Parallel lines of roof, wall, box, or house can thus be easily corrected. But what of the upright lines—the lintel of the door, the frame of a window, the sides of a wall? How shall we prove whether we have drawn these correctly?
Take a piece of thick silk or cotton, preferably of a dark tint, and weighted with a lump of lead (or some similar heavy substance), and you have one of man's oldest tools, the plumb-line.
Hold this at arm's length and between the finger and thumb before the object of your drawing.
The plumb-line will prove whether the door or window is perfectly upright (or perpendicular). Pull your drawing-block, or drawing-board, forward and let the plumb-line hang before the doubtful line of your sketch. The plumb-line always finds the true perpendicular.
When, however, you are drawing complicated subjects such as a large box, pieces of furniture, a portion of a room, house, or a street, you are faced with greater difficulties.
"How are things in a drawing made to go back?" is a question that requires a little more elucidation.
Probably as a small child you began to appreciate that as objects retire or recede, so must they become apparently smaller—a first rule of perspective.
Did you not sometimes play at the game of hiding from your sight a house or a tree by putting your finger, or even a single hair, close to your eye?
You must have noticed that the boat becomes smaller and smaller as it nears the horizon; that a man climbing a distant hill or mountain is reduced eventually to a mere speck; that a huge aeroplane looks no larger than a tiny fly among the clouds?
Therefore you have fully convinced yourself that objects must become smaller as they recede.
In other words, as objects retire, or are further from the eye, they occupy less space upon the field of vision.
The objects in the nearest part of your picture—that is
