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 Fraunhofer, 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 inches diameter. For his researches published in the Denkschriften der Münchener Akademie for 1814-15, by which he laid the foundation of solar and stellar chemistry, Fraunhofer is especially known. The dark lines of the spectrum of sunlight, earliest noted by Dr 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 hence 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 his belief that the dark lines D of the solar spectrum coincide with the bright lines of the sodium flame, a fact subsequently established by Brewster, Foucault, and Miller, and successfully accounted for by Kirchhoff. Fraunhofer was a member of the Academy of Sciences at Munich, and of the university of Erlangen. 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, June 7, 1826, and was buried near Reichenbach, whose decease had taken place eight years previously. On his tomb is the inscription “Approximavit sidera.” His scientific papers were published in the Denkschriften der Münchener Akademie, Gilbert’s Annalen der Physik, and Schumacher’s Astronomische Nachrichten.

See Optics; J. von Utzschneider, Kurzer Umriss der Lebcns-geschichte des Herrn Dr J. von Fraunhofer, Munich, 1826; and Merz, Das Leben und Wirken Fraunhofers, Landshut, I860.