are in profound darkness. The exhibitor now pushes in the lens with one hand, whilst with the other he draws back the apparatus, causing the image to enlarge upon the screen. The gradual enlargement of the image causes the spectators, who have no means of judging of its exact location, to suppose that the image is approaching them.

It has been suggested that a similar effect, produced by a combination of mirrors, was used in ancient times for the purpose of frightening those who were initiated into the mysteries of Isis and Ceres, by causing images of the dead to appear when invoked.

The Polyrama and Dissolving Views.

330. The POLYRAMA consists of a double magic lantern, as shown in Fig. 223. The DISSOLVING VIEWS are obtained by using both lanterns. Thus, if a picture of a daylight scene be painted upon one of the slides, and of the same scene by moonlight be painted on the other, the first picture is thrown upon the screen strongly illuminated, the other one being entirely excluded by a screen which covers the second lens. By an arrangement operated by the exhibitor, the light is gradually cut off from the first picture and adnitted upon the second, the first fading away insensibly, whilst the second as insensibly grows brighter. In this way all of the effects intermediate between full daylight and full moonlight may be obtained in succession.

In this way a volcano, calm, and only surmounted by a light r'oud of smoke, may be followed by a picture of the same volcano sending forth volumes of flame and smoke. A storm may be made to succeed a smiling landscape, and so on; the illusion is complete.

331.

The Photo-Electric Microscope.

The PHOTO-ELECTRIC MICROSCOPE is constructed on the same optical principles as the magic lantern, except

(330.) What is the Polyrama? Explain the method of producing the Dissolving Views. Illustrate. (331.) What is the Photo-Electric Microscope?

that the light employed is obtained by passing an electric current between two charcoal points. The pictures on the shades are also made smaller than in the magic lantern, which requires a greater illumination.

Fig. 224 represents in detail the arrangement of this instrument. At the foot of the apparatus is a battery for generating electricity, which will be described hereafter. The electricity is conveyed to the charcoal points in the box, B, by means of two copper wires, one going to the

upper, and the other to the lower point. The points being slightly separated, the circuit is completed only by the electricity passing across the interval, which gives rise to a light of extreme brilliancy.

In the figure, I represents a parabolic reflector for concentrating the light upon the slide, X, through a lens, C. D is a lens which forms a magnified image of the minute object on a screen. The tube in which the lens, D, is placed, may be drawn out or pushed in to vary the magnifying power of the apparatus.

The magnifying power of this instrument may be made extremely great, and by suitable management it serves to show to a large company the wonders of the microscopic world.

One of the most remarkable experiments made with it, is to show the circulation of the blood. Instead of a picture on the slide, let the tail of a tadpole be placed between two plates of glass and introduced. There will appear upon the screen what seems an illuminated map, all of whose streams flow with a rapid current. It is but the blood circulating with great velocity through the arteries and veins.

The phenomena of crystallization are exceedingly beautiful when seen by this microscope. If a drop of a solution of sal ammoniac, for example, be poured upon a plate of glass, and then introduced into the instrument, the heat will cause the water to evaporate, producing one of the most beautiful examples of crystallization that can be exhibited.

The minute animalculæ of solutions and stagnant water can de shown by this microscope.

When the light of the sun is used instead of the electric light, the apparatus is called the solar microscope.

332.

The Diorama.

The DIORAMA Consists of two pictures, one on each side of a transparent muslin screen, these pictures, as in

How is the magnifying power varied? What are its advantages? How is the circulation of the blood shown? The phenomena of crystallization? Animalculo? What is a solar microscope? (332.) What is the Diorama?

the polyrama, being different effects of the same scene. One of these pictures is seen directly, and the other by transmitted light, and the illusion arises from the light being

managed so as to produce either of these effects at pleasure. Fig. 225 explains the manner of exhibiting this kind of