Gravesande, Willem Jacob 's
,
An essay on perspective
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on PERSPECTIVE.
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Priſm is greater than the Height of the Eye, the
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precedent Method is the ſhorteſt.</
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<
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IX.</
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">Having firſt ſound the Perſpective of the ſaid
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Body, afterwards find the Appearance of its
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Cavity, in conſidering the Cavity as a new
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Body.</
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<
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X.</
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">then the Point I the Perſpective of the Centre
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muſt be found, and the Line IV drawn to
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Point of Sight V. </
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">This being done, raiſe V F per-
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pendicular to V I, which make equal to the
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Diſtance from the Eye to the perſpective Plane;
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</
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xml:space
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">and in this Perpendicular continued, take V P
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equal to the Diſtance from the Centre of the
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Sphere to the perſpective Plane. </
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Point P draw P Q parallel to V I cutting a Line
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drawn from F through I, in Q; </
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xml:space
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">and about Q as
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a Centre, with the Semidiameter of the Sphere,
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draw the Circle C B, to which from the Point F,
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draw the Tangents F C and F B, cutting the
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Line I V in the Points G and E. </
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">On the Line
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G E deſcribe the ſemicircle E D T G, wherein
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draw the Line G D perpendicular to F I, which
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biſect in H, and about H, as a Centre with the Ra-
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dius H D, deſcribe the Arc of a Circle, L D R, cut-
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ting the Line F I in the Points L and R. </
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the Chord G T in the Semicircle E D T G equal
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to R L, and deſcribe a Semicircle T m G upon
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G T; </
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">in which Semicircle draw ſeveral Lines,
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as m n Perpendicular to G T; </
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