The difference is as great between
The optics seeing, as the object seen. Pope.

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

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.