Johannes Robert Rydberg (8.11.1854 —28.12.1919) was a Swedish physicist and mathematician and a member of the Royal Swedish Academy of Sciences (1919). He was born in Halmstad. He graduated from Lund University (1879), where he worked (professor from 1897).
His main work concerned the systematics of atomic spectra and atomic physics. He was the first to operate with the wave number, introduced the universal constant R (the Rydberg constant), established the relations between the various series that form the spectrum of an element, and gave a formula describing any spectral line (1890). Rydberg's ideas played a significant part in the establishment in 1908, by W. Ritz, of the so-called combination principle, according to which the wave number of any spectral line can be represented as the difference of two terms from the set of spectral terms characterising the element. This law is now known in physics as the Rydberg — Ritz combination principle. In 1889 he suggested that the emission spectra of the chemical elements should lead to an understanding of the periodic system. He put forward the idea that the independent variable is not the atomic weight of an element but its atomic number.