The Artist's Assistant, In the Study and Practice of Mechanical Sciences: Calculated for the Improvement of Genius. Illustrated with Copper-Plates — Birmingham, [ca. 1785]

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THE ARTIST'S ASSISTANT. 4i

laws of this fort of projection are thefe : 1. The rays
by which the eye at an infinite diftance perceives
any object, are parallel. 2. A right line perpendicu-
lar to the plane of the projection, is projected into a
point, where that right line cuts the plane of the
projection. 3. A right line not perpendicular, but
either parallel or oblique to the plane of the projec-
tion, is projected into a right line, and is always
comprehended between the extreme perpendiculars.
4. The projection of the right line is the greateft,
when that line is parallel to the plane of the pro-
jection. 5. Hence it is evident, that a line parallel
to the plane of the projection, is projected into a
right line equal to itfelf; but if it be oblique to the
plane of the projection, it is projected into one
which is lefs. 6. A plane furface, at right angles to
the plane of the projection, is projected into that
right line, in which it cuts the plane of the projec-
tion. Hence it is evident, that a circle ftanding at
right angles to the plane of the projection which,
paffes through its center, is projected into that
diameter, in which it cuts the plane of the projec-
tion. 7. A circle parallel to the plane of the pro-
jection, is projected into a circle equal to itfelf; and
a circle oblique to the plane of the projection, is
projected into an elliphs.

Stereographic projection, is that wherein the fur-
face and circles of the fphere are drawn upon the
plane of a great circle, the eye being in the pole of
that circle. As to the properties of this fort of pro-
jection : 1. In this projection a right circle is pro-
jected into a line of half, tangents. 2. The repre-
sentation of a right circle perpendicularly oppofed
to the eye, will be a circle in the plane of the pro-

F je&ion.
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