08 THE UNIVERSE A VAST SYSTEM OF PARTS 36 inches to 72.5 inches, and more than forty large refractors in use throughout the world. STUDY QUESTIONS 1. Give the principles of the two types of telescopes. 2. What type of telescope is used at (a) the Mount Wilson Observatory, (b) the Yerkes Observatory, (c) the Lick Observatory? 3. Which type of telescope should be used for observing the details of the surface of the moon, and why should that tyjje be selected? 4. What type of telescope did Galileo use? 5. Why are many telescopes located on mountains? 6. How are modern telescopes kept focused on a given star as the earth moves? 7. What is a light-year? 8. Of what possible value will the 200-inch telescope be to humanity? 9. Why is the 200-inch mirror made from "Pyrex" glass? 10. Why are the largest telescopes reflectors rather than refractors? 11. What determines the magnifying power of a telescope? 12. What are the relative advantages and disadvantages of reflecting and refracting telescopes? 13. What are the functions of the objectives in telescopes? 14. Inasmuch as telescopes do not depend upon magnification to see distant stars, how do they enable us to see stars invisible to the unaided eye? 15. What is spherical aberration, and how is it overcome? 16. What is chromatic aberration, and how is it overcome? 17. What is meant by the resolving-power of a lens? 18. Why are very large refractors less practical than very large reflectors?
UNIT II SECTION 3 MANY INSTRUMENTS SUPPLEMENT THE TELESCOPE IN EXPLORING THE UNIVERSE Introduction. Nowhere is man's ingenuity shown to better advantage than in his invention of instruments to aid him in making observations. In many cases the information obtained requires interpretation in the Hght of modern knowledge, and it is always possible that, as more knowledge becomes available, these interpretations will have to be changed. It is important, therefore, to keep in mind that the interpretations presented in this section are in some cases several years old and it quite possible that some of them now need revision. The Camera Is More Sensitive Than the Eye. By substituting a camera for the eye, the telescope becomes much more useful, because it not only enables man to gather all of the light from a star for many consecutive hours, but it provides a lasting record which may be studied at leisure and be compared with similar photographs taken from time to time. The Spectrometer Is One of Man's Most Valuable Instruments. To Newton we owe the discovery that light rays are bent when they enter a prism, the visible and invisible rays forming a continuous spectrum in which all of the rays originally present are separated. We now know that the rays are separated because the rays of short wave length are bent more than the rays of long wave length as they pass through the prism. If red light rays are absent in the original light rays, there will be a gap in the spectrum where red should occur, and so on for all of the visible and invisible light rays. The invisible short ultraviolet and long infrared rays, as well as the visible rays, may be photographed, so that a camera generally replaces the eye in a spectrometer. The spectrometer has two tubes, one of which collects a thin beam of light through a narrow slit and renders the light parallel by means of 69 is