In all probability you will say, "I can't tell whether the lines are running up or down."
Can you see the top of the box? If you sit about the same height as myself, you will say, "Yes, I can see a little bit of the top."
If we see even a small portion of the top, the eye is above the top, and if the eye is above the box, what must the lines do but rise to the level of the eye?
The top of the box is nearest to the level of the eye, and
the lower part of the box is farther away (the depth of the box is between). Therefore the top lines of the box rise a little, but the bottom lines rise a great deal.
If we have two lines which go gently on, one at a slight angle and one at a stiff angle, what must be the final result? They will meet at the height of your eye, or at your horizon.
The higher you sit on your chair, the more you can see of the top of the box. The lower you sit, the less you can see.
Place the same box on the floor, sit on a chair, and make a drawing of it.
Then place the box at a height—say on the top of a cupboard about 6 ½ feet high—and draw it again.
In the first drawing, up come the lines to meet the level of your eye. In the second sketch, down come the lines to the same level. In other words, your horizon in the first drawing
