Rydberg, Johannes Robert (yōˈhänəs rôˈbərt rüdˈbĕryə) [key], 1854–1919, Swedish physicist. Rydberg was a professor at Lund from 1901 to 1919. He is best known for his grouping of the frequencies of certain lines of the emission spectra of the elements into simple series characterized by a running integer and a universal "Rydberg" constant. These series helped guide the development of atomic physics. Rydberg also wrote on the structure of the periodic table of the elements.