If the line _AB_ produced would pass through some point beneath or above
the station-point, _CD_ is to _DT_ as _C′D′_ is to _D′T_; in which case
the point _c_ coincides with the point _c′_, and the line _ab_ 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.[13]
[11] For by the construction _AT_ ∶ _aT_ ∷ _BT_ ∶ _bT_; and therefore
the two triangles _ABT_, _abT_, (having a common angle _ATB_,)
are similar.
[12] The demonstration is in Appendix II. Article I.
[13] The reflection in water of any luminous point or isolated object
(such as the sun or moon) is therefore, in perspective, a
vertical line; since such reflection, if produced, would pass
under the feet of the spectator. Many artists (Claude among the
rest) knowing something of optics, but nothing of perspective,
have been led occasionally to draw such reflections towards a
point at the center of the base of the picture.