A discontinuous spectrum characteristic of excited atoms, ions, and certain molecules in the gaseous phase at low pressures, to be distinguished from band spectra, emitted by most free molecules, and continuous spectra, emitted by matter in the solid, liquid, and sometimes gaseous phase. If an electric arc or spark between metallic electrodes, or an electric discharge through a low-pressure gas, is viewed through a spectroscope, images of the spectroscope slit are seen in the characteristic colors emitted by the atoms or ions present.