Reflecting circle, probably made by George Dollond

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Reflecting circles were used to measure angles necessary for finding longitude. This example was made around 1830, probably by George Dollond (1774-1852), a scientific instrument maker based in London.

With the index arm clamped, the observer first sights directly on the object to the right, and by using the mirror, by reflection on the left, moving the telescope arm until this is achieved. Then, freeing the index arm, he sights directly on the left object, moving the index arm until both are again in coincidence in the mirror. The vernier arm is inscribed 'Dollond / London'.

This form of reflecting circle was a development of the Borda circle. It was costly and difficult to manage and its popularity was greater with Continental seaman than with the Royal Navy. As more sextants were produced with machine-divided scales, the need to use the inherently more accurate reflecting circle diminished.

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• Online ID: 000-180-000-900-C
• Image Rights Holder: National Museums Scotland
• Project: National Museums Scotland Part 2
  Project description   View all records in project
• Ref: National Museums Scotland  T.1967.95
• Date: 1800 - 1850
  Around 1830
  
• Material: Brass, platinum scale, brass pillar stand, wooden handles / wooden mercury bottle. Inscription: Dollond / London
  Reflecting circle, navigation / accessory / box
  
• Dimensions: Box 11.50" x 10.50" x 7.50"
  
• What: Reflecting circle, navigation / accessory / box
  
• Subject: 9. NAVIGATION (Departmental Classification)
  
• Who: Dollond, London (Maker)
  
• Where: England, London
  
• Event:
• Description: Reflecting circle, 7 3/4 inch, used in navigation, with accessories in a mahogany box, by Dollond of London, 1st half 19th century
  
• References:
  • Dollond, G. Description of an Improved Repeating Reflecting Circle. Gleanings in Science, 2 (1830), pp 9-13. 
  • Edinburgh Encyclopedia 1830 Vol VI pp 484-92 
  • Stanley, W.F. Surveying and Levelling Instruments Theoretically and Practically Described. 3rd edition, London, 1901, pp 385-6. 
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