Noting that the solar spectrum was crossed by "innumerable" dark lines, Fraunhofer mapped their relative positions with precision.  He developed an alphabetic system for labeling selected reference lines, assigning A to a line near the red end of the spectrum, D to the pair of dark lines associated with the bright orange streak he had observed in flame spectra, H to a line near the end of the visible violet, and I to a line in the ultraviolet.  Fraunhofer's lines puzzled practitioners and theorists alike for over four decades after the publication of his spectral maps.

Noting that the solar spectrum was crossed by "innumerable" dark lines, Fraunhofer mapped their relative positions with precision.  He developed an alphabetic system for labeling selected reference lines, assigning A to a line near the red end of the spectrum, D to the pair of dark lines associated with the bright orange streak he had observed in flame spectra, H to a line near the end of the visible violet, and I to a line in the ultraviolet.  Fraunhofer's lines puzzled practitioners and theorists alike for over four decades after the publication of his spectral maps.

Fraunhofer Absorption Lines in the Sun’s Spectrum Na D lines Orange street lamps contain sodium Harry Kroto 2004