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8CIOPTICON MANUAL. 45 Taking the instrument under his left arm, he should go up pretty close to the screen, and adjust the focus with his right hand; the image of course will be very small; he must then walk slowly backwards, at the same time adjusting the focus. As the image increases in size, it will appear to the spectators to be coming towards them; and then again let him walk up towards the screen, thus diminishing the image, and it will appear to them as if receding. The screen not being seen, the image appears to be suspended in the air, and! the de- ception is complete, even to those accustomed to the exhibition. The focusing is most evenly and easily effected by prying the extension; front out and in with the thumb and fingers of the- right hand. Slides producing the best phantasmagoria^ effect are those containing but one or two- figures with, a black background. In ancient times, the images from the phantasmagoria were thrown on the smoke arising from a chafing dish in which odors and drugs were burning, and by means of which many surprising and; apparently supernatural effects were produced.. As a, relief &om so closely following practical details, let us advert to the probable use made by ancient magicians, necro- mancers, and sorcerers,, of these optical contrivances for producing supernatural illusions. In this w« cannot do better than to quote from that eminent authority on optical science, Sir David Brewster : » " In the imperfect accounts wliich have reached us of these rep- resentations, we can trace all the elements of optical illusion. In the ancient temple of Hercules,.at Tyre, £Hny N mentions that there was a seat made of consecrated stone, 'from .which the gods easily arose.' Esculapius often exhibited hirnself, to, his worshipers in the temple at Tarsus ; ; and the Temple a.t.Enguinum^in. Sicily, was