In nature there is always movement and sound. Even on those rare days when the wind has ceased and the air seems still and dead, there is motion with noise of some kind. A brook trickles by, insects buzz their zigzag way, and shadows vary as the sun mounts or descends. But most commonly there is a breeze to rustle the trees and shrubs, to ripple the surface of the water, and to throw over the scene evidence of life in its ever charming variety. The painter cannot reproduce these movements and sounds. All he represents is silent and still as if nature had suddenly suspended her work--stayed the tree as it bent to the breeze, stopped the bird in the act of flight, fixed the water, and fastened the shadows to the ground. What is there then to compensate the artist for this limitation? Why, surely he can represent nature as she is at a particular moment, over the hills and valleys, or across great plains, with sunlight and atmosphere to mark the breadth and distance and so produce an illusion of movement to delight the eyes of the observer with bewitching surprise. For the eye as it involuntarily travels from the foreground of the picture to the background, proceeds from sign to sign, each decreasing in definition in conformity with the changes in nature, till vague suggestions of form announce that far distance has been reached. The effect is precisely that of the cinematograph, except that the eye moves instead of the picture. The apparent movement corresponds closely with the opening of distance in nature when one proceeds in a fast moving vehicle along a road from which a considerable stretch of country may be observed. Very rarely is the illusion so marked that the apparent movement is identified to the senses. When it is so marked the distance seems to come forward, but is instantaneously stayed before consideration can be brought to bear upon it. Clearly if one specially seek the illusion, it becomes impossible because search implies reason and an examination slow out of all proportion with the rapidity of the sensorial effect. Accident alone will bring about the illusion, for it can only arise when the eye travels at a certain rate over the picture, the minimum of which rate is indeterminable. It is evident that any landscape of fair size in which considerable depth is indicated must necessarily produce an illusion of opening distance if the varying signs are sufficiently numerous and properly painted in accordance with the aerial perspective; and this illusion is undoubtedly the key to the extraordinary beauty observed in the works of the great masters of landscape since Claude unveiled the secrets of distance painting. That the apparent movement is rarely actually defined is immaterial, for it must be there and must act upon the eye, producing an involuntary sensation which we interpret as pleasure arising from admiration of the skill of the artist in giving us so good a representation of distance in his imitation.