The survey that proved the Parthenon has almost no straight lines. Penrose measured the buildings of the Athenian Acropolis with surveying instruments and showed that their curves, inclinations and thickenings were deliberate, systematic and far too regular to be accidents of settlement.
Penrose (1817–1903) was a young architect and a keen amateur astronomer when the Society of Dilettanti sent him to Athens in 1845–47. Earlier visitors (the English architect John Pennethorne and the architect Josef Hoffer) had noticed that the steps of the Parthenon rose toward the middle. Penrose set out to settle whether this was real, and if so how exact it was. With a theodolite, levels and carefully checked rods he measured the stylobate, entablatures and columns of the Parthenon, the Propylaia, the Erechtheion and the temple of Hephaistos, recording dimensions to the thousandth of an English foot.
The results were published in a great folio in 1851, with plates that remain the standard illustration of the “optical refinements”: the upward curvature of the platform and entablature, the inward lean of the columns and walls, the extra thickness and closer spacing of the corner columns, and the delicate convex swelling of every shaft (entasis). Penrose argued that the curves were laid out on geometric principles and that the Greek architects intended them to correct, or to enliven, the way a large building is seen.
His measurements gave substance to passages of Vitruvius that had long puzzled commentators, above all the “unequal steps” (scamilli impares) by which the stylobate is to be raised in the middle. Penrose later became Surveyor of the Fabric of St Paul’s Cathedral and the first Director of the British School at Athens (1886–87); a second, enlarged edition of the book appeared in 1888.
Whether the refinements were meant to correct optical illusions, as Vitruvius says, or to give the building a subtle tension and life, as many modern writers prefer, is still argued. What Penrose established beyond doubt is that they are there, and that they were built on purpose.
The book, part by part
I The horizontal curves
The stylobate and the entablature of the Parthenon rise toward the middle of every side, by about 11 cm along the flanks and about 6 cm across the fronts. The curves are continuous, run through every course, and are repeated in the entablature, so they cannot be the result of settlement.
II The inclination of the columns and walls
The columns lean inward toward the cella by some 6–7 cm over their height; the corner columns lean diagonally. The inner faces of the architrave and the walls lean too, so that the whole peristyle gently converges.
III Entasis and the corner columns
Every shaft swells slightly in a smooth convex curve, the greatest departure from a straight taper being under 2 cm. The corner columns are thicker than the rest and the end intercolumniations narrower.
Rules: The Greek entasis: a barely perceptible swelling · Corner columns a fiftieth thicker · Contract the corner bays
IV The measured buildings
Plans, elevations and details of the Parthenon, the Propylaia, the Erechtheion and the temple of Hephaistos (then called the Theseum), with tables of dimensions and a discussion of the curves used for echinus and mouldings.