Source : Webster's Revised Unabridged Dictionary (1913)

, the boundary line between Pennsylvania and Maryland, as run
before the Revolution (1764-1767) by two English astronomers
named Charles Mason and Jeremiah Dixon. In an extended sense,
the line between the free and the slave States.

   {On the line}, on a level with the eye of the spectator; --
      said of a picture, as hung in an exhibition of pictures.