pace with his eye, and rise higher upon the win dow; and the contrary, if he places it ever so low. And so in every situation of the eye, the objects upon the window will seem to rise higher or lower; and consequently, 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 horizontal line, or that 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 removed from the same place.

Let us now suppose two planes A B GD, a b gd, (fig. 3), of the same height and parallel to each other, one to pass through the eye E, and the other through any line as a b, and both to be perpendicular to the ground A B a b; and let us imagine another plane G D g d to be laid upon these two planes A B G D, a b g d, as in the figure, it will be evident that this plane G D g d, is parallel to the ground A B a b, because it lies upon two planes, ABGD, abg d, of the same height. Now if we suppose the plane G D g d to be continued to an infinite distance, and the line g d to represent a part of the real horizon, and then imagine a picture KM LH to be placed between the eye E, and the horizon gd; then its section H L, with the horizontal plane G D g d, will be the indefinite rerepresentation of the horizon g d, upon the picture; and this representation is called the horizontal line. Now since all objects which lie flat upon the ground, or are parallel to it, seem to vanish into the real horizon, therefore the repre

sentations of all such objects upon the picture must vanish into this horizontal line, because it is the perspective representation of the real horizon; and, for the same reason, the ground, or whole extent between the eye and the real horizon, will not appear to lie flat, but to rise upwards. For let E be the eye, K M a b the ground, and K M a b the utmost extent which the eye can distinguish; now, I say, the ground will not appear to lie flat, like K M a b, but to rise upwards, like K Mg d, till it cuts the plane G D g d, that is drawn through the eye E, parallel to the original or ground plane A Bab; and the section g d, which the planes K M g d and G D g d make with each other, will represent the real horizon. And, as before, if we suppose a picture, K M L H, to be fixed between the eye and the said horizon; then the section HL, which the picture makes with the parallel plane GD g d, will be the indefinite representation of the horizontal line upon the picture; because the rays of light, g E, d E in their passage from the section g d, will cut the picture in the line H L.

From hence, then, we may see the grand principle upon which perspective depends; namely, in finding the lines and points into which objects seem to vanish upon the picture: and the chief difficulty in perspective only requires to have a clear idea of the nature and property of vanishing lines and vanishing points, and a few other requisites which he may partly conceive by what has been said already, and by considering, that as the horizontal line HL is produced by means of the plane G D g d, which passes through the eye parallel to the ground, or original plane; so, in the very same

manner, all other vanishing lines are determined, namely, by imagining a plane to pass through the eye parallel to those planes whose representations are required upon the picture. Again, in regard to vanishing points; these are determined by drawing lines from the eye, parallel to the original lines, till they cut the picture; in order to which we must always suppose these lines to lie in some plane, and then having found the vanishing line of that plane, the vanishing point of any line in that plane may be found also. From hence we may observe, that the horizontal line is of the same nature with any other vanishing lines, and differs from them only in being more useful; because many more objects are perpendicular and parallel to the picture, than oblique with it: and therefore the great stress, which hath been laid upon this line by most writers, is not so very significant as they apprehended; for, in some cases, it is of no use at all in a picture. For let us consider a little : if vanishing lines upon the picture are always to be produced by planes passing through the eye parallel to the original figures, then no original plane can have its vanishing line in the horizontal line, unless it is parallel to the ground; but, if any object be obliquely situated with regard to the ground, then the plane, which is to pass through the eye parallel to the original, in order to determine its vanishing line, will be oblique with the ground also; and therefore it cannot pass through the horizontal line, but will be either above, below, perpendicular to it, or cross it in an oblique manner. The situations of objects may be reduced under the following heads. 1. When they are perpendicular to the

picture or parallel to the ground, that is to the plane of the horizon. 2. When they are parallel to the picture, or perpendicular to the ground. 3. When they are obliquely situated, both as to the picture and the ground.

The following definitions should be carefully attended to. "The point of sight," is that point, where the spectator's eye is placed to view the picture. Thus E, in the foregoing figures is the point of sight, or place of the eye.

If from the point of sight E, a line E C be drawn perpendicular to the picture, the point C, where that line cuts is called "The centre of the

picture."

"The distance of the picture" is the length E C, which line is drawn from the eye, perpendicular to the picture.

By the phrase "Original Object" is meant the real object, whose representation is sought whether it be a line, point, or plane figure: and by " Original Plane" is meant that plane upon which the real object is situated: thus the ground plane O P, (fig. 1.) is the original plane, and R S T V the original object.

If an original line S R be continued so as to cut the picture, the point F, where it cuts the picture, is called the intersection of the original line or its intersecting point."

"The vanishing line" of an original plane is that line where a plane passing through the eye, parallel to that original plane, cuts the picture; thus, HL, HL, &c. are the vanishing lines of their several original planes, R S T V.

"The vanishing point" of an original line is that

point where a line drawn from the eye parallel to that original line cuts the picture.

If from a point of sight E, a line E C be drawn perpendicular to any vanishing line H L, the point C, where that line cuts the vanishing line, is called "The centre of that vanishing-line." "The distance of a vanishing line" is the length E C, which is drawn from the eye perpendicular to the said line. PERSPECTIVE, aerial, the art of giving a due diminution or degradation to the strength of the light, shade, and colours of objects, according to their different distances, the quantity of light which falls on them, and the nature of the medium through which they are seen. As the eye does not judge of the distance of objects entirely by their apparent size, but also by their strength of colour and distinction of parts; so it is not sufficient to give an object its due apparent bulk according to the rules of stereography, unless at the same time it is expressed with that proper faintness and degradation of colour which the distance requires: thus if a man at a distance were painted of a proper magnitude for the place, but with too great distinction of parts or too great strength of colour, it would appear to stand forward, and seem proportionally less, so as to represent a dwarf situated near the eye, and out of the plane on which the painter intended it should stand.

By the original colour of an object is meant, that colour which it exhibits to the eye when duly exposed to it in a full, uniform light, at such a moderate distance as to be clearly seen: now this colour receives an alteration from many causes, the principal of which are the following: