Concerning the date, we know that the play was written after 475 B.C., the year in which occurred that eruption of Etna described by Prometheus (vv. 363-72). Further, it is usually regarded as later than the _Seven_ owing to the increased preponderance of dialogue over lyrics. Also, the supposition that three actors are required has led some scholars to believe that the _Prometheus_ belongs to the period when Sophocles had introduced a third actor, and so to place it in the last part of the poet’s life.[226] The static nature of the drama might seem to forbid such a view, but possibly it formed the centre of the trilogy, the most likely place for an equilibrium of the tragic forces. And the theological basis of the whole series is so profound, that an approximation in date to the _Oresteia_ is not unreasonable. On the whole, then, the _Prometheus_ may be conjecturally assigned to about the year 465 B.C. As for the division of the parts among the actors, we find in the opening scene three[227] persons engaged, Prometheus, Hephæstus, and Cratos (“Strength”). Prometheus, however, does not utter a word until his tormentors have retired, and it has been held that only two actors are needed here (as in the rest of the work). On this view, Prometheus would be represented by a lay-figure, either Hephæstus or Cratos would return unseen, delivering the later speeches of Prometheus from behind the figure, through a mouth-piece in the head. But as there was no curtain in the theatre, it would be necessary for the executioners to carry the lay-figure forth in view of the audience before the action began. The true objection to this is not its absurdity; an audience will tolerate much awkwardness in stage-management, if only it is accustomed to such conventions. But it would scarcely have harmed the play if the poet had dispensed with Cratos; the actor thus disengaged could have impersonated Prometheus from the beginning. That Æschylus saw this possibility cannot be doubted; therefore he did not feel bound to use a lay-figure; therefore he did not, and we must assume that he employed three actors.