EXPLORING THE GLORIES OF THE FIRMAMENT 



159 



Photograph from Lick Observatory 



HOW THE MOON LOOKS THROUGH A 36-INCH TELESCOPE 



A clay on the moon is four of our weeks long. If our mountains were as high in propor- 

 tion to the size of the earth as those on the moon, they would he fifteen miles high: a man 

 there would weigh only as much as a five-year-old hoy here. Note the size of the sphere in 

 the telescope by extending the arc in the upper left-hand corner into a circle (see page 165). 



build an artificial pupil that serves the 

 same purpose. Men call such artificial 

 pupils telescopes. Imagine trying to fill 

 a narrow-necked bottle by catching rain- 

 drops as they fall. Rain falls all around, 

 but only a few drops go into the bottle. 

 Put a wide-mouthed funnel into the neck 

 of the bottle and see how much more 

 water you catch. The telescope is merely 

 a light funnel, wide-mouthed enough to 

 catch many rays of light and to bring 

 them so close together that they can all 

 enter the pupil of the human eye. 



Many of these huge instruments have 

 tubes of greater diameter and length than 

 the dimensions of the most powerful gun 

 ever built. They have grown larger and 

 stronger in a way that is startling. In 

 1861 the 18-inch Dearborn telescope was 



the biggest in existence. It was when 

 adjusting that instrument that Alvan G. 

 Clark discovered the elusive companion 

 of gay Sirius (see pages 154 and 155). 



THE BIG YERKES INSTRUMENT 



Typical of the big refracting telescopes 

 is the 40-inch equatorial at the Yerkes Ob- 

 servatory. The outstanding impression 

 one gets when studying the surpassing 

 delicacy of its mechanical manipulation 

 is that our knowledge of the infinitely 

 large comes from our mastery of the in- 

 finitely small (see page 161). 



The big lens of this instrument weighs 

 a thousand pounds and is carried in the 

 upper end of the six-ton. 62-foot tube, 

 which is -,2 inches in diameter at the 

 center. To train this big spyglass on a 



