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