October 22, 1891] 



NATURE 



607 



Milky Way, whilst stars of other types are fairly divided over 

 the sky. 



Now, >tars of iha Sirius type are very white stars, very rich, 

 relative to other stars, in the rays which act most strongly on a 

 photographic plate. Here, then, is the explanation of the 

 results of our photographic star-charting and of the discordance 

 between the photographic and visual magnitudes in the Milky 

 Way. 



The results of the Cape charting further show that it is not 

 alone to the brighter stars that this discordance extends, but it 

 \tei>ds also, though in a rather less degree, to the fainter stars 

 r the Milky W^ay. Therefore, we may come to the very re- 

 markable conclusion that the Milky Way is a thing apart, and 

 that it has been developed perhaps in a different manner, or 

 more probably at a different and probably later epoch, from the 

 rest of the sidereal universe. 



Here is another interesting cosmical revelation which we owe 

 to photography. 



You all know the beautiful constellation Orion, and many in 

 this theatre have before seen the photograph of the nebula which 

 is now on the screen, taken by Mr. Roberts. 



Here is another photograph of the same object, taken with 

 a much longer exposure. You see how over-exposed, in fact 

 burnt out, the brightest part of the picture is, and yet what a 

 wonderful development of faint additional nebulous matter is 

 revealed. 



But I do not think that many persons in this room have seen 

 tliis picture, and probably very few have any idea what it repre- 

 sents. It is from the original negative taken by Prof. Pic'vcring, 

 with a small photographic lens of short focu-, after six hours' 

 exposure in the clear air of the Andes, 10,000 feet above sea- 

 level. 



The field embraces the three well-known stars in the belt 

 of Orion, on the one hand, and ;3 Orionis (Rigel) on the other. 

 You can hardly recognize these great white patches as stars ; 

 their illdefined character is simply the result of excessive over- 

 exposure. But mark the wonders which this long exposure with 

 a lens of high intrinsic brilliancy of image has revealed. Here 

 is the great nebula, of course terribly over-exposed ; but note 

 its wonderful fainter ramifications. See how the whole area is 

 more or less nebulous, and surrounded as it were with a rin^ 

 fence of nebulous matter. This nebulosity shows a special 

 concentration about 3 Orionis. 



Well, when Prof. Pickering got this wonderful picture, know- 

 ing that I was occupied with investigations on the distances of 

 the fixed stars, he wrote to ask whether I had made any obser- 

 vations to determine the distance of /3 Orionis, as it would be of 

 great interest to know, from independent evidence, whether this 

 very bright star was really near to us or not. It so happens 

 that the observations were made, and their definitive reduction 

 has shown that )3 Orionis is really at the same distance from us 

 as are the faint comparison stars. ^ Orionis is, therefore, prob- 

 ably part and parcel of an enormous system in an advanced but 

 incomplete state of stellar evolution, and that what we have seen 

 in this wonderful picture is all a part of that system. 



I should explain what I mean by an elementary or by an 

 advanced state of stellar evolution. There is but one theory of 

 celestial evolution which has so far survived the test of time and 

 comparison with observed facts, viz. the nebular hypothesis of 

 Laplace. Laplace supposed that the sun was originally a huge 

 gaseous or nebulous mass, of a diameter far greater than the 

 orbit of Neptune. I say originally — do not misunderstand me. 

 We have finite minds ; we can imagine a condition of things 

 which might be supposed to occur at any particular instant of 

 time however remote, and at any particular distance of space 

 however great, and we may frame a theory beginning at another 

 time St. II more remote, and so on. But we can never imagine a 

 theory beginning at an infinite distance of time or at an in- 

 finitely di.-taut point in space. Thus, in any theory which man 

 with his finite mind can devise, when we talk ol originally we 

 simply mean at or during the tiine considered in our theory. 



Now, Laplace's theory begins at a time, millions on millions 

 of years ago, when the sun had so far disentangled itself from 

 chaos, and its component gaseous particles had by mutual at- 

 traction so far coalesced, as to form an enormous gaseous ball, 

 far greater in diameter than the orbit of the remotest planet of 

 our present system. The central part of this ball was certainly 

 much more condensed than the rest, and the whole ball revolved. 

 There is nothing improbable in this hypothesis. If gaseous 



matter came together from different parts of space, such coali- 

 tion wou!d unquestionably occur, and as in the meeting of oppo- 

 site streams of water or of opposite curients of wind, vortices 

 would be created, and revolution about an axis set up, such as 

 we are familiar with in the case of whirlpools or cyclones. The 

 resultant would be rotation of the whole globular gaseous mass, 

 about an axis. 



Now this gaseous globe begins to cool, and as it cools it 

 necessarily contracts. Then follows a necessary result of con- 

 traction, viz the rotation becomes more rapid. This is a well- 

 known fact in dynamics, about which there is no doubt. Thus, 

 the cooling and the contracting go on, and, simultaneously, the 

 velocity of rotation becomes greater and greater. At last the 

 time arrives when, for the outside particles, the velocity of rota- 

 tion becomes such that the centrifugal force is greater than the 

 attractive force, and so the outside particles break off and form 

 a ring. Then, as the process of cooling and contraction pro- 

 ceeds still further, another ring is formed, and so on, till we 

 have, finally, a succession of rings and a condensed central ball. 

 If from any cause the cooling of any of these rings does not go 

 on uniformly, or if some of the gaseous matter of the ring is 

 more easily liquefied than others, then probably a single nucleus 

 of liquid matter will be formed in that ring, and this nucleus 

 will finally, by attraction, absorb the whole of the matter of 

 which the ring is composed — at first as a ga eous ball with a 

 condensed nucleus, and this will finally solidify into a planet. 

 Or, meanwhile, this yet unformed planet may repeat the history 

 of its parent sun. By contraction, and con>equent acceleration 

 of its rotation, it may throw off one or more rings, which in like 

 manner condense into satellites like our moon, or those of 

 Jupiter, Saturn, Uranus, or Neptune. Such, very briefly out- 

 lined, is the celebrated nebular hypothesis of Laplace. No one 

 can positively say that the hypothesis is true, still less can any- 

 one say that it is untrue. Time does not permit me to enter 

 into the very strong proofs which Laplace urged in favour of its 

 acceptance. 



But I beg you for one moment to cast your imaginations back 

 to a period of time long antecedent to that when our sun had 

 begun to disentangle itself from chaos, and when the fleecy 

 clouds of cosmic stuff had but commenced to rush together. 

 What should we see in such a case, were there a true basis for 

 the theory of Laplace? Certainly, in the first place,we should 

 have a huge whirlpool or cyclone of cosmic gaseous stuff, the 

 formation of rings, and the condensation of these rings into 

 gaseous glole?. 



Remembering this, look now on this wonderful photograph 

 of the nebula in Andromeda, made by Mr. Roberts. In the 

 largest telescopes this nebula appears simply as an oval patch of 

 nearly uniform light, with a few dark canals through it, but no 

 idea of its true form can be obtained, no trace can be found of 

 the significant story which this photograph tells. It is a picture 

 that no human eye, unaided by photography, has ever seen. It 

 is a true picture drawn without the intervention of the hand of 

 fallible man, and uninfluenced by his bias or imagination. 

 Have we not here, so at least it --eems to me, a picture of a very 

 early stage in the evolution of a star-cluster or sun-system — a 

 phase in the history of another star system similar to that which 

 once occurred in our own— millions and millions of years ago, 

 when our earth, nay, even our sun itself, " was without form 

 and void," and " darkness was on the face of the deep." 



During this lecture I have been able to trace but very imper- 

 fectly the bare outlines of an astronomer's woik in a modern 

 observatory, and to give you a very few of its latest results — 

 results which do not come by chance, but by hard labour, and 

 to men who have patience to face dull daily routine for the love 

 of science— to men who realize the imperfections of their 

 methods, and are constantly on the alert to improve them. 



The mills of the astronomer grind slowly, and he must be 

 infinitely careful and watchful if he would have them, like the 

 mills of God, to grind exceeding small. 



I think he may well take for his motto these beautiful lines — 



*' Like the star 

 Which shines afar, 

 VViih jut haste, 

 Without rest, 

 Let each man wheel 

 With steady sway, 

 Round the task 

 Which rules the day. 

 And do his best." 



NO. II47, VOL. 44] 



