probably made in London

Reflecting circle

Reflecting circles were used to measure angles necessary for finding longitude. It is based on the same principle as the sextant. The arc is taken to the full circle, and several readings are taken to narrow instrumental error. This brass example was made around 1820, probably by James Allen, a scientific instrument maker based in London.

This reflecting circle has a wooden handle and a silver engraved scale, marked below 'Allan, London'. The scale is marked from 0-180 degrees four times, and originally had four verniers (to measure fractions of a degree) and four microscopes (one missing, one broken). The telescope is mounted over the verniers by a series of linking arms; there is a half-silvered mirror and six filters.

Allen published his improvements to the instrument in 1812, for which he was awarded a silver medal by the Society of Arts in 1811. Although these instruments were more commonly made in France, English examples were made, both by Allan and Dollond. Allan had also won a gold medal for his dividing engine in 1810.

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Online ID:	000-180-001-174-C
Image Rights Holder:	National Museums Scotland
Project:	0504: National Museums Scotland Part 2 Project description \| View all records in project
Ref:	National Museums Scotland T.1981.19
Date:	Around 1820
Material:	Brass Inscription: Allan, London Silver Wood
Dimensions:	300 mm D x 140 mm
What:	Reflecting circle
Subject:	9. NAVIGATION (Departmental Classification)
Who:	Allen, London (Maker)
Where:	England, London
Event:
Description:	Reflecting circle in brass with wooden handle and silver scale, by James Allan, London, c. 1820
References:	For James Allan, see Gloria Clifton, Directory of British Scientific Instrument Makers 1550-1851, London, 1995, pp 5-6 James Allan, 'Description of an Improvement on La Borda's Reflecting Circle', Philosophical Magazine, Philosophical Magazine 39 (1812), pp 249-53 See article 'Circle', ch. 1 'On the Reflecting Circle', in the Edinburgh Encyclopaedia, edited by David Brewster, Edinburgh, 1830, vol VI, pp 486-92
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