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Johannes Rydberg organized atomic spectral lines into series using wavenumbers. In modern notation, a typical series has the form ${\tilde{\nu }}_n={\tilde{\nu }}_{\infty }-R/(n+\delta {)}^2$, with successive integers $n$ and a fitted offset $\delta$. The relation predicts a limiting wavenumber and increasingly close spacing of spectral lines. His empirical comparisons across chemical elements preceded the later explanation through transitions between quantized atomic energy levels, illustrating how precise regularities can guide a physical theory.^[ref]

Physicist.^[ref]