HADLEY, John, English mathematician and astronomer: b. 1682; d. 14 Feb. 1743. He became a Fellow of the Royal Society in 1717, and was the inventor of Hadley's quadrant (see Sextant) and of a reflecting telescope (1723). The credit of having invented the sextant is claimed for Hadley, Godfrey and Newton, but each seems, nevertheless, to have made his own discovery independently. Hadley described his instrument, which he called an “octant,” to the Royal Society in May 1731.