5"4 



NA JURE 



{April 15, 1886 



as " very distinct and immediately conspicuous " (Tempel, 

 Monthly Notices, vol. xl. p. 622). 



The truth seems to be that, whether variable or not, it 

 is one of the most sensitive objects in the sky. The 

 slightest haze suffices to obliterate it. Air so translucent 

 as to allow 13th or 14th magnitude stars to shine clearly 

 may still contain mist enough to shroud the iVIeropc 

 nebula. Nor will it endure high magnification. Its scanty 

 rays need to be condensed into a small image to become 

 sensible, while, in the restricted field of a great telescope, 

 they are apt to leave the eye unaffected for want of a con- 

 trasted black background. Even the enormous light- 

 grasp of the Rosse reflector was unavailing to show this 

 delicate object until the lowest possible powers were 

 applied. Idiosyncracies both of instruments and ob- 

 servers have besides tended to widen divergences of 

 opinion. Some eyes appear to be incapable of discerning 

 an illumination so faint and diffused. Nay, telescopes of 

 equal apertures are not perhaps devoid of " personality " 

 regarding it. 



Still more difficult to explain than its anomalous invisi- 

 bility, are the differences in its aspect when seen. Gold- 

 schmidt made the supposed discovery in 1863 {Les 

 Mondes, t. iii. p. 529) that it was no isolated forma- 

 tion, but a spur or projection inwards from a vast 

 nebulosity 5° in diameter, in which a blank central space 

 similar to that left clear for the trapezium in the Orion 

 nebula, was occupied by the entire group of the Pleiades 

 And Wolf, after careful consideration, adopted this view 

 in 1876. Other observers have seen several distinct 

 patches in lieu of the large misty ellipse, about 35' by 20', 

 in which M. Tempel could just distinguish the beginnings 

 of two nuclear condensations. Engelmann's map of the 

 Pleiades, published in 1876 (in Bd. ii. of Bessei's ^M(;«(/- 

 lungen), shows a mere nebulous wisp to the south of, and 

 apparently unconnected with, Merope. Two such were 

 observed by MM. Baillaud and Andri^, at Paris, March 7, 

 1874, and form the regular aspect of the nebula as viewed 

 with Mr. Pratt's S-inch mirror. Mr. Common's great 

 speculum disclosed to him, February 3, 1880, a triple 

 and considerably scattered group, with unmistakable 

 symptoms of an extension north-west towards Electra. 

 Now at last the camera steps in as arbiter between con- 

 flicting observations. The Paris photographs decide at 

 once the Merope nebula to be no such illusory object as 

 has sometimes been supposed. It really exists ; but in a 

 shape at present strangely varied from that of its earlier 

 appearances. Only its position on either side of the star 

 Merope identifies the irregularly striated formation visible 

 on the plates, with the uniform train of almost evanescent 

 luminosity recorded in M. Tempel's skilful drawings. 



Comparison with future autographic pictures will 

 quickly and easily settle the question of its variability. 

 As yet there is little positive, though some presumptive, 

 evidence for the affirmative. Hind's and Chacornac's 

 admittedly variable nebulae are situated not far off, one in 

 the head, the other near the tip of the right horn, of 

 Taurus. And it is unquestionable that some kinds of 

 sidereal phenomena tend to localise themselves in certain 

 quarters of the sky. There is, moreover, reason to 

 believe that a nebula has vanished from the heart of the 

 cluster itself Such an object is marked on Jeaurat's 

 chart of the Pleiades (published in 1782) in connection 

 with a pair of small stars nnmbered by Bessel 31 and 32. 

 These lie about half a lunar diameter north of Pleione, in 

 a quarter where no vestige of nebulosity can now be dis- 

 tinguished ; so that the possibility is excluded of Jeaurat's 

 having merely anticipated subsequent discoveries. The 

 stars 31 and 32 form a binary system already giving 

 signs of mutual revolution ; and one of them (No. 32) is 

 considered by Lindemann as variable. If any weight 

 attached to Jeaurat's estimates of brightness, there could 

 be no doubt about the matter, since he stated them to be 

 respectively of 7th and 4th magnitudes, while Bessel 



found them both of the 8th. Jeaurat's authority in photo- 

 metry, it is true, is of the lowest ; yet it is hard to believe 

 that his eye can have represented to him a difference of 

 three whole orders between two equal stars, visible side 

 by side in one field of his telescope. 



Stellar fluctuations have so frequently been observed 

 to be associated with nebulous surroundings, that it is 

 worth noting, as at least a coincidence, that neither 

 Merope nor Maia is exempt from a strong suspicion of 

 variability. The latter, according to M. Wolf, is slowly 

 gaining lustre ; the former oscillates through a range of 

 one magnitude. 



The stars of the Pleiades are immeasurably far off. 

 None of them has any sensible parallax ; and we are thus 

 uninformed as to their intrinsic lustre, mutual distance, or 

 gravitating mass. It is, however, easy to compute the 

 dimensions of the group relatively to its remoteness from 

 ourselves.^ A circle described from Alcyone with a radius 

 of 48' includes all its principal stars. Only one of Bessel's 

 53 falls outside it. We may then take the globular mass 

 of the cluster to be of this apparent size, disregarding 

 the stellar streams external to it as being, more or less, 

 outliers. Now, since the sine of an angle of 48' is to 

 radius as (in round numbers) I : 71, it follows that the 

 furthest of the suns congregated into the nuclear group 

 under consideration, are just 71 times as distant from us 

 as from the centre of their own system. Consequently, 

 Alcyone blazes upon them with 5000 times the lustre 

 it displays to us, or as a star about 86 times the brilliancy 

 of Sirius. It would still, however, seem a star rather 

 than a sun. Even from the distance of Neptune, our 

 own central luminary must outshine Alcyone, as viewed, 

 say, from Atlas or Taygeta, fully 77,000 times. 



The glimpse afforded by recent investigations of the 

 structure of the Pleiades group is a very surprising one. 

 We find in it a miniature sidereal system, the richness 

 and variety of which bewilder theoretical conceptions, 

 and recall, as analogous, the accumulated wonders of the 

 Magellanic clouds. Nebul* are discovered in most inti- 

 mate connection with lucid stars, and in suspicious rela- 

 tions to their luminous vicissitudes, while themselves 

 possibly subject to strange alternations of visibility. 

 Stars of all orders are included in one vast assemblage, 

 some doubtless magnificent orbs, of many times the 

 radiance of our sun ; others as inferior to them perhaps 

 as the moons of Mars to Jupiter. The distribution of 

 these bodies appears to be no less varied than their size. 

 Groups are collected within the main group, systems 

 revolve apart, the subordination of which to the laws of 

 a general federative union leaves their internal liberty of 

 movement unshackled. It is not, indeed, certain that a 

 dynamical equilibrium of the whole subsists. Hints of 

 a centrifugal tendency have been caught by M. Wolf, 

 suggesting that an impulse of separation may at present 

 be the predominating one. Possibly, then, Mr. Stone's 

 curious observations on the slowly divergent proper 

 motions of some southern stars, apparently the remnants 

 of broken-up systems, may exemplify the inscrutable 

 destinies in store for the unnumbered stars of the 

 Pleiades. A. M. Clerke 



NOTES 



Science was en fete -aX the Mansion House on Tuesday night, 

 when the Lord Mayor and Lady Mayoress received the Presi- 

 dents and Members of Council of the Royal and other principal 

 Learned Societies. Everything passed off in the most admir- 

 able way, and the presence of about 100 ladies, as well as of 

 200 of our representative men of science, made the gathering a 

 very brilliant as well as a very remarkable one. The present 



" As was done by the Rev. J. Michcll in 1767 (Phil. Tnvis., vol. Ivii. 

 P- 257)- 



