Ruskin, John; Cook, Edward T. [Editor]: The works of John Ruskin: The elements of drawing. The elements of perspective. And the laws of Fésole (London, 1904)

CURVE IN HORIZONTAL PLANE

281

constructions, it may be inconvenient to draw E & parallel
to the sight-line, the square being perhaps hrst constructed
in some oblique direction. In such cases, Q & and E p must
be determined in perspective ratio by the dividing-point, the
line E G being used as a measuring-line.

(0&. In drawing Fig. 31, the station-point has been taken much nearer
the paper than is usuahy advisable, in order to show the character of the
curve in a very distinct form.
If the student turns the book so that E G may be vertical, Fig. 31 will
represent the construction for drawing a circle in a vertical plane, the
sight-line being then of course parallel to G L ; and the semicircles A D B,
A c B, on each side of the diameter A B, will represent ordinary semi-
circular arches seen in perspective. In that case, if the book be held so
that the line E n is the top of the square, the upper semicircle will repre-
sent a semicircular arch, %5o?'e the eye, drawn in perspective. But if
the book be held so that the line G F is the top of the square, the upper
semicircle will represent a semicircular arch, the eye, drawn in
perspective.
If the book be turned upside down, the figure will represent a circle
drawn on the ceiling, or any other horizontal plane above the eye; and
the construction is, of course, accurate in every case.)