<p>
	The octant was invented in 1731 by Edmond Halley. It is a reflecting instrument used in navigation to measure the heights and distances of stars.&nbsp; It was made in wood, ivory and brass and the early mirrors were in polished metal until progress in glass-making permitted the use of <a href="http://www.antiques-in-france.com/antiquites/-/-/-/mercury-mirror">mercury-back mirrors</a> followed by silver-back mirrors in the 19th C.</p>