The Theory and Practice of Perspective | Page 9

understood before we can begin a perspective drawing. First, the position of the eye in front of the picture, which is called the +Station-point+, and of course is not in the picture itself, but its position is indicated by a point on the picture which is exactly opposite the eye of the spectator, and is called the +Point of Sight+, or +Principal Point+, or +Centre of Vision+, but we will keep to the first of these.
[Illustration: Fig. 9.]
[Illustration: Fig. 10.]
If our picture plane is a sheet of glass, and is so placed that we can see the landscape behind it or a sea-view, we shall find that the distant line of the horizon passes through that point of sight, and we therefore draw a line on our picture which exactly corresponds with it, and which we call the +Horizontal-line+ or +Horizon+.[3] The height of the horizon then depends entirely upon the position of the eye of the spectator: if he rises, so does the horizon; if he stoops or descends to lower ground, so does the horizon follow his movements. You may sit in a boat on a calm sea, and the horizon will be as low down as you are, or you may go to the top of a high cliff, and still the horizon will be on the same level as your eye.
[Footnote 3: In a sea-view, owing to the rotundity of the earth, the real horizontal line is slightly below the sea line, which is noted in Chapter I.]
This is an important line for the draughtsman to consider, for the effect of his picture greatly depends upon the position of the horizon. If you wish to give height and dignity to a mountain or a building, the horizon should be low down, so that these things may appear to tower above you. If you wish to show a wide expanse of landscape, then you must survey it from a height. In a composition of figures, you select your horizon according to the subject, and with a view to help the grouping. Again, in portraits and decorative work to be placed high up, a low horizon is desirable, but I have already spoken of this subject in the chapter on the necessity of the study of perspective.
III
POINT OF DISTANCE
Fig. 11. The distance of the spectator from the picture is of great importance; as the distortions and disproportions arising from too near a view are to be avoided, the object of drawing being to make things look natural; thus, the floor should look level, and not as if it were running up hill--the top of a table flat, and not on a slant, as if cups and what not, placed upon it, would fall off.
In this figure we have a geometrical or ground plan of two squares at different distances from the picture, which is represented by the line KK. The spectator is first at A, the corner of the near square Acd. If from A we draw a diagonal of that square and produce it to the line KK (which may represent the horizontal-line in the picture), where it intersects that line at A�� marks the distance that the spectator is from the point of sight S. For it will be seen that line SA equals line SA��. In like manner, if the spectator is at B, his distance from the point S is also found on the horizon by means of the diagonal BB��, so that all lines or diagonals at 45�� are drawn to the point of distance (see Rule 6).
Figs. 12 and 13. In these two figures the difference is shown between the effect of the short-distance point A�� and the long-distance point B��; the first, Acd, does not appear to lie so flat on the ground as the second square, Bef.
From this it will be seen how important it is to choose the right point of distance: if we take it too near the point of sight, as in Fig. 12, the square looks unnatural and distorted. This, I may note, is a common fault with photographs taken with a wide-angle lens, which throws everything out of proportion, and will make the east end of a church or a cathedral appear higher than the steeple or tower; but as soon as we make our line of distance sufficiently long, as at Fig. 13, objects take their right proportions and no distortion is noticeable.
[Illustration: Fig. 11.]
[Illustration: Fig. 12.]
[Illustration: Fig. 13.]
In some books on perspective we are told to make the angle of vision 60��, so that the distance SD (Fig. 14) is to be rather less than the length or height of the picture, as at A. The French recommend an angle of 28��, and to make the distance about double the length of the