KEEPING.
867.
If linear perspective displays the gradation of objects in their apparent size as affected by distance, aërial perspective shows us their gradation in greater or less distinctness, as affected by the same cause.
868.
Although from the nature of the organ of sight, we cannot see distant objects so distinctly as nearer ones, yet aërial perspective is grounded strictly on the important fact that all mediums called transparent are in some degree dim.
869.
The atmosphere is thus always, more or less, semi-transparent. This quality is remarkable in southern climates, even when the barometer is high, the weather dry, and the sky cloudless, for a very pronounced gradation is observable between objects but little removed from each other.
870.
The appearance on a large scale is known to every one; the painter, however, sees or believes he sees, the gradation in the slightest varieties of distance. He exemplifies it practically by making a distinction, for instance, in the features of a face according to their relative position as regards the plane of the picture. The direction of the light is attended to in like manner. This is considered to produce a gradation from side to side, while keeping has reference to depth, to the comparative distinctness of near and distant things.