Gravesande, Willem Jacob 's
,
An essay on perspective
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on PERSPECTIVE.
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<
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<
s
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xml:space
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">It is manifeſt , that T R may be
<
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xml:space
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">19.</
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equal to {1/2} or {1/3} & </
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<
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">c. </
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<
s
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">of what it is taken here,
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provided likewiſe that then D N be aſſumed
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equal to a correſpondent Part of the propoſed
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Line.</
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<
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IV.</
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s
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">To throw a Sphere into Perſpective.</
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<
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<
s
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">The Method of ſolving this Problem before
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laid down , muſt be uſed here, but with
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">63.</
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Difference; </
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<
s
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">that inſtead of uſing the Point of
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Sight, the Point wherein a Perpendicular drawn
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from the Eye to the perſpective Plane, meets
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the ſaid Plane, muſt be uſed. </
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<
s
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">And you muſt
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obſerve, that this Perpendicular meaſures the
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Eye’s Diſtance from the perſpective Plane.</
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<
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V.</
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<
s
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">To find the accidental Point of any Number
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<
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">Fig. 48.</
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of Lines inclined to the Geometrical Plane.</
s
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<
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<
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<
s
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">Let A B be the Direction of one of the in-
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clined Lines, O the Eye in the Horizontal Plane,
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and S the Station Point.</
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<
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.</
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<
s
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">Draw the Line O D, thro’ the Eye O parallel
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to A B, meeting the Horizontal Line in D, which
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will be the Accidental Point of the
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of the given Line; </
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<
s
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