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How was indium discovered through spectroscopy?

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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).

Indium was discovered in 1863 by German chemists Ferdinand Reich and Hieronymus Theodor Richter while they were analyzing zinc ore samples.1

The discovery was made through spectroscopy, a method involving the analysis of light emitted by or passed through substances containing the element. During their examination, the chemists observed a predominant indigo [spectral line], which led to the identification of the new element and suggested its name, indium. Like other members of the boron group, such as gallium and thallium, indium was first detected using this spectroscopic technique in the late 19th century.21