An opisometer is a curious instrument. As soon as you handle one it is pretty obvious how it works. A handle, in this case made of bone, probably from a cow, with a small milled wheel that turns easily on a supported fine thread. There is also a small metal pointer to aid in locating whatever it is you are measuring.
What is essential is a scale to read off against once a line has been precisely tracked. You wind the wheel to one end of the thread, then wheel it along a line on a map, or along any distance you are measuring, then turn the wheel backwards against a known scale. With maps, the scale usually appears at the bottom. Simple to use and effective. Opisometers are still made today but seldom for use with maps, more as an aid in the medical or surveying worlds.
Having first stated as an employee of Trelawney Saunders at 26-27 Cockspur Street, Charing Cross, London in 1848, Edward Stanford became a partner in 1852. This business relationship never lasted the year however and Stanford became sole proprietor in 1853, expanding his shop, that sold maps and charts, to 7-8 Charing Cross. The business further expanded and a printing works in nearby Trinity Place was purchased. Stanfords became the map maker and seller in London. They still enjoy an enviable reputation to this day supplying maps for countless expeditions across the globe.
As well as maps and charts, Stanfords sold many accessories including map measurers. The opisometer shown here is unlikely to have actually been made by Stanfords. It was likely supplied to them by a manufacturer of fine mathematical or surveying instruments. It is difficult to date, certainly they were being sold in the 1870s and the example shown probably dates from around then, but examples were still available for purchase for much of the first half of the twentieth century. Though an all metal construction became more in favour at the turn of the century.
The opisometer is pretty generic in design. Other sellers of surveying and mathematical equipment would also have stocked these, each seller having their own address stamped onto the bespoke leather case that held these quite delicate instruments. The Stanford example is quite small when compared against others that were available. It measures just 97mm (3 13/16″) in length. The turned and pointed finial on the end of the bone handle shows that this is an English made instrument as each country had their own particular design. It will measure a line 958mm (37 3/4″), so over a yard/almost a metre, which is quite remarkable for such a small instrument.