The Theory and Practice of Perspective | Page 9

Also, it is at the same time a perspective and a
geometrical figure, and can therefore be measured with the compasses.
Or in other words, we can touch the square fj, because it is on the
surface of the picture, but we cannot touch the square ghmb at the other
end of the cube and can only measure it by the rules of perspective.
II
THE POINT OF SIGHT, THE HORIZON, AND THE POINT OF
DISTANCE
There are three things to be considered and 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