Gravesande, Willem Jacob 's
,
An essay on perspective
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An ESSAY
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<
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. III.</
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xml:space
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">To find the Repreſentation of a Figure in the
<
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Geometrical Plane, when the Perſpective Plane is
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placed above the Eye.</
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<
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<
s
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xml:space
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">When the perſpective Plane is ſituated above
<
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the Eye, we ſuppoſe the Geometrical Plane to
<
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paſs through the Top of the Perſpective Plane;
<
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</
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<
s
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xml:space
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">upon which Geometrical Plane are drawn the
<
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Figures of Objects meeting it; </
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<
s
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xml:space
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">as alſo the Seats
<
lb
/>
of thoſe Objects that are underneath it, by
<
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Means of Perpendiculars; </
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<
s
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xml:space
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">and the Height of the
<
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Eye is here meaſur’d by a Perpendicular drawn
<
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from the Eye to the Geometrical Plane; </
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<
s
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xml:space
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">whence
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the perſpective Plane, elevated in reſpect to the
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Eye, is the ſame thing, as an Eye elevated in
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regard to the perſpective Plane.</
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<
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<
s
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">Let I L be the Baſe Line, and H the Foot of
<
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the vertical Line; </
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<
s
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xml:space
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">then in the Baſe Line aſ-
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ſume the Points I and L at Pleaſure, towards
<
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the Sides of the perſpective Plane. </
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<
s
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">Make I S
<
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equal to {1/3} or {1/4} Part of I H, and raiſe the Per-
<
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/>
pendicular S X, in the Point S, to the Baſe Line,
<
lb
/>
equal to a correſpondent Part of the Height and
<
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Diſtance of the Eye taken together; </
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>
<
s
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xml:space
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">draw the
<
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Line X I G, and moreover Y L Q, in aſſuming
<
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L T equal to {1/3} or {1/4} & </
s
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<
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">c. </
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<
s
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">of L H. </
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<
s
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xml:space
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">Again draw
<
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the Line G Q in the Geometrical Plane, pa-
<
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/>
rallel to the Baſe Line, and diſtant therefrom
<
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/>
(for Example) a third Part of the Height of
<
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the Eye; </
s
>
<
s
xml:id
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xml:space
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">draw alſo F P in the perſpective Plane,
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parallel to the Baſe Line, and diſtant therefrom,
<
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/>
a fourth Part of the Eye’s Diſtance; </
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>
<
s
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xml:space
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">theſe two
<
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Lines will cut X I in G and F, and Y L in Q
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and P. </
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>
<
s
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xml:space
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">Note, if the Diſtance of G Q from the
<
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Baſe Line, had been aſſumed equal to a fourth
<
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Part of the Eye’s Diſtance; </
s
>
<
s
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xml:space
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">then F P muſt </
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