Produce it to cut the horizontal sight-line in X.
Therefore X is the vanishing-point of horizontal lines in the given inclined plane. ([Problem XVIII.])
Join T X, and draw T Y at right angles to T X.
Therefore Y is the rectangular vanishing-point corresponding to X.[Footnote 29] ]
From Y erect the vertical Y P, cutting the sight-line of the inclined plane in P.
[p60]
]Then P is the vanishing-point of steepest lines in the plane.
All lines drawn to it, as Q P, R P, N P, etc., are the steepest possible in the plane; and all lines drawn to X, as Q X, O X, etc., are horizontal, and at right angles to the lines P Q, P R, etc.
[Footnote 29: ] That is to say, the vanishing-point of horizontal lines drawn at right angles to the lines whose vanishing-point is X.] [Return to text]