[Illustration: PLATE 17 (See page xii) Arcadian Landscape, by Claude Lorraine (_National Gallery, London_)] As will presently be seen there are other kinds of illusion of motion which may be produced in landscape, but this illusion of opening distance is the most important, and it should be produced wherever distance is represented. In nature the effect of the unfolding of distance is caused by a sequence of signs apparently diminishing in size and clearness as the eye travels back, and a sequence of this kind should be produced by the artist in his picture. It is not sufficient that patches of colour of the tone and shape of sections of vegetation, trees, varied soils, and so on, be given, for while these may indicate distance as any perspective must do, yet an illusion cannot be produced by such signs because they are not sufficiently numerous for the eye to experience a cinematographic effect when passing over them. It is not distance that gives the beauty, but an illusion of opening distance, without which, and presuming the absence of any other illusion, only simple harmonies of tone and inanimate forms are possible. Moreover the patches of colour do not properly represent nature either as she appears to the eye, or as she is understood from experience. If one were to take a momentary glance at a view specially to receive the general colour impression, he might conceivably retain on his mind a collection of colour masses such as is often put forward as a landscape, but natural scenes are not observed in this way, and the artist has no right to imply that a view should be painted as it is observed at an instantaneous glance. One cannot be supposed to keep his eyes closed, except for a moment, when in front of nature, and he cannot be in front of nature for more than a moment without involuntarily recognizing thousands of signs. There must necessarily be a certain clearness of the atmosphere for distance to be represented, and in the minimum clearness, trees, bushes, rivulets, and buildings of every kind, are well defined at least to the middle distance. These can and should be painted, and there can be no object whatever in omitting them, except the ignominious end of saving trouble.