Fine Prismatic Surveying Compass by Francis Barker c.1920

A very good oxidised and lacquered brass prismatic surveying compass by Francis Barker for Negretti & Zambra, London c1920. Prismatic surveying compass having 3¾” engine divided, polished and engraved compass ring, 0° - 360°, with cross bar indicating ‘N,’ graduated in degrees with 0° set at south, jewelled suspension, under flat glass. All maintained in a drum form case. Convex-faced 45 degree two-position reflecting prism with height (focus) adjustment having rotating lens cover, red and green solar shades. Folding sighting vane (stowed position arresting the circle), fine wire sight, sliding and tilting reflecting mirror. The verso with mounting point. Push fit lid, engraved “Negretti & Zambra, London.” Together with its original dark tan leather and maroon velvet lined travel case with strap and buckle-secured crescent lid. Comments: Unsigned, but by Francis Barker for Negretti & Zambra - this exact instrument appears in the Barker catalogues for both 1909 and 1919. The surveyor’s compass evolved as a specialised type of ‘bearing compass’ in the early 17th century and was commonly in use for surveying large-scale civil engineering works such as canals, roads and water supply by the early 18th century. The railway construction boom of the 19th century further expanded its use and helped stimulate numerous design improvements, including improved portability with smaller and lighter instruments that could be carried and operated in one hand. Following Charles Schmalcalder’s design and patent of the prismatic compass in 1812, the first specific prismatic surveyor’s compass design was patented in 1885, incorporating a viewing prism and lens attachment that enabled the user to more accurately sight the heading of geographical landmarks or an alignment when laying out the route of roads, railways, canals or aqueducts. These instruments are seldom seen – far less in this fine condition and in this large 4" diameter size. Very much a collector’s piece.