n :  a nearly achromatic optical system that contains two plano-convex lenses with the convex surfaces facing each other and is used esp. in instruments fitted with micrometer wires or a scale
 
Ramsden, Jesse (1735-1800), British instrument maker. Ramsden was a pioneer in the early invention and manufacture of precision instruments. He designed improved, highly accurate sextants and theodolites. He introduced the Ramsden eyepiece in 1779.
 
 
 
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