_An Explanation of the Lines of the Plan and Horizon, and of the Points
of Sight and Distance; but more especially of this last._
For beginning any Design in Perspective, there are principally requir’d
three Lines, and two Points: One Line where the Feet stand, which is
call’d the Line of the Plan, or Ground-line: The second where the Eye is
plac’d, call’d the horizontal Line: I shall speak of the third in the
Elevations. Of the Points, one is assign’d to the Eye, the other to the
Distance. The first of these is generally known, the latter not so well
understood, though of great Use for giving the Removal or Depth of every
Object. I shall therefore insist a while on the Explanation of the Point
of Distance; and that I may more clearly shew what it is, I have chosen
the Geometrical Description of a Church, which is divided into three
Parts; _viz._ the Plan, Profile, and inner Face; in the midst of which
Face one would paint a Piece of Perspective, that should seem to recede
as much as the Square P in the Plan, and the Depth Q in the Profile.
On the Face CCCC, which suppose that of the Design, you see the Manner
of disposing the two Lines and the two Points. HI is the Ground-line.
NON is the horizontal Line, which is usually made a Man’s Height above
the Ground line, as in B. The Point of Sight is O, the Point of Distance
N, on which side you will. This Point N must be as far from O, as the
Distance you determine to place yourself at for viewing the Depth of the
Square PQ; as is exemplify’d in the Plan and Profile, where you see the
thing as in its natural Position: And in them N is distant from O, as far
as the Man in A is remov’d from DE; or the Man B in the Profile from FG,
which is the Wall to be drawn or painted on.
If you farther and more strictly examine this Description, you’ll discern
how well the Square of the Plan P, and the Elevation Q, correspond as if
naturally put into Perspective on the Face CCCC, which is the Draught.
For you see the Visuals which cut the Space RN in the Plan, cut the
same Space TV in the Upright; and the Segment of the Visuals XZ in the
Profile, answer that of YK in the Elevation; which needs no Demonstration.