double vision

or diplopia

Perception of two images of an object, usually caused by temporary or permanent eye-muscle paralysis. Normally, the brain fuses slightly different images from each eye by matching corresponding points on each retina. When an eye muscle is paralyzed, the image falls at a different point and the images do not correspond. Double vision may be an early symptom of botulism or myasthenia gravis and occurs in other infections, head injuries, and nerve or muscle disorders.

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Pair of stars in orbit around a common centre of gravity. Their relative sizes and brightnesses and the distance between them vary widely. Perhaps half of all stars in the Milky Way Galaxy are binaries or members of more complex multiple systems. Some binaries form a class of variable stars (see eclipsing variable star). Stars can be identified as binaries in various ways—visually by telescope, through spectroscopic observation, by changes in apparent brightness (when the dimmer star eclipses its companion), or by changes in the proper motion of the visible member (owing to the gravitational pull of the invisible companion).

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Monetary standard or system based on the use of two metals, traditionally gold and silver, rather than one (monometallism). In the 19th century, a bimetallic system defined a nation's monetary unit by law in terms of fixed quantities of gold and silver (thus automatically establishing a rate of exchange between the two metals). The system provided a free and unlimited market for the two metals, imposed no restrictions on the use and coinage of either metal, and made all other money in circulation redeemable in either gold or silver. Because each nation independently set its own rate of exchange between the two metals, the resulting rates of exchange often differed widely from country to country. When the ratio of the official prices proved different from the ratio of prices in the open market, Gresham's law operated in such a way that coins of only one metal remained in circulation. A monometallic system using the gold standard proved more responsive to changes in supply and demand and was widely adopted after 1867. Seealso exchange rate; silver standard.

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or birefringence

Optical property in which a single ray of unpolarized light (see polarization) splits into two components traveling at different velocities and in different directions. One ray is refracted (see refraction) at an angle as it travels through the medium, while the other passes through unchanged. The splitting occurs because the speed of the ray through the medium is determined by the orientation of the light compared with the crystal lattice of the medium. Since unpolarized light consists of waves that vibrate in all directions, some will pass through the lattice without being affected, while others will be refracted and change direction. Materials that exhibit double refraction include ice, quartz, and sugar.

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In law, the prosecution of a person for an offense for which he or she already has been prosecuted. In U.S. law, double jeopardy is prohibited by the 5th Amendment to the Constitution of the United States, which states that no person shall “be subject for the same offense to be twice put in jeopardy of life and limb.” The clause bars second prosecutions after acquittal or conviction and prohibits multiple convictions for the same offense. Thus a person cannot be guilty of both murder and manslaughter for the same homicide, nor can a person be retried for the same crime after the case has been resolved. A person can, however, be convicted of both murder and robbery if the murder arose from the robbery. The prohibition against double jeopardy is not violated when an individual is charged for behaviour stemming from an offense for which he has been charged in a different jurisdiction or in a different court (e.g., a civil court as opposed to a criminal court). Seealso rights of the accused; due process.

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Double bass, viol-shaped, side and front views.

Lowest-pitched of the modern stringed instruments. It varies in size, up to 80 inches (200 cm) tall. Its shape also varies; its shoulders usually slope more than those of the violin, reflecting its status as a hybrid of the viol and violin families (the name comes from the double-bass viol). It emerged from these families in the late Renaissance, and it has always been less standardized in form than its cousins in the violin family. It normally has four strings; the orchestral instrument often has a lower fifth string (more often, an extension is added to the fourth string), and the jazz instrument has a higher fifth string. Its range is an octave below that of the cello. It is normally bowed in orchestral music and plucked in jazz. In rock bands and some jazz bands, the electric bass is used instead.

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A double-barreled shotgun is a shotgun with two parallel barrels, allowing two shots to be fired in quick succession.


Double-barreled shotguns, often known as doubles, are almost universally break open actions, with the barrels tilting up at the rear to expose the breech ends of the barrels for unloading and reloading. Since there is no reciprocating action needed to eject and reload the shells, doubles are more compact than repeating designs such as pump action or semi-automatic shotguns.

Barrel configuration

Doubles come in two basic configurations, the side by side or SxS and the over/under or O/U, indicating the arrangement of barrels. The original double barreled guns were nearly all SxS designs, which was a more practical design in the days of muzzle-loading firearms. Early cartridge shotguns also used the SxS action, because they kept the exposed hammers of the earlier muzzle-loading shotguns they evolved from. When hammerless designs started to become common, the O/U design was introduced, and most modern sporting doubles are O/U designs.

One significant advantage that doubles have over single barrel repeating shotguns is the ability to use more than one choke at a time. Some shotgun sports, such as skeet, use crossing targets presented in a narrow range of distance, and only require one level of choke. Others, like sporting clays, give the shooter targets at differing ranges, and targets that might approach or recede from the shooter, and so must be engaged at differing ranges. Having two barrels lets the shooter use a more open choke for near targets, and a tighter choke for distant targets, providing the optimal shot pattern for each distance.

Trigger mechanism

The early doubles used two triggers, one for each barrel. These were located front to back inside the trigger guard, and generally the index finger operated the front trigger and the middle finger the rear trigger. In double trigger designs, it is often possible to pull both triggers at once, firing both barrels simultaneously, though this is generally not recommended as it doubles the recoil, battering both shooter and shotgun.

Later models used a single trigger that would alternately fire both barrels, called a single selective trigger or SST. The SST does not allow firing of both barrels at once, since it must be pulled twice in order to fire both barrels. The change from one barrel to the other may be done by a clockwork type system, where a cam alternates between barrels, or by an inertial system where the recoil of firing the first barrel selects the next barrel. Generally there is a method of selecting the order in which the barrels of an SST shotgun fire; commonly this is done through manipulation of the safety, pushing to one side to select top barrel first and the other side to select bottom barrel first.

One of the advantages of the double, with double triggers or SST, is that a second shot can be taken almost immediately after the first. Pump-action and semiautomatic guns require that the fired round be ejected and a new one be loaded, which takes time. Even the few fully automatic shotguns which have been built, such as the USAS-12 have cyclic rates that can be exceeded by a quick shooter with a double.


Regulation is a term used for multi-barreled firearms that indicates how close to the same point of aim the barrels will shoot. Regulation is very important, because a poorly regulated gun may hit consistently with one barrel, but miss consistently with the other, making the gun nearly useless for anything requiring two shots. Fortunately, the short ranges and spread of shot provide a significant overlap, so a small error in regulation in a double will often be too small to be noticed. Generally the shotguns are regulated to hit the point of aim at a given distance, usually the maximum expected range since that is the range at which a full choke would be used, and where precise regulation matters most.

See also


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