Great architectural and engineering achievements of ancient Rome, like roads, aqueducts, the use of concrete and the invention of a dome, were preceded by centuries of the architectural development of ancient Greece. The Greek builders implemented numerous innovative solutions, which were then taken over and further developed by the Romans. Many of those were intended for the optical correction of buildings, which would look completely different without the use of these nearly invisible modifications.
One of the first and most basic techniques implemented originally by the Greeks was entasis. Named so by Vitruvius after the Greek word eteíno (εντείνω), which means to stretch, it relief on producing a column in such a way that its biggest circumference was located approximately one-third of the height from the base. According to Hero of Alexandria, such a bulge was created in order to prevent an optical illusion causing a perfectly straight column to look concave. This technique also allowed for a building to look bigger than it really was. The temple of Aphaea from the archaic period, located on the island of Aigina, is believed to be the first example of the use of such a swelling but the most pronounced implementation of the Doric temple complex in Paestum, where it can be observed in the temples of Hera and Athena.
Some contemporary researchers do not agree with Hero’s opinion. According to some, convex columns were supposed to stress their strength, the strength of the whole building or the weight of the roof. There is also a theory that says that the shape of columns represents the swelling of strained muscles (strain is another meaning of the word eteíno). An alternative theory states that the reasons for the shape were of a purely technical nature – a bulged column is stronger than a straight one or one with diameter linearly decreasing from bottom to top. It is however unknown whether ancient Greek were aware of this fact because the earliest source concerning this type of architectural correction was Vitruvius, who lived in the 1st century CE.
Over time even more subtle corrections were introduced, some of which are invisible to the naked eye. The Parthenon of Athens is one of the chief examples. In order to avoid the illusion of the central part of the façade sinking, it was built in such a way that the middle part of the pedestal is 6 cm higher than the corner sections. Only thanks to this solution the line of the base of the building seems straight to an observer and the difference can be seen only when looking from close to one of the corners. What is more, such a correction was implemented also on the sides and the back of the building which makes the surface of the base of the Parthenon convex upwards at the centre of the building.
Another example of the optical corrections is leaning off the columns inwards toward the centre of the building and at the same time towards the centreline of each side – of the central axes of the corner columns of the façade were extended, they would cross several kilometres above the Parthenon. The corner columns are also slightly thicker than the other columns and their neighbouring columns are closer – the spacing between the corner columns and those closest to them is smaller than that between the rest of the columns. In case of the lack of such a modification, the corner columns would appear thinner and more distant from the others.
The Roman architecture scholars used to believe that optical corrections of this type were abandoned with time and altogether ignored in the imperial period. However, towards the end of the 19th century, examples were discovered in one of the best-preserved Roman temples – the so-called Square House in Nîmes, France (dedicated to Gaius and Lucius, the sons of Marcus Agrippa and adopted grandsons of Octavian). In general, though it should be noted that the Roman buildings were usually higher than the Greek ones and thus their columns were longer and thinner.