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1554800847Code 6393 Brass vintage sextantBrass sextant signed Negretti & Zambra made in the second half of the nineteenth century, complete with lenses and compass placed in its original mahogany box complete with key lock and brass hinges. Silver flap and vernier, wooden handle, 3 colored glasses for the fixed mirror and 4 for the movable one, two telescopes, a microscope for reading the vernier, graduated from 0 to 150 °, index and mirror horizon. Box cm 27.3x25.3 h 12 - inches 10.74x9.96 h 4.72. Very good condition.
Enrico (Henry) Negretti (Como 1817 - London 1879), emigrated to London in 1829, where he attended the London Mechanics Institute, at the same time starting an apprenticeship in a laboratory of precision optical instruments. In 1859 he joined Joseph Zambra (1822-1897), a manufacturer of scientific instruments, and they established the Negretti & Zambra Ltd, with numerous branches in London, and soon they had a good reputation both at home and abroad because of the excellence of their products. In the early years of the company, they limited production to meteorological instruments, or, as they defined them, "philosophical instruments". About in 1857 Negretti & Zambra were commissioned by the British government to investigate about the temperature of the sea along the coasts, to clarify which influence the temperature had on migration and habits of fishes and which were the seasons and temperatures more suitable for fishing. For these investigations, the Negretti & Zambra realized for the Admiral Fitzroy a mercury thermometer that was the most accurate ever made. Towards the end of the nineteenth century, they diversified their products by manufacturing optical instruments such as astronomical and terrestrial telescopes, levels, theodolites, sights, as well as further improvements in barometers and other meteorological instruments. The company passed from father to son until 1946.
In order to measure a height of a star (for example, the Sun), you have to set the instrument in a vertical plane and, looking through the sight device, to mark out the line of the horizon visible through the non-silvered half of the fixed mirror. Moving the alidade that is together with this mirror, make sure that the light rays coming from the star, and then reflected by the mobile mirror and by the silver half of the fixed mirror, be directed from the fixed mirror to the observer: if you look through the sight device, you see the image of the star, obtained for double reflection, coinciding with the line of the horizon. The height of the star is expressed by the angle whose value is read on the graduated scale. The filter is used when the star is the Sun.
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Nautical antiques
Code 6393 Brass vintage sextant
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