MEASUREMENT OF STAR DIAMETERS BY THE 

 INTERFEROMETER METHOD. 



By F. G. pease. 



{Read April 22, 1921.) 



The idea of measuring the angular diameter of a fixed star by 

 the method of interference of light beams was suggested by Fizeau 

 in his report on the Bordin Prize to the French National Academy 

 in 1868. Stephan spent the year 1874 examining all the brighter 

 stars by this method with the 31.5-inch telescope of the Marseilles 

 Observatory, and rightly found that the telescope was altogether too 

 small for the purpose. The papers relating to this work will be 

 found in Comptes Rendiis, LXVI., p. 1008, 1873, and LXVIII., p. 

 1008, 1874. 



Nothing further appeared on this subject until Michelson in 

 1890 published a masterly paper in the Philosophical Magazine, " On 

 the Application of Interference Methods to Astronomical Meas- 

 ures." He pointed out that the telescope itself need not be large, 

 and that the results desired might be accomplished by the use of a 

 double periscopic arrangement consisting of four small auxiliary 

 mirrors placed in a frame on the end of the telescope. Designs for 

 a periscopic attachment are shown by Michelson in his paper but 

 his successful measures of the satellites of Jupiter with the 12-inch 

 refractor of the Lick Observatory in 1891 were made with two 

 apertures placed directly in front of the objective. Aside from this 

 question of size, the interferometer was considered an instrument 

 of the utmost precision requiring the best of optical conditions, and 

 the belief was general that the disturbances of the atmosphere 

 would probably be such as to prevent its successful use even if a 

 large telescope should be built. After the 100-inch Hooker tele- 

 scope had been completed and excellent results obtained with it at 

 the full aperture of the mirror, Director Hale invited Dr. Michel- 

 son to investigate the question of interference with this large instru- 



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