Gravesande, Willem Jacob 's
,
An essay on perspective
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on PERSPECTIVE.
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<
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xml:space
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">The Torus of the Column muſt be conceiv’d as
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made up of an Infinite Number of Circular Planes,
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lying one upon another. </
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<
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the Reaſon why each of thoſe Circles cannot be wholly
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ſeen, is becauſe that which is immediately under it
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hides a Part thereof; </
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<
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">from whence it follows, that
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if the Plane of one oſ theſe Circles be every way
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continu’d, and the Circle immediately under it, be
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thrown into Perſpective upon it, (which
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is alſo a Circle) the two Points of Interſection of this
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Repreſentation, and the Circle in the Plane, will deter-
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mine the viſible Part of the ſaid Repreſentation; </
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conſequently if the Repreſentation of theſe two Points
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of Interſection be found upon the Perſpective Plane, we
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ſhall have two Points of the Perſpective of the Torus of
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the propoſed Column. </
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<
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Solution of the Problem, as we ſhall now Analytically
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demonſtrate.</
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<
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">Let O be the Eye, A M a part of the Torus of
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<
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the Column, A P a Perpendicular to the Baſe paſſing
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through the Center of the Column, and A B a
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Parallel to the Baſe, drawn thro’ the Center B of
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the Semicircle Concavity of the Torus. </
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<
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xml:space
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a Semidiameter of one of the Circles ſpoken of in the
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the beginning of this Demonſtration. </
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Line m p be drawn parallel and infinitely near C M P
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and the Lines m O and p O are drawn cutting M P
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in D and T, it is evident that D T, which is in the
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Plane of the Circle paſſing thro’ M P, will be the
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Semidiameter of the Perſpective of the Circle imme-
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diately underneath.</
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to the Line A B, and continue the Lines M P and
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m p, till they meet the ſaid Perpendicular in the Points
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Q and q. </
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