John Harrison, Jesse Ramsden 97 



these, Graham, whose name has already been mentioned in 

 connexion with clocks, worked for Halley and Bradley at 

 Greenwich, and supplied an instrument to the Paris Academy 

 of Sciences. The second, John Bird (1709-1776), divided 

 a number of quadrants for several public observatories, and 

 his method of working was considered so good that the 

 Government purchased the right of employing it. 



Further improvements were introduced by Jes^e Ramsden 

 (1735-1800), the son-in-law of John Dollond, who designed 

 an engine for dividing mathematical instruments and re- 

 ceived a premium for 315 from the Government for this 

 invention. Ramsden was a remarkable man. The son 

 of an innkeeper at Halifax, he became a clerk in a cloth- 

 maker's warehouse, after having completed a three years' 

 apprenticeship. Two years later, when twenty-three years 

 old, he again apprenticed himself, this time with a mathe- 

 matical instrument maker, and afterwards established him- 

 self independently. His shop, first opened in 1762, in the 

 Haymarket, was transferred later to Piccadilly. He soon 

 acquired fame for the excellence of his workmanship, and 

 we are told that, though ultimately sixty workmen were 

 employed by him, the demand from all parts of Europe for 

 his instruments was greater than could be satisfied. He 

 was highly successful in constructing a new equatorial 

 mounting for telescopes and a clockwork which drove the 

 mirror of a siderostat so accurately that a star could be 

 followed for twelve hours ; but it was his skill in dividing circles 

 to which he mainly owed his great reputation. There can 

 be no doubt that his practice of substituting entire circles 

 for the usual quadrants and sectors was sound in principle 

 and contributed much to his success. Every student of 

 optics knows " Ramsden's eyepiece," and he also invented 

 a double image micrometer. The Royal Society recognized 

 his work by awarding him the Copley medal in 1795. 



While clocks and divided circles are necessary parts of 

 an astronomer's equipment, he depends primarily on the 

 optical performance of his telescopes. Newton had used 

 mirrors to focus the beams of light, as he considered it 

 to be impossible to do so accurately by means of lenses, 

 because rays of different colours, being ^refracted to a different 



G 



