April 15, 1 8 86 J 



NATURE 



gradually becoming available. The presence of minute 

 displacements connected with the internal economy of 

 the system, emerged pretty clearly from Wolf's inquiries ; 

 although their direction and amount remained little more 

 than conjectural. The main fact indicated is that of an 

 extraordinary complexity in the governing plan of the as- 

 semblage. It appears to embrace a great number of 

 binaries, and at least one triple star, which pursue their 

 separate revolutions independently of the higher systemic 

 relations which no doubt bind and sway them. Each of 

 the pairs, for instance, numbered 9, 10, and 31, 32, on 

 Bessel's list, gives signs of orbital movement ; while the 

 beautiful little triangle of Sth-magnitude stars close by 

 Alcyone is (seemingly) in slow process of deformation. 



Fresh evidence was deduced from a set of elaborate 

 measurements of forty stars in the Pleiades, completed by 

 Prof Pritchard in 1884. To fourteen amongst the 

 number, two independent processes were found applic- 

 able. Meridian observations extending over 130 years 

 afforded, when reduced, the means of ascertaining their 

 absolute co-ordinates and proper motions ; and these 

 were compared \\ith the results of micrometrical deter- 

 minations of relative position at Konigsberg, Paris, and 

 Oxford, 1838 to 1 880. Both methods agreed in pointing 

 to certain shiftings inter sc,]ml, as it were, nascent, and 

 demanding a further lapse of time for the development of 

 the scheme they are conducted upon. Enough, however, 

 was made out to show plainly that no mere incongruities 

 of proper motion, or perspective displacements consequent 

 on change in our own point of view, were concerned, but 

 real effects of gravitative action amongst a group of 

 mutually connected bodies. 



Thus at last we seem to be on the verge of learning 

 something of the interior mechanism of a star-cluster, the 

 extraordinary difficulty of the problems presented by 

 which has hitherto almost silenced speculation. The 

 facilities for collecting the necessary data offered by the 

 recent enormous improvements in stellar photography, 

 will doubtless help to stimulate the inquiry, as well as to 

 assure its conclusions. 



Our readers are already familiar with the first results 

 in photographic star-charting secured by means of an 

 apparatus constructed (as to its optical part) by MM. 

 Paul and Prosper Henry, and mounted in the garden of 

 the Paris Observatory in April 18S5. These have been fol- 

 lowed by four photographs of the Pleiades, taken respect- 

 ively on November 16, December 8 and 9,and January 8, of 

 surpassing beauty and interest. The exposure was in 

 each case of three hours, during which long period the 

 following of the diurnal movement appears to have been 

 absolutely perfect. No mechanism is adequate to effect 

 this with the requisite nicety ; the eye and hand of the 

 operator are an indispensable adjunct. An ili-inch 

 telescope, adapted for visual use, inclosed in the same 

 metallic tube with a photographic object-glass 134 inches 

 in diameter, serves accordingly as a channel of communi- 

 cation with the sky, through which the progress of the 

 operation is surveyed, and timely notice derived of the 

 need for controlling incipient inequalities. 



On the plates thus exposed, above a thousand stars — all 

 presumably belonging to the same magnificent cluster — 

 are clearly imprinted. They range down to the 17th 

 magnitude, many of them being beyond the power of any 

 telescope hitherto constructed to disclose to the eye. But 

 the retentive "photographic retina" has time on its side. 

 Such extraordinary success in registering the faintest 

 objects necessarily implies considerable over-exposure in 

 regard to bright ones. It is indeed found that the time 

 of poxc for a star of the sixteenth is no less than one 

 million times than for one of the first order of lustre (MM. 

 W^xvcy, La Nature. December 5, 1885). This disparity 

 constitutes perhaps the most serious drawback to the 

 photographic metliod of charting. It has, however, the 

 counterbalancing advantage of supplying tolerably accu- 



rate indications of magnitude in the varying size of the 

 stellar disks. 



The importance of these remarkable pictures is one 

 which the lapse of centuries must tend to heighten. They 

 will place future astronomers in possession of documentary 

 evidence of ever-growing value. Their historical function, 

 however, does not stand alone. They have also unex- 

 pectedly served the purpose of present discovery. A 

 small nebula of a spiral form, encircling the 5th-magnitude 

 star Maia, of which no visual trace had ever been per- 

 ceived, comes out with surprising intensity on all four 

 plates. It consists of a single whorl resembling a strongly- 

 curved cometary train, and extends on one side so as to 

 involve a minute adjacent star, which might be thought 

 to play the part of a secondary nucleus. The configura- 

 tion recalls the tendency to a duplicate structure visible 

 in the great spiral in Canes Venatici, as well as in other 

 similar formations. The photographic strength, in pro- 

 portion to the curious optical weakness of the new nebula, 

 suggests that its rays are situated mainly in the upper 

 part of the spectrum, and that it is of a gaseous constitu- 

 tion. Spiral nebute conform to no fixed rule in this 

 respect. The first recognised and most striking member 

 of the class (that in Canes, 51 Messier) emits continuous 

 light, while several others show bright lines. Amongst 

 these would most probably be found the specimen just 

 discovered, were it possible tq submit its feeble light to 

 analysis. This probability is greatly strengthened by 

 Mr. Lockyer's recent detection in the spectrum of Maia 

 of bright lines, as yet, however, undetermined in regard 

 to position. 



Photographic discovery led the way to, and was quickly 

 followed by, visual detection. On February 8 Admiral 

 Mouchez communicated to the Paris Academy of Sciences 

 a telegram from M. Struve announcing that the Maia 

 nebula had just been successfully observed with the 30- 

 inch Clark achromatic recently mounted at Pulkova. 

 This promising debut by the largest refractor yet con- 

 structed, encourages the hope that the limit of useful size 

 has not yet been reached. 



The singular vortex round Maia is not the only nebula 

 in the Pleiades. At Venice, on October 19, 1859, M. 

 Tempel, who had then lately exchanged his profession of 

 an engraver for that of an astronomer, discovered an ex- 

 tensive nebulosity of an elliptical form, encompassing, and 

 stretching southward from, the star Merope. The history 

 of its subsequent observation is not a little curious. Un- 

 accountable discrepancies have perplexed the evidence 

 of its existence. Some of the finest instruments in the 

 world have persistently refused to disclose it, while at 

 times it has been plainly visible with glasses too insig- 

 nificant to serve as their finders. Messrs. Hough and 

 Burnham have uniformly failed to perceive it with the 

 great Chicago refractor. Prof. Pritchard, during the 

 whole course of his assiduous study of the group, has 

 never detected a nebulous indication connected with any 

 of the stars composing it. D'Arrest only succeeded in 

 seeing it in 1862 after two years of fruitless searching, and 

 considered it the faintest object he had ever viewed with 

 the I i-inch Copenhagen refractor {Astr. Nach., No. 1393). 

 Nevertheless Schiaparelli, February 25, 1875, found it to 

 extend past the star Electra as far as Ceteno, and gave 

 it as his opinion that it was a striking object in a clear 

 sky {Astr. Nach., No. 2045). The late Dr. Schmidt, of 

 Athens, had no doubt at all of its variability. Mr. Lewis 

 Swift, of Rochester (N.Y.), on the other hand, who, 

 unaware that it e.xisted, " ran upon it " accidentally in 

 1874, watched it carefully during seven years without 

 perceiving a sign of change (^Monthly Notices, vol. xlii. 

 p. 107). Its presence is to him palpable. A 2-inch 

 aperture with a power of 25 suffices to reveal it. M. 

 Tempel himself has always seen it in its original form. 

 Mr. Maxwell Hall, in Jamaica, has never looked for it in 

 vain with his 4-inch achromatic. Schonfeld described it 



