The cleverness of the architects in insuring to the Parthenon, after its completion, the appearance of absolute harmony of proportion in all its outward lines, is one of their best claims to that celebrity which they have justly earned. As it goes so far toward illustrating their great professional skill, the reader may be interested in reading the language used by Professor Roger Smith of London in explaining the measures adopted by Ictinus and possibly Callicrates also, to correct the optical defects which the Parthenon might otherwise have possessed when completed. “The delicacy and subtlety of these [optical illusions] are extreme, but there can be no manner of doubt that they existed. The best known correction is the diminution in diameter or taper, and the _entasis_ or convex curve of the tapered outline of the shaft of the column. Without the taper, which is perceptible enough in the order of this building, and much more marked in the order of earlier buildings, the columns would look top-heavy; but the _entasis_ is an additional optical correction to prevent their outline from appearing hollowed, which it would have done had there been no curve. The columns of the Parthenon have shafts that are over thirty-four feet high, and diminish from a diameter of 6.15 feet at the bottom to 4.81 feet at the top. The outline between these points is convex, but so slightly so that the curve departs at the point of greatest curvature not more than three-quarters of an inch from the straight line joining the top and bottom. This is, however, just sufficient to correct the tendency to look hollow in the middle. “A second correction is intended to overcome the apparent tendency of a building to spread outward toward the top. This is met by inclining the columns slightly inward. So slight, however, is the inclination, that were the axes of two columns on opposite sides of the Parthenon continued upward till they met, the meeting point would be 1952 yards, or, in other words, more than one mile from the ground. “Another optical correction is applied to the horizontal lines. In order to overcome a tendency which exists in all long lines to seem as though they drop in the middle, the lines of the architrave of the top step and of other horizontal features of the building are all slightly curved. The difference between the outline of the top step of the Parthenon and a straight line joining its two ends is at the greatest only just two inches.” Still another correction which Professor Smith alludes to, in respect to the vertical proportions of the building, he does not discuss more than to say: “The small additions, amounting in the entire length of the order to less than five inches, were made to the heights of the various members of the order, with a view to secure that from one definite point of view the effect of foreshortening should be exactly compensated, and so the building should appear to the spectator to be perfectly proportioned.”