It was not till 447 B.C., when Pericles was at the height of his power, that the building of the temple was actually commenced; and it took about ten years to finish. Pericles had previously made an appeal to other Greek cities to unite with Athens in some such commemoration of their victory over the Persians, but the response was disappointing. Fortunately, however, other means were available, owing to many of the states allied with Athens in the Delian League commuting into money-payments the obligations they were under to contribute ships to the defensive navy of which Athens was the head. It was this Delian fund mainly which enabled Pericles to carry out his great project for glorifying the Acropolis as the throne of Athena and the rallying-point of Hellenic patriotism. Of all the architectural monuments of the Periclean age the Parthenon is by far the grandest, producing a wonderful impression of strength and dignity and grace. There is a charm in the subtle harmony of its proportions quite apart from the rich decoration of frieze and pediment. The perfect unity of plan which it [Illustration: THE PARTHENON FROM THE NORTH END OF THE EASTERN PORTICO OF THE PROPYLÆA (EVENING LIGHT) The local colour of the rocky surface of the Acropolis intensifies the long blue shadows. The effect of the golden-brown weathering of the surface of the marble on the west front of the Parthenon is faithfully given.] exhibits was no doubt due to the genius of Pheidias, assisted by the architectural skill of Ictinus and Callicrates, while the mechanical precision and careful finish in the execution prove the competency of the sculptors and masons who were employed under their supervision. It is surprising how much attention was paid to nice optical considerations, which must have been very difficult to calculate, though they enhance greatly the general effect. For example, there is scarcely a straight line in the whole edifice, quadrangular as it is. There is a slight convex curve on the line of the steps and of the substructure, and the same is the case with the architrave. There is a gentle swelling of the columns towards their centre, and the axes of the columns incline slightly inwards.