The Elements of Perspective / arranged for the use of schools and intended to be read in connection with the first three books of Euclid

by John Ruskin

Join T V. Then the student will find experimentally that T V is parallel to A B.[Footnote 12] ]

[p16]
][COROLLARY III.]

If the line A B produced would pass through some point beneath or above the station-point, C D is to D T as C′ D′ is to D′ T; in which case the point

c

coincides with the point

c′

, and the line

a b

is vertical.

Therefore every vertical line in a picture is, or may be, the perspective representation of a horizontal one which, produced, would pass beneath the feet or above the head of the spectator.[Footnote 13] ]