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 ori ginal 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, EL, &c. are the vanishing lines of their several original planes, R S T V. "The vanishing point" of an original line is that VOL. IV. 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: 1. The removal of the object to a greater distance from the eye, whereby the rays of light which it reflects are less vivid, and the colour becomes more ́diluted, and tinged in some measure, by the faint bluish cast, or dimness or haziness of the body of air through which the rays pass. 2. The greater or less degree of light with which the object is enlightened; the same original colour, at an equal distance from the eye, having a different appearance in the shade, proportioned to the degree, from what it has in the light. 3. The colour of the light that falls on it, resulting from the reflection of coloured light from an adjacent object, or its passage through a coloured medium, which will exhibit a colour compounded of the original one of the object, and itself. 4. The position of the surface of the object, or of its several parts, with respect to the eye; the colours that front the eye appearing more lively and distinct than those that are seen obliquely. 5. The closeness or openness of the place where the object is situated; the light being much more variously directed and reflected within a room than in the open air. 6. The natural reflection of light by some original colours, in greater proportion than others, though equally exposed to its rays; whereby the degradation of these, at any given distance, will be different from that of those others. From these several causes it happens that the colours of objects are seldom seen pure and unmixed, but generally arrive at the eye broken and softened by each other; and therefore in painting, where the natural appearances of objects are to be described, all hard or sharp colouring should be avoided.. A painter who would succeed in aërial perspee tive, ought carefully to study the effects which distance in its different degrees, or accidental colours of light, have on each particular colour; so that in a picture of various coloured objects, he may know how to give each original colour its proper diminution or degradation, As all objects in a picture are proportioned to those placed in front; so in aërial perspective, the strength of light, and the brightness of the colours of objects close to the eye, that is, in the front-ground of the picture, must serve as a standard with respect to which all the same colours, at different distances, must have a proportional degradation in like circumstances. In order to give any colour its proper degradation in proportion to its distance, it ought to be known what the appearance of that colour would be, were it close to the eye, regard being had to that degree of light which is chosen as the principal light of the picture: for if any colour is made too bright for another, or for the general colouring of the picture, it will have a glaring appearance, seem to start out of the picture, and throw a flatness and damp upon the rest of the work; or, in the technical phrase, the brightness of that colour will kill the rest. PERSPIRATION, in medicine, evacuation of the juices of the body through the pores of the skin. Perspiration is distinguished into sensible and insensible; the former of which is vulgarly called "sweating," and the latter "perspiration” only. |