178 



EARTH AND HEAVENLY BODIES 



Many years elapsed, however, before the true pos- 

 sibilities of the telescope were perceived. Today two 

 types of astronomical telescopes are in use : the re- 

 fractor and the reflector. In the refracting telescope (see 

 Fig. 292) a convex lens is mounted at the upper end of 

 the tube which is pointed toward the heavenly body. 

 The light, after passing through the lens, is brought 



Courtesy Mount Wilson Observatory 



FIG. 294. LARGE REFLECTING TELESCOPE AT MOUNT WILSON 

 OBSERVATORY 



to a point near the lower end of the tube and forms an 

 image of the heavenly body. The image may be viewed 

 through an eyepiece at the lower end of the tube or it 

 may be photographed on a sensitive plate. One of the 

 largest instruments of this type is the 36-inch refrac- 

 tor of the Lick Observatory. 



Reflecting telescopes are constructed quite differ- 

 ently. In this type of instrument a concave mirror (see 

 Fig. 292) is used in place of a lens to collect the light. 

 The reflection occurs on the upper surface of the 

 mirror, which is covered with a coat of pure silver. 

 At the time this is written the largest instrument of 

 this type is the 100-inch reflector of the Mount Wilson 

 Observatory 'in California. However, a 200-inch re- 

 flector is in the process of construction. In modern 

 astronomical observatories both refracting and re- 

 flecting telescopes are used, each for the purpose for 

 which it is best adapted. Since it is much easier to 



make a mirror than a lens, it is a favorite exercise 

 for amateur astronomers to make their own reflecting 

 telescope. To grind and polish a mirror requires pa- 

 tience, care, and skill, but many good ones have thus 

 been made. 



REFERENCES FOR FURTHER STUDY 



Texts 



Caldwell and Curtis, Science for Today, Chap. 15 



Clement, Collister, and Thurston, Our Surroundings, Chap. 11 



Hunter and Whitman, Science in Our World of Progress, 



Unit 8 

 Lake, Harley, and Welton, Exploring the World of Science, 



Unit 5 

 Pieper and Beauchamp, Everyday Problems in Science, Unit 



1 

 Powers, Neuner, and Bruner, This Changing World, Chap. 1 ; 



Man's Control of His Environment, Chaps. 31 and 32 

 Skilling, Tours through the World of Science, Tour 6 

 Van Buskirk and Smith, The Science of Everyday Life, pp. 



293-299 



Watkins and Bedell, General Science for Today, Chap. 11 

 Webb and Beauchamp, Science by Observation and Experi- 

 ment, Unit 6 

 Wood and Carpenter, Our 'Environment: How We Adapt 



Ourselves to It, Topic 1 



Special references 



Collins, The Book of Stars 



Martin, The Friendly Stars 



McPherson, The Romance of Modern Astronomy 



Ingalls, Amateur Telescope Making 



Downing, Our Physical World 



Bond, The American Boys' Engineering Book 



Courtesy Corning Glass Works 



FIG. 295. THE 200-INCH REFLECTOR 



