Gravesande, Willem Jacob 's - An essay on perspective

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4218An ESSAY and drawing a Line from the Point I to the
other Point of Diſtance; which, by its Inter-
ſection with E D, will give the Appearance of
the Point A.

Method II.

26. γ is the Horizontal Plane, X the Perſpe-
ctive Plane, Z the Geometrical Plane, O the Eye,
D C the Horizontal Line, B E the Baſe Line, and
A the given Point.

11Fig. 8.

Operation.

Draw a Line from the Point A, to the Eye O, cut-
ting the Baſe Line in the Point B, and the Horizon-
tal Line in the Point C: Then aſſume B E in the
Baſe Line, equal to B A; and C D in the Hori-
zontal Line, equal to C O; and join the Points
E and D, by a Line cutting the Line A O in
the Point a; which will be the Appearance
ſought.

Demonstration.

27. The Triangle O D C in the Horizontal
Plane, is ſimilar to the Triangle A B E in the
Geometrical Plane; and conſequently A B is pa-
rallel to O C, and A E to O D. But the Appear-
ance of A muſt be in the Lines B C, and E D; 2213. and therefore it will be in a, their Interſection.

Remarks.

28. If the Place wherein the Eye ought to be in
the Horizontal Plane be not known, but the Point
of Sight is; then, to find the Place of the Eye,
a Perpendicular muſt be rais’d from the Point

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Scan	Original
41	17
42	18
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44	20
45
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48	21
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50	23
51	24
52
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55	25
56	26
57
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61	28
62	29
63	30
64
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70	31

Scan	Original
41	17
42	18
43	19
44	20
45
46
47
48	21
49	22
50	23
51	24
52
53
54
55	25
56	26
57
58
59
60	27
61	28
62	29
63	30
64
65
66
67
68
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70	31