This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
19. The Horizon Line or Eye Level This, as the name implies, is an imaginary horizontal line on a level with the eye, It is of great importance in representation, as all objects appear to change their shape as they are seen above or below the horizon line.
The following experiments should be made before beginning to draw any of the exercises in freehand perspective. Fasten two square tablets together at right angles to each other so that the adjacent corners exactly coincide, giving two sides of a cube. Hold it at arm's length with the edge where the two planes touch. parallel to the eyes and the upper plane level. Lower it as far as the arms allow, then raise it gradually to the height of the eyes, and above as far as possible, holding it as far out as possible. Observe that the level tablet appears to become narrower as it approaches the eye level, and when it is opposite the eye it becomes only a line showing the thickness of the cardboard. Observe that this line or front edge of the tablet always appears its actual length while the side edges have been gradually appearing to become shorter. As the tablet is lifted above the horizon the lower side begins to appear very narrow at first, but widening gradually the higher the tablet is lifted. It will be seen also that when the tablet is below the horizon line the side edges appear to run upward, and when the tablet is above the eye its side edges, appear to run downward, toward the horizon.
That they and similar lines appear to con -verge and vanish in the horizon line is proved by the following experiment: Place a book on a table about two feet away with its bound edge toward the spectator and exactly horizontal to the eye. that is. with either end equally distant from the eye. Between the cover and the first page and as near the back as possible place a string, leaving about two feet of it on either side. Hold the left end of the
Fig. 4. Book with Strings.
String in the right hand and move it until it coincides with or covers the left edge of the book. Hold the right end of the string in the left hand and move it until it covers the right edge of the book The two strings will be seen to form two converging or vanishing lines which meet at a point on the level of the eye, that is, in the horizon line. This and the preceding experiment illustrate the following rule
Rule, I. Horizontal retreating lines above the eye appear to descend or vanish downward, and horizontal retreating lines below the eye appear to ascend or vanish, upward. The vanishing point of any set of parallel, retreating, horizontal lines is at the level of the eye.
It is necessary to remember that the horizon line is changed when the spectator's position is changed. This is very noticeable when one stands on a high hill and observes that the roof lines of houses which one is accustomed to see vanishing downward to the level of the eye, now vanish upward, since the eyes have been raised above the roofs.
Retreating lines are those which have one end nearer the eye than the other.
Exercise 1. Foreshortened Planes and Lines. Cut from paper a tracing of the square tablet, which is a part of the set of drawing models, and leave a projecting flap as at A, Fig. 5. Paste the flap on the under side of the slate, with the edges of the square parallel to the edges of the slate, and trace the actual shape of the square.
Holding the slate vertical and so that half the square is above and half below the level of the eye, turn the square somewhat away from the slate and trace the appearance. Turn it still farther and trace. Turn it so that the surface disap-pears and becomes a line.
Fig. 5. Slates with Square and Circle Flaps.
ENTRANCE TO A "PEOPLE'S PALACE".
Composition and Drawing by R. Binet, Architect, Paris, France Detail of Column Shown at Left; of Wrought-Iron Grille with Medallions of Bronze, in Lower Right-Hand Corner. Reproduced from "Esquisses Decoratives de Binet, Architecte."
Trace a circular tablet and cut it out of paper, leaving a flap as at B, Fig. 5. Paste the flap on the back of the slate, as with the square, and trace its real appearance. Turn the circle away at a moderate angle and trace its appearance. Trace it as it appears at a greater angle and finally place it so that it appears as a line.
Try similar experiments with the triangle, the pentagon, and the hexagon and observe that these exercises all show that lines and surfaces under certain conditions appear less than their true dimensions, and that this diminution takes place as soon as the surfaces are turned away from the glass slate.
"When the square rests against the slate, with the centers of the square and slate coinciding, and the slate held so that half is above and half below the horizon line, all four corners of the square will be at equal distances from the eye so that a line from the eye to the center of the slate and of the square is at right angles to the surface of the slate, the latter represents in these experiments what in scientific perspective is called the picture plane. Thus a surface or plane appears its true relative dimensions only when it is at right angles to the direction in which it is seen.
It is for this reason that it is always necessary to arrange the surface on which a drawing is made, at right angles to the eye, otherwise the surface and drawing upon it become foreshortened; that is, they appear less than their true dimensions.