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 the screen; we must, therefore, cause the machine so to recede or approach, that the two motions, being duly combined, the image may be distinctly formed.
These phantasmagoria are furnished with a great number of transparencies, in each of which, several changes may be made by slackening their springs. Thus we may change at every instant, the form, the magnitude, and the distance of the spectres, as they appear to the spectator.
What has been said hitherto, relates only to the images of transparent figures. To obtain those of opaque bodies, first place the gauze and box, at the distance of about six feet one from the other, and adapt to the orifice of the box, an apparatus of two tubes furnished with two bi-convex lenses. An opaque body, such, for example, as a medal, or a picture, is attached to a little support, posited in the middle of the box; the lamp with its supply of air, situated in one of the foremost corners of the box, illuminates that object, and the reflected rays, crossing the lenses, proceed till they trace the image upon the gauze, with an amplification which is in the ratio of the distances.
If the image be not distinct, we must infer that it is not at the focus; but it may be adjusted in three different ways; 1. By moving the box to or from the gauze; 2. By moving the object nearer to, or farther from, the lenses within the box; 3. By slowly moving the tubes, to cause a variation in the distance between the lenses.—See Haüy's Philosophy, translated by Gregory, vol. ii, p. 390.
Mr. Nicholson, however, witnessed an exhibition of the phantasmagoria at the London Lyceum by Philipstal, who took out a patent for his improvements in the apparatus and machinery. He observes, that the novelty consists in placing the lantern on the opposite side of the screen which receives the images, instead of on the same side as the spectator, and suffering no light to appear but what passes through, and tends to form those images. His sliders are therefore perfectly opaque, except that portion upon which the transparent figures are drawn, and the exhibition is thus conducted.
All the lights of the small theatre of exhibition were removed, except one hanging lamp, which could be drawn up, so that its flame should be perfectly enveloped in a cylindrical chimney, or opaque shade. In this gloomy and wavering light, the curtain was drawn up, and presented to the spectator a cave or place exhibiting skeletons, and other figures or terror, in relief, and painted on the sides or walls. After a short interval, the lamp was drawn up, and the audience were in total darkness, succeeded by thunder and lightning; which last appearance was formed by the magic lantern upon a thin cloth or screen, let down after the disappearance of the light, and consequently unknown to most of the spectators. These appearances were followed by figures of departed men, ghosts, skeletons, transmutations, &c. produced on the screen by the magic lantern on the other side, and moving their eyes, mouth, &c. by the well known contrivance of two or more sliders. The transformations are effected by moving the adjusting tube of the lantern out of focus, and changing the slider during the moment of the confused appearance.
It must be again remarked, that these figures appear without any surrounding circle of illumination, and that the spectators, having no previous view or knowledge of the screen, nor any visible object of comparison, are each left to imagine the distance according to their respective fancy. After a very short time of exhibiting the first figure, it was seen to contract gradually in all its dimensions, until it became extremely small, and then vanished. This effect, as may easily be imagined, is produced by bringing the lantern nearer and nearer the screen, taking care at the same time to preserve the distinctness, and at last closing the aperture altogether: and the process being (except as to brightness) exactly the same as happens when visible objects become more remote, the mind is irresistibly led to consider the figures, as if they were receding to an immense distance.
Several figures of celebrated men were thus exhibited with some transformations; such as the head of Dr. Franklin being converted into a skull, and these were succeeded by phantoms, skeletons, and various terrific figures, which instead of seeming to recede and then vanish, were (by enlargement) made suddenly to advance; to the surprise and astonishment of the audience, and then disappear by seeming to sink into the ground.
This part of the exhibition, which by the agitation of the spectators appeared to be much the most impressive, had less effect with me than the receding of the figures; doubtless because it was more easy for me to imagine the screen to be withdrawn than brought forward. But among the young people who were with me, the judgments were various. Some thought they could have touched the figures, others had a different notion of their distance, and a few apprehended that they had not advanced beyond the first row of the audience.
The whole, as well as certain mechanical inventions, were managed with dexterity and address. The lightning, being produced by the camera, was tame, and had not the brisk transient appearance of the lightning at the theatres, which is produced by rosin, or lycopodium powder, thrown through a light, which in Mr. P's utter darkness might easily have been concealed in a kind of dark lantern.
A plate of thin sheet iron, such as German stoves are made of, is an excellent instrument for producing the noise of thunder. It may be three or four feet long, and the usual width. When this plate is held between the finger and thumb by one corner, and suffered to hang at liberty, if the hand be then moved or shaken horizontally, so as to agitate the corner at right angles to the surface, a great variety of sounds will be produced; from the low rumbling swell of distant thunder, to the succession of loud explosive bursts of thunder from elevated clouds. This simple instrument is very manageable, so that the operator soon feels his power of producing whatever character of sound he may desire; and notwithstanding this description may seem extravagant, whoever tries it for the first time will be surprised at the resemblance. If the plate be too small, the sound will be short, acute, and metallic.
We may remark also, that the magic lantern, by new contrived sliders and machinery, may be applied to important uses, by employing it with such figures as will explain the general principles of optics, astronomy, botany, &c.
The experiment mentioned by Ferguson, with a concave mirror, reflecting into the air the appearance of fire, &c. into a focal point, (founded on a general principle of concave reflectors,) is productive of many agreeable deceptions, and which exhibited