It explains Rydberg-Ritz combination principle. A Swiss physicist Walther Ritz resolved the relationship between the spectral lines of the atom in terms of their frequencies in 1908 and named it as Rydberg-Ritz combination theory.
And it successfully calculated the positions of unknown spectral lines in the spectra of a chemical element with the help of frequencies of known spectral lines of the same substance.
“The principle states that the frequency of mysterious unknown spectral emission of an atom is nothing but the sum or difference of frequencies of its closely related known spectral emission lines.”
Ritz took advantage of this quantized energy transferences of an atom and explained its spectral data in terms of their frequencies.