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.