Inventory no. 43827 - Former Display Label
English OPTICAL SQUARE
c.1850
Signed: "Dancer, Manchester."
Brass; diameter 53 mm
The optical square is a 19th-century improvement to the surveyor's cross-staff, used to measure "off-sets". When running out a chain in a straight line, the distance of a feature to one side is measured from a point exactly 90° to the line of the chain. In the optical square there is a half-silvered mirror, to enable the surveyor to see the end of the chain line and the feature at right angles superimposed in the mirror, so giving a more accurate starting point for the off-set.
[c.999]
Clay Collection