Decejibek 2, 1895.] 



KNOWLEDGE 



281 



the left foot of Perseus. Yet the fragmentary nebulje 

 registered in this neighbourhood cannot, however vastly 

 remote, be dissevered from the rest. The precise nature of 

 their connection needs further investigation ; but the con- 

 jecture seems plausible that, on the sensitive plates of the 

 future, aU will combine into a spiral structure not far 

 inferior in dimensions to the stupendous coils involving a 

 large majority of the Orion-brilHants. 



The large, round, burnt-up surface on the plate is denoted 

 on the map by a circle enclosing the star-group, with its 

 now well-known appendages. Relatively to the entire span 

 of the gigantic nebulous system half disclosed, half 

 suggested in the new photograph, it is surprisingly small. 

 Yet there beyond question lies the nucleus, and there 

 possibly resides the dominating power of the grand exterior 

 formation. That they are parts of a whole is most clearly 

 indicated. One cannot fail to remark, however, that within 

 the precincts of the cluster, individual nebulre adhere, in 

 the main, to individual stars ; while outside of them, the 

 relations are more generalized. Maia resembles a diamond 

 clasp on a curving plume ; Electra protrudes a luminous 

 tentacle straight towards Alcyone ; Merope, besides its 

 sweeping gauze-train, has a close nebulous satellite, round 

 and condensed, like a distant comet, as yet unprovided 

 with perihelion-trappings. This singular couple, separated 

 by a clear interval of barely ten seconds, was first expressly 

 observed by Prof. Barnard, with the Lick thirty-six 

 inch, in November, 1890; but was then proved to have 

 been already photographed at Oxford and Lick. If for no 

 other reason, it deserves attention for its illustrative 



S. 



N. 



Fig. 2. — Photograph of the Ph-iudos, bv E. 

 1st, lb93. Scale, 1mm = 4'-5 of ai-c. 

 reversed, right and left. 



E. Barnard. Exposiu-e = 41i., December 

 In reproduction the photograph has been 



bearing upon speculations regarding the origin of double 

 stars. Our present purpose, however, is to point out that 

 while the Pleiades nebula; show a detailed connection with 

 the members of the group, the new ones, to which Prof. 

 Barnard introduces us, seem to depend upon the group as 

 a whole. This large structural affinity is manifest in the 

 single fact that the two chief nebular branches issue from 

 the cluster at opposite ends of one of its diameters. 



" At opposite ends of one of its diameters," we repeat ; 



since the photograph we are engaged in considering 

 intimates, or rather asserts, the shape of the central 

 stellar collection to be roughly globular. Yet its com- 

 ponents give no sign of condensation towards a centre. 

 They are then, it may be presumed, scattered very 

 sparsely throughout the congeries of attached nebula, the 

 whole conceivably rotating with extreme slowness on a 

 common axis. In this not impossible, though scarcely 

 probable case, a statical equilibrium would prevail, and the 

 stars should continue relatively fixed. And we recall that 

 the measures hitherto executed upon them have afforded 

 no proof of systemic circulation. It would, none the less, 

 be the height of rashness to infer its absence on negative 

 evidence collected during little more than half a century — 

 a mere moment of cosmical time. The mechanism of 

 clusters oft'ers indeed a problem for the present insoluble 

 — scarcely to be reached by so much as a conjecture. 



Part of a photograph taken by Prof. Barnard in four 

 hours, December 1st, 1893, is shown in Fig. 2. The 

 halation rings, so curiously conspicuous in it, are, we need 

 hardly say, a purely photographic effect, and each seems 

 to be associated with several stars. It is an inverse 

 representation. The stars are printed black on a white 

 ground, as in the original negative, the object in view 

 being to bring out more clearly the peculiarities of their 

 distribution. Away to the north-west, on a part of the plate 

 not included in the accompanying reproduction, a meteor 

 has inscribed its "adsum" by means of a ;liort, broken trail, 

 indicating movement, so far as it deviated from the line of 

 sight, towards the south-east. But the apparition was 

 doubtless a mere flash, although a 

 tolerably bright one, since it was 

 registered in — at the most — one seven 

 thousandth part of the time allotted 

 to the stars. 



Turning again to the plate, we 

 perceive with amazement that the sky- 

 vista, which after four hours of ex- 

 posure lay comparatively open, became, 

 in the course of ten, almost choked 

 with star-discs. They exhibit no trace 

 of condensation towards the cluster, or 

 of any other kind of arrangement 

 w. relative to it ; and must be taken 

 therefore to represent the contents of 

 the backward-lying abyss. Clearly, 

 then, the Pleiades are projected upon 

 a stratum of small stars, to which 

 probably belong the great majority of 

 the two thousand three himdred and 

 twenty-six mapped by the MM. Henry. 

 It follows that we are stOI in the 

 dark as regards the true nature of 

 this wonderful aggregation. For its 

 genuine members must be associated, 

 indistinguishably, except through the 

 effects of proper motion, with a 

 multitude of fictitious Pleiades. Of 

 these, half a dozen have been already 

 expelled by Dr. Elkin's remeasure- 

 ment, in 1886, of thirty-two stars determined by Bessel 

 during the years 1829-1841, and the proportion of intruders 

 may be assumed to increase enormously with descent to the 

 lower ranks of brightness. But about twenty years must 

 elapse before they can be surely identified. The cluster, 

 as a whole, drifts nearly six seconds a century in a 

 direction sensibly opposite to that of the sun's advance 

 through space. Stars not belonging to it will, accordingly, 

 be left behind ; and when the lag amounts to two second^, 



