optic axis

definition: a line that passes through the center of curvature of a lens so that light is neither reflected nor refracted
example: in a normal eye the optic axis is the direction in which objects are seen most distinctly
speech part: noun
synonyms: principal axis
definition: in a doubly refracting crystal, the line in the direction of which no double refraction occurs
speech part: noun

How optic axis is being used?