How did Robert Bunsen and Gustav Kirchhoff discover cesium and rubidium?

Robert Bunsen and Gustav Kirchhoff discovered cesium and rubidium by pioneering the field of spectrum analysis, a technique that identifies elements by the characteristic light they emit when heated to incandescence.12

In 1860, while investigating mineral waters in the Palatinate, the researchers observed two distinct blue lines in the spectrum of a filtrate. They concluded these lines indicated a new element, which they named cesium after the Latin caesius, meaning "sky-blue".34

Shortly thereafter, in 1861, Bunsen and Kirchhoff extracted alkalies from the mineral lepidolite. Upon analyzing the solution, they identified two unique red spectral lines. They named this second new element rubidium, derived from the Latin rubidus, signifying the darkest red color.4

By demonstrating that every element produces a specific pattern of wavelengths when vaporized in a flame, Bunsen and Kirchhoff established a powerful analytical tool that allowed for the detection of elements that were otherwise difficult to isolate or identify using traditional chemical methods.524