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THE POPULAR SCIENCE MONTHLY 



Pig. 14. Head of the 75-foot Spectrograph of the 150-foot Tower Telescope. 



this is successful, it will not only show beyond doubt whether the sun 

 is a magnet : it will also permit the polarity of the sun to be compared 

 with that of the earth, give a measure of the strength of the field at 

 different latitudes, and indicate the sign of the charge that a rotating 

 sphere must possess if it is to produce a similar field. 



I first endeavored to apply this test with the 60-foot tower telescope 

 in 1908, but the results were too uncertain to command confidence. 



Thanks to additional appropriations from the Carnegie Institution 

 of Washington, a new and powerful instrument was available on Mount 

 Wilson for a continuation of the investigation in January, 1912. The 

 new tower telescope has a focal length of 150 feet (Fig. 13) . To prevent 

 vibration in the wind, the ccelostat, second mirror and object-glass are 

 carried by a skeleton tower, each vertical and diagonal member of which 

 is enclosed within the corresponding member of an outer skeleton tower, 

 which also carries a dome to shield the instruments from the weather. 

 In the photograph, we see only the hollow members of the outer tower. 

 But within each of them, well separated from possible contact, a sec- 

 tional view would show the similar, but more slender members of the 

 tower that suppoits the instruments. The plan has proved to be suc- 

 cessful, permitting observations demanding the greatest steadiness of 

 the solar image to be made. 



The arrangements are similar to those of the 60-foot tower. The 

 solar image, 16J inches in diameter, falls on the slit of a spectrograph 



