'Very rare and unusual 19th century Circular Protractor with Tangent screw adjustment and clamp to Vernier, made in England by Dollond, London. 1820. This is a brass precision measuring tool with accuracy measurement that can reach 1 minute of an angle through the vernier scale. Diameter: 152 mm (6 inches). This fine instrument comes in its original velvet lined red Moroccan case. This important instrument demonstrates the high level of accuracy achieved in the 19th century. Dollond London, a well known scientific instrument makers and opticians founded by John Dollond (1706-1761) the inventor of the achromatic lens for telescopes. His sons Peter (1731-1820) and John (1733-1821) continued the business.