Gravesande, Willem Jacob 's
,
An essay on perspective
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on PERSPECTIVE.
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a Hole at its End, through which the above-men-
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tion’d Screw may paſs, and ſo the Mirrour H be
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fixed to any Inclination before the Aperture N.</
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<
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<
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">Beſides the Mirrour H, there is another leſſer
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one, L, faſten’d near its Middle to a Ruler going
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out through the Middle of the Top of the Box.
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</
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<
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">This Ruler may Screw on, and ſerves to raiſe or
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lower the Mirrour faſten’d to it, ſo that it may be
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fixed to all Angles of Inclination.</
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<
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.</
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<
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<
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<
s
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">be not thought
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ſufficient for giving Air to the Machine, a ſmall
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Pair of Bellows may be put under the Seat,
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which may be blown by one’s Foot. </
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<
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xml:space
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">And by
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this means the Air within the Machine may be
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continually remov’d; </
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<
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xml:space
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">for the Bellows driving the
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Air out of the Machine, obliges the external Air
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to enter through the Tubes therein.</
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<
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I.</
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ſition.</
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<
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<
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">When Objects are to be repreſented within the
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">Fig. 70.</
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Machine, we extend a Sheet of Paper upon the
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Table A; </
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<
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">or, which is better, we lay a Sheet of
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Paper upon another Board, ſo that it ſpreads be-
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yond the Edges of the Board; </
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<
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">then we ſqueeze
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the ſaid Paper and Board into a Frame, ſo that
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it be fixed therein by means of two Dove-tail’d
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Rulers. </
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<
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">This being done, we place a Convex
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Glaſs in the Cylinder C, which ſcrews into
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Top of the Machine, having its Focal </
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