NO. 1865 OUR knowledge; of the sun — HALE 337 



spectroscopes and other instruments developed during the preceding 

 quarter of a century in the physical laboratory, but still unused in 

 the observatory. 



2. The development of the spectroheliograph and of other research 

 methods involving new principles. 



3. The development of the reflecting telescope, in forms adapted 

 for solar research and for physical investigations of the stars and 

 nebulae. 



4. The more adequate recognition of the close union which should 

 unite laboratory researches with solar and stellar investigations. 



The opportunities enumerated above relate to the possibility of 

 improving and extending the methods of astrophysical research. An- 

 other special opportunity had its origin in the basic principles which 

 underlie the Carnegie Institution. A large proportion of the world's 

 observatories are connected with universities or with institu- 

 tions affected by local interests. The Carnegie Institution es- 

 tablishes its laboratories and observatories on the islands of the 

 Carribbean Sea, the deserts of Arizona, the mountains of Cali- 

 fornia, and at other points where their work can be done 

 most efifectively. On Mount Wilson, the long periods of cloud- 

 less weather, the purity of the atmosphere, and the absence, 

 during a large part of the year, of winds and atmospheric 

 fluctuations which seriously hamper astronomical work in most 

 parts of the world afford great advantages. To illustrate the purity 

 of the night sky, two photographs of the Pleiades, one made with an 

 exposure of 9'' 47"^ at Williams Bay, Wisconsin (1,200 feet), the 

 other made at Mount Wilson (5,886 feet), with an exposure of only 

 3^ 48™, are reproduced in Plate xxiv. These were both taken by 

 Professor Barnard with the lo-inch Bruce photographic telescope, on 

 plates of equal sensitiveness and on nights of normal clearness at 

 each station. Though the exposure time was two and one-half 

 times longer at Williams Bay, yet the number of stars recorded at 

 Mount Wilson is fully as great and the details of the nebula much 

 sharper. Other proofs of the fine quality of the Mount Wilson at- 

 mosphere are afforded by many visual and photographic observa- 

 tions, made by night and by day, during the past three or four 

 years. 



Plate XXV shows the summit of Mount Wilson, where a large tract 

 of land has been set apart for the purposes of the observatory. This 

 site commands a magnificent view of southern California, extending 

 on the east to the snowy peaks of the San Bernardino Range, on 

 the west to islands lying far out in the Pacific, on the north to an 



