two planoconvex lenses with the curved sides facing each other and separated by a gap of about 2/3 of their focal length. It had the additional advantage of allowing a greater distance (or eye relief) between the lens and the eye. It thus also allowed sunshades and prisms to be placed before it. The exit pupil of an eyepiece was once called the Ramsden disc in his honour. In 1791 he completed the Shuckburgh telescope, an equatorial mounted refracting telescope. His most celebrated work was a 5-feet vertical circle, which was finished in 1789 and was used by Giuseppe Piazzi at the Palermo Astronomical Observatory in constructing his catalogue of stars and in the discovery of the dwarf planet Ceres on 1 January 1801. Micrometers He was the first to carry out in practice a method of reading off angles (first suggested in 1768 by the Duc de Chaulnes) by measuring the distance of the index from the nearest division line by means of a micrometer screw which moves one or two fine threads placed in the focus of a microscope. Honours Ramsden Rock in Antarctica is named after Jesse Ramsden. Notes and references Bibliography External links 1735 births 1800 deaths British scientific instrument makers Optical engineers Recipients of the Copley Medal Fellows of the Royal Society of Edinburgh People from Halifax, West Yorkshire 18th-century English people Fellows of the Royal Society Honorary members of the Saint Petersburg Academy of Sciences English engravers English inventors