148 



KNOWLEDGE 



[August 1, 1892. 



snappings and much slipping of rocks past each other. 

 Probably the slipping movements — the existence of which 

 is abundantly proved by the numerous " faults " so familiar 

 to geologists — took place gradually, that is, only a few 

 inches at one time ; so that a single fracture may have 

 given rise to hundreds, or even thousands, of earthquakes. 

 There is in mountain building a chance for many slight 

 shocks with but a small amount of motion. In the 

 formation of such folds as those composing ]\Iont Blanc 

 the tremors may have been numbered by the million. 

 If earthquakes are associated with the raising up of 

 mountains, who shall say that they are of no use ? 



PLANETARY NEBULAE. 



By Miss A. M. Clerke, ^i/iAoj-e.ss nf'A Popular History of 



AKtronomy durini/ the Nineteenth Centiiry ," and "The System of 



the Stars." 



THE question "What is a Nebula?" put by our 

 editor to himself and his readers in the last 

 number of Knowledge, must have brought home 

 to many minds, with startling distinctness, the 

 extreme difficulty of forming a rational conception 

 as to the real physical status of cosmical clouds and 

 cloudlets. They lie indeed so far beyond the range of our 

 immediate experience, that observations of them are apt to 

 become more perplexing the more they are rendered minute 

 and detailed. Speculations regarding their nature have 

 hitherto been sternly checked by the collection and verifi- 

 cation of facts. They flourish only under the shelter of a 

 certain amount of vague generality, while the test- questions 

 which every theory worthy of the name must sooner or 

 later put to experience, have so far uniformly been answered 

 in the negative. Added knowledge has not, accordingly, 

 in this direction, brought clearer understanding. In the 

 registration, for instance, especially by pliotographic means, 

 of- lines in the nebular spectrum, and in the accurate 

 determination of their places, much has of late been done ; 

 but their interpretation remains as backward as ever. 

 Nebular chemistry has scarcely yet entered upon the path 

 of progress ; what it has done hitherto has been mostly to 

 put a veto upon mistaken identifications, leaving affirmative 

 propositions to the future. It is acquainted with but one 

 terrestrial element ; almost the only positive assertion 

 warranted by the evidence at its command is that hydrogen 

 is extensively present in the " flre-mist " of the skies, and 

 present in a state of high molecular agitation. This 

 indeed is a most important piece of information ; the 

 possibility of obtaining it proves nebular to be separated 

 by no impassable gulf from terrestrial conditions, and 

 gives good hope, accordingly, of eventual advance along 

 this line of investigation. 



A peculiar interest, then, attaches to the detection and 

 demonstration of structural resemblances between nebulae 

 and other heavenly bodies. Mr. Ranyard pointed out some 

 few years ago the faithful imitation of solar prominence- 

 forms by some of the gigantic outgrowths of shining fluid 

 from the trapezium of stars in Orion ; and the resemblance 

 is accentuated by the undoubted presence in the nebula of 

 the solar element helium. Cometary analogies, on the 

 other hand, are backed by no well-ascertained chemical 

 coincidences ; yet they are prevalent and remarkable. The 

 wings and wisps, the taOs, rays, and trains, the complicated 

 varieties of tenuous veils and envelopes that develop 

 together or in succession during the rapid sweep of a 

 great comet round the sun, are reproduced over and over 

 again, with endless modifications, in the " lucid matter" of 

 remote space. But they are stereotyped in being repro- 



duced. Forms that might well be concluded to be purely 

 transitional, and that are in the highest degree suggestive 

 of evanescence, are nevertheless maintained, decade after 

 decade, without appreciable modification. Processes of 

 change are no doubt meanwhile progressing ceaselessly, 

 but on so vast a scale in space, and at so leisurely a rate 

 in time, as completely to baffle human observations during 

 the short span available for them. Hints as to their 

 character may, however, in the course of years, be gathered 

 by studying the distribution of matter brought about by 

 their action. The curious discovery, moreover, has of late 

 been made that data on this subject may difi'er widely, and 

 lead to widely different conclusions, accordmg as visual or 

 photographic means are employed to procure them. This 

 incongruity between effects to the eye and impressions on 

 the sensitive plate first became apparent in M. Trepied's 

 photograph of the annular nebula in Lyra (Knowledge, 

 vol. xiii., p. 253) ; it has now been found to cliaracterize 

 two well-known planetaries, and may be inferred to belong, 

 more or less, to all members of the same class. 



The distinction between annular and planetary nebuhe 

 has been to a great extent abolished by the use of improved 

 optical appliances. Each kind seems to be made up of 

 three essential parts ; a faintly shining disc — or globe pro- 

 jected into a disc — a ring-like condensation near its outer 

 margin, and a central nucleus, presenting the appearance 

 of a star. This last feature is often seen only with extreme 

 difficulty, but there is reason to believe that it always 

 exists. Mr. Burnham, who has measured a large number 

 of these objects with the Lick Sfi-inch, for the purpose of 

 providing a standard of comparison for the determination 

 of their possible future movements, goes so far as to 

 suggest that the presence of a central star should be 

 regarded as the criterion of classification for planetary 

 nebulae.* He adds : — " Various powers have been used in 

 studying these central stars, and particularly the brighter 

 ones. In no instance has any one of these stars presented 

 under any power any peculiar appearance. Bo far as it 

 can be determined in this way, they all appear to be true 

 stars, differing in no sense from the comparison stars. 

 Many of the nights on which these measures were made 

 were of the best quality, and any nebulous or other unusual 

 appearance should have been apparent if it really exists." 

 Nevertheless, photography has a very different story to 

 tell, as we shall see presently. 



A small bluish disc observed by the elder Herschel near 

 the star v Aquarii was estimated by his son as equal in 

 light to a 6-7 magnitude star, and took its place as No. 

 4628 in his General Catalogue. It is No. 7009 of Dreyer's 

 New General Catalogue. With the Parsonstown reflector, 

 in 1850, the surface of the nebula appeared tolerably 

 uniform ; it was interrupted by no certifiable perforation, 

 and emblazoned by no central star. It gave the effect, 

 however, in 1878, not of strict eUipticity, but of being made 

 up of two overlapping circular segments. f It was seen, 

 besides, to possess a pair of singular appendages like the 

 " ansae " of the ringed planet in our own system ; and Mr. 

 Lassell's first impression of the object at Malta was that 

 of " a sky-blue likeness of Saturn." An interior ring, too, 

 measuring 26" by 16,j", and projected upon a hazy back- 

 ground, was plainly visible ; while a small star marked 

 the middle-point of the entire formation. Yet the great 

 Vienna refractor failed, in 1883, to display the star to Vogel, 

 although a power of 1500 brought out complexities of 

 internal nebulous arrangement strongly suggestive of an 

 essentially spiral structure. The results of Professor 



* Monthlif Notices, Tol. lii., p. 

 t Trans. R. IJuhlin Society, vol. ii 



159. 



