While everything thus seemed to promise a life of experimentation, Müller's imagination had a powerful hold on him, and he gave himself up for some time to certain mystical theoretical questions and problems of introspection which, for a time, threatened to take him away from his real calling of an experimental physiologist. Fortunately for Müller, as we shall see, though at the moment he doubtless {230} thought it a serious misfortune, these excursions into a too introspective psychology were followed by nervous troubles, what we could now call neurasthenia, and he was consequently led back to the study of external nature. Just after Müller's promotion to the doctorate in medicine, the Rhenish universities came once more under the authority of the Prussian government, and Berlin became a Mecca for students, who looked upon it in a way as the mother university. After his graduation at Bonn, then, Müller was attracted to Berlin, and came especially under the influence of Rudolphi, who recognized his talents and gave him special opportunities for original investigation. Rudolphi's private library and his collection were placed at the command of this young original worker, who had already proved his power of investigation and his capacity for following a subject to its ultimate conclusions, even though those were not yet extrinsically known. While at Berlin, too, Müller came under the influence of the younger Meckel, whom he learned to respect very much. After Meckel's death the _Archives of Physiology_, previously edited by Meckel, fell into Müller's hands, who successfully continued it for many years. At Müller's departure from Berlin he was presented by Rudolphi with an English microscope, as a testimonial of the old professor's appreciation of the young man's labors while under his observation. As Müller's pecuniary resources were very limited, this must have been an especially acceptable gift, since it enabled him to continue his researches in embryology, and it was not long before these began to bear fruit. At Bonn, to which Müller returned, he set up as a Privat-Docent in the University, and for several years eked out by teaching the allowance his mother could give him, and even by the practice of medicine.