If objects of equal size are placed side by side the one with the fainter outline will seem farther away. If you make the objects overlap so that the more faintly drawn ones are partly covered you have started creating the illusion of recession in space. This illusion can be considerably increased by reducing the size of the partly hidden objects.
All these factors, heavy, light, and fading lines and dark, light, and fading areas, are the external media of perspective, of the art of conveying space and volume. This "art" rests on clear mathematical rules which make it possible to construct exact perspective. Every painter and draftsman should be acquainted with at least the main features of the construction of perspective. Even if in practice he does not wish to. carry out these constructions exactly, he must support his subjective picture with constructional guiding lines if he is to avoid gross errors.
The easiest way to' grasp the essentials of perspective is to look through a square of glass held directly in front of one eye, and copy onto it all visible outlines. If you then examine the lines, you will see that all the surfaces with rectangular outlines running parallel with the glass have parallel horizontal and vertical outlines on the glass.
All lines leading into the distance, if they run perpendicular to your glass, meet like rays at one point. The outlines of surfaces which stand at an angle to the glass collect at other points. These points are called vanishing points.
However many there are, these points all meet on a horizontal line, the, horizon. This applies only to horizontal and vertical planes. If the planes are sloping, their vanishing points lie above or below the horizon. This horizon is always on eye level and is flat. When we speak of "worm's or bird's eye view" the eye level is simply unusually low or high.
If you stand in the center of a room which stretches in front of you with parallel and vertical walls, the vanishing point will lie on the central vertical of your picture. This is central perspective, frequently used for ceremonial and sacred pictures.
You should now work from nature without a glass; transferring what you see, directly onto your picture. This is easiest when you hold your drawing so that you can see above it, and your eye always meets it at a perpendicular, exactly as it did the glass.
Begin with a group of buildings. You then have only one horizon line to find and two vanishing points. You find the horizon line by seeking architectural elements, such as window sills and ridges,
which lie on a horizontal line right across the view.
Put these horizontals, which are synonymous with the horizon, down on your paper, and from them fix the placing of the verticals. At first you find it helpful to use a viewer, a frame of stiff gray board through which you can isolate any part of the subject, holding it closer or farther from your eye according to the scale of the subject matter in your picture.
In all these manipulations, especially when using a viewer, you should look only with one eye. If you use both eyes, you see the viewer double, since the separation of the eyes which is adapted to giving stereoscopic vision, presents a clear vision of near and far at the same time.