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.
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.
Fig. 398.—Formation of an image by an opera-glass. a-b is the virtual image.
Fig. 399.—Diagram of the Zeiss binocular or prism field glass.
398. The Prism Field Glass or Binocular.—This instrument. has come into use in recent years. It possesses the wide field of view of the spy glass but is as compact as the opera glass. This compact form is secured by causing the light to pass back and forth between two right-angle prisms (as shown in Fig. 399). This device permits the use of an objective lens with a focal length three times that of the tube, securing much greater magnifying power than the short instrument would otherwise possess. A further advantage is secured by the total reflection from the two prisms, one of which is placed so as to reverse the image right for left and the other inverts it, so that when viewed in the eyepiece it is in its proper position.