Sextant in brass signed R. Jmme Berlin no. 2578 second mid 19th Century

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Sextant in brass signed R. Jmme Berlin no. 2578 second mid 19th Century

About the Item

Sextant in brass signed R. Jmme Berlin no. 2578 of the second c. mid-19th century, complete with two optics and housed in its beautiful original mahogany wood box with brass handle, locking hooks and hinges. Silver flap and vernier engraved, wooden handle, 3 colored glasses for fixed mirror and 4 for movable mirror, two telescopes, one of which is long with rack and pinion focusing with adapted thread, a microscope for reading vernier graduated from o to 150°, and index and horizon mirror. A plate on the inside of the lid reads: R. JMME Mechanische optische Werkstaff fur nautische Precision Instrument BERLIN N.W. 6 Marien Strasse No. 29. Conservation status: Good , fully functional, complete with custom-made stand base. Box measurements 28.5x 26.7x14 cm - inches 11.3x10.5x5.5. 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 the value of which is read on the graduated scale. The filter is used when the star to be looked at is the Sun. 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 conducted a series of sea trials to test a new method that relied on lunar distance as a means of calculating longitude. This is how the sextant was developed. Initially made of brass, they had scales divided with great precision by mathematicians who built scientific instruments.

Dimensions:
Height: 5.5 in (13.97 cm)Width: 10.5 in (26.67 cm)Depth: 11.3 in (28.71 cm)
Materials and Techniques:
Brass
Period:
1870-1879
Date of Manufacture:
1870
Condition:
Wear consistent with age and use.
Seller Location:
Milan, IT
Reference Number:
1stDibs: LU1020239832892

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