70 



KNOWLEDGE 



[Ai>Bii, 1. 1893. 



p. 38, as "a siugular lemniscate-oval vacuity," and says 

 that a thin nehulous veil extends over a part of its area on 

 the preceding side. In the photographs it appears much 

 narrower, and with a form which reminds one of a solar 

 prominence. This is no doubt the blackest and densest 

 part of the dark area drawn by Sir John Herschel. That 

 there still exists a wider dark area with an outline some- 

 ■what similar to that drawn by Sir .John Herschel is 

 evidenced by the drawing of Mr. Peek made in 1882, as 

 "well as by the drawing of Mr. Le Sueur made in 1862, 

 given in Mr. ^Macgeorge's paper. 



In the photographs several narrow, somewhat similar 

 curved structures will be noticed extending into the 

 northern nebulous mass. They should, in the first 

 instance, be looked for on Dr. Gill's photograph, and 

 should then be examined on the less dense photograph of 

 Mr. Russell published in the March number of Knowledge. 

 The largest lies a little to the north of Herschel's lemniscate 

 structure, and is situated in column 24 and row 14. It 

 seems to extend downwards from an area about two bright 

 stars, and it has a spreading and branching head like the 

 largest of the structures which has attracted the attention 

 of all observers. A third of these curving structures is cut 

 by the line of the reticule between rows 13 and 14, on the 

 south preceding side of the great nebulous mass. It, too, 

 seems to have a branching head and a curving stem, which 

 is broadest and darkest near its base. A similar but rather 

 smaller dark branching structure will be found in row 14, 

 column 26, and another in row 15, column 20, entering the 

 great nebulous mass from its north following side, and 

 again another on the south preceding side of the great 

 nebulous mass in row 12, column 25. 



The appearance of many of these smaller dark structures 

 is somewhat similar to the appearance which the largest 

 lemniscate structure of Herschel assumes on the dense 

 photograph taken with Mr. Russell's camera, which was 

 reproduced in the March number and faces his larger 

 photograph. In the camera photogi'aph it will be noticed 

 that the great lemniscate structure is reduced to a 

 very narrow, dark, almost linear structure ; this may 

 possibly be due to the irradiation and encroachment 

 of the brighter parts caused by the want of sharpness in 

 the image thrown by the short-focussed camera lenses, or 

 it may be due to the partial transparency of the edges of 

 the dark structures. That the dark structures are partially 

 transparent is, I think, rendered evident by a comparison 

 of them as shown upon Dr. GiU's photograph and Mr. 

 Russell's less dense large photograph published in the March 

 number. In the latter photograph they appear broader 

 and far less sharply defined at their edges than in Dr. Gill's 

 photograph. 



The connection between the dark areas in this nebula 

 and alineations of stars along their borders was noticed by 

 Sir John Herschel. Such bordering lines of stars are 

 noticeable about the dark area that lies in rows 5 and 0, 

 column 26, which has been referred to as being shaped 

 like a " kidney bean." A similar bordering of a dark area 

 by lines of stars is noticeable about the dark structure 

 ■which lies in row 13, columns 28 and 29, and there are 

 also very evident lines of stars along the borders of the 

 broad dark channel on the south preceding side of the 

 northern nebulous mass. 



Some light may be thrown on the nature of the dark 

 structures in the MilKy Way by the forms they assume in 

 this nebula. The prominence-like dark forms all appear 

 to spring from the outside of the bright masses, and to bo 

 projected into the brighter matter. In the nebular mass 

 to the north of the trifid nebula (a photograph of which 

 ■was published in the January number, 1893) three dark 



prominence forms seem to spring from two central dark 

 patches within a bright nebulous cloud. Similarly, in the 

 photograph of the Sagittarius region, published in the 

 December number of Kniiwi.edge for ls91, three dark 

 branching structures appear to spring from a dark area 

 surrounded by bright stars and star clusters at the lower 

 right-hand corner of the picture. In the region about 

 a Cygni, a photograph of which was published in the October 

 number of Knowledge for 1891, the dark branching 

 structures (with the exception of the tree-like structure to 

 the north of a Cygni) seem to spring from the dark region 

 on the following side of a Cygni, and the tree-like structure 

 to the north of a Cygni is connected with a long dark 

 channel running east and west upon the plate, and 

 associated with lines of stars. In all these cases the dark 

 structures seem to represent matter projected into a 

 resisting medium. 



There are many ways of accounting for an opaque 

 or partially opaque cloud in space. We have, in the 

 chromosphere of our own sun, dark prominence forms, 

 which are occasionally seen, and have been photographed 

 by Prof. Hale. These, we have reason to believe, are due 

 to cool masses of gas cutting out the light derived from 

 similar incandescent gas behind them. But in the case of 

 the Milky Way it seems more probable that we have to 

 deal with matter at a lower temperature than that in the 



' solar chromosphere, and it seems reasonable to suppose 

 that such moving streams of gas would be derived from a 

 heated source, and would be hotter than the medium 

 surrounding them. We have, therefore, to account for an 

 opaque condition of matter when it is comparatively hot, 

 and a luminous condition as it cools. 



A mass of mixed gases and vapours, on expanding and 

 radiating into space, would no doubt, in the first instance, 

 be precipitated into very fine particles surrounded by 

 gaseous matter corresponding to the materials which only 

 condense at a still lower temperature. The transparency 

 of such a cloud would depend upon the size of the particles 

 mto which the less volatile constituents were precipitated, 

 and on the distance between such particles. The amount of 

 matter in a unit of volume remaining unchanged, the 

 smaller the particles into which the matter condensed the 

 more opacjue would be the cloud ; thus, if the earth were 

 broken into cubical masses an inch in diameter, which 

 were distributed uniformly throughout a sphere with a dia- 

 meter as large as the diameter of the earth's orbit, and the 

 sun were situated at the centre of such a spherical cluster 



] of stones, it may be shown that only about half his light 

 would emerge from the cluster. If the fragments were 

 very irregular in shape, though of the same average weight 

 as before, a larger proportion of the sun's light would be 

 intercepted ; and if the stones were broken into fragments 

 as large as sea sand, a planet outside such a cluster would 

 revolve in practical darkness. It is evident that a cloud or 

 fog of precipitated particles would become more and more 

 transparent as condensation and aggregation of the particles 

 went on. 



The microscopic structure of meteors seems to indicate 

 that they originally consisted of minute separate particles, 

 which have become aggregated and cemented together. As 

 such aggregation went on, a cloud of meteoric matter would 

 become more and more transparent, and the uncondensed 

 gaseous residue would tend, as it cooled to a low tempera- 

 ture, to collect around such meteoric bodies in loosely 

 packed atmospheres. I assume that the incandescence 

 must be caused by occasional impacts between such atmos- 

 pheres, which, if the meteors are moving, must come into 

 collision much more frequently than the meteoric bodies 

 themselves. 



