The Hundredth Figure.
The Method of drawing the Net or Lattice-Work on Vaults.
For Works on a flat Superficies, two Net-works are sufficient; as has been already intimated: One drawn on the Copy; the other on the Table to be painted. But for arch’d Surfaces, or Vaults, three are requir’d: One made on the Copy, which I suppose drawn according to Rules of horizontal Perspective. The second consists of a Frame of small Cords or Threads, to be hung up; the Geometrical Form of which is M. The Lines AB, EF, shew the Place where this Frame is to be fix’d, in the same manner as the Perspective N. The Point of Sight is O; of Distance, LO. Therefore, if you imagine a Lamp or Candle fix’d in the Night-time at the Point O; the Shadows of the Thread, thrown thereby on the Vault, being trac’d by a Pencil, make the third Net-work requir’d for painting the same.
I say, if you imagine a Lamp thus fix’d; because either the Scaffold to the Vault, or the great Distance of the Vault from the Net-work, or the greater of both from the Light, may prevent the Shadows from being thrown at all, or at least, may render them so faint, as not to be distinct enough for the purpose. Therefore, where this happens, instead of the Light fix one End of a Thread in the Point O; and extending the other to the Vault, make use of it as a Ray from the Lamp or Candle, for describing the Place of the Shadows. It will be also of great use, to second the Motion of the Thread with the Light of a Candle you may have by you on the Scaffold, holding the same near the Thread itself. By this, and other such Helps, which your own Industry will suggest, you may lay these Shadows in Colours, and complete the third Net-work requir’d. The Frame of Threads may also be fix’d nearer the Vault at some Distance above the Cornice, as at GH, where the painted Architecture begins; for the Shadows thrown on the Arch will by that means become more visible and distinct.
You must be very careful, that the Measures of your Copy are exactly the same with those of the Vault, that the Net-work thrown into the Angles, Arches and Lunettes of the Vault, may perfectly correspond with that of your Copy. Lastly, if you would not run into inextricable Errors; assure yourself, that all these Rules of horizontal Perspective are as strictly to be observ’d in the Figures of Men or Animals, as in painting Columns, Cornices, or the like.