A Cupola in horizontal Perspective.
In the Execution of this Work, you are to begin with the Geometrical Plan; in which the two Ranges of Circles denote the Columns, the other Lines shew the Pedestals, with the Projectures and Breaks of the Bases and Cornices. The Line of the Plan is AB, that of the Horizon CD, the perpendicular Line is AD. The Point of Sight is O, that of Distance D; wherefore this Figure ought to be plac’d as much above the Eye, as the Height DO. I have set the Point of Sight something without the Cupola, that the Eye might be less weary’d in viewing the Work, and embrace more of the Architecture, than it could have done, had the Point of Sight been in the midst. The Points of the Line EF are transferr’d into AG, part of the Line AD. The Center of the Plan I is continu’d to H, and from all these Points visual Lines are drawn to O. Then placing the Heights of every part both of the Cupola and Lantern on the Line AB, from the Points of those Divisions draw Lines to the Point of Distance D; and where they cut the Visual AO, erect Perpendiculars intersecting the Line HO; which Points are the Centers of the several Circles. On the Visuals, between AG, must be describ’d the Out-lines of the Columns and Cornices, in like manner as when a Geometrical Upright is rais’d from a Plan. This done, you proceed to the Delineation of the Cupola itself in Perspective, by transferring into the Perpendicular EO the several Centers of HO, by Parallels to HI, as LN, &c. On the Center, with the Interval LM, describe the Circle NP, for the Nose of the Cornice; and with the Semidiameter ST describe the Circle QR, and so of the rest. The Numbers 1, 2, 3, 4, shew how the Breaks of the Cornice are determin’d by Lines from the Angles of the Geometrical Plan tending to the Point of Sight, till they intersect the Circle: The Returns of which Breaks are made by Lines tending to the Centers of their respective Circles; as is plain from N 3, and N 4. In this Plan I have omitted the Corbels, lest I should too much encumber the Work.
Hence appears the Necessity of making the Geometrical Plan of the whole Cupola, the Plan of a single Column not being sufficient; by reason each requires its particular Delineation. And when the Work itself is to be drawn for painting, you can’t well take it from a small Draught by way of Net-work or Squares, but the visual Lines should be drawn in their proper places, and the several Centers found; in which, by fixing Strings, you may readily describe the Circumferences of all the Circles.
Fig. xci.
Figura Nonagesimaprima.
Tholus figuræ nonagesimæ, cum luminibus & umbris.
