FIGURA PRIMA,
Explicatio linearum Plani & Horizontis, ac Punctorum Oculi & Diftantiaj. T principia Terfpettiva facilius intelligas, pono tibi ob oculos Templum, in cujus in- teriori facie, prater c cetera, pingendum fit aliquid ad c PerJpettivam pertinens. Tern- pli hujus vejtigium geometrkum eft A, elevatio geometrica in longum eft B, in la- tum eft C. In A eft locus Hominis a/picientis line am D E, cut paries pingendus in- cumbit. In B idem Homo ex eddem diftantia intuetur lineam F G, qua refert ele- vationetnparietis. In figura C fupponimus Homiuem confiftere e regione ipfius pa- rte t is : eafdemque proportions menfurarum tranftatas ejfe ex vero pariete in figu- ram C, qua ipjum in parvo reprafentat. Trima ergo linea H I dicitur linea terra vel plani, ex qua incipit, eidemque in- cumbit Aidificium. Secunda linea N O N priori parallela, dicitur horizontalis, in qua ponitur O punttum oculi, & N punttum diftantia. T)uo autem puntta diftantia a nobis pofitafunt, ut unum adhibeas ex qua parte volueris ; nam ad figuras opticb tontrahendas fujficit unum punttum diftantia : nee fieri pot eft ulla optica delineatio, quin primo loco defignentur dua parallela, una plani feu terra, altera horizontis, notando in lined horizontis punttum oculi, feu opticum, & punttum diftantia. Torrb unam eandemque rem triplici Schemate reprafentare oportuit, tit videas, locum ex quo afpicienda eft figura C ejfe punttum N unius ex rettis N O, quam concipere debemus veluti normaliter infix am in O ; ac diftantiam inter O SJ N eandem ejfe deb ere cum diftantia inter A & D E, inter B © G F. In pitturis multum fpatii occupant ibus, punttum oculi poni folet in medio linea horizontalis : at que ubi altitudo pittura fit major latitudine, diftantia NO fiet aqualis altitudini. Si latitudo pittura fit major altitudine, diftantia NO fiet aqualis latitudini ; it a enim unico intuitu totum pittura fpatium comprehendi poterit. Torrb quamvis eadem dijtantia diverfo modo adhibeatur in vejtigio A, & in elevationibus B & C ; nihilominus fiettiones vijiialium cum pariete ueftigii A, & elevationis B, omninb confpirant cum fettionibus vifualium figura C. Jam fi velimus Ut fpettatori in A & B paries depittus videatur diftare a lineis DE S G F, quanta eft Ion* qitudo quadrati P, cujus elevatio eft Q_; ex punttis A & B fiant vijuales ad puntta extrema quadrat i, notando fiettiones vijualium cum pariete D E & G F, qui ab aliis vocatur velum, vitrum diaphanum, jettio, tela, vel ta- bula. Invenies autem, line as RS acTV ejfe a quale s, ac fimi liter lineas XZ £sYK; & fie de aliis.
The Firft Figure.
Explication of the Lines of the T/an and Horizon, and of the Points of the Eye and of the Diflance.
HAT you may the better underftand the Principles of Perlpedive, here is prefented to your View a Temple, on the inner Wall of which, amongft other Things, one would paint fomething in Perfpecl:ive. The Geometrical Plan of this Church is A, the Geo- metrical Elevation, or Upright, lengthwife is B, breadthwife is C. In A is the Place from whence a Man beholds the Line DE, which is the Plan Of the Wall that is to be painted : in B the fame Man, from the fame Diftance, looks upon the Line FG, that reprefents the Elevation of the Wall. In Fig. C, the Man is iuppos'd to fland oppofite to the faid Wall ; and this Figure contains in little, the very fame Propor- tions of Meafures transferr'd from the real Wall. The firft Line therefore HI is call'd the Ground-line, or Line of the Plan, at which | the Edifice begins, and on which it ftands. The fecond Line N O N, parallel to the former, is call'd the Horizontal Line, wherein is plac'd O the Point of the Eye, and N the Point of the Diftance. Two Points of Diftance are here laid down, that you may make ufe of which you pleafe : for that on one Side only is fufficient for the fore-fhort'ning Figures in Perfpective : Neither can any Optick Delineation, or Perfpective, be defcrib'd, without firft making two Parallels ; one of the Plan or Ground-line, the other of the Horizon ; marking, in the Line of the Horizon, the Point of the Eye, or Sight, and the Point of Diftance. It was thought befides expedient to put one and the fame Thing into three Schemes or Defigns, to let you fee, that the Place, from which the Figure C is to be look'd upon, is the Point N, one of the right Lines NO, which muft be conceived as fixt at right Angles into O ; the Diftance O N being the fame as that between A and DE in the Plan, or between B and GF in the Upright. In Pictures taking up a great deal of Room, the Point of Sight ought to be made in the middle of the Horizontal Line ; and where the Height of the Picture happens to be greater than the Breadth, the Diftance N O muft be made equal to the Height. If the Breadth of the Picture exceed the Height, the Diftance N O muft be made equal to the Breadth : For fo will the Extent of the Picture be the better comprehended, or receiv'd, at one View. And altho the fame Diftance may feem to be ufed in a different manner in the Plan A, and in the Elevation B, from what it is in C ; neverthelefs the Sections of the vifual Rays, with the Wall of the Plan A, and of the Elevation B, have a perfect Correfpondence with the Sections of thofe of the Figure C. Now, if to the Spectator in A and B, we would have the fartheft part of the Work feem to recede from the Lines DE and GF, as much -as the Square P does, whofe Elevation is Q_; draw from the Points A and B, the vi- fual Rays to the extreme Points of the Square P and Qj noting the Sections they make with the Walls D E and G F ; which by fome is call'd the Veil, Tranfparent Medium, Section, Cloth, or Table : and you'll find R S equal to T V, X Z equal toYK; and fo of the reft. 43
