Page:The wonders of optics (1869).djvu/240

 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.

Fig. 66 will show the way in which these changes are managed. The large picture, which is hanging vertically, is painted both in front and behind. The front is illuminated by reflection from a semi-transparent screen placed over it, which receives the light of the floor above. The back is lighted from the windows behind, which are provided with blinds to regulate the amount of light. The effects produced by the diorama were truly marvellous, and Daguerre had a special talent for this kind of painting. His famous ''Midnight Mass'', which was exhibited at the Regent's Park, was one of the most renowned of his works. The scene first represented a dark, empty church, feebly lighted by a