When two identical towers rise in parallel the eyes expect their corresponding outlines to converge in the retinal image due to perspective, but in the identical images of the Leaning Tower the corresponding outlines do not converge and are therefore assumed to be non-parallel, i.e. diverging.
The eyes expect their corresponding outlines. This is to converge in the retinal image due to perspective. The corresponding outlines do not converge. This is in the identical images of the Leaning Tower. The eyes are therefore assumed to be non-parallel, i.e. diverging. Two identical towers rise in parallel.