221116An ESSAY on
ſemblance.
The Reaſon of which is, that the Li-
neaments appear lively and diſtinct within the
Machine, when the Re-union of the Rays pro-
ceeding from a given Point in the Face, happens
exactly upon the Paper in one Point only: But
the leaſt Diſtance that one Point is more than
another from the Convex Glaſs, (when the Di-
ſtance of the Face from the Glaſs is ſo ſmall, as
it muſt be to repreſent it in its natural Bigneſs)
ſo alters the Place of the ſaid Re-union, that for
different Parts of the Face, thoſe Places of Re-
union will differ about two Inches and a half.
Whence it is no wonder that all the Lineaments
be not repreſented as could be wiſhed; ſince in
all Diſtances choſen, there will be always a great
many Rays, whoſe Re-union will fall above an
Inch beſides the Paper. The Confuſion ariſing
from this Diverſity, though not being very di-
ſtinguiſhable by the Eye, yet is prejudicial, and
hinders our getting the exact Reſemblance of the
Face. We have obſerved this, in order to give
an exact Idea of the Goodneſs of this Machine,
in equally ſhewing wherein it may be really
uſeful, and wherein its apparent Uſefulneſs is
ſubject to an Error rather diſcovered by Experi-
rience than Reaſon.
neaments appear lively and diſtinct within the
Machine, when the Re-union of the Rays pro-
ceeding from a given Point in the Face, happens
exactly upon the Paper in one Point only: But
the leaſt Diſtance that one Point is more than
another from the Convex Glaſs, (when the Di-
ſtance of the Face from the Glaſs is ſo ſmall, as
it muſt be to repreſent it in its natural Bigneſs)
ſo alters the Place of the ſaid Re-union, that for
different Parts of the Face, thoſe Places of Re-
union will differ about two Inches and a half.
Whence it is no wonder that all the Lineaments
be not repreſented as could be wiſhed; ſince in
all Diſtances choſen, there will be always a great
many Rays, whoſe Re-union will fall above an
Inch beſides the Paper. The Confuſion ariſing
from this Diverſity, though not being very di-
ſtinguiſhable by the Eye, yet is prejudicial, and
hinders our getting the exact Reſemblance of the
Face. We have obſerved this, in order to give
an exact Idea of the Goodneſs of this Machine,
in equally ſhewing wherein it may be really
uſeful, and wherein its apparent Uſefulneſs is
ſubject to an Error rather diſcovered by Experi-
rience than Reaſon.
38.
We muſt not forget in all the precedent
Problems, to examine the Aperture the Convex
Glaſs ought to have; for although we cannot re-
duce this Aperture to a fixed Meaſure, yet it is
proper to obſerve the following Remarks. 1. The
Convex Glaſs may commonly have the ſame
Aperture, as we would give a Perſpective Glaſs,
having the ſaid Glaſs for its Object Glaſs. 2. When
Objects are very much enlightned, the ſaid A-
perture muſt be leſſened; and contrariwiſe, when
they are expoſed to a weaker Light, it muſt be
made greater; and when any Repreſentation
Problems, to examine the Aperture the Convex
Glaſs ought to have; for although we cannot re-
duce this Aperture to a fixed Meaſure, yet it is
proper to obſerve the following Remarks. 1. The
Convex Glaſs may commonly have the ſame
Aperture, as we would give a Perſpective Glaſs,
having the ſaid Glaſs for its Object Glaſs. 2. When
Objects are very much enlightned, the ſaid A-
perture muſt be leſſened; and contrariwiſe, when
they are expoſed to a weaker Light, it muſt be
made greater; and when any Repreſentation
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