Gravesande, Willem Jacob 's
,
An essay on perspective
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An ESSAY
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<
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xml:space
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">In order to underſtand well how Mathema-
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ticks may be apply’d to Drawing; </
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<
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xml:space
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">let us ſup-
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poſe a Man A, viewing an Object; </
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xml:space
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<
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">Fig. 1.</
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tween him and the Object he looks at, let us
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imagine a tranſparent Plane C. </
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over, that Lines be drawn upon this Plane, as
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in D, which cover the Bounds of the Object B
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in reſpect of the Spectator A, and each Part that
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he ſees thereof. </
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<
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xml:space
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">Now, ſince all Objects are ſeen
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by the Rays of Light coming from every of
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their Points, and terminating at the Eye, and
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not otherwiſe; </
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<
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xml:space
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">and ſince that here all the Rays
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proceeding from the Object B, likewiſe paſs
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thro’ every Point of the Repreſentation D; </
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<
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manifeſt, that this Repreſentation will have the
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ſame Effect upon the Spectator’s Eye, as the
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ſaid Object B hath. </
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<
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">Now, by means of Geo-
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metry, we can find the Points of the Figure D,
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on the Plane C, placed in a given Situation,
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thro’ which the Rays coming from the Object B
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to the Eye of the Spectator A, do paſs; </
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<
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xml:space
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theſe Points are the Interſections of the Rays
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and the Plane. </
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<
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">Alſo, (as others have very well
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obſerv’d) a Perſpective Plane, or Picture in Paint-
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ing, may be conceiv’d as a Window, upon which
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the Objects ſeen thro’ it are repreſented.</
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<
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">Now, without Mathematicks, this Repreſen-
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tation cannot be well found: </
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xml:space
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are drawn by only viewing, or looking at them;
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</
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<
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">their true Repreſentations after this way, will
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be very often miſs’d on; </
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<
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try, we can always obtain them.</
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<
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">This Obſervation only, is ſufficient to eſta-
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bliſh the Neceſſity of Perſpective: </
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<
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">Tho’ there are
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ſome Painters, who (according to the common
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Maxim) affirm, That what they do not know
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of this Art, is not worth the Pains of learn-
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ing.</
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