Astronomical Observatories 

 mical research afforded by the atmospheric con- 

 ditions in the Pacific area, with the result that many 

 observatories, public and private, have been estab- 

 lished here. 



The most important of these is the great Solar 

 Observatory of the Carnegie Institution of Washing- 

 ton. This is located on Mount Wilson (altitude 

 5885 feet), near Pasadena, California. (See PI. 

 XXVI.) As its name implies this observatory was 

 established primarily for investigations of the sun, 

 considered not only as the central body of our own 

 system, but as a typical star, the only one near 

 enough to us to be studied in detail. The equip- 

 ment was planned with direct reference to the pro- 

 posed investigations. The principal instruments on 

 Mount Wilson are: 



A 60-inch reflecting telescope, equatorially 

 mounted and equipped with secondary mirrors to 

 convert it into the Newtonian and Cassegrain forms, 

 and with a variety of spectrographs and other aux- 

 iliary apparatus. 



A horizontal reflecting telescope, aperture 24 

 inches, whose focal length can be varied from 60 

 feet to 145 feet. It is fed by means of coelostat 

 mirrors and supplied with spectro-heliographs, etc. 



Two "tower" telescopes, in which the coelostat 

 mirrors on the top of a tower receive light from 

 the sun and send it vertically dow^nward through 

 a lens to form an image near the ground at the top 

 of a "well." The well contains spectrographs 

 mounted verticallv in such positions that their slits, 

 at the top of the well, are in the focal plane of the 

 lens on the tower. The first tower telescope had a 

 lens with a focal length of 60 feet, and a well 30 

 feet deep. It proved so successful that, in 1910, a 

 similar telescope 150 feet high, supplied with a 

 well 75 feet deep was constructed. The lens on the 

 tower supplies an image of the sun about 17 inches 

 in diameter, and the large scale spectrographs in the 

 well enable exceedingly minute details of the solar 

 image to be subjected to powerful analysis. 



A reflecting telescope with a mirror 100 inches 

 in diameter and an equatorial mounting is ap- 

 proaching completion. 



A unique feature of the Solar Observatory con- 

 sists in the maintenance of a physical laboratory, 

 equipped with the most refined and powerful in- 

 struments of their class, whose principal function 

 is to assist in the interpretation of phenomena ob- 

 served in the sun and the stars. 



The mountain station is used to secure the as- 

 tronomical observations, nearly all of which are 



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