-359]

Phantasmagoria.

359

toms, for it was originally used to produce fright, by making spectres appear in darkness.

tern.

The internal arrangement of the phantasmagoria is just the same as in the magic lantern, the only difference being, that in the magic lantern the image projected on the screen is always of the same size, while, in the case of the phantasmagoria, this size may be varied at pleasure. To understand how this is effected, let us refer to fig. 279, which represents the arrangement of glasses in the magic lanThe lenses, m, which are used to project the images on the screen, being always at the same distance from the painted glass, ab, the image, AB, is always at the same distance, and is always therefore of the same size. Now if one of the lenses, m, be brought nearer the glass, ab, it follows from the properties of lenses (328) that the image will be found at a greater distance, and will be larger. Hence the effect sought requires two movements; one which brings the system of lenses, m, nearer the painted glass, to amplify the image; the other, which makes the whole apparatus re

cede, so that the image, while being moved away, is always formed upon the same screen as at first.

To obtain this double effect the whole apparatus is mounted upon four small wooden wheels covered with cloth, so that they roll noiselessly on the floor. Figure 280 represents a phantasmagoria thus arranged, with the difference that in the figure it is double, that is, consists of two apparatus united. We shall presently see the reason for this double use, and for the moment we shall only consider one of the parts. The front of the box is provided with a conical brass tube: in this tube is the lens of projection, which is

not fixed, but may be advanced or receded by means of a milled head and screw, which the experimenter turns with the hand.

-360]

Polyorama.

361

A large white sheet is stretched in front of the apparatus, and the spectators are on the other side of the sheet. The whole being in profound darkness, the experimenter is careful first of all to keep the projecting lens away from the glass, on which are painted the objects he desires to show. Thus there is at first formed on the sheet a very small image of the object. Then, with one hand, the experimenter brings the lens near the painted glass, while with the other he draws towards himself the apparatus, and away from the cloth; the image projected on the latter gradually increases, and ultimately becomes very large. But the spectators, who are prevented from seeing whether the position of the image changes or not, and who see the image very distinctly through the cloth, fall into the illusion that its increase in size is due to its coming nearer them. Some authors have supposed that use was made of the ⚫ phantasmagoria in remote antiquity, and, by means of apparatus of this kind, those initiated into the mysteries of Isis and Ceres were terrified, and the infernal deities evoked were made to appear. Yet nothing indicates that lenses were then known, but concave mirrors would be sufficient for producing effects analogous to those of the phantasmagoria.

360. Polyorama, or dissolving views.-The polyorama is an application of the phantasmagoria. This is then double, as represented in fig. 280, and the two systems of lenses converge towards the same point of the cloth which receives the image. Two pictures on glass are used representing the same view under different conditions; for example, Mount Vesuvius seen at daytime, calm, and with a slight cloud of smoke rising from it; the other volcano seen at night vomiting forth flames and torrents of fiery lava. Having arranged these glasses, each in one of the phantasmagoria, and the lenses being so arranged as to project the magnified images on exactly the same part of the cloth, the diaphragm of the one containing the picture representing the effect of day is opened; the other remaining closed. Then when the image has for some time been exposed to the view of the spectators, a mechanism, a, is worked, which gently closes the one which has been exposed, and opens the other. It follows, that in gradually passing through all the shades of light, the image which produces the effect of day disappears, while it is gradually succeeded by the effect of night represented on the other. In like manner, too, the effect of the moon rising may be made to succeed to sunset; to a calm and transparent sea, a tempest; to a smiling landscape, a snow effect, and so

forth. Hence the name polyorama, from two Greek words, which signify several views.

361. Photo-electrical microscope. This apparatus is based on the same principles as the magic lantern and the phantasmagoria. But, as in these apparatus, the subjects painted on glass have at once size, no great enlargement is required, and therefore the illumination need not be very intense. Whereas objects, the image of

which is reproduced by the photo-electric microscope, being very small, should be considerably magnified, and the light must therefore be very powerful, or else the image will be confused and indistinct. Hence the apparatus is illuminated by the powerful light which the electric battery yields.

-362]

Photo-Electric Microscope.

363

Figure 281 represents the use of the photo-electric microscope. At the base of the vessel is a series of vessels which serve for the disengagement of electricity, and which we shall presently describe as the electric battery. From these vessels the electricity passes by two stout copper wires to two rods of charcoal, contained in the box B. Thus charged with electricity, these carbons become heated to incandescence, and to emit such a bright light that the eye cannot support it. A reflector, I, sends the luminous rays in the direction of the tube, C, where they meet two condensing lenses, which concentrate them on the very small object which is to be magnified, and which is arranged between two glass plates, X. The rays pass from thence into a tube D, where there is a system of condensing lenses intended to produce the same effect of projection as the lenses, m, in the magic lantern (358); that is, it is a system of lenses which produces on a white screen at a distance an extremely magnified image of the small object placed between the glass plates. The tube, D, is movable, and may be approached to or receded from the object, so as to vary the magnification.

In the adjacent figure, the image projected on the screen is that of the infusoria which are found in paste when it has fermented. A small quantity is mixed with water, and a few drops put in a small glass box with parallel faces, which is placed at X. A multitude of these animalculæ are seen on the screen, ten or twelve inches in length, which move about in a confused mass, and soon die in consequence of the heat which is concentrated along with the light in the focus of the lenses.

362. Experiments with the photo-electric microscope are among the most interesting in the whole range of physics. By its means, objects of extreme minuteness may be exhibited, greatly magnified, to a large number of spectators. A hair, for example, looks like a broomstick; a flea like a sheep; the itch-tick, an animalcule found in itch pustules, and by which this disease is propagated, appears like a man's head; the same is the case with the animalcules found in decayed cheese, although these cannot be seen by the naked eye. One of the most remarkable experiments is that showing the circulation of the blood. This is made by placing between two glass plates the tail of a living tadpole, that is to say, the young of a frog, before its upper and lower limbs are developed. There is then observed on the screen a kind of illuminated map, all the rivers in which appear to flow very rapidly this is the blood which circulates in the veins. A very beautiful experiment is the