We may also remark, that optical exhibitions sometimes accompany those of fire, when performed on a small scale. In the phantasmagoria, for instance, whether before, or at the time the exhibition commences, as well as after, thunder and lightning, if well imitated, produces a good effect. The mechanism of the phantasmagoria is concealed from the spectators, who have only before their eyes a screen of gauze or gummed muslin posited vertically, which serves as the ground of a picture, where the images are depicted by reason of the transparency. The apartment is deprived of all light, except that which proceeds from an apparatus hid behind the screen. At the moment when the operation commences, a spectre appears (as a skeleton, the head of a celebrated person, &c.), at first extremely small, but which afterwards increases rapidly, and thus seems to advance at a great rate towards the spectators. And when the scene passes before them in a room representing a cave hung with black, a solemn silence being occasionally interrupted by mournful sounds from an appropriate musical instrument, it is not easy for an observer to defend himself from the impression of terror, at the sight of an object, in itself formed to produce the illusion, and which finds in the imagination a place already prepared for the reception of phantoms. The instrument placed behind the gauze screen is in fact a peculiar construction of the magic lantern: only in the former, it is necessary that the lenses should run over a much greater space, and that the instrument may be susceptible of approaching to, and receding from, the frame of gauze, in such manner, that each luminous pencil may be depicted there in a single point. The general construction is this: In a square box, a lamp is placed, the luminous rays proceeding from which, are reflected by a conical mirror, towards an orifice made in the box. At this orifice is placed a tube, blackened within, and composed of several tubes which slide one into another, like those of a pocket telescope. This tube is furnished with two bi-convex lenses of about five inches diameter; one of these is fixed, the other is at the outer extremity of the tube, and is separated from the former in proportion as the tube is lengthened by the aid of a hooked lever situated along the tube, between the lamp and the lenses. A groove is properly adapted to the tube, destined to receive transparent figures; lastly, the box rests upon a table moveable on four wheels, that slide in two channels perpendicularly to the frame on which the images are depicted. It is manifest, that we may augment or diminish the dimensions of the images, and consequently make the spectre appear more or less near to the spectator, by separating farther, or by bringing nearer together, the two lenses; but then the focus of the diverging rays, which proceed from the same point of the transparent body, will be no longer upon