The universities most popular with the Poles were the Protestant universities rather than the Catholic. The relative proportion of Poles attending German Protestant and Catholic universities was, in the sixteenth century, six to one.[240] The most popular Catholic university was Ingolstadt, registering in that century three hundred and sixty-five Polish students and occupying the fifth place among the German universities frequented by Poles.[241] Freiburg in the course of fifty-six years, from 1575 to 1631, during a period when the Catholic reaction had already set in, registered less than a hundred Polish students.[242] Dillingen, Würzburg, Mainz, founded by Catholic bishops for the purpose of counteracting the influence of the Protestant universities and under the control of the Jesuit Order, began to draw Polish students with the rise of the Catholic reaction after 1564.[243] Of the Protestant universities the most popular with the Poles were Wittenberg, Königsberg, Heidelberg, and Frankfort on the Oder,--all centres of Lutheranism. Of the Swiss universities the one most largely attended by Poles was the University of Basel. The Swiss universities together with Altdorf, Herborn, and Marburg in Germany were centres of Calvinism, and were sought and frequented by Calvinistic sympathizers from among the Poles.[244] At Altdorf, where the number of students was comparatively small, the Poles constituted in some years one-fourth of the total student body. Owing to their numerical strength, the honorary rectorship of the university was held twice by one of their number, in 1583-1584 by Nicholas Ostrorog, and in 1609-1610 by Adam Sienieński.[245]