280 



KNOWLEDGE 



[December 2, 1895. 



arc spectrum the absorption line does not always occupy the 

 exact centre of the bright band (;/). Now, if already in 

 the dififerent layers of the voltaic arc the lines belonging 

 to the same matter show such clear changes of refrangi- 

 bility, how much greater changes can be expected in the 

 difl'erent layers of stars, which differ so far more widely. 



AVe have here to deal with a vastly more complicated 

 piece of mechanism than has hitherto generally been 

 supposed. Chemical and electrical luminescence, cold 

 riames with discontinuous spectra, displacement of lines 

 without any motion in the line of sight, are facts which can 

 no longer be discarded. They must be taken into account. 

 So we shall get more trustworthy results than by continu- 

 ing to solve all spectral problems by the principles of 

 Kirchhoi? and Doppler only. 



THE EXTERIOR NEBULOSITIES OF THE 

 PLEIADES. 



By Miss A. M. Clerke, Authoress of " The System of the 



Stars" and " A Popular History of Astrotwmy during the 



Nineteenth Centttry," tfc, ih. 



WHAT a powerful telescope is to a multitude of 

 remote star-clusters, a common opera-glass is 

 to the Pleiades. Its use just suffices to tran- 

 quillize their images, while leaving intact the 

 visible evidence of their close association. A 

 single glance embraces the entire brilliant group, and takes 

 in the peculiarities of its configuration, which, once seen, 

 are not easily forgotten. The scenic effect is splendid, and 

 ought to be familiar. At the first instant, one is almost 

 startled at the beauty and wonder of such an assemblage of 

 lustrously white orbs, poised, as it were, in the breathless 

 stillness of an apparition, and distributed with a semblance 

 of express arrangement, very different from that of a 

 casually collected crowd. Behind them, the sky, thus seen, 

 appears intensely black ; yet the "silver braid " of the poet's 

 fancy is now perceived to have been an unconscious fore- 

 cast of coming knowledge. The Pleiades are, in very fact, 

 "tangled" and wreathed in cosmic material. Tempel's dis- 

 covery, in 1859, of the great Maia nebula gave the first hint 

 of this state of things. He intuitively apprehended its 

 importance. That " breath-stain " (Hauch) on the vault 

 of heaven would, he used to declare, perpetuate his name. 

 And its significance, both in itself and as an earnest of 

 further disclosures, is still very far from being exhausted. 

 The reproduction, in Knowledge for May, 1891, of Dr. 

 Koberts's astonishing photograph of this cluster, gave our 

 readers the opportunity of judging for themselves as to the 

 nebulous relations of its component stars. They are now 

 invited to look further afield. Between the Pleiades and the 

 Milky Way to the north, the elder Herschel located one 

 of those tarnished sky-areas which he alone among his 

 contemporaries and immediate successors was capable of 

 perceiving. " Diifused nebulosity," as he called it, evades, 

 as a rule, direct observation, so that the experience recently 

 gained of its considerable self-recording power is parti- 

 cularly welcome ; all the more so that this kind of dim 

 phosphorescence seems to denote the presence, not of 

 mere vague outflows of tenuous matter, but of more or less 

 definite constructions, modelled by the play of, to us, 

 inscrutable forces. In obtaining pictures of them, telescopes 

 are left nowhere by ordmary portrait lenses. Dr. Max 

 Wolf has shown,'- both practically and theoretically, that 

 his two and a quarter inch aplanatic doublet is five times 



(y) CAem. of the Sun, y,. 221. * Sirius, 1891, ^. 106. 



more effective for the chemical delineation of nebulse than 

 the Henry thirteen-inch photographic refractor, although 

 thirty-two times its inferior in getting impressions of stars. 

 With this unpretending instrument he obtained in 1890 

 indications of extensive nebulosities near, and plainly not 

 unrelated to the Pleiades.! For the intervening space, 

 although wide, was spanned by nebulous " bridges," 

 survivals perhaps of an antique condition, if not the means 

 of present communication. 



In the course of his comet-searches, meanwhile. Prof. 

 Barnard had become telescopically familiar with Herschel's 

 " much-aftected " region north of the Pleiades (KxcnxEDOE, 

 •January, 1892), and formed the project of letting his 

 famous " Willard lens " have a good long look at it on the 

 earliest available occasion. This was on December Gth, 

 1893, and the experiment was continued on the night but 



Fia. 1. — Key-Map to the Exterior Nebulosities of the Pleiades. 



one following, giving a total exposure of more than ten 

 hours. With what results can be seen by turning to our 

 plate, in which an enlargement, made by Prof. Barnard 

 himself from the central portion of his negative, is repro- 

 duced. Obliterated by its own brilliancy, the actual 

 cluster, with its filmy domestic furniture, is merged in a 

 featureless mass ; but it is easy to discern several nebulous 

 outflows from it through the dense throng of surrounding 

 stars. The most conspicuous of these run eastward, one 

 from the northern, the other from the southern side of the 

 central mass ; and an effusion to the south-west is likewise 

 manifest. 



The picture, however, which should, for the purpose of 

 comparison, be turned sideways with the north uppermost, 

 is best elucidated by the accompanying sketch-map (Fig. 

 1), made by the artist from the original negative, with 

 which it is co-extensive. It covers, in fact, about one 

 hundred and fifty-eight square degrees, and reaches up to 



f Axfr. Nachrichfen, No. 3275. 



