by what seemed then and still seems a chance , the suggestion of a moment 's idle thought followed up upon familiar lines and paths that i had tracked a hundred times already , the great truth burst upon me , and i saw , mapped out in lines of sight , a whole world , a sphere unknown ; continents and islands , and great oceans in which no ship has sailed ( to my belief ) since a man first lifted up his eyes and beheld the sun , and the stars of heaven , and the quiet earth beneath .