Rods and cones

Rod \Rod\, n. [The same word as rood. See {Rood}.]
   1. A straight and slender stick; a wand; hence, any slender
      bar, as of wood or metal (applied to various purposes).
      Specifically:
      (a) An instrument of punishment or correction;
          figuratively, chastisement.
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                He that spareth his rod hateth his son. --Prov.
                                                  xiii. 24.
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      (b) A kind of sceptor, or badge of office; hence,
          figuratively, power; authority; tyranny; oppression.
          "The rod, and bird of peace." --Shak.
      (c) A support for a fishing line; a fish pole. --Gay.
      (d) (Mach. & Structure) A member used in tension, as for
          sustaining a suspended weight, or in tension and
          compression, as for transmitting reciprocating motion,
          etc.; a connecting bar.
      (e) An instrument for measuring.
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   2. A measure of length containing sixteen and a half feet; --
      called also {perch}, and {pole}.
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   {Black rod}. See in the Vocabulary.

   {Rods and cones} (Anat.), the elongated cells or elements of
      the sensory layer of the retina, some of which are
      cylindrical, others somewhat conical.
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