19th century illustration showing a man using a compound spectroscope. This compound spectroscope was used by German physicist Gustav Kirchhoff (1824-1887) and German chemist Robert Bunsen (1811-1899) to identify chemical elements. The elements were identified by the characteristic spectrum of radiation they emit when heated. This method is used both in the laboratory and to analyse distant stars. Kirchhoff and Bunsen used this sort of apparatus to analyse the spectrum of elements such as sodium, potassium, lithium, barium, strontium and calcium. They also discovered new elements, such as rubidium (1861) and caesium (1860). Illustration from 'Spectrum Analysis in its Application to Terrestrial Substances, and the Physical Constitution of the Heavenly Bodies', 1872.