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338 THE ADVANCE OF PHOTOGRAPHY
be enlarged. The images obtained vary in size with the distance of the screen from the lens.
This instrument was formerly nothing but a plaything, but it has latterly become an important auxiliary in instruction. Photographs of microscopic preparations, of animals, plants, minerals, landscapes, national types, and architecture, may be in this manner more faithfully represented than by maps, which are in general very imperfectly designed."
Universal Projection Apparatus. — We will not consider next one of the many excellent lanterns fitted with limelight or arc-light lamps, nor even a biunial or triple lantern, such as are in frequent use not only where dissolving views are exhibited, but also for many other purposes ; but we will briefly describe a projection apparatus suitable for projecting not only images of slides, but also images of apparatus and experimental arrangements, etc., upon the screen.
Such a piece of apparatus is that known as the universal projection apparatus. (Fig. 148.)
This picture gives a good idea of the general appearance of the apparatus. The frame, which is of great rigidity, is an iron one, and the parts of the frame are connected by means of steel tubes. While the instrument is in use it is enclosed in a light-tight screen, so that the rest of the room can be kept in complete darkness if desired. The lamp used is one of the simple arc lamps previously described, and is especially suited for this kind of work, since it regulates itself automatically, and the light is always perfectly centred ; besides, from the arrangement of the carbons, the lighting capacity of the lamp is increased enormously.
Let us now examine a few of the many uses to which this apparatus may be put. It will be seen on looking at the illustration, p. 337, that the instrument is there shown with an attachment for projecting images of microscopic