What is "binocular parallax"

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.

   The moon, whose orb Through optic glass the Tuscan artist views.
   --Milton.

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]

3. Relating to the science of optics or to devices designed to assist vision; as, optical works; optical equipment. 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.)

   1. 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.

   2. 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.