Full text: Gravesande, Willem Jacob: An essay on perspective

on PERSPECTIVE. a Hole at its End, through which the above-men-
tion’d Screw may paſs, and ſo the Mirrour H be
fixed to any Inclination before the Aperture N.

200.1.

7.

14. Beſides the Mirrour H, there is another leſſer
one, L, faſten’d near its Middle to a Ruler going
out through the Middle of the Top of the Box. This Ruler may Screw on, and ſerves to raiſe or
lower the Mirrour faſten’d to it, ſo that it may be
fixed to all Angles of Inclination.

201. Remarks .

If the Tubes mention’d in N. 6. be not thought
ſufficient for giving Air to the Machine, a ſmall
Pair of Bellows may be put under the Seat,
which may be blown by one’s Foot. And by
this means the Air within the Machine may be
continually remov’d; for the Bellows driving the
Air out of the Machine, obliges the external Air
to enter through the Tubes therein.

202. Uſe of the Machine.
Problem I.

15. To repreſent Objects in their natural Diſpo-
ſition.

When Objects are to be repreſented within the
Machine, we extend a Sheet of Paper upon the
Table A; or, which is better, we lay a Sheet of
Paper upon another Board, ſo that it ſpreads be-
yond the Edges of the Board; then we ſqueeze
the ſaid Paper and Board into a Frame, ſo that
it be fixed therein by means of two Dove-tail’d
Rulers. This being done, we place a Convex
Glaſs in the Cylinder C, which ſcrews into the Top of the Machine, having its Focal Length

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