PERSPEC"TIVE, the science which tenches the representation of an object m objects on a definite surface so as to a fleet the eye when viewed front a given point, in the same manner as the object or ob jects themselves. Correctly defined, a perspective delineation is a section, by the plane or other surface, on which the delineation is made, of the cone of rays proceeding from every part of t he object to the eye of the spectator. It is inti mately connected with the arts of design, and is indispensable in architecture, en gineering, fortification, sculpture, and generally all the mechanical arts; but it is particularly necessary in the art of 1 painting, as without is correct observance of the rules of perspective no picture can hare truth and life. Perspective alone enables us to represent foreshortenings with accuracy, and it is requisite in de lineating even the simplest positions of ob jects. Suppose we view a point situated beyond an upright transparent plane, as a glass window, the spot where a straight line from the eye to this point will go through the window is the perspective representation of it : for the eye views all objects by means of rays of light, which proceed from it, to the different points of the object, in straight lines. Let us then imagine a spectator to be looking at a prospect without doors, from within, through a glass window ; he will per ceive not only the vast extent which so small an aperture will admit to be seen by his eye, but also the shape, size, and situation of every object upon the glass. If the objects arc near the window, the spaces which they take upon the glass will be proportionably larger than when they are at a greater distance ; if they are parallel to the• window, then their shapes upon the glass will be parallel also; but if they are oblique, then their shapes will be oblique, and so on. And he will always perceive, that as he alters the situation of his eye, the situation of the objects upon the window will be altered also : if he raises his eye, the objects will seem to keep pace with it, and rise higher upon the window ; and the contrary if he lowers it. And so in every situation of the eye, the objects upon the window will seem to rise higher or lower ; and con sequently the depth of the whole prospect will be proportionably greater or less, as the eye is elevated or depressed : and the horizon will, in every situation of the eye, be upon a level with it : that is, the imaginary line which parts the earth and sky will seem to be raised as far above the ground upon which the spectator stands as his eye is. Now suppose the person at the window, keeping his head steady, draws the figure of an object seen through it upon the glass with a pencil, as if the point of the pencil touched the ob ject: he would then have a true represen tation of the objectin perspective, as it ap pears to his eye : for as vision is occasion ed by pencils of rays coming in straight lines to the eye from every point of the visible object, it is plain that, by join ing the points in the transparent plane through which all those pencils of rays respectively pass, an exact representation must be formed of the object, as it ap pears to the eve in that particular posi tion, and at that determined distance.
And were pictures of things to be always first drawn on transparent planes, this simple operation, with the principle on which it is founded, would comprise the whole theory and practice of perspective. Perspective is divided into two branches, linear and ecrial. Linear perspective has reference to the position, form, wag. nitude, Ito. of the several lines or con tours of objects, ,1c. The outlines of such objects as buildings, machinery, and most works of human labor which consist of geometrical forms, or can be re duced to them, may be most aezturately ob tained by the rules of linear perspective, since the intersection with an interposed plane of the rays of light proceeding from every point of such objects may be obtained by the principles of geometr).
Linear perspective includes the various kinds of projection; as scenographic, orthographic, ichnographic, stereograph ic projections, perspective teaches how to give due diminution to the strength of light, shade, and colors of objects according to their distances, and the quantity of light falling on them, and to the medium through which they are seen.—Perspective plane, the surface on which the object or picture is delinea ted, or it is the transparent surface or plane through which we suppose objects to he viewed ; it also termed the plane of pro. jection, and the plane of the picture.— Parallel perspective is where the picture which is supposed to be so situated, as to bo parallel to the side of the principal object in the picture ; as a building, for instance.—Oblique perspective, is when the plane of the picture is supposed to stand oblique to the sides of the object represented ; in which ease the represen tations of the lines upon those sides will not be parallel among themselves, but will tend toward their vanishing point. —Isontetrical perspective, a kind of por speetive on the principles of orthographic projection invented by Professor Parish of Cambridge, by which solids, of the form of rectangular parallelopipeds, or such as are reducible to this form, can be represented with their three pair of planes in one figure, which gives a more intelligible idea of their form than can be done by a separate plan and elevation. At the same time, this method admits of their dimensions being measured by a scale as directly as by the usual mode of delineation. As applied to machinery, it gives the elevation and ground plait in one view. It is considered for such pur poses, to be preferable to the methods in common use, as it is easier and simpler in its application.