optical square

Optic \Op"tic\ ([o^]p"t[i^]k), Optical \Op"tic*al\
   ([o^]p"t[i^]*kal), a. [F. optique, Gr. 'optiko`s; akin to
   'o`psis sight, 'o`pwpa I have seen, 'o`psomai I shall see,
   and to 'o`sse the two eyes, 'o`ps face, L. oculus eye. See
   {Ocular}, {Eye}, and cf. {Canopy}, {Ophthalmia}.]
   1. Of, pertaining to, or using vision or sight; as, optical
      illusions. [WordNet sense 2]

   Syn: ocular, optic, visual.
        [1913 Webster + WordNet 1.5]

              The moon, whose orb
              Through optic glass the Tuscan artist views.
                                                  --Milton.
        [1913 Webster]

   2. Of or pertaining to the eye; ocular; as, the optic nerves
      (the first pair of cranial nerves) which are distributed
      to the retina; the optic (or optical) axis of the eye. See
      Illust. of {Brain}, and {Eye}. [WordNet sense 3]
      [1913 Webster]

   3. Relating to the science of optics or to devices designed
      to assist vision; as, optical works; optical equipment.
      [WordNet sense 1]
      [1913 Webster +PJC]

   {Optic angle} (Opt.), the angle included between the optic
      axes of the two eyes when directed to the same point; --
      sometimes called {binocular parallax}.

   {Optic axis}. (Opt.)
      (a) A line drawn through the center of the eye
          perpendicular to its anterior and posterior surfaces.
          In a normal eye it is in the direction of the optic
          axis that objects are most distinctly seen.
      (b) The line in a doubly refracting crystal, in the
          direction of which no double refraction occurs. A
          uniaxial crystal has one such line, a biaxial crystal
          has two.

   {Optical circle} (Opt.), a graduated circle used for the
      measurement of angles in optical experiments.

   {Optical square}, a surveyor's instrument with reflectors for
      laying off right angles.
      [1913 Webster]