At the beginning of the 12th century began a series of translations from Arabic into Latin of scientific and philosophic treatises, partly original works of the Arabs, partly Arabic translations of the Greek books. One of the most active of the translators was _Plato of Tivoli_, who studied Arabic in Spain about it 1116, and translated Albategnius’s _Astronomy_ (§ 59), as well as other astronomical books. At about the same time Euclid’s _Elements_, among other books, was translated by _Athelard of Bath_. _Gherardo of Cremona_ (1114-1187) was even more industrious, and is said to have made translations of about 70 scientific treatises, including the _Almagest_, and the _Toletan Tables_ of Arzachel (§ 61). The beginning of the 13th century was marked by the foundation of several Universities, and at that of Naples (founded in 1224) the Emperor Frederick II., who had come into contact with the Mahometan learning in Sicily, gathered together a number of scholars whom he directed to make a fresh series of translations from the Arabic. Aristotle’s writings on logic had been preserved in Latin translations from classical times, and were already much esteemed by the scholars of the 11th and 12th centuries. His other writings were first met with in Arabic versions, and were translated into Latin during the end of the 12th and during the 13th centuries; in one or two cases translations were also made from the original Greek. The influence of Aristotle over mediæval thought, already considerable, soon became almost supreme, and his works were by many scholars regarded with a reverence equal to or greater than that felt for the Christian Fathers. Western knowledge of Arab astronomy was very much increased by the activity of _Alfonso X._ of Leon and Castile (1223-1284), who collected at Toledo, a recent conquest from the Arabs, a body of scholars, Jews and Christians, who calculated under his general superintendence a set of new astronomical tables to supersede the _Toletan Tables_. These _Alfonsine Tables_ were published in 1252, on the day of Alfonso’s accession, and spread rapidly through Europe. They embodied no new ideas, but several numerical data, notably the length of the year, were given with greater accuracy than before. To Alfonso is due also the publication of the _Libros del Saber_, a voluminous encyclopædia of the astronomical knowledge of the time, which, though compiled largely from Arab sources, was not, as has sometimes been thought, a mere collection of translations. One of the curiosities in this book is a diagram representing Mercury’s orbit as an ellipse, the earth being in the _centre_ (cf. chapter VII., § 140), this being probably the first trace of the idea of representing the celestial motions by means of curves other than circles.