=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. [Illustration: 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.] The length of the telescope tube depends upon the focal length of the objective, since the distance between the two lenses must equal the _sum_ of their focal lengths. [Illustration: FIG. 397.--Formation of an image by a telescope. _b_-_a_ is the real image; _d_-_c_ is the virtual image seen by the observer.] =397. The opera glass= consists of a convex lens as objective and a _concave_ lens as an eyepiece. The former tends to form a real image but the latter diverges the rays before a real image can be formed, the action of the two lenses producing an enlarged virtual image (as in Fig. 398) which is viewed by the one using the glass. The compact size of the opera glass is due to the fact that the distance between the two lenses is the _difference_ of the focal lengths. [Illustration: FIG. 398.--Formation of an image by an opera-glass. _a_-_b_ is the virtual image.] [Illustration: FIG. 399.--Diagram of the Zeiss binocular or prism field glass.]