By means of concave mirrors, made of highly polished metal, the priests were able to project images upon walls, in the air, or upon the smoke arising from burning incense. In speaking of the art of casting specula of persons upon smoke, the ingenious Salverte says: “The Theurgists caused the appearance of the gods in the air in the midst of gaseous vapors disengaged from fire. Porphyrus admires this secret; Iamblichus censures the employment of it, but he confesses its existence and grants it to be worthy the attention of the inquirer after truth. The Theurgist Maximus undoubtedly made use of a secret analogous to this, when, in the fumes of the incense which he burned before the statute of Hecate, the image was seen to laugh so naturally as to fill the spectators with terror.” A. Rich, in his _Dictionary of Roman and Greek Antiquities_, relates, under the heading of the word “Adytum,” that many of the ancient temples possessed chambers the existence of which was known only to the priests, and which served for the {3} production of their illusions. He visited one at Alba, upon the lake of Fucius. It was located amid the ruins of a temple, and was in a perfect state of preservation. This chamber of mysteries was formed under the apsis—that is to say, under the large semi-circular niche which usually sheltered the image of the god, at the far extremity of the edifice. “One part of this chamber,” says he, “is sunk beneath the pavement of the principal part of the temple (_cella_) and the other rises above it. The latter, then, must have appeared to the worshipers gathered together in the temple merely like a base that occupied the lower portion of the apsis, and that was designed to hold in an elevated position the statue of the god or goddess whose name was borne by the edifice. This sanctuary, moreover, had no door or visible communication that opened into the body of the building. Entrance therein was effected through a secret door in an enclosure of walls at the rear of the temple. It was through this that the priests introduced themselves and their machinery without being observed by the _hoi polloi_. But there is one remarkable fact that proves beyond the shadow of a doubt the purpose of the adytum. One discovers here a number of tubes or pipes which pierce the walls between the hiding-place and the interior of the temple. These tubes debouch at different places in the partitions of the cella, and thus permit a voice to be heard in any part of the building, while the person and place from which the sound issues remain unknown to the auditors.”