A Theodolite is a surveying instrument for measuring both horizontal and vertical angles with the aid of a telescope and graduated circles.

This instrument has the telescope mounted in a pivoting holder (trunnion) above a bubble level and a 50-0-50 degree arc with Vernier scale. The Vernier scale is silver and has clamping and tangent screws to facilitate more precise readings.

The instrument is signed "Jecker a Paris" for Francois-Antoine Jecker. Jecker had worked in England with the great instrument maker Jesse Ramsden for five years before opening an atelier with his two brothers in Paris where he made great use of the dividing engines devised by Ramsden.

Similar surveying instruments had been in use since the 17th century. However, telescopic sights were first added in 1725 by London instrument maker Jonathan Sisson. The sophistication and high quality of this early instrument demonstrates the rapidity of development that was being encouraged by the economic and scientific demands at the turn of the 19th century.

After the 1840’s the modern transit theodolite was created by giving the telescope a 360 vertical circle instead of an arc. This change allowed a quicker and more accurate taking of both back and fore sights because the instrument could be rotated through 180 degrees.