Second, during the whole period under discussion there was an active production of new text-books on the established subjects, some of which were widely used in the universities. Among the grammars was the _Doctrinale_ of Alexander da Villa Dei, written in 1199. This rhyming grammar was enormously popular, and continued to be so, well into the sixteenth century. The _Grecismus_ and _Labyrinthus_ of Eberhard of Bethune (early thirteenth century), also grammars in rhyme, were widely used. Logical treatises often mentioned in university programs of study were _De Sex Principiis_ (On the Six Principles), written about 1150 by Gilbert de la Porree, a teacher of John of Salisbury; and the _Summulae_ of Petrus Hispanus (thirteenth century). In the thirteenth century Albertus Magnus made a digest of all the works of Aristotle, which proved to be easier for students than the originals, and which were sometimes used in place of them. Among mathematical works of this century were the _Algorismus_ (Arithmetic) and the _Libellus de Sphaera_ (On the Sphere) by John Holywood (Sacrobosco); and the _Perspectiva Communis_, i.e. Optics, by John (Peckham) of Pisa. A treatise on Music by John de Muris of Paris was produced in the early part of the fourteenth century. All of these were well-known university text-books. They appear in the list at Leipzig throughout the fifteenth century (see p. 139). 4. UNIVERSITY PRIVILEGES The privileges granted by civil and ecclesiastical powers constitute a fourth important influence upon the growth of universities. Beginning with the year 1158 a long series of immunities, liberties, and exemptions was bestowed by State and Church upon masters and students as a class, and upon universities as corporations. Masters and scholars were, for example, often taken under the special protection of the sovereign of the country in which they were studying; they were exempted from taxation, and from military service; most important of all, they were placed under the jurisdiction of special courts, in which alone they could be tried. Universities as corporations were given, among other privileges, the right to confer upon their graduates the license to teach "anywhere in the world" without further examination, and the very important right to suspend lectures, i.e. to strike, pending the settlement of grievances against State or Church. They had, of course, the general legal powers of corporations. Thus fortified, the universities attained an astonishing degree of independence and power; and their members were enabled to live in unusual liberty and security. This fact in itself unquestionably tended to increase the university population.