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

An ESSAY are to be repreſented upon the Paper in the Frame

43. There are two pieces of Wood, (one of
which is repreſented in Fig. 77.) ſerving to
ſuſtain the Box upon its Support or Foot. One
of theſe Pieces may be faſtned to one Side of the
Support, and the other to the other Side thereof,
by means of four Iron Pins, two of which go thro
the Holes N and P in the Side of the Support,
and the Holes T and V in the Piece R, and the
other two in like manner, through Holes made
in the other Side of the Support, and the other
Piece, when we have Mind the Bottom of the
Box ſhould be parallel to the Horizon; but when
the Box is to be a little inclin’d, that Pin going
through the Hole P, muſt be put through the
Hole O, in the Piece R. Underſtand the ſame
of the other Piece.

44. We are ſometimes obliged to ſet the Box
forwarder on its Support; and this is done, in
uſing the Holes Q and S, inſtead of N and P. It
is likewiſe ſomething neceſſary to incline the
Box a little backwards; which may be done, by
putting the Pin in S, into the Hole X, made in
a Piece of Wood faſtned to the Back-ſide of the
Box, and the correſpondent Pin on the other Side,
into another Hole made on the other Side of the
ſaid Piece.

45. The Box ♈ ſlides upon the Top of the
Machine, and is like that already deſcribed, but with this Difference only, that it is leſſer. On
the Top of the Box are two little Staples Z Z,
in which a Ruler ſlides, having a Mirrour faſtned
to it, in the Manner as is mentioned Numb. 13. and ſo by this Means the ſaid Mirrour may be
put in the ſame Situation, as that in the Figure
of the firſt Machine it hath in H.


10, 11,

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