March 1, 1894.] 



KNOWLEDGE 



61 



and bluish-grey between the wings, while the tail coverts 

 are grey. The tail and wings are bluish-grey, shading off 

 to a leaden-grey. 



The female, as is the case with most of the pigeon family, 

 is smaller and less brilliant in colouring than the male ; 

 but otherwise she resembles her mate. Young birds, before 

 their first moult, have not the beautiful metallic tints 

 spoken of above on their necks. 



THE STRUCTURE OF THE MILKY WAY. 



By A. C. Ranyaed. 



THE beautiful photograph of the Milky Way in 

 Cephens, reproduced in our plates, will well repay 

 very careful study. The plate on the left hand will 

 bear examination with a magnifying lens, and if 

 there still remain any readers of Knowledge who 

 are sceptical as to the actual existence of dark structures 

 in the Milky Way, they will find ample evidence in this 

 plate to convince them of the existence of narrow streams 

 and branching tree-like structures of semi-opaque material, 

 which evidently acts like a fog in space, cutting out the 

 light of the bright nebulous matter on which the dark 

 structures are seen superposed — evidence which, if 

 thoughtfully studied, must completely satisfy the objectors 

 who have rather hastily advocated the theory that the dark 

 channels and tree-like structures may be explained as 

 accidental interspaces between bright nebulous clouds. 



The plate on the right hand has been made on an 

 enlarged scale (about two and one-eighth diameters) from 

 the central region of the ^shotograph represented in the 

 opposite plate. It has also been treated so as to bring out 

 with greater contrast the gradation of faint tints, so that 

 some of the long lines of stars and associated dark channels 

 are more strikingly exhibited in the enlarged picture than 

 in the smaller but sharper reproduction of Mr. Barnard's 

 original photograph. 



I will ask the reader to devote a little time to the careful 

 examination of both of the large plates of the region repro- 

 duced in Fig. 1. Close to the bottom edge of Fig. 1, over 



the words " a little," is 

 a narrow dark channel 

 which leads into a large 

 dark area, within which 

 a few isolated stars are 

 sprinkled. From this 

 dark area spring 

 several branching dark 

 structures. The most 

 notable of them is one 

 which reaches nearly 

 to the top of the block 

 in Fig. 1. From it 

 there spring several 

 small dark branches or 

 expansions of the main 

 stream on either hand. 

 There are at least 

 three such expanding 

 heads, or opposite pairs 

 of branches, one above 

 the above. I would ask 



Fi&. 1. — Region a little to the right 

 of the centre ou the enlarged plate, anj 

 a little below the line joining the words 

 " East " and " Wej^t " on the opposite 

 plate. 



the reader to compare it with the great branching structure 

 in the Orion nebula, referred to later on. This is not by 

 any means the only dark branching structure which springs 

 from the large dark area shown in Fig. 1. Another series 

 of dark branching channels may be traced springing from 

 the upper part of the dark area on the left hand. They 



branch in a complicated manner, and mostly end in 

 expanded heads. There are also some narrow, dark, 

 branching channels which run into the nebulosity on the 

 left-hand side of the dark area. 



It will be seen that all the linear dark channels and 

 dark structures branch or fork in a direction away from 

 the dark area, and it seems difficult to resist the conclusion 

 that we have evidence here of streams of dark nebulous or 

 semi-opaque matter, which have been projected from the 

 dark region into a surrounding resisting medium. I would 

 ask the reader to compare this region with the region of the 

 Milky Way in Sagittarius, shown in the photographs 

 reproduced in the January number of Knowledge for 

 1893. In each case we seem to have a black area, from 

 which dark structures radiate into surrounding bright 

 nebulosity, and it will be remembered that we found a 

 similar radiation of dark structures from the larger dark 

 region in Sagittarius shown in Knowledge for December, 

 1.S91, and discussed on page 232. 



In the enlarged plate, several narrow dark channels, 

 associated with lines of stars, will be recognized running 

 in undulating lines nearly horizontally across the plate. 

 One of the most striking of these runs through a small dark 

 area to the south of the dark area shown in Fig. 1. This 

 darlc channel, like most of the others, is very narrow and 

 intensely black, and runs in a wavy line nearly from one 

 side of the page to the other. It cannot be reasonably 

 suggested that such a dark channel is merely the inter- 

 space between two bright nebulous areas, for such a theory 

 would involve the assumption that both of the bright 

 nebulous areas have irregularly curving edges which 

 accurately fit into each other, leaving everywhere the same 

 breadth of narrow interspace between them. The chains of 

 stars running parallel with the dark channels also bear 

 witness to their having a physical existence, and not 

 being due to a mere effect of perspective. 



Several such dark channels are to be found in this 

 region of the Milky Way. One rims horizontally from a 

 to h across the middle of the region comprised Ln Fig. 2, but 

 it is best shown on the larger plates and on the glass 

 transparencies sent by Mr. Barnard. In this region there 

 are several minute dark channels alineated with stars, 

 which branch away from the annular dark channel. Many 

 of them can be recognized in Fig. 2. 



Fig. 2. — Region to the north of that shown in Fig. 1, showing 

 bright nebulosity surrounded by a dark channel. 



In trying to interpret the vast phenomena presented to 

 us by the Milky Way, we must in the first instance note 

 facts, and where we can recognize them we must carefully 

 note analogies ; and though the structures which have been 

 photographed in comets, in the corona, and in the Orion 

 nebula are probably on a diminutive scale compared with 

 those presented to us in the Milky Way, it is interesting 

 and may be very useful to note analogies between them. 



