August i, 1895] 



NA TURE 



329 



simply this. Suppose I light a match, the smaller the match 

 the sooner will it go out, and similarly the larger a fire the 

 longer will it last. So if you are dealing in space with those 

 illuminations which disappear in hours, days, or weeks, you 

 cannot be dealing with any large mass ; therefore the collisions 

 in question cannot be between large masses of matter, but it 

 must be a question of collisions amongst the smallest particles 

 of matter we can conceive. 



It is interesting to consider one of the possibilities which may 

 explain why small nebulae may be overlooked in telescopic 

 observations. In the so-called achromatic telescope, all the rays 

 of light are not brought to quite the same focus, so that when 

 ordinary stellar observations are being made, the focus is ad- 

 justed for yellow rays which are most luminous to the eye. Now 

 the greater part of the visual light of a planetary nebula is con- 

 lined to a single line of the spectrum in the green, so that the 

 focus which is best adapted for observations of stars is not 

 suitable for the observation of a small nebula, the nebula being 

 out of focus, and its feeble light thus reduced by the diffusion 

 of the image. This difference is much more marked in large 

 than small telescopes, and Prof Campbell has pointed out that a 

 small nebula like Nova Auriga; will in general appear relatively 

 brighter in a small telescope than a large one. 



I will next go into some details touching the phenomena of the 

 Nova; in relation to the hypothesis. 



First let us see the crucial phenomena we have to explain. 

 We have (l) the sudden bursting out of light and accompanying 

 spectra ; (2) the indication of the existence of two bodies revealed 

 by the spectra ; (3) the variations and dimming of the light and 

 accompan)'ing spectral changes ; and (4) the final stage giving us 

 the spectrum of a nebula. 



Since the new era of spectroscopic work has begun. Nova 

 .\urigx' and Nova Normns have proved to us that the sudden 

 illumination was, to say the least, associated with two bodies, 

 and that these were in different stages of condensation. On the 

 meteoritic hypothesis it was shown that the main differences 

 between bodies giving bright and dark line spectra is one of con- 

 densation only : a sparse swarm gives us bright lines because the 

 number of meteorites in unit volume is small and the interspaces 

 are great ; a more condensed swarm gives us dark lines because 

 the number of meteorites in unit volume is greater, and the 

 atmospheres of cooler vapour round each meteorite in collision 

 begins to tell because the interspaces are reduced. I am the 

 more justified in insisting upon the importance of this view 

 that two bodies in difierent stages of condensation are involved, 

 because years after it was formulated Dr. Muggins apparently 

 arrived at it independently — at all events he makes no reference 

 to my prior announcements when he brings it forward as an 

 explanation of the phenomena. 



The following quotations will show how this matter stands : — 



"If we assume a brightening of the meteor-swarm due to 

 collisions as the cause of the so-called nesv stars, we have good 

 grounds for supposing that in these bodies the phenomena should 

 be mixed, for the reason that we should have in one part of the 

 swarm a number of collisions probably of close meteorites, while 

 among the outliers the collisions would be few. We shall, in 

 fact, have in one part the conditions represented in Class Ilia, 

 and in the other .such a condition as we get in 7 Cassiopeix."' 



" The discussion of the observations which have been made 

 of the changes that take place in the spectra of new stars, has 

 already shown that the sequence of phenomena is strikingly 

 similar to that which occurs in cometary spectra after perihelion 

 passage. In general, however, there will be a diH'erence : 

 namely, that in comets there is usually only one swarm to be 

 considered, whereas in new stars, there are two, which may or 

 may not be equally dense. In new stars, we have accordingly the 

 ntegration of two spectra, and the spectrum we see will depend 

 upon the densities and relative velocities of the two swarms." - 



" The spectrum of Nova Aurig.v would .suggest that a dense 

 swarm is moving towards the earth with a great velocity, and 

 passing through a sparser swarm, which is receding." ' 



" The circumstance that the receding body emitted bright 

 Imes, while the cme approaching us gave a continuous spectnmi 

 with broad absorption lines similar to a white star, may, per- 

 haps, be accounted for by the two bodies being in different 

 evolutionary stages, and consequently differing in diffuseness and 

 temperature." ■* 



• November, 1887. Lockyer. Proc. R.S., vol. xliii. p. 147. 

 2 November, 1890. Lockyer. /"A//. 7"ra«i., 182 A, p. 407. 

 » February 11, 1892. Lockyer. Proc. R.S., vol. 1. p. 435. 



* May 16, 1892. Dr. Muggins. /"»•«. R.S., vol. li. p. 494. 



NO. 1344, VOL. 52] 



Now two sheets or streams of meteorites interpenetrating and 

 thus causing collisions will produce luminosities which will in- 

 dicate the condensation of each, and the spectra of the two 

 Nov:e we are considering thus indicate that the colliding swarms 

 were of different degrees of condensation, and the variations of 

 light observed indicate several such encounters between less 

 dense swarms after the most dense one had somewhat cooled 

 down. The final stage was arrived at and the pure nebula 

 spectrum produced when the most condensed swarm had ceased 

 to indicate any disturbance, after all the others had returned to 

 their pristine quiet and in\isibility. 



It is important to insist upon the fact that the nebula; are now 

 almost generally conceded to represent " early evolutionary 

 forms." We have then from the first appearance of a Nova to 

 the la.st a "backwardation" in the phenomena ending in an 

 "early evolutionary form." Increase of temperature is accom- 

 panied by spectral changes in a certain order ; if the temperature 

 is reduced the changes occur in reverse order, until finally we 

 reach the " early evolutionary form," which cannot be a mass of 

 gas because its temperature is lower than that of the sun, which 

 it is potentially, and it must contain all the substances eventually 

 to appear in the atmosphere of the sun. 



On the hypothesis, then, we imagine a nebula in the position 

 occupied by Nova Auriga; not chronicled for the reason stated. 

 This nebula is approaching us. It was distiirbed by a much 

 sparser stream leaving us, the relative velocity being over 500 

 miles a second. During the time of impact, the disturbances 

 ]3roduced in the two swarms gave rise to bright-line spectra in 

 the sparse swarm, and to dark-line spectra in the more condensed 

 one. The spectrum of the sparse swarm disappears, the spectrum 

 of the dense swarm changes gradually from dark to bright lines, 

 and ultimately it puts on the original nebula spectrum. It is 

 still there, and still approaching us. 



We have next to consider the objections which have been 

 urged against this hypothesis. They are of a most trivial nature. 

 hn. objection made by Vogel is that it is improbable that the 

 velocities could have been so great after collisions. The reply is 

 easy. The light was produced by the disturbed members of the 

 two swarms which escaped end-on collision. On the meteoritic 

 hypothesis we can escape from the difficulties produced by the 

 old idea of collisions en bloi. Such objectors would urge that the 

 velocity of a comet as a whole would be retarded by passing 

 through the sun's corona, but we have instances to the contrar)'. 



Another objection has been raised by Dr. Vogel because in 

 relation to the Nova I did not restate all I had preWously 

 written concerning the origin of the cause of bright and dark 

 line spectra in stars. It has been difficult for him to understand 

 how one (temporary) star should have bright lines in its sjiectrum, 

 and another (temporary) star should have dark lines. All I can 

 say is that upon such objectors lies the onus of producing a more 

 simple (and yet sufficient) explanation than that I have suggested. ' 



J. NoRM.vN Lockyer. 

 ( To be continued. ) 



THE INTERNATIONAL GEOGRAPHICAL 

 CONGRESS. 



'X'HE International geographical Congress, now a recognised 

 -^ institution, has this year met for the first time on British 

 ground. Originating in a festival organised to celebrate the 

 inauguration of statues of Mercator and Ortelius at -Antwerp and 

 Rupelmond, the first Congress was held at Antwerp in .\ugust 



^ It h.xs been st.ated that the meteoritic hj-pothesis has received a fatal 

 blow from the observations of the Nova ^.Astronomy and .'Istrophysics^ 

 1892J p. jog). Capable and unprejudiced persons I think will not Ik: of this 

 opinion, r append a quotation from an article by Prof. C.vnpbcll. which has 

 appeared since the lectures were delivered. 



" As bearing upon .any possible theory of Nova .\uriga;, perhaps it will 

 not be out of place to say here what 1 said last winter in another journal 

 (Pub. .\.S P. vi., 52, 133.) The Harvard College Observatory has shown that 

 both Nov.i .-Vuriga; and Nova Norma: at discovery possessed substantially 

 identical spectra of bright and dark lines, similarly and equally dispLiced. 

 Both diminished in brightness, and both assumed the nebular type of 

 spectrum. The new star of 1876 in Cygnus probably had nearly an identical 

 history : passing from a bright star with a spectrum of bright and dark lines, 

 to a faint object with a spectrum consisting of one bright line (undoubtedly 

 the nebular line A 5010, or the two nebular lines A 5010 and A 4960 combined). 

 We may say that only five 'new stars' have been discovered since the 

 application of the spectroscope to astronomical investigations, and that three 

 of these have had substantially identical spectroscopic histories. This is a 

 remarkable fact. We cinnot say what the full significance of this fact is. 

 One result, however, is very cic.ir : the special theories propcunded by 

 various spectroscopisls to account for the phenomena observed in Nova 

 .\uriga; must unquestionably give way to the more i'C«tfra/ theories." (.-isiro- 

 physical Journal, Jan. 1895, p. 51.) 



