is unsupported by any sufficient evidence. Lastly, there is Robert’s hypothesis. Robert denies the existence of a stage during the fifth century; but supposes that one was erected in the course of the fourth century for the performance of new plays, in which there was practically no chorus. Henceforth new plays were acted on the stage, old plays in front of it, in the orchestra.[537] But it is impossible to suppose that in the same theatre, and at the same festival, the proscenium should have served at one time as a stage, and at another time as a background. Nor is there anything in the ancient authorities to support such a view. § 14. _Various Details._ To return to the subject of the construction of the theatre in general. It is obvious that, considering the enormous size of the building, and the immense numbers of spectators which it was intended to accommodate, the greatest attention must have been bestowed upon its acoustic properties. Vitruvius is most emphatic upon the necessity of keeping this object in view, when choosing a site for a theatre. The situation against the side of a hill, and the gentle and symmetrical upward slope of the tiers of seats, are mentioned as qualities by which acoustic excellence was ensured. The height of the stage-buildings was also of great importance. It was found that the best results were obtained by making them exactly the same height as the uppermost parts of the auditorium.[538] That this was the ordinary practice during the Roman period is proved by the remains of various theatres, such as those of Aspendos and Orange. But whether, at any time during the Greek period, stage-buildings were constructed on this enormous scale is very doubtful. Another matter on which the ancient architects insisted was the wooden flooring of the stage, which tended to make the voices of the actors more audible. When Alexander the Great wished to have a stage built entirely of bronze, it was pointed out to him that this material would be fatal from the acoustic point of view.[539] Vitruvius mentions a peculiar practice which was adopted for the purpose of adding resonance to the voices of the actors. Hollow vessels of bronze, of different tones, were suspended in niches in various parts of the auditorium. When a sound was uttered of the same tone as that of any of the vessels, its resonance was increased. He states that this custom, though not adopted in Rome, existed in many Greek and Italian theatres; and that Mummius, after his capture of Corinth, brought back several of these vessels from the theatre there.[540] In the remains of the existing theatres no traces are to be found of the niches he describes. It is probable that the whole plan was merely an experiment adopted in a few special cases. As far as Athens was concerned, no such extraneous assistance to the voice was necessary. Experiments at the present day have shown that the acoustic properties of the theatre of Dionysus are excellent; and this must have