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.