Jesse Ramsden

Jesse Ramsden FRS FRSE (6 October 1735 – 5 November 1800) was a British mathematician, astronomical and scientific instrument maker. His reputation was built on the engraving and design of dividing engines which allowed high accuracy measurements of angles and lengths in instruments. He produced instruments for astronomy that were especially well known for maritime use where they were needed for the measurement of latitudes and for his surveying instruments which were widely used for cartography and land survey both across the British Empire and outside. An achromatic eyepiece that he invented for telescopes and microscopes continues to be known as the Ramsden eyepiece.

Life Ramsden was born at Salterhebble, Halifax, West Riding of Yorkshire, England

Having attended the free school at Halifax from 1744 to 1747, he was sent at the age of twelve to his maternal uncle, Mr Craven, in the North Riding, and there studied mathematics under the Rev. Mr. Hall. After serving his apprenticeship as a cloth-worker in Halifax, he went to London where, in 1755, he became a clerk in a cloth warehouse. In 1758 he was apprenticed to a mathematical instrument maker and he proved so proficient that he was able to set up his own business only four years later. The quality and accuracy of his instruments established his reputation as the most able instrument maker in Europe for the next forty years until his death in 1800. The Ramsdens had two sons and two daughters with only one, John, living past infancy. John later became a commander in the East India Company's navy. Many delays could be attributed to Ramsden's quest for perfection, as he continually refined his designs as the slightest shortcomings were revealed. The Copley Medal of the Royal Society was bestowed upon him in 1795 for his 'various inventions and improvements in philosophical instruments.’

Ramsden's health began to fail and he traveled to Brighton on the south coast to try to benefit from its better climate; he died there on 5 November 1800. He was buried at St James's Church, Piccadilly on 13 November. His instrument-making business in London was taken over by his foreman, Matthew Berge until his death in 1819. The estate passed on to his son.

Ramsden's instruments

The 5-foot diameter Palermo circle manufactured by Jesse Ramsden to measure apparent positions of astronomical objects.

Dividing engines Ramsden created one of the first high-quality dividing engines. This led to his speciality in dividing circles, which began to supersede the quadrants in observatories towards the end of the 18th century. He published a Description of an Engine for dividing Mathematical Instruments in 1777.

Other instruments He also built an early plate electrostatic generator in 1768.

Surveying instruments In about 1785, Ramsden provided General William Roy a new large theodolite which was used for the measurement of the latitude and longitude separations of London (Greenwich) and Paris and later for the Principal Triangulation of Great Britain. This work provided the basis for the subsequent Ordnance Survey of the counties of Britain.

Telescopes

Ramsden is also responsible for the achromatic eyepiece named after him. In its simplest form it consists of 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