July i i, 1895] 



NATURE 



25: 



THE SUN'S PLACE IN NATURE} 



VIH. 



'T'WCJ objections, however, have Ijeen niaile to these hyjiolhetical 

 _ two swarms. It has been tirgefl that the secondary swarm 

 which we saw moving in a closed orliit round the primary one 

 woidd soon sprea<l out into a line along the orbit, st) that there 

 would always be some parts of it mixed up with the constituents 

 of the parent swarm. That is a perfectly fair objectirm, sup- 

 jio.sing we are dealing with millions and billions of years, but I 

 think that those who have made it do nfit know the history of 

 astronomy. Let us take, for instance, the history of the 

 November swarm which cuts the earth's orbit, so that in certain 

 Novembers, generally about thirty-three years apart, we gel this 

 swarm of meteorites passing through our atmosphere, getting 

 burnt out in that passage, and giving us one of the most magnificent 

 siijhts which it is possible for mortals to see — a whole hemisphere 

 of sky filled with shooting stars. Some of you may remember 

 such a phenomenon as that in the year 1866, some of us are 

 hoping to see the recurrence of it in 1899, for which we have not 

 long to wait. But the fact that we only get this appearance every 

 thirty-three years shows that, at all events, that swarm of 

 meteorites to which the phenomena are due has not changed 

 (luring our life-time - nay, it has not changed during the last 

 thousand years, for man has known of that November swarm 

 for more than a thousand years, and we have only kiKnvn of the 

 varialiility of .\I ira for 300 years ; so that you see such an objection 

 a.s that is entirely out of court, because it lacks the historical 

 touch. 



Another objection which has been urged is that there are 

 certain irregularities in the light-curves of these l)odies ; that 

 Mira. for instance, does not always come up to the same amount 

 oi brightness at its maximinn,and jierhaps, for all we know, does 

 not always go down to the same low magnitude when it is at its 

 low'est. That also is ])erfectly true, and on this account : there 

 is no reason why we should suppose that these phenomena of the 

 waxing and waning light of the body are produced by the move- 

 ment of one body oid\' : suppose, for instance, that there is some 

 cosmic eye a billion miles away from our solar system, so beauti- 

 fully and exquisitely wrought, so <lelicate in its cvjnslruclion, that 

 it can see an increase in the light of the sun every lime a big comet 

 5Joes round it. Now v\e know fr<mi our own exjierience of comets 

 that it would lie absolutely impossible for that delicately constructed 

 eye to see anything like a constant variability in the light of the 

 sun under these conditions, because sometimes the brightest 

 ■comets which come to us are absolutely unpredicted, they come 

 at irregular times. It must also l)e pointed out in coimeclion 

 with this objection that there are other obvious causes for 

 ■considerable variations in the light, both at the maximum and 

 at the minimum, ^'ou remember that I showed you those beau- 

 tiful spiral nebula; of which Dr. Roberts has given us such 

 magnificent photographs : suppose them to represent the irarenl 

 swarms, and that another minor swarm tries to pass them ; it is 

 imjinssibic t<> imagine that the minor swarm woidd exactly ])ass 

 through all the intricacies of those magnificent spirals, and go and 

 ■come through it precisely on the same |Mth. It would be certain 

 that in consequence of perturbations, the secondary swarm would 

 Tiometimes go through a denser portion, at other times through a 

 less dense portion, and then you see that would be quite sufficient 

 to give us a considerable difierence of luminosity. 



I have another interesting series of diagrams, which will 

 .show you that almost any amount of variability and irngiilar 

 variability in the light curves of these bodies may be explained 

 on very simple grounds, supposing we acknowledge that we are 

 ■ilealing with the movements of more than two bodies. For in- 

 stance, suppose we have one cause at work which gives us a 

 maximum and minimum, and another cause which gives us 

 two very nuich smaller maxima and minima occurring at a 

 ilirtereni period rei)re.sented in Kig. 34 in the upper part of the 

 • liagram. 



If we add these two together, we gel the irregular light curve 

 shown below the two simple curves in the diagram. But the 

 amount of irregularity may possibly only reveal the amount of our 

 ignorance, and when the time comes when we can isolate these 

 t«o causes, and thus see how the addition of them should be made, 

 we shall find that every i>art of this curve is really the result of a 



' Revised from shonh.iiul notes of a course of Lectures to Working Men 

 -« the Museum of Pr.-ictic.-\l Ceolosy during Xovemtjer and I)ecenit)er, 

 '694. (Cotuinued from |).'ige J07). 



NO. I 34 I, VOL. t2] 



most beautiful law. I am very glad to say that quite recently Mr. 

 Maxwell Read, of the Harvard Observatory, has put forward 

 this very same suggestion, so that we may hope that it will soon 

 be worked out on pretty broafl lines. 



But sujipose for a niojiient that this vie« of two bodies is not 

 accejned. What have we got in place of it ? W'ell, we have to 

 explain all the phenomena of variability by one body. That has 

 been attempted more or less liapjjily. Suppose, for instance, we 

 have the case of a liody waxing and waning quite regularly ; you 

 have only to say that body is like a soup-plate, and rotates cm an 

 axis, so that sometimes you see the face, sometimes only the edge. 

 But that is not very satisfactory, because we do not know any stars 

 which are like soup-plates, .\nother way is to .say that the stars 

 whichare variable have great dark p.atchesonone.side of them, great 

 bright patches on the other. Well, of course you can get a varia- 

 tion of light by such a scheme as that ; but we have not observed 

 that, we are sim])ly inventing, merely suggesting ideas to nature 

 that I fancy nature will tell us by and by are (juite erroneous. 

 P'or instance, I have .shown you the facts with regard to 3 Lyra;. 

 What is the explanation put forward for the variability of that 

 star? Simply this, that it is a surface of revolution, the ratio 



-indicating lunv jippnrenliy irregular light-curves may be due to 

 the summation of two regular light variations. 



of the axes being 5 to 3, i.t-. elliplic beyond any experience of 

 ours with regard to any other bodies; there is a dark portion 

 at one end of the axis symmetrically situated. This thing then 

 has to turn and twist with its axes and the bl.ack spot, and .soon, 

 and at the end of the chajiter you are to have such a light curve 

 as that of 3 Lyr.v. That you see is blown into thin air by the 

 spectral facts. I think you will .acknowledge that these things 

 are irrational, because they have no true basis of fact, and we 

 must remember that in all this work we must deal strictly with 

 the facts in accordance with the rules of philosophising ; i.e. we 

 must never have a complicated explanation until we are perfectly 

 certain that a simjiler explanation will nol do, and the simplest 

 ex]ilanation of all is that which occurs most fretjuently in the 

 region of facts. That puts the soup-plate theory with regard to 

 variable stars entirely out of court. Further, remember that 

 supjwsing those gentlemen who still hold to the one-body theory, 

 one star, one variability, olyect to the possible explanation of 

 variability by the meteoritic hypothesis, they will fiiul it very much 

 more difficult to ex]ilain the ileparture from regularity by any 

 getjmetric system, because a geometric system must certainly be 



