TELESCOPE 



5743 



TELESCOPE 



the telescope was so great that it can hardly 

 be exaggerated.- 



The transforming power of the telescope on 

 man's thinking was shown from the very in- 

 vention of the instrument in the seventeenth 

 century. Though he was not the original in- 

 ventor, Galileo is given credit for bringing out 

 the first practical instrument. His first tele- 

 scope was a crude affair, and the best one he 

 was able to devise before his death magnified 

 but thirty times. Nevertheless, he was able to 

 discern four of the satellites of Jupiter and to 

 discover characteristics of the sun, the moon 

 and other heavenly bodies that had not been 

 known before. 



Under the action of a powerful lens, the sum- 

 mer heavens are transformed, and they swarm 

 with countless glittering points of light. The 

 naked eye may see between two thousand and 

 three thousand of these stars or suns; the tele- 

 scope reveals a multitude that has been esti- 

 mated at a hundred million. This great num- 

 ber is of course only an estimate, but patient 

 astronomers have charted about 600,000. Stars 

 that seem single to the unaided vision are split 

 up by the telescope into two or three stars 

 turning about one another in space. The 

 Milky Way, which makes but a luminous patch 

 across the sky, is dissolved into countless multi- 

 tudes of distinct stars. Among the seven chief 

 stars of the Pleiades the telescope shows a star 

 cluster in which 2,326 stars have been counted. 



Structure. The essential parts of a telescope 

 are an objective (also called object glass and 

 object lens) for the formation of an image of 

 the object under observation, and an eyepiece for 

 magnifying the image. These parts are set in 



o 



PRINCIPLE OF THE TELESCOPE 

 Explanation appears in text. 



a tube so constructed that the observer can 

 lengthen or shorten the distance between them. 

 Astronomical telescopes are of two types, re- 

 fracting and reflecting. In refractors the ob- 

 jective is a large convex lens of long focus, and 

 the eyepiece a convex lens of short focus. In 

 reflectors the object glass is a concave mirror 

 which reflects the rays of light to a focus. 

 " The accompanying diagram shows the prin- 

 ciple of the astronomical refractor. The object 

 viewed is marked CD; m is the objective, and 



it forms an inverted image cd. This is viewed 

 through the eyepiece M, which produces a 

 magnified image, ab, of the first image. The 

 first image, cd, is as much smaller than the 

 magnified image, ab, as the focal distance of 

 the eyepiece is smaller than that of the ob- 

 jective. The objective has to be made large in 

 order to collect enough light to permit con- 

 siderable magnifying of the image without too 

 much loss of distinctness. 



Terrestrial telescopes, which are constructed 

 for viewing objects on the earth, have two 

 double-convex lenses between the eyepiece and 

 the objective, and as the rays diverge from the 

 inverted image they cross and form an erect, 

 magnified image. The inversion of the image 

 which takes place in astronomical instruments 

 does not interfere with the accuracy of obser- 

 vations made by the astronomer. 



Great Modern Telescopes. The largest re- 

 fracting telescope in the world is that in the 

 Yerkes Observatory, on the north shore of 

 Lake Geneva, Wisconsin. It is the property of 

 the University of Chicago. The object lens has 

 an aperture of forty inches and weighs about 

 760 pounds. The tube is sixty-two feet in 

 length, has a weight of six tons, and is mounted 

 upon a massive cast-iron column, which rests 

 upon a solid concrete foundation. Clockwork 

 located in an upper section of the column 

 drives the mechanism. The apparatus occupies 

 a dome ninety feet in diameter, which has a 

 rising floor seventy-five feet across. This floor 

 moves through a range of twenty-three feet be- 

 tween two balconies, and is close to the lowest 

 point when the observer is viewing a star near 

 the zenith (see YERKES OBSERVATORY). Second 

 only to the Yerkes refractor is the thirty-six- 

 inch telescope of the Lick Observatory at 

 Mount Hamilton, California (see page 3406). 



The great reflecting telescope of Lord Rosse, 

 at Birr Castle, in Ireland, was for a long time 

 the largest of its kind in the world. Since 1914 

 it has been a possession of the South Kensing- 

 ton Museum of Science. The reflecting mirror 

 of this instrument is six feet across, but the 

 telescope is surpassed in size by those of Mount 

 Wilson Solar Observatory (8 feet, 4 inches) and 

 the Dominion Observatory, near Victoria, Brit- 

 ish Columbia (6 feet, 1 inch). Other notable 

 reflectors are those at Harvard University, Mel- 

 bourne and Paris. The telescope at Harvard, 

 which has a mirror five feet across, has been 

 used with remarkable success in photographing 

 the stars. How telescopes are used in photog- 

 raphy is told in the article ASTRONOMY. G.B.D. 



