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.
Op"tic*al*ly, adv. By optics or sight; with reference to optics.
Optically active, Optically inactive (Chem. Physics), terms used of
certain metameric substances which, while identical with each other in
other respects, differ in this, viz., that they do or do not produce
right- handed or left-handed circular polarization of light. --
Optically positive, Optically negative. See under Refraction.