The Rockefeller Foundation offered an extension of Bethe's fellowship, allowing him to return to Italy in 1932. In the meantime, Bethe worked for Sommerfeld in Munich as a privatdozent. Since Bethe was fluent in English, Sommerfeld had Bethe supervise all his English-speaking postdoctoral fellows, including Lloyd P. Smith from Cornell University. Bethe accepted a request from Karl Scheel to write an article for the Handbuch der Physik on the quantum mechanics of hydrogen and helium. Reviewing the article decades later, Robert Bacher and Victor Weisskopf noted that it was unusual in the depth and breadth of its treatment of the subject that required very little updating for the 1959 edition. Bethe was then asked by Sommerfeld to help him with the handbuch article on electrons in metals. The article covered the basis of what is now called solid state physics. Bethe took a very new field and provided a clear, coherent, and complete coverage of it. His work on the handbuch articles occupied most of his time in Rome, but he also co-wrote a paper with Fermi on another new field, quantum electrodynamics, describing the relativistic interactions of charged particles.