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at the heavenly bodies; but in a telescope to be used on objects on the surface of the earth, it is more convenient to have the image upright. This result is secured usually by putting two converging lenses, at proper distances, between the object glass and the eyepiece.
560. Galileo's Telescope; the Opera Glass. The simplest, and also the oldest form of telescope is Galileo's tel
escope. This has but two lenses; namely, a convex object glass, and a concave eyepiece (Fig. 527). The eyepiece is placed between the object glass and the image formed
FIG. 527.-Galileo's Telescope
by it. Being concave, the eyepiece causes the rays to diverge and appear to the eye to come from ab, which thus forms an enlarged upright image. Opera glasses consist of a pair of these telescopes.
561. The Prism Binocular. - The insertion of the two converging lenses between the object glass and the eyepiece insures an upright image in the terrestrial telescope, but increases the absorption of light. The use of a pair of Porro prisms gives an upright image and shortens the tube of the instrument so that it can be used, in the form of a binocular, as an opera glass. Figure 528 represents such an
FIG. 528. Prism
opera glass, half in cross section. The arrow represents the path of a ray of light through the instrument. This ray is totally reflected from the inner surfaces of the prisms four times.
562. The Optical Lantern is used for the purpose of throwing an enlarged picture of a lantern slide upon a screen. The
essential parts are some brilliant source of light, a condensing lens C, formed of two plano-convex lenses with curved surfaces toward each other, a glass lantern slide S, upon which is the picture to be enlarged, and a projecting lens P, a combination of lenses for enlarging the picture. The electric arc is the best artificial light for projection, since the light is intense and its size is small. The lime light, in which a cylinder of lime is raised to incandescence by the heat from an oxyhydrogen blowpipe, is a good light. In cases where the lantern is to be used in a small room, an incandescent lamp with a spiral filament is satisfactory. Such lamps are made to give from 50 to 100 candle power, and are most convenient wherever there is an incandescent circuit.
Book illustrations or opaque objects can be used for projec
tion instead of lantern slides by directing the light from an arc light or other brilliant source against the picture. An image
of this picture is thrown upon the screen by a projecting lens. Some lanterns, like the balopticon, are so arranged that both lantern slides and pictures can be used.
Figure 530 shows the use of two mirrors with the lantern slide. When a picture is to be projected, the lower mirror is raised and the light falls directly upon the picture as in Fig. 531. The complete apparatus is shown in Fig. 532.
The moving-picture machine is a form of
lantern used to throw upon a screen in rapid succession a series of pictures taken at intervals of a small fraction of a second. Sixteen pictures per second is the usual number. The retina of the eye re
tains each image until the next is presented and links them together to show continuous motion. (Figs. 533, 535.)
563. The Camera, used in photography, consists of a lighttight box having at one end an achromatic lens and at the other a ground-glass plate and a space in which a plateholder containing a sensitized plate can be placed. In order to regulate the amount of light passing through the lens and to increase the distinctness by cutting off the outside rays, the lens is provided with a series of diaphragms with different-sized openings. The camera is so constructed that the distance