18289PERSPECTIVE.
the Diſtance of the two Points whereat the
Thread is faſten’d on the Ruler, deſcribe an
Arc cutting the Geometrical Line in F; then
faſten the Extremity of one of the aforeſaid
Threads in the ſaid Point F, and the Extremity
of the other in the Point O: Take moreover
another Thread, put through the Eye of a
Needle, as in the aforegoing Method; one End
of which, faſten in F, and afterwards put it a-
bout a Pin placed in O. Then the only Dif-
ference between this Way and the precedent one,
is, that the Ruler M N muſt be uſed, by always
keeping the Threads M F, and N O tight, in-
ſtead of one turning about a Point.
Thread is faſten’d on the Ruler, deſcribe an
Arc cutting the Geometrical Line in F; then
faſten the Extremity of one of the aforeſaid
Threads in the ſaid Point F, and the Extremity
of the other in the Point O: Take moreover
another Thread, put through the Eye of a
Needle, as in the aforegoing Method; one End
of which, faſten in F, and afterwards put it a-
bout a Pin placed in O. Then the only Dif-
ference between this Way and the precedent one,
is, that the Ruler M N muſt be uſed, by always
keeping the Threads M F, and N O tight, in-
ſtead of one turning about a Point.
For the Demonſtration of this, vide n.
39.
Prob. II.
111.
To find the Perſpective of one or more Lines
perpendicular to the Geometrical Plane.
perpendicular to the Geometrical Plane.
Take two Rulers L C and N Z, having the
11Fig. 60. four Ends of two Threads, or rather four Braſs
or Steel Wires of equal Length fixed on them,
at the Places L, I, N and M, ſo that L I be equal
to M N. Then fix one oſ theſe Rulers upon the
Edge of the perſpective Plane, perpendicular to
the Baſe Line. Now take a Thread, put thro’
the Eye of a Needle, hang a Plummet at one
End, for the ſame Uſe as in Problem I , 22109. faſten the other End to the Slider or Curſor D,
freely moveable on the Ruler L C; put this
Thread about a Pin, ſet up againſt the Ruler C L
inc, ſo that C H be equal to the Height of the Eye.
11Fig. 60. four Ends of two Threads, or rather four Braſs
or Steel Wires of equal Length fixed on them,
at the Places L, I, N and M, ſo that L I be equal
to M N. Then fix one oſ theſe Rulers upon the
Edge of the perſpective Plane, perpendicular to
the Baſe Line. Now take a Thread, put thro’
the Eye of a Needle, hang a Plummet at one
End, for the ſame Uſe as in Problem I , 22109. faſten the other End to the Slider or Curſor D,
freely moveable on the Ruler L C; put this
Thread about a Pin, ſet up againſt the Ruler C L
inc, ſo that C H be equal to the Height of the Eye.
Operation.
Let T be the Repreſentation of the Foot of a
Perpendicular. Move the Curſor D along C
Perpendicular. Move the Curſor D along C