Gravesande, Willem Jacob 's
,
An essay on perspective
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on PERSPECTIVE.
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<
s
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xml:space
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">And the Triangles D P m and D G n are ſimilar:
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D G: </
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And
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G H: </
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<
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<
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">: G n: </
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<
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">The Triangles B A N and L A M are ſimilar:
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<
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">Therefore,
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BA: </
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But the three firſt Terms of the two laſt Proportions,
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are equal between themſelves; </
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">whence P m is alſo
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equal to A M. </
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<
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">Which was to be demonſtrated.</
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">When the Height of the Cone is greater
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than the Height of the Eye, the Points, G and
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H, will fall below the Point D; </
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<
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">in which Caſe,
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the Lines A B and A C muſt be produc’d, till
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they cut the Circle in the Points l and f, oppo-
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ſite to L and F: </
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Part of the Baſe.</
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<
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">When the Cone is inclin’d, ſo that T (for Ex-
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ample) is the Seat of its Vertex; </
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drawn: </
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">And then having aſſum’d P D equal to
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the perpendicular Height of the Cone, and Pt
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equal to A T; </
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<
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</
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<
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">and the Part T X, taken in A T, equal to G x. </
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Alſo, X S muſt be drawn, and A s, equal and
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parallel thereto.</
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<
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">This being done; </
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apply’d here, that I have laid down for the up-
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right Cone; </
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<
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">with this Difference only, that the
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Point s muſt be us’d inſtead of the Station Point
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S. </
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<
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">But when the Height of the Cone is greater
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than the Height of the Eye, the Point X muſt
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be aſſum’d in the Line T A, between the Points
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T and A.</
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