A bull's eye level is a type of

spirit level A spirit level, bubble level, or simply a level, is an instrument designed to indicate whether a surface is horizontal (level) or vertical (plumb). Different types of spirit levels may be used by carpenters, stonemasons, bricklayers, other ...

that allows for the leveling of planes in two dimensions — both the 'pitch' and 'roll' in nautical terms. Standard

tubular level Tubular may refer to: *having the form of a hollow cylinder, or tube *having the form of a cylinder *''Tubular'', a television-related entertainment blog on the ''Houston Chronicle'' website *''Tubular'', a level in the video game ''Super Mario Wo ...

s only consider one dimension. Bull's eye levels are used primarily by

carpenter Carpentry is a skilled trade and a craft in which the primary work performed is the cutting, shaping and installation of building materials during the construction of buildings, Shipbuilding, ships, timber bridges, concrete formwork, etc. ...

s in construction, but can also be found as features of

compass A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with ...

es or other devices that need to be kept from tipping in certain directions (whether it be for functionality or precision of measurements). Small bull's eye levels are also found incorporated into

tripods A tripod is a portable three-legged frame or stand, used as a platform for supporting the weight and maintaining the stability of some other object. The three-legged (triangular stance) design provides good stability against gravitational loads ...

. Another name for a bull's eye level is a "circular bubble" which is the name used by surveyors in the United Kingdom. Surveying instruments such as

theodolite A theodolite () is a precision optical instrument for measuring angles between designated visible points in the horizontal and vertical planes. The traditional use has been for land surveying, but it is also used extensively for building and i ...

s (transits) and

total station A total station (TS) or total station theodolite (TST) is an electronic/optical instrument used for surveying and building construction. It is an electronic transit theodolite integrated with electronic distance measurement (EDM) to measure b ...

s often have a circular bubble as well as a tubular level or "plate level". The circular bubble is used to roughly level the instrument in two dimensions. The plate level is then used, rotating it 90 degrees between readings to level the instrument more precisely. The Fell All-Way precision level, one of the first successful American made bull's eye levels for machine tool use, was invented by William B. Fell, Rockford, Illinois prior to World War II in 1939. The device was unique in that it could be placed on a machine bed and show tilt on x and y axes simultaneously; eliminating the need to rotate the level 90 degrees. The device set a new standard of 0.0005 inches per foot resolution (five ten thousands per foot or five arc seconds tilt). The level's production stopped around 1970. Production restarted in 1980s by Thomas Butler Technology, Rockford, Illinois, but finally ended in the mid 1990s. However, there are still hundreds of the highly prized devices in existence.

References

Inclinometers Woodworking measuring instruments {{Measurement-stub de:Libelle (Messtechnik)#Dosenlibelle