19094An ESSAY
Rulers, keeping all the Threads tight, ſo that
the two Threads fix’d in the Points C and F,
croſs each other in the Point A; and then the
Point a, wherein the two other Threads croſs
each other, is the Perſpective ſought. And in
this Manner may the Repreſentations of any
Number of Points be ſound.
the two Threads fix’d in the Points C and F,
croſs each other in the Point A; and then the
Point a, wherein the two other Threads croſs
each other, is the Perſpective ſought. And in
this Manner may the Repreſentations of any
Number of Points be ſound.
Demonstration.
118.
The Triangle D a E, is ſimilar to the
Triangle C A F: And becauſe all the Triangles
formed for finding the Repreſentations of diffe-
rent Points, have the ſame Baſes D E and C F,
which are between themſelves, as the Eye’s Di-
ſtance from the perſpective Plane, to its Diſtance
from the Geometrical Plane; whence their Ver-
tices form ſimilar Figures, whoſe correſpondent
Lines are in the ſame Proportion; and which
conſequently , are the Appearances ſought.
118, 9.Triangle C A F: And becauſe all the Triangles
formed for finding the Repreſentations of diffe-
rent Points, have the ſame Baſes D E and C F,
which are between themſelves, as the Eye’s Di-
ſtance from the perſpective Plane, to its Diſtance
from the Geometrical Plane; whence their Ver-
tices form ſimilar Figures, whoſe correſpondent
Lines are in the ſame Proportion; and which
conſequently , are the Appearances ſought.
Remarks.
It will be convenient to have the two Threads
P E, and M D of one Colour, and the two
Threads Q F, and C N of another.
P E, and M D of one Colour, and the two
Threads Q F, and C N of another.
Prob. VI.
119.
To find the Repreſentations of any Number
of Lines, equal and perpendicular to the Geometri-
cal Plane.
of Lines, equal and perpendicular to the Geometri-
cal Plane.
Let C D E F G I H, be the Points denoted
22Fig. 64. with the ſame Letters in the precedent Figure,
as alſo let P Q and M N be the Rulers: More-
over, let B be the Point, wherein a
22Fig. 64. with the ſame Letters in the precedent Figure,
as alſo let P Q and M N be the Rulers: More-
over, let B be the Point, wherein a