The need to make more accurate measurements of the angles required instruments more precise than the balestriglia or the Davis backstaff . In the late seventeenth century appears the first document that mentions the principle of reflection on plane mirrors, as used to measure the angles, ie History of the Royal Society of London by Birch. It mentions the reflective instrument of Robert Hooke (1635-1703), created around 1665 and presented to the scientific community the subsequent year. This was however fitted with a single mirror that allowed a single reflection, and therefore was inadequate for the purpose, mainly due to the instability of the images of the horizon and of the star.
Hooke understood the problem, but failed to improve his instrument, abandoning the principle of reflection and presenting, years later, a dial with glasses and mirrors, that was not applicable to navigation.
The idea of using the reflection of mirrors was considered by other researchers, that studied the way of correcting the drawbacks of the instrument of Hooke. In 1731, John Hadley (1682-1744) presented to the Royal Society of London his octant, based on double reflection by two mirrors and a telescope, formed by an arc of 45°, divided into 90 half degrees, each of them corresponding to one degree in the observations.
The astronomer Halley, who attended the presentation of Hadley, informed the Royal Society that Sir Isaac Newton, around the year 1700, had already reported his invention of a similar instrument, of which however he had not provided a description. This omission may be explained by the known Newton's reluctance to make public his inventions, and therefore the merit of the "primacy" on Hadley remains unchanged. Only in 1742, the descendants of Newton discovered among his papers a written description, illustrated by a drawing, that was read on Oct. 28 at the Royal Society.
In 1732 the British Admiralty was offering the yacht Chatham for experimenting Hadley's tool in navigation. This received the applause of the most distinguished astronomers of the time: brothers Halley, the royal astronomer Bradely, Flamsteed , G.B. Airy, Director of the Greenwich Observatory.
The merit of having invented the reflective instruments is given in America to Thomas Godfrey, of Philadelphia, who at the end of 1730 built a tool similar to Hadley's one, and was presented to the Royal Society two years later. But neither the Society nor Godfrey never put in doubt the originality of Hadley's invention, proven by conclusive documents.
In France, the octant was used in 1736 during a trip to Far East, and was described in 1739 by the explorer Jean-Baptiste d'Après de Mannevillette , hydrographer of the East India Company. The first french specimens, however, should be roughly contemporary with the production of Hadley, as the Hydrographic Institute of the Navy has an octant produced by French mathematician Nicolas Bion (1652-1733), "engineer of the King" and renowned manufacturer of nautical, topographical and optical instruments and of armillary and terrestrial spheres. In this instrument, signed on the graduated limb, the mirrors are missing, but it remains an instrument of utmost interest, as from the vast production of Bion a few instruments have been saved, and no other octants seem to be known.
A second octant is at the Hydrographic Institute: it is made of oak, is signed by William Duff and is equipped with two mirrors, while the collimation is through three small holes into the brass viewfinder. The graduation is engraved on ivory and is numbered from 0 to 90 degrees.
In 1757, on a suggestion of captain J. Campbell, some changes were made to the octant, that took the form of the sextant, in which the amplitude of the arc goes from 45° to 60°; moreover, the telescope is fixed to the right arm where begins the graduated limb, and is focused on the fixed mirror in a direction nearly normal to this arm, whereas previously it was fixed on the same arm as the fixed mirror.
The first instruments were made of precious wood, usually ebony, and the graduation was on ivory. Later they were made of brass or bronze, with the axle in steel, while the graduation was on a strip of silver. On instruments equipped with this type of graduation, this was extending from 0° to 130° or 140°, the maximum angle measured, and was obtained with great precision processes, improved over time.
At first, rather large radii were used, such as the three feet long octant of Newton, then the art of obtaining precise graduations progressed, especially by British and German manufacturers - such as Ramsden , Reichembach, Throughton, Pistor and Martins, to the high degree of perfection that have been achieved in recent types of sextants, still in use until a few years ago: they have the silver scale with divisions of 10 minutes and are provided with a vernier microscope, which allows to make readings approximated to 10 seconds, that are in fact 5' and 5", as a result of doubling the angle reading.