Is a direction through an anisotropic crystal structure parallel to which single refraction occurs.
(from the article `double refraction`) ...ray is seen to split into the ordinary ray CO and the extraordinary ray CE upon entering the crystal face at C. If the incident ray enters the ...
the direction in a birefringent crystal along which no splitting of the beam occurs
in an anisotropie medium, a direction of propagation in which the two waves having orthogonal polarizations have the same phase velocity NOTE - In English the term 'optic axis' should be distinguished from optical axis.
The axis of the eye connecting the anterior and posterior poles; it usually diverges from the visual axis by five degrees or more. ... Synonym: axis opticus. ... (05 Mar 2000) ... 
a line connecting the center of the anterior curvature of the cornea (anterior pole) with that of the posterior curvature of the sclera (posterior pole). Axes of the eye. the hypothetical straight line passing through the centers of curvature of the front and back surfaces of...
Type: Term Definitions: 1. the axis of the eye connecting the anterior and posterior poles; it usually diverges from the visual axis by 5° or more. Synonyms: axis opticus
A line passing through the center of a curved mirror and the center of the sphere from which the mirror is made. See also: Focal Plane.
(in a crystal exhibiting double refraction) the direction or directions, uniaxial or biaxial, respectively, along which this phenomenon does not occur.