III. — The Butherfurd Photographic Measures of Sixty-Jive Stars 



near 61 Gygni. 



BY HERMAN S. DAVIS. 



Read May, 1897. 



1. It was but natural that Mr. Rutherfurd, in developing the 

 art of astronomical photography, should try his skill upon that 

 star which has attracted the attention of so many investigators 

 ever since Bessel proved by it the possibility of determining 

 stellar parallax. 



Of these photographs of 61 Gygni and its surrounding stars 

 taken by Mr. Rutherfurd, nineteen, exposed between 1871, Nov. 

 9, and 1874, June 13, were measured by Miss Ida Martin more 

 than twenty years ago, but have remained unreduced until re- 

 cently placed in my hands for that purpose by Professor J. K. 

 Rees, Director of the Observatory. The present paper contains 

 the results of measures of position of stars surrounding 61 Gygni, 

 and will be followed by a second paper containing the results of 

 an investigation of the Parallax of 61 l Gygni. The methods of 

 reduction used so far as measures of distance are concerned are 

 those presented by Dr. Harold Jacoby in earlier Contributions 

 from this Observatory. 



2. In Table I are given the general data of exposure of the 

 plates, including the computed values of the zenith-distance, par- 

 allactic angle and refraction factor. 



3. Table II contains the means of the refractions computed for 

 the Eastern and Western impressions from the data of Table I by 

 the formulae 



■k. [tan 2 C cos 2 (p — q) + 1] 



s 

 tt —p = — J/c cosec 1 " tan 2 ? sin 2 ( p — q) . 



The argument for entering this table is p. 



4. Table III. — The corrections to the position-angle due to pre- 

 cession, nutation and aberration will be found in column two. 

 These were computed bj^ the formulae 



