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Prism Spectrograph, Inv. No. 4034. Photo: Deutsches Museum | Konrad Rainer
With this prism spectrograph (Inv. No. 4034), Fraunhofer dispersed sunlight into its color spectrum. Within the continuous band of colors, he made a far-reaching discovery around 1814: dark lines that were apparently a hidden property of sunlight. Decades later, it became clear that these lines reveal the substances that the sun is made of. Our entire knowledge of the composition of the sun and other stars stems from these lines.
Today, we can also recognize the broad spectrum of Fraunhofer's work in this instrument. At the center of the apparatus is a glass prism, because glass was the focal point of Fraunhofer's work. Although the dark lines piqued his interest, his task at the "Optical Institute" in Benediktbeuern was to perfect glass manufacturing. Thus, the dark lines became a useful tool for him to precisely determine the chromatic dispersion of different types of glass. With this knowledge, he was able to accurately calculate the properties of the lenses for his optical instruments.
The apparatus itself is a modified surveying instrument, a theodolite: glass production was not an end in itself. In the early 19th century, territories and plots of land were being surveyed all across Europe. High-quality measuring instruments were required for these projects and for setting up surveying offices. A key factor: the quality of the glass used for the lenses in the telescopes.
