The sensitive taste of the Greeks led them into other refinements than those of mere mechanical perfection. In the Parthenon especially, but also in lesser degree in other temples, the seemingly straight lines of the building were all slightly curved, and the vertical faces inclined. This was done to correct the monotony and stiffness of absolutely straight lines and right angles, and certain optical illusions which their acute observation had detected. The long horizontal lines of the stylobate and cornice were made convex upward; a similar convexity in the horizontal corona of the pediment counteracted the seeming concavity otherwise resulting from its meeting with the multiplied inclined lines of the raking cornice. The columns were almost imperceptibly inclined toward the cella, and the corner intercolumniations made a trifle narrower than the rest; while the vertical lines of the arrises of the flutings were made convex outward with a curve of the utmost beauty and delicacy. By these and other like refinements there was imparted to the monument an elasticity and vigor of aspect, an elusive and surprising beauty impossible to describe and not to be explained by the mere composition and general proportions, yet manifest to every cultivated eye.[10] [Footnote 10: These refinements, first noticed by Allason in 1814, and later confirmed by Cockerell and Haller as to the columns, were published to the world in 1838 by Hoffer, verified by Penrose in 1846, and further developed by the investigations of Ziller and later observers.] CHAPTER VII. GREEK ARCHITECTURE--_Continued_. BOOKS RECOMMENDED: Same as for Chapter VI. Also, Bacon and Clarke, _Investigations at Assos_. Espouy, _Fragments d’architecture antique_. Harrison and Verrall, _Mythology and Monuments of Ancient Athens_. Hitorff et Zanth, _Recueil des Monuments de Ségeste et Sélinonte_. Magne, _Le Parthénon_. Koldewey and Puchstein, _Die griechischen Tempel in Unteritalien und Sicilien_. Waldstein, _The Argive Heræum_. +HISTORIC DEVELOPMENT.+ The history of Greek architecture, subsequent to the Heroic or Primitive Age, may be divided into periods as follows: The ARCHAIC; from 650 to 500 B.C. The TRANSITIONAL; from 500 to 460 B.C., or to the revival of prosperity after the Persian wars. The PERICLEAN; from 460 to 400 B.C. The FLORID or ALEXANDRIAN; from 400 to 300 B.C. The DECADENT; 300 to 100 B.C. The ROMAN; 100 B.C. to 200 A.D. These dates are, of course, somewhat arbitrary; it is impossible to set exact bounds to style-periods, which must inevitably overlap at certain points, but the dates, as given above, will assist in distinguishing the successive phases of the history.