Illustration of the apparatus used by the German physicist Joseph von Fraunhofer (1787-1826) to detect absorption lines in the solar spectrum. Six lamps (B, C) are placed behind a shutter with six slits in it. The light from the lamps is directed to a prism (A), which refracts the light through another shutter. Some of the refracted light is directed towards a modified theodolite (top right). This enabled Fraunhofer to see the entire spectrum and measure each colour's dispersion. Absorption lines, known as Fraunhofer lines, indicate the presence of certain elements in the Sun's atmosphere and were discovered by Fraunhofer in 1814. From Determination of the Refractive and Dispersive Indices of Differing Types of Glass in Relation to the Perfection of Achromatic Telescopes (1817).