The origins of entasis: illusion, aesthetics or engineering?

A typical characteristic of columns in Doric temples is entasis; a slight convexity in the body of a column. Often, and particularly in guide-books, it is suggested that entasis is intended to compensate for an illusion of concavity in columns with truly straight sides. We have investigated whether any such visual illusion exists, both in parallel sided and in tapering columns in a series of experiments, finding little evidence to support any illusion-compensation theory. Further, we explored the possibility that entasis was employed for purely aesthetic reasons, but the results do not support this conclusion. Finally, evidence supporting an engineering role for entasis is presented.