Main Entry: Rams·den eyepiece

Pronunciation: \ˈramz-dən-\

Function: noun

: a nearly achromatic optical system that contains two plano-convex lenses with the convex surfaces facing each other and is used especially 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.