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])

go THE ARTIST'S ASSISTANT.

the vifual point ; which term point, is ufed for va-
rious parts, or places, with regard to the perlpettive
plane.—The point of light, or of the eye. is a point
on the plane, marked out by a right line drawn from
the eye, perpendicular to the plane : This is aifo
called the principal point. This point is in the in-
terfe&ion of the horizontal and vertical planes.
Some authors call it the principal point: and give
the name point of fight, or virion, to the point wherein
the eye is aftually placed, and where all the rays
terminate.—The third point, is apoint taken at dii
cretion in the line of diftance. wherein all the diago-
nals drawn from the di virions of the geometrical plane,
whofe rcprefentation is required on the perfpeftive
plane.—An accidental point, is apoint in the horizon-
tal lines, where lines parallel to one another, though
not perpendicular to the picture, or repefentation
meet.—A vifual point, is a point in the horizontal
line, wherein all the ocular rays unite. Thus a per-
ion Handing in a {trait long gallery, and looking for-
wards, the fide, the floor, and ceiling feem to meet,
and touch one another in a point, or common centre.

Thefe things prcvioufly confidcred, I'llpafs to the
Explanation of the different forts of perfpeftive.

I • From a point in a given line AB,/s raife a perpendicular.

See fig. II.

Draw with the ruler the given line A B : then fet
one loot of your compaffes in B, and extending
them to rather more than half the length of the
>:ne A B, fweep the arch c d ; and with the fame
extent of compaffes fet one point in c, and fweep
the arch e r; then, without altering the compaffes*

fet