Meanwhile, it is the interior of the building that presents the chief impressiveness. Here the walls, which originally were faced with precious Oriental marbles, extend to a height of only two stories, crowned by the vast dome, which in the interior has a height equal to its diameter--one hundred forty-two and one-half feet. It is embellished with coffers, which in order to assist the perspective effect are foreshortened, diminishing in width as they ascend. Thus the gaze is carried up with a sweep to the central aperture at the summit, an open circle twenty-seven feet in diameter, the sole source of light to the interior. “One great eye opening upon Heaven--by far the noblest conception for lighting a building to be found in Europe.” It is as if the soaring imagination of the architect could brook no limit to its vision and must incorporate with his vault the firmament itself. In this magnificent audacity men have seen a symbolic reference to the ancient worship of Jupiter, the god of gods, beneath the open vault of heaven. Meanwhile, the architect may have derived the idea from the old nymphæum with its court open to the sky. And of the two, some will prefer to believe the latter, seeing in it a beautiful illustration of how the artist can and sometimes will use the requirements of practical conditions as an inspiration to the creativeness of his own imagination. * * * * * From structures circular in plan, we may pass to those in which the plan had the form of an ellipse, or comprised as its chief feature portions of a circle. In the first class belong the amphitheatres and to the latter the various circuses and theatres. The prototype of all these was the Hellenic Theatre, in the construction of which the architect took advantage of a sloping site. The Romans, on the other hand, with their general use of arch and vaulting, were independent of natural assistance and usually built their circuses and amphitheatres and theatres in the open. =Circus.=--The Roman circus was an adaptation of the Hellenic Stadium, which, however, was used chiefly for athletic games, while the Circus was employed for horse and chariot races. The oldest was the =Circus Maximus=, situated between the Palatine and Aventine; but the one of which most remains have been preserved is the =Circus Maxentius=, near the tomb of Cæcilia Metella on the Appian Way. Its plan presents a long rectangle terminating at one end in a semicircle. Surrounding this were tiers of marble seats, supported by raking vaults and an external wall of concrete. At the square end were situated the _Carceres_ or stables and down the centre of the rectangle ran a _spina_ or barricade, with a _meta_ or post at each end to mark the turning points. “To graze the meta” was a Roman saying for the taking of great chances. The course was seven times round and on the top of the spina were oval objects, one of which was removed on the completion of each lap of the race.