Ante descriptionem opticam quadrati A, quod fingimus delineatum esse in papyro separatâ, ducendæ sunt duæ lineæ parallelæ; altera plani, altera horizontis, ut jam docuimus; notando in linea horizontis punctum oculi O, & punctum distantiæ E. Tum translatâ in lineam plani latitudine ac longitudine ipsius quadrati A, ita ut linea CB sit æqualis latitudini, & DC sit æqualis longitudini. Ex punctis B & C fiunt visuales BO, CO ad punctum oculi; ex puncto D fit recta DE ad punctum distantiæ. Demum ubi visualem CO secat recta DE, fit GF parallela ad CB; habesque quadratum opticè contractum.
Compendium temporis & laboris facies, præsertim in schematibus quæ abundant lineis, si chartulam in medio complicaveris, eademque utaris, ut latitudinem ac longitudinem quadrati transferas in lineam plani.
The Second Figure.
Manner of delineating a Square in Perspective.
Before the Square A, which is supposed to be drawn on a separate Paper, can be laid down in Perspective, two parallel Lines must be drawn; one of the Plan, the other of the Horizon, as is already intimated; noting in the Horizontal Line the Point of Sight O, and the Point of Distance E. Then, when the Length and Breadth of the Square A shall be transferr’d into the Line of the Plan, so that the Line CB be equal to the Breadth, and DC be equal to the Length, let the visual Lines BO, CO be drawn from the Points B and C to the Point of Sight O, and the right Line DE from the Point D to the Point of Distance. Lastly, where the Line DE cuts the Visual CO, make GF parallel to CB: and you have the Square Optically contracted, or fore-shorten’d in Perspective.
To spare Time and Pains, especially in Figures that abound in Lines, fold your Paper in the middle, and make use of it to transfer the Breadth and Length of the Square, into the Line of the Plan.
FIG. III.