he picture, is
the perspective of little _a_. I now draw another line from _g_ to _S_,
and thus form the shaded figure _ga'Po_, which is the perspective of
_aAa'g_.

Let it be remarked that in the shaded perspective figure the lines _a'P_
and _go_ are both drawn towards _S_, the point of sight, and that they
represent parallel lines _Aa'_ and _ag_, which are at right angles to
the picture plane. This is the most important fact in perspective, and
will be more fully explained farther on, when we speak of retreating or
so-called vanishing lines.

RULES

VII

THE RULES AND CONDITIONS OF PERSPECTIVE

The conditions of linear perspective are somewhat rigid. In the first
place, we are supposed to look at objects with one eye only; that is,
the visual rays are drawn from a single point, and not from two. Of this
we shall speak later on. Then again, the eye must be placed in a certain
position, as at _E_ (Fig. 22), at a given height from the ground, _S'E_,
and at a given distance from the picture, as _SE_. In the next place,
the picture or picture plane itself must be vertical and perpendicular
to the ground or horizontal plane, which plane is supposed to be as
level as a billiard-table, and to extend from the base line, _ef_,
of the picture to the horizon, that is, to infinity, for it does not
partake of the rotundity of the earth.

We can only work out our propositions and figures in space with
mathematical precision by adopting such conditions as the above. But
afterwards the artist or draughtsman may modify and suit them to a more
elastic view of things; that is, he can make his figures separate from
one another, instead of their outlines coming close together as they do
when we look at them with only one eye. Also he will allow for the
unevenness of the ground and the roundness of our globe; he may even
move his head and his eyes, and use both of them, and in fact make
himself quite at his ease when he is out sketching, for Nature does all
his perspective for him. At the same time, a knowledge of this rigid
perspective is the sure and unerring basis of his freehand drawing.

[Illustration: Fig. 22.]

[Illustration: Fig. 23. Front view of above figure.]

RULE 1

All straight lines remain straight in their perspective appearance.[4]

[Footnote 4: Some will tell us that Nature abhors a straight line,
that all long straight lines in space appear curved, &c., owing to
certain optical condition