222117PERSPECTIVE.
to be copyed, the Convex Glaſs muſt have the
leaſt Aperture poſſible; but yet with this Cau-
tion, that the Light coming into the Machine,
muſt not be too much extenuated. From theſe
Obſervations it is manifeſt, that we ought to be
provided with ſeveral round Pieces of Tin or
thin Braſs, having round Holes of different big-
neſſes therein, in order to give a neceſſary A-
perture to the Glaſs; or Holes of different big-
neſſes may be made in a long thin Piece of Braſs
which may ſlide upon the Convex Glaſs: Or
elſe, we may uſe a round Plate, having Holes
of different bigneſſes therein, which turning a-
bout its Centre, may bring any deſired Hole for
the Glaſſes Aperture.
leaſt Aperture poſſible; but yet with this Cau-
tion, that the Light coming into the Machine,
muſt not be too much extenuated. From theſe
Obſervations it is manifeſt, that we ought to be
provided with ſeveral round Pieces of Tin or
thin Braſs, having round Holes of different big-
neſſes therein, in order to give a neceſſary A-
perture to the Glaſs; or Holes of different big-
neſſes may be made in a long thin Piece of Braſs
which may ſlide upon the Convex Glaſs: Or
elſe, we may uſe a round Plate, having Holes
of different bigneſſes therein, which turning a-
bout its Centre, may bring any deſired Hole for
the Glaſſes Aperture.
A Deſcription of the Second Machine.
39.
This Machine is a kind of Box, the Side
11Fig. 78. A C G B being open, whoſe breadth B D, and
height AB, are equal, each being about 18 Inches:
Its greateſt width F B, is 10 Inches, and the Side
E F is ſloping, ſo that A E is but about 6 Inches.
11Fig. 78. A C G B being open, whoſe breadth B D, and
height AB, are equal, each being about 18 Inches:
Its greateſt width F B, is 10 Inches, and the Side
E F is ſloping, ſo that A E is but about 6 Inches.
40.
The Frame G ſlides at the Bottom of the
ſaid Box, in which the Paper is faſtned.
2215.ſaid Box, in which the Paper is faſtned.
41.
There is a round Hole in the Middle of
the Top of the Box, in which the Cylinder
carrying the Convex Glaſs ſcrews.
339.the Top of the Box, in which the Cylinder
carrying the Convex Glaſs ſcrews.
42.
The two Sticks H I and L M, ſlide in four
little Iron Staples fixed to the Inſide of the Top
of the Box, like thoſe mentioned in Numb. 7.
Theſe Sticks come about two Feet without the
Box, and the Diſtance of their Extremities I and
M, is equal, or ſomething greater than the
Length of the Box. Their Uſe is to hang a black
Cloth upon, which is faſtned to the Three Sides,
B A, A C and C D, of the opening of the Box,
that ſo the Box may be darkned, when
little Iron Staples fixed to the Inſide of the Top
of the Box, like thoſe mentioned in Numb. 7.
Theſe Sticks come about two Feet without the
Box, and the Diſtance of their Extremities I and
M, is equal, or ſomething greater than the
Length of the Box. Their Uſe is to hang a black
Cloth upon, which is faſtned to the Three Sides,
B A, A C and C D, of the opening of the Box,
that ſo the Box may be darkned, when