stellar clusters' 



5 



tlie mode of investigation used in tliis memoir is not very well adapted. The 

 remaining 143 clusters are catalogued in the table 1 below. The most precious 

 information derived from the memoir of Melotte is the value of the diameter of 

 the clusters estimated at a uniform scale, through which we are put in position to 

 get an approximate appreciation of the relative distance of the different clusters. 



I have examined on the F. A. C. a great part of the clusters examined by 

 Melotte on the F. A. P. Most of them are visible on the charts, though their 

 diameter generally is smaller than on the plates *, evidently because of the two 

 magnitudes lost at the reproduction. The »eluster-hunting» is much easier on the 

 charts than on the heaven, a glance at the chart being indeed sufficient for detecting 

 objects of this sort. A more detailed study of the clusters is with excellent outcome 

 possible to perform on the carts and I will come back below to this point. For 

 the present I confine me to use the results obtained by Melotte from the Franelin- 

 Adams Chart plates. 



3. Relative distance of the clusters. Each attribute of an object varying 

 with the distance from the observer may be used for determining the relative 

 distance of the objects. The mean error in such a determination of the distance 

 s dependent on the frecjuency distribution of the objects in space and of the 

 attribute considered. I shall consider this point more closely below. 



The attributes that may be taken into consideration when the question is about 

 clusters are 1) the apparent diameter of a cluster, 2) the apparent magnitude of the 

 stars in a cluster, 3) the spectral type or colour index of the stars. Regarding glo- 

 bular clusters there are in addition one or two attributes more that may advanta- 

 geously be taken into consideration and that will be considered in the next chapter. 



As to 2) and 3) little or nothing is as yet known for ordinary clusters at least. 

 Some of the globular clusters have in the last years been carefully studied regarded 

 2) and even regarding the colour index, but similar researches on the ordinary clusters 

 fail, as far as I know, totally, if we leave the very large clusters (Pleiades etc.) out 

 of consideration. In any case we have no systematic observations of that kind. 



We are thus for the clusters confined to consider the apparent diameter. How 

 is the relative distance of a cluster to be deduced from its apparent diameter? 



4. The apparent diameter of a cluster is inversely proportional to the distance, 

 if we suppose the cluster placed at different distances, but is the inverse statement 

 also true, so that the distances were inversely proportional to the apparent diameter? 

 If all clusters have identically the same (absolute) diameter the statement is evidently 

 true. If the dispersion in the diameters is very small, it must be approximately 

 true. If the dispersion is considerable the correlation is more complicated. 



* This reinark relates princijjally tu the globular clusters. 



