PERSPECTIVE (Fr. perspective, from Lat. perm/wet/is, p.p. of perspierre, to see through, from per, through specere, to see). The art of representing natural objects upon a plane sur face in such manner that the representation shall affect the eye in the same 'Way as the objects themselves. The distance and position of objects affect both their distinctness and apparent form, giving rise to a subdivision of perspective into linear perspective. which is strictly seientifie and a branch of applied geometry. and which, as its name denotes, eonsiders exehisively the effeet prothwod by the position and dist:tiny of the ob server upon the a pp:111'1a form and grouping of objects; and perspective. which confines itself to their dist inet nes., as modi tied by distance and light, and belongs to the non-scientilie sphere of pictorial representation. After the 'scope' (i.e. the number of objects to be introduced. and the distance at which they are to be viewed) of the picture has been determined, and before the de sign is eommeneed. it. is necessary to draw upon perspective plan three lines: ( I ) The base line, or ground line, which limits the sketch toward the operator. and is the base line of the picture. (2. Tl e lwri:ontal line, which represents the or.linary position of the sensible horizon. The height of the horizontal line is about one-third of the height of the picture. when the sketcher is placed at or a little above the level of the hori zon: but it may rise in a degree corresponding to his increase of elevation till it reaches near to the top of the perspective plan. The general rule is to have a high horizontal line when the view is taken, or supposed to be taken, from an emi nence; but when the station is on a level, either actual or assumed, the horizontal line must be low. The horizontal line in nearly all cases is supposed to be level with the spectator's eye. (1) The rertical line, which is drawn from the supposed position of the sketcher, perpendicular to the ground and horizontal lines, meeting the latter in a point which is called the point of sight, or centre of the picture. The vertical line has no representative in nature, and is merely a mechanical adjunct to the construction of the picture, all vertical lines in nature being parallel to it in the picture. The point of sight, being the point directly opposite to the observer, is often placed in the centre of the picture; but we very frequently find it on the right or left side, though always, of course, on the horizontal line. All lines which in nature are perpendicular to the ground line, or to a vertical plane which is raised upon it as a base, meet in the point of sight, which is thus their vanishing point (see the line of the tops and bottoms of the pillars in Fig. 1 ). The points of distance are two points
in the horizontal line on each side of the point of sight, and in a 'direct' sketch are at a distance from it equal to the horizontal distance of the sketcher's eye from the ground line. The equality of distance of these points from the point of sight is not, however, necessary, as it occurs only in those cases where the lines, of which the points of distance are the vanishing points, are inclined (in nature )at an angle of to the base line; but, in all cases, the two points of distance are about twice as far apart as the eye is from the picture. One important use of the points of dis tance is to define the distance of objects in a row (Fig. 1) from each other. For this purpose. two points of distance are not necessary, as, when the position of one pillar is found, that of the op posite is at once obtained by drawing a line parallel to the base or ground line.
There are many other groups of parallel lines in a picture which have different situations, and therefore different vanish ing points. Such lines with their vanishing points (called, for distinction's sake. accidental points) are represented in Fig. 2. If the accidental point is above the horizontal line, it is called the accidental point aerial—if below, the accidental point terrestrial: and a little consideration makes it evident that these points may or may not be situated within the plane of the picture. such are the points and lines neces sary for the construction of a plan in true per spective: and from the above explanation, we may deduce the two general principles: (1) That all parallel straight lines in nature are tio longer parallel when projected on the perspec tive plane, but meet in a point which is called the vanishing point, and is some one of the three above described, unless these lines happen to be also parallel to the ground line or the vertical line, in which case they remain parallel when transferred to the picture; and (2) that since the bodies drawn below the horizontal line are seen as if from above, those above as if from below, and those to the right and left of the point of sight as if observed from the left and right, it follows that straight lines which in the picture are above the hori zontal line lower them selves, and those below raise themselves to it: those to the left, following the same law, direct them selves to the right, and vice versa.