The Encyclopedia Americana (1920)/Fraunhofer, Joseph von

FRAUNHOFER, frown'hō-fĕr, Joseph von, German mathematician: b. Straubing, Bavaria, 6 March 1787; d. Munich, 7 June 1826. In 1799 he was placed with a looking-glass maker and glass-grinder at Munich. After various vicissitudes he received an appointment as optician in the mathematical and mechanical institute of Reichenbach and Utzschneider at Benedictbeuren, and in 1809 the mechanical part of the optical institute was chiefly under his direction. In the same year he became one of the members of the firm under which the business was conducted, and in 1818 its director. In 1823, after it was moved to Munich, Fraunhofer became a member of the Academy of Science, its conservator of physics and was made a member of the nobility in 1824. One of the most difficult operations of practical optics was to polish the spherical surfaces of large object-glasses accurately. Fraunhofer invented a machine which obviated this difficulty, and rendered the surface more accurate than it was left by the grinding. He invented also other grinding and polishing machines, and introduced many improvements into the manufacture of the different kinds of glass used for optical instruments, and which he found to be always injured by flaws and irregularities of various sorts. In 1811 he constructed a new kind of furnace, and on the second occasion when he melted a large quantity found that he

could produce flint-glass, which, taken from the bottom of a vessel containing two hundred-weight of glass, had the same refractive power as glass taken from the surface. He found that the English crown-glass and the German table-glass both contained defects occasioning irregular refraction. In the thicker and larger glasses there would be more of such defects, so that in larger telescopes this kind of glass would not be fit for object-glasses. Fraunhofer therefore made his own crown-glass. The cause which had hitherto prevented the accurate determination of the power of a given medium to refract the rays of light and separate the different colors which they contain was chiefly the circumstance that the colors of the spectrum have no precise limits, and that the transition from one to another is gradual and not immediate; hence, the angle of refraction cannot in the case of large spectra be measured within 10 feet or 15 feet. To obviate this, Fraunhofer made a series of experiments for the purpose of producing homogeneous light artificially, and unable to effect his object in a direct way, he did so by means of lamps and prisms. In the course of these experiments he discovered that bright fixed line which appears in the orange color of the spectrum when it is produced by the light of fire. This line enabled him afterward to determine the absolute power of refraction in different substances. Experiments to ascertain whether the solar spectrum contains the same bright line in the orange as that produced by the light of fire led him to the discovery of the innumerable dark fixed lines in the solar spectrum, consisting of perfectly homogeneous colors and now bearing his name. The importance of this discovery can scarcely be overestimated. It led to the invention and use of the spectroscope (q.v.), to the science of spectroscopy (q.v.), and to all our present knowledge of solar and stellar chemistry. Fraunhofer also made a variety of other important discoveries and inventions. He made the great refracting telescope for Dorpat His writings were edited by Lommel as &lsquo;Gesammelte Schriften&rsquo; (Munich 1888). Some of his writings were translated into English by J. S. Ames and published as the second volume of &lsquo;Harper's Scientific Memoirs&rsquo; under the title &lsquo;Prismatic and Diffraction Spectra&rsquo; (New York 1898). Consult Anon., &lsquo;Life of Fraunhofer&rsquo; (in American Journal of Science and Art, Vol. XVI, p. 304, New Haven 1829); Voit, &lsquo;Joseph Fraunhofer&rsquo; (Munich 1887).