Aerial, is sometimes employed as a general denomination for that which in a more restricted sense is called aerial perspective, or the art of giving a due diminution or degradation to the strength of light, shade, and colour, according to the different distances of objects, the quantity of light which falls upon them, and the medium through which they are seen; the chiaro oscuro, which consists in expressing the different degrees of light, shade, and colour of bodies, arising from their own shape, and the position of their parts with respect to the eye and neighbouring objects, by which their light or colours are affected; and keeping, which is the observance of a due proportion in the general light and colouring of the whole picture, so that no light or colour in one part may be too bright or too strong for another. A painter who would succeed in aerial perspective ought carefully to study the effects which distance, or different degrees or colours of light, have on each particular original colour, to know how its hue or strength is changed in the several circumstances which occur, and to represent it accordingly. As all objects in a picture take their measures in proportion to those which are placed in the front, so, in aerial perspective, the strength of light, and the brightness of the colours of objects close to the picture, must serve as a measure, with respect to which all the same colours at several distances must have a proportional degradation in like circumstances.

Bird's eye view in Perspective, is that which supposes the eye to be placed above any building, or other object, as in the air at a considerable distance from it. This is applied in drawing the representations of fortifications, when it is necessary not only to exhibit one view as seen from the ground, but also so much of the several buildings as the eye can possibly take in at one time from any situation. In order to this, we must suppose the eye to be removed to a considerable height above the ground, and to be placed, as it were, in the air, so as to look down into the building like a bird when flying. In representations of this kind, the higher the horizontal line is placed, the more of the fortification will be seen, and vice versa.