June 1. 1893.] 



KNOWLEDGE 



111 



Fio. 7.— Forked 

 Struct are. 



to their motion through a fine rain of falling njatter, 

 but we can hardly conceive of a spiral form being 

 given to an outrushing filament unless it was projected 

 from the centre of a cyclonic storm, or was deflected in 

 its passage through a resisting medium by lateral currents. 

 The fact that the stellar points follow the lines of the 

 nebulous curves, seems to indicate that 



Wthe stellar aggregations have formed 

 within the nebulous filaments after 

 their deflection ; for if the stellar aggre- 

 gations differ appreciably in density 

 from the nebulosity joining them, the 

 resisting medium would have a less 

 effect in diminishing the momentum of 

 the denser portions, and the denser 



Pn5 fi BraucliiiuT masses would drift on in front of the 



striictiirr. '^ less dense portions and get separated 

 from the general line of nebulosity. 

 Fig. 6 represents a branching structure adjacent to the 

 spiral form shown in Fig. 5. The 

 narrow bands of nebulosity linking the 

 stellar regions is well shown in it, as 

 also in the branching structure shown in 

 Fig. 7. 



In addition to the bright structures 

 shown in these illustrations, there seems 

 to be evidence of the existence of dark 

 branching forms associated with the cluster. 

 The Earl of Rosse at Parsonstown in 

 1850, noticed three "dark lanes" meeting 

 at a point considerably removed from the centre of the 

 Hercules cluster, and the Rev. T. W. Webb describes 

 these dark lanes in his " Celestial Objects " as visible with 

 a nine-inch reflector. They form quite a striking feature 

 of the cluster as seen with larger apertures, and are 

 distinctly traceable on a dense photograph of the cluster 

 taken by Mr. Isaac Roberts in May, 1887 — a photograph 

 of which has been well reproduced in Miss Gierke's " System 

 of the Stars,' plate 9. But the photographs now repro- 

 duced do not extend quite far enough to show the triple 

 dark lanes discovered by the Earl of Rosse, though they 

 can be distinctly made out upon the original copies from 

 which the illustrations for this paper have been prepared. 

 In these. and in the glass dia-positive of the Lick photograph 

 lent me by Mr. McClean, there appear to be several other 

 small dark branching structures which can be distinctly 

 made out in different parts of the cluster. They are 

 clearly not merely lacun» between bright structures. They 

 interfere with the nebulous background and with the bright 

 regions in a way that can only be accounted for by 

 supposing that a branching stream of light - absorbing 

 matter is situated between the eye and the cluster, 

 probably in the immediate neighbourhood of the cluster ; 

 a few such branching channels may be traced in the central 

 parts of Fig. 1, though they are not very conspicuous. It, 

 however, seems probable that the cluster is a group of 

 nebulous structures, some bright and some light-absorbing; 

 the bright structures being studded with brighter regions, 

 which appear as stellar points in the telescope. 



If the cluster is situated at a distance seven and a half 

 times as great as a Centauri, the bright regions might have 

 a diameter .500 times as great as that of our sun, and yet 

 hardly be distinguishable, except in the largest instru- 

 ments, from stars. For our sun, if it were removed to a 

 distance seven and a half times as great as a Centauri, would 

 only appear to have a diameter of 0-00096" as seen from 

 the earth. Consequently, nebulous clouds with a diameter 

 twice as great as the diameter of the earth's orbit would, 

 at such a distance, appear to have a diamete of less than 



half a second of arc, and would be hardly distinguishable, 

 except in the largest instruments, from stellar points. We 

 are, therefore, hardly in a position to say that the stellar 

 points are sun-like bodies with a brilliant photosphere, and 

 are not merely brighter regions in the nebulous streams or 

 structures radiating from the cluster. 



Mr. P>urnham, in commenting on a close pair of star- 

 like points near to the centre of this cluster, which he 

 examined with the 8(j-inch Lick refractor, and found to be 

 separated by a distance of only 0-88", says: "This is one 

 of the principal stars, near the central portion of the great 

 cluster in Hercules. It was the only pair close enough to 

 be called a double star I could find on this occasion, but 

 the conditions were not specially favourable. Of course, 

 there are many stars within, say, 2" of each other, but 

 in all of the bright compressed clusters which I have 

 examined with this and other instruments there seems to 

 be a remarkable absence of real double stars ; and this 

 seems to be true of star clusters generally." 



Mr. Burnham notes four or five comparatively close 

 pairs in the Pleiades and in the cluster in Perseus, but 

 they may (as well as the pair in the Hercules cluster 

 above referred to) not belong to the clusters but only be 

 seen by projection upon them. If we accept Prof. See's 

 theory as to the evolution of double stars from nebular 

 masses whose velocity of rotation has been accelerated by 

 the shrinkage of the mass due to cooling, the existence of 

 a binary star in a cluster must be taken as indicating that 

 the secular changes which give rise to binaries have not 

 had time, since the isolation of the stars as separate masses, 

 to produce their effects ; and the absence of binaries may 

 be taken as indicating the comparatively recent separation 

 of the stellar masses. 



Noti«s of Boofes. 



I » I — 



Observational Astmnomii : a Book for Beginners. By 

 Arthur Mee.F.R.A.S. (Cardiff: DanielOwen &Co., 1893.) 

 — This is an excellent, honest little book, containing a good 

 deal of out-of-the-way information about things astro- 

 nomical, especially amateur astronomers and their instru- 

 ments. It is profusely illustrated, though the diagrams 

 are sometimes rather rough. The series of drawings of 

 lunar craters by Mr. Mee, Mr. Stanley Williams, Mr. Gwyn 

 Elger, and Mr. G. T. Davis are worthy of special attention. 

 One of the quaintest illustrations in the book is a picture 

 of the Orion nebula as seen by Mr. Mee with an 8^ inch 

 reflector. It is as unlike the photographs of the nebula as 

 any of the drawings by early astronomers collected in 

 Prof. Holden's memoir, from which he and some other 

 astronomers have argued that striking changes are taking 

 place in the comparatively short period during which man 

 has been observing the nebula. One of the most interest- 

 ing parts of the book is the memoir of the Rev. T. W. 

 Webb, Prebendary of Hereford and author of " Celestial 

 Objects for common Telescopes." It contains a few too 

 short extracts from some of his letters, of which there 

 must be a great number treasured up in different parts of the 

 world, for he was a most voluminous and interesting letter 

 writer, always ready to assist the amateur astronomer. 

 His letters were written in a neat small-hand which 

 enabled him to put a greac deal into the page, and they 

 were frequently illustrated with charming little drawings 

 as well as enlivened by the quaintest humour, which, of its 

 kind, was as curious as De Morgan's. The woodcut from a 

 photograph of Mr. Webb is unfortunately not a success, and 



* " Astronoiuische Xacliriohten." Bd. 127, p. 382. The place of 

 tlie double star referred to is, R.A. IHli. a7in. Dec. + 36^ 41'. 



