=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. Important Topics 1. The eye: parts, formation of image, kind, how, where. 2. Eye defects, how remedied. Visual angle. 3. Simple microscope, camera; images, kind, how formed. 4. Compound microscope, telescope and opera glass; images, action of each lens. Exercises 1. Name three instruments in which lenses form virtual images and three in which _real_ images are formed. 2. In what direction is an oar in water apparently bent? Explain by a diagram. 3. What optical instruments have you used? Is the _visible_ image formed by each of these _real_ or _virtual_? 4. The focal length of a copying camera lens is 14 in. Where must a drawing be placed so that an image of the same size may be formed upon the ground glass screen? What must be the distance of the screen from the lens? 5. What are two methods by which you can determine the focal lengths of the lens of a photographic camera? 6. The critical angle for water is 48-1/2 degrees. Show by a diagram how much of the sky can be seen by a diver who looks upward through the water. 7. How is near-sightedness caused? How is it corrected? Illustrate by a diagram. 8. How is the eye accommodated (focused) as an object gradually approaches it? 9. Explain why a simple microscope assists in looking at the parts of a flower or insect. 10. Why do people who have good eyesight when young require glasses as they grow old? (7) COLOR AND SPECTRA [Illustration: GUGLIELMO MARCONI "Copyright by Underwood & Underwood, N. Y." Guglielmo Marconi (Italy). Inventor of wireless telegraphy.] [Illustration: ALEXANDER GRAHAM BELL "Copyright by Underwood & Underwood, N. Y." Alexander Graham Bell, Washington, D. C. Inventor of the telephone.]