The point of sight is the fixed point from which the spectator views the per spective plane.
Vanishing points are the points which are marked down in the picture, by sup posing lines to be drawn from the spec tator's eye, parallel to any original lines,. and produced until they touch the pic ture.
The centre of a picture is that point on the perspective plane where a line, drawn from the eye perpendicular to the picture, would cut it ; consequently it is that part of the picture which is nearest to the eye of the spectator.
The distance of the picture is the dis tance from the eye to the centre of the picture. If what has been already said and repeated, regarding the angle of 60 degrees, is understood, the spectator will never bring the picture so near to himself as to occasion the eyes to expand, indeed to strain, so as to embrace more than that angle.
The distance of a vanishing point is the distance from the eye of the spectator to that point where the converging lines meet, and after gradually diminishing all the objects which come within their di-. rection and proportion, are reduced so as in fact to terminate in nothing. All paral lel lines have the same vanishing point ; that is to say, all such as are in a build ing, parallel to each other, when not re presented exactly opposite to, and paral lel with the eye, will appear to converge towards some remote point, i. e. their va nishing point. Circles, when retiring in such manner, are represented by ellipses, proportioned to their distances : their di mensions in perspective are ascertained by enclosing them, or the nearest of them, where a regular succession is to be pour trayed within a square, which being di vided into any number of equal parts or chequers, will show all the proportions of those more remote. We trust it scarcely requires to be repeated, that the further any object is from the eye or foreground of a picture, the less it will appear in na ture, and the more it must be reduced in exhibiting its perspective.
A bird's-eye view is supposed to be taken from some elevated spot, which commands such a prospect as nearly re. sembles the plane or ichnography of the places seen. Thus the view from a high tower, or from a mountain, whence the altitudes of the several objects on the plane below appear much diminished, gives nearly the same representation as is offered to a bird flying over them ; whence the term. Some idea may be formed of
this by standing on any height, and ob serving how low those objects, which are near thereto, will appear, when compared •with those more distant, taking, however, the perspective diminution of the latter into consideration.
We shall now explain the five figures included in the Plate of Perspective. The first figure chews a base line, A B, divid. ed into eight equal parts, whose perspec tive proportions on the lines, A C and B C, are shewn, by drawing, from the several divisions, 1, 2, 3, 4, &c. on A B, rays to the vanishing points, D and E, situated on the horizon. If A C and B C were of equal length, the several squares thus made in the area, A C B, would shew trapezia regularly diminishing towards C, having their opposite angles intersecta. ble by perpendiculars from the base line, A B, and the other opposite angles inter sectable by horizontal lines parallel to A B. But A C being longer than B C, gives the whole of the trapezia a cast to wards E. This shews that the two va nishing points, while, (in this instance) they serve to intersect each other, con tain distances, considered perspectively, in proportion to their brevity ; they are under the same parallels, but the angle, B A C, being smaller than the angle C B A, causes the divisions on A C to be more extensive than those on B C, as may be seen by referring to the lesser spaces occupied by the standard on the latter. The figures 1, 2, 3, &c. correspond with those on the base line, exhibiting their due perspective distances on the lines A C and B C. It will also be observed, that as the trapezia become more distant, they become smaller, while their angles point ing towards C, and towards the base line, that is, their perpendicular angles become more obtuse, and their horizontal angles, 1. e. those on the right and left, become more acute : were it otherwise, they could not produce a diminution of the trapezia in proportion to distance.