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OPPEN2EIM OPTICS 653 to the empire, and subsequently it became part of Prussia. OPPENHEIM, a town of Germany, in the rnd duchy of Hesse, on the Rhine, 10 m. S. E. of Mentz; pop. in 1871, 2,926. It occu- pies the site of an ancient Roman castle, and at one time was one of the most important cities of the Rhine; but it was almost en- tirely destroyed by the French in 1689. The Protestant St. Catharine's church, one of the most magnificent Gothic edifices of Germany, and especially celebrated for its windows, is in ruins, excepting the E. part, which was re- stored in 1843. Nier stein and other places fa- mous for excellent vintages are in the vicinity. OPPERT, Jules, a French orientalist, born in Hamburg, of Jewish parents, July 9, 1825. He received a classical education, studied law at Heidelberg, and Sanskrit and Arabic at Bonn. He next studied the Zend and the ancient Per- sian, and published a treatise at Berlin on the vocal system of the latter language. His re- ligion incapacitating him for a professorship in a German university, he went to France in 1847, obtained the professorship of German at the lyceums of Laval and Rheims, and was ap- pointed on the scientific expedition sent by the government to Mesopotamia. After his return in 1854, he submitted to the institute a new system of interpreting the inscriptions. He also laid before the geographical society of the institute a plan of ancient Babylon. For 20 years he has devoted himself chiefly to the study of cuneiform inscriptions. In 1857 he was appointed professor of Sanskrit in the school of languages attached to the imperial library. Among his works are: Les inscrip- tions des Achemenides (1852); fitudes assyri- ennes ; L 1 Expedition scientifique de France en Mesopotamia (185 8-' 64); Grammaire san- scrite (1859) ; Lesfastes de Sargon, in company with M. J. M6nant (1863); Grande inscription du palais de Khorsabad (1864) ; Histoire des empires de Chaldee et d'Assyrie, d'apres les monuments (1866); and IS Immortalite de Vdme chez les Chaldeens, suivi d'une traduction de la descente aux enfers de la deesse Istar Astarte (1875). OPPIAN, a Greek poet, born in Cilicia, flour- ished about A. D. 180. He belonged to a dis- tinguished family. His father having been banished to the island of Melita, Oppian ac- companied him, and there wrote his Halieu- tica, a poem on fishing, in 3,500 verses. A poem entitled Cynegetica, "On Hunting," at- tributed to him, modern critics suppose to have been written by another person of the same name. The best edition of the two is that of Schneider (Strasburg, 1776). OPTICS, the science which treats of the na- ture of light, and of the laws of the phe- nomena of light and vision. For the theo- ries of light, and other branches of the sub- ject, see the articles ABERRATION, CHROMATICS, FLUORESCENCE, LIGHT, SPECTRUM, SPECTRUM ANALYSIS, SPECTACLES, STEREOSCOPE, and VI- SION. The present article will be devoted chiefly to the laws of reflection (catoptrics) and those of simple refraction (dioptrics). These form a large portion of geometrical or for- mal optics, in which, without regard to any theory, the actual phenomena of light are ob- served and generalized, and the laws of the changes effected in the rays by surfaces and media are ascertained. In connection with the transmission of light one other general fact may be noticed. It is that, with the ex- ception that some degree of dimness will arise when the interposed body of air is of great extent, a given surface, as that of the side of a house, illuminated in the same degree, appears equally luminous, at whatever dis- tance it may be regarded. This equal bright- ness at different distances is readily explained when we remember that the actual intensity of light from a point or unit of surface dimin- ishes in inverse ratio as the square of the dis- tance increases; and that, since any linear magnitude diminishes in the inverse ratio of ,the simple distance, so a surface must also appear lessened in the ratio of the square of distance; less light comes to the eye from a given surface at increased distance, but the actual surface becomes contracted into an ap- parent surface less in the same proportion; and thus one effect balances the other, and the actual illumination is reduced by the effect of the aerial perspective only. The ancient Greeks and the Arabians made considerable progress in formal optics, but chiefly in the dis- covery of the law of reflection, and of conse- quences flowing from it. They had attained the idea of rays of light, the fact of their or- dinary straight-lined transmission, and the law of equality of the angles of reflection and inci- dence, and deduced with much completeness the properties of shadows, perspective, and the convergence of rays by concave mirrors. Eu- clid and the followers of Plato, however, taught that these rays proceed from the eye, not from the visible object. Aristotle reasoned that an interposed medium was necessary to vision; this he considered to be light, and defined as " the transparent in action." Of special trea- tises on light, the earliest known are the " Op- tics" of Euclid, Heron's "Catoptrics," and Ptolemy's " Optics." In the last of these oc- curs an elaborate collection of measurements of the refraction at different angles, from air to glass, and from glass to water tables of much interest, as furnishing the oldest extant ex- ample of an accurately conducted physical in- vestigation by experiment. Tycho Brahe in- troduced a correction for atmospheric refrac- tion into astronomical calculations; the tele- scope appears to have been invented separately by Metius and Jansen about the year 1608; and Kepler, with his usual fertility of mathe- matical elements and of hypotheses, and incited by these advances, strove earnestly to find the true law of relation of the angle of refraction to that of incidence, but reached only a near