The theodolite was invented in the 16th century and by the end of the 18th was regarded as the most important surveying instrument. It enables the user to measure, at the same time, the horizontal angle between two points and their angles of elevation. This example is made of brass. The horizontal circle and the vertical semi-circle are both divided into degrees, with verniers (small movable scales) for subdivision into minutes of arc. There are also microscopes for reading the verniers, telescopes for sighting and a compass for orientation.

The Dollond family established one of the leading firms of optical instrument makers, which remained under their control between 1750 and 1870. The name is still familiar in Britain as a chain of opticians.