DEFINITIONS.-1. An oriyinal object is any object what ever, which is rendered the subject of picture.
2. Original planes or lines are the surthees or lines of original objects.
Perspectire plane is the surface on which a picture is delineated. It may be here observed, that painters regard the frame of a picture merely as an aperture through which original n objects are see; and they therefore consider the perspective plane to be transparent, to admit of this view. It is on this account that the perspective plane is called the truiLspyrent plane.
4. Ground -plane is the earth or surface on which stand the objects to be delineated, as well as the spectator.
5. Grounddinc is the line on which the perspective plane is supposed to rest.
6. Visual rays arc those which, passing through the trans parent plane., render original objects visible.
7. Principal visual ray is that which passes through the axis or centre of the ey e, and the course of which, thereflire, front the perspective plane, is shorter than any ether, be cause it is perfectly direct. Its height above the ground-line is, of course, alway s the same as that of the eve.
8. Point of sight is that fixed point from which the spec tator looks upon the perspective plane, when any original object is delineated.
9. Centre of the picture is that point of the perspective plane which is exactly. opposite the point of sight, that is, the principal visual ray enters the transparent or perspective plane. It must, therefore, be carefully distin guished from the measured centre of any picture, as it can Dever exceed the height of the eye from the ground-tine.
10. The distance of the picture, or point of distance, is the distance between the eye and point of sight, and the centre of the picture.
11. Vanishing points are those points to which all lines inclined to the picture appear to converge, and which those lines meet when produced. Vanishing points have no place in a finished picture; they are used to facilitate drawing in persliective.
12. The horizontal line is a line parallel with the horizon, at the height of the eye,—that is, it passes horizontally through the centre of the picture.
Distance of a vanishing point is the distance upon the vanishing point on the picture to the eye of the spectator. It may also be proper to remind some, of the difference be tween a perpendicular and a vertical line or plane : a vertical line points directly to the centre of the earth ; it is therefore at right angles to the plane of the horizon, and is the same with the direction of a plumb-line: a perpendicular line is any line which is at right angles to another; it may therefore be sometimes a vertical line, sometimes a horizontal une, or in any other position, according to the direction of the line or surface with which it forms a right angle.
Methods of putting squares into perspective.—Suppose square to be traced upon the ground at some distance before us; that we find, upon admeasurement, the length of each side to be 8 feet, and that we are opposite the centre of the nearest side, at the distance of 18 feet. We know, that if we wish to obtain what is called a ground-plan of this square, we must represent it by a square upon paper, as in Figure 4, and thus we shall have its real appearance, sup posing the eye to be looking down upon it, just over its cen tre; hut looking upon it obliquely, as we have stated, and with the eye at the height of 6 feet from the ground, we are convinced, from the nature of perspective, as before ex plained, that the side nearest to us will make a longer line upon the retina than any of the rest. The question is, there fore, to obtain the true appearance of the whole square,— that is, the tiue form of the image it makes on the retina. In the first place, determine the scale to be observcd,—that is, what space shall correspond to a foot of the original. For example, suppose one-tenth of an inch; then draw a line, A B, Figure 5, eight-tenths of an inch long, and another line, it D, parallel with this base-line, at the height of six-tenths of an inch from it. Raise a perpendicular from the centre of the line A B, and the point c, in which cuts the horizontal line, will be the centre of the picture. From c on the horizontal line, set off the distance at which the square is seen, which will here be eighteen-tenths of an inch. and the point of distance D will be From A. draw the line A c; and from 11, the line 13 e; then from A, Iraw the line A D, and to the point It, in which A D intersects 13.C, (bast a line y le, parallel with the eromid-line .t at ; then will A II n It tomb the perspective out line of the square required. Let it be supposed that the above described is viewed by an eye situated opposite one of its corners, fni in Figure Draw a baseline, 13 L. as before, and on each side of any assumed point. A', set off half the measured length of the diagonal of the square, viz., half the distance between the corners y z, Figure 4. Parallel to the base-line, at the height of six-tenths of an inch from it, draw the hitriziaital line p 11 to, and raise from A. the perpen dicular A' c. F1'0111 F, draw the line F C, and front G, the line G C. On each side of the centre c, set off on the horizontal line the points of distance p u, and from each side of them draw lines to the centre of the base then from a, draw the line a e, and from It, the line z D, and the diagonal view, a b f A., of the square, will be completed.