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

PERSPECTIVE. more conveniently go in at the Door in the Side
of the Machine.

6. There are two Tin-Tubes bent at each End,
(one of which is repreſented in Fig. 76. becauſe
they could not be ſhewn in the Figure of the
Machine) placed in the Furniture, near the
Back-ſide of the Machine, each having one End
without the Machine. Theſe Tubes ſerve to give
Air to Perſons ſhut up in the Machine, yet ſo,
that no Light may enter through them.

7. At the Places c, c, c, c, on the Out-ſide of
the Back-part of the Machine, are four Iron-Sta-
ples, in which ſlide two Wooden Rulers D E, D E,
about 3 Inches broad, having two other thin
Rulers going through holes made at their Tops, at
the Places D, D, to which thin Rulers the Board
F is fixed; and ſo by this Means the ſaid Board
may be vertically moved backwards or for-
wards.

8. On the Top of the Machine, there is a
Board about 15 Inches long, and 9 Inches broad,
having an hole PMOQ quite through it, about
9 or 10 Inches long, and four Inches broad.

9. Upon the aforeſaid Board are fix’d two Dove-
tail’d Rulers, between which another Board ſlides
of the ſame Length as that, and whoſe breadth is
about 6 Inches. In the middle of this ſecond
Board is a round Hole about three Inches Diame-
ter, hollowed into a female Screw, in which is
fitted a Cylinder about four Inches long, which
carries the Convex Glaſs; Of which more here-
after.

10 The Figure X is a ſquare Box about 7 or
8 Inches broad, and 10 in height, which ſlides
upon the plain Board mentioned in Numb. 8. the
Side B ſerving as a Door, is next to the fore-ſide
of the Machine, as it appears in the Figure; and

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