Kirchhoff and Bunsen invent spectroscopy

In 1859 at the University of Heidelberg, the physicist Gustav Kirchhoff and the chemist Robert Bunsen showed that each chemical element, when heated until it glows, gives off light at its own sharp set of colors. Passed through a prism in a device called a spectroscope, the light forms bright lines that act as a fingerprint for the element. Bunsen's burner, designed with the instrument maker Peter Desaga in 1855, supplied a clean, almost colorless flame that made the lines easy to see. Kirchhoff also explained the dark lines Joseph von Fraunhofer had mapped in sunlight in 1814: a cool gas absorbs the same colors it would emit when hot, so the dark lines reveal the elements in the Sun's atmosphere.¹

Newton had split sunlight with a prism in 1666, and Fraunhofer had counted hundreds of dark lines without understanding them. Dalton's atomic theory had made chemists think of elements as distinct kinds of matter. Kirchhoff and Bunsen joined these threads. In 1860 and 1861 they used the method to discover two new elements, caesium and rubidium, named for their sky-blue and deep-red lines, which they found in traces in mineral spring water. Others soon found thallium and indium the same way.¹

Astronomers seized on the technique. William Huggins and Angelo Secchi examined starlight and found familiar elements such as hydrogen, sodium and iron, overturning the earlier belief that the makeup of stars could never be known. Spectral lines also posed a puzzle that classical physics could not solve: why atoms emit only certain colors. The pattern in hydrogen's lines, found by Johann Balmer in 1885, led to Niels Bohr's atomic model of 1913 and to quantum mechanics.