I 



Nov. I, 1888] 



NATURE 



13 



conveying a current in a magnetic field is certainly more 

 or less strained by mechanical forces, and hence heat will 

 be developed unequally in different parts, by a sort of 

 Peltier effect ; and the result of this will be to modify the 

 resistance in patches and so to produce a disturbance of 

 the flow which may easily result partly in a transverse 

 1..M.F. This has been pointed out by Mr. Shelford 

 llidwell. 



The more direct effect of magnetism on conductivity 

 may be negligibly small in many metals, but in bismuth 

 it is certainly large. Both of these spurious effects seem 

 to be large in bismuth, and probably quite mask any true 

 Hall effect there may be in that metal. In all cases the 

 existence of these spurious effects makes it difficult to be 

 sure of the magnitude and sign of the real rotational 

 effect. 



But, it may be asked, what right have we to distinguish 

 between a real and a spurious Hall effect? If a transverse 

 E.M.F. can be predicted by reason of known strains and 

 thermo-electric properties, as well as by known rotation of 

 light effects, why should the two things be considered 

 different .^ Why should they not be different modes of 

 regarding one and the same phenomenon .-* 



In other words, may not the Faraday rotation of light 

 vibration be due to infinitesimal temporary strains and 

 heatings in the medium caused by the fact that minute 

 electric displacements are occurring in a violent magnetic 

 field ? This is a question capable of being answered by 

 a quantitative determination of the amounts and direction 

 of the effects to be e.xpected, and a comparison with those 

 actually observed. I do not know of data at present 

 obtained sufficient to enable us to answer it. If the 

 answer should turn out to be in the affirmative, the phe- 

 nomenon of hysteresis will be at once linked, by an 

 underground path, with those of thermo-electricity and 

 strain. 1 



Oliver J. Lodge. 

 (7!? be continued.) 



IRREGULAR STAR CLUSTERS. 

 T T is not always easy to distinguish between a casual 

 ■*- " sprinkle " of stars and a genuine cluster. The 

 movement-test, by which so many physical have been 

 discriminated from optical double stars, is here inapplic- 

 able. The Pleiades are the only considerable group 

 possessing an ascertained common proper motion. All 

 other clusters, debarred as yet from the appeal to this de- 

 monstrative argument of their physical nature, have to 

 depend solely upon evidence from probability, with its in- 

 definite variations of conclusiveness according to the 

 circumstances of each particular case. It is, however, in 

 general, amply sufficient. Among five hundred clusters 

 registered as such, there are few indeed as to which there 

 can be any doubt of their forming separate systems ; 

 although many real aggregations may exist unrecognized, 

 owing to their loosely scattered character. 



Two inferences may be safely derived from the results 

 of recent inquiries into the constitution of the Pleiades. 

 First, that interstitial movements in clusters are likely to 

 be so extremely slow that centuries must elapse before 

 they can become conspicuous ; next, that stars showing 

 somewhat marked displacements are presumably mere 

 travellers across, and no genuine components of, the 

 cluster they seem to belon'g to. An example of this kind 

 of temporary association is almost certainly furnished by 

 an apparent member of a scattered group in Ophiuchus 

 ("Gen. Cat." 1440), the position of which was found, by the 

 comparison of photographs taken by M. von (iothard in 

 1886 with Vogel's measures of eighteen years previously, 



' Perhaps I ought to caution students not to accept my connection of 

 Faraday's or Hall's effect with hysteresis as in any way authoritative. Until 

 these views have been criticized it will be wise to place no reliance on them. 



to have changed to the extent of 45", or at the rate of 

 2^' annually {Asir. Nac/i., No. 2777). Itsmotion, if recti- 

 linear, would carry it from end to end of the collection it 

 is projected upon, in 360 years ; and its eventual detach- 

 ment from it may have become palpably inevitable withir^ 

 ten. The star is of the eleventh magnitude, and is by 

 far the swiftest-moving yet known of so small a size. 



Several of the stellar gems surrounding k Crucis are 

 suspected of considerable mobility. Sir John Herschel, 

 during his visit to the Cape, determined the relative 

 places of 110, all included in an area of about ^ of a 

 square degree ("Cape Observations," p. 17); and the 

 process was, by Mr. H. C. Russell, of Sydney, in 1872, 

 repeated and extended to 130 components {Monthly 

 Notices, vol. xxxiii. p. 66). The result was to bring out 

 discrepancies 'which, if really due to movements of the 

 grouped stars, would be of extreme interest. Herschel's 

 measurements, however, were necessarily too hasty to be 

 minutely reliable ; so that changes depending upon their 

 authority need to be confirmed by continuance before 

 they can be unreservedly accepted. The same qualifica- 

 tion applies to M. Cruls's discovery of orbital revolution 

 in three double stars within the precincts of the cluster 

 {Comptes rendus, t. Ixxxix. p. 435). 



The stars about k Crucis are famous for the loveliness' 

 of their varied hues. Blue and green, red and sulphur- 

 coloured orbs shine together in a matchless sidereal 

 picture, setting at the same time a problem in sidereal 

 chromatics by no means easy to solve. There is no evi- 

 dence of change of tint among them since Herschel's 

 time, but there is some, tolerably conclusive, as to change 

 of brightness. 



Many irregular clusters seem to be throughout made up 

 of star-streams and reticulations exactly similar to the 

 inflected appendages of globular clusters. A collection 

 (M 24) visible to the naked eye as a dim cloudlet near 

 \i Sagittarii, and regarded by Sir John Herschel as in- 

 timately connected with, if not an actual part of, the 

 Milky Way, was named by Father Secchi " Delle Cau- 

 stiche," from the peculiar arrangement of its stars in rays, 

 arches, caustic curves, and intertwined spirals. Closely 

 adjacent to it, he noted a group of eleventh magnitude 

 stars forming three spokes, as it were, and the nave of a 

 wheel, the axis of which was occupied by a much brighter 

 close pair {Atti dell' Accad. Pont., t. vii. p. 72). 



The same kind of radiated structure is apparent in a 

 stellar swarm near the right foot of Castor I'M 35), 

 which, with Lassell's 24-inch mirror, showed as so 

 " marvellously striking an object that no one could see 

 it for the first time without an exclamation." A field 

 19' in diameter " is perfectly full of brilliant stars, un- 

 usually equal in magnitude and distribution over the 

 whole area. Nothing but a sight of the object itself can 

 convey an adequate idea of its excjuisite beauty " {Monthly 

 Notices^ vol. xiv. p. 76). Admiral Smyth described it as 

 " a gorgeous field of stars from the ninth to the sixteenth 

 magnitudes, but with the centre of the mass less rich 

 than the rest. From the small stars being inclined to 

 form curves of three or four, and often with a large one 

 at the root of the curve, it somewhat reminds one of the 

 bursting of a sky-rocket " (" Cycle of Celestial Objects," 

 p. 168, Chambers). A marvellously perfect photograph 

 of this cluster, taken by the MM. Henry, March 10, 1886, 

 exhibits not less than two thousand stars disposed in a 

 roughly-indicated, eight-rayed figure, the branches often 

 connected by drooping chains, and composed in detail of 

 sinuous lines, or "fantastically crossing arcs'' of stars 

 (Secchi, loc. cit,). 



About one hundred connected stars in Ophiuchus 

 (" G. C." 4346) " run in lines and arches " (J. Herschel, 

 Phil. Trans., vol. cxxiii. p. 460) ; a collection of eleventh 

 magnitude ones in Sagittarius (" G. C." 4323) are scattered 

 along " zigzag lines." The constituents of a large cluster 

 near the Poop of Argo ("G. C." 1649) struck the elder 



