English physicist and mathematician Sir Isaac Newton, working with an early spectrometer—a graduated glass prism—noted in 1666 that the seven rainbow colors, dispersed when white light was passed through the prism, could not be subdivided into more colors. A second breakthrough in light research occurred when German glassmaker Joseph von Fraunhofer found that the solar spectrum contained dark absorption lines of constant wavelength.
Robert Bunsen (1811-1899), Professor of Chemistry at the University of Heidelberg, working with Gustav Kirchhoff (1824-1887), used a prism spectrometer to reveal the spectral emission lines produced by elements when heated in a flame. In 1859 they became convinced that elements were uniquely characterized by their line spectra, and this led to the discovery of cesium and rubidium. The researchers also realized that the orange Fraunhofer D lines in the solar spectrum were at the same wavelength as the lines emitted by laboratory sodium. So it was understood that spectrometers could be used to analyze the composition of the sun and the stars.
From the 1870s many European firms began to manufacture prism spectrometers. These were fitted with circular scales so that the positions (and thus wavelengths) of spectral lines could be measured. Resolution was improved by passing the light through sequences of prisms. At first, no one had any idea as to how spectra were produced. However, in 1913 the Danish physicist Niels Bohr introduced a model of the atom in which spectral lines resulted from electrons moving from one stable orbit to another.
