THE GREAT STAR MAP 245 



measured over 2000 stars clearly belonging to the cluster ; and 

 just as this particular plate recorded more stars than others 

 taken with inferior instruments, so a further improvement on 

 the great Yerkes telescope would probably show an increase 

 in the number of members of the cluster. There is every 

 chance that the advance has already been made. Within the 

 last few months we have seen the first results of the new 

 60-inch reflector of the Solar Observatory established by the 

 Carnegie Institution on Mount Wilson, Cal., U.S.A. They are 

 wonderful examples of what may be done in a really fine 

 climate by a master in the construction and use of instruments. 

 For the moment the latest photograph of the cluster in Hercules 

 is not available. But we are indebted to Professor Ritchey, 

 who made the telescope and took the photograph, for permission 

 to reproduce his picture of the globular cluster in Canes 

 Venatici^ which admirably illustrates our text. There is no 

 reason why a single cluster should not contain millions of stars ; 

 from this point of view, instead of the limits of our cluster 

 being reached at about the 6th magnitude, there is no reason 

 why they should not extend to the 7th, 8th or even much 

 fainter magnitudes. But there are two considerations which 

 make us hesitate to extend very far in this direction. The 

 first is that by doing so we diminish the resemblance to 

 other observed clusters in an important particular. In the 

 clusters which we see in the sky the stars are thickest in 

 the central portions. The law of condensation towards the 

 centre has not been exactly formulated (there is room for an 

 interesting research here) : but something has been done, for 

 instance, by Mr. Plummer in his paper ^ on the Hercules 

 cluster ; from the figures he gives we can infer that if an 

 observer could be placed at the centre of the cluster to count 

 the number of stars of successive magnitudes (as we have 

 been doing for the stars visible from the earth), then the 

 numbers would increase very slowly indeed. The ratio, instead 

 of being 4 or 3, would probably be less than 2. Now, if we 

 look at the numbers assigned to our "solar cluster" by the 

 crude supposition just made, we shall find that the ratio is 

 greater than 2. We can reduce it by reducing the dimensions 

 of the cluster (supposing the cluster to extend no further than 

 the 5th magnitude, say) but the more we extend the cluster 

 ' Mon. Not. K.A.S. Ixv. p. 812. 



