Full text: Gravesande, Willem Jacob: An essay on perspective

PERSPECTIVE. 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.

175.1.

Fig. 61.
Figure 1. page 88.
Plate. 25.
Fig. 60
O G F f Z L R P D I T S M a Q E R H N A C B

For the Demonſtration of this, vide n. 39.

176. Prob . II.

111. To find the Perſpective of one or more Lines
perpendicular to the Geometrical Plane.

Take two Rulers L C and N Z, having the
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 , and 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.

176.1.

Fig. 60.
109.

177. Operation .

Let T be the Repreſentation of the Foot of a
Perpendicular. Move the Curſor D along C L,

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