562 



NATURE 



{April IS, 1886 



heed of stellar configurations. Yet all over the world, in 

 the northern and southern hemispheres, amongst Poly- 

 nesian and Australian savages, as well as under the sway 

 of Egyptian, Peruvian, Mexican, early Hellenic, and 

 Indian civilisations, traces are found of a primitive 

 calendar regulated by the risings and settings of the so- 

 called '' Seven Stars." ' 



Only 6 Pleiades, indeed, are usually visible, although 

 12, 14, even perhaps 16, have been made out without 

 optical aid by exceptionally keen and vigilant observers. 

 Hipparchus is probably the only astronomer of antiquity 

 who mentions the possibility of discerning a seventh 

 member of the family ; and he admits that it could be 

 seen only under a transparent sky and in the absence of 

 the moon. Thus Ovid's well-known line in reference to 

 the group, — 



" Quae septem did, se.'i tamen esse solent," — 



quite correctly describes its ordinary visual aspect. 

 Nevertheless, it figures as septuple in the folk-lore of 

 well-nigh all the peoples of the globe, from the Baltic to 

 the Tropic of Capricorn ; and the aborigines of Victoria, 

 no less than Greek poets, have sought, by appropriate 

 inventions, to reconcile what was apparent with what was 

 assumed. It is altogether uncertain whether the story of 

 the " lost Pleiad" is a tradition or a myth, — whether it 

 commemorated an actual prehistoric occurrence, or 

 merely supplied by fable the unit wanting to complete a 

 consecrated number. There is no manner of doubt, 

 however, that it is cosmopolitan and immemorially an- 

 tique. Before dismissing it as an idle fancy, it may be 

 worth while to inquire into the probability of a real loss 

 of lustre in an originally manifest companion of Alcyone. 



The spectroscope affords some grounds for a prhmi 

 facie presumption against marked variability in these 

 associated stars With one or two quite insignificant ex- 

 ceptions, they all shine with the bluish-white radiance, 

 and display the brilliant spectrum obscured only by 

 hydrogen-absorption, of which steadfast emission is 

 usually the concomitant. Usually, but not invariably. 

 There are noted instances to the contrary, which further 

 experience may perhaps multiply. It is, at any rate, cer- 

 tain that perplexing anomalies have hitherto been found 

 to affect photometric estimates of the Pleiades. 



Very little reUance can in general be placed upon early 

 accounts of relative stellar brightness ; yet it is startling 

 to find that Ptolemy enumerates four individuals of the 

 group, none of which can be identified with its now pre- 

 eminent member. Either, then, his description is strangely 

 misleading, or Alcyone was not, 1750 years ago, the 

 lucida of the collection. Francis Baily, who well knew 

 how to make allowance for ancient inaccuracy, considered 

 that this star, if observed at all by the Alexandrian 

 astronomer, must then hive been of far less magnitude 

 than now {Memoirs R. Astr. Soc, vol. xiii. p. 9). Further, 

 Abdurrahman Sufi, who professed, in the tenth century, 

 to have corrected, from personal observations, the cata- 

 logue of his predecessor, expressly states that Ptolemy's 

 quartette were re-specified by him as being the most con- 

 spicuous of the Pleiades (Flammarion, " Les Etoiles," 

 p. 294). But Alcyone, as we have seen, was certainly not 

 amongst them. The leading position it still occupies was 

 first, some six centuries later, assigned to it by Tycho 

 Brahe. That there have been fluctuations of lustre 

 among its attendant stars has, by the recent inquiries of 

 Wolf and Lindemann, been rendered certain in a few 

 cases, and highly probable in many more. Room for 

 doubt on the point will presumably ere long be narrowly 

 limited. By means of his " wedge photometer," invented 

 for the special purpose of introducing harmony into the 

 light-measurements of the Pleiades, Prof Pritchard has 

 accumulated materials for future comparisons, vouched 



' See R^ G. Haliburton's " Festival cf the Tead," aid Bunsen's "Die 

 Plyaden und der Thierkreis." 



for as trustworthy by the satisfactory agreement be- 

 tween his estimates of magnitude and those arrived at 

 by Profs. Lindemann and Pickering. In the meantime 

 there is sufficient authentic evidence of variability in the 

 group to render the literal explanation of the disappear- 

 ance of the seventh daughter of Atlas a plausible one. 



The oldest existing map of the Pleiades was constructed 

 by Maestlin in 1579. It deserves attention as a curiosity 

 of archaeological astronomy, and as exhibiting eleven stars, 

 discerned, of course, with the naked eye.' Galileo made 

 the first telescopic survey of the group, of which he 

 detected, with his feeble instrument, nearly 50, and 

 graphically recorded the positions of 36 components. De 

 la Hire, Cassini, and Jeaurat followed, the last mapping 

 and cataloguing at the Ecole Militaire, in 1782, 64 

 leading Atlantids {Mimoires de I' Ac. lies Sciences, 1779, 

 pub. 1782, p. 505). In publishing, in 1841, differential 

 measures, with the Konigsberg heliometer, of 52 of these 

 stars {Astr. Nach., No. 430) — besides Alcyone, the place 

 of which was fixed by observations in the meridian — 

 Bessel had another end in view than mere enumeration. 

 He designed to establish a term of comparison from 

 which their mutual displacements might hereafter be 

 determined. And, in fact, the prospect of gaining some 

 real knowledge of such displacements would be still 

 remote, were it not for this anticipatory labour. Dr. 

 Gould was the first to turn it to account. He obtained, 

 it is true, only a negative result, but one memorable as 

 the earliest in sidereal science derived from the use of a 

 method then in its infancy, but now, within a score of 

 years, grown to be of overwhelming importance. 



With an object-glass II inches in diameter, corrected 

 for the ultra-violet rays, Lewis M. Rutherfurd, of New 

 York, took, in 1S65, some admirable photographs of the 

 Pleiades {Observatory, vol. ii. p. 16). One of them, now 

 in the possession of the Royal Astronomical Society, re- 

 markably exemplifies the capabilities of the old collodion- 

 process. The time of exposure is not known, but was 

 probably short, since there is scarcely a trace of irradia- 

 tion, and the stellar impressions are minute and beautifully 

 distinct. They are thus peculiarly susceptible of exact 

 measurement. The right ascensions and declinations of 

 nearly fifty Pleiades, hence deduced by Gould, showed so 

 close an agreement with Bessel's, as to make it fairly 

 certain that no appreciable change in their relations had 

 taken place within a quarter of a century. 



This conclusion was somewhat modified by the results 

 of M. C. Wolf's laborious investigation at Paris ten years 

 later {Annates de I'Observatoire, t. xiv. 1877)— probably 

 the last aiming at exhaustiveness for which merely visual 

 data will furnish the materials. His chart includes all 

 stars down to the 14th magnitude, to the number of 625, 

 contained in a rectangle 135' by 90', in which Alcyone 

 occupies a nearly central position. Of these, 571 are cata- 

 logued, while the places of 79 are determined with the 

 utmost nicety, and compared with those assigned at 

 Konigsberg. The upshot went to show unmistakably a 

 community of backward drift, reflected from our own 

 advance through space. Alcyone has a well-marked, 

 though small, proper motion, in a direction exactly oppo- 

 site to that of the solar translation. The invariability of 

 relative situation in its crowd of companions, inferred 

 by Gould in 1866, hence really amounted to a demonstra- 

 tion of the existence of a physical tie between them. For 

 it proved that the whole cluster was pursuing an identical 

 line of march in the sky. Even though that march be 

 purely a parallactic effect, the force of the argument 

 remains untouched. Unanimity in apparent movement 

 implies a real aggregation equally with unanimity in rapid 

 actual progress. 



Other, and if possible more cogent, proofs of the close 

 relationship of these clustered stars are now, however, 



■ For a map by Miss Airy of 12 Pleiades from ocular view see Monilily 

 Notices, vol. xxiii. p. 175. 



