s; but this is not apparent in short straight
lines, so if our drawing is small it would be wrong to curve them; if
it is large, like a scene or diorama, the same optical condition which
applies to the line in space would also apply to the line in the
picture.]

RULE 2

Vertical lines remain vertical in perspective, and are divided in the
same proportion as _AB_ (Fig. 24), the original line, and _a'b'_, the
perspective line, and if the one is divided at _O_ the other is divided
at _o'_ in the same way.

[Illustration: Fig. 24.]

It is not an uncommon error to suppose that the vertical lines of a high
building should converge towards the top; so they would if we stood at
the foot of that building and looked up, for then we should alter the
conditions of our perspective, and our point of sight, instead of being
on the horizon, would be up in the sky. But if we stood sufficiently far
away, so as to bring the whole of the building within our angle of
vision, and the point of sight down to the horizon, then these same
lines would appear perfectly parallel, and the different stories in
their true proportion.

RULE 3

Horizontals parallel to the base of the picture are also parallel to
that base in the picture. Thus _a'b'_ (Fig. 25) is parallel to _AB_, and
to _GL_, the base of the picture. Indeed, the same argument may be used
with regard to horizontal lines as with verticals. If we look at a
straight wall in front of us, its top and its rows of bricks, &c., are
parallel and horizontal; but if we look along it sideways, then we alter
the conditions, and the parallel lines converge to whichever point we
direct the eye.

[Illustration: Fig. 25.]

[Illustration: Fig. 26.]

This rule is important, as we shall see when we come to the
consideration of the perspective vanishing scale. Its use may be
illustrated by this sketch, where the houses, walls, &c., are parallel
to the base of the picture. When that is the case, then objects exactly
facing us, such as windows, doors, rows of boards, or of bricks or
palings, &c., are drawn with their horizontal lines parallel to the
base; hence it is called parallel perspective.

RULE 4

All lines situated in a plane that is parallel to the picture plane
diminish in proportion as they become more distant, but do not undergo
any perspective deformation; and remain in the same relation and
proportion each to each as the original lines. This is called the front