view.

[Illustration: Fig. 27.]

RULE 5

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

Thus the lines _AB_ and _CD_ (Fig. 28) are horizontal or parallel to the
ground plane, and are also at right angles to the picture plane _K_. It
will be seen that the perspective lines _Ba'_, _Dc'_, must, according to
the laws of projection, be drawn to the point of sight.

This is the most important rule in perspective (see Fig. 7 at beginning
of Definitions).

An arrangement such as there indicated is the best means of illustrating
this rule. But instead of tracing the outline of the square or cube on
the glass, as there shown, I have a hole drilled through at the point
_S_ (Fig. 29), which I select for the point of sight, and through which
I pass two loose strings _A_ and _B_, fixing their ends at _S_.

[Illustration: Fig. 28.]

[Illustration: Fig. 29.]

As _SD_ represents the distance the spectator is from the glass or
picture, I make string _SA_ equal in length to _SD_. Now if the pupil
takes this string in one hand and holds it at right angles to the glass,
that is, exactly in front of _S_, and then places one eye at the end _A_
(of course with the string extended), he will be at the proper distance
from the picture. Let him then take the other string, _SB_, in the other
hand, and apply it to point _b"_ where the square touches the glass, and
he will find that it exactly tallies with the side _b"f_ of the square
_a'b"fe_. If he applies the same string to _a'_, the other corner of the
square, his string will exactly tally or cover the side _a'e_, and he
will thus have ocular demonstration of this important rule.

In this little picture (Fig. 30) in parallel perspective it will be seen
that the lines which retreat from us at right angles to the picture
plane are directed to the point of sight _S_.

[Illustration: Fig. 30.]

RULE 6

All horizontals which are at 45 deg, or half a right angle to the picture
plane, are drawn to the point of distance.

We have already seen that the diagonal of the perspective square, if
produced to meet the horizon on the picture, will mark on that horizon
the distance that the spectator is from the point of sight (see
definition, p. 16). This point of distance becomes then the measuring
point for all horizontals at right angles to the picture plane.

Thus in Fig. 31 lines _AS_ and _BS_ are drawn to the point of sight _