Fig. 393.—Correction of far-sightedness by a convex lens.
Fig. 394.—Test card for astigmatism.
393. The Photographic Camera.—This is a light-tight box, provided with a convex lens in front, covering an aperture and a ground glass screen at the back. The distance between the lens and the screen is adjusted until a sharp image is obtained upon the latter, which is then replaced by a sensitive plate or film. The sensitized surface of the plate or film contains a salt of silver which is changed by the action of light. After the plate has been "exposed" to the action of light, it is "developed" by the use of chemicals producing a negative image. From "negative," by the use of sensitized paper, "positive" prints may be secured which resemble the object photographed.
Fig. 395.—Diagram of the projecting lantern.
394. The projecting lantern (see Fig. 395) employs a strong source of light, as an electric arc lamp L, to strongly illuminate a transparent picture, or lantern slide, S, a real image (I) of which is formed upon a large screen. Two large plano-convex lenses (C), called condensing lenses, are placed near the lamp to concentrate the light upon the "slide" S. The convex lens forming the image is called the "objective" (O).
395. The compound microscope consists of two lenses. One called the objective is placed near the object to be viewed. This lens has a short focal length usually less than a centimeter. It forms a real image of the object. A´-B´. The other lens, the eyepiece forms a virtual image of this real image. A´´-B´´. (See Fig. 396.)
396. The telescope consists of two lenses, the eyepiece and the objective. As in the compound microscope, the objective of the telescope forms a real image of the distant object, the eyepiece forming an enlarged virtual image of the real image. It is the virtual image that is viewed by the observer. (See Fig. 397.) In order to collect sufficient light from distant stars the objective is made large, sometimes 50 in. in diameter.
Fig. 396.—Formation of an image by a microscope. A-B is the object. B´-A´ the real image formed by the "objective." B´´-A´´ is the virtual image formed by the eyepiece. The eye sees the virtual image.