Gravesande, Willem Jacob 's
,
An essay on perspective
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An ESSAY
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draw S N perpendicular to the ſaid Line A B,
<
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cutting the Baſe Line in N, and draw the Line
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N D. </
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<
s
xml:id
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echoid-s1726
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xml:space
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">Then about the Point D, as a Center, and
<
lb
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with the Radius D O, deſcribe the Circular Arc
<
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O o: </
s
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<
s
xml:id
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echoid-s1727
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xml:space
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">And about N, as a Center, with the Radius
<
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N S, draw the Circular Arc S s. </
s
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<
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xml:space
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">This being done,
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draw the Line s o touching the ſaid two Arcs,
<
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/>
and the Line D o perpendicular to s o. </
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<
s
xml:id
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echoid-s1729
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xml:space
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">Then
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draw o F, making an Angle with o D, equal to
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the Angle of the Inclination of the Lines given,
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and cutting N D continued in F: </
s
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<
s
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xml:space
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">Now I ſay F
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is the Accidental Point ſought when the Lines
<
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do not incline towards the Perſpective Plane:
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</
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s
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">But if they do, o F muſt be drawn below o D.</
s
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<
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emph
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.</
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s
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">If a Plane be ſuppoſed to paſs thro’ the Eye
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parallel to theinclin’d Lines; </
s
>
<
s
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xml:space
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">the common Secti-
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ons of this Plane, and the Horizontal and Geo-
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metrical Planes, will be O D and S N. </
s
>
<
s
xml:id
="
echoid-s1735
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xml:space
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preserve
">It is
<
lb
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now manifeſt, that if a Line be drawn in the
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/>
ſaid Plane, below the Horizontal Plane, when
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the Lines incline towards the perſpective Plane,
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/>
and above it when they incline the other way,
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making an Angle with O D equal to the Angle
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of Inclination of the propoſed Lines; </
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>
<
s
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xml:space
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">I ſay it is
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manifeſt, that the ſaid Line will be parallel to
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the propoſed Lines, and will meet the
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">13, 14.</
note
>
ctive Plane in the Accidental Point ſought. </
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>
<
s
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xml:space
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">If
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now the before ſuppoſed Plane be conceiv’d to
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turn about the Line N D, the Eye, and the
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Station Point in the ſaid Plain, will then meet
<
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/>
the perſpective Plane in the Points o and s; </
s
>
<
s
xml:id
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xml:space
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">for
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the Lines D o and N s are equal to D O and N S,
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and form right Angles with the Line s o joyning
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their Extremities. </
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>
<
s
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xml:space
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">Now the two Points s and o
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anſwer to the Situation of the Eye and </
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