Gravesande, Willem Jacob 's
,
An essay on perspective
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(23)
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50
23
on PERSPECTIVE.
Perpendiculars
f
P
,
O
H
,
muſt
be
let
fall
from
the
Points
f
and
O
,
on
the
Baſe
Line
,
and
the
Line
P
g
drawn
;
then
the
Point
V
,
wherein
it
cuts
the
Perpendicular
O
H
,
is
the
Point
of
Sight
ſought
,
and
the
Parts
O
V
,
and
V
H
determine
the
Height
and
Diſtance
of
the
Eye
.
Method
V
.
35
.
When
the
Appearance
of
a
Point
is
known
,
Let
A
be
a
Point
in
the
Geometrical
Plane
,
1
1
Fig
.
11
.
and
a
its
Repreſentation
in
the
perſpective
Plane
,
it
is
requir’d
to
find
the
Appearance
of
the
Point
B
.
Operation
,
Without
Compaſſes
.
Draw
a
Line
from
the
Point
B
to
the
Eye
O
,
and
another
from
the
Point
E
,
wherein
the
ſaid
Line
continued
,
cuts
the
Baſe
Line
,
to
the
Point
A
;
then
draw
the
Line
E
a
,
and
where
it
cuts
B
O
,
is
the
Point
b
ſought
.
Demonstration
.
The
Point
E
is
its
own
Repreſentation
;
and
becauſe
the
Point
a
is
the
Repreſentation
of
A
,
the
Line
E
a
is
that
of
E
A
.
Now
ſince
the
Point
B
is
in
the
Line
E
A
,
the
Appearance
of
this
Point
will
be
likewiſe
in
E
a
,
as
alſo
2
2
27
.
B
O
;
therefore
it
is
in
b
the
Interſection
of
the
Lines
E
a
,
and
B
O
.
Remark
.
37
.
If
the
Point
A
be
in
the
Line
B
O
,
or
the
Line
B
A
be
parallel
,
or
a
very
little
inclined
to
the
Baſe
Line
,
we
cannot
then
uſe
this
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