Hadley, John

(b. Hertfordshire, England, 16 April 1682; d. 14 February 1744)

optical instrumentation.

Hadley was the son of George Hadley, a deputy lieutenant and, after 1691, high sheriff of Hertfordshire, England, and Katherine Fitzjames. Nothing is known of his early life or of the places of his education.

Hadley was the first to develop the form of reflecting telescope introduced by Newton in 1668. By 1719 he had produced paraboloidal mirrors of speculum metal superior to any made by the London master opticians. He then constructed two Newtonian reflectors with an aperture of 5 7/8 inches and a focus of 5 1/4 feet and, in 1726, a small Gregorian reflector. He presented one of the Newtonian reflectors to the Royal Society of London, where it evoked great interest. James Bradley and James Pound compared it with an eight—inch object glass of 123–foot focus which Christiaan Huygens had made and presented to the Society. The reflector outperformed Huygens’ refractor in both manageability and definition.

Hadley communicated his grinding and polishing methods to Bradley and Samuel Molyneux, who in turn instructed some of the London master opticians. He also befriended the Scottish optician James Short, then about to set up in London as a maker of Gregorian reflectors.

In 1731 Thomas Godfrey, a young American glazier, made a reflecting octant. In the same year, quite independently, Hadley produced a similar instrument. Both instruments, precursors of the modern nautical sextant, were based on a mirror arrangement proposed by Newton but not described in print until 1742. After Bradley had tested Hadley’s octant at sea and obtained altitude readings down to one minute of arc, the instrument was universally adopted.

Hadley played an active part in the affairs of the Royal Society. Elected fellow in 1717, he was annually elected a member of council from 1726 until the year of his death and became vice-president in 1728. In 1726 he was one of the committee appointed by the Society to examine and report on the new instruments which Edmond Halley had obtained for the Royal Greenwich Observatory.

BIBLIOGRAPHY

I. Original Works. Hadley’s Newtonian reflector is described by him in “An Account of a Catadioptrick Telescope, Made by John Hadley, Esq; F.R.S. With the Description of a Machine Contriv’d by Him for Applying It to Use,” in Philosophical Transactions of the Royal Society, 32 (1723), 303–312. Also see “A Letter from the Rev. Mr. James Pound, Rector of Wanstead, F.R.S., to Dr. Jurin, Secretary R.S. Concerning Observations Made With. Mr. Hadley’s Reflecting Telescope,” ibid., 382–384. The Society still possesses Hadley’s mirror, five eyepieces, and the reflecting octant which is described in “The Description of a New Instrument for Taking Angles,” ibid., 37 (1731) 147–157. Hadley’s own account of the grinding, polishing, and testing of concave specula comprises most of bk. 3, ch. 2, of R. Smith, A Compleat System of Opticks (Cambridge, 1738).

II. Secondary Literature. A most complete sketch of Hadley’s work is contained in Biographical Account of John Hadley, an unsigned and undated tract in the library of the Royal Astronomical Society, London. The library also contains a similar tract, The Invention and History of Hadley’s Quadrant. The significance of Hadley’s instruments in the history of applied optics and astronomy is discussed by H. C. King, The History of the Telescope (London, 1955). pp. 77–84.

H. C. King

sextant, a double-reflecting instrument for measuring angles at sea, primarily the altitude of astronomical bodies in celestial navigation. The instrument may also be used for measuring other angles at sea such as for distance-off and horizontal angles for coastal navigation. Although its arc subtends an angle of only 60°, by double reflection the sextant measure angles up to 120°. The instrument was designed by Captain (later Admiral) John Campbell of the British Navy in 1757 and based on Hadley's reflecting quadrant (or octant), a double-reflection instrument proposed in principle by Newton and in 1731 demonstrated to the Royal Society by Robert Hooke. The principal requirement to measure more than 90° arose largely from the need to make lunar distance observations to determine longitude at sea. However, the overwhelming advantage of the double-reflecting principle was that, in a seaway, once the body observed has been brought down to the horizon it will not leave it no matter what the weather. Observations could now be made to within a few minutes of arc. A vernier, which enabled single minutes of arc to be read off the scale, became normal, but was replaced much later by the micrometer drum.

At the same time as the development of Hadley's quadrant a young American optician, Thomas Godfrey, who was engaged with the problem of taking lunar distance observations at sea, had submitted an instrument of his own design based on the same principle as Hadley's, and he is now widely regarded as the joint inventor of the sextant.

The modern sextant employs the same optical principles as Hadley's reflecting quadrant. There are two mirrors: a fixed mirror, the horizon glass, is mounted on the frame of the sextant in line with the observer's telescope. Then there is the index mirror at the top of the arm which, when the index arm is set to zero on the scale, will lie precisely parallel to the horizon glass. The horizon glass is half silvered, so that the horizon can be seen through the plain half and the reflected object seen in the silvered half. To take a sight the navigator, with the index arm set to zero, faces the body to be observed and, with the sextant held vertically, looks through the telescope and moves or rotates the sextant as necessary to get the horizon in view. He then moves the index arm along the scale until the body observed appears in the silvered half of the horizon glass and then adjusts the image so that the limb of the body coincides with the horizon.

See celestial navigation.

Mike Richey

Hadley's reflecting quadrant, the replacement of the far less accurate Davis's quadrant for taking a sight for the purposes of celestial navigation. It was invented by the English mathematician and scientist John Hadley (1682–1744) and was in fact an octant. However, as the principle of double reflection made one degree of the octant's arc represent two degrees between the observed objects, it was called a quadrant. After details of Hadley's new invention had been announced in the Royal Society in 1731 the British Admiralty ordered a series of observations to be made to test the instrument, and as a result the quadrant was widely accepted at sea as a vastly improved aid to navigation and hydrography. The incorporation of a spirit level in 1734 made it possible to take a meridional altitude at sea without the horizon being visible.

John Hadley 1682–1744, English instrument maker. An optician by trade, Hadley built reflecting telescopes, based on Newton's model, that had greater resolution than the cumbersome refractors then in use. In 1731 he built a reflecting octant, based on Newton's sketch, that prefigured the modern nautical sextant. Hadley was a leading member of the Royal Society.