158 



NA TURE 



[> 



?, 1884 



From similar observations with these comparison stars I 

 obtained for the parallax of a Centauri d'"jdo, a result which is 

 identical with the last within the limits of the probable error of 

 either. 



My friend Dr. Elkin selected the stars ab and a b' as his com- 

 parison stars, and in a precisely similar way he obtained as the 

 mean of his results a parallax of o"752, a result identical with 

 my own, so that we may conclude as one of the most certainly 

 established facts of astronomy that the parallax of a Centauri 

 relative to an average star of the seventh or eighth magnitude is 

 three-quarters of a second of arc. 



It is therefore beyond all doubt that Henderson's discovery 

 was a real one. Herschel's verdict must therefore be confirmed, 

 and the palm for first breaking down the barriers that separated 

 us from any knowledge of the distances of the fixed stars be 

 accorded to the memory of the Cape Astronomer Henderson. 



So far as all existing researches go, a Centauri is the nearest 

 of the fixed stars. Regarding the faint comparison stars as 

 practically infinitely distant, let us try to realise how near or 

 how far distant a Centauri really is. 



There are, of course, an infinite number of illustrations whi li 

 one might employ to convey some idea of such a distance. I 

 shall content myself with one of them — something akin to 

 which has already been used by Dr. Ball within these 

 walls. 



We are a commercial people, we like to make our estimates 

 in pounds sterling. We shall suppose that some wealthy 

 directors have failed in getting Parliamentary sanction to cut a 

 sub-Atlantic tunnel to America, ami so for want of some other 

 outlet for their energy and capital they construct a railway to 

 a Centauri. We shall neglect for the present the engineering 

 difficulties — a mere detail — and suppose them overcome and the 

 railway open for traffic. 



We shall go further, and suppose that the directors have 

 found the construction of such a railway to have been peculiarly 

 easy, and that the proprietors of interstellar space had not been 

 exorbitant in their terms for right of way. Therefore, with a 

 view to encourage traffic, the directors had made the fares ex- 

 ceedingly moderate, viz. first class at one penny per 100 

 miles. 



Desiring to take advantage of these facilities, an American 

 gentleman, by way of providing himself with small change for 

 the journey, buys up the National Debt of England and of a few 

 other countries, and, presenting himself at the booking-office, 

 demands a first-class single to a Centauri. For this he tenders 

 in payment the scrip of the National Debt of England, which 

 just covers the cost of his ticket ; but I should explain that at 

 this lime the National Debt, from little wars coupled with some 

 unremunerative Government investments in landed property, had 

 run up the National Debt from 700 millions to 1 100 millions 

 sterling. Having taken his seat, it occurs to him to ask — 



At what rate do you travel? 



Sixty miles an hour, sir, including stoppages, is the answer. 



Then when shall we reach a Centauri ? 



In forty-eight million six hundred and sixty-three thousand 

 years, sir. 



Humph, rather a long journey. 



But enough of joking. If we wish to deal with distances 

 so immense, we must adopt a more convenient unit of 

 measure. 



The most convenient unit for our purpose is the number of 

 years that light would take to reach us. Light takes almost 

 exactly 500 seconds of time to come from the sun ; this is a 

 figure easy to remember, and is probably exact to a single unit. 

 The sun is ninety-three millions of miles distant, and this 

 figure I believe to be exact within 200,000 miles. 



Quite recently the accuracy of these figures has been confirmed 

 in a very remarkable way by different kinds of investigations by 

 different observers ; otherwise I should not have quoted them 

 with so much confidence. 



The parallax of a Centauri is three-quarters of a second of arc ; 

 therefore its distance is 275,000 times the distance of the earth 

 from the sun, and therefore light, which travels to the earth from 

 the sun in 500 seconds {i.e. in 84 minutes) would take 4"36, or a 

 little more than 4^ years to come from a Centauri. 



You will find in the accompanying table a specific account of 

 the other results which were arrived at by Dr. Elkin and myself 

 by precisely similar means, and you will find on the wall dia- 

 grams representing my own detailed observations in the case of 

 Sirius and 1 Indi. 



Table II. — Rtsulls of Recent Researches on the Parallax of Stars 

 in the Southern Hemisphere 



Time does not permit me to go into more detail as to each of 

 these separate results, full of interest though they are, and each 

 of them representing months of labour. 



My object now is to generalise, to point out the conclusions 

 that must be drawn from these two tables of parallax (Tables I. 

 and II.), and to see what are the broad lessons that they teach us. 



A glance is sufficient to show that neither apparent magni- 

 tude nor apparent proper motion can afford a definitive criterion 

 o| the distance of any fixed star — that different stars really differ 

 greatly in absolute brightness and in absolute motion. 



And now, what is the work before us in the future ? 



The great cosmical problem that we have to solve is not so 

 much what is the parallax of this or that particular star, but we 

 have to solve the much broader questions — 



1. What are the average parallaxes of stars of the first, second, 

 third, and fourth magnitudes, compared with those of fainter 

 magnitude ? 



2. What connection does there subsist between the parallax of 

 a star and the amount and direction of its proper motion, or can 

 it be proved that there is no such relation or connection? 



With any approximate answer to these questions we should 

 probably be able to determine the law of absorption of starlight 

 in space, and be provided with the data at present wanting for 

 determining with more precision the constant of precession and 

 tlie amount and direction of the solar motion in space. And 

 who can predict what hitherto unknown cosmical laws might 

 reveal themselves in the course of such an investigation? 



It is important to consider whether such a scheme of research 

 is one that can be realised in the immediate future, or one that 

 can only be carried to completion by the accumulated labours of 

 successive astronomers. 



I have very carefully considered this question from a practical 

 point of view, and I have prepared a scheme, founded on the 

 results of my past experience. I have submitted that scheme for 

 the opinion of the most competent judges, and in their opinion, 

 11 I, my own, the work can be done, with honi 1 hard 

 work for one hemisphere, within ten years. I have offered to 

 do that work for the southern hemisphere with my own hands, 

 and a proposal for the necessary instruments and appliances is 

 now under the consideration of my Lords Commissioners of the 

 Admiralty. I need hardly add that in this matter I look 

 confidently for that complete consideration and that efficient 

 support which I have never failed to receive at their hands since 

 I have had the honour to serve them. 



The like work will be undertaken for the northern hemisphere 

 by my friend Dr. Elkin, who is now in charge of the heliometer 

 at Yale College in America. It is at present the finest instru- 

 ment of the kind in the world, and a photograph of it you have 

 already seen upon the screen. 



I most earnestly trust that we may be granted health and 

 strength for this work, and that no unforeseen circumstances will 

 prevent its complete accomplishment. 



Before closing this lecture I wish briefly to allude to another 

 engine of research in sidereal astronomy which quite recently 



