ture plane
diminish in proportion as they become more distant, but do not undergo
any perspective deformation. This is called the front view.

RULE 5

All horizontal lines which are at right angles to the picture plane are
drawn to the point of sight.

RULE 6

All horizontals which are at 45 deg to the picture plane are drawn to the
point of distance.

RULE 7

All horizontals forming any other angles but the above are drawn to some
other points on the horizontal line.

RULE 8

Lines which incline upwards have their vanishing points above the
horizon, and those which incline downwards, below it. In both cases they
are on the vertical which passes through the vanishing point of their
ground-plan or horizontal projections.

RULE 9

The farther a point is removed from the picture plane the nearer does it
appear to approach the horizon, so long as it is viewed from the same
position.

RULE 10

Horizontals in the same plane which are drawn to the same point on the
horizon are perspectively parallel to each other.

BOOK SECOND

THE PRACTICE OF PERSPECTIVE

In the foregoing book we have explained the theory or science of
perspective; we now have to make use of our knowledge and to apply it to
the drawing of figures and the various objects that we wish to depict.

The first of these will be a square with two of its sides parallel to
the picture plane and the other two at right angles to it, and which we
call

IX

THE SQUARE IN PARALLEL PERSPECTIVE

From a given point on the base line of the picture draw a line at right
angles to that base. Let _P_ be the given point on the base line _AB_,
and _S_ the point of sight. We simply draw a line along the ground to
the point of sight _S_, and this line will be at right angles to the
base, as explained in Rule 5, and consequently angle _APS_ will be equal
to angle _SPB_, although it does not look so here. This is our first
difficulty, but one that we shall soon get over.

[Illustration: Fig. 43.]

In like manner we can draw any number of lines at right angles to the
base, or we may suppose the point _P_ to be placed at so many different
positions, our only difficulty being to conceive these lines to be
parallel to each other. See Rule 10.

[Illustration: Fig. 44.]

X

THE DIAGONAL

From a given point on the base line draw a line at 45 deg, or half a
right angle, to that base. Let _P_ be the given point. Draw a line from
_