Source : Webster's Revised Unabridged Dictionary (1913)

   {Collimating eyepiece}, an eyepiece with a diagonal reflector
      for illumination, used to determine the error of
      collimation in a transit instrument by observing the image
      of a cross wire reflected from mercury, and comparing its
      position in the field with that of the same wire seen
      directly.

   {Collimating lens} (Optics), a lens used for producing
      parallel rays of light.