What is "vision purple"

Visual \Vis"u*al\, a. [L. visualis, from visus a seeing, sight: cf. F. visuel. See Vision.]

1. Of or pertaining to sight; used in sight; serving as the instrument of seeing; as, the visual nerve.

   The air, Nowhere so clear, sharpened his visual ray.
   --Milton.

2. That can be seen; visible. [R.]

   Visual angle. (Opt.) See under Angle.

   Visual cone (Persp.), a cone whose vertex is at the point of sight, or the eye.

   Visual plane, any plane passing through the point of sight.

   Visual point, the point at which the visual rays unite; the position of the eye.

   Visual purple (Physiol.), a photochemical substance, of a purplish red color, contained in the retina of human eyes and in the eyes of most animals. It is quickly bleached by light, passing through the colors, red, orange, and yellow, and then disappearing. Also called rhodopsin, and vision purple. See Optography.

   Visual ray, a line from the eye, or point of sight.

   Visual white (Physiol.), the final product in the action of light on visual purple. It is reconverted into visual purple by the regenerating action of the choroidal epithelium.

   Visual yellow (Physiol.), a product intermediate between visual purple and visual white, formed in the photochemical action of light on visual purple.

n. (alternative form of visual purple English)