Encyclopædia Britannica Australia -

Encyclopedias since 1768

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operon ... optometry

operon: genetic regulatory system found in bacteria and their viruses in which genes coding for functionally related proteins are clustered along the DNA. This feature allows protein synthesis to be controlled coordinately in response to the needs of the cell. By ...
Opet: ancient Egyptian festival of the New Year. In the celebration of Opet, the god Amon together with Mut, his consort, and Khons, their son, made a ritual journey from their shrines at Karnak to the temple of Luxor (called Ipet ...
ophicleide: brass wind musical instrument with a cup-shaped mouthpiece and padded keys, the bass version of the old keyed bugle. The name (from Greek ophis and kleid, "serpent" and "key") alludes to its improvement on the military band "upright serpents" (now-obsolete ...
Ophioglossaceae: only family in the fern order Ophioglossales, a primitive group not closely related to other ferns. The family contains three genera and about 55 species. Its members are characterized by leaves (fronds) that are divided into two parts, a sterile ...
Ophir: unidentified region famous in Old Testament times for its fine gold. The geographic list of Genesis 10 apparently places it in Arabia, but in the time of Solomon (c. 920 BC), Ophir was thought of as being overseas. Gold, almug ...
Ophite: (from Greek ophis, "serpent"), member of any of several Gnostic sects that flourished in the Roman Empire during the 2nd century AD and for several centuries thereafter. A variety of Gnostic sects, such as the Naassenes and Cainites, are included ...
Ophrys: genus of orchids, family Orchidaceae, containing approximately 30 species of plants native to Eurasia and North Africa. All have metallic-coloured, hairy flowers that resemble insects. Each plant is less than 30 cm (1 foot) tall and bears several flowers on ...
ophthalmology: medical specialty dealing with the diagnosis and treatment of diseases and disorders of the eye. The first ophthalmologists were oculists. These paramedical specialists practiced on an itinerant basis during the Middle Ages. Georg Bartisch, a German physician who wrote on ...
ophthalmoplegia: paralysis of the extraocular muscles, which control the movements of the eye and of the pupil. Ophthalmoplegia usually involves either the third (oculomotor), fourth (trochlear), or sixth (abducent) cranial nerves. In oculomotor paralysis, the muscles controlling one pupil are affected ...
ophthalmoscope: instrument for inspecting the interior of the eye, invented in 1851 by the German physiologist Hermann von Helmholtz. The device consists of a strong light that can be directed into the eye by a small mirror. The light reflects off ...
Ophuls, Max: German motion-picture director whose mastery of fluid camera movement gave his films a characteristic lyrical flow. He was one of the first truly international directors, sensitive to national differences and to the human qualities common to all his characters.
Opie, Eugene Lindsay: American pathologist who conducted important research on the causes, transmission, and diagnosis of tuberculosis and on immunization against the disease.
Opie, John: English portrait and historical painter popular in England during the late 18th century.
Opik, Ernest Julius: Estonian astronomer best known for his studies of meteors and meteorites, and whose life work was devoted to understanding the structure and evolution of the cosmos.
opisthobranch: any marine gastropod of the approximately 2,000 species of the subclass Opisthobranchia. These gastropods, sometimes called sea slugs, breathe either through gills, which are located behind the heart, or through the body surface. The shell and mantle cavity are reduced ...
Opitz, Martin: German poet and literary theorist who introduced foreign literary models and rules into German poetry.
opium: narcotic drug that is obtained from the unripe seedpods of the opium poppy (Papaver somniferum), a plant of the family Papaveraceae. (See poppy.) Opium is obtained by slightly incising the seed capsules of the poppy after the plant's flower petals ...
opium trade: in Chinese history, the traffic that developed in the 18th and 19th centuries in which Western nations, mostly Great Britain, exported opium grown in India and sold it to China. The British used the profits from the sale of opium ...
Opium Wars: two trading wars in the mid-19th century in which Western nations gained commercial privileges in China. The first Opium War (1839-42) was between China and Britain, and the second Opium War (1856-60), also known as the "Arrow" War, or the ...
Opole: city, capital of Opolskie wojewodztwo (province), southwestern Poland, situated on the Oder River. Opole began as the home of the Slavic Opolanie tribe; the earliest mention of it was in the 9th century. In 1202 it became ...
Opolskie: wojewodztwo (province), southern Poland. It is bordered by the provinces of Wielkopolskie and Lodzkie to the north and Slaskie to the east, by the Czech Republic to the south, and by the province of Dolnoslaskie to the ...
opossum: any of about 66 species of New World mammals constituting the family Didelphidae of the superorder Marsupialia. For ordinal relatives in American tropics, see rat opossum; for Australasian marsupials called possum, see phalanger.
opossum shrimp: any member of the crustacean order Mysidacea. Most of the 450 known species live in the sea; a few live in brackish water; and fewer still live in fresh water. Most are 1 to 3 cm (about 0.4 to 1.2 ...
Oppen, George: American poet and political activist, one of the chief proponents of Objectivism, a variation on Imagism.
Oppenheim, E. Phillips: internationally popular British author of novels and short stories dealing with international espionage and intrigue.
Oppenheim, Lassa Francis Lawrence: German jurist and teacher of law who was best known for his Positivist approach to international law.
Oppenheimer, J. Robert: American theoretical physicist and science administrator, noted as director of the Los Alamos laboratory during development of the atomic bomb (1943-45) and as director of the Institute for Advanced Study, Princeton (1947-66). Accusations of disloyalty led to a government hearing ...
Oppenheimer, Sir Ernest: German-born industrialist, financier, and one of the most successful leaders in the mining industry in South Africa and Rhodesia.
Oppland: fylke (county), south-central Norway. Bisected by Gudbrand's Valley, it extends from Mjosa and Randsfjorden lakes northwestward to the Jotunheimen, Dovrefjell, and Rondane mountains. Much of the fylke is made up of high plateaus; large areas of the mountainous north are ...
Opportunity: American magazine associated with the Harlem Renaissance, published from 1923 to 1949. The editor, Charles S. Johnson, aimed to give voice to black culture, hitherto neglected by mainstream American publishing.
opposites, table of: in Pythagorean philosophy, a set of 10 pairs of contrary qualities. The earliest reference is in Aristotle, who said that it was in use among some contemporary Pythagoreans. But Aristotle provided no real information about its function in Pythagorean practice ...
opposition: in astronomy, the circumstance in which two celestial bodies appear in opposite directions in the sky. The Moon, when full, is said to be in opposition to the Sun; the Earth is then approximately between them. A superior planet (one ...
opposition, square of: in traditional logic, a diagram exhibiting four forms of a categorical proposition (q.v.), or statement, with the same subject and predicate, together with their pairwise relationships:
oprichnina: private court or household created by Tsar Ivan IV the Terrible (1565) that administered those Russian lands (also known as oprichnina) that had been separated from the rest of Muscovy and placed under the tsar's direct control. The term also ...
optic atrophy: degeneration of the optic (second cranial) nerve, which carries visual data from the retina of the eye to a relay station in the centre of the brain (the lateral geniculate body) for transmission to a cortical area at the back ...
optic nerve: second cranial nerve that carries sensory nerve impulses from the retina to the visual cortex of the brain. In the eye, the nerve forms from the convergence of visual nerve fibres in the optic disk at the rear of the ...
optic neuritis: inflammation of the optic (second cranial) nerve, the nerve that carries visual data from the retina of the eye to a relay station in the centre of the brain for transmission to a cortical area at the back of the ...
optical activity: the ability of a substance to rotate the plane of polarization of a beam of light that is passed through it. (In plane-polarized light, the vibrations of the electric field are confined to a single plane.) The intensity of optical ...
optical axis: the straight line passing through the geometrical centre of a lens and joining the two centres of curvature of its surfaces. Sometimes the optical axis of a lens is called its principal axis. The path of a light ray along ...
optical ceramics: advanced industrial materials developed for use in optical applications.
optical crystallography: branch of crystallography that deals with the optical properties of crystals. It is of considerable interest theoretically and has the greatest practical importance. The science of petrography is largely based on the study of the appearance of thin, transparent sections ...
optical image: the apparent reproduction of an object, formed by a lens or mirror system from reflected, refracted, or diffracted light waves. There are two kinds of images, real and virtual. In a real image the light rays actually are brought to ...
optical interferometer: instrument for making precise measurements for beams of light of such factors as length, surface irregularities, and index of refraction. It divides a beam of light into a number of beams that travel unequal paths and whose intensities, when reunited, ...
optical model: in physics, description of atomic nuclei as similar to cloudy crystal balls in that, when struck by a beam of particles, they partially absorb the beam, partially scatter it, and partially transmit it in a way analogous to the behaviour ...
optical pumping: in physics, the use of light energy to raise the atoms of a system from one energy level to another. A system may consist of atoms having a random orientation of their individual magnetic fields. When optically pumped, the atoms ...
optical sound recording: use of an optical system for registering sound on photographic film; it is a technique widely used in making the sound track (q.v.) of motion pictures.
optical storage: electronic storage medium that uses low-power laser beams to record and retrieve digital (binary) data. In optical-storage technology, a laser beam encodes digital data onto an optical, or laser, disk in the form of tiny pits arranged in concentric tracks ...
optics: science concerned with the genesis and propagation of light, the changes that it undergoes and produces, and other phenomena closely associated with it. There are two major branches of optics, physical and geometrical. Physical optics deals primarily with the nature ...
Optimates and Populares: (Latin: respectively, "Best Ones," or "Aristocrats", and "Demagogues," or "Populists"), two principal patrician political groups during the later Roman Republic from about 133 to 27 BC. The members of both groups belonged to the wealthier classes.
optimization: collection of mathematical principles and methods used for solving quantitative problems in many disciplines, including physics, biology, engineering, economics, and business. The subject grew from a realization that quantitative problems in manifestly different disciplines have important mathematical elements in common. ...
optometry: health-care profession concerned with examining the eyes for defects of vision and diagnosing and treating such conditions. Optometrists prescribe and supply eyeglasses, contact lenses, and other optical aids that correct the focusing of the eyes. They also examine the eyes ...