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Reflecting circle, probably made by George Dollond

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probably made in London

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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