MOUNT WILSON OBSERVATORY. 



George E. Hale, Director. 



SUMMARY OF THE YEAR'S WORK. 



The 100-inch Hooker telescope, yielding results that are of service 

 in all departments of the Observatory's work, has remained a center of 

 interest throughout the year. In light-gathering power, in the pho- 

 tographic registration of minute details of structure, and in the separa- 

 tion and measm'ement, by Michelson's interference method, of double 

 stars previously unresolved, this instrument has responded admirably 

 to a wide variety of tests. Through its aid we have been enabled to 

 carry our researches into new and profitable fields, thus considerably 

 enlarging our research program. Before fixing the details of this pro- 

 gram, it has seemed advisable to determine in what ways the capacity 

 of the telescope could be most fully utilized, and for this reason much 

 attention has been given to experiments with promising accessory 

 apparatus and methods. 



Altogether the most significant of the auxiliaries thus developed is 

 Michelson's interferometer, which promises to play an important part 

 in the future of sidereal astronomy. Its possibilities in this field were 

 clearly foreseen and fully described by Michelson in 1890, and one of 

 them was successfully tested by him in his measurement of the satellites 

 of Jupiter in the following year. The fact that no astronomical appli- 

 cations of the method have since been made is not easily explained. 

 Astronomers acquainted with the extreme sensitiveness of the inter- 

 ferometer, and constantly hampered by atmospheric disturbances, 

 have naturally feared that differences in optical path would obliterate 

 the fringes. But it turns out that they can be clearly observed with 

 large apertures even when the seeing is poor. 



A crucial test was made September 18, 1919, when Professor Michel- 

 son, at the first trial on Altair, had no difficulty in seeing the fringes 

 with the full aperture of the 60-inch and 100-inch telescopes. The 

 essence of the method lies in the use of tvv^o slits, symmetrically placed 

 on either side of the axis of the telescope, and so mounted in a rotating 

 support that their distance apart can be varied. As the mirror is other- 

 wise covered, the only light entering into the focal image is that which 

 passes through the slits. The conditions are thus precisely analogous 

 to those of Fizeau's classical experiment, in which two pencils of 

 light, derived from a single point-source, are brought to interference. 

 Observed with a high-power eyepiece (2,000 to 10,000 diameters) the 

 fringes appear as sharp, narrow lines on a fluctuating background. 

 In a case like that of the close double star Capella, the components of 

 which, about 0T05 apart, are not visible in any telescope, there are 



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