An Optical Square rangefinder, dating from c.1905, complete with its original fitted leather case. Reputed to have been invented by George Adams Senior in the middle of the 18th century, the optical square appeared in the 1795 catalogue of George Adams Junior as 'A small instrument for surveying by right angles'. In the 1907 Negretti & Zambra catalogue it was described as being used 'for fixing a right angle, and in the laying off of offsets'.
A small, portable surveying and engineering instrument used to set out or measure precise right angles, the optical square utilizes two mirrors within a circular brass box case to reflect light, allowing a user to sight a target at a 90 degree angle to their current line of sight.
There are no maker's or retailers marks, but this is a high quality instrument, produced by one of the best makers of the time, such as J. H. Steward, Negretti & Zambra, or Francis Barker. (almost all of Barker's instruments were supplied unsigned to the retail trade).
This optical square, along with a Verner's compass and several other items, was part of a set of navigation instruments owned by Captain P. F. M. Michelli, a regular army officer, who served with the Royal Engineers during WW1. The Verner's compass was inscribed 'P. F. M. Michelli, Eton College, 1905'.
Condition
In very good condition, and full working order. The leather case is in very good condition.
Dimensions : 55mm diameter x 25mm height, (leather case: 70mm)
