In 1648, he went to Sweden, summoned by Queen Christina to take up a position as her court librarian, and was accompanied by Cornelius Tollius as his amanuensis. There he enriched the library that had been founded by Gustavus Adolphus, partly as booty of war from the library of Prague, with judicious purchases, but incurred the enmity of the French philologist Claudius Salmasius.