In the case of more remote objects, there is no dissimilarity of the retinal pictures or feelings of convergence to assist the eye in determining distance. Here its judgment, which now becomes more of a process of _conscious_ inference, is determined by a number of circumstances which, through experience and association, have become the signs of differences of depth in space. Among these are the degree of indistinctness of the impression, the apparent or retinal magnitude (if the object is a familiar one), the relations of linear perspective, as the interruption of the outline of far objects by that of near objects, and so on. In a process so complicated there is clearly ample room for error, and wrong estimates of distance whenever unusual circumstances are present are familiar to all. Thus, the inexperienced English tourist, when in the clear atmosphere of Switzerland, where the impressions from distant objects are more distinct than at home, naturally falls into the illusion that the mountains are much nearer than they are, and so fails to realize their true altitude. _Illusions of Art._ The imitation of solidity and depth by art is a curious and interesting illustration of the mode of production of illusion. Here we are not, of course, concerned with the question how far illusion is desirable in art, but only with its capabilities of illusory presentment; which capabilities, it may be added, have been fully illustrated in the history of art. The full treatment of this subject would form a chapter in itself; here I can only touch on its main features. Pictorial art working on a flat surface cannot, it is plain, imitate the stereoscope, and produce a perfect sense of solidity. Yet it manages to produce a pretty strong illusion. It illustrates in a striking manner the ease with which the eye conceives relations of depth or relief and solidity. If, for example, on a carpet, wall-paper, or dress, bright lines are laid on a dark colour as ground, we easily imagine that they are advancing. The reason of this seems to be that in our daily experience advancing surfaces catch and reflect the light, whereas retiring surfaces are in shadow.[38] The same principle is illustrated in one of the means used by the artist to produce a strong sense of relief, namely, the cast shadow. A circle drawn with chalk with a powerful cast shadow on one side will, without any shading or modelling of the form, appear to stand out from the paper, thus: [Illustration: FIG. 1.] The reason is that the presence of such a shadow so forcibly suggests to the mind that the object is a prominent one intervening between the light and the shaded surface.[39]