Gravesande, Willem Jacob 's
,
An essay on perspective
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An ESSAY
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to the Length of the ſaid Perpendiculars: </
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<
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therefore</
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<
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xml:space
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<
s
xml:id
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xml:space
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">But in the Conſtruction of this Problem, be-
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cauſe the Triangles T C D and T H G, are
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ſimilar;</
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<
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">H G = H I: </
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<
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">T H:</
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<
s
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">: C D = a X: </
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<
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xml:space
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">T D = a T;
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</
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<
s
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xml:space
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">and conſequently H I and a X, repreſent Perpen-
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diculars of the ſame Length. </
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<
s
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">Which was to be
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demonſtrated.</
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<
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. III.</
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">To find the accidental Point of any Number
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of parallel Lines inclined to the Geometrical Plane.</
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<
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<
s
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">Let a b be the Perſpective of the Direction
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<
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of one of the given Lines.</
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<
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.</
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<
s
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">Draw the Line F T L, parallel to a b, through
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the accidental Point T of the Lines perpendicu-
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lar to the Geometrical Plane; </
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<
s
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xml:space
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">and at the Point
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T, raiſe the Perpendicular T G, which make e-
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qual to the Diſtance of the Eye from the per-
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ſpective Plane; </
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<
s
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xml:space
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">then draw the Line G L, or
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G F, ſo that the Angle T L G, or T F G, be e-
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qual to the Angle of the Inclination of the given
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Lines; </
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<
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xml:space
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">and the Point L, will be the Accidental
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Point ſought, if the given Lines incline to-
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wards b; </
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<
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">but if they incline towards a, F will
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be the Accidental Point.</
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<
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.</
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<
s
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xml:space
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">It is manifeſt by Conſtruction, that if T G be
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ſuppoſed to be raiſed perpendicularly to the Geo-
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metrical Plane, G L or G F, will be parallel </
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