FRAUNHOFER, JOSEPH, a distinguished optician of Bavaria, was born at Straubiog in that kingdom, in 1787, of parents iu humble life ; and by their death he was left an orphan when eleven years of age.
He had been accustomed to labour from his childhood, and he was early engaged as an apprentice to a manufacturer, who exacted from him an unremitting attention to the mechanical operations connected with his calling ; yet the youth found means, without the aid of an instructor, to supply in a certain degree the deficiencies of his educa tion, and to make some progress in the study of mathematics.
An accident, which nearly cost him his life, was the cause that the merit of Fraunhofer became known : an old house in which he lodged fell down one day and buried him in the ruins; be was extricated however, and happily came out unhurt. The interest excited by the danger which the young man had escaped drew upon him the notice of several persons of rank and fortune; and these, being struck with admiration on discovering the efforts be bad made in the midst of many adverse circumstances to cultivate the sciences, procured for him an introduction to the celebrated Reichenbach, who received him, he being then about twenty years of age, into the great manufactory for the construction of mathematical and philosophical instruments which he had established at Benedietbalern, near Munich. In this situation Fraunhofer had ample scope for the exercise of his talents ; and he distinguished himself as much by his inventive genius as by his skill in executing the mechanical processes on which he was employed. Enabled now to study optics as a science, he used the means at his disposal to make many important experiments on light, and to construct instruments of superior kinds for celestial observa tions. By his discoveries and improvements be greatly increased the reputation of the establishment to which he belonged; and at length it became his own property.
Fraunhofer was a member of the University of Erlangen and of the Royal Academy of Sciences at Munich ; and in 1822 this academy appointed him keeper of the Museum of Physics. The king of Bavaria conferred on him the order of Civil Merit, and the king of Denmark that of Dsnebrog. Ile died in 1826, in the thirty-ninth year of his age.
His most remarkable discovery was that of the existence of a series of dark lines in the spectrum produced by the refraction of the sun's light in a prism of glass or other transparent medium. The prisms were formed of a material free from veins, and in the experiments they were disposed so that the light entered and emerged at equal angles with their sides, by which means each of the coloured spaces in the solar spectrum on the screen were homogeneous : on examining these with a telescope it was perceived that they contained many black lines parallel to one another and to the breadth of the spectrum ; and Fraunhofer ascertained that they amounted in number to about 354; Sir David Brewster has since discovered mauy more. By means of .a
theodolite he measured the angular distances between the most strongly marked of these lines in every two of the differently coloured spaces in the spectrum produced by each of the prisms employed in the experiments; and thus he was enabled to determine with great accuracy the indices of refraction for the mean rays of the prismatic colours in each of the media of which the prisms were formed, as well as the dispersive powers of those media. He observed similar black lines in the spectra of the moon, Mars, Venus, and some of the fixed stars; also in the spectra formed by the two polarised pencils produced by a prism of Iceland spar. An account of the observations on spectra was published in a pamphlet entitled Bestimmung des Brechungsund Farbenzer-Streutings-Verinfigens verechiedener Glasarten B-zug auf die Vervollkommnung achromatischer Fernriihre,' 4to, Munich, 1815. Fraunhofer also made many highly curious and interesting experi ments on the phenomena arising from the interference of light in passing through small apertures of different forms, and through wire gratings. An account of these experiments on the inflexion 'of light was published in 4to, at Munich, under the title of ' Neue Modifikation des Lichtes durch gegenseitige Einwirkung und Bcuguog der Strahlen und Gesetze derselben.' Fraunhofer executed an equatorially-mounted telescope for the observatory at Dorpat. The diameter of the object-glass is nearly 10 inches, and its focal length about 16 feet ; it consists, as usual, of a convex lens of crown glass, and a concave lens of flint glass, but the materials were compounded by himself, and the performance of the instrument is said to be superior to that of any which had been made before. A description of the telescope is given in the Astrouomische Nachrichten; Nos. 74, 75, 76, with a memoir on the refractive and dispersive powers of different kinds of glass. In the same work is a memoir by Fraunhofer ou halos, parhelia, and the like phenomena : in this he ascribes the formation of the small solar and lunar halos to the inflexion of light in tho vapour of the atmosphere; and that of the larger kind to the refraction in hexagonal prisms of ice.