English mathematician and inventor. Fellow of the Royal Society of London. In the 1720s, Hadley began to build and improve Newtonian and Gregorian reflecting telescopes. Perfected methods for grinding and polishing telescope lenses, training several successful London instrument-makers in these skills. His name is linked to a reflection instrument: the octant. Introduced c. 1730, it became a fairly common means of measuring the altitude of the Sun or a celestial body above the horizon at sea. Hadley's octant was the ancestor of the modern nautical sextant.