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How was thallium discovered using spectral lines?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — boron group element Modern version of the periodic table of the elements (printable).

In 1861, the British chemist Sir William Crookes discovered thallium by applying the technique of spectrochemical analysis to selenium-bearing pyrites used in the manufacture of sulfuric acid.12

By observing the light emitted from these substances, Crookes identified a prominent, characteristic green spectral line that did not correspond to any known element. This distinct green line is the origin of the element's name, which is derived from the Greek word thallos, meaning "green shoot". Following this spectroscopic detection, Crookes and the French chemist Claude-Auguste Lamy independently isolated the element in 1862, confirming it to be a metal.123