stretch of the lines and you note that the rails—wide as they are below your feet—seem to narrow down to a point in the far distance on the horizon. That point is called (what else could it be called?) the Vanishing Point. And perspective says: All retiring lines have vanishing points.
Have you not observed the long straight street and its rows of lamp-posts or electric-light standards and noted that they diminish in size as they recede, though you know for a fact that they are all uniform in size?
As the lamps lessen in height, so does the pavement narrow, and the houses on each side of the street diminish. For all lines that lie parallel disappear to the same point on the horizon—the vanishing point.
The lines of the long esplanade by the sea, of the long buildings, of the long passage or tunnel, all recede, and if continued in our imagination meet at the level of the eye, which is the horizon. For the vanishing point is that point on the surface of the picture where retiring lines if continued would meet.
A large picture in a frame is perhaps the easiest example of parallel lines diminishing.
You are well aware that the sides of the picture are parallel. They are equal. Measure with an inch measure if you have any doubts on that point.
Now hang the picture on a wall; stand aside and at one end, several paces away, and make a quick sketch of it in its frame. Does not the near end of the frame appear larger than the far end? In other words, the picture-frame appears smaller as it recedes.
Measure the farthest end against the nearest by holding the pencil at arm's length. Continue the diminishing lines until they meet. Again we get the vanishing point resting on the (imaginary) line of the horizon at the height of our eye.
Let us procure a cardboard-box, and placing it on a table, three-quarter view, and about the height of the eye, take up our pencils and proceed to sketch it.