the world > matter > physics > electromagnetic radiation > light > emission of light, radiation > 			[noun]		 > line of vision (3)		
= axis, n.1. The central line, e.g. the axis of vision.
Optics. A ray passing through the centre of the eye or of a lense, or falling perpendicularly on it; the line which passes through the centres of…
Optics. The optical axis of a lens.