Page:The American Cyclopædia (1879) Volume XII.djvu/674

 660 OPOTTIA ORACLE tal dispersion and length of spectrum for a hollow glass prism filled with oil of cassia, are about four times those of crown glass ; and of flint glass, 1 time those of crown glass. Now, lenses, like prisms, must disperse or de- compose light. The different colors are real- ly brought to foci that, in the case of convex lenses, lie in the following order : The focus of the least refrangible or red ray corresponds with the true place of the principal focus ; and the more refrangible rays are brought to foci within this, as the orange, yellow, green, blue, indigo, and violet, lying nearer and nearer to the lens. These colored rays cross at their foci, and again diverge ; the effect is a colored border or fringe, mainly blue or red, as the case may be, surrounding the image, and more marked as the aperture of the lens is great- er, and in objects toward the margin of the field of view. This is chromatic aberration of lenses. It is almost perfectly corrected by combining lenses in various ways, thus form- ing achromatic combinations. The principal of these is usually that of correcting, for ex- ample, the less dispersion of crown glass by the greater dispersive power of flint glass. To do this, a concave of flint of less entire curvature is combined with a crown glass, convex, and of the greater entire curvature. The disper- sion is corrected ; but part of the refractive or lens effects remains undestroyed, and the focal distance becomes greater. (See AOHEOMATIC LENS.) OPUNTIA. See CACTUS. OPZOOMER, Carolns Wilhelnms, a Dutch philos- opher, born in Rotterdam, Sept. 20, 1821. He studied at Leyden, where he wrote a " Letter to Da Costa," and " Examination of the Dutch Annals of Theology," both attacking Christian- ity. In 1846 he became professor of philos- ophy in the university of Utrecht. Among his philosophical writings is De weg der weten- schapen (" The Path of Knowledge," Utrecht, 1851 ; German translation, 1852), in which he advocated the most absolute rational empiri- cism. A revised edition was published as Het wezen der Icennis (Amsterdam, 1863). ORACLE (Lat. oraculum, from or are, to speak), in ancient religion, a revelation believed to be made by some divinity in reply to the questions of men ; applied also to the place where such revelations were communicated. The responses were given either by the mouths of priests or priestesses, or by other signs. Responses of the oracle at Dodona were given either by the movement of leaves, the noise of brazen ves- sels, or the murmuring of the waters of a fountain. Springs, fountains, grottoes, and caves, the waters of which were discovered to have delirious or narcotic effects, were select- ed to be the sites of oracles. At Didyma the vapor of the water affected both the priestess and the person who came to consult her ; at Colophon the priest drank of the water of a secret well in a cavern ; at Delphi the priest- ess, called the Pythia, delivered her utterances from a tripod placed over a chasm from which intoxicating vapors arose. In some of the oracles artificial fumigations were employed. Oracular responses were in general remarka- ble for obscurity and equivocation, yet they exerted great political as well as religious in- fluence. The responses of the Pythia were not authoritative till they had been written and interpreted by the presiding officer. Del- phi, which was the common centre of all the oracles of Apollo, thus became the religious and political metropolis of Greece, and after- ward extended its authority over the Romans. The Neo-Platonists referred the origin of ora- cles to demons, as did also the early Christians. The theurgists sought to revive them and to oppose their power to Christianity. Eusebius and others affirmed that they became silent at the birth of Christ, and assigned as the rea- son that Christ put an end to the power and the worship of Satan on the earth. There were 22 oracles for the consultation of Apollo, the most important of which was at Delphi. (See DELPHI.) The principal others were that at Aba3 in Phocis, which, though burned by Xerxes, continued to be held in repute as late as the reign of Hadrian ; that of the Branchi- dse at Didyma, near Miletus, which was ad- ministered by a family having the hereditary gift of prophecy, received from Croesus as rich presents as that at Delphi, and was burned by the Persians, but continued to be consulted; that at Clarus, in the territory of Colophon; that at Ismenium, in Boeotia, the national sanc- tuary of the Thebans, which interpreted signs instead of speaking from inspiration; that at Patara, in Lycia, which was consulted only in winter, and where the prophetess was obliged to wait a whole night in the temple before making communications; and that at Telmes- sus, also in Lycia, the priests of which inter- preted dreams and other marvellous events. The most important oracles of Jupiter were at Olympia in Elis, and Dodona in Epirus. That at Olympia was chiefly consulted by those in- tending to take part in the Olympic games. That at Dodona was one of the most ancient and celebrated. The responses, in sounds pro- duced by the rustling of the wind in an oak tree ("the speaking oak"), were interpreted in early times by men, but afterward by old women. Its sacred oaks were cut down and its temple demolished by the JEtolians in 219 B. C., but it was consulted until the 3d cen- tury A. D. There was also an oracle of Ju- piter Ammon in Libya, which was first made known to the Greeks by the Cyrenseans ; it was in decay in the time of Strabo. The other divinities were consulted by oracles only on the special departments over which they presided. Thus, Ceres foretold at Patrse in Achaia the fate of sick persons by means of a mirror suspended in a well; Mercury was consulted at Pharse in Achaia, the person go- ing away after a ceremony, and accepting the first remark that he heard from any one as