Ramsden eyepiece

noun

Optics.

1. 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.

Word History and Origins

Origin of Ramsden eyepiece¹

1840–50; named after Jesse Ramsden (1735–1800), English maker of astronomical instruments

Example Sentences

In the Ramsden eyepiece two plano-convex lenses are also used, but they are of equal focal length, are placed at a distance apart equal to two thirds of the focal length of either, and have their convex sides facing one another.

Read more on Project Gutenberg

The equivalent focal length of a positive or Ramsden eyepiece is equal to three fourths of the focal length of either of the lenses.

Read more on Project Gutenberg