4 Inch sextant geen serie Nr., waarschijnlijk voor reddingsboot, 1792
Jesse Ramsden, London
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Een van de zeer kleine en nauwkeurige sextanten van de befaamde Jesse Ramsden
die met een automatisch graveer- en verdeelmachine de nauwkeurigheid van de schaal enorm kon verbeterde.
A MINIATURE 4IN. RADIUS SEXTANT BY RAMSDEN, LONDON,
CIRCA 1795 constructed from lacquered brass, signed on the T-frame
as per title (upside down!), the arc divided to 140°, vernier with magnifier,
secondary mirror adjustment switch, index mirror and two shades, brass pin feet, contained within fitted keystone box with detachable lid and containing two sighting tubes.
According to Piazzi, Ramsden made sextants from 15inches radius down to 1.5 inches, but very few of the smaller sizes are known. A 5. 25 inches radius instrument employing a similar simple crossed-strut frame however is conserved in the Royal Museums of Greenwich (see Morzer Bruyns, 173 N° 150). Particularities of the present instrument are its lack of shades for the horizon mirror, the removable index shades, the absence of a handle or provision for attaching one or otherwise mounting the instrument, and the use of Peter Dollond's 1772 extended lever and rising piece for adjusting the horizon glass. The small size of the instrument and these peculiarities suggest that it may have been intended for non-marine use, but since it is currently without parallel in the corpus of Ramsden's sextants, there can only be speculation as to what this use might have been.