18491on PERSPECTIVE.
in the Perpendicular R S, continued equal to R S,
and draw f i parallel to the Baſe Line, in which
take the Point f in E I, the ſame as F in F I ; 11110. if the Threads which before were faſten’d in F,
being placed in f, and by uſing them thus faſten’d,
as likewiſe S Q for a Baſe Line, the Repreſen-
tations of the Feet of the Perpendiculars be
found , you will have the Repreſentations 22110. their Extremities.
and draw f i parallel to the Baſe Line, in which
take the Point f in E I, the ſame as F in F I ; 11110. if the Threads which before were faſten’d in F,
being placed in f, and by uſing them thus faſten’d,
as likewiſe S Q for a Baſe Line, the Repreſen-
tations of the Feet of the Perpendiculars be
found , you will have the Repreſentations 22110. their Extremities.
The Demonſtration of the two laſt Ways.
114.
If a Plane be imagined to paſs through
33Fig. 60,
and 61. the Extremities of the equal Perpendiculars, it
will be parallel to the Geometrical Plane, and
will meet the perſpective Plane in S Q; becauſe
R S is equal to the ſaid Perpendiculars: Moreover,
the Extremities of theſe Perpendiculars form a
Figure in this ſuppoſed Plane, ſimilar to that
which their Feet form in the Geometrical Plane;
and the ſaid Figure hath the ſame Situation with
regard to the Line Q S, as that on the Geome-
trical Plane hath in reſpect of H R: And conſe-
quently, if the Figure in the Geometrical Plane
be ſo raiſed up, that it hath the ſame Reſpect to
Q S, as it had to H R, and if the Appearances
of the Feet of the propoſed Perpendiculars be
found, the Repreſentations of their Extremties
will be had. But the before ſuppoſed Tranſpo-
ſition of the Figure in the Geometrical Plane,
gives it the requiſite Situation with regard to
Q S, and the Repreſentation of the Figure
conſider’d in this new Geometrical Plane is
found ; becauſe S Q, is taken for the Baſe 4432, 39. O f (Fig. 60.) equal to the Height of the Eye
above this Plane, and fi (Fig. 61.) is the Geo-
metrical Line in the ſaid Plane.
33Fig. 60,
and 61. the Extremities of the equal Perpendiculars, it
will be parallel to the Geometrical Plane, and
will meet the perſpective Plane in S Q; becauſe
R S is equal to the ſaid Perpendiculars: Moreover,
the Extremities of theſe Perpendiculars form a
Figure in this ſuppoſed Plane, ſimilar to that
which their Feet form in the Geometrical Plane;
and the ſaid Figure hath the ſame Situation with
regard to the Line Q S, as that on the Geome-
trical Plane hath in reſpect of H R: And conſe-
quently, if the Figure in the Geometrical Plane
be ſo raiſed up, that it hath the ſame Reſpect to
Q S, as it had to H R, and if the Appearances
of the Feet of the propoſed Perpendiculars be
found, the Repreſentations of their Extremties
will be had. But the before ſuppoſed Tranſpo-
ſition of the Figure in the Geometrical Plane,
gives it the requiſite Situation with regard to
Q S, and the Repreſentation of the Figure
conſider’d in this new Geometrical Plane is
found ; becauſe S Q, is taken for the Baſe 4432, 39. O f (Fig. 60.) equal to the Height of the Eye
above this Plane, and fi (Fig. 61.) is the Geo-
metrical Line in the ſaid Plane.