Henry Moseley measured characteristic X-ray frequencies across a sequence of chemical elements. For a given series, the square root of the frequency is approximately linear in atomic number after a screening offset is included. The regularity connects a spectral measurement with a discrete nuclear-charge label, rather than merely correlating with atomic weight. Missing positions in that sequence also made X-ray spectroscopy a way to test and extend the periodic ordering of chemical elements.[ref]
The High-Frequency Spectra of the Elements. Part II by Henry Gwyn Jeffreys Moseley. 1914 paper, pp. 703–713: characteristic lines, atomic-number sequence and spectral regularity
A physicist.[ref]
Moseley and X Rays by University of Oxford Department of Physics. Independent professional biography and identity
Portrait of Henry Moseley
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Henry Moseley was a British physicist and chemist, born on November 23, 1887, and who died on April 10, 1915. He is best known for his work in atomic physics, particularly for formulating Moseley's Law, which relates the atomic number of an element to its X-ray emission spectra. His research demonstrated that the atomic number, rather than the atomic mass, should be the organizing principle for the periodic table of elements.