(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.