If, being a Painter, you were requir’d, against the Solemnity of the Holy-Week, to alter for a while the Architecture of some Altar-piece, by joining Painting to the real Work; as I have often done, both at Rome and Milan, to the great Satisfaction and Surprize of the Beholders: I shall briefly shew the Method to be observ’d in performing the same.
The Dissection of the solid Cornice, which I here suppose shall appear continu’d in that painted on the Canvass, is A; the Geometrical Elevation of the Cornice, and other Parts to be drawn, is B; the Geometrical Plan is C. The Plan and Elevation of the Length are put in Perspective after the usual manner, in C and B; from those the finish’d Cornice, with the Pillar and Pilaster, are delineated on the Canvass; and the Picture is then conjoin’d, at right Angles, to the true Cornice.
For adjusting the Members so, that the painted Cornice may seem to be the real one continu’d, (which can’t be done by the Perspective Upright) you must transfer the Section A to D; and from the terminating Points of the several Members thereof, draw visual Lines, till they meet those of their respective Members in the Perspective. And if the Colours are laid by a skilful Hand, the Angle at E, tho’ painted only, will appear as real; and on the contrary, the Angles which the Members of the painted Cornice make with the different Projectures of those of the true, will never be discern’d, unless in the very uppermost Fillet; but the Conjunction of the real with the painted Architecture, will be altogether imperceptible.
Fig. xxxi.
Figura Trigesimaprima.
Optica projectio coronicis Corinthiæ, cum capitello & summitate columnæ.
In hoc schemate linea plani est CIE, horizontis est DFO; punctum oculi est O, distantiæ est D. Elevatio geometrica capitelli Corinthii cum sua coronice est A, quorum divisiones cernuntur in perpendiculari CD. Vestigium geometricum B habet longitudinem æqualem latitudini: opticè autem contrahitur methodo consueta. Nimirum, translatis divisionibus latitudinis & longitudinis in lineam plani CIE; ex punctis latitudinis fiunt visuales ad punctum oculi; ex punctis verò longitudinis fiunt occultæ ad punctum distantiæ: hoc modo habes quicquid necessarium est ad contractionem opticam vestigii. Nam lineæ longitudinum sunt partes visualium, ut patet in GN, HL: lineæ latitudinum, parallelæ ad lineam plani fiunt ex punctis in quibus lineæ tendentes ad punctum distantiæ secant visualem HO, ut vides in NL. Porro, si tantundem prolongaretur horizontalis DO, ita ut haberet duo puncta distantiæ remota æqualiter ab O, medietas diagonalium, quæ sunt in quadrato majori GNLH opticè deformato, & in quadratis ejus minoribus, tendent ad unum punctum distantiæ; altera medietas ad aliud punctum distantiæ.