From The Collaborative International Dictionary of English v.0.48:

eyepiece \eye"piece`\ eye-piece \eye"-piece`\, n. (Opt.)
   The lens, or combination of lenses, at the eye end of a
   microscope, telescope or other optical instrument, through
   which the image formed by the mirror or object glass is

   Syn: ocular.
        [1913 Webster]

   Collimating eyepiece. See under Collimate.

   Negative, or Huyghenian, eyepiece, an eyepiece
      consisting of two plano-convex lenses with their curved
      surfaces turned toward the object glass, and separated
      from each other by about half the sum of their focal
      distances, the image viewed by the eye being formed
      between the two lenses. it was devised by Huyghens, who
      applied it to the telescope. Campani applied it to the
      microscope, whence it is sometimes called {Campani's

   Positive eyepiece, an eyepiece consisting of two
      plano-convex lenses placed with their curved surfaces
      toward each other, and separated by a distance somewhat
      less than the focal distance of the one nearest eye, the
      image of the object viewed being beyond both lenses; --
      called also, from the name of the inventor, {Ramsden's

   terrestrial, or Erecting eyepiece, an eyepiece used in
      telescopes for viewing terrestrial objects, consisting of
      three, or usually four, lenses, so arranged as to present
      the image of the object viewed in an erect position.
      [1913 Webster]
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