Note: Do not rely on this information. It is very old.
Hadley's Quadrant, or the Reflecting Quadrant, was introduced by John Hadley in 1731 to take the place of the backstaff, or Davis's quadrant. It was formerly used for taking the altitudes of the sun and stars, and for taking angles in surveying. Its form was that of an octagonal sector of a circle. The arc, therefore, contained 45°, but, there being double reflection, the limb was divided into 90°. The quadrant is now superseded by the sextant.