FRAUNHOFER, JOSEPH VON (1787–1826), German optician and physicist, was born at Straubing in Bavaria on the 6th of March 1787, the son of a glazier who died in 1798. He was apprenticed in 1799 to Weichselberger, a glass-polisher and looking-glass maker. On the 21st of July 1801 he nearly lost his life by the fall of the house in which he lodged, and the elector of Bavaria, Maximilian Joseph, who was present at his extrication from the ruins, gave him 18 ducats. With a portion of this sum he obtained release from the last six months of his apprenticeship, and with the rest he purchased a glass-polishing machine. He now employed himself in making optical glasses, and in engraving on metal, devoting his spare time to the perusal of works on mathematics and optics. In 1806 he obtained the place of optician in the mathematical institute which in 1804 had been founded at Munich by Joseph von Utzschneider, G. Reichenbach and J. Liebherr; and in 1807 arrangements were made by Utzschneider for his instruction by Pierre Louis Guinand, a skilled optician, in the fabrication of flint and crown glass, in which he soon became an adept (see R. Wolf, Gesch. der Wissensch. in Deutschl. bd. xvi. p. 586). With Reichenbach and Utzschneider, Fraunhofer established in 1809 an optical institute at Benedictbeuern, near Munich, of which he in 1818 became sole manager. The institute was in 1819 removed to Munich, and on Fraunhofer’s death came under the direction of G. Merz.
Amongst the earliest mechanical contrivances of Fraunhofer was a machine for polishing mathematically uniform spherical surfaces. He was the inventor of the stage-micrometer, and of a form of heliometer; and in 1816 he succeeded in constructing for the microscope achromatic glasses of long focus, consisting of a single lens, the constituent glasses of which were in juxtaposition, but not cemented together. The great reflecting telescope at Dorpat was manufactured by him, and so great was the skill he attained in the making of lenses for achromatic telescopes that, in a letter to Sir David Brewster, he expressed his willingness to furnish an achromatic glass of 18 in. diameter. Fraunhofer is especially known for the researches, published in the Denkschriften der Münchener Akademie for 1814–1815, by which he laid the foundation of solar and stellar chemistry. The dark lines of the spectrum of sunlight, earliest noted by Dr W. H. Wollaston (Phil. Trans., 1802, p. 378), were independently discovered, and, by means of the telescope of a theodolite, between which and a distant slit admitting the light a prism was interposed, were for the first time carefully observed by Fraunhofer, and have on that account been designated “Fraunhofer’s lines.” He constructed a map of as many as 576 of these lines, the principal of which he denoted by the letters of the alphabet from A to G; and by ascertaining their refractive indices he determined that their relative positions are constant, whether in spectra produced by the direct rays of the sun, or by the reflected light of the moon and planets. The spectra of the stars he obtained by using, outside the object-glass of his telescope, a large prism, through which the light passed to be brought to a focus in front of the eye-piece. He showed that in the spectra of the fixed stars many of the dark lines were different from those of the solar spectrum, whilst other well-known solar lines were wanting; and he concluded that it was not by any action of the terrestrial atmosphere upon the light passing through it that the lines were produced. He further expressed the belief that the dark lines D of the solar spectrum coincide with the bright lines of the sodium flame. He was also the inventor of the diffraction grating.
In 1823 he was appointed conservator of the physical cabinet at Munich, and in the following year he received from the king of Bavaria the civil order of merit. He died at Munich on the 7th of June 1826, and was buried near Reichenbach, whose decease had taken place eight years previously. On his tomb is the inscription “Approximavit sidera.”
See J. von Utzschneider, Kurzer Umriss der Lebensgeschichte des Herrn Dr J. von Fraunhofer (Munich, 1826); and G. Merz, Das Leben und Wirken Fraunhofers (Landshut, 1865).