Complex sentence: When two identical towers rise in parallel the eyes expect perspective convergence of their corresponding lines, but with the identical images of the Leaning Tower, the corresponding lines do not converge and are therefore assumed to be non-parallel, i.e. diverging.
Split into simpler sentences: When two identical towers rise in parallel the eyes expect perspective convergence of their corresponding lines. However with the identical images of the Leaning Tower, the corresponding lines do not converge and are therefore assumed to be non-parallel, i.e. diverging.