82 



KNOWLEDGE. 



[April, 1903. 



most justly honoured of living men of science. The 

 Fortnightly Re i-iev for March, 1903, opens with an article, 

 at once striking and attractive, ou " Man's Place in the 

 Universe, as indicated bv the New Astronomy," by Alfred 

 Russel Wallace, d.c.l., f.r.s. The article leads up to a 

 theological application which it does not lie within our 

 ]>rovince here to discuss ; it is the astronomical basis of 

 the article alone with which we are concerned. 



Dr. Wallace's first thesis is that the stellar universe is 

 finite iu extent and the stars finite in number. His next 

 point is that the proper motions of stars furnish the best 

 indication of their distances. Then he argues that the 

 solar svstem is about equally distant from all parts of the 

 Milky Way, and exactly in its plane. Then that the sun 

 is one of the central orbs of a globular star cluster, and 

 that this star cluster occupies a nearly exactly central 

 position in the exact plane of the Milky Way. " Our sun 

 is thus shown to occupy a position very near to, if not 

 actually at the centre of the whole visible universe, and 

 therefore in all probability in the centre of the whole 

 material universe." 



This completes the first part of Dr. Wallace's enquiry ; 

 the second part deals with the earth's position in the solar 

 system as regards its adaptability for organic life. Here 

 Dr. Wallace leaves strictly astronomical questions and is 

 on his own ground. His point is that the conditions for 

 the development of organic life are far more stringent than 

 has been generally recognised. The surface temperature 

 of the planet must remain stable within a very limited 

 range, not for hundreds or thousands of years, but for 

 millions, perhaps for hundreds of millions. The chief 

 favourable conditions which in their combination appear 

 to have rendered the development of a complex system 

 of organic life possible on our earth are, its distance from 

 the sun securing the equability of temperature just 

 mentioned ; an atmosphere of sufficient density ; broad 

 and deep oceans, stirred into tides by the action of a large 

 satellite ; and the presence of deserts and volcanoes for 

 the distribution of atmospheric dust. The stringency of 

 these conditions appears to indicate that our earth is the 

 only home of organic life within the solar system, and 

 Dr." Wallace considers that suns near the confines of the 

 stellar universe cannot have systems sufficiently stable for 

 their planets to fulfil these conditions there. In his view, 

 therefore, the position of the solar system in the centre of 

 the material universe renders it probable that here, and 

 here alone, has organic life reached its full development. 



It will be seen that the entire argument falls to the 

 iTOund if the first point, the demonstration that our 

 universe is finite, is not complete. As Dr. Wallace himself 

 remarks : " Infinite space has been well defined as a circle, 

 or rather a sphere whose centre is everywhere and circum- 

 ference nowhere," and unless the material universe can be 

 proved to be finite, we certainly cannot prove that any 

 particular body occupies its centre. Dr. Wallace's argument 

 is, first, that the telescopes of greatest size have failed to 

 reveal to us fainter stars in anything like the same 

 proportion which smaller telescopes had done ; as if we 

 were looking right through the stellar universe, and out 

 into the blackness of space beyond. This is partly 

 accounted for by the fact that the increase in aperture of a 

 refractor is necessarily accompanied by an increase in 

 the absorption of its object-glass, and we are approaching 

 the limit where the gain and loss will be balanced. 

 So too with the photographic plate. For medium 

 luminosities it is perfectly true that an increase of 

 exposure will compensate for inferiority of light in a 

 strictly commensurable degree ; but the correspondence 

 ceases to hold good when we are dealing with very faint 

 lights. 



Dr. Wallace's next argument is an extraordinary one. 

 He tells us, and quotes Prof. Newcomb* in his support, 

 that were the stars infinite in number, then we should 

 receive an infinite amount of light from them. A reference 

 to what Prof. Newcomb actually has written shows that 

 Dr. Wallace has omitted two important limitations which 

 Prof. Newcomb attaches to this conclusion. It rests upon 

 the hypotheses " that light is nevei- lost in its passage to 

 any distance however great," and " that every region of 

 space of some great but finite extent is. on the average, 

 occupied by at least one star." In short. Prof. Newcoinb's 

 demonstration rests on the two conditions that light must 

 come through space to us without any loss, and that the 

 stellar universe must, on the whole, be uniform in 

 constitution ; it must not be structured. We know that 

 neither of these conditions holds good. As there are 

 bright bodies in space, so are there dai'k bodies. If the 

 first be infinite iu number, so must also be the second ; 

 we may almost say that the infinity of the second must be 

 of a higher order. As Sir Robert Ball recently put it, 

 " the dark stars are incomparably more numerous than 

 those that we can see . . . and to attempt to number 

 the stars of the universe by those whose transitory bright- 

 ness we can perceive would be like estimating the number 

 of horseshoes in England by those which are red hot." 

 The same line of argument which would infer that from 

 an infinity of bright suns the background of the sky 

 should shine as the sun at noonday, will lead yet more 

 forcibly to the conclusion, when the dark stars are the 

 basis of the argument, that we are shut in by a veil which 

 no light from an infinite distance can jjierce. On the 

 second point, that of structure, we need only the evidence 

 of our eyes. The existence of the Milky Way is proof 

 that our stellar system has a strongly marked form. 

 There is no approach to uniformity of the stars as to 

 direction, why should we assume that there is in distance? 

 But Dr. Wallace does not see that these two conditions 

 are vital. He writes : " Even if we make an ample 

 allowance for the stoppage of light by intervening dark 

 bodies, or by cosmic dust, or by imperfect transparency of 

 the ether, we should at least receive quite as much light 

 from them as the sun gives us at noonday," forgetful that 

 the entire argument depends upon the exclusion of these 

 three causes of absorption. 



The attempted demonstration of the finite nature of the 

 universe thus breaks down entirely ; it is based upon a 

 careless reading of Prof. Newcomb's book. In his next 

 point Dr. Wallace again rests upon Prof. Newcomb, whilst 

 again ignoring his deductions. He quotes : — " If we should 

 blot out from the sky all the stars having no proper 

 motion large enough to be detected we should find 

 remaining stars of all magnitudes, but they would be 

 scattered almost uniformly over the sky, and show no 

 tendency towards the Milky Way." Prof. Newcomb's 

 words are actually somewhat difi^ereut. He writes, " show 

 little or no tendency to crowd towards the Galaxy, unless, 

 perhaps, in the region near 19 hrs. of E.A. From this, 

 again, it follows that the stars belonging to the Galaxy lie 

 farther away than those whose proper motions can be 

 detected." This conclusion of Prof. Newcomb's cannot 

 be disputed, but Dr Wallace substitutes for it another, 

 viz. : that stars with measured proper motions constitute 

 a globular mass, and that we must be situated very near 

 indeed to the centre of this solar cluster. 



The points upon which Dr. Wallace lays stress as to 

 the Galaxy, namely, that the sun is situated in its central 

 plane, and nearly centrally with regard to it, are indeed 



* " The Stars : A Study of the Universe." By Prof. Simon Newcomb. 



