May, 1903.] 



KNOWLEDGE 



109 



north of the plane of the Milky Way. Now the " apex " 

 of the solar motion lies roui,'hly yO"^ from this position, 

 and judging from the position of the apex found by Sir 

 William Herschel, Argelander and Airy (about 17h. 30ni.), 

 and that indicated by recent researches (about 18h. 30m.), 

 there may perhaps be a shift of the apex towards the 

 centre of the Milky Way, which should be the case if the 

 sun were revolving round a centre. This supposed "shift" 

 may of course be more apparent than real, and may, 

 perhaps, be partly or altogether due to errors of calculation. 

 The various positions, however, assigned to the apex show 

 a tendency at least to shift towards the supposed centre of 

 the Milky Way. However this may be, it seems not im- 

 proljal)le that the sun may be revolving round the centre 

 of gravity of the Milky VVay, which may also be the centre 

 of gravity of the whole system of stars composing our 

 visible universe. But if this be so, the sun is probably 

 at a great distance from the centre. For its measured 

 velocity is considerable, and in a system of stars, like a 

 globular cluster, those near the boundary would have a 

 greater velocity than those near the centre, the law of force 

 in such a system varying directly as the distance from 

 the centre. J. E. G-oee. 



Dublin, Aprd 14th. 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — Some further remarks on the question recently 

 mooted between Dr. Wallace, on the one side, and Mr. 

 Maunder and Dr. Turner, on the other, may be of interest 

 to the re id^rs of KNOwLEOfiE. First, all the astronomical 

 arguments in favour of a finite universe seem to me to turn 

 on the assumption that the ether is infinite Admittedly 

 it is only by the light of very distant objects that we learn 

 their existence ; anil for the transmission of this light, ether 

 is necessary. Our knowledge is, therefore, confined within 

 the limits of the eth r, whether it bs finite or infinite. But 

 how can we draw any conclusion as to the finiteness or 

 infiniteness of the universe if all that we can know of it is 

 limited to the finite sphere of the ether ':" With an infinite 

 and perfectly transparent ether, a limited numtier of shining 

 bodies would he suggestive of a limited vmi verse of matter; 

 but if the ether itself he finite, we would, in drawing this 

 conclusion, act like an islander who imagined that his 

 island was the only bit of solid land in creation, because, 

 when looking round on every side, he could see nothing 

 but water. His universe would be limited by the curva- 

 ture of the water as ours (on this assumption) is by the 

 finiteness of the ether. But. Dr. Wallace fails to see this. 

 He speaks of bodies wandering outside the limits of the 

 ether, but assiunes that they would be cold and dead, &c. 

 Why ? If ether is necessary to convey heat to them, it is 

 equally necessary to convey it away. They would, there- 

 fore, siuqily retain their heat without change. That they 

 would l)e ilark I admit, liut light is not necessary to animal 

 life. Then, if our ether be finite — if it be like a vast 

 attenuated nebula — why may there not be other great 

 iiebuliB of ether in external s|iacc, some of them, perhaps, 

 much larger and better supplied with stai's than our own? 

 A finite ether practically excludes us from knowing any- 

 thing lieyoud its limits, and for that very reason it allows 

 scope for all kinds of conjecture as to what is really 

 beyond them. Dr. Wallace does not suppose it to be a 

 vacuum. There are liodies in it which once belonged to 

 our universe ; and why should there not also be bodies in 

 it that never formed part of our universe — never entered 

 our ether ? There are, at all events, no grounds for 

 dogmatising on this subject. (But I do not think any 

 observation has shown such appearances and disappear- 

 ances of stars as would arise from their entering or leaving 

 the ethor.) 



We are, according to Dr. Wallace, in the centre of the 

 universe. He infers this from our position in relation to 

 the visible stars. I need hardly say that when our know- 

 ledge of the visible stars is carried further, our present 

 ideas as to their distribution may be largely modified, and 

 even as it is I doubt if any astronomer could go within 

 1000 light-years (if I am to use that ill-selected unit) of 

 the centre of the star-system as at present known. But 

 so far as we know, while heat, light, and some other 

 influences are dependent on the ether, gravitation is not, 

 and whatever the distance may be its transmission is 

 instantaneous. Hence if there are bodies outside of the 

 ether, as Dr. Wallace supposes, these bodies are as influential 

 in determining the centre of the universe as the bodies 

 which are situated within the limits of the ether. No 

 doul)t the distance of these outlying bodies must be very 

 great, but their number and mass may also be very great, 

 and some of them may not be more distant than some of 

 those witbiu the ether but near itsconfines. A finite material 

 world in an infinite ether is a hypothesis which finds some 

 support in the facts of astronomy. A finite ether with 

 matter (perhaps infinite) beyond it, is also to a certain extent 

 a feasible hypothesis ; but a finite ether and a finite material 

 world having distinct and independent limits seems to me 

 to be the worst hypothesis of the three, and is, moreover, one 

 that does not assist us in fixing the centre of this finite 

 universe of matter. 



1 therefore reject the astronomical basis of Dr. Wallace's 

 theory, leaving it to others to deal -with the i-onditions ni 

 animal life and mental development. I confess, however, 

 that I have sufficient faith in the principle of evolution to 

 think that man mght accommodate himself to the con- 

 ditions of life ou almost any of the planets, provided that 

 the change were sufficiently gradual, and a sufficient time 

 were allowed to elapse. But some arguments used by 

 Mr. Maunder and Prof. T inier seem to me also unsatis- 

 factory. The first of these is the argument drawn from 

 the number of dark bodies in space in an^wer to the proofs 

 of finiteness deduced from the faintne^s of the general 

 illumination of the sky. What would be the consequence 

 if the dark bodies were on the average as numerous and 

 as large as the bright ones, while the sun were adopted 

 as an average specimen of the latter? Simply that the 

 average illumination of the entire sky would be half that 

 of the sun. The dark bodies would be as often behind 

 the lii'ight ones as before them. Now compare this 

 with the fact. The highest estimate that I have seen of 

 the total light of the full moon is TfnoVoo of ^li"t of the 

 sun, the discs of the two bodies being about equal. 

 Suppose that the dark bodies were 150,000 times as 

 numerous as the bright ones, instead of being equal in 

 number. Then the whole sky ought to l>e as l>right as the 

 illuminated portion of the moon. Everyone knows that 

 this is not so. But it is said that the stars, though infinite, 

 may only extend to infinity in particular directions, e.g., iu 

 that of the Galaxy. Be it so. AVhere in the very brightest 

 portion of the (jalaxy will we find a portion equal in 

 angular magnitude to the moon which affords us the same 

 quantity of light? In the very brightest spot, the light 

 probably does not amount to -^l^ part of that of the moon 

 (when full), thus raising the proportion to fifteen millions 

 to one. Dark bodies may explain why the sky-light is not 

 infinite — which in fact it could not be even if all bodies 

 were bright, and their number infinite — but the extremely 

 small amount of sky-light which actually exists is a 

 startling fact, and one difficult to reconcile with the theory 

 of an infinite material universe. 



Again, I think too much stress has been laid by Dr. 

 Wallace's opponents on the motion of the solar .system iu 

 space. In the first place, the proof of this motion rests ou 



