PERSPECTIVE, aerial, is 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 medium through which they are seen.
As the eye does not judge of the dis tance of objects entirely by their apparent size, but also by their strength of colours, and distinction of parts : so it is not suf ficient to give an object its due apparent bulk according to the rules of stereogra phy, unless at the same time it be express ed with that proper faintness and degra dation of colour which the distance re quires. Thus if the figure of a man, at a distance, were painted of a proper magni tude for the place, but with too great a distinction of parts, or too strong colours, it would appear to stand fOrward, and seem proportionally less, so as to repre sent a dwarf situated nearer the eye, and out of the plane on which the painter in tended 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 open uniform light, at such a moderate distance as to be clearly and distinctly seen. This colour. receives an alteration from many causes, the principal of which are the rol lowing.
1. From the objects being removed to a greater distance from the eye, whereby the rays of light which it reflects are less vivid, and the colour becomes more di luted and tinged, in some measure, by the taint bluish cast, or with the dimness or haziness of the body of air through which the rays pass.
2. From the greater or less degree of light with which the object is enlighten ed ; the same original colour having a different appearance in the shades from what it has in the light, although at all equal distance from the eye, and so in proportion to the strength of the light or shade.
3. From the colour of the light itself which falls upon it, whether it be from the reflection of coloured light from any adjacent object, or by its passage through a coloured medium, which will exhibit a colour compounded of the original colour of the object, and the other accidental co lours which the light brings with it.
4. From the position of the surface of the object, or of its several parts with re spect to the eye ; such parts of it appear ing more lively and distinct than those which are seen obliquely.
5. From the closeness or openness of the place where the object is situated; the light being much more variously di rected and reflected within a room, than in the open air.
6. Some original colours naturally re flect light in a greater proportion than others, though equally exposed to the same degrees of it ; whereby their degra dation at several distances will be differ ent from that of other colours which re flect less light.
From these several causes it happens that the colours of objects are seldom seen pure and unmixed, but generally ar rive at the eye broken and softened by each other ; and, therefore, in painting, where the natural appearances of objects are to be described, all bard or sharp co louring should be carefully avoided.
It painter, therefore, who would suc ceed in aerial perspective, ought careful ly to study the effects which distance, or the different degrees or colours of light, have on each particular original colour, to know how its appearance or strength is changed in the several circumstances above mentioned, and represent it accord ingly ; so that, in a picture of various co loured objects, he may be able to give each original colour its own proper dimi nution or degradation, according to its place.
Now, as all objects in a picture are pro portioned to those placed in the front ; so in aerial perspective the strength of light, and the brightness of the colours of ob jects close to 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, therefore, to give any colour its proper diminution in proportion to its distance, it ought to be known what the appearance of that colour would be, were it close to the picture, regard being had to that degree of light which is chosen as the principal light of the picture. For it' any colour should be made too bright for another, or for the general colours em ployed in the rest of the picture, it will appear too glaring, seem to start out of its place, and throw a flatness and damp upon the rest of the work ; or, as the painters express it, the brightness of that colour will kill the rest.