Sextants4

The instruments were fitted with two sets of experimenting with thicker gauge metals and other forms of coloured glass shades for use with the sun and the earlier type manufacture to ensure rigidity. Curiously enough, the most of instrument had a second pinnule fitted on the opposite limb difficult part of the instrument to make was the plane mirrors. so that when the horizon below the sun was ill-defined, the The two aces had to be ground parallel and silvered in the old opposite horizon could be used. This was of greater use at way with mercury and tinfoil. The problem lay in the grinding of anchor off unexplored coasts where latitude had to be the glass, for if it was not absolutely flat, the instrument would determined. The size of the instrument was controlled by the be inaccurate. fact that the scales on the arc had to be calibrated by hand. The eyepieces are either of the Huygens or the When Jesse Ramsden (1735-1800) invented his dividing Ramsden type which work on diff e rent optical machine in 1771, this operation could be swiftly, accurately principles, although the results are almost the same. and economically carried out in a smaller area, so the size The main diff e rence is that the Ramsden can be used of the instruments shrank from a radius of approximately with cro s s w i re for measurements whereas the s 45cm (18") to one of approximately 20cm (8"). In Huygens may have cro s s w i res but only so as to this compact form, the octant was in use until the mark the centre. The object glass is achro m a t i c end of the 19th century. They were cheap to buy to avoid spherical and chromatic aberrations (they were offered in a catalogue at 30 shillings) which the four spherical surfaces of the and were less vulnerable than the brass meniscus or concave flint glass lens and the instruments, that ultimately replaced them, biconvex crown glass lens overcome when when used on board a small freighter or fishing bonded together. boat. The handsome quadrant or octant was The pocket sextant was a useful tool for the well established as a practical 'no nonsense' tool 19th century surveyor, which he used for a by about 1790. quick meridian bearing. A handy size for the Perhaps the onslaught of the Napoleonic pocket, they were not intended to have the wars and the enormous demand for accuracy of a navigational instrument. instruments for the hastily impressed battle fleet, plus the new mass-produced scales, Leonard Honey

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