Fig. lxxiii.
FIGURA Septuagesimatertia.
Aliud vestigium theatri, ubi de modo inveniendi ejus punctum.
Si pingendæ sint scenæ theatri alicujus antea constructi, delineare oportebit vestigium geometricum ex ipso erutum, (ad formam vestigii quod cernis in hac paginâ) ut inveniatur longitudo theatri, seu distantia quam ejus punctum habet à puncto A: id autem nullo negotio fiet, accipiendo distantias BC inter primos canales, & DE inter ultimos, ac ducendo visuales MO, NO: nam theatrum habebit longitudinem AO, ac punctum perspectivæ in vestigio theatri erit O. Præterea scire oportebit longitudinem & latitudinem canalium, eorumque numerum, distantias, & flexus; ac præcipuè curandum est, ut licet sint obliqui ad lineam MN, sint invicem paralleli in unoquoque latere, ac singuli tangant lineas MO, NO. Jam si rectæ AO fiat æqualis recta FA, in F erit punctum distantiæ: adeoque si theatrum juxta methodum à nobis tradendam depictum fuerit, spectatori qui consistat in F apparebit veluti tabula picta juxta leges perspectivæ, posita in A.
The Seventy-third FIGURE.
Another Plan of a Theater, with the Method of finding the Point of Sight therein.
If it be requir’d to paint the Scenes of some Theater already built, the Geometrical Plan thereof must first be carefully drawn, (as you see, for Example, in this Plate) that the Length of the Theater may be found; or the Distance of its Point from that of A; which is easily done, by taking the Interval BC of the first Grooves, and DE of the latter; and drawing the Visuals MO, NO: for AO is the Length of the Theater, and the Point of Sight, or Perspective, therein, is O. Moreover the Length and Breadth of the Grooves must be known, as also their Numbers, Distance, and Obliquity; and especial Care must be taken, that though they be oblique to the Line MN, that on each Side they be Parallels between themselves, and that they all touch the Lines MO, NO. If you then make AO equal to FA, the Point of Distance will be F; and if the Theater be painted according to the Rules hereafter given, it will appear to him that views it from F, as a regular Piece of Perspective plac’d in A.