CHAPTERS ON THE STARS. 419 



the course of the galaxy from that constellation in the opposite direc- 

 tion from that which we have been following. South of Cassiopeia we 

 see, near the central line, the well-known cluster forming the sword- 

 handle of Perseus. Farther south the belt grows narrower and fainter; 

 although the irregularities of structure continue, they are far less strik- 

 ing than on the other side. On a moonlight evening it will scarcely be 

 visible at all. If the moon is absent and the air clear we shall see 

 that it grows slightly brighter toward the southern horizon, near which 

 will be the narrowest part of its entire course. Below is the broad and 

 diffused region in Argo. 



One conclusion from the inequalities of structure which we have 

 described will be quite obvious. The Milky Way is something more 

 than the result of the general tendency of the stars to increase in 

 number as we approach its central line. There must be large local 

 aggregations of stars, because, as we have already pointed out, there 

 cannot be such diversity of structure shown in a view of a very widely 

 stretched stratum of stars. When, instead of a naked eye view of the 

 belt, we study the photographs of the Milky Way, we find this evidence 

 of clustering to grow still stronger. It is shown very strikingly in the 

 photograph by Barnard, showing the singular rifts in the Milky Way 

 in the constellation Ophiuchus. Yet more singular are three-minute 

 openings in the constellation Aquila, the positions of which are: 



(1). R. A. = 19h. 35.0m. 



" = 19h. 36.5m. 



= 19h. 37.2m. 



Dec. = + 10° 17' 

 " = + 10° 37' 

 " = + 11° 2' 



The fundamental question which we meet in our farther study of 

 this subject is: At what magnitude do these agglomerations of stars 

 begin? Admitting, as we must, that they are local, are they composed 

 altogether of stars so distant as to be faint, or do they include stars of 

 considerable brightness? We consider this question in a way quite 

 similar to that in which we discussed the clustering of the stars in the 

 last chapter. We mark out on a map of the Milky Way the brightest 

 regions — that is, those which include the densest agglomeration of very 

 faint stars. We also mark out the darkest regions, including the coal 

 sack. For this purpose I have taken the maps found in Heis's Atlas 

 Ccelestis for the northern portion of the Milky Way and the Atlas of 

 Gould's Uranometria Argentina for the southern portion. In order 

 to enable any one to repeat and verify the work I give the position of 

 the central part of each patch or region studied. This serves simply 

 for the purpose of identification. The outlines can be drawn by any 

 one when the patch is identified. The third column of the table is 

 given, approximately, the number of square degrees in the patch as 

 outlined. Then follows the number of stars as found on the map. 

 Here are included stars somewhat fainter than those regarded as lucid. 



