August 18, 1887] 



NATURE 



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It has been known for some time that the stars of the 

 Pleiades possess a small identical proper motion. Its 

 direction, as ascertained by Newcomb in 1878, is about 

 south-south-east ; its amount is somewhat less than six 

 seconds of arc in a century. The double-star 61 Cygni, 

 in fact, is displaced very nearly as much in one year as 

 Alcyone with its train in one hundred. Nor is there 

 much probability that this slow secular shifting is other 

 than apparent : since it pretty accurately reverses the 

 course of the sun's translation through space, it may be 

 presumed that the backwani current of movement in 

 which the Pleiades seem to float is purely an effect of 

 our own omvard ix?iM€[\'\ng. 



Now the curious fixct emerges from Dr. Elkin's 

 inquiries that six of Bessel's stars are exempt from the 

 general drift of the group. They are being progressively 

 left behind. The inference is obvious, that they do 

 not in reality belong to, but are merely accidentally 

 projected upon, it : or, rather, that it is projected 

 upon them ; for their apparent immobility (which, in 

 two of the six, may be called absolute) shows them 

 with tolerable certainty to be indefinitely more remote — 

 so remote that the p^th, moderately estimated at 

 21,000,000,000 miles in length, traversed by the solar 

 system during the forty-five years elapsed since the 

 Kdnigsberg measures, dwindles into visual insensibility 

 when beheld from them ! The brightest of these six far-off 

 stars is just above the eighth (7 "9) magnitude ; the others 

 range from 8*5 down to below the ninth. 



A chart of the relative displacements indicated for 

 Bessel's stars by the differences in their inter-mutual posi- 

 tions as determined at Konigsberg and Yale, accompanies 

 the paper before us. Divergences exceeding o"40 (taken 

 as the limit of probable error) are regarded as due to real 

 motion ; and this is the case with twenty- six stars besides 

 the half-dozen already mentioned as destined deserters 

 from the group. With these last may be associated two 

 stars surmised, for an opposite reason, to stand aloof 

 from it. Instead of tarrying behind, they are hurrying on 

 in front. An excess of the proper movement of their 

 companions belongs to them ; and since that movement 

 is presumably an effect of secular parallax, we are justified 

 in inferring their possession of an extra share of it to 

 signify their greater proximity to the sun. Hence, of all 

 the stars in the Pleiades these are the most likely to have 

 a measurable annual parallax. One is a star a little above 

 the seventh magnitude, distinguished as s Pleiadum ; the 

 other, of about the eighth, is numbered 25 in Bessel's 

 list. Dr. Elkin has not omitted to remark that the con- 

 jecture of their disconnexion from the cluster is confirmed 

 by the circumstance that its typical spectrum (as shown 

 on Prof. Pickering's plates) is varied in s by the marked 

 character of the K line. The spectrum of its fellow- 

 traveller (No. 25) is still undetermined. 



It is improbable, however, that even these nearer stars 

 are practicable subjects for the direct determination of 

 annual parallax. By indirect means, however, we can 

 obtain some idea of their distance. All that we want to 

 know for the purpose is the rale of the sun's motion ; its 

 direction we may consider as given with approximate 

 accuracy by Airy's investigation. Now, spectroscopic 

 measurements of stellar movements of approach and 

 recession will eventually afford ample materials from 

 which to deduce the solar velocity ; though they are as 

 yet not accurate or numerous enough to found any defi- 

 nitive conclusion upon. Nevertheless, M. Homann's pre- 

 liminary result of fifteen miles a second as the speed with 

 which our system travels in its vast orbit, inspires con- 

 fidence both from the trustworthiness of the determina- 

 tions (Mr. Seabroke's) serving as its basis, and from its 

 intrinsic probability. Accepting it provisionally, we find 

 the parallax .of Alcyone = about o"-02, implying a distance 

 of 954,000,000,000,000 miles, and a light-journey of 163 

 years. It is assumed that the whole of its proper motion 



of 2""6i in forty-five years is the visual projection of our 

 own movement towards a point in R.A. 261'', Decl. + 25°. 



Thus, the parallax of the two stars which we suspect 

 to lie between us and the stars forming the genuine group 

 of the Pleiades, at perhaps two-thirds of their distance, 

 can hardly exceed o""03. This is just half that found by Dr. 

 Gill for f Toucani, which may be regarded as, up to this, 

 the smallest annual displacement at all satisfactorily deter- 

 mined. And the error of the present estimate is more 

 likely to be on the side of excess than of defect. That 

 is, the stars in question can hardly be much nearer to us 

 than is implied by an annual parallax of o"*03, and they 

 may be considerably more remote. 



Dr. Elkin concludes, from the minuteness of the de- 

 tected changes of position among the Pleiades, that " the 

 hopes of obtaining any clue to the internal mechanism of 

 this cluster seem not likely to be realized in an imme- 

 diate future ;" remarking further : "The bright stars in 

 especial seem to form an almost rigid system, as for only 

 one is there really much evidence of motion, and in this 

 case the total amount is barely 1" per century." This one 

 mobile member of the naked-eye group is Electra ; and 

 it is noticeable that the apparent direction of its displace- 

 ment favours the hypothesis of leisurely orbital circula- 

 tion round the leading star. The larger movements, 

 however, ascribed to some of the fainter associated stars 

 are far from harmonizing with this preconceived notion 

 of what they ought to be. On the contrary, so far as they 

 are known at present, they force upon our minds the idea 

 that the cluster may be undergoing some slow process of 

 disintegration. M. Wolfs impression of incipient centri- 

 fugal tendencies among its components certainly derives 

 some confirmation from Dr. Elkin's chart. Divergent 

 niovements are the most strongly marked ; and the region 

 round Alcyone suggests, at the first glance, rather a very 

 confused area of radiation for a flight of meteors, than 

 the central seat of attraction of a revolving throng of 

 suns. 



There are many signs, however, that adjacent stars in 

 the cluster do not pursue independent courses. " Com- 

 munity of drift " is visible in many distinct sets ; while 

 there is as yet no perceptible evidence, from orbital 

 motion, of association into subordinate systems. The 

 three eighth-magnitude stars, for instance, arranged in a 

 small isosceles triangle near Alcyone, do not, as might 

 have been expected a priori, constitute a real ternary 

 group. They are all apparently travelling directly away 

 from the large star close by them, in straight lines which 

 may of course be the projections of closed curves ; but 

 their rates of travel are so different as to involve certain 

 progressive separation. Obviously, the order and method 

 of such movements as are just beginning to develop to 

 our apprehension among the Pleiades will not prove 

 easy to divine. A. M. Clerke. 



NOTES. 

 Strenuous eflforts have been made to secure that the arrange- 

 ments for the observation of the total solar eclipse of August 19 

 shall be adequate. " A large number of astronomers," says the 

 Times o{ \he 15th inst., "will be distributed along the central 

 line, fully equipped with instruments suited to the particular 

 work they intend to do. The Russians themselves have most 

 energetically organized a very complete set of observations, 

 meteorological and otherwise, at widely-distant stations, viz. 

 Krasnoiarsk in Siberia, Perm in the Ural Mountains, and Viatka 

 in Central Russia ; while Prof. Mendeljew goes to Pavlovsk, 

 near St. Petersburg ; Prof. Bredichin, of the Moscow Ob- 

 servatory, to Kineshma ; and Dr. Podsolnotschnaja will be 

 stationed near Tver. Several foreign astronomers will also visit 

 Russia, and have received very hospitable treatment at the hands 

 of Prof. Struve and the other Russian authorities. From 



