= Solution
\b[<Spectrum analysis> identifies substances by the positions and patterns of characteristic <spectral lines>, rather than by the overall colour of a flame alone.] A small amount of a suitable salt is introduced into a hot, weakly luminous flame. In the historical prism <spectroscope>, light enters a narrow slit, a collimator makes the emerging rays nearly parallel, a prism separates them by wavelength, and an observing telescope shows images of the slit at different spectral positions. A reference scale or comparison spectrum makes positions reproducible.
A clean <platinum> wire and reagent blank help distinguish the sample's signal from contamination. A reference sample establishes the expected positions, while repeated cleaning and testing check whether an unfamiliar line belongs to the sample. Several substances can be recognized in one mixture because their characteristic line patterns can coexist, although overlapping lines and changing excitation conditions complicate identification. <Robert Bunsen> and <Gustav Kirchhoff>'s original account explicitly emphasizes the faint background of the flame, the prism instrument and the characteristic patterns of alkali and alkaline-earth salts.<Chemical Analysis by Observation of Spectra by Gustav Kirchhoff and Robert Bunsen>[1860 paper, apparatus and comparison of salt spectra]{ref}
In modern terms, <emission lines> arise when excited <chemical atoms> or <ions> release photons at energies corresponding to allowed differences between <energy levels>:
$$
\boxed{h\nu=E_{\rm upper}-E_{\rm lower},\qquad\lambda=c/\nu.}
$$
The <quantum mechanics> behind this equation is a later explanation, not the theory available to an 1866 candidate. Similarly, <absorption lines> appear when a cooler foreground gas removes particular wavelengths from a brighter background. Agreement between laboratory line positions and dark lines in the <solar spectrum> permits identification of constituents of the solar atmosphere without bringing a solar sample into the laboratory. It does not establish the detailed chemical constitution of every layer of the Sun.
By December 1866, striking examples included \b[<caesium> (1860), <rubidium> (1861), <thallium> (1861) and <indium> (1863)]. The first two were recognized in the work of <Robert Bunsen> and <Gustav Kirchhoff>, using conspicuous blue and deep-red spectral features respectively. <William Crookes> recognized the characteristic green line associated with <thallium>, while <Ferdinand Reich> and <Hieronymus Theodor Richter> identified <indium> through its indigo-blue spectral signature. These observations motivated chemical separation and further characterization; an unexpected line by itself is a clue to a new <chemical element>, not a complete proof.<Periodic Table by Royal Society of Chemistry/Caesium — Element information, properties and uses>[History, discovery in 1860]{ref}<Periodic Table by Royal Society of Chemistry/Rubidium — Element information, properties and uses>[History, discovery in 1861]{ref}<Periodic Table by Royal Society of Chemistry/Thallium — Element information, properties and uses>[History, green spectral line and 1861 announcement]{ref}<Periodic Table by Royal Society of Chemistry/Indium — Element information, properties and uses>[History, 1863 discovery]{ref}
The corresponding recognition of familiar <chemical elements> in the <solar spectrum> opened stellar chemical analysis. Later discoveries such as solar <helium> should not be credited to the 1866 state of the subject.
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