Full text: Gravesande, Willem Jacob: An essay on perspective

on 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 the 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.

204.1.

23: [?]

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 perpendicu- lar Picture, one of the Mirrours muſt make an
Angle of 112 {1/2} Degrees with the Horizon; and the other, L, muſt be inclin’d towards the firſt,

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