the second treatise, but so far from having just quoted any special work by Ptolemy he has said nothing at all about him since the third chapter, and then it was only his opinion and name that were mentioned. I have not been able to trace the quotation given in the _Quæstio de Aqua et Terra_ (xxi. 29-31), where Ptolemy is said to have asserted that things on earth resemble things in heaven, but this probably comes also (or was supposed to come), from the “judicial astronomy” of the _Tetrabiblios_. It is not taken from the _Almagest_. ------------------------------------------------------------------------ [94] _Conv._ II. xiv. 198-202. _Ibid._ 249-253. [95] “Ptolemy says in the book above cited.” ------------------------------------------------------------------------ The elements of Ptolemy’s system, however, could be learned indirectly, and it seems that Dante had recourse to the excellent epitome of the Arab astronomer Alfraganus. It is true that his name is only mentioned once, and his book once; but nearly all Dante’s astronomical data appear to have been taken from him, and his very expressions are sometimes repeated. One of the most striking examples of borrowing from the _Elementa Astronomica_ of Alfraganus occurs in the _Paradiso_, where Dante likens twenty-four spirits to as many brilliant stars.[96] He makes up the number by taking fifteen specially bright stars from different parts of the sky, and adding to them the stars of Ursa Major and two stars of Ursa Minor. The seven chief stars of Ursa Major are well known, but Beta and Gamma Ursæ Minoris are not conspicuous nor specially familiar; and why, if he takes fifteen unnamed, should he name any of the twenty-four? Turning to chapter 19 of Alfraganus’ book, we find that he follows Ptolemy in enumerating fifteen first-magnitude stars in different parts of the sky, and then he gives as examples of the second magnitude (_i.e._ next in brightness) Benet Naax and Alfarcatein, Arab constellations which correspond with the tail of Ursa Major, and Beta and Gamma of Ursa Minor. ------------------------------------------------------------------------ [96] _Par._ xiii. 1-13. ------------------------------------------------------------------------ There is no reason to suppose that Dante knew Arabic, but there were several versions of Alfraganus in Latin, and it is even possible to determine with some certainty which of them he used. Speaking of the movements of the planet Venus, he says that they may be found, summarized from the best demonstrations of the astrologers, in the _Book of the Collection of the Stars_:—