What is "arc of vision"

Vision \Vi"sion\, n. [OE. visioun, F. vision, fr. L. visio, from videre, visum, to see: akin to Gr. ? to see, ? I know, and E. wit. See Wit, v., and cf. Advice, Clairvoyant, Envy, Evident, Provide, Revise, Survey, View, Visage, Visit.]

1. The act of seeing external objects; actual sight.

   Faith here is turned into vision there.
   --Hammond.

2. (Physiol.) The faculty of seeing; sight; one of the five senses, by which colors and the physical qualities of external objects are appreciated as a result of the stimulating action of light on the sensitive retina, an expansion of the optic nerve.

3. That which is seen; an object of sight.
   --Shak.

4. Especially, that which is seen otherwise than by the ordinary sight, or the rational eye; a supernatural, prophetic, or imaginary sight; an apparition; a phantom; a specter; as, the visions of Isaiah.

   The baseless fabric of this vision.
   --Shak.

   No dreams, but visions strange.
   --Sir P. Sidney.

5. Hence, something unreal or imaginary; a creation of fancy.
   --Locke.

   Arc of vision (Astron.), the arc which measures the least distance from the sun at which, when the sun is below the horizon, a star or planet emerging from his rays becomes visible.

   Beatific vision (Theol.), the immediate sight of God in heaven.

   Direct vision (Opt.), vision when the image of the object falls directly on the yellow spot (see under Yellow); also, vision by means of rays which are not deviated from their original direction.

   Field of vision, field of view. See under Field.

   Indirect vision (Opt.), vision when the rays of light from an object fall upon the peripheral parts of the retina.

   Reflected vision, or Refracted vision, vision by rays reflected from mirrors, or refracted by lenses or prisms, respectively.

   Vision purple. (Physiol.) See Visual purple, under Visual.