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.