© Science Museum / Science & Society Picture Library
Illustration of a spectroscope taken from 'Untersuchungen uber das Sonnenspectrum und die Spectren der chemischen Elemente' (c 1861) by the German physicist Gustav Robert Kirchhoff (1824-1887). Kirchhoff and Robert Bunsen (1811-1899) established the theory of spectrum analysis, which Kirchhoff applied to determine the chemical composition of matter on Earth, the Sun and the stars. They developed the Bunsen-Kirchhoff spectroscope for this purpose in 1859. The spectroscope works by dispersing light into a spectrum, enabling it to be observed and analysed. They used it to discover the chemical elements rubidium and caesium in the Sun's atmosphere in 1860.