In 1802, William Hyde Wollaston discovered dark lines in Newton
Absorption and Emission
Spectral line shape - Wikipedia
spectral lines, explained by RP; emission, absorption lines, gas discharge lamps, linewidth, line shape
Formation of Spectral Lines
File:Spectral lines in spectral classes.svg - Wikimedia Commons
Spectral Lines
Spectral Lines of Hydrogen
Spectral Lines
The Classification of Stellar Spectra
Atomic Spectra