550 



NATURE 



[April 26, 1877 



burg, on May 17, and was observed by Messier and Maraldi at 

 Paris until July 2. When first seen it was just visible to the 

 naked eye. The interval between the perihelion passages is 

 1 14 "9 1 years, and with such period of revolution, with the other 

 elements of 1762, the descending node would fall about 0*27 

 from the orbit of Mars and the ascending node at a radius- 

 vector of 3'3S» or in the region occupied by the minor planets ; 

 thus the difference of inclination will not be easily explained on 

 the supposition of identity of the comets, though it must be re- 

 marked that elements of the present comet founded upon the 

 first few days' observations may be open to more sensible correc- 

 tion than is usually the case. 



"The Observatory, a Monthly Review of Astro- 

 nomy." — There is ample room for the new astronomical periodi- 

 cal, which has been launched by Mr. Christie, the First Assistant 

 of the Royal Observatory, Greenwich, under the above title, 

 during the last week. Its aim is to present in a popular form a 

 general survey of the progress of astronomy and to afford early 

 intimation of recent advances. Such a publication ought to be 

 well supported in this country, where astronomical amateurs are 

 in great force. The first number holds out good augury for the 

 future ; amongst the contents are a report of the proceedings at 

 the last meeting of the Royal Astronomical Society, proceedings 

 which are not detailed in the Monthly Notices, where the discus- 

 sions following the reading of papers are, as a rule, ignored, but 

 which, as everyone knows who has been in the habit of attending 

 the meetings of our scientific societies, are frequently the most in- 

 teresting feature in the evening's proceedings ; and we hope this 

 point will not be lost sight of in the new periodical. There is an 

 article on the photographic spectra of stars, a subject known to have 

 lately much occupied the attention of the president, by whom it is 

 furnished ; the first part of a contribution from Mr. Gill, on the 

 determination of the solar parallax j remarks on the nebular 

 hypothesis, by Mr. Darwin, being an account of an inquiry in- 

 tended to suggest a cause which may fill up a hiatus in the theory, 

 and an outline of the results of Dr. von Asten's [researches on 

 the motion of Encke's Comet, recently communicated to the 

 St. Petersburg Academy ; also, ephemerides for physical obser- 

 vations of the moon and of Jupiter, by Mr. Marth, whose 

 assistance in this direction deserves the high appreciation of 

 observers. We will further express the hope that accuracy of 

 typography may characterise the future numbers of Mr. Christie's 

 publication ; it is most important that this should be the case if 

 the confidence of the practical astronomer is to be secured for it, 

 and we are induced to offer this suggestion from remarking one 

 or two inaccuracies in the first number, as on p. 4, where the 

 search for an intra-mercurial planet by the Rev. S. J. Perry is 

 dated in April instead of in March, and on p. 27, where Mr. 

 Swift's discovery of the comet subsequently found by M. Bor- 

 relly, is erroneously referred to Apiil 5, which was the date of 

 discovery of the previous comet. 



THE NEBULA— WHAT ARE THEYf^ 



BEFORE the announcement of Mr. Huggins's dis- 

 covery of the presence of bright lines in the spectra 

 of nebulae, it was generally, if not universally, accepted 

 as a fact that nebulae were merely stellar clusters irresolv- 

 able on account of their great distances from us. This 

 view had become impressed on the minds of many of our 

 greatest observing astronomers in the progress of their 

 work, and is one therefore which should not lightly be 

 abandoned. 



It appears to me that Mr. Huggins's observations in- 

 stead of being inconsistent with the view formerly held 

 by astronomers, are rather confirmatory of the correct- 

 ness of that view. 



' On a Cause for the Appearance of Bright Lines in the Spectra of Irre- 

 solvable Star Clusters. Paper read at the Royal Society by E. J. Stone, 

 M.A., F.R.S., Her Majesty's Astronomer, Cape of Good Hope. 



The sun is known to be surrounded by a gaseous i 

 envelope of very considerable extent. Similar envelopes j 

 must surround the stars generally. Conceive a close j 

 stellar cluster. Each star, if isolated, would be sur-| 

 rounded by its own gaseous envelope. These gaseous^ 

 envelopes might, in the case of a cluster, form over the , 

 whole, or a part of the cluster, a continuous mass of gas. , 

 So long as such a cluster was within a certain distance 

 from us the light from the stellar masses would predomi- 

 nate over that of the gaseous envelopes. The spectrum 

 would therefore be an ordinary stellar spectrum. Suppose 

 such a cluster to be removed further and further from us, 

 the light from each star would be diminished in the pro- 

 portion of the inverse square of the distance ; but such 

 would not be the case with the light from the enveloping - 

 surface formed by the gaseous envelopes. The light from 

 this envelope received on a slit in the focus of an object- 

 glass would be sensibly constant because the contributing 

 area would be increased in the same proportion that the^ 

 light received from each part is diminished. The result 

 would be that at some definite distance, and all greater 

 distances, the preponderating light received from such a 

 cluster would be derived from the gaseous envelopes and 

 not from the isolated stellar masses. The spectrum of 

 the cluster would therefore become a linear one, like that 

 from the gaseous surroundings of our own sun. The 

 linear spectrum might, of course, under certain circum- 

 stances, be seen mixed up with a feeble continuous spec- 

 trum from the light of the stars themselves. 



It should be noticed that, in this view of the subject, 

 the linear spectrum can only appear when the resolva- 

 bility of the cluster is at least injuriously afTected by the 

 light of the gaseous envelopes, becoming sensibly pro- 

 portional to that from the stellar masses, and that in the 

 great majority of such cases it would only be in the light 

 from the irresolvable portions of the cluster that bright 

 lines could be seen in the spectrum. 



The changes in form which would be presented to us 

 by such a nebula might be expected to be small. These 

 changes would depend chiefly upon changes in the distri- 

 bution of the stellar masses constituting the cluster. It 

 has always appeared to me difficult to realise the con- 

 ditions under which isolated irregular masses of gas, 

 presenting to us sharp angular points, could exist uncon- 

 trolled by any central gravitational mass without showing 

 larger changes in form than appear to have been the case 

 with many of the nebulae. In my view of the nature of 

 nebulae this difficulty no longer exists. 



THE RACES AND TRIBES OF THE CHAD 

 BASIN 



ON this subject a most valuable paper has been con- 

 tributed to the last number of the Zeitschrift der 

 Gesellschaft fiir Erdkunde by Dr. G. Nachtigal, one of 

 the few living writers entitled to speak v/ith authority on 

 the ethnography of Sudan. While the great problems 

 now being rapidly solved in the portion of Africa lying 

 south of the equator are almost exclusively of a strictly 

 geographical nature, those still awaiting solution in the 

 northern half of the Continent are on the contrary mainly 

 of an ethnological character. The reason of this pointed 

 difference is very obvious. Although there are vast 

 regions south of the line still unexplored, enough is already 

 known to warrant the conclusion that what remains to be 

 there discovered is peopled by the same great race hold- 

 ing almost exclusive possession of the parts already 

 opened up by the spirit of modern enterprise. With the 

 sole exception of the extreme south-western corner, occu- 

 pied by the Namaqua and Cape Hottentots, and of some 

 districts also in the south still haunted by a few straggHng 

 Bushman tribes, the whole of Africa from the equator 

 southwards would seem to be the domain of what iSfi 

 pow conventionally known to philologists as the Bantui 



