PERSPECTIVE

(Lat. perspicio, / look through.) In the Fine Arts, the art of delineating on a given transparent plane or superficies objects as they appear to an eye placed at a given height and distance. From the definition, it is evident that, to delineate the true appearance of an object on a plane surface, it becomes nece ary to know the laws according to which the apparent linear dimensions of an object increase or decrease; and they are these generally: 1. The visual angle, or the apparent magnitude of a line, will be le the greater the distance, and the converse; 2. It will be le the more obliquely a line is viewed; 3. The law of diminution will be nearly in proportion to the'obliquity and distance conjointly. In the following diagram, let the eye of the spectator be at I, and -let E F G H be the plane on which the appearance of objects is observed. This is called the perspective plane, or plane of the picture. Now the appearance of every object to be delineated will vary according to the plane in which it stands, considered with respect to.tlie perspective plane;■ hence the particular situations of object-planes are the main points for consideration. It is manifest that any plane pa ing through the eye can only be seen on the perspective plane as a line: for the eye, having neither elevation above nor depre ion below such plane, can see no part of its surface, its edge being all that is visible tqthe eye. Of such planes two are of primary importance in perspective, viz., the horizontal plane, O K L M, parallel to the horizon; and the vertical plane, Q. P R -\, perpendicular to the last. The first intersects the perspective plane in the line L K, called the horizontal line; and the last in the line Q, P, called the vertical line. Planes not pa ing through the eye must have a direct or an oblique situation relative thereto. If the former, it must be parallel to the perspective plane, which is supposed to be placed directly before the eye; thus, the plane A B C D is a direct one, and parallel to the perspective plane G E. Of the planes situated obliquely to the eye, the most considerable is A E H D, which is called the ground plane, and is parallel to that of the horizon. From the foregoing observations, then, it appears that objects in the surfaces of the horizon tal and vertical planes cannot be seen by the eye at I, and therefore cannot be represented on the perspective plane If O B he tin object in the direct plane, and from the extreme points O and B the visual rays O 1, B I be drawn to the eye at I, they will pa through the perspective plane in the points o and b, and by joining them the right line o b will be the representation of the line or object O B on the perspective plane. In like manner, the representation of O A is o a, and b r and a » will be the representation of U R and A N, and consequently rAonofRBAN, , &.c. So all lines parallel to A B or C D in the object-plane will have their perspective lines parallel to a ft and c d in the picture on the perspective plane; and, however the object plane A C may be divided, their representations on the perspective-plane or planes of the picture will divide that in a similar mannor. Any point B in a direct plane lias the same ratio of distance from the horizontal and vertical planes as its perspective has from the horizontal and vertical planes, viz., that of the distances of the planes from the eye, from the nature of similar triangles: hence the forms of objects on the perspective plane, when they are presented in a direct view, may be drawn with facility. The last species of plane whereon it is "apposed we may view the natural object is that of the grou-'d itself, as A D H E. above which the eye has more or lest <in elevation; as t P, equal [s. 934]