onychophoran V.a-ni-'ka-fo-ronX Any of about 90 species of free- living terrestrial invertebrates in the class Onychophora (sometimes con¬ sidered a phylum). They are sometimes called velvet worms for their velvety skin. The common genus Peripatus occurs in the West Indies, Central America, and northern South America. Onychophorans are slen¬ der and segmented; each segment has a pair of short legs. Species range from 0.6 to 6 in. (14-150 mm) long. They live in humid, hidden spots: in forest litter, wood crevices, termite nests, or the soil, sometimes to a depth of more than 3 ft (1 m). They use their jaws to open captured prey (often small insects) and suck out the juices.
onyx \'a-niks\ Striped, semiprecious variety of the silica mineral agate with white and black alternating bands. Other varieties include carnelian onyx, with white and red bands, and sardonyx, with white and brown bands. Its properties are the same as those of quartz. Onyx is used in carved cameos and intaglios because its layers can be cut to show a colour contrast between the design and the background. It is found worldwide, but chiefly in India and South America.
Oort cloud Vast spherical cloud of small, icy bodies orbiting the Sun at distances ranging from about 0.3 light-year to one light-year or more that is probably the source of most long-period comets. In 1950 the Dutch astronomer Jan Hendrik Oort (1900-92) noted that no comets have orbits that would indicate an interstellar origin. He proposed that the Sun is sur¬ rounded by billions of these objects, which are only occasionally detect¬ able when they enter the inner solar system. The Oort cloud is believed to be composed of primordial bodies dating from the formation of the solar system (see solar nebula). Whether the Oort cloud merges, in its inner region, into the disk-shaped Kuiper belt is not known.
Op art or Optical art Branch of mid-20th-century geometric abstract art that deals with optical illusion. Op art painters devised complex opti¬ cal spaces by manipulating repetitive forms such as parallel lines, check¬ erboard patterns, and concentric circles or by creating chromatic tension from the juxtaposition of complementary colours, thereby creating the illusion of movement. Principal artists of the Op movement in the late 1950s and the ’60s include Victor Vasarely, Bridget Riley (b. 1931), and Larry Poons (b. 1937).
opal A hydrated, noncrystalline silica mineral used extensively as a gem¬ stone. Its chemical composition is similar to that of quartz but generally with a variable water content. Pure opal is colourless, but impurities generally give it various dull colours ranging from yellow and red to black. Black opal is especially rare and valuable. White opal and fire opal, characterized by yellow, orange, or red colour, are much more common. Various forms of common opal are widely used as abrasives, insulation material, and ceramic ingredients. Opal is most abundant in volcanic rocks, especially in areas of hot-spring activity. The finest gem opals have been found in Australia; other areas that yield gem material include Japan, Mexico, Honduras,
India, New Zealand, and the U.S.
OPEC in full Organization of the Petroleum Exporting Countries Multinational organiza¬ tion established in 1960 to coordinate the petroleum production and export policies of its members. Iran, Iraq, Kuwait, Saudi Arabia, and Venezuela were the original members; they were joined by Qatar (1961), Indonesia and Libya (1962), Abu Dhabi (1967; membership transferred to the United
Arab Emirates, 1974), Algeria (1967), and Nigeria (1971). Ecuador (1973) and Gabon (1975) are no longer OPEC members. Policy decisions are taken by consensus at its Vienna headquarters. In 1973 OPEC began a series of oil price increases in retaliation for Western support of Israel in the 1973 Arab-Israeli war, and OPEC members’ income greatly increased as a result. Internal dissent, the development of alternative energy sources in the West, and Western exploitation of oil sources in non-OPEC coun¬ tries subsequently combined to reduce the organization’s influence. OPEC countries supply about two-fifths of the world’s oil consumption and pos¬ sess about two-thirds of the world’s proven oil reserves.
open cluster or galactic cluster Any group of Population I (see Populations I and II) stars with a common origin, held together by mutual gravitation (not to be confused with a cluster of galaxies). Stars in open clusters are much more scattered than those in globular clusters. All known open clusters contain from about 10 to 1,000 or more stars (about half contain fewer than 100) and have diameters of 5-75 light-years. More than 1,000 have been discovered in the Milky Way Galaxy; well-known examples include the Pleiades and the Hyades.
Open Door policy Statement of U.S. foreign policy toward China. Issued by U.S. secretary of state John Hay (1899), the statement reaffirmed the principle that all countries should have equal access to any Chinese port open to trade. The U.S. sent notes to Britain, Germany, France, Italy, Japan, and Russia explaining the policy to prevent them from establish¬ ing separate spheres of influence in China. Their replies were evasive, but the U.S. considered them acceptances of the policy. Japan’s violation of the policy in 1937 led the U.S. to impose an oil embargo. The policy was discontinued with the communist takeover of China in 1949.
open-heart surgery Any surgical procedure opening the heart and exposing one or more of its chambers, most often to repair valve disease or correct congenital heart malformations (see congenital heart disease). Invention of the heart-lung machine (see artificial heart), which allows the heart to be stopped during surgery, made it possible. The first successful open-heart surgery was performed in the U.S. in 1953 by John H. Gib¬ bon, Jr., to close an atrial septal defect.
open-hearth process or Siemens-Martin process Steelmaking technique that for most of the 20th century accounted for most steel made in the world. William Siemens made steel from pig iron in a reverberatory furnace of his design in 1867. The same year the French manufacturer Pierre-Emile Martin (1824-1915) used the idea to produce steel by melt¬ ing wrought iron with steel scrap. Siemens used the waste heat given off by the furnace: he directed the fumes from the furnace through a brick checkerwork, heating it to a high temperature, and then used the same path to introduce air into the furnace; the preheated air significantly increased the flame temperature. The open-hearth process furnace (which replaced the Bessemer process) has itself been replaced in most industrial¬ ized countries by the basic oxygen process and the electric furnace. See also
REVERBERATORY FURNACE.
open market operation Any of the purchases and sales of govern¬ ment securities and commercial paper by a central bank in an effort to regulate the money supply and credit conditions. Open market operations can also be used to stabilize the prices of government securities. When the central bank buys securities on the open market, it increases the reserves of commercial banks, making it possible for them to expand their loans and investments. It also increases the price of government securi¬ ties, equivalent to reducing their interest rates, and decreases interest rates generally, thus encouraging investment. If the central bank sells securi¬ ties, the effects are reversed. Open market operations are usually per¬ formed with short-term government securities such as treasury bills.
opera Musical drama made up of vocal pieces with orchestral accom¬ paniment, overtures, and interludes. Opera was invented at the end of the 16th century in an attempt by the Camerata (an academy of Florentine poets, musicians, and scholars) to imitate ancient Greek drama, which was known to have been largely sung or chanted. Since no actual Greek music was known, composers had considerable freedom in reconceiving it. Imi¬ tations of Greek pastoral poetry became the basis for early opera libretti. The first operas, Dafne by Jacopo Peri (1561-1633) in 1598 and by Giuuo Caccini about the same time, are now lost; the earliest surviving opera is Peri’s Euridice (1600). They consisted of lightly accompanied vocal melody closely imitating inflected speech. Claudio Monteverdi, the great¬ est early operatic figure, composed the first masterpiece, Orfeo, in 1607; unlike its predecessors, it is scored for a small orchestra. With this work,