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Co Lta 113 Frenchurch St. London 1890
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Burnished brass sextant D. Mc Lean
Co Lta 113 Frenchurch St. London 1890
$3,314.82
£2,476.42
€2,800
CA$4,615.72
A$4,959.33
CHF 2,660.67
MX$58,426.63
About the Item
Burnished brass sextant D. Mc Lean
Co Lta 113 Frenchurch St. London, datatable around the end of the 19th century, instrument complete with optics and housed in its original square-shaped mahogany box with hinges, locking hooks and handle in brass.
Brass frame with engraved silver goniometric scale embedded in the arch, with scale from 0 to + 150, silver flap and vernier, wooden handle, 3 colored glasses for fixed mirror and 4 for movable mirror, three telescopes, sun filter and adjustment key, a microscope for reading vernier, index and horizon mirror.
A label on the inside of the box lid indicates that the instrument was serviced in 1914.
Box size cm 26x23.5x12.7 - inches 10.3x9.3x5.
Conservation status: very good, complete with stand base made of custom wood and brass.
The last photo is the gift box.
The sextant is an optical instrument used in astronomical navigation to measure the height of the stars on the horizon in order to derive geographical coordinates relative to the ship's point. It is in the shape of a circular sector of 60°, that is, one-sixth of a circumference, hence the name, at the apex of which is pivoted a movable alidade on which is fixed a mirror that rotates with it. On the back of the mirror is a stand with a telescope oriented toward a second mirror, only one half of which is silvered, making it possible to simultaneously observe the sea horizon, in alignment, and the pointed star, whose image is reflected by the mirror attached to the alidade and subsequently by the silvered part of the mirror.
By adjusting the position of the index of the alidade, it is possible to collimate the image of the horizon with that of the star and to derive on the graduated scale of the 60° sector the angle between the horizon and the star.
To make a sextant measurement of the height of a star (for example, the Sun), one places the instrument in a vertical plane and, looking through the sighting device, aims at the horizon line visible through the unsilvered half of the fixed mirror. Moving the alidade, with which the mirror is integral, causes the light rays coming from the star and subsequently reflected by the moving mirror and the silvered half of the fixed mirror to be sent back by the latter in the direction of observation: if you look through the aiming device, you see the image of the star, obtained by double reflection, coincide with the horizon line. The height of the astro is expressed by the angle whose value is read on the graduated scale. The filter is used when the star to be looked at is the Sun.
It was Sir Isaac Newton who invented the principle of double reflection in navigation instruments, but this research was never published. Subsequently, two men, independently of each other, discovered the sextant around 1730 : John Hadley (1682-1744), an English mathematician, and Thomas Godfrey, (1704-1749), an American inventor. But it was not until 1758 that Admiral John Campbell carried out a series of offshore trials to test a new method that relied on lunar distance as a means of calculating longitude. This was how the sextant was developed. Initially made of brass, they had scales divided with great precision by mathematicians who made scientific instruments.
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