A mathematician.[ref]
https://mathshistory.st-andrews.ac.uk/Biographies/Balmer/. Independent professional profile
Portrait of Johann Jakob Balmer
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Johann Jakob Balmer (1825–1898) was a Swiss mathematician and physicist best known for his work in the field of spectroscopy and for formulating the Balmer series, which describes the wavelengths of light emitted by hydrogen atoms. In 1885, Balmer derived a simple mathematical formula that accurately predicts the wavelengths of the visible spectral lines of hydrogen.