They say of the Acropolis where the Parthenon is...
All the apparently straightlines of the Parthenon are, in truth, slightly curved -- but the architects knew these lines would give the impression of being correctly linear. To compensate for the eye's tendency to see a column as thinner in the middle, the designers bowed each column. The columns were also slanted inward slightly. In a final refinement, the columns at the temple's corners were made thicker, since they catch more sunlight than other columns and so would appear thinner unless the architects compensated.