Here the tree AD would not be visible to the spectator C, even if he were to approach as far as B; because no visible perpendicular can be inserted between the two lines AC, DC, till after the point B is past; when the tree will appear like a very little speck.
Q. Why do telescopes enable us to see objects invisible to the naked eye?
A. Because they concentrate several rays within the tube of the telescope, and bend them upon the mirror or lens, which acts as a magnifying glass.
Q. When a ship (out at sea) is approaching the shore, why do we see the small masts before we see the bulky hull?
A. Because the earth is round, and the curve of the sea hides the hull from our eyes, after the tall masts have become visible.
Here only that part of the ship above the line AC can be seen by the spectator A; the rest of the ship is hidden by the swell of the curve DE.