Gravesande, Willem Jacob 's - An essay on perspective

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216111on PERSPECTIVE. Angle with the Horizon, equal to half the
Inclination of the Picture leſs {1/4} of a right Angle.
This Angle is found exactly enough for Practice,
by inclining the Mirrour L, until the Repreſenta-
tion of the Top of the Machine, after a double Re-
flection, appears in the other Mirrour under an
Angle with the Horizon, equal to the Inclination
one would give the Picture. Note, If the Incli-
nation of the Picture be leſſer than {1/4} of 90 De-
grees, the Looking-Glaſs L muſt not be inclin’d
towards the other, as is directed, but 1123: contrary Way, in making the Angle of the In-
clination of the Looking-Glaſs, equal to the
Difference of the Inclination of the Picture, and
{1/4} of 90 Degrees.

25. When the Objects are to be repreſented
for a parallel Picture, the Looking-Glaſs L muſt
be placed in the Diſpoſition of Numb. 15. and
the Looking-Glaſs H in that mentioned, Numb.
13. by inclining it towards the Horizon, under an
Angle of 45 Degrees; the poliſhed Side thereof
facing downwards, when the Picture is ſuppoſed
underneath the Eye; and upwards, when it is
ſuppoſed above the Eye.

26. This Diſpoſition of the Machine may be
likewiſe uſeſul for inclin’d Pictures, making very
ſmall Angles with the Horizon; in which Caſe,
the Inclination of one of the Looking-Glaſſes
muſt be diminiſh’d, by half of the Inclination
of the Picture.

27. A Demonſtration of the Inclination of the
Mirrours.

We have mentioned, that for a 2222. lar Picture, one of the Mirrours muſt make an
Angle of 112 {1/2} Degrees with the Horizon; 3311. the other, L, muſt be inclin’d towards the

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Scan	Original
101	47
102	48
103
104
105
106	49
107	50
108	51
109	52
110
111
112
113	53
114	54
115	55
116	56
117
118
119
120	57
121	58
122
123
124
125	59
126	60
127
128
129
130	61

Scan	Original
101	47
102	48
103
104
105
106	49
107	50
108	51
109	52
110
111
112
113	53
114	54
115	55
116	56
117
118
119
120	57
121	58
122
123
124
125	59
126	60
127
128
129
130	61

Scan	Original
101	47
102	48
103
104
105
106	49
107	50
108	51
109	52
110
111
112
113	53
114	54
115	55
116	56
117
118
119
120	57
121	58
122
123
124
125	59
126	60
127
128
129
130	61