September 1, 1894.] 



KNOWLEDGE. 



205 



illustrate this paper have been reproduced from photo- 

 graphs given me by Mr. W. E. Wilson, of Daromoua. 



The light-grasping power of Mr. Wilson's instrument 

 is so great that, with only an hour's exposure, smaller 

 stars and a considerably larger area of nebu- 

 losity have left their imprint upon the photo- 

 graphic plate than are to be traced upon the 

 photographs made at the Lick and Paris 

 observatories with exposures three times as 

 long. Therefore, in comparing these photo- 

 graphs of the Hercules cluster with the photo- 

 graphs pubUshed in the June number of 

 Knowledge for 1893, the reader must bear in 

 mind that the structures and star-streams 

 which appear to project from the edge of the 

 spherical cluster in the Lick and Paris photo- 

 graphs are in Mr. 

 Wilson's photo- 



graphs to be found 

 imbedded in nebu- 

 losity at some dis- 

 tance below the 

 boundary where in- 

 dividual stars be- 

 come separately 

 visible. 



This wUl be more 



clearly recognized if 



v.vawi we take an example ; 



Fig. 1. — Promiuence-like thus, the prominence- 

 Structure, from photograph like structure shown 

 by the Brothers Henry. ^^ p-„_ 1, which in 



the Henry photograph projects from the upper 



part of the nebulous mass, will be found in the 



Wilson photographs imbedded in nebulosity. 



The scale on which the Wilson photograph 



is here reproduced is smaller than that 



of Fig. 1, so that the prominence - like 



structure about corresponds with the scale of Fig. 4. 



This structure, as seen in Mr. Wilson's photographp, is 



horizontally across the page. It is one of the three dark 

 lanes, diverging from a point, which were first noted by the 

 Earl of Eosse, and have been described by Webb and several 

 observers since his day. These diverging dark lanes are 



Fig 



Fig. 4. — Prominenee- 

 like Structure. Ke- 

 producecl from the 

 Henry photograph. 



Fia. 2. — Untouched etched block made from a photograpli of 

 the Cluster in Hercules taken by the Brothers Henrv on the 

 23rd June, 1886. 



somewhat interfered with by a dark bar or lane extending 



— Untouched etched block made from a photograph of the Hercules 

 Cluster taken by Mr. W. E. Wilson on the 5th August, 1894. 



pretty obvious when the cluster is 

 observed in a large telescope. It wiU 

 be noticed that the outer lane (which 

 is nearly radial with the centre of the 

 cluster) is much the most marked of 

 the three, as shown in Mr. Wilson's 

 photographs, and that there are some 

 small, branching, dark structures 

 which diverge from it on either hand. 

 Mr. Wilson's photographs also show 



several other small dark areas within the cluster, which 



should be compared with the dark areas within the 



.'. Centauri cluster, a photograph of which is reproduced 



in Knowledge for May, 1893. Ikit one of the most 



interesting features in Mr. Wilson's 



photographs is that many of the stars 



in the outer parts of the cluster are 



distinctly seen upon the photographs 



to be united by ligatures of nebulosity. 



It has been very difficult to reproduce 



these nebulous ligatures in the etched 



blocks or in the collotype plate. They 



are, perhaps, most marked and easily 



recognizable in the upper right hand 



quadrant (see Fig. 3). But there is 



no doubt about their existence, and 



in the silver prints and platinotype 



prints made from Mr. Wilson's original enlargements they 



are very clearly shown. Mr. Wilson himself has no doubt 



as to their actual existence ; that is, he believes that they 



are not due to any optical or photographic defect. 



Fit. -^ llrancliing 

 Structure, from photo- 

 grapli by the Brothers 

 Henrv. 



