112 



KNOWLEDGE 



[June 1, 1891. 



The photograph of a and fi ( 'i-m-is ami the Coal-Sack 

 Region which we are enabled, by the kindness of Mr. H. C. 

 Russell, Director of the Sydney Observatory, to lay before 

 the readers of Knowledge, shows that the Coal-Sack is far 

 from being absolutely void of stars ; the greater part of it 

 is bridged over by a network or veil of stars which are 

 relatively minute compared with the larger stars that are 

 scattered over the region surrounding the Coal-Sack. Tliis 

 network of minute stars does not cover the whole of the 

 Coal-Sack area. There are small regions, one notably 

 towards the northern end of the Coal-Sack and one near to 

 a ( 'nici'x, which sliow a comparatively black and starless 

 background. It is evident that the structure of this portion 

 of the Milky Way cannot be accounted for by the simple 

 cylindrical streams imagined by Mr. Proctor. 



The relative blackness of the Coal- Sack Region, as seen 

 by the naked eye, seems to be due to a breach in a net- 

 work or structure of larger stars, which are evidently 

 distributed in a different manner to the network or struc- 

 ture of small stars which .stretches across the Coal-Sack 

 Region. The two systems or groupings of stars need not 

 necessarily lie at greatly different distances, but they seem 

 to be as independent of one another as the distribution of 

 two flocks of birds. There may be some general law 

 connecting the association of two flocks or flights of birds, 

 and yet each may have its own independent arrangement, 

 as for example, one set of larger birds might be attacking 

 or following a flight of smaller birds or insects. We 

 should then expect to And the two flights intermixed as 

 seen on the sky and yet distinctly separate both by their 

 grouping as well as by their size. Thus we find in the 

 photograph stream lines of minute stars as well as stream 

 lines of stars of larger magnitude. 



In this region of the Milky Way, stars of similar magni- 

 tude seem generally to be associated together in the stream 

 lines. There is a very remarkable series of such diverging 

 stresim lines about the bright star a ( 'nicis (the lowest 

 of the bright stars on the plate). A circle of small stars 

 may be traced around the large star, within the sharply' 

 defined bright ring, which, as was explained in Knowledge 

 for .July of last year, p. 174, is due to the reflection of the 

 light of the bright star from the back of the plate." Con- 

 verging towards this circle of minute stars are several 

 curving stream- of minute stars, which it seems hardly 

 possible to doubt are associated together. The s)'mmetry 

 of their arrangement with respect to the large star is also 

 very curious. From a probability point of view, it is over- 

 poweringly evident that these diverging streams of small 

 stars, together with the circular stream of small stars from 

 which they spring, form a cluster and are all at about the 

 same distance from us. The fact that a. Crncis falls 

 apparently at the centre of this curiously symmetrical 

 cluster may be a mere coincidence, and may not mdicate 

 any physical connection between the cluster and the large 

 star, but the improbability of such a chance coincidence in 

 position is considerable. The evidence, whatever it is 

 worth, is another link in the chain of evidence tending to 

 prove that the girdle of large stars referred to in my paper 

 in the last number is intimately associated with the 



* During thr expoMirc of tlir iihotogi-upliic- plate, a point ou the 

 sensitive film was lit up by the image of the bright star, and slione like 

 a little lamp. The rays from this illuminated region, nhieh fell 

 nearly per])endicularly on the back of the plate emerged, whereas 

 those whieh fell more obliquely would be in greater and greater 

 jn-oportion reflected back again", and produce their effect on the 

 sensitive film. The inner edge of the bright ring corresponds to the 

 light reflected at the critical angle where the whole of the light is 

 reflected back again to the sensitive film, and none gets out at the 

 back of the plate. 



Milky Way, and that a Cnwi.i and the other stars of the 

 girdle are giant suns compared with the cloud of smaller 

 suns which compose the Milky Way. a ( 'ntcis is a double 

 star with components 5" apart which move about one 

 another very slowly, for their position seems hardly to 

 have changed since Sir John Herschel observed them in 

 1834. Tliey give a spectrum of Sirimi Type. The stars 

 of the cluster about a ( 'n(ris seem to belong to the class 

 of smaller stars which stretch across the Coal-Sack ; one of 

 the branches of the cluster seems to merge into a curiously 

 curving stream of small stars, which stretches a long way 

 if not right across the Coal-Sack area. There is also, on a 

 glass positive sent me by Mr. Russell, a narrow black 

 channel, bordered by small stars, which stretches right 

 across tlie Coal-Sack area. 



The other photograph reproduced represents the Milky 

 Way, about rj Argus, a region which Sir John Herschel 

 thought was composed of stars of larger average magni- 

 tudes than other parts of the Milky Way ( see Tlic ( 'n/ie 

 Obsi'i-rationx, p. 33). He also thought that it differed from 

 other regions of the Milky Way, in that the stars appeared 

 distinctly separated on a perfectly dark background. It is 

 seen here to be composed of a series of clusters separated 

 by a number of darker patches and lanes. It seems clear 

 that the Milky Way is by no means homogeneous ; different 

 parts exhibit great differences of star grouping — just as 

 one region of the Milky Way is rich in red stars, while 

 another is rich in stars having bright line .spectra, so one 

 region seems to be composed of large stars, and another of 

 relatively small stars, and the method of their grouping in 

 different regions is also strikingly different. This was 

 long ago noticed by Sir John Herschel. He says,+ 

 speaking of the region at 17h. .50m. : " The Milky Way is 

 here composed of separate, or slightly or strongly connected 

 cloitds of semi-nebulous light, and as the telescope moves, 

 the appearance is that of clouds passing in a .sc?/rf, as the 

 sailors call it ... . The Milky Way is like sand, not 

 strewed evenly as with a sieve, but as if flung down by 

 handfuls (and both hands at once) leaving dark intervals." 

 The nebula about r; An/iix, though comparatively brilliant 

 in the telescope, seems to be deficient in actinic or photo- 

 graphic light, and consequently to need very long exposures 

 as compared with the great nebula in Oriim. Mr. Russell 

 says that he can trace on his original negative curving 

 structures, which he says remind him of the great nebula 

 in AniJroiiu'da. Only three stars are shown in the great 

 dark oval patch in this nebula where Herschel's drawings 

 give four. The dark region, known as the Lemmiscate, also 

 appears quite void of light and of stars, except the one 

 small star observed by Herschel, but the scale of the photo- 

 graph is too small to show much detail. The scale of both 

 photographs is 1" 0'56(j inch. They were taken with a 

 Dallmeyer six-inch portrait lens attached to the mounting 

 of the telescope, which will be used for taking part in the 

 photographic survey of the heavens. 



FLYING ANIMALS. 



]?y R. Lydekkee, B.A.Cantab. 



ONLY in certain members of two great groups of 

 iinimals do we meet with the faculty of flight, or 

 the power of supporting their bodies in the air 

 for longer or shorter periods by the aid of mem- 

 branous or other expansions developed therefrom. 

 These two groups are the Insects, which constitute a class 

 of the larger group known as the Arthropodous sub- 



t Cape Obneri'ations, p. 388. 



