Practical perspective, or, Perspective made easie teaching [brace] by the opticks, how to delineate all bodies, buildings, or landskips, &c., by the catoptricks, how to delineate confused appearences, so as when seen in a mirror or pollisht body of any intended shape, the reflection shall shew a designe, by the dioptircks [sic], how to draw parts of many figures into one, when seen through a glass or christal cut into many faces : usefull for all painters, engravers, architects, &c., and all others that are any waies inclined to speculatory ingenuity / by Joseph Moxon ...

CHAPTER. II. (Book 2)

Containing Definitions used in this Art:

DEFINITION I.

The Base. Is the Plain whereon any Object is placed, which for Example sake we will suppose to be a huge great plain Field, or if you will 〈◊〉〈◊〉 the flat Floor or Pavement of some Hall 〈◊〉〈◊〉 Chamber. As in Defin. VI. of this Chapter, f g •••• is the Plain: This Plain or Floor we shall all along hereafter in this Book call the Base.

DEFINITION II.

The Height. Is the Perpendicular distance be∣tween the Base and the Ey, or which is all one The Height of the Visual point above the Base, be∣cause the Ey lies alwaies in the Horizontal line, as well as the Visual point. Thus the height from

a to b in the aforesaid Figure is called the Height of the Ey.

DEFINITION III.

The Visual Point. This point is not that point the Ey is placed in, (as most young Studients in this Art are apt to think:) But is a point in the Horizontal line wherein all the Occular raies u∣nite. For Example, If you look on a long straight Brick wall, you will perceive the several laies of Mortar between the several courses of Bricks seem to incline towards each other, as if they ran into one common Center: Or as if you were in a long straight Gallerey where the two Sides, the Floor, and Ceiling, seem to incline and touch each other in a point or common Center, which point reflected on a Glass erected perpendicularly on the Base, is called the Visual point. This you may experiment if you lay the Muscovie Glass in Definition VI. flat upon the Ob∣ject a b c d, for then so much of your Scenogra∣phick Figure as is made of Visual raies, as the lines a b and c d, shall run into a point opposite to the point e: which point elevated to the height of the Ey is the Visual point, as a foresaid. This point is by some Authors called also the Center: and by others the Principal Point.

Note, that where an Object is reflected, it is al∣so reverted, as you may see in Looking Glasses and cleer water. And for this reason it is that Orthographick Figures are inverted in their Sce∣nographic. As you may see in most of the Fi∣gures throughout this Book.

DEFINITION IIII.

The Hozriontal Line. By the Horizontal line is

not meant the Base or Line of Earth, though in Astronomy &c. it is generally so taken; But it is a Line proceeding from the Ey, (which in Men of a common size is placed about five foot above the Base or Ground they stand upon,) into the Visual point, and is Parallel to the Base, as the Line b c in Chap. 1. Defin. 8. is the Horizontal Line.

DEFINITION V.

The Distance. The Distance is that space on the Base contained between the Glass and the point in the Base that lies directly under the Ey. As from the point a in the Base, to o at the Glass is called the Distance. Note that in many Fi∣gures two points of Distance is required, as in O∣peration 10, 15, 16, 17, 18, and several others throughout this Book.

DEFINITION VI.

The Section Is a Plain of transparent matter erected Perpendicularly on the plain of the Base, and stands directly before you, that is is Parallel to a straight line imagined to pass through the convex points of your two Eys: As is this Plain of Muscovie Glass erected before the Fi∣gure

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e. This Plain is by some Authors called the Section, by others the Table, and by others the Glass. They that call it a Section call it most properly, Because it cuts or intersects the Plain whereon it stands, as here it cuts the Plain or Base a b c d. And they that call it a Table, call it so allusively as they do other Planis whereon any thing is ei∣ther painted or written. But they that call it a Glass have some good reason for it too, Because we know as yet no other matter so apt to de∣monstrate the changing of an Orthographick Fi∣gure into a Scenographick as Glass: And there∣fore though I confess the word Section to be more proper than the word Glass, yet because (as I said before) my design is Practice: I shall all a∣long in this Book (when I have occasion to name this Plain) call it Glass and not Section.

This Glass or Section is absolutely necessary to be understood by all young Studients in the Art of Perspective. Because (as by what hath al∣ready been said) without knowing what it is they cannot know what Perspective means: Neither can they know the reason why the Scenogra∣phick Figure alters from the Orthographick, or one Scenographick Figure from another. But by rightly understanding this Glass they may ea∣sily perceive the Various affections of Sceno∣graphick appearences. For first, If the Glass be placed near the Visual point and far from the Object, the Figure appearing therein will be ve∣ry small: Because all the raies which inclose both Orthographick and Scenographick Fi∣gures though wider asunder at the Object run into the Visual point as into a common Center: As you may see in the first Figure, where the Cir∣cle a appears much smaler in the Plain or Glass d e f g, which stands neer the Visual point and

farther from the Object than in the Glass h i k l, which stands farther from the Visual point and neerer the Object. Secondly, If the Visual point be placed higher, though at the same distance from the Object the greater the Scenographick form appears: As you may see if you place the Visual point at m, and delineate the same Circle Scenographically as d, it becomes bigger in length and bredth; for the Visual raies being e∣levated the several Perpendiculars erected on the Glass intersect them in wider distances, Be∣cause futher from the Glass. As you may see in the Scenographick Circle d. Thirdly, If this Glass were set oblique to the Plain, the Figure would again alter; For if it Incline to the Visu∣al point the Scenographick appearence of this Circle (for Example,) shall on the Glass lengthen between the Visual point and the Object: Because the Plain of the Glass gathers in more of the Vi∣sual raies: And if the Glass Recline from the Vi∣sual point the Scenographick appearence shall grow rounder and more like the Orthographick Figure. But if the Glass be placed parallel to the Plain the Object lies on, the Scenographick and Orthographick Figure shall appear the same; Because the Figure is not seen in Section. And all this you may easily experiment by erecting the Muscovy Glass in Defin. VI. of this Chap. be∣tween your Ey and the Figure: For if you keep your Ey steddy in one point, and with a black-lead pencil trace about the edges of the same Fi∣gure as you find it on the Glass, you will have the true Scenographie of that Figure: As you may try by comparing it afterwards with the same Figure drawn by the Rules of Perspective, as they are taught in this Book.

DEFINITION VII.

The Visual Raies, or Diametrals, are those lines which proceed from the Visual point through a Glass to any point either above or below the Horizon. As in Oper. II. the line c f e is a Visual Raie or a Diametral: So is the lines c k g, and c l f in Oper. III.

DEFINITION VIII.

Lines of Distance, or Diagonals, are those Lines that are drawn from the point of Distance, to any point either above or below the Hori∣zon. As in Oper. III. the line b k i, is a line of Di∣stance: So is the lines b d l h.

DEFINITION IX.

The Object. The Object is that Figure, Body, or Building you intend to describe in Perspe∣ctive. As in Oper. II. the point d is the Object: So are the Squares d e f g, and l m n o, in Oper. IIII.