AERIAL PERSPECTIVE, the relative colours of visible objects, as modified by distance, by accidental varieties of light, and by the intervention of atmosphere. This term is, however, more generally employed, to signify the art of imitating these colours in painting. The laws by which this art is regulated, by no means admit of that precision which accompanies those of lineal per spective. In the latter of these, which has for its object to determine the apparent form, place, and bulk, of the various parts of his scene, the artist avails himself of in variable principles, which admit of mathematical demon stration ; in the practice of the former, he must rely al most entirely on the delicate and cultivated accuracy of his immediate perceptions. How very far these percep tions are wide of the truth, till corrected by the closest attention, and by long practice, every artist who has studi ed to detect the real appearances of nature, will readily admit. Habituated from our earliest years to correct the information of the senses by the knowledge we have previously acquired, it becomes afterwards a difficult ex ertion to give, even for a moment, our undivided atten tion to the real sensation communicated by the exter nal organ, and to prevent that sensation from being mo dified by the insensible, and almost mechanical sugges tions of the judgment. It is by means of this habitual process in the economy of our perceptions, that we ascribe the same colour and the same dimensions to any known ohject,although seen at very different distances ; a process which, though absolutely essential to the truth and consistency of our judgments, it is the business of the artist to counteract and suspend at will. It is by this acquired faculty that he is enabled to look at nature under that aspect which his art and to mark, Without MN sensible exertion, those delicate and evane scent varieti,!s of form and colour, which escape the or dinary observer.
It is the most obvious and general fact in aerial per spective, that objects assume a fainter tone of colour, the farther they are removed from the eye of the spec tator. This fact is founded on the imperfect transpa rency of the atmosphere, and on the divergency of the rays of light reflected from objects of an unpolished sur face. Among objects of this description, we find that the strong opposition of light and shadow which they exhibit when near the eye, is gradually reduced nearer to equality as we recede horn them. The shady side becomes lighter; the illuminated part of it becomes less brilliant. In the case of flat polished surfaces, such as that of still water, which reflect the sun's rays without any sensible divergency, we find that the light loses lit tle of its force and brilliancy by distance, except when diminished by the imperfect transparency of the me dium through which it is transmitted. But as to all the nicer distinctions exhibited in the aerial perspective of nature, it is impossible to lay down any rule which ad mits of general application. Not only the infinite variety of the local colours of objects themselves, but every ac cidental distribution of light', every transient change in the state of the air, creates new appearances, and forms new relations in the apparent colouring of the different parts of the scene. In some of the most sublime effects observable in nature, we occasionally find the middle ground of a lighter hue than the distance. In order to assist the exertions of the eve, on which the artist, in co pying nature, must ultimately depend, it may bc recom mended as an useful practice, to hold up against any ob ject another resembling it in real colour, and thus, by comparing them together, we shall the more readily dis cover the change produced, even by the distance of a few yards. (t)