218113on PERSPECTIVE.
Angle B C P, and its equal R C Q, is 22 {1/2} plus
{1/2} of D I A. Now if the Angle R Q C be added
to this Angle, their Sum will be equal to the An-
gle D I A; whence it follows, that the Angle
C R Q is equal to D I R. Which was to be demon-
ſtrated.
{1/2} of D I A. Now if the Angle R Q C be added
to this Angle, their Sum will be equal to the An-
gle D I A; whence it follows, that the Angle
C R Q is equal to D I R. Which was to be demon-
ſtrated.
30.
If the Angle R B N be alter’d, and it be
called a, the Angle D I A, b, and the right An-
gle d; then the Angle C Q R = d + {1/2} b — a.
called a, the Angle D I A, b, and the right An-
gle d; then the Angle C Q R = d + {1/2} b — a.
31.
When a Picture is parallel, it appears ma-
11Fig. 74. nifeſt, the Mirrours GH and M N being each in-
clin’d under an Angle of 45 Degrees, that a Ray,
which is perpendicular to the Horizon, likewiſe
falls, after a double Reflection, perpendicularly
upon the Table A.
11Fig. 74. nifeſt, the Mirrours GH and M N being each in-
clin’d under an Angle of 45 Degrees, that a Ray,
which is perpendicular to the Horizon, likewiſe
falls, after a double Reflection, perpendicularly
upon the Table A.
Prop. III.
32.
To repreſent Objects which are roun’d about
the Machine, and make them appear erect to the Per-
ſon ſeated within the ſame.
the Machine, and make them appear erect to the Per-
ſon ſeated within the ſame.
The Back-ſide of the Machine muſt be turned
towards the Sun, and the Objects behind the
ſame repreſented by one Reflection only; 2215. their Appearance will always be clearer, altho
they be in the Shade, than the Appearance of the
Objects on the other Sides of the Machine, which
cannot be perceiv’d unleſs by a double Refle-
ction.
towards the Sun, and the Objects behind the
ſame repreſented by one Reflection only; 2215. their Appearance will always be clearer, altho
they be in the Shade, than the Appearance of the
Objects on the other Sides of the Machine, which
cannot be perceiv’d unleſs by a double Refle-
ction.
33.
The Objects that are on the right and left
33Fig. 70. of the Machine, may be repreſented by means
of the Mirrour H, ſituated as per Figure; but 4412. ſaid Mirrour muſt be cover’d with a Paſtboard Caſe,
having two Apertures therein; the one next to
the Objects, and the other next to the Aperture
N, of the Box X. The Reaſon of our uſing this
@recaution is, becauſe when the Mirrour is not co-
ver’d at all, it reflects the Rays of Light
33Fig. 70. of the Machine, may be repreſented by means
of the Mirrour H, ſituated as per Figure; but 4412. ſaid Mirrour muſt be cover’d with a Paſtboard Caſe,
having two Apertures therein; the one next to
the Objects, and the other next to the Aperture
N, of the Box X. The Reaſon of our uſing this
@recaution is, becauſe when the Mirrour is not co-
ver’d at all, it reflects the Rays of Light