CHAPTER II. 



Globular clusters. 



11. The most exhaustive examination of the globuhir clusters lias been made 

 by Solon I. Bailey at the observatories belonging to the Harvard astronomical de- 

 partment. For a great part of these clusters he has made a thorough study of the 

 luminosities of the stars and of their distribution at different distances from the 

 centre. Moreover he has examined all dusters designated in the New General 

 Catalogue of Dreyer as globular on photographs taken with the 24-inch Bruce 

 telescope and having, generally, an exposure of about one hour. The results of 

 his researches on these objects are collected in the Annals of the Harvard College 

 observatory Vol. 76, N:o 4. 



One of the most remarkable facts stated by this examination is that the list 

 of these objects may at present be regarded as complete or nearly complete. More 

 precisely we may conclude that all globular clusters having a diameter exceeding 

 a minute of arc may be considered as registered in the catalogue of Dreyer and 

 in that of Bailey. As to the globular clusters having a diameter of one minute 

 of arc or smaller the interpretation of the observations may be somewhat uncertain 

 owing to the difficulty of discerning between a small globular cluster and a round 

 nebulous or hazy star. Meanwhile it is stated by Bailey that »such objects (hazy 

 objects) have rarely, if ever, been shown to be globular dusters». The statistical 

 investigation regarding their distribution in space makes it also probable that the 

 number of globular clusters is nearly exhausted through the existing catalogues. 



As to the distribution of the globular clusters on the sky it has been pointed 

 out by BoHLiN (K. Svenska Vet. Akad. Handlingar Bd. 43) that these objects are 

 most frequently to be found in the south sky (principally in the scjuare Dq). Hinks 

 has found the remarkable fact (M. N. Vol. 71, 1911) that »the clusters are contained 

 very nearly in one hemisphere of the sky, whose pole is on the galactic plane in 

 galactic longitude about 300**». Hertzsprung makes in A. N. 4600 (1912) a more 

 complete computation of the point of concentration and finds as its coordinates 

 / = 307", h^ — 14«. 



The catalogue of Bailey of globular dusters contains 113 objects. He gives 

 regarding all of them the description in the N. G. C. o I; Dreyer with the abbre- 



