219114An ESSAY
Side-ways upon the Mirrour L, which being again
reflected by the ſaid Mirrour L, and going through
the Convex Glaſs, extremely weakens the Repre-
ſentation.
reflected by the ſaid Mirrour L, and going through
the Convex Glaſs, extremely weakens the Repre-
ſentation.
34.
The Objects before the Machine are re-
preſented according to N. 22, and 28.
preſented according to N. 22, and 28.
Problem IV.
35.
To repreſent Pictures or Prints.
If we have a mind to repreſent Pictures and
11Fig. 70. Prints, they muſt be faſten’d againſt the Board
F on that Side, regarding the Back of the Ma-
chine, which muſt be ſo turned, that the Pictures
be expoſed to the Sun. Then they are repreſent-
ed in this Situation as the other Objects, 2215. with this Difference, that the Convex Glaſs in the
Cylinder C muſt be changed: For if Prints are
requir’d to have their true Bigneſs, the focal Di-
ſtance of the Convex Glaſs muſt be equal to half
the Height of the Machine above the Table;
that is, equal to half A C. Again, if the ſaid
Pictures or Prints are requir’d to be repreſented
greater than they really are, the focal Diſtance
of the Convex Glaſs muſt ſtill be leſſer. And
if, on the contrary, they are to be repreſented
leſſer than they really are, the focal Length of
the Glaſs muſt be greater than the Length A C.
Moreover, the proper Diſtance whereat the Pi-
ctures or Prints muſt be placed, may be found
in ſliding the Board F backwards or forwards,
until they diſtinctly appear within the Machine.
This Diſtance alſo may be determin’d by the
following Proportion:
11Fig. 70. Prints, they muſt be faſten’d againſt the Board
F on that Side, regarding the Back of the Ma-
chine, which muſt be ſo turned, that the Pictures
be expoſed to the Sun. Then they are repreſent-
ed in this Situation as the other Objects, 2215. with this Difference, that the Convex Glaſs in the
Cylinder C muſt be changed: For if Prints are
requir’d to have their true Bigneſs, the focal Di-
ſtance of the Convex Glaſs muſt be equal to half
the Height of the Machine above the Table;
that is, equal to half A C. Again, if the ſaid
Pictures or Prints are requir’d to be repreſented
greater than they really are, the focal Diſtance
of the Convex Glaſs muſt ſtill be leſſer. And
if, on the contrary, they are to be repreſented
leſſer than they really are, the focal Length of
the Glaſs muſt be greater than the Length A C.
Moreover, the proper Diſtance whereat the Pi-
ctures or Prints muſt be placed, may be found
in ſliding the Board F backwards or forwards,
until they diſtinctly appear within the Machine.
This Diſtance alſo may be determin’d by the
following Proportion:
As the Machine’s Height above the Table, leſs the
Glaſs’s focal Length,
is to
Glaſs’s focal Length,
is to
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