Ramsden eyepiece
noun Optics.
an eyepiece consisting of two plano-convex crown-glass lenses of equal focal length, placed with the convex sides facing each other and with a separation between the lenses of about two-thirds of the focal length of each.
Origin of Ramsden eyepiece
1840–50; named after Jesse
Ramsden (1735–1800), English maker of astronomical instruments