IV. — The Itutherfurd Photographic Measures of the 

 Group of the Pleiades. 



BY HAROLD JACOBY. 



Bead Dec. 7, 1891. 



INTRODUCTION, 



The present paper contains the results of Rutherfurd's photo- 

 graphic observations of the Pleiades group, made in 1872 and 1874. 

 The history of the plates, together with a description of the appa- 

 ratus with which they were made and measured, will form the 

 subject of a separate paper by Professor J. K. Rees, at whose 

 suggestion, it is proper to say, the entire investigation was under- 

 taken by me. The Pleiades have been selected for reduction from 

 among a large number of clusters photographed by Rutherfurd 

 with his 13 inch telescope, because this group offers the best op- 

 portunity for judging of the accuracy obtainable by the methods 

 employed. It is hoped that the near future will see the publication 

 of all the other Rutherfurd measures, many of which constitute 

 the earliest accurate observations of the clusters in question. The 

 measures, as contained in the observation books, are in the form 

 of position angles and distances from the star 24 p. This star was 

 selected as the origin of coordinates, for it is situated near the centre 

 of the cluster, and is otherwise better suited to measurement than 

 the neighboring large star Alcyone. Accordingly, the method of 

 reduction in its general course is similar to that in use for ordinary 

 micrometric measures, aud to that adopted by Gould in his reduc- 

 tion of the earlier Pleiades observations.* The author takes this 

 opportunity to thank Dr. Gould for his kindness in explaining 

 many of the details of the methods used by him. It has been 

 possible to obtain a very accurate determination of the scale values 

 for the various plates by a comparison with Bessel's measures, and 

 the recent ones of ELKiN.f These scale values will be of very high 



* National Academy of Sciences, vol. iv, third memoir. 

 f Transactions of the Astronomical Observatory of Yale University, vol. i, 

 part i. 



Annals N. Y. Acad. Sci., VI, Feb. 1892.— 17 



