January 2, 1893.] 



KNOWLEDGE 



11 



would, as the cooling proceeded, cease to glow, but they 

 would still be surrounded by the more volatile gaseous 

 constituents of the original nebulous mass. We should 

 expect to find the uncondensed gases arrdngiag themselves 

 round the li(]uid or solid particles, forming a sort of 

 atmosphere about them, very loosely packed, owing to 

 the small attracting power of the particles. The particles 

 would be moving with the general velocity of the vapour 

 from which they were condensed, each carrying around it 

 an atmosphere of uncondensed gas of considerable diameter, 

 compared with the diameter of the solid or liquid particle, 

 and wherever two streams of the original nebulous matter 

 impinged upon one another, we should have far more 

 frequent collisions of the atmospheres surrounding the 

 particles than collisions of the particles themselves. The 

 collisions of the particles themselves, if moving with 

 planetary velosity, as supposed by Mr. Lockycr in his 

 mjteoric hypothesis, would give rise to a continuous 

 spectrum, but the much more frequent collisions of their 

 atmospheres, would, it seems probable, give rise to a 

 gaseous spectrum, even though the matter of the nebula 

 were extremely cold. 



Before endeavouring to make a further step in the 



Sorrn 



Fig. 1.— Blofk etclied by a laliotogvaphic proL'ess from the central 

 region of Prof. Barnard's photograph, enlarged about three 

 diameters. 



enquiry " What is a Nebula?" I will ask the reader to 

 devote a little time to a careful examination of the remark- 

 able photographs reproduced in our plate. Near the 

 centre of Prof. Barnard's large photograph is a bright 

 nebulous patch, thickly strewn with stars, and within the 

 bright region, or projected upon it, is a darker patch with 

 straggling arms or projections, which appear to have 

 spnmg from the dark area and spread out in their upper 

 parts somewhat after the manner of solar prominences, 

 though on a vastly larger scale. Again, within the dark 

 area there are two black or nearly black starless regions, 

 which appear like holes in the stellar cluster and its 

 associated nebulous matter, except that one star is seen 

 projected upon the larger of the two black regions. 



The question whether the dark structure is nearer to 

 us than the bright nebulous cloud and its associated 

 stars — that is, whether it is only by chance seen projected 

 upon the bright background, or whether the dark structure 

 is surrounded by the nebulous haze and stars, and is 



only partly seen through the bright nebulous matter — 

 is one of very great iutarest and importance in deter- 

 mining the theory which we may be led to adopt in order 

 to explain the phenomena observed. 



The facta referred to in the paper on " Dark Structures 

 in the Milky Way," published in the December number of 

 Knowledge for 1891, would lead us to conclude that the 

 dark and bright structures of the part of the Milky Way 

 there described are all at about the same distance from us, 

 and are intimately associated together. The dark arch 

 and dark tree-like structures, referred to in the December 

 number, 1891 (Fig. 5, page 232), spring from a large dark 

 area in Sagittarius, about ten degrees to the south of the 

 region shown at the centre of Prof. Barnard's photograph, 

 which we are now discussing, and the dark structures there 

 appear to have been projected into surrounding bright 

 nebulous matter. Similarly, in the case now before us, 

 the dark structures seem to be intimately associated with 

 lines or streams of stars in the stellar cluster, and the 

 stellar cluster appears to be associated with a general 

 nebulosity which seems to be brightest where the stellar 

 points cluster most thicldy. 



One of the most remarkable of these branching lines or 

 streams of stars is situated between the dark structures 

 marked a and b in Fig. 2. It is best seen in the enlarged 

 copy of Prof. Barnard's photograph, and is marked with 



South. 



Fu 



, 2. — Block etched by a i5hotograj)hic process from the central 

 region of Prof. Barnard's photogi-aph. Tlie same scale as the 

 original. 



the letter A in Fig. 1, but it can be distinctly traced in Prof. 

 Max Wolf's photograph. It seems to aflbrd evidence of a 

 stream of matter which has rushed outward from the dark 

 region, for it forks or branches in a direction awav from 

 that region, as also do the dark structures marked () and /' 

 in Fig. 2, and the almost linear dark branching structure 

 marked C in Fig. 1. So also does the dark structure 

 marked r in Fig. 2, for it is connected with the central 

 dark region by a narrow dark stem, which, after haviu" 

 attained a certain height, spreads into a broader head, 

 from which diverge narrow branching streams which fork 

 outwards. Stretching westward from the base of '■ is a 

 remarkable hne of stars, with an adjacent narrow black 

 channel. In view of these facts, we are led to the con- 

 clusion that there is an intimate connection between the 

 dark structures and the stellar cluster, and that the dark 

 region is probably surrounded by bright nebulosity. 



It seems to me that the easiest explanation of the two 

 black holes is to assume that they correspond to the places 

 where two dark structures similar to a, i or c (Fig. 2) 

 stretch out from the central mass in a direction towards 

 the observer, and that they reach to the surface of the 



