MOUNT WILSON OBSERVATORY. ' 



George E. Hale, Director. 

 SUMMARY OF THE YEAR'S WORK. 



Three outstanding events, combined with vigorous and successful 

 work in all departments of research, render the past year a memor- 

 able one in the history of the Observatory. The first of these is the 

 publication by Dr. Adams and his associates of the absolute mag- 

 nitudes and parallaxes of 1,646 stars, and the deduction from these 

 results of the impK)rtant generalizations outlined in this report. The 

 second advance of great significance is the successful application of 

 the interferometer by Messrs. Michelson and Pease to the measure- 

 ment of star diameters. No less important in its future possibili- 

 ties is the establishment in Pasadena of the Norman Bridge Physical 

 Laboratory of the California Institute of Technology and the accep- 

 tance by Dr. R. A. Millikan of its directorship. It will be advantage- 

 ous to consider the bearing of these events on the progress of the 

 Observatory before summarizing the other activities of the year. 



The two first-named advances, and in a different sense the third, 

 illustrate the value in research of the development of new instruments 

 and methods. Prior to 1900, only 60 stellar parallaxes had been meas- 

 ured by the laborious methods, for the most part visual, applied up 

 to that time. The work of Dr. Schlesinger with the 40-inch Yerkes 

 refractor initiated a school of parallax measurers, whose efficient use 

 of photographic methods added new and more precise determinations 

 at such a rapid rate that the total number of trigonometric parallaxes 

 is now about 1,400. In 1915, Dr. Adams began systematic application 

 with the 60-inch reflector of an entirely new method, which gives a 

 parallax mea'sure, of high precision, from the simple comparison of the 

 relative intensities of two lines on a stellar spectrum photograph. In 

 five years 2,000 stellar parallaxes have been determined at Mount 

 Wilson by this beautiful process, which has been applied with the 

 100-inch reflector to stars as faint as the 9th magnitude, and could be 

 pushed to much fainter objects. Thus, while the value of trigono- 

 metric parallaxes is by no means diminished, but rather increased, 

 by the introduction of the spectroscopic method, the range of action 

 and the rate of progress have been advanced in very high decree. 



The best evidence of this advance is afforded by the conclusions 

 based on the new measures. In complete confirmation of the earlier 

 work of Adams and Joy, these establish beyond doubt the validity of 

 Russell's views on giant and dwarf stars. In their early oi giant stage 

 the stars are immensely inflated gaseous masses, so tenuous that their 

 density may be as low as one-thousandth that of atmospheric air. 

 As they condense, their temperature rises and their reddish color 



^ Situated on Mount Wilson, California. Address, Pasadena, California. 



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