John Hadley, English optical instrument maker, 1728.
© Science Museum / Science & Society Picture Library
Lithograph from an original painting. In 1716 Hadley (1682-1744) became a Fellow of the Royal Society and was appointed vice-president in 1728. In 1721 he invented the first Gregorian reflecting telescope. In 1730 he invented a quadrant which worked on a double-reflecting principle allowing an accurate measurement of the altitude of stars and the Sun. This instrument was the precursor of the sextant. A spirit level was attached to the device enabling it to determine a ship's position at sea.