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John Hadley. Engraving of John Hadley (1682- 1744), English mathematician and optical instrument maker. In the 1720s Hadley improved Newton's reflecting telescope of 1668. As a result of Hadley's work, this type of telescope subsequently dominated optical astronomy. In 1731 Hadley made a reflecting quadrant which could accurately measure the position of a star above the horizon. This forerunner of the sextant was used in conjunction with chronometers and astronomical tables to discover a ship's position. Hadley was voted a Fellow of the Royal Society in 1717 and became its vice-president in 1728.
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