Gravesande, Willem Jacob 's
,
An essay on perspective
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on PERSPECTIVE.
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fore the Point X is the Repreſentation of the
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ſaid Extremity.</
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<
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Perſpective of a Line, perpendicular to the
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Geometrical Plane is once found, it is eaſy af-
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terwards to find the Repreſentations of any Per-
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pendiculars of the ſame Length as that.</
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II.</
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">The Perſpective Plane being conſider’d as the Geo-
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metrical Plane.</
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Point of perpendicular Lines to the Geometri-
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cal Plane; </
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ter is T, and ſemidiameter the Eye’s Diſtance
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from the perſpective Plane: </
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Point where the Perpendicular, whoſe Appear-
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ance is ſought, meets the perſpective Plane, and
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B C the Length of this Perpendicular.</
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">About the Point a, as a Center, and with the
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Semidiameter B C, deſcribe the Circle L F, and
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draw the Line I L, or H F, touching each of the
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Circles H I, and F L; </
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">and then a X or a x, is
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the Appearance ſought, viz. </
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pendicular is above that Surface of the per-
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ſpective Plane next to the Eye, and a x, when
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the Perpendicular is on the oppoſite Side.</
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<
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Points of Contact F, L, I, and H. </
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