At the same time, it is found that the opposite mode of conversion, that is to say, the transformation of the raised into the depressed form, though occurring occasionally, is much less frequent.  Now, it may be asked, why should we tend to transform the concave into the convex, rather than the convex into the concave?  The reader may easily anticipate the answer from what has been said about the deeply fixed tendency of the eye to solidify a plane surface.  We are rendered much more familiar, both by nature and by art, with raised (cameo) design than with depressed design (intaglio), and we instinctively interpret the less familiar form by the more familiar.  This explanation appears to be borne out by the fact emphasized by Schroeder that the illusion is much more powerful if the design is that of some well-known object, as the human head or figure, or an animal form, or leaves.[43]

Another illustration of this kind of illusion recently occurred in my own experience.  Nearly opposite to my window came a narrow space between two detached houses.  This was, of course, darker than the front of the houses, and the receding parallel lines of the bricks appeared to cross this marrow vertical shaft obliquely.  I could never look at this without seeing it as a convex column, round which the parallel lines wound obliquely.  Others saw it as I did, though not always with the same overpowering effect.  I can only account for this illusion by help of the general tendency of the eye to solidify impressions drawn from the flat, together with the effect of special types of experience, more particularly the perception of cylindrical forms in trees, columns, etc.

It may be added that a somewhat similar illustration of the action of special types of experience on the perception of individual form may be found in the region of hearing.  The powerful disposition to take the finely graduated cadences of sound produced by the wind for the utterances of a Iranian voice, is due to the fact that this particular form and arrangement of sound has deeply impressed itself on our minds, in connection with numberless utterances of human feeling.

Illusions of Recognition.

As a last illustration of comparatively passive illusions, I may refer to the errors which we occasionally commit in recognizing objects.  As I have already observed, the process of full and clear recognition, specific and individual, involves a classing of a number of distinct aspects of the object, such as colour, form, etc.  Accordingly, when in a perfectly calm state of mind we fall into illusion with respect to any object plainly visible, it must be through some accidental resemblance between the object and the other object or class of objects with which we identify it.  In the case of individual identification such illusions are, of course, comparatively rare, since here there are involved so many characteristic differences.  On the other hand, in the case of specific recognition there is ample room for error, especially in those kinds of more subtle recognition to which I have already referred.  To “recognize” a person as a Frenchman or a military man, for example, is often an erroneous process.  Logicians have included this kind of error under what they call “fallacies of observation.”