June 5, 1884] 



NA TURE 



135 



have taken place near coasts, some species have been gradually 

 carried down to deep water, have accommodated themselves to 

 the new conditions, and have gradually migrated to the regions 

 far from land. A few species may thus have migrated to the 

 deep sea during each geological period. In this way the origin 

 and distribution of the deep-sea fauna in the present oceans may 

 in some measure be explained. In like manner, the pelagic 

 fauna and flora of the ocean is most probably derived originally 

 from the shore and shallow water. During each period of the 

 earth's history a few animals and plants have been carried to 

 sea, and have ultimately adopted a pelagic mode of life. 



Without insisting strongly on the correctness of some of these 

 deductions and conclusions, we present them for the considera- 

 tion of naturalists and geologists, as the result of a long, careful, 

 but as yet incomplete, investigation. 



T 



THE FIXED STARS 1 

 'HERE is no science which has so long and so continuously 

 occupied the thoughtful minds of successive generations of 

 men as has astronomy ; and of its various branches there is one 

 which has for all ages possessed a special fascination, viz. that of 

 sidereal astronomy. 



There has ever been a desire to burst aside the constraints 

 imposed upon our research by the distances of space, to pass 

 from the study of the planets of our solar system to that of the 

 suns and galaxies that surround us, to determine the position and 

 relative importance of our own system in the scheme of the uni- 

 verse and the whence we have come and the whither we are 

 drifting through the realms of space. 



Questions without number crowd upon the mind. The galaxy 

 or Milky Way — what is it ? Is our sun one of its members ? 

 What is the shape of that galaxy? What are its dimensions? 

 What is the position of our sun in it ? 



The star-clusters — what are they, these wondrous aggregations 

 where hundreds and even thousands of suns may be seen in the 

 limited field of view of a powerful telescope ? Are these clusters 

 galaxies ? Have these suns real dimensions comparable with 

 those of our sun, and is it distance alone that renders their light 

 and dimension so insignificant to the naked eye ? Or are the 

 real dimensions of the clusters small as compared with our 

 galaxy ? Are their component suns but the fragments of some 

 great sun that has been shattered by forces unknown to us, or 

 have they originated from chaotic matter, which, instead of 

 torming one great whirlpool and condensing by vortex action 

 into one great sun, has been disturbed into numerous minor 

 vortices, and so become rolled up into numerous small suns ? 



The nebulae — what are they? Are they too condensing into 

 clusters or stars, or will their ghost-like forms remain for ever 

 unchanged amongst the stars ? or do they play some part in the 

 scheme of nature of which we have as yet no conception ? 



These and many others are the questions which press on the 

 ardent mind that contemplates the subject ; and there arises the 

 intense desire to answer such questions, and where facts are 

 wanting to supply facts by fancy. The history of deep and pro- 

 found thought in some of these subjects goes back through 2000 

 years, but the history of real progress is but as of yesterday. The 

 loundation of sidereal astronomy may be said to have begun with 

 the art of accurate observation. Bradley's meridian observations 

 at Greenwich about 1 750, his previous discovery of the aberration 

 of light in 1727, and Herschel's discovery of the binary nature of 

 double stars, his surveys of the heavens, and his catalogues of 

 double stars — these are solid facts, facts that have contributed 

 more to the advancement of sidereal astronomy than all the 

 speculations of preceding centuries. They point to us the lesson 

 that "art is long and life is short/' that human knowledge, in 

 the slow developing phenomena of sidereal astronomy, must be 

 content to progress by the accumulating labours of successive 

 generations of men, that progress will be measured for genera- 

 tions yet to come more by the amount of honest, well-directed 

 and systematically-discussed observation than by the most brilliant 

 speculation, and that in observation concentrated systematic 

 effort on a special thoughtfully-selected problem will be of more 

 avail than the most brilliant but disconnected work. 



I hope that no one present thinks from what I have said that 

 I undervalue the imaginative fervid mind that longs for the truth, 



' Lecture on Friday evening. May 23, at the Royal Institution, " On Recent 

 Researches on the Distances of the Fixed Stars, and on some Future Problems 

 in Sidereal Astronomy," by David Gill, LL.D., K.K.S.. II: M. 

 Astronomer at the Cape of Good Hope. 



and whose fancy delights to speculate on these great subjects. 

 On the contrary, I think and I believe that without that fervid 

 mind, without that longing for the truth, no man is fitted for the 

 work required of him in such a field — for it is such a mind and 

 such desires that alone can sweeten the long watches of the night, 

 and transform such work from drudgery into a noble labour 

 of love. 



It is for like reasons that I ask you to leave with me the 

 captivating realms of fancy this evening, and to enter the more 

 substantial realms of fact. And if at any time I should become 

 too technical or dry I beg that you too will remember the noble 

 problems for the solution of which such dry work is undertaken. 



We suppose ourselves then face to face with all the problems 

 of sidereal astronomy to which I have hastily referred — the 

 human mind is lost in speculation, and we are anxious to establish 

 a solid groundwork of fact. 



Now what in such circumstances would be the instinct of the 

 scientific mind ? 



The answer is unquestionable — viz. to measure — and no sooner 

 were astronomical instruments made of reasonable exactness than 

 astronomers did begin to measure, and to ask, are the distances 

 of the fixed stars measurable ? 



I should like to have given a short history of the early attempts 

 of astronomers to measure the distance of a fixed star. I had 

 indeed prepared such an account, but I remembered that there 

 is in this theatre a relentless clock that has curbed the exuberant 

 verbosity of many a lecturer before me, and I found that if the 

 real subject-matter of this evening's lecture were to be reached 

 and dealt with before 10 o'clock, I must pass over this earlier 

 history, instructive and interesting though it is, and come at once 

 to the time when the long baffled labours of astronomers began 

 to be crowned with success. 



Perhaps I cannot summarise it better than in the words of Sir 

 John Herschel. In one of his presidential addresses he says : — 

 ' ' The distance of every individual body in the universe from us 

 is necessarily admitted to be finite. But though the distance of 

 each particular star be not in strictness infinite, it is yet a real 

 and immense accession to our knowledge to have measured it in 

 any one case. To accomplish this has been the object of every 

 astronomer's highest aspirations ever since sidereal astronomy 

 acquired any degree of precision. But hitherto it has been an 

 object which, like the fleeting fires that dazzle and mislead the 

 benighted wanderer, has seemed to suffer the semblance of an 

 approach only to elude his seizure when apparently just within 

 his grasp, continually hovering just beyond the limits of his dis- 

 tinct apprehension, and so leading him on in hopeless, endless, 

 and exhausting pursuit." 



Those who have read the history of exact astronomy from the 

 days of Flamsteed — i.e. from 16S9 — down to 1832, will under- 

 stand how exactly these words of Sir John Herschel describe the 

 position of the problem. 



But these laborious pursuits, like all honest researches in quest 

 of truth, were not without reward, even though the immediate 

 object in view was not attained. Bradley was rewarded by his 

 greal discovery of aberration, and Sir William Herschel by the 

 ■ I his great discoveries, the binary nature of double 

 stars, when engaged in vain attempts to measure the distance of 

 a fixed star. Time forbids that I should tell more of this in- 

 structive story — for the story of failure is often fully as instructive 

 as that of success — and I must begin the history of our problem 

 between 1S32 and 1842, when success was first attained. 



But before I begin it will save both time and circumlocution 

 if I define a word that we must frequently use — viz. the word 

 parallax. 



Here on the table is a large ball representing the sun, and 

 here, travelling on a circular railway round the larger ball, is a 

 smaller ball which we shall suppose to represent the earth. The 

 larger ball is suspended from the ceiling by a white string, the small 

 ball is suspended from the same point by a red string. At the far 

 end of the white string you can suppose a star whose true direc- 

 tion is represented by this white string, and whose apparent 

 direction as looked at from the earth is represented by the red 

 string. Now if the star is within a measurable distance, the 

 red string which indicates the star's apparent direction as seen 

 from the earth will always be displaced inwards towards the sun. 

 This displacement is called "parallax." li may be defined as 

 the change in the apparent place of a star produced by viewdng 

 it from a point other than that of reference. Our point of refer- 

 ence for stars is the sun, and as we view the stars now from one 

 side of the sun, and six months afterwards from a point on the 



