THE description of the polyorama naturally follows that of the phantasmagoria, being a practical application of precisely the same principles. In the case of the polyorama, however, two or even more lanterns of the best construction, are used. There are therefore two sets of lenses identical in every particular, placed side by side, in the same line, the foci of both being adjusted for the same spot, so that the images refracted from each may superpose each other without difficulty. In each instrument there are the same pictures, but they differ in certain particulars, as we shall see presently. In the phantascopes shown in figs. 52 and 54 there are two sets of lenses; the first carries a glass bearing the image of a skeleton in a winding sheet, while on the glass belonging to the second a naked skeleton is portrayed. If, therefore, at a given instant the first lantern is shut off, the spectators see the winding sheet torn, as it were, suddenly from the spectre before them. The first lantern being turned on once more, the skeleton is instantly reclothed in its hideous garb. It is of course not necessary always to choose such horrible subjects for representation, as it is possible to produce changes of a much more agreeable nature. For instance, a volcano may be depicted during its tranquillity, with the sun shining on its verdant sides, and surmounted with a gently rising wreath of smoke. Then it may be shown at night, with its crater vomiting flames and red-hot stones, while streams of lava are flowing beneath. By proper mechanism, one lantern may be gradually shut and the other as gradually opened, producing an effect that appears perfectly natural, from the gentle change which takes place. Daylight, twilight, and moonlight effects may be easily made to succeed each other in their proper order, and the most opposite scenes may be made to change each other by proper appliances. Those who have seen the dissolving views at the Polytechnic, know what effects are produced by this very simple means. A virgin forest changes to a crowded church, which in turn dissolves into a scene on the Alps. The diorama, properly so called, invented by the illustrious Daguerre, differs completely in principle from the apparatus we have just been describing. As its etymology indicates, the pictures shown are seen _through_. As in the case of the polyorama, there are two different effects painted upon the cloth, which are brought out by a double system of illumination. [Illustration: FIG. 66.—Diorama. ]