An optical plummet made by Hilger & Watts between around 1950 and 1970. Its base was made so that it could be mounted onto a tripod and used with a theodolite for surveying.
Optical plummets were used to ensure that theodolites were properly centred before any measurements were made. They were particularly useful in poor weather conditions and could guarantee an accuracy of measurement up to 1mm.
The theodolite was used by mine surveyors to measure horizontal and vertical angles, helping them to make detailed plans of mines. The Coal Mines Act of 1911 made it compulsory for mine owners to keep records of where their mines were sunk, and it was a mine surveyor's job to make these detailed plans of colliery sites.