430 



PERSPECTIVE. 



jects diilers esentially from Umt which teaches the 

 gradation of colours according to tlie relative distance 

 of objects. Hence perspective is divided into mathe- 

 matical, or linear perspective, and the perspective 

 our, or aerial perspective, Both are of the 

 greatest importance to painters, architects, sculptors, 

 &c. Without a correct observance of the rules of 

 perspective, no picture can have truth and life. 

 I't r-j't 'dive alone enables us to represent foreshort- 

 enings with accuracy, and it is requisite in deline- 

 ating even the simplest positions of objects. As 

 limir, therefore, as its rules were unknown, the act 

 of drawing; necessarily remained in its infancy. This 

 art has been most cultivated in modern times ; yet 

 the paintings found in Herculaneuni prove that the 

 ancient (iret-k painters were acquainted with it in 

 some measure. 



The contour of an object, drawn upon paper or 

 canvass, represents nothing more than such an inter- 

 section of the visual rays, sent from the extremities 

 of it to the eye, as would arise on a glass put in the 

 place of the paper or canvass. Now the situation 

 of an object at the other side of a glass being given, 

 the delineation of it on the glass itself depends entire- 

 ly on the situation of the eye on this side of the 

 glass ; in other words, on the rules of perspective. 

 Suppose a spectator to be looking at a prospect with- 

 out doors from within through a glass window ; he 

 will perceive the shape, size and situation of every 

 object visible upon the glas"s. If the objects are 

 near the window the spaces they occupy on the 

 glass will be larger than when they are at a greater 

 distance ; if they are parallel to the window, their 

 shapes upon the glass will be parallel likewise ; if 

 they are oblique, their shapes will be oblique ; and 

 so on. 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 rise higher upon the window, and the con- 

 trary if he lowers it. 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 sup- 

 pose 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 object ; he would then have 

 a true representation of the object in perspective, 

 as it appears to his eye. To every person who 

 possesses a general knowledge of the principles of 

 optics, this must be self-evident ; for as vision is 

 occasioned by pencils of rays coming in straight 

 lines to the eye from every point of the visible ob- 

 ject, it is plain that, by joining the points in the 

 transparent plane through which all those pencils 

 respectively pass, an exact representation must be 

 formed of the object, as it appears to the eye in that 

 particular position, 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. As 

 this, however, is far from being the case, rules must 

 be deduced from optics and geometry for drawing 

 representations of visible objects on opaque planes ; 

 and the application of these rules constitutes what 

 is properly called the art of perspective. There are 

 used in perspective a certain number of terms pecu- 

 liar to the art, definitions of which are necessary to 

 an intelligent use of them. The original object 

 is that wliich is made the subject of the picture. 

 Original planes, or lines are the surfaces or lines of 

 original objects. The point of view is the situation 

 of the eye. The point of sight is the point in the 



perfective plane which is nearest to the eye. As 

 l;ir tis the picture is concerned, these two points 

 coincide, so that some authors h;ive used them indis- 

 criminately one for the other. The point of sight 

 is also called the centre of the picture. A visual 

 ray is a line from the object to the eye. If the ob- 

 ject is a point, there is but one visual ray ; if it is a 

 line, the visual rays form a triangle ; if it is a square, 

 they form a pyramid ; if a circle, a cone, &c. The 

 principal visual ray is that from the nearest point in 

 the picture, or point of sight. The perspective 

 plane is the surface on which the picture is deline- 

 ated ; or it is the transparent surface through wliich 

 we suppose objects to be viewed. The directing 

 plane is a plane supposed to pass through the eye of 

 the spectator, parallel to the perspective plane. 

 The ground plane is the earth, or the plane surface 

 on which the spectator and objects are situated. 

 The horizon, or horizontal plane, is one parallel to 

 the ground plane, and at the height of the spectator's 

 eye. The horizontal line is the intersection of the 

 picture, or perspective plane, with the horizontal 

 plane. The ground line is the intersection of the 

 perspective plane with the ground plane ; or it is 

 the line on which the picture is supposed to stand. 

 The perpendicular is a line on the perspective plane, 

 drawn through the point of sight, perpendicular to 

 the ground line and horizontal line. The points of 

 distance are points on the perspective plane, set off 

 from the point of sight, sometimes on the horizontal 

 line, and sometimes on the perpendicular, at the same 

 distance from the point of sight that the eye is sup- 

 posed to be at from the perspective plane. 



Projections. The projections of a body are the 

 different modes by which it may be delineated on a 

 plane surface. Scenographic projection represents 

 objects as they actually appear to the eye at limited 

 distances. Orthographic projection represents ob- 

 jects as they would appear to the eye at an infinite 

 distance, the rays which proceed from them being 

 parallel, instead of converging. The shadow which 

 a body casts in the rays of the sun may be consider- 

 ed as an orthographic projection. In this projec- 

 tion, lines which are parallel in the original are 

 parallel in the picture, and do not converge to any 

 vanishing point. Their comparative length, also, is 

 not affected by difference of apparent -distance. 

 Orthographic projection is much used in delineating 

 buildings, machinery, &c., because those parts of the 

 drawing which are not foreshortened maintain their 

 true relative size, so that measures can be taken 

 from them. The term ichnographic projection is 

 sometimes used to express the horizontal deline- 

 ation, or ground plan, of an object. A bird's eye 

 view is a scenographic or orthographic projection, 

 taken from an elevated point in the air, from wliich 

 the eye is supposed to look down upon the objects. 

 Geometrical and mechanical methods will enable a 

 person not previously conversant with the art to ob- 

 tain correct perspective representations of any object. 

 But by long practice in drawing from nature a certain 

 tact is acquired by painters, which enables them, by 

 the accuracy of the eye and judgment alone, to make 

 correct views of objects, without the aid of any 

 computation or mechanical process. Thus miniature 

 painters produce the nicest resemblance of the human 

 countenance, in any position, with no other guide 

 than the faculty obtained by experience of estimating 

 the exact shape and proportion which each part of 

 the original should bear upon the picture. 



Equally important with the linear perspective is 

 aerial perspective, though not founded on equally 

 demonstrable rules. It teaches how to judge of the 

 degree of light which objects reflect in proportion to 

 their distance, and of the gradation of their tints in 



