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.
FIGURA SECUNDA.
Quadratum opticè delineatum.
Postquam descripseris in papyro separata quadratum geometricum A, facies duas lineas parallelas inter se distantes altitudine, quam dederis puncto oculi; linea inferior erit linea plani, linea superior erit linea horizontalis, super quam ponuntur puncta oculi O, & distantiæ E, quod sit ex parte quam mavis: linea distantiæ non debet esse brevior magnitudine rerum describendarum. Transfer posteà circino latitudinem quadrati A in CB, unà cum visualibus ad punctum O; & similiter transfer longitudinem ipsius quadrati in DC, ducens lineam à puncto D ad punctum distantiæ E, transeuntem per visualem CO, & ubi illa secat, habebis terminum quadrati optici GFCB, ducens parallelam ad lineam plani in F.
Ut autem hoc idem citiùs absolverem, sæpius chartam complicavi, ut habes in A.
SECOND FIGURE.
A Square in Perspective.
After you have drawn, on a separate Paper, the Geometrical Square A, make two parallel Lines as much distant one from the other, as you would have the Height of the Eye. The under Line is the Plan or Ground-line; the upper Line is that of the Horizon, on which are plac’d the Points of Sight O, and of Distance E, on which side you please. The Line of Distance should not be shorter than the Extent of the thing to be describ’d in Perspective. Then with your Compasses set the Breadth of the Square A on CB, and draw Visuals to the Point O; and from the Length of the Square transferr’d into DC, draw a Line from the Point D to the Distance E; and where that cuts the Visual CO, by drawing a Line parallel to GF, you describe the Square in Perspective GFCB.