ASTRONOMICAL OBSERVATORIES 



been devoted to determining the law of the solar 

 rotation. 



While the sun has formed the chief object of 

 study at Mount Wilson, the great 60-inch reflector 

 has heen used to photograph the more prominent 

 spiral and irregular nebulae, and the principal star 

 clusters, to obtain .spectra of special classes of 

 stars and of the Milky Way, to determine the radial 

 velocities of stars, and to establish a scale of photo- 

 graphic magnitudes. 



The photographs of nebulae and of clusters are 

 of the highest excellence. They show that nebulous 

 stars in great numbers are associated with many of 

 the spiral nebulae and in the clusters reveal unex- 

 pectedly great numbers of stars fainter than those 

 photographed with smaller reflecting telescopes. 



Some evidence has been obtained to show that 

 light from the stars is absorbed appreciably in its 

 passage through interstellar space. Many investiga- 

 tions are in progress to determine the arrangement 

 of stars in space and the relations to each other of 

 great groups of stars. The great reflector has also 

 proved to be an admirable instrument for visual 

 observations of planetary surface features. 



The extensive researches in the phvsical labora- 

 tory at Pasadena relate chiefly to the effects of vary- 

 ing temperatures, pressures, magnetic fields and 

 other factors on the spectra of the principal chemi- 

 cal elements. In many cases the results have been 

 applied to the interpretation of solar and stellar 

 spectra. 



The observatory of the Smithsonian Institution 

 has maintained a Branch station on Mount Wilson, 

 within the grounds controlled by the Solar Observa- 

 tory, since 1905. It is occupied in the summer 

 months for the purpose of making measures of the 

 sun's radiation, the radiation from the sky, clouds, 

 etc., to compare with similar researches made in 

 Washington, in Algiers (for two years), and on 

 Mount Whitney. These researches have determined 

 the average intensity of the solar radiation as a 

 function of the spottedness of the sun and have 

 made it very probable that the radiation varies as 

 much as 8 or 9 per cent in irregular periods of from 

 7 to 10 days. 



In 1909, a three-room shelter of stone, steel and 

 glass was built by the Smithsonian Institution upon 

 the summit of Mount W T hitney (altitude 14,500 

 feet). It is intended that this shall be available to 

 scientific parties who wish to occupy the summit of 

 the mountain for research purposes. Detailed in- 

 fomation regarding the conditions may be obtained 



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