Council of Constance
In 1874, he finished his dissertation about the Alliance of Canterbury and its significance for the Hundred Years' War and the Council of Constance. In the following year he started working for the secret state archives in Marburg with the duty of editing the correspondence of Philip I, Landgrave of Hesse. It would be published in three volumes from 1880 to 1891. Meanwhile, he also wrote his habilitation about the Council of Constance, which he finished in 1876. In the following decade, he studied the Protestant Reformation, publishing a biography of Martin Luther in 1883.