An octant is a portable instrument that uses a small mirror to bring two
images together--those of the sun and the horizon, for instance--to determine latitude at sea by observing the altitude of celestial bodies. It has an arc of 45o or more that measures angles of 90o
or more. John Hadley described an instrument of this sort to the Royal
Society of London in 1731 and obtained a British patent in 1734, and so octants are sometimes known as Hadley quadrants. They were still in use in the early twentieth century.
Early octants have mahogany frames and boxwood scales read by
diagonals. Those made after around 1800 have ebony frames, brass index arms, and ivory scales read by verniers. Although early examples were large, heavy, and costly, Ramsden's invention of the dividing engine in 1777 led to the production of smaller and less expensive instruments.
Ref: "Octant" in Robert Bud and Deborah Warner, eds., Instruments
of Science. An Historical Encyclopaedia (New York & London, 1998), pp.
John Hadley and His Reflecting Quadrant, National Maritime Museum,
Occasional Technical Paper No. 2 (Greenwich, 1975).