To the hindmost wall of the scene an upper story could be added; whenever, for instance, it was wished to represent a tower with a wide prospect, or the like. Behind the great middle entrance there was a space for the Exostra, a machine of a semicircular form, and covered above, which represented the objects contained in it as in a house. This was used for grand strokes of theatrical effect, as we may see from many pieces. On such occasions the folding-doors of the entrance would naturally be open, or the curtain which covered it withdrawn. A stage curtain, which, we clearly see from a description of Ovid, was not dropped, but drawn upwards, is mentioned both by Greek and Roman writers, and the Latin appellation, _aulaeum_, is even borrowed from the Greeks. I suspect, however, that the curtain was not much used at first on the Attic stage. In the pieces of Aeschylus and Sophocles, the scene is evidently empty at the opening as well as the conclusion, and seems therefore to have required no preparation which needed to be shut out from the view of the spectators. However, in many of the pieces of Euripides, and perhaps also in the _Oedipus Tyrannus_, the stage is filled from the very first, and presents a standing group which could not well have been assembled under the very eyes of the spectators. It must, besides, be remembered, that it was only the comparatively small proscenium, and not the logeum, which was covered by the curtain which disappeared through a narrow opening between two of the boards of the flooring, being wound up on a roller beneath the stage. The entrances of the chorus were beneath in the orchestra, in which it generally remained, and in which also it performed its solemn dance, moving backwards and forwards during the choral songs. In the front of the orchestra, opposite to the middle of the scene, there was an elevation with steps, resembling an altar, as high as the stage, which was called the _Thymele_. This was the station of the chorus when it did not sing, but merely looked on as an interested spectator of the action. At such times the choragus, or leader of the chorus, took his station on the top of the thymele, to see what was passing on the stage, and to converse with the characters there present. For though the choral song was common to the whole, yet when it took part in the dialogue, one usually spoke for all the rest; and hence we may account for the shifting from _thou_ to _ye_ in addressing them. The thymele was situated in the very centre of the building; all the measurements were made from it, and the semicircle of the amphitheatre was described round it as the centre. It was, therefore, an excellent contrivance to place the chorus, who were the ideal representatives of the spectators, in the very spot where all the radii converged.