the base line to the horizon. Of this we shall speak further on. In
nature it is not really level, but partakes in extended views of the
rotundity of the earth, though in small areas such as ponds the
roundness is infinitesimal.

[Illustration: Fig. 8.]

Fig. 8. This is a side view of the previous figure, the picture plane
_K_ being represented edgeways, and the line _SE_ its full length.
It also shows the position of the eye in front of the point of sight
_S_. The horizontal-line _HD_ and the base or ground-line _AB_ are
represented as receding from us, and in that case are called vanishing
lines, a not quite satisfactory term.

It is to be noted that the cube _C_ is placed close to the transparent
picture plane, indeed touches it, and that the square _fj_ faces the
spectator _E_, and although here drawn in perspective it appears to him
as in the other figure. 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 t