Gravesande, Willem Jacob 's
,
An essay on perspective
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of this laſt Line, and the Line E D G, which was
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to be demonſtrated.</
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<
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">By this Demonſtration it appears, that there
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is no Neceſſity in taking G O exactly equal to
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the Eye’s Diſtance, and O F equal to its Height:
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<
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">But it is ſufficient if they have the ſame Propor-
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tion, as the aforeſaid Diſtance has to the Height. </
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Likewiſe there is no Neceſſity in aſſuming the
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Points G and F, in a Line parallel to the Baſe
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Line; </
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">for any other Line paſſing through the
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Eye O may be uſed at Pleaſure. </
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<
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">For Example,
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let g O f be a Line any how drawn through the
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Eye O, and take the Point g at Pleaſure in this
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Line, through which draw alſo the Line g N I
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at Pleaſure, cutting the Horizontal Line in N,
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and the Baſe Line in I; </
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xml:space
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">and draw the Line O N,
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and through the Point I, draw the Line I f pa-
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rallelthereto, cutting the Line g O f in f.</
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<
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">This being done, the Points g and f may be
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uſed inſtead of G and F: </
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<
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">for among all the
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Lines that can be drawn (as G N I) it is mani-
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ſeſt, that g N will always be to g I:</
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">g f,
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which is ſufficient for the Demonſtration.</
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<
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">If the Point f be firſt determin’d, the Point g
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muſt be found by an Operation quite contrary
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to that we have laid down.</
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<
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may (a Baſe Line being firſt drawn) take at
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Pleaſure, in another Line any how drawn,
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the three Points g O f; </
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<
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">ſo that in this Caſe, there
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is no Manner of Neceſſity to uſe Compaſſes, in
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throwing any Figure whatſoever, which is on
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the Geometrical Plane, into Perſpective. </
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if after having thus work’d, the Point of Sight,
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Height and Diſtance of the Eye be requir’d, </
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